Chapter-01

INTRODUCTION

1.1Origins Of Fuel CellThe seminal work of William Grove on fuel cells in 1839 is well known nowadays but at the time of his invention, the scientist rather called the device a “gas voltaic battery”. Therefore, while year 1839 is unambiguously considered at the birth date of fuel cells, the first official appearance of the term “fuel cell” will only be found in a publication of the Transactions of the Faraday Society almost one century later, in 1922. Grove’s first invention, a battery called “Grove cell” was used by the American Telegraph Company due to its high current output until 1860. After having started as a lawyer Grove will turn to a professor of physics, and after then switch between a legal and a scientific carrier several times. With probably some help from the German chemist Christian Schönbein, a friend and colleague with whom he is exchanging fruitful ideas; he successfully reverses the electrolysis of water that has been discovered in the early 1800s by English chemists William Nicholson, Anthony Carlisle and experimented by Humphry Davy. Grove constructs a cell consisting of two separate sealed compartments, each one having a porous platinum foil electrode dipped in aqueous sulphuric acid and being fed by hydrogen gas and by oxygen gas, respectively. The experiment however, does not generate enough electricity to do useful work. Therefore, Grove combines in series several sets of electrodes and obtains the actual “gas battery”. He shows this way that a constant current can be drawn between electrodes and observes that water and heat are produced as by products. Yet, he is unable to quantify the reaction products and study in much more detail the system he has created. The reason is that these questions could not possibly be answered due to the lack of a comprehensive theory and adequate equipment in the 1840s. He is also conscious that the chief issue is to increase the “surface of action” between the components and comes close to the idea of gas electrodes as used in current fuel cells. It really seems that the man was too much ahead of his time! Unlike batteries such as the Grove cell, the gas battery is left as a scientific curiosity during a large part of the XIXth century. Times were not up to, should we say. Despite continued technological advancements, applications in the real world have not come to the mind of engineers and inventors. Nevertheless, during the same period the debate is lively among scientists trying hard to clarify the basic principles of electrochemical phenomena. Grove’s gas battery becomes a perfect practical illustration of these theoretical discussions. In order to explain the origin of current flow between certain materials, a “contact” theory involving mere physical contact and a “chemical” theory involving a chemical reaction are opposed. Schönbein and Grove are in favour of the chemical theory. The truth is actually in between,‐ and after a long controversy it is eventually established that in a gas battery the reaction will only occur in the contact zone between reactant, electrode and electrolyte. None but the main founder of modern physical chemistry, the Russian German ‐ Friedrich Ostwald eventually brings a decisive contribution to the theoretical and experimental understanding of fuel cell reactions in the 1880s. By skilfully associating measurements of physical properties and chemical analysis, and thanks to his work on metal catalysis, he succeeds to explain how fuel cells operate: not only the function of the different components (electrodes, electrolyte and gas reactants) but also their thermodynamically‐

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and kinetically driven interactions at the interface between the electrode and the electrolyte‐ under the presence of a fuel. This breakthrough in the scientific knowledge about gas batteries is the necessary open door for practical attempts to make them become a reality. At the turning point between the XIXth and XXth centuries, the first systems begins to come out from European and US laboratories as researchers are examining the possibility of converting coal or coal gas directly into electricity. As coal is a fuel, Grove’s gas battery changes its name into “fuel battery” and finally “fuel cell” as we know today.

In agreement with Grove’s request for a higher contact area between fuel cell components, early developments (50 years after Grove’s experiments!) are devoted to experimental improvements in the design, e.g., introduction of more efficient platinum black powder as a catalyst coated on the bulk platinum electrodes and addition of a porous diaphragm in order to impregnate the liquid electrolyte. In 1889 German chemist Ludwig Mond and his assistant Charles Langer build a device running on air and coal gas known as the “Mond gas”. At the same time other systems are setup by US and French teams. Typical problems experienced by researchers are due to materials chosen in the different parts or gas leakages between compartments, which prevent from reaching high voltages upon series combination of the unit cells and cause limited durability. It is also observed that only high cost precious metals‐ such as platinum can make the reaction with a valuable efficiency. This is obviously deleterious for practical applications of the process.

The end of the XIXth century is also the time for a prospective debate about the possible direct production of electricity from inexpensive coal and combustible gasses: such a perspective is asserted by some authors nothing less than a revolution while strongly tempered by others, (e.g., controversy in the Electrical World Journal in 1895). Finally, consensus is made on the fact that gas batteries are complex and costly systems unable to compete with more simple batteries. In the following period batteries undergo continued development for several applications including cars, whereas gas batteries are put away back in the lab for a few more decades. Let us remind that in the first quarter of the XX th century one third of all automobiles were battery powered electric vehicles. What visionary mind‐ could have predicted at this time that it would require hundred more years or so before gas batteries, now called fuel cells, could come close to being “ready for market”?

It is therefore at the lab level that the next improvements are obtained on fuel cells. The importance of the kinetics in the electrochemical reactions is discovered. At the beginning of the XXth century, new electrolyte materials performing at higher temperatures than aqueous solutions are explored that will lead to the various types of modern fuel cells: melt carbonates, solid oxides, phosphoric acid. Further historic information about practical developments of a specific fuel cell technology will be found in the corresponding pages of this website. For example, Francis Bacon’s pioneering work on acid phosphoric fuel cells is reported in the AFC page.

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1.2 History Of Fuel Cell

In 1889, the term “fuel cell” was first coined by Ludwig Mond and Charles Langer, who attempted to build a working fuel cell using air and industrial coal gas. Another source states that it was William White Jacques who first coined the term "fuel cell." Jacques was also the first researcher to use phosphoric acid in the electrolyte bath.

In the 1920s, fuel cell research in Germany paved the way to the development of the carbonate cycle and solid oxide fuel cells of today.

In 1932, engineer Francis T Bacon began his vital research into fuels cells. Early cell designers used porous platinum electrodes and sulphuric acid as the electrolyte bath. Using platinum was expansive and using sulphuric acid was corrosive. Bacon improved on the expensive platinum catalysts with a hydrogen and oxygen cell using a less corrosive alkaline electrolyte and inexpensive nickel electrodes.

It took Bacon until 1959 to perfect his design, when he demonstrated a five-kilowatt fuel cell that could power a welding machine. Francis T. Bacon, a direct descendent of the other well known Francis Bacon, named his famous fuel cell design the "Bacon Cell."

In October of 1959, Harry Karl Ihrig, an engineer for the Allis - Chalmers Manufacturing Company, demonstrated a 20-horsepower tractor that was the first vehicle ever powered by a fuel cell.

During the early 1960s, General Electric produced the fuel-cell-based electrical power system for NASA's Gemini and Apollo space capsules. General Electric used the principles found in the "Bacon Cell" as the basis of its design. Today, the Space Shuttle's electricity is provided by fuel cells, and the same fuel cells provide drinking water for the crew.

NASA decided that using nuclear reactors was too high a risk, and using batteries or solar power was too bulky to use in space vehicles. NASA has funded more than 200 research contracts exploring fuel-cell technology, bringing the technology to a level now viable for the private sector.

The first bus powered by a fuel cell was completed in 1993, and several fuel-cell cars are now being built in Europe and in the United States. Daimler Benz and Toyota launched prototype fuel-cell powered cars in 1997.

Maybe the answer to "What's so great about fuel cells?" should be the question "What's so great about pollution, changing the climate or running out of oil, natural gas and coal?" As we head into the next millennium, it is time to put renewable energy and planet-friendly technology at the top of our priorities.

Fuel cells have been around for over 150 years and offer a source of energy that is inexhaustible, environmentally safe and always available. So why aren't they being used everywhere already? Until recently, it has been because of the cost. The cells were too expensive to make. That has now changed.

In the United States, several pieces of legislation have promoted the current explosion in hydrogen fuel cell development: namely, the congressional Hydrogen Future Act of 1996

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and several state laws promoting zero emission levels for cars. Worldwide, different types of fuel cells have been developed with extensive public funding. The United States alone has sunk more than one billion dollars into fuel-cell research in the last thirty years.

In 1998, Iceland announced plans to create a hydrogen economy in cooperation with German car maker Daimler Benz and Canadian fuel cell developer Ballard Power Systems. The 10-year plan would convert all transportation vehicles, including Iceland's fishing fleet, over to fuel-cell-powered vehicles. In March, 1999, Iceland, Shell Oil, Daimler Chrysler, and Norsk Hydro formed a company to further develop Iceland's hydrogen economy.

In February, 1999, Europe's first public commercial hydrogen fuel station for cars and trucks opened for business in Hamburg, Germany. In April, 1999, Daimler Chrysler unveiled the liquid hydrogen vehicle NECAR 4. With a top speed of 90 mph and a 280-mile tank capacity, the car wowed the press. The company plans to have fuel-cell vehicles in limited production by the year 2004. By that time, Daimler Chrysler will have spent $1.4 billion more on fuel-cell technology development.

In August, 1999, Singapore physicists announced a new hydrogen storage method of alkali doped carbon nano tubes that would increase hydrogen storage and safety. A Taiwanese company, San Yang, is developing the first fuel cell powered motorcycle.

1.3 Basic Principle Of A Fuel Cell

Basically, a fuel cell is a device that converts directly the chemical energy stored in gaseous molecules of fuel and oxidant into electrical energy. When the fuel is hydrogen the only by‐ products are pure water and heat. The overall process is the reverse of water electrolysis. In electrolysis, an electric current applied to water produces hydrogen and oxygen; by reversing the process, hydrogen and oxygen are combined to produce electricity and water (and heat).

A fuel cell can be seen with profit as a “chemical factory” that continuously transforms fuel energy into electricity as long as fuel is supplied. However, unlike internal combustion engines that can be regarded as factories as well, fuel cells rely on a chemical reaction involving the fuel, and not on its combustion.

During combustion, molecular hydrogen and oxygen bonds are broken and electrons reconfigure into molecular water bonds at a picoseconds length scale. There is no possible way to “catch up” these free electrons and the net energy difference between molecular bonds in products vs. reactants can only be recovered in the most degraded form of energy, i.e. heat. A Carnot cycle involving the transformation of heat into mechanical and electrical energy is then involved in conventional methods for generating electricity: these successive steps of transformation of energy severely limit the overall efficiency of the process (which is by definition the product of the efficiency of the different steps).

In a fuel cell the direct conversion of the chemical energy of covalent bonds into electrical energy is made possible by the spatial separation of the hydrogen and oxygen reactants by the electrolyte. The electron transfer necessary to complete the bonding reconfiguration into water molecules occurs over a much longer length scale. This allows direct collection of electrons as a current in fuel cells and leads to fuel efficiencies two to three times higher than in internal combustion engines (depending on the fuel cell technology).

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Unlike batteries, there is no chemical transformation of any component of the fuel cell device during operation and it can generate power without recharging, as long as it is being fed with fuel.

The unit fuel cell structure called the membrane electrode assembly (MEA) typically consists of an electrolyte in contact on its both sides with two electrodes, one negative electrode (anode) and one positive electrode (cathode). Fuel is continuously fed to the anode side and oxidant is continuously fed to the cathode side.

Fuel cell reactants are classified as fuels and oxidants on the basis on their electron donor and electron acceptor properties. Oxidants mainly include pure oxygen and oxygen‐ containing gases e.g. air, or halogens e.g. chlorine. Fuels include pure hydrogen and hydrogen‐containing gases, e.g. methanol, ethanol, natural gas, gasoline, biogas, diesel, etc.

In the most straightforward case, i.e. the hydrogen fuel cell the combustion of hydrogen into water is split into two electrochemical reactions occurring at the anode and cathode, respectively, which are termed as the two half‐cell reactions:

H2 = 2 H+ + 2 e‐½ O2 + 2 H+ + 2 e‐ = H2O

Combination of the two half‐cell reactions gives the overall combustion reaction:

H2 + ½ O2 → H2O

In any fuel cell configuration the role of the electrolyte is crucial because it must insulate the two half‐cell reactions electrically in a strict sense while allowing the ionic passage of protons produced at the anode to the cathode side where they will combine and form a molecule of water. As a consequence, electrolytes are both good proton conductors and electric insulators. The third requirement for electrolytes is impermeability to gases in order to separate the anodic and the cathodes compartments, and thus prevent parasitic reactions due to gas crossover. Finally, the electrolyte has to be chemically resistant to any reactant or product during the process.

As passage of electrons is hindered through the electrolyte, they are forced to flow another way. To this purpose, electrodes are connected to an external electrical circuit and instead to follow protons the electrons take this second pathway. This allows direct collection of electricity. Depending on the type of fuel cell, the most suitable electrode materials are of various natures: metals or oxides, catalyzed or not. They are described in the section relative to the members of the fuel cell family. The common feature of fuel cell electrodes is a high surface area in order to maximize each half‐cell reaction zone; therefore they are relatively porous compounds.

Every type of fuel cell is characterized by its own particular geometry, dimensions, and materials; yet, the core of the device remains the same: it consists of an electrolyte, two electrodes, and two gas backing layers and most often, bipolar plates separating unit cells.

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For the gas backings not less than five different requirements must be fulfilled:

1. Good electronic conductivity to transport the electrons from the electrochemical oxidation of hydrogen most efficiently;

2. High gas permeability to allow easy access of the gas reactants from the feeding source to the reaction zone;

3. High porosity to optimize product water management in the system; 4. Good resistance strength to give a mechanical support to the MEA; 5. High corrosion resistance to the acidic environment in the fuel cell.

The bipolar plates are the interconnecting components that collect the electrons and drive them to the external circuit. They are grooved with channels for gas flow input and output and must manage water as well as possible. The design and the geometric dimensions of the channels (in the order of 1 mm) are crucial for obtaining a homogeneous transport of gases on the whole surface of electrodes, evacuate liquid water droplets formed by the fuel cell reaction, thus achieving stable continuous operation. As every component in a fuel cell, they must be corrosion‐resistant; but unlike gas backings, the bipolar plates must be gastight.

Fig.1 Fuel Cell

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Chapter-02

FUEL CELL TECHNOLOGY

2.1 Introduction

A fuel cell is a galvanic cell that efficiently converts chemical energy to electrical energy and useful heat. Stationary fuel cells can be used for backup power as well as distributed power. Modularity of fuel cells makes them useful for almost any portable application that typically uses batteries. Fuel cells have proved to be very effective in the transportation sector from personal vehicles to marine vessels.

There are two important types of fuel cells, namely, hydrogen fuel cells and microbial fuel cells. This study will be focused on hydrogen fuel cells. These fuel cells directly convert the chemical energy in hydrogen to electricity. The only by-products of this reaction are pure water and useful heat. Hydrogen fuel cells are more efficient than traditional combustion engines and are pollution free, given that one has a source of hydrogen. A traditional combustion power plant is 33% - 35% efficient in generating electricity, whereas fuel cells have been known to be 60% efficient without cogeneration [2]. In addition to that, fuel cell engines have fewer moving parts when compared to a traditional combustion engine, and this helps in their quieter operation.

Fuel cells are expected to be suitable for a wide range of applications. Transportation applications include vehicle propulsion and on-board auxiliary power generation. Portable applications include consumer electronics, business machinery, and recreational devices. Stationary power applications include stand-alone power plants, distributed generation, cogeneration, back-up power units, and power for remote locations. Within CASCADE MINTS, we focus on applications that might achieve a relevant share with respect to their overall energy consumption in a future energy system, i.e. stationary applications in power plants (mainly decentralised cogeneration), and vehicle propulsion.There are several different fuel cell technology paths being pursued. These divide into low temperature and high temperature technologies. Low temperature technologies, including phosphoric acid (PAFC) and polymer electrolyte membrane (PEMFC) fuel cells, target transportation, portable power, and lower-capacity distributed power applications. High temperature technologies, including molten carbonate (MCFC) and solid oxide (SOFC) fuel cells, focus on larger stationary power applications, niche stationary and distributed power, and certain mobile applications (e.g. APU). A combination of technology developments and market forces will determine which of these technologies are successful. Currently, phosphoric acid fuel cells are the only commercially available fuel cells. While more than 200 units of the 200 kW PC25 fuel cell manufactured by UTC Fuel Cells are in operation around the world, UTC decided recently to stop the production of the PAFC fuel cell, because cost targets could not be achieved, and to focus on PEMFC development in the future.

2.2 Working of Fuel Cell[7]

Figure 1 shows the basic working principle of a hydrogen fuel cell. It consists of two electrodes separated by an electrolyte. When hydrogen gas, in channels, flows to the anode, a catalyst (usually platinum based) causes the hydrogen molecule to split into protons and electrons. These electrons follow an external circuit to the cathode, whereas the protons get conducted through the electrolyte. This flow of electrons through the external circuit is the produced electricity that can be used to do work.

Fig.2.1 Working of hydrogen fuel cell

2.3 Classification of Hydrogen Fuel Cells

In this section, we will be discussing the classification criterion of hydrogen fuel cells followed by a detailed description of each hydrogen fuel cell. Table 1 provides us with the classification of the types of hydrogen fuel cells that are currently in use and development. Fuel cells are usually classified depending on the electrolyte that is used in them, with one exception: the direct methanol fuel cell in which methanol is directly fed to the anode in the course of the reaction. Methanol acts as a fuel in these types of fuel cells eliminating the need to reform the fuel to hydrogen. Fuel cells can also be classified on the basis of operating temperature for the fuel cell. Alkaline fuel cells, Polymer electrolyte membrane fuel cell, direct methanol fuel cell and Phosphoric acid fuel are low temperature fuel cells: the operating temperature is below 220C. Molten carbonate fuel cells and Solid oxide fuel cells are high temperature fuel cells, with an operating temperature of around 600-1000C.

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Table 2.1: Classification of fuel cells

Fuel Cell Type Operating System Output Efficiency ApplicationTemperature

(C)

53-58% Backup Power,

Polymer 50-100 1kW – 250kW(transportation) Portable Power,

Transportation,Electrolyte 25-35% (stationary) Small Distributed,

Generation

Direct Methanol 60-90 1W – 100W 25-35% Small PortablePower

Alkaline 90-100 10kW – 60% Military, Space100kW

Phosphoric Acid 150-200 50kW – 1MW >40% DistributedGeneration

Large Distributed

Molten Carbonate 600-700 1kW – 1MW 45-47%Generation, Electric

Utility.

Auxiliary Power,

Solid Oxide 600-1000 1kW – 3MW 35-43%Large Distributed

Generation, ElectricUtility

2.3.1 Stationary fuel cell systemsThe following sections provide an overview on the state of the art and the current most relevant development paths with regard to stationary fuel cells. While there is still a considerable variety of competing fuel cell technologies, there are currently two main fields of application for stationary fuel cells: small scale house supply fuel cell heating appliances with a capacity of 1 to 5 kWel, and fuel cells of the 200 to 300 kWel class which can be used as a district heating CHP plant or for power and heat supply for commercial or industrial applications. It is possible that after a first successful market integration of high temperature fuel cells (MCFC,SOFC) of the 300 kWel class these technologies will expand towards the multi-Megawatt class.

All the fuel cell systems discussed below are in a very early phase of market introduction. The UTC 200 kW PAFC is the only stationary fuel cell system commercially available. Most of the other systems are currently tested in field trial programs, while for some fuel

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cell types there exist only first proof-of-concept demonstration plants. As a consequence, the availability of reliable data is poor. Taking into account the context of early market introduction, it is obvious that technical and economical data are quite sensitive, and thus in general a matter of confidentiality. It is difficult to assess to which extent specific data published by fuel cell manufacturers can be generalised or transferred into another context. Any data on current investment cost is uncertain, and the information on the cost structure given by different manufacturers seems to be partly contradicting.The data given in the following sections was compiled largely during a recent German research project on long term perspectives of stationary fuel cells in Germany (Krewitt et al. 2004). The technical data and cost data underlying the analysis were reviewed in a workshop that took place in June 2003 in Stuttgart, with participation from fuel cell manufacturers and utilities. Although the workshop did not claim to achieve an explicit agreement on each data set, workshop participants in general agreed to the basic database used in the project. We thus conclude that an updated version of the database elaborated in (Krewitt et al. 2004) is a sound basis for the future work in CASCADE MINTS.It is difficult to derive reliable estimates on operation and maintenance costs from the current pilot and demonstration plants. We expect that O&M costs during the fuel cell market introduction phase are higher than those of a conventional CHP unit, but that on the lung run O&M costs for the fuel cell system are lower because there are less moving components. We thus assume ‘generic’ O&M costs that are similar for all types of fuel cells, which are 5% of the investment costs per year during the early market introduction phase, and 2% of investment costs per year for a mature system.Based on the description of the individual fuel cell technologies, section 1.1.6 provides a summary of the main technical and economic data for each type of fuel cell which can be used as an input to the energy models in CASCADE MINTS. Some of the data which are required for a consistent characterisation of the fuel cell technology in the model are not always directly available from the data sources (e.g. availability, stack lifetime), and are thus specified based on expert judgement. As some of the models to be used in CASCADE MINTS might not differ between different fuel cell technologies for the same applications, section 1.1.6 suggests also the specification of two ‘generic’ fuel cells for small scale heating appliances, and for cogeneration units of the 300 kW-class.

2.3.2 Polymer electrolyte membrane fuel cell (PEMFC)

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In this section we discuss the polymer electrolyte membrane fuel cell (also known as proton exchange membrane fuel cell). We start with a basic introduction of the fuel cell followed with the specifics of the electrodes and electrolyte used in the PEMFC.

PEMFC are a type of the low temperature fuel cell with an operating temperature in the range of 85C - 105C. The low temperature operation delivers high current density and high power density. This allows the cell to have a compact design, lightweight and faster response time when compared to other fuel cells.

Cell components for PEMFC

As the name suggests, a solid proton exchange membrane is used as electrolyte in a PEMFC. The proton conducting membrane is an important component of the fuel cell. Using a solid electrolyte has its advantages. The sealing of the anode and cathode gases becomes easier, which in turn makes the manufacturing economical. Unlike liquid electrolytes, solid electrolytes are less prone to corrosion allowing the system to have longer cell and stack life. Figure 2 here shows the working of a PEMFC. Platinum impregnated porous gas diffused electrodes are usually used in PEMFC’s to ensure the regular supply of reactant gases to the system. The back of the electrodes is coated with polytetrafluoroethylene (PTFE) that provides a waterproof path for diffusion of gas to the catalyst. The gas supply, the catalyst particle, and the ionic conductor form a three-phase boundary.

Membranes usually operate in a very limited temperature range. Nafionis the most studied membrane in the PEMFCs [3]. Membranes in this fuel cell are generally filled with water that keeps the conductivity high. Thus, water management becomes major issue in the fuel cell. Solidifying the gases coming into the fuel cell can solve this problem.

Fig.2.2 PEM

Fields of application

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A variety of stationary PEM-fuel cell systems are being developed (for mobile applications). Various manufacturers (including e.g. Buderus/IFC, HGC, IDATECH, Nuvera, Vaillant, Plug Power, Ballard, Viessmann) aim at small scale house supply cogeneration systems with a capacity of 1 to 5 kWel. The fuel cell systems in general run on natural gas, and are equipped with a gas reformer integrated into the system. The systems are designed to operate to feed electricity into the utility grid (‘virtual power plant’).Ballard Generation Systems developed a stationary PEMFC generator with a capacity of 250 kWel (P2B-PEM-FC). The system is designed to deliver power to the onsite power grid and to provide heat. A field trial programme with several units installed in the US and Europe was launched in 1999, and is expected to continue until 2004 (Ballard 2004).

Technical data

Table 1.2 summarises the key technical data of PEMFC systems used for the two different types of application described above. Data are based on a review of technical data published from the various manufacturers. It should be noted that the electrical, thermal and overall efficiency depend on the intended application of the system (e.g. level of heat supply).With regard to the house supply system, there are data available e.g. from the Vaillant’s field trial programme in Europe, which is supported by the European Commission. The Vaillant system is based on a PEM- fuel cell provided by Plug Power. A total of 31 Vaillant fuel -cell heating appliances have been installed by now in several European countries and linked with each other via a control centre to analyse how fuel cells can be run as a virtual power plant. 13 units of a 4.7 kWel system (“EURO1” version) were installed in the first half of 2003, and the installation of further 43 units (“EURO2”) started in November 2003. Measurement data from the EURO2 systems suggest that an electrical efficiency of up to 31 %, and a total efficiency of 88-89 % can be achieved. The lifetime of the stack is currently about 6,000 h, according to Vaillant a stack lifetime of 12,000 h seems to be realistic with an acceptable degradation (Klinder 2003). There are no published data available on the system’s availability.Measured data from the Ballard 250 kWel P2B-PEM-FC operated in Berlin since June 2000, which was the first system installed in Europe, indicate that an electrical net efficiency of 33 to 34 % was achieved, with a total efficiency of more than 70 % (Pokojski 2001). The target electrical efficiency of a pressurised system is 40 %. The availability of the first pilot plant during the first time of operation was still low. There are no data open to the public on stack degradation.

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Table 2.2: Technical data of PEMFC systems1-5 kWel PEMFC fuel 250 kWel PEMFC

cell heat appliancestate-of- long term state-of- long termthe-art target the-art target

Performance dataElectrical capacity kWel 1-5 1-5 250 250Electrical net efficiency % 28 32 35 42Overall efficiency % 80 90 76 84Stack lifetime h 10,000 40,000 40,000Availability %

Fuel processingFuel natural gas natural gas, biogasReforming steam reformer Steam reformer

Economic dataThe cost of the Plug Power 5 kWel fuel cell, which is used e.g. in the Vaillant system, is about 55,000 $, the specific costs are thus 11,000 $/kW (Fuel Cell Today 2002). Plug Power expects that still 4 to 7 years are required until a fuel cell heat appliance system is an economically viable option for the end-use costumer. Since January 2001, Vaillant could reduce the manufacturing costs of its fuel cell system by 50 % within 1.5 years (Klinder 2002). Target costs for the Vaillant fuel cell heat appliance system are given as 1,500 €/kW.

Based on a detailed costs analysis, Siemens concludes that assuming a production volume of 50,000 units per year the costs for the individual components of a 5 kWel PEMFC system amount to at least 1,550 €/kW. It was assumed that the stack with 100 €/kW contributes to only about 6 % of the total system costs (Kabs 2002).

Data from Viessmann indicate that the material costs for the current 2 kW el prototype are 6,100 €/kW if a product volume of 10,000 units per year is achieved, the material costs of the stack contribute about 20% of the total material costs (Britz 2003). The reformer costs contribute to about 8 to 10% of the total costs.The total cost of the Ballard 250 kWel P2B-PEM- FC pilot plant which was put into operation in June 2000 in Berlin was about 3.8 Mill €, with about 5 % of the total costs allocated for planning and other services (Pokojski 2001). The specific system costs were thus about 14,000 €/kWel. There are no data available on operation and maintenance cost.According to (Schmidt et al., 2002), stack costs of around 30 $/kW can be achieved in an automated mass production, leading to system costs of about 100 to 200 € /kW. We take this optimistic view as an indication of floor costs for small scale (1-5 kWel) PEM-systems for house applications.

2.3.3 Phosphoric acid fuel cell (PAFC)

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In this section we discuss the Phosphoric Acid fuel cell. We start with a basic introduction of the fuel cell followed with the specifics of the electrodes and electrolyte used in a PAFC.

The phosphoric acid fuel cell is a low temperature fuel cell with an operating temperature of about 200 ºC. It is the most advanced fuel cell system with its main application in stationary power plants. PAFC is amongst the first few commercialized fuel cell technology with worldwide installed capacity of 75 MW . These cells are expected to find a position in the market for applications of about 1 MW as they are very reliable and can be used for cogeneration of low-temperature steam.

Cell components for PAFC

Figure 5 below shows the general working of a PAFC. Electrodes of a PAFC are Pt bonded PTFE (Polytetrafluoroethylene). At the cathode, relatively higher loading of Pt is necessary for the oxygen reduction reaction [3, 55]. It was the development of supported platinum electro catalysts that helped to reduce the platinum loading. In recent times, platinum supported carbon black electrodes are also used along with the porous PTFE electrode structure as electro catalyst . The carbon not only increases the conductivity of the electrodes but it also help in dispersing the Pt catalyst and ensuring the proper utilization of the catalyst .

Phosphoric acid is used as the electrolyte in the PAFC. The ionic conductivity of phosphoric acid is low at low temperatures, thus PAFC’s are operated in temperature range of 150 - 200 ºC. In the beginning diluted PAFC was used to avoid the corrosion of the cell elements, but with the advent in technology and with development of better materials 100% concentrated acid is used. The higher the concentration of the acid, higher is the conductivity of the electrolyte. Operating temperature and concentration of the acid have increased in order to achieve better performance.

Figure 2.3: Phosphoric Acid Fuel Cell

Fields of Application

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The PAFC is available for commercial applications. UTC as the main manufacturer introduced the PC-25 200 kWel system, the world’s first commercially available fuel cell power plant, in 1991. Today, more than 250 units of the PC-25 have been delivered. These power plants operate independently or connected to the grid, partly as a co-generator with heat recovery, and it is used in numerous applications that require ‘premium’ power supply.

Technical dataTable 1.4 summarises the technical data of the current PC25TM as specified by UTC Fuel Cells (UTC 2004). The efficiency of the system is high over a wide range of load settings. The standard PC25 delivers product heat to a heat exchanger designed to provide 60 oC hot water to customer loads, a high-grade heat option is also provided that makes approximately half of the power plant heat available at temperatures up to 120 oC. Overhaul after 40,000 h includes replacement or factory refurbishment of the stack. Experience with the PC25 C system shows that power plant availability is > 90% (King and McDonald 2003). The system is designed to run on natural gas or anaerobic digester gas.As the costs of the PC25 system could not be reduced as required to achieve significant market penetration, UTC announced in 2002 to phase out the production of PAFC systems. As a consequence we do not report any long term technical development targets.

Table 2.3: Technical data of PAFC system

state-of-the-art long term targetPerformance dataElectrical capacity kWel 200Electrical net efficiency % 37Overall efficiency % 87Stack lifetime h 40,000Availability % > 90

Fuel processingFuel natural gas, anaerobic digester gasReforming steam reformer

Economic dataUTC reported costs for the 200 kWel PC25 unit to be approximately 850,000 $ , the specific costs for the installed system are thus about 4,250 $/kW (UTC 2002). The cost of the PAFC power plant is approximately three times higher than required for significant market penetration. There is no unique component or system in the power plant that dominates the overall system costs. As mentioned above, in 2002 UTC announced to phase out the production of PAFCs because there is no realistic perspective to bring the costs down to the target costs, and to focus future fuel cell development activities towards the development of PEM-fuel cells.

2.3.4 Molten carbonate fuel cell (MCFC)

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In this section we discuss the Molten Carbonate fuel cell in detail. We start with a basic introduction of the fuel cell followed with the specifics of the electrodes and electrolyte used in a MCFC.

The molten carbonate fuel cell is a high temperature fuel cell having an operating temperature of about 600 - 700C. The high temperatures are needed to improve the conductivity of its carbonate electrolyte and still work with low cost metal cell components. The high temperature also improves the oxidation - reduction processes at the electrodes. The high temperature has two major disadvantages. It places a great demand on corrosion stability of the cell and it adversely affects the life span of the cell components. MCFCs have proven to attain an electrical efficiency of approximately 50%.

Cell components for MCFC

Figure 4 below shows the working of a MCFC. Electrodes for a MCFC are usually made from Nickel. The cathode for MCFC is made of Nickel Oxide (NiO) and Ni-Al or Ni-Cr alloys [3, 10, 55]. The problem with nickel oxide cathodes is that particles of nickel oxide creep into the molten carbonate over a period of time which reduces its conductivity. Hence, lithium oxide material in combination with nickel oxide is used to avoid this problem. Nickel oxide is used because it is very active at high temperatures for oxygen reduction which eliminates the need for a Pt based catalyst.

The electrolyte used in the MCFC is alumina based and it is in the form of a stabilized matrix. Till the 1990’s, the electrolyte was prepared by fabricating it into a tile using a hot pressing process. Nowadays tape casting methods are used for the preparation of the matrix. Ceria based electrolytes with better stability at higher temperatures are being used as electrolyte, but ceria-based materials are very expensive. Thus, mixtures of lithium and potassium carbonate salts that melt at high temperatures are also being considered .

Figure2.4: Molten Carbonate Fuel Cell

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The electrochemical reactions occurring in MCFCs can be summarised as follows:

Anode: H2 + CO32- H2O + CO2 + 2e-

Cathode: ½O2 + CO2 + 2e- CO32-

Overall cell reaction: H2 + ½O2 + CO2 (cathode) H2O + CO2 (anode)

High operating temperatures enable internal reforming, usually carried out with steam and promoted by a catalyst. Various concepts have been developed to use coal gas, biogas or liquid hydrocarbons as a fuel.

Fields of application

While several years ago according to market requirements development of MCFC power plants aimed at the multi-MW class, currently the 250 kW class for decentralised combined heat and power production is close to commercialisation. The largest number of demonstration plants today is based on the ‘HotModule’ concept developed by MTU CFC Solutions (which is based on cells from FCE), with currently 14 systems installed. Some of them are operated to provide premium power supply and combined heat and power for hospitals, which is a quite challenging supply task in terms of reliability. We thus take the technical data of the HotModule as a reference. Future developments will include an up- scaling of the system (MTU announced to produce a 1 MW system in 2006 (Berger 2003)), and the development of hybrid systems with a steam or gas turbine.

Technical dataMeasurement data on the HotModule demonstration plants, which run on natural gas, confirm that the relatively high electrical efficiency of 47% could be reached (Berger 2003). We assume that the CHP unit achieves an overall efficiency of 83%, with depleted air available at a temperature of 400 oC. There are no public data on availability and on stack degradation, but it is reported that the stack lifetime is still one of the main challenges. Similar to other fuel cell manufacturers, the target for the stack lifetime is given as 40,000 h.As development activities mainly focus on increasing the electrical efficiency, it is expected that future developments will lead to an increase of the electrical efficiency by 5 percent points to 52%, while the overall efficiency increases to 86%. It is expected that a future hybrid system which combines a MCFC with a steam turbine will achieve an electrical efficiency of 55 to 56%, with an overall efficiency of 90%.

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Table 2.4: Technical data system250 kWel MCFC hybrid MCFC with steam

turbinestate-of-the- long term 2010 (?) long term

art target targetPerformance data

Electrical power kWel 250 250 1,000 1,000Electrical net efficiency % 47 52 55 56Overall efficiency % 83 86 90 90Stack lifetime h << 40,000 40,000 40,000availability % 80 (?) 90 (80) 90

Fuel processingFuel natural gas, biogas natural gas, biogasDesulphurisationReforming internal reforming internal reforming

Economic data

The system costs of the Hot Module 2, which was put into operation 1999, and was the first system which was operated by the customer, were 2.9 Mill €, which is equivalent to specific investment costs of about 12.000 €/kW. The cost of the Hot Module installed at the Los Angeles Department for Water and Power (LADWP) headquarters in 2000 was reported to amount to 2.4 Mill $ (MTU 2000). The stack costs are estimated to contribute about 30% of the total system costs. No data are available on operation and maintenance costs.According to MTU, the target costs for a commercial product are below 1,250 €/kW. MTU expects to achieve this target once the production volume amount to 40 to 50 MW/a (Berger 2003), which is equivalent to the production of 160 to 200 systems per year.

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2.3.5 Solid oxide fuel cell (SOFC)

In this section we discuss the Solid Oxide fuel cell. We start with a basic introduction of the fuel cell followed with the specifics of the electrodes and electrolyte used in a SOFC.

The SOFCs are the latest entry to the high temperature fuel cells with an operating temperature of 1000°C. SOFCs are a two-phase gas-solid system, which is a major advantage over other fuel cells i.e. the absence of a liquid electrolyte eliminates the need for elaborate systems for water management or flooding of the catalyst layer. SOFCs have demonstrated high power densities that help in the compact designing of its system. Due to the operating condition of the system, special materials are required to withstand the high operating temperature. Thus, the development of low cost ceramic structures (which would work efficiently under such high temperatures) is the key to commercialize SOFCs.

Cell components for SOFC

Figure 7 below shows the basic working of a SOFC. The electrodes in the SOFCs have to perform under severe operating conditions. Thus, right from the beginning LSM (Lanthanum Strontium Magnetite) cathodes have been used, since they are stable under SOFC operating temperatures and show high activity for oxygen reduction at high temperatures. The anodes of the SOFCs are Ni based usually Ni-Zr (nickel – zirconia cermet). Applying a thin layer of zirconia particles improves the conductivity and stability of the electrodes.

The SOFCs use solid oxide ceramics, usually perovskites, as the electrolyte that operates at temperatures as high as 1000C. Electrolytes supported with Zirconium oxide (ZrO2) have proven to be highly conductive and stable . Compared to other cell components, the electrolyte layer exhibits high ionic and low electronic conductivities.

The solid state character of the SOFCs, enable us to shape the cell according to the type of application. Also the solid electrolyte eliminates the need of a water management system (like in PEMFC) and helps in avoiding the corrosion of cell components. But due to the high temperature of the SOFCs, it is difficult to find suitable materials that would have the necessary thermal and stability properties. Thus, it is one of the major contributors to the cost of SOFC.

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Figure 2.5: Solid Oxide Fuel Cell

Fields of Application

Current SOFC development aims primarily at stationary applications. The two main manufacturers which are closest to SOFC commercialisation developed two different SOFC concepts for different applications for the combined heat and power sector:

• The Sulzer Hexis system, which is based on the planar SOFC concept, generates 1 kW of electrical power and is intended to cover the thermal requirements of a one-family house. A modular auxiliary burner integrated into the system is switched on automatically on higher heat demand, thus the system replaces a conventional gas boiler. The system is designed for use with natural gas, it is operated in parallel to the grid. Sulzer Hexis is commercialising the fuel cell system “HXS 1000 Premiere” with an energy contracting programme in close cooperation with utilities. A pre-series phase was launched towards the end of 2001. It is planned to manufacture a near-series product starting from the end 2004/beginning 2005.

• Siemens-Westinghouse is planning two major product lines based on the tubular SOFC concept. The first product will be a 250 kW cogeneration system operating at atmospheric pressure. The system will be a direct scale-up of a 100 kW demonstration plant, which was operated in the Netherlands since 1997, and later in Germany. The 250 kWel system will be followed by a pressurised SOFC/gas turbine hybrid system of approximately 500 kWel. A 220 kWel proof-of-concept hybrid system is operated at the University of California, Irvine. After initial production, larger systems are expected as well.

Current developments also aim at using small scale (1-10 kW) SOFCs as Auxiliary Power Units (APU) in vehicles and aircrafts. While this application might open up a potential high value mass market for fuel cells, it is not relevant in terms of installed capacity from the overall energy system’s perspective, so that we do not consider it in the context of CASCADE-MINTS.

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Technical Data

The technical data of the relevant SOFC systems are summarised in Table 1.8. Data from the pre-series phase of the Sulzer Hexis system indicate that an electrical net efficiency between 25 and 30 % were achieved. The development target is to achieve an electrical efficiency > 30 %, so that we assume a 32 % net electrical efficiency and a 90 % overall efficiency as a long term development perspective. The main problem of the per-series product is the high stack degradation, resulting in an average lifetime of the stack of only 2000 hours (Raak 2003). More than 90 % of the demonstration systems had an availability of more than 90 %.There are currently two demonstration plants that can be used to get an indication of performance data of the Siemens-Westinghouse concept. A 100 kWel SOFC cogeneration unit was first operated by EDB/ELSAM in the Netherlands, and was moved in 2001 to Essen, Germany for operation by RWE. The demonstration plant has been operated since more than 20,000 hours, and achieved an electrical efficiency of 46 %. According to information from Siemens, a stack lifetime of more than 80,000 hours seems to be achievable already now with the Siemens-Westinghouse tubular stack concept (as a conservative estimate, we assume a stack lifetime of 40,000 hours for current state-of-the-art).The first SOFC/gas turbine hybrid system is operated by Southern California Edison at the University of California. As of January 2002 the system has operated for more than 900 hours and has demonstrated 53 % electrical efficiency (Siemens 2004). It is expected that hybrid systems with a capacity of 1 MW can achieve an electrical capacity of 60 % using simple small gas turbines. At the 2 to 3 MW capacity level with larger, more sophisticated gas turbines analysis indicates that electrical efficiencies of 70 % or more are possible. However, the construction of a 1 MW “Mega-SOFC” in Marbach (near Stuttgart, Germany) which was planned to be brought into operation in 2003, had to be cancelled; the ‘official’ reason was that there was no appropriate micro-gas turbine available, but there are also doubts on whether cost targets could have been achieved.

Table 2.5: Technical data of SOFC systems

1 kWel SOFC 250 kWel pressurised hybridsingle-family house atmospheric SOFC SOFC

applicationstate-of- long term state-of- long term state-of- long termthe-art target the-art target the-art target

Performance dataElectrical power (fuel cell) kWel 1 1 250 250 244 400Gas turbine AC power kWel

28 32 47 4965 100

Electrical net efficiency % 58 60Overall efficiency % 80 90 80 85 80 85Stack lifetime h 2,000 40,000 100,000 40,000 100,000availability % > 90

Fuel processingFuel natural gas natural gas, biogas natural gas, biogasDesulphurisation activated carbonReforming steam reformer or internal reforming internal reforming

catalytic partialoxidation

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Economic dataThe costs for the 100 kW Siemens-Westinghouse demonstration plant which was put into operation in the Netherlands in 1998 were estimated to about 100,000 €/kWel by (Kuipers 1998). Investment costs for a today’s demonstration plant are estimated by Siemens to 10,000 to 25,000 $/kW. Costs for a first commercial market introduction shall be reduced to 6,000 $/kW, and the long term target costs are 1,000 kW for a CHP unit, and 500 $/kW for a small scale SOFC system (Kabs 2002), which we consider here as extremely optimistic. Based on the analysis of component costs, Siemens-Westinghouse expects that a turn-key-price of 1,300 $/kW for a 1 -3 MW hybrid system (SOFC/gas turbine) can be realised if series production of 50 units per year is achieved (George a. Bessette 1998). Tanaka et al. (Tanaka et al. 2000) however estimate the investment costs for a 1 MW pressurised hybrid system to 4,900 $/kW. The costs for the stack are estimated to about 25 to 50 % of the total system costs, while the share of the gas turbine is about 35 %.For planar concepts, prices for electrolyte-supported cells produced in batch wise series are in the range of 2,500 to 5,000 $ /kW, whereas the prices of prototypes of anode-supported cells are in the order of 12,000 $/kW (Stöver et al. 2003). It is obvious that an order of magnitude cost reduction is still required to achieve commercially acceptable prices for the cells. Sulzer Hexis expects to reduce costs for the 1 kWel HXS Premiere system to 3,600 CHF until 2010 (Sulzer Hexis 2001).

2.4 Other Fuel Cell Technologies

In addition to the fuel cell technologies described above, there are other fuel cell technologies that had importance in the past, or are an important future option. In this section we briefly describe these fuel cell technologies and give some reasoning for not including them in our study.

Alkaline Fuel cellIs a low temperature fuel cell that was amongst the first fuel cells to be used in the Apollo space missions that led to its application in the European Hermes Project [3, 4]. The AFC uses aqueous solution of potassium hydroxide as electrolyte and Pt-Co (Platinum-Cobalt) and Pt-Pd (Platinum Palladium) alloys electrodes. Major operating constraints for AFCs are that they work well only with pure gases and it requires for low carbon dioxide concentrations in the feed.

AFC’s are known to have the highest electrical efficiency among fuel cells, but interest in these types of fuel cells has diminished over the years as they were considered too costly for commercial applications and also there are no significant advantages over PEMFCs.

Direct Methanol fuel cells

It is actually a subset of Polymer electrolyte membrane fuel cells, and are typically used for small portable applications having low operating temperature. Methanol is directly fed to the anode in these fuel cells. This eliminates the need for a fuel reformer to convert the fuel to hydrogen. This makes the DMFC a very promising candidate for portable power sources, electric vehicles and transportation application. The working of a DMFC is similar to that of the PEMFC and it also uses a selective membrane as its electrolyte.

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The DMFCs are typically used for small portable applications having low operating temperatures. Thus, it is highly unlikely that these types of fuel cells could be used in a wind farm for storage purposes.

2.5 Fuel Cell Systems For Automotive Application

2.5.1 Polymer Electrolyte Fuel Cell (PEFC) System

Basic mechanism

The basic mechanism of polymer electrolyte membrane fuel cells (PEFC) is described in section 1.1.1. on stationary fuel cell systems. For automotive applications PEFC can be applied also, however the main difference between stationary and automotive applications lies in the high degree of integration of system components in order to comply with the available space in a passenger car.In order to operate a fuel cell stack in general, supplementary technical equipment is necessary. These equipment is composed of a stack cooling system, a cathode air supply compressor, sensors and control valves. In addition so called ancillary components or ‘Balance of plant’ (BOP) and efforts are needed to operate the fuel cell stack within an application: e.g. an electronic controller, a start-up battery, electrical equipment, safety equipment and system packaging. The figure below illustrates such a PEFC system with the stack and the surrounding controls.

Figure 2.6: 1.5 kW PEFC system (Source: DLR-FK)

Fields of applicationPolymer electrolyte fuel cell stacks are preferably used in fuel cell engines which are designed to be easily integrated into electric vehicles. Today, most of the fuel cell demonstration vehicles are custom build with an individual integration of components in the vehicle. Development towards modularisation and system integration however is the way forward in the future in order to fit in the concept of automobile manufacturing.

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Fuel cell engines typically consist of the system module, the stack module, a control unit, power distribution unit (PDU) and a cooling pump, as shown in Figure 1.7 from Ballard. Ballard is today the world leader as a fuel cell system integrator.

Figure 2.7: Light duty fuel cell engine Xcellsis HY-80 from Ballard

To date a small fleet of fuel cell vehicles has been demonstrated (see Table 1.12 for vehicles of the time 2001-2003). However no fuel cell vehicle is commercially available yet. The next generation of demonstration vehicles is expected to be introduced between 2005-2007. Commercially available fuel cell vehicles are expected to be manufactured after 2012. Market penetration however depends on several other factors like e.g. the existence of a hydrogen supply infrastructure.

Table 2.6: Hydrogen fuel cell demonstration vehicles 2001-2003.

Car manufacturer Fuel Cell Nr. of Project Yearmanufacturer demonstration

projectsDaimler Chrysler Ballard 14 F-Cell (A - Class) 2002Ford Ballard 5 Advanced Focus FCV 2002

General Motors Hydrogenise/GM 9 Diesel Hybrid Electric 2003Military truck

Honda Ballard 5 FCX 2002Toyota Toyota 10 FCHV-Bus2 2003Nissan Ballard/UTC 3 X-TRAIL (SUV) 2002PSA Peugeot Citroen Nuvera, H Power 2 Peugeot Fuel Cell Cab "Taxi 2001

PAC"BMW UTC 2 BMW 745h 2001

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Technical data Polymer electrolyte fuel cell stacksConsiderable improvements in power densities have been achieved in PEFC stacks through reduction of electrolyte thickness and architectural control of the composite electrodes. Current research efforts are targeted on developing water-free membranes. This technology significantly reduces the system complexity and would result in lowering costs. A second target is to increase the operation temperature from at present 70°C to about 150-170°C. Higher operating temperatures result in higher efficiency and greater fuel flexibility due to enhanced electrode activity (Haile 2003). This would solve also currently existing problems with the catalyst and help to reduce platinum loadings. State-of-the-art platinum loadings are 0.3 mg Pt/cm2 for pure hydrogen as fuel, while for reformat 0.6 mg Pt/cm2 are used (Rice 2004).

Reformers

Direct hydrogen allows for simpler system architecture but storage problems limit the operation range of vehicles. The use of small reformers to produce hydrogen onboard of FCVs could overcome this and omit major infrastructure changes and thus support the transition towards fuel cell propulsion at low costs. Onboard reformers convert a liquid (or other gaseous fuel) to hydrogen, however natural gas reforming is more difficult than liquid reforming, and therefore research has focused on liquids reforming so far. Methanol reforming is also possible, however to check whether reforming is economically a possibility at all, CASCADE focuses on gasoline reformers. It is also important to note that the funding for R&D of onboard fuel processing in the US has been recently severely reduced since it was judged uncertain if the technical goals can be reached until 2015. The independent review panel concluded, that due to the advanced state of competing technology (gasoline ICE/battery hybrids) on-board reforming technology does not offer clear advantages over hybrid vehicle technology available today (DoE Team Report, 2004). Table 1.13 shows key technical data for gasoline reformers.

Table 2.7: Technical data for gasoline reformers

Characteristics Units Status (2003)Target for year Ultimate

2005 2010 targetEfficiency % 78 78 80 >80Power density We/L 700 700 800 2,000Specific power W/kg 600 700 800 n.a.Durability hours 2000 4000 8000 8,000Source: Thompson et al (2005), DoE Team Report (2004)

Economic data

Cost data for current PEFC stackFor automotive applications, it is still rather difficult to purchase a fuel cell stack with an electric power above 50 kWhel today. Stacks that are build in demonstration vehicles are mostly obtained through research co-operations. This technology cannot be purchased in the wholesale or retail market yet and therefore no clear price is available. Currently available information on the unit cost is thus not based on a “true” business calculation. Someindicative cost data of for fuel cells implemented in demonstration projects are summarised in Table 1.14.

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Table 2.8 : Cost data for PEFC stacks for molecular hydrogen.

Stack characteristics Power Year Units Stack cost Cost

kW net units / year T€ €/kWel

ZSW "Water cooled stack" n.a. n.a. n.a. 30 n.a.

Ballard 70kW-Stack 37 2003 n.a. 230 6,000Nuvera "Direct water stack" 10 7,000Nuvera "Air cooled stack" 0.5 2 4,000

Smart Fuel Cell DMFC § 0.05 5 100,000Föller H2-System 0.08 5 63,000Source: all data DLR-VC internal information. § methanol, all others hydrogen stacks

Cost data for current fuel cell engines

A fuel cell engine such as the Xcellsis HY-80 comprises the stack and the system equipment to run the fuel cell. The production volume of fuel cell engines from Ballard in 2003 was < 10 units with “costs” of about 37,000 US$/kW. The term “costs” is here used in the sense of the expenses which have to be paid to suppliers for purchasing specialised components by the system integrator. This includes however not the accumulated expenses accruing from own research of Ballard. In 2004, the production volume is expected to be approximately 100 fuel cell engines. For this number of sales already cost reductions in one order of magnitude to 3,700 US$/kW are anticipated (Anonymous 2004). Of the total costs only 40% are allocated to the stack manufacturing while the remaining 60% originate from system components.

Modelled cost projections for PEFC fuel cellsTotal fuel cell module costs are projected by ADL (2001) to US$ 181/kW for a PEFC-Stack with 50 kWel (unit cell voltage 0.8V, power density 250 mW/cm2). These costs are driven by the membrane electrode assembly (MEA) (82%), while approx. 12% arise from the bipolar coolant plate and interconnect. Cost projections of several other authors lead to contributions of MEA in the range of 50-80% to the total cost of a PEFC-stack. This is due to the complicated manufacturing processes and the contribution of membrane and catalyst material cost.Current carbon paper electrodes are thermoset resin impregnates carbon fibre paper moulded and graphitised at temperatures in excess of 2000°C. Typically, these electrodes are 0.2 – 0.3 mm thick, but provide the required electrical conductivity and structural strength (James 1999). Costs for such components are approx. 50 $/m2 at an annual production volume of 2000 m2, but may potentially drop to 20 $/m2 provided further standardization and market introduction.The configuration of the catalyst in the PEFC depends to a large extent on the type and quality of fuel used. Recent research has focused on CO-tolerant catalysts, which are needed at the Anode, if hydrogen contains residual CO from the reforming process. Pt-Ru catalysts are the state-of-the-art here.Almost all of the membranes contain the toxic element fluor. Therefore, the manufacturing process needs dedicated precautions to protect the environment, which adds also to the total costs for such systems. Research on fluor- free membranes is currently under way (e.g. by AXIVA). However interim results are not yet available since such membranes are still in development (Gerl 2002).

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Chapter-03

Hydrogen Engines For Automotive Application

3.1 Basic mechanismHydrogen can be used as a fuel for internal combustion engines (H2 ICE). The combination of hydrogen as an energy carrier with proven and well understood engine technologies is seen as an ultimate near-term solution which could bridge the gap up to the full development of fuel cell vehicles. Combustion of H2 in air produces water, non-methane volatile organic compounds and carbon monoxide below SULEV2 standard, but significant amounts of oxides of nitrogen. However given the inherent inefficiency of the internal combustion engine, H2ICE will not be able to provide such substantial emission reductions ‘well-to-wheel’ and efficient energy use than a fuel cell powered vehicle.The use of hydrogen as an engine fuel has been addressed on a rather limited basis with varying degrees of success over several decades (Karim 2003). Hydrogen can be applied in conventional spark ignited piston engines as well as in compression ignition engines of dual fuel type, homogenous charge compression ignition engines (HCCI) and engines where ignition is effected through surface or catalytic ignition (Karim 2003).There are numerous positive features of hydrogen as an ICE fuel, mostly related to its high burning rates. However there are also some disadvantages and limitations. Engines fuelled with hydrogen suffer from reduced power output, due mainly to the very low heating value of hydrogen on volume basis (Karim 2003). A hydrogen engine when operated stoichiometrically needs to be 40-60% larger in size than for gasoline for the same power and torque output. Knock is seen as problem in SI engines caused by auto-ignition of unburned mixture of gas (Karim 2003) however seems to be solved by engine manufacturers. Due to the volatile nature of the gas, there are serious potential operational problems with uncontrolled pre-ignition and back-firing on the intake manifold, which need to be addressed.High temperatures may lead to high emissions of oxides of nitrogen. NOx emissions of 0.5 g/km are reported (RICARDO 2002) thus requiring a lean NOx trap or other NOx after treatment to meet EURO 4 and 5 standards.

3.2 Examples of applicationPrototype vehicles using hydrogen internal combustion engines have been demonstrated in the past by BMW, Daimler Crysler, Mazda and Ford. BMW certainly has put the highest effort in H2 ICEs, starting in 1979 and meanwhile presenting vehicles in the fourth Generation (Scheuerer 2003). BMW presented a dual-fuel V12 hydrogen engine at IAA Frankfurt Motor Show 2003. Daimler Crysler has stopped research on H2ICE and switched to FC engines.Mazda, the world’s only manufacturer of rotary engines, has presented at the 2003 Tokyo Motor Show the hydrogen -powered version of the RENESIS as an example of a hydrogen-gasoline dual-fuel system. The rotary engine can run on hydrogen with very little modification. It has the injection, compression, ignition and exhaust areas separated from each other so no pre-ignition of the gas occurs. Since the engine requires fewmodifications to run on hydrogen, it could enable production of a relatively low-cost hydrogen hydrogen-powered alternative-fuel vehicle.

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Technical dataFrom the demonstration vehicles and engines, some technical data are available. For general conclusions there is less information available. Some generic data for application in energy system models is summarized in Fehler! Verweisquelle konnte nicht gefunden werden..The BMW Group has presented a dual-fuel V12 hydrogen engine at the IAA Frankfurt Motor Show 2003. The 12-cylinder concept engine with a capacity of six litres yields 170 kW (231 HP) at 5500/min. The BMW 745h is powered by a 4.4-liter V8, featuring variable valve timing, variable intake runners, and a fully variable intake manifold. The 745h can use either hydrogen or premium unleaded gasoline. Running on hydrogen, the 745h produces 137 kW and can achieve a top speed of 214 km/h. The cruising range is 300 km. Added to the 645-km range of the normal fuel tank, the 745h can go 945 km between fill-ups .The hydrogen-powered MINI features a possible new injection process in which super-cooled liquid hydrogen is injected into the intake ducts where it mixes with air before entering the cylinders for ignition. Previously, the liquid hydrogen was heated to ambient temperature before combustion. This measure lowers also NOx emissions (RICARDO 2002).Ford’s introduced the Model U concept SUV, including a hydrogen-fueled 2.3-liter, four-cylinder internal combustion engine (based on the 2.3- liter I-4 engine used in the Ford Ranger and European Ford Mondeo) that is supercharged and optimized to run on hydrogen. The powertrain is coupled with a hybrid electric transmission. The overall efficiency can reach 38 percent, which is 25 percent more fuel efficient than gasoline fueled ICE vehicles. The Model U was designed for mass production and affordability, and has a range of approximately 300 miles (Ford 2003).

Table 3.1: ‘Generic’ technical data for H2 internal combustion engines (dedicated engine)

‘dual-fuel’Energy consumption MJ/100 km 151 ‡

Average efficiency improvement $+ 14% §City cycles

Highway cycles + 6% §

Range km ++

Technical system lifetime a -as conventional-§ Ford P2000 demonstration vehicle results with 2.0 litre Zetec dedicate hydrogen engine (RICARDO 2002); $ compared to gasoline engine; ++ depending on storage; ‡ own calculation based on storage and range data.

Today’s hydrogen IC engines do not achieve the specific power of conventional gasoline engines. The reason is that the volume of hydrogen in the combustion chamber is one third of a gasoline mixture and due to this charge loss the heating value of the total compound is lower. However with exterior mixture formation or high pressure injection with turbo charging fuel consumption of EURO IV calibrated turbo charged diesel engines are feasible (Pischinger 2005).

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Economic dataEconomic data on hydrogen internal combustion engines are scarce. Difiglio (2003) suggest additional cost in a range between of $0-1,000 over conventional ICE. RICARDO (2002)used a reduction of 400 €2000 in their analysis for the year 2008 for an advanced H2ICE compared to a Diesel engine with expensive fuel injection system. However at this stagealready 800 € were include for the lean NOx trap (LNT) and a Diesel particle filter. RICARDO (2002) concludes also: “there would be a cost penalty due to the introduction of an LNT and addition of a boosting system”.Therefore we suggest until better data is available to use additional costs of 750 € compared to conventional gasoline engines due to the boosting system and LNT for energy system modelling. On the long run, no additional costs for the engine compared to a future diesel engine with supercharging can be assumed.We don’t see any significant changes on O&M fixed and O&M variable costs for the H2ICE compared to conventional gasoline engines. Operation on hydrogen has no fundamental implications on cost, durability and recyclability of the engine (RICARD 2002).

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3.3 Hydrogen Production OptionsHydrogen is a high quality secondary energy carrier, not a primary fuel, and thus has to be produced from primary energy sources such as thermal or electric power. Hydrogen can be made from a variety of widely available feedstocks, including various fossil and renewable energy sources (Figure 3.1). Today almost 50 % of hydrogen is produced by steam methane reforming, which for large scale hydrogen production is the most economical route. To comply with key sustainability targets (climate protection, protection of non-renewable resources), a future hydrogen economy will however be based on alternative hydrogen production options. The following sections review the current technical and economic status of hydrogen production technologies, taking into account their respective potential for future prospects for improvement.

Figure 3.1: Hydrogen production Path

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Renewable non-renewable

Primary Biomass OrganicSolar,

Wood Natural Uranium Coal Crude oilHydro,

energy ResidualsWind gas

Electricity

ConversionGeneration

Fermen- Fermen-Electrolysis Gasification Reformer Gasification Refinerytation tation

SecondaryEthanol Biogas Hydrogen Natural Methanol Gasoline/

energy (I) gas diesel

Conversion Reformer Reformer Reformer Reformer Reformer

Secondary Hydrogenenergy(II)

Conversion Burner Fuel Cell Combustionengine/turbine

Useful energy Heat Power/heat Power/heat

3.4 Hydrogen from Water

3.4.1 Electrolysis

Basic mechanism

Water is electrolysed in an electrochemical cell to produce hydrogen and oxygen. Water electrolysis was one of the main means of hydrogen production before the steam reforming process was introduced..Water can be electrolysed by passing direct current (DC) through it in the presence of a suitable electrolyte, causing positively charged hydrogen ions to migrate to the negatively charged cathode, where they reduce to form hydrogen. Similarly, oxygen is formed at the positively charged anode. Chemically inert conductors such as platinum are used as electrodes to avoid unwanted reactions and the production of impurities in the hydrogen gas.

Anode: 2 H2O O2 + 4 H+ + 4 e-

Cathode: 4 H+ + 4 e- 2 H2

Overall reaction: 2 H2O 2 H2 + O2Three different process versions have been developed for electrolytic water dissociation:

• Alkaline water electrolysers are in use commercially for long times. Alkaline electrolysers work at atmospheric or low pressure (0-8 bar) or at pressure (up to 30 bar).

• Proton Exchange Membrane (PEM) electrolysers (reverse reaction of PEM-fuel cell) use an organic membrane instead of alkaline aqueous solution, which leads to a significant volume reduction. PEM electrolysers exhibit higher efficiency while operating at significantly higher current densities when compared to the advanced alkaline electrolysers (but system costs are still very high, see below). Mitsubishi Corporation is developing a ‘High-pressure Hydrogen Energy Generator’ (HHEG), which is based on a proton exchange membrane stack in a high pressure vessel, which stores high-compressed hydrogen generated by electrolysis. The advantage of the system is that it produces high-pressure hydrogen by itself without the need for further compression.

• High-temperature steam electrolysis operates between 700 and 1000 oC, using oxygen ion-conducting stabilised ZrO2-ceramics as an electrolyte (reverse reaction of Solid Oxide Fuel Cell). The water to be dissociated is entered on the cathode side as steam, which forms a steam hydrogen mixture during electrolytic dissociation (Winter and Nitsch 1988). The dissociation of steam theoretically requires less electrical energy than the dissociation of water. The energy required for water evaporation can be supplied by thermal energy and thus allows the use of various energy sources. The driving force for the development of the high-temperature electrolysis process in the 1980s was to use waste heat from high temperature nuclear reactors for hydrogen production, but most research activities were reduced significantly since then. Because of the improvements in Solid Oxide Fuel Cell technologies, there is a new interest in this technology, which can be combined with high temperature solar heat. Solid Oxide Fuel Cells (SOFC) used in electrolysis

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mode, called Solid Oxide Electrolyser Cells (SOEC), have the potential to become an efficient electrolyser technology.

Fields of applicationElectrolysis is scaleable from few Nm3/h to several 10,000 Nm3 /h, so that it can be used for decentralised on-site hydrogen production as well as for large scale centralised hydrogen plants. On-site hydrogen production can use existing electrical infrastructure and has the advantage that no additional infrastructure for hydrogen distribution is required. Due to their flexibility in operation (start and stop on demand) electrolysers can support the load management in the electricity grid and thus facilitate the integration of fluctuating energy sources.Because of the additional high temperature heat requirements from a nuclear reactor or a solar thermal facility, early process design studies for a high temperature steam electrolysis process assumed a hydrogen production capacity of several 10,000 Nm3/h (Dönitz 1988). A Solid Oxide Electrolyser Cell will be scaleable over a broad capacity range, thus also allowing the decentralised hydrogen production.

Technical data

As mentioned before, the capacity of electrolysers is scaleable from small-scale on site hydrogen production to large-scale centralised hydrogen plants near the electricity source. The efficiency of the process is largely unaffected by the variation in size (EUCAR 2003), so that we do not have to differentiate between different size classes. The outlet pressure of commercially available pressurised electrolysers range between 1.1 and 30 bar, and the efficiency ranges between 62 and 70 % related to the LHV of the delivered hydrogen (see e.g. EUCAR 2003; Stuart Energy 2004; BOC 2004). Future long term developments are expected to result in an efficiency of close to 80 % (Dreier and Wagner, 2000).Membrane electrolysers are still in the development stage. A version is offered by Proton Energy Systems, USA, for the production of ultra-high purity hydrogen gas for industrial and laboratory applications, but the energy efficiency of 45 % is rather low (Proton 2004), and the lifetime of the stack is only 3 to 4 years. Mitsubishi Corporation recently announced the successful application of the ‘High-pressure Hydrogen Energy Generator’ (HHEG) prototype PERM-electrolyser (Mitsubishi 2004), producing hydrogen at a pressure of up to 350 bar without a compressor. The capacity of the prototype is 2.5 Nm3/h, and Mitsubishi plans to have a commercial product with a capacity of 30 Nm3/h and a pressure of 400 bar available in the next years. There are currently no data available on efficiency, stack lifetime, or costs of HHEG system.Bench scale tests and a pilot plant design of a high temperature steam electrolysis process have been performed in the 1980s in particular in Germany, but research activities were slowed down (high costs, termination of the high temperature nuclear reactor programme in Germany). As discussed above, current research activities (e.g. at RISO, Denmark) on high temperature electrolysis focus on Solid Oxide Electrolyser Cells. The splitting of steam into hydrogen and oxygen at 1000 oC in a SOFC operated in the electrolysis mode has been successfully demonstrated in the laboratory (Jensen et al. 2003). The durability of the system is however limited, and much more work is necessary to achieve an acceptable lifetime of the cells.

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Economic dataA summary of cost data for 100 MW scale plants built in the early 1990s suggest that the investment costs were in the range of ∼600 $1997/kWel (Basye and Swaminathan 1997). Stuart Energy, one of the main manufacturers of alkaline electrolysers, report capital costs for small scale systems with a capacity below 5 MW as 500 $/kWel, while the costs for larger systems (> 5 MW) are 400 $/kW (Stuart Energy 2004). Additional system costs from connections, building, and other infrastructure are estimated to amount to about 15% of the electrolyser. Long term target costs of the US DOE for electrolysers are given as 300 $/kW (Myers et al. 2003), which we consider here as very optimistic. While (Angloher and Dreier, 2000) estimate that capital costs of a high pressure electrolyser will be reduced by 6% until 2025, we assume a long term cost reduction of 10 to 15%.The O&M costs for electrolysis, apart from electricity, are small, as electrolysis plants are well automated. Electrode reactivation is normally carried out every five to seven years for alkaline electrolysers. According to Stuart Energy the annual operation and maintenance costs amount to 2 % of the investment costs (Stuart Energy 2004).Costs for PEM electrolysers are currently a factor of about 3 higher than conventional alkaline electrolysers. It is expected that due to spillovers from the development of PEM fuel cells – if manufacturers succeed to reduce fuel cell costs significantly – the competitiveness of PEM-electrolysers might be enhanced in the future.In the early phase of high -temperature steam electrolysis development, system target costs were estimated to amount to about 700 €2000/kWH2 (Dönitz 1988). (Mogensen and Bagger al. 1998) estimate investment costs for a SOEC system to around 1,000 €/kWel, which is well in line with the long term SOFC target costs (see section 1.1.4.4). Because of the lower electricity demand of the high-temperature steam electrolysis, the competitiveness of the process over other electrolysis technologies gains from an increase in electricity costs.Oxygen is the by-product of water electrolysis. Whether any credit can be taken for the oxygen produced depends very much on the oxygen demand in close proximity to the plant. In the context of CASCADE MINTS we do not consider a by-product credit for oxygen.

3.5 Hydrogen from Fossil Fuels[33]

3.5.1 Natural gas steam reforming

Basic mechanismSteam reforming of natural gas (SMR) is the most commonly used process for producing hydrogen in large quantities. Natural gas is reacted with steam at elevated temperatures (> 800 oC) in the presence of a nickel catalyst. The reaction product (syn-gas) is a mixture of hydrogen and carbon monoxide:

CxHy + xH2O xCO + (x + y/2) H2

The syngas from the reformer is cooled and fed to a water-gas shift reactor in which carbon monoxide reacts with steam over a catalyst to hydrogen and carbon dioxide:

xCO + xH2O xCO2 + xH2

To achieve pure hydrogen, the gas is finally cleaned in a pressure swing absorption unit (PSA) (Figure 3.2).A novel concept called ‘Sorption Enhanced Reaction Process’ (SERP) aims at developing a more cost-effective steam methane reforming process. The reactants steam and methane are fed at 440-550 oC into a reactor containing an admixture of reforming catalyst and an adsorbent for removing carbon dioxide from the reaction zone (Reijers et al. 2003, Hufton et al. 2000). The potential benefit from this process is the production of hydrogen at relatively high purity (∼90% H2) directly from the reactor, thus leading to a reduction or even elimination of downstream hydrogen purification steps.Möller et al. (Möller et al. 2004) suggest that fuel saving of up to 40 % can be achieved by using a solar reforming process, which provides process heat by solar energy. The off-gas, which is burnt to provide process heat in the conventional process, can be recycled, which increases the overall process efficiency and reduces CO2 emissions. A concentrating solar system with a heliostat field and a solar tower is used to supply high temperature solar process heat. A first proof of concept solar volumetric reactor receiver has been tested (Möller et al. 2004).

Steam Steam

NaturalDesulphuri- HydrogenGas

Reformer CO-shift CO2 removal PSAsation

CO2

Figure 3.2: Schematic diagram of natural gas steam reforming

Fields of applicationToday, hydrogen is produced industrially in large-scale steam reformers at rates in the order of 100,000 Nm3 /h at high pressures (20-40 bar). Decentralised on-site production e.g. at a gas station or in a residential district requires small-scale SMR facilities with a production rate between 1,000 and 4,000 Nm3/h. Small scale stationary and mobile reformers which are developed as being part of a fuel cell package are not considered here.

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Technical dataTechnical data for two steam reforming facilities of different size, which are considered as representative for small scale on-site hydrogen production, and for large scale industrial applications. As SMR is a well established and mature technology, the potential for technical improvement is limited. According to (Dreier and Wagner 2000) we assume that today’s process efficiency of 67% (small scale) and 74% (large scale) (related to LHV of hydrogen) can still be slightly increased in the future.Results from first Sorption Enhanced Reaction Process laboratory plants do not yet fulfil the expected performance objectives (Hufton et al. 2000), so that the potential for improvement is difficult to assess. We suggest to not consider this new process here as long as there is no further technical evidence that the theoretical targets can be met.Technical data for the solar reforming process are based on a system design study by (Möller et al. 2004). The plant has a solar thermal capacity of 50 MW th, and a hydrogen production rate of 50,000 Nm3/h. The current concept aims at a ‘solar only’ operation mode without storage for night time operation, resulting in only 2,000 full load hours per year. The system requires 0.25 Nm3 of CH4 per Nm3 of H2, and 0.32 kWh solarthermal energy per Nm3 of H2, resulting in an overall reformer efficiency of 87 %.

Economic dataA literature review by (Basye and Swaminathan 1997) suggest capital costs for large hydrogen production plants (1.4 to 2.8 million Nm3 /d) in the range of 160 to 490 $1997/kWH2, indicating that economies of scale plays an important role even in these large scale plants. More recent data from (Dreier and Wagner 2000) suggest capital costs of 335 €2000/kWH2 for large scale steam methane reforming plants. Capital costs for decentralised small scale applications are significantly higher, and according to (Dreier and Wagner 2000)amount to 690 €2000/kWH2 for the 1,000 Nm3/h reference plant. As the process of steam reforming is well established, (Angloher and Dreier, 2000) do not expect a substantialpotential for further cost reduction. We assume that on the long term specific investment costs will be reduced by 5%.A review of O&M costs from various studies in (Basye and Swaminathan 1997) suggeststhat normalised O&M costs for large scale plants are between 0.45 and 0.6 $-cents 1997/Nm3. We thus assume O&M costs of 0.55 Euro-Cent2000/Nm3, not including feedstock costs. There are no reliable data available on O&M costs for small scale facilities. As theoperating costs for smaller SMR plants do not decrease with plant size and the raw material needs per unit of hydrogen produced remains similar, we assume the same O&M costs for all plant sizes.The economics of a first Sorption Enhanced Reaction demonstration process were found to be non-competitive with conventional technology (Hufton 2000).The costs of the solar natural gas reforming process were estimated by (Möller et al. 2004) to amount to 370 € /kWH2, including the heliostat field, the solar tower and receiver, but not including the land cost. We consider the cost estimates of Möller et al., which are only slightly higher as the investment cost of a current large scale SMR plant, as a long term target. Annual O&M costs are estimated to amount to 7 % of the investment costs

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3.5.2 Partial oxidation of heavy oil

Basic mechanismPartial oxidation is used in refineries for the conversion of residues (liquid, highly viscous hydrocarbons) essentially into hydrogen, CO, CO2 and water. Heavy hydrocarbon's low volatility and often high sulphur content prevents using steam reforming. Instead, heavy hydrocarbons are transferred to hydrogen by partial oxidation, leading to an exothermic conversion of heavy fuel oil with oxygen:

CxHy + x/2 O2 x CO + y/2 H2

Partial oxidation takes place at a temperature of 1300-1500 oC and a pressure of 30 to 100 bar without the need for a catalyst. The partial oxidation process is followed by adesulphurisation process, the CO-shift and a final CO2-removal (Figure 3.3). In contrast to steam reforming, the partial oxidation process does not pose any specific requirements towards the quality of the feedstock.

Fields of applicationPartial oxidation is not widely utilized for the production of hydrogen unless the only available feedstock is a heavy hydrocarbon. The process is most commonly considered for the production of carbon monoxide or for applications where the desired H2/CO ratio is 1.6 - 1.8 or less. Partial oxidation is used in refineries for the conversion of residues (liquid, highly viscous hydrocarbons) essentially into hydrogen, CO, CO2 and water. The products can be used as synthesis gas, fuel gas or feed for hydrogen recovery. Partial oxidation is relevant only for large scale hydrogen production.

Technical dataPartial oxidation of heavy hydrocarbons is a well established process based on the Texaco Gasification Process (TGP) (see e.g. EPA 1995) or the Shell Gasification Process (SGP) (see e.g. Heurich and Higman 1993). As a reference, we take technical data given by (Angloher and Dreier 2000) for a partial oxidation plant with a capacity of 100,000 Nm3/h hydrogen output. The data given in Table 3.6 refer to a plant in which electricity covers about 5 % of the energy demand. As the primary energy for electricity generation is not considered when calculating the process efficiency it results in a higher efficiency compared to steam reforming.Although partial oxidation is a well established process, it is expected that a further optimisation of system integration may result in a reduction of fuel input by 5 %, and a reduction of the electricity requirement by 10 %, leading to an improvement of the total efficiency from today 73 % to 77 % in the future.

Economic data

Capital costs for partial oxidation are higher than for steam reforming, while the feedstock price (heavy oil) is lower. Consequently, capital costs begin to contribute more to the overall production costs. Partial oxidation of heavy oil is a commercial process, so that cost estimates are based on considerable experiences. According to

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(Angloher and Dreier 2000),capital costs for a current heavy fuel partial oxidation plant are 500 €2000/kWH2. They do not expect that capital costs can significantly be reduced in the future due to technologicalimprovements, but like in the case of steam reforming we assume a future 5% reduction of capital costs.

There are unfortunately no reliable data on O&M costs available. (Basye and Swaminathan 1997) report O&M costs of 1.9 $1997/GJ based on a quite old study by (Steinberg and Cheng 1989), which are also quoted elsewhere throughout the relevant literature. We consider annual O&M costs in the range of 10 % of the capital costs as too high, and suggest to use fixed annual O&M costs of 5 % of total capital costs, which is more in line with similar facilities in the energy sector. Variable operation costs resulting from electricity consumption depend on the electricity costs. Assuming electricity costs of 5 ct/kWh, the electricity costs amount to 0.95 ct/Nm3

H2.

3.6 Other methods for hydrogen productionThe following sections summarise further processes for hydrogen production, which today are in a basic research phase and thus quite far away from a technical implementation. Although some authors already provide cost estimates for specific processes, we consider them as being highly unreliable, as there is still significant basic research required to understand and to set up a reliable technical process. This does not mean that some of these processes might not evolve further during the 50 years time horizon covered in CASCADE MINTS, but for the time being there is no basis for a sound technical and economic characterisation that is required to include these processes into the energy system models addressed in CASCADE MINTS.

3.6.1 Photo electrochemical water splittingPhoto electrolysis of water is the process whereby light is used to directly split water into hydrogen and oxygen. This can be achieved by illuminating a wet semiconductor device either directly or via dye sensitisation. Such systems eliminate the need for two separate systems regarding power generation and electrolysis, and hence offer the potential for cost reduction of electrolytically produced hydrogen compared with conventional two-step technologies. The direct production of hydrogen via water splitting by sunlight requires a light-harvesting device in conjunction with water-splitting catalysts. The necessary semiconductor-based light-sensitive device is similar to a PV solar cell.For direct photo electrochemical decomposition of water to occur, several key criteria of the semiconductor must be met: the semiconductor system must generate sufficient voltage to split water, the energetic of the semiconductor must overlap that of the hydrogen and oxygen redox reactions, the semiconductor must be stable in aqueous electrolytes, and finally the charge transfer from the surface of the semiconductor must be fast enough not only to prevent corrosion, but also reduce energy losses due to overvoltage.

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Research and development programs on Photo electrolysis of water are in place in several countries such as the United States, Japan, Sweden or Switzerland. Photo electrochemical hydrogen production is in an early stage of development, and it depends on a breakthrough in materials development. One of the main technical problems related to the use of photo electrochemical cells is the stability of electrodes against corrosion. Research activities focus on the development of appropriate semiconducting materials and surface treatment to increase the semiconductor’s stability. Amorphous silicon (Varner et al. 2002) or gallium based materials (Turner 2003 et al.) are being considered for this technology. Different coating options are analysed to increase the stability of the semiconductor.While the efficiency of the electrolysis in a photoelectrochemical system can be higher than in a conventional electrolysis due to the lower voltage required, theoretically the overall efficiency for photoelectrochemical water splitting is around 24 % (Winter and Nitsch 1988). System efficiencies of 10 to 12 % have been achieved in the laboratory, but these high efficiency systems have extremely short lifetimes (< 20 hours). A solar-to-hydrogenefficiency of 16%—the highest reported efficiency to date—was achieved using a tandem photo-electrochemical cell (IEA 2004a). The materials used in this cell are still too costly for this to be an economically competitive technology in the near term. A potential future system will likely be a compromise between efficiency and lifetime. Current research targets are to achieve a 10 % solar-to-hydrogen efficiency at a 10 year cell lifetime, with hydrogen production costs of 3 $/kg (Turner 2003).

Assuming an efficiency of 5 to 10 %, the potential for hydrogen production with photo-electrochemical water splitting methods is comparable to that of solar cells combined with electrolysis. Today, with research still focused on the development of appropriate stable semiconductor materials, it is not possible to give a definitive statement about the technical feasibility or a possible point in time when such a system might become practical.

3.6.2 Biological hydrogen productionGreen algae capture energy from sunlight. Under anaerobic conditions, the green algae produce a hydrogenase enzyme which produces hydrogen from water by a process known as bio-photolysis. The conditions must be carefully regulated, since the hydrogenase enzyme works in the dark phase and is sensitive to the presence of oxygen produced from photosynthesis. Basic research, which is progressing under the IEA Task 15 ‘Photobiological Production of Hydrogen’ is still in a very early stage. The overall objective set out for the IEA Task 15 program is to ‘sufficiently advance the basic and early stage applied science in this area of research to allow an evaluation of the potential of such a technology to be developed as a practical renewable energy source for the 21st century’(IEA 2004b). Current laboratory work demonstrates the feasibility of using algal cultures to photoproduce hydrogen continuously for 3-4 days (Ghirardi et al. 2002). The major research challenges however still are to increase hydrogen production by a factor of at least 10, and to increase the solar energy conversion efficiency from 5 % to at least 10 % (Dutton 2002).

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The second main biological pathway to hydrogen uses fermentation without the need for light. This is a dark, anaerobic process carried out by many species of bacteria, one of which (Clostridia) has been singled out for particular attention. The reaction involves a hydrogenase enzyme acting on carbohydrates to produce hydrogen and carbon dioxide (Dutton 2002):

C6H12O6 + 2H2O CH3COOH + 2CO2 + 4H2

Laboratory studies have concentrated on pure microbial cultures, sterile feedstocks, and batch operation. Potential feedstocks include sugar cane or the organic fraction of municipal waste. The theoretical yield is 0.5 m3 H2/kg carbohydrate, although this is yet to be achieved, largely because the reaction is hindered by increasing hydrogen concentration. Fermentative bacteria can multiply rapidly and produce high quantities of hydrogen, but the design and the operational parameters of the process are not yet well established, so that there is still basic research required to further understand and improve the process.

3.6.3 Hydrogen production from thermochemical cycle processesWater can be split thermally only at very high temperatures above 2,000 K. To avoid materials problems occurring at these high temperatures, in contrast to direct thermal water splitting, thermochemical cycle processes utilise the coupling of several equilibrium reactions which in separate process steps and at different temperature levels permit the separate release of hydrogen and oxygen at temperatures of less than 1,200 K. Thermochemical cycles are Carnot-limited, for an upper temperature of 1,000 K and a lower temperature of 300 K the maximum theoretical efficiency is 88 %.During the 1970/80ies, some 2,000 to 3,000 cycles have been ‘invented’ and analysed in theory (Wendt and Bauer 1988). After examining their feasibility with respect to reaction and process technology, only some 20 to 30 remained, some of which were examined in some detail as to their reaction technology. They were grouped in ‘families’ according to the respective working substances, such chlorides, oxides, metals sulphates or other elements with multiple valences common to the process of one family. High temperature process heat can be provided either from solar energy or from a high temperature nuclear reactor. As an illustrative example the following equations describe the high-temperature dissociation of a metal oxide to oxygen and metal, which is coupled to the low-temperature reaction of the same metal with water releasing hydrogen:

Me2O + high temperature heat 2Me + ½O2 2Me + H2O + low temperature heat Me2O + H2

Theoretical calculations based largely on entropy balance considerations postulate that such thermo chemical cycles shall not be limited to two reactions to achieve an attractive energy yield at an upper maximum temperature of 1,200 K, but must include more than two reactions operating at more than two separate temperature levels. This on the other hand implies complicated process engineering. In all cycles that use metal oxides, metal chlorides or metal sulphates as working materials, solids must be reacted, cooled or heated and circulated, leading to significant technical difficulties in process engineering. After some ten years of research and technical and economic analysis of process specific technical data, because of these problems further development work on this type of thermo chemical cycles was virtually stopped.

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In recent years, significant progress has been accomplished both in the field of materials science, and in the development of optical systems for large scale collection and concentration of solar energy, using solar tower and tower-reflector technology at power levels of several megawatts. These concentrating systems are capable of achieving power flux intensities equivalent to solar concentration ratios of 5000 suns and high by applying non-imaging secondary concentrators in tandem with the primary focusing heliostat field. Such high radiation fluxes correspond to temperatures exceeding 3,000 K and allow the conversion of concentrated solar radiation to thermal reservoirs at 2,000 K and above. Thus, the door was opened for the more efficient two-step thermo chemical cycles for splitting water using solar energy (Seinfeld 2002; Palumbo et al. 2004). Several two-step water-splitting cycles based on metal oxides reactions have been proposed. Of specific interest is the solar thermo chemical cycle based on ZnO/Zn redox reactions. The first endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O2 at 2,300 K using concentrated solar energy. The second, exothermic step is the hydrolysis of Zn(l) at 700 K to form H2 and ZnO(s), the latter separates naturally and is recycled to the first step:

ZnO Zn + ½O2Zn + H2O ZnO + H2

Hydrogen and oxygen are derived in different steps, thereby avoiding the need for high-temperature gas separation. According to (Steinfeld 2002), a maximum exergy conversion efficiency of 29 % is achieved when using s solar cavity-receiver operated at 2,300 K and subjected to a solar flux concentration ratio of 5,000. Steinfeld estimate the costs of solar hydrogen produced in a large scale chemical plant equipped with a 90 MWth solar reactor and a hydrogen production of 61 million kWh/a between 0.13 and 0.15 $/kWh, thus suggesting might be competitive with other renewables-based routes for hydrogennproduction. However, currently there are only exploratory test carried out e.g. at PSI/ETH (Switzerland) on the dissociation of ZnO, and the economic feasibility strongly depends on the availability of an effective Zn/O2 separation technique. The cost estimates by (Steinfeld 2002) thus are at least highly uncertain. Similar to the ‘old’-type of thermochemical cycleprocesses, the ZnO/Zn process has to face the challenge of handling large volume streams of solid materials for producing a relevant amount of hydrogen. Further development work is currently focused on solar chemical reactor modelling, and on designing a better quench method for recovering Zn. The benchmark against which the process has to prove its technical and economic viability in the future is hydrogen production from electrolysis with electricity from solar thermal power plants.

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Chapter-04

HYDROGEN STORAGE TECHNOLOGIES

4.1Introduction

In the development of fuel cell vehicles, hydrogen storage is “the biggest remaining research problem” according to the January 2003 Office of Technology Policy report,Fuel Cell Vehicles: Race to a New Automotive Future. Current hydrogen storage systems are inadequate to meet the needs of consumers in a fuel cell vehicle. The OTP report continues, “Hydrogen’s low energy-density makes it difficult to store enough on board a vehicle to achieve sufficient vehicle range without the storage container being too large or too heavy.”

Existing and proposed technologies for hydrogen storage include (1) physical storage: pressurized tanks for gaseous hydrogen and pressurized cryotanks for liquid hydrogen;(2) reversible hydrogen uptake in various metal-based compounds including hydrides, nitrides, and imides;(3) chemical storage in irreversible hydrogen carriers such as methanol;(4) cryoadsorption with activated carbon as the most common adsorbent; and(5) advanced carbon materials absorption, including carbon nanotubes, alkali-doped carbon nanotubes, and graphite nanofibers.

The Department of Energy timeline for development of storage systems projects that high pressure and cryogenic storage will be demonstrated in 2002- 3, cost-effective hydride storage systems in 2003-6, and carbon-based storage systems in 2006-11 [24].

Goals for hydrogen storage systems for 2010 that were established in the Freedom CAR initiative include

available capacity of 6 wt% hydrogen specific energy of 2 kWh/kg energy density of 1.1 kWh/L cost ≤ $5/kWh or $1.25/gal (gas equiv.) in CY2001 dollars

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4.1.1 Pressurized Tank Storage

Pressurized tanks of adequate strength, including impact resistance for safety in collisions, have been made of carbon-fiber wrapped cylinders. Compressed gas storage in such tanks has been demonstrated at a pressure of 34 MPa (5,000 psi) with a mass of 32.5 kg and volume of 186 L, sufficient for a 500-km range. Note, however, that this tank volume is about 90% of a 55-gallon drum, rather large for individual automobiles. So while the 6 wt% goal can be achieved, tank volume is problematic. Pressures of 70 MPa (10,000 psi) have been reached, and in 2002 Germany certified Quantum Technology’s 10,000 psi on-board storage tank . A footnote in the OTP report cited above says, “The Toyota and Honda vehicles available for lease in late 2002 use hydrogen stored in high-pressure containers. However, their range will be less than optimal because hydrogen’s low density does not permit a sufficient amount to be stored (unlike CNG [compressed natural gas], which has a higher energy density for the same volume).”

Low temperature storage of liquid hydrogen does not appear to be suitable for normal vehicle use, although research on this possibility is being conducted at a low level by several automobile manufacturers. Furthermore, “a liquid hydrogen storage system loses up to 1% a day by boiling and up to 30% during filling, as well as requiring insulation to keep the hydrogen at 20 K.”

4.1.2Hydrogen Uptake in Metal-Based Compounds

Metal hydridation can be used to store hydrogen above room temperature and below 3 or 4 MPa. However, the metals introduce too much additional weight for most vehicle uses. They are also expensive .

Recent work by P. Chen, et al. has shown that lithium nitride can reversibly take up large amounts of hydrogen . This material takes up hydrogen rapidly in the temperature range 170-210ºC, and achieved 9.3 wt% uptake when the sample was held at 255ºC for 30 minutes. Under high vacuum (10-9 MPa, 10-5 mbar, or 10-5 torr) about two -thirds of the hydrogen was released at temperatures below 200ºC. The remaining third of thestored hydrogen required temperatures above 320ºC for release. The hydrogen was taken up as lithium imide (LiNH2) and lithium hydride (LiH). These researchers suggest that related metal-N-H systems should be investigated to find a hydrogen storage system that works at more practical temperatures and pressures.

4.1.3 Cryoadsorption Hydrogen Storage

While having potential weight and volume advantages, cryoadsorption with activated carbon as adsorbant requires liquid nitrogen temperatures and 2 MPa (300 psi) to hold the physically adsorbed hydrogen. It does not appear to be suitable for vehicle use.

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4.1.4 Carbon Nanotube and Related Storage Technologies

The status of hydrogen storage in advanced carbon materials is still unclear. In this subsection, we review briefly the status of carbon nanotube storage, both single-walled and double-walled, and graphite nanofiber stack storage. Other carbon-based storage technologies that have been proposed include alkali-doped graphite, fullerenes, and activated carbon.

High surface area and abundant pore volume in the nanostructured materials make these especially attractive as potential absorption storage materials. Some early work gave tantalizing results for hydrogen storage in carbon nanotubes. Ogden reported various conflicting, some excessively optimistic, results . A query by this subcommittee to Prof. Mildred Dresselhaus of MIT about the achievable wt% (6.5 wt% has been suggested) brought this response:

1. It is hard to say what is a reliable estimate of the hydrogen uptake number because of the differences in the reported levels by different groups, presumably doing similar measurements. The reasons for the different results between groups are not understood.

2. The 6.5% value is not yet achievable in my opinion.

3. The problem seems to be hard to me, arguing from a theoretical standpoint. However I would not discount the possibility of a breakthrough that might change the situation dramatically. So far it doesn't seem to me that there is yet much available carefully controlled work.

A 2001 review of carbon nanostructure storage research, sponsored by the German Federal Ministry for Education and Research (BMBF), found that follow-up work “has been unable to reproduce any of the high-capacity results.” [29] They concluded “In view of today’s knowledge, it is unlikely that carbon nanostructures can store the required amount of hydrogen. In any case, this calls into doubt whether carbon nanostructures would have any advantage over high-pressure tank storage.”

The German study presented the following summary figure on the capacity of current and future hydrogen storage systems:

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The considerable doubt that has been cast upon carbon nanotube storage capacity makes it highly likely that their position on this chart will have to be moved to a much less favourable point.

4.2 HYDROGEN FUEL STORAGE SAFETY

Hydrogen has a reputation for being explosive and therefore raises concerns about the safety of carrying a substantial quantity of H2 in a vehicle fuel tank. However, because H2 is the lightest gas, it has a tendency to diffuse away quickly in case its container is breached and consequently may represent less of a hazard than gasoline.

The simplest way to carry hydrogen fuel in a car or other vehicle is as a high-pressure gas 3-10 kpsi (21-69 MPa) in metal or composite-reinforced (fibreglass, carbon fibre, Kevlar) tanks. This is similar to the way compressed natural gas (CNG) vehicles operate.

There is an interesting report on H2 for energy use by the Norwegian environmental organization. These authors conclude that “hydrogen is no more or less dangerous than any other energy carrier and furthermore that hydrogen has properties that in certain areas make it safer than other energy carriers: it is not poisonous, and has the ability to dissipate quickly into the atmosphere because of its light weight compared to air.” They describe tests by Lockheed and Arthur D. Little that indicated that H2 is, if anything, safer than gasoline or jet fuel. There are a number of references to crash testing by BMW that say BMW demonstrated the safety of H2 fuel for cars.

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4.3Benefits Of The Fuel Cell Technology

4.3.1 Stacks and systems

Now moving from the single fuel cell unit to real‐world systems, what do we have to add to get them all setup and why?

Similar to all electrical devices the output power of a fuel cell is equal to the current multiplied by the voltage. While the current may be in theory indefinitely increased by increasing the reaction area between hydrogen‐ and oxygen‐containing reactants, the voltage, i.e. the potential difference between the anode and cathode, is thermodynamically limited to a little more than 1 V by the nature of the two half‐cell reactions in a fuel cell: hydrogen oxidation reaction (HOR) at the anode and oxygen reduction reaction (ORR) at the cathode. Moreover, losses inevitably occur in a fuel cell due to slow kinetics of the electrode reactions (activation over potential), intrinsic resistances of the different components and contact resistances between one other (ohmic over potential), and transport resistances of the reactants (concentration over potential). Therefore, under operational load the actual voltage of a single fuel cell is in the 0.6‐0.7 V range.

Useful voltages are generally achieved by interconnecting multiple unit fuel cells in series. This is the concept of “stacking”. The stack’s final output voltage will depend on the number of cells and the available current will be proportional of the total surface area of the cells. In this configuration, the conductive interconnecting element is in contact with both the anode of one cell and with the cathode of the adjacent cell, hence the name “bipolar plate”. Flow channels are grooved on each side for gas distribution and water removal. Bipolar plate materials are highly impermeable to gases in order to avoid harmful fuel and oxidant mixtures: these materials are mainly graphite, polymer‐graphite composites and metals such as stainless steel or aluminum (most often coated with a corrosion‐resistant alloy).

Bipolar stacking It has been up to now the most simple and the most conventional configuration in most types of fuel cell systems, particularly low‐temperature systems. For high‐temperature systems such as SOFCs however, sealing issues due to large temperature gradients during operation have driven research toward alternative arrangements, leading to the development of a tubular design.

Tubular stacking In the different elements of the fuel cell unit (anode/electrolyte/cathode) are arranged concentrically forming a hollow cylinder. Fuel is fed on the anode side, either through the inside or along the outside of the cylinder, and oxidant is fed on the cathode side. Series connection is accomplished by vertical addition of the cells (in the height direction) while parallel connection is accomplished by horizontal addition of the cells (in the same plan). The tubular design is well suited for high‐temperature applications since it minimizes the number of seals in the fuel cell system thus alleviating problems due to unmatching expansion coefficients.

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Planar stacking Is a second alternative to the bipolar arrangement, in which cells are connected laterally rather than vertically. Several planar designs have been explored, mostly for small‐scale systems: the banded‐membrane design, in which the anode of one cell is connected to the cathode of the adjacent cell across the band; and the flip‐flop design, in which there is interconnection of unit cells on the same side of the band thanks to alternateanodes and cathodes. The main advantage of this third arrangement is a better volumetric packaging, yet at the expense of increased resistance losses.

Besides the fuel cell stack, referred to as the fuel cell subsystem, the other subsystems that are needed to keep the whole system running can be classified into three categories:

• The thermal management (cooling) system • The fuel delivery/processing system • The power electronics (and safety) system for power regulation and monitoring

The components that draw electrical power from the fuel cell thereby causing parasitic power losses are called ancillaries. For example, an actively cooled fuel cell system will employ an ancillary device like a fan, a blower or a pump for cooling fluid circulation.

1. As fuel cells are usually about 30‐60% electrically efficient (depending of the type of fuel cell), the balance of energy is released in the form of heat and this has to be managed by the system in order to maintain the thermal gradients inside the stack at the lowest possible level (within a few °C) and ensure stable operation. A cooling system is required for fuel cells that cannot benefit from natural heat regulation by the ambient, i.e. all systems except small PEMFCs (output power < 100 W). The cooling fluid can be either a gas (air), or a liquid (distilled water or aqueous glycol‐based solution) depending on the heat dissipation capacity needs and the other characteristics of the fuel cell system. Given that the heat capacity of liquids is much greater than that of gases; consequently, small liquid‐cooled devices will generally be far more efficient than their massive gas‐cooled equivalents.

In advanced fuel cell systems, the heat released by the stack can be purposely recovered for internal (1) or external (2) heating. Examples follow:

• Heat can be used for conditioning reactant gases = pre‐heating + humidification; • Heat can be used for providing space and/or water heating in a house, or passenger

compartment warming in a car; Cogeneration by heat recovery is a powerful means to increase the overall efficiency of fuel cells systems up to 80‐85%. It is very advantageous in high temperature fuel cell systems, mainly PAFC and SOFC.

2. Given that almost all practical fuel cells today use hydrogen or compounds containing hydrogen as a fuel, there are two primary options to feed a fuel cell:(a) in a direct way by pure hydrogen or (b) by a hydrogen carrier after upstream processing.

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(a) In the first case, hydrogen is produced outside the fuel cell system in an industrial process (steam reforming for example), and is ready for direct use. The fuel management subsystem will include a hydrogen reservoir related to the physical state of hydrogen stored: high‐pressure gas cylinder (up to 700 bars) for compressed gas, double‐walled insulator under cryogenic conditions (22 K) for liquid hydrogen in extreme situations where mass storage capacity is especially important, e.g. space conditions, or low‐pressure container for metal hydrid compounds ground into extremely fine powders. The advantages of direct hydrogen feeding include high performance, simplicity, and the elimination of impurityconcerns. But the current storage options, mainly in the form of compressed gas or reversible metal hydride, are not optimal yet.

(b) In the second case, the system is more complex. Since hydrogen is not available as is, it must be derived from hydrogen‐containing fuels called “hydrogen carriers” that are widely available in the industry, like methane, methanol, diesel or gasoline. Except a few hydrogen carriers that are directly usable in fuel cells systems including methanol in DMFCs and methane in SOFCs of MCFCs, a vast majority of them must be processed before they enter the fuel cell. This is possibly achieved in two different ways:

• by direct electro‐oxidation • by chemical reforming

A further distinction must be made between external reforming whether reforming occurs in a chemical reactor separated from the fuel cell, and internal reforming whether the reaction takes place at the catalyst surface inside the fuel cell.

i. Direct electro‐oxidation of the carrier fuel into hydrogen is attractive because it avoids the extra step of reforming it prior to the fuel cell reaction and all chemical reactors associated with it. Direct Methanol Fuel Cells are based on this principle, and other simple compounds like ethanol and formic acid can also be employed. Unfortunately, the overall electrical efficiency of this category of fuel cells is significantly reduced due to the complexity of the reactions. As a result, the energy density gained by the absence of a reformer or a fuel reservoir can be largely offset by the low fuel efficiency and the need for larger stacks.

Direct electro‐oxidation is best applied in portable applications, where simple systems, minimal ancillaries, and low power are needed.

ii. External reforming systems are composed of several devices for successively treating chemically or physically the gas reactant (hydrogen carrier) and the products (including hydrogen). Several ways are possible, and the exact conditions will vary with the process and the hydrogen carrier. The most used process is steam reforming: fuel molecules are burned over a catalyst (nickel‐, copper oxide‐ or zinc‐based) under the presence of water steam at a few hundred degrees C, according to the reaction:

CxHy + x H2O(g) ↔ x CO + (1/2y + x) H2 ⇒ H2, CO, CO2, H2O

The yield of this endothermic reaction can be further increased in the presence of excess water vapor via the water‐gas shift reaction involving the product CO:

CO + H2O(g) ↔ CO2 + H2

Before reforming, the fuel may have to be desulfurized and heated. After reaction, a

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hydrogen containing gas mixture is obtained that in certain cases has to be purified in‐ multi processing steps. This additional post treatment is required for removing‐ ‐ poisons and be able to feed the fuel cell with a pure hydrogen gas, what is especially important for low temperature systems.‐

iii. Internal fuel reforming is possible for high‐temperature fuel cells with certain fuels. In this case, the fuel is mixed with steam prior entering the fuel cell anode where it is both reformed into hydrogen and the usual co‐products CO and CO2 and then split into protons in the fuel cell reaction. Under these high‐temperature conditions, the presence of carbon oxides is not an issue anymore. These gases are even further processed in situ thus contributing to the fuel cell efficiency. Although the different interplaying parameters are difficult to optimize, internal reforming is a promising solution because it gives an elegant (and economically winning) answer to a complex question.

Fuel reforming is best applied in stationary applications, where fuel flexibility is important and the excess heat can be managed inside or outside the system. However, fuel reforming technology is not a current choice of authorities for transportation applications since the existing technologies do not meet the technical or economic targets, and only marginal improvement is expected in efficiency and emissions between a hybrid vehicle and an FC vehicle equipped with on‐board reforming.

For any of the fuel delivery/processing systems considered previously, gas pumps are used to feed the gas reactants in the fuel cell, and a water purge system must also be integrated.

3. Last but not least, it is necessary to manage the direct power output of the fuel cell into usable power. The power electronics subsystem consists of:

a) Power regulation; b) Power inversion; c) Power monitoring and control; d) Power supply management.

Power conditioning corresponds to regulation and inversion of the fuel cell power output.

a) Regulation allows delivery of power at a voltage level that is stable over time from a fuel cell output power that most often is not. Fuel cell power is generally regulated by DC/DC converters, which transform the fluctuating Direct Current (DC) voltage input into a fixed, stable DC voltage output. A DC/DC converter is generally 85‐98% efficient.

b) Inversion means converting the DC power provided by the fuel cell into Alternative Current (AC) power consumed by most electronic devices, electric motors, and the electrical grid. This task is performed by a DC/AC converter. Similar to DC/DC converters, DC/AC converters are 85‐97% efficient. Consequently, these devices taken together yield a 5‐20% decrease in fuel cell electrical efficiency, which is far from negligible. Selections of optimal options for a given fuel cell stack technology, geometry and size in a given environment is therefore an essential point.

c) Power monitoring and control includes system monitoring by gauges, sensors, etc. that measure the conditions of the fuel cell, system actuation by valves, pumps, switches, fans, etc. that regulate them, and a central control unit that mediates the interaction between

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sensors and actuators. Most control systems are based on feedback algorithms to maintain stable fuel cell operation, i.e. different sets of feedback loops are implemented between stack monitoring elements and ensuing corrective actions by actuators.

(4) Power supply management is the part of the power electronics subsystem that adapts the electrical power output of the fuel cell to the load requirements. Depending on the application, the demand may be driven be toward short‐time and/or large‐scale load changes that the fuel cell alone is not able to answer due to lag times in system ancillary components such as compressors and pumps. The dynamic response of the fuel cell can be enhanced by energy storage buffers like batteries or capacitors. The response time will be reduced from seconds or even hundreds of seconds to milliseconds. In the case of stationary fuel cell applications, the power supply management subsystems must also incorporate a special control device for interacting with the local grid, allowing for example shutdown or disconnection during a power outage.

The target application ultimately dictates the fuel cell system design and the choice of fuel delivery. In portable systems for instance, there is a strong incentive to minimize the size of components and avoid the use of ancillaries. Direct or reformed methanol fuel systems may provide energy density improvements compared to direct hydrogen storage solutions. A delicate trade‐off is necessary between the size of the fuel cell unit and the size of the fuel reservoir. In utility‐scale stationary power generation, durability and efficiency are of prime importance. Reformed natural gas and biogas are the leading fuel solutions due to their wide availability and low cost.

4.4 ADVANTAGES Of Hydrogen Fuel Cell

• Efficiency Fuel cells combine many of the advantages of both internal combustion engines (ICE) and batteries. Thanks to the direct conversion of chemical energy into electrical energy, fuel cells are 2‐3 times as efficient as ICEs for vehicle propulsion: the net electrical efficiency of a PEMFC ranges between 40 and above 50% in a driving schedule, which is favorably compared to the 21‐24% efficiency range of ICEs “from well to wheel”, i.e. accounting for the type of fuel used and its entire life cycle. Now if we add in the calculations the reforming process of gasoline and methanol or the use of compressed hydrogen in the calculations the efficiencies are 33, 38, and about 50%, respectively.

Interestingly, the fuel cell efficiency does not drop for small systems because it does not depend on its size: unlike gas turbines for example that suffer from scale effects, small fuel cell devices are quite as efficient as larger ones. Accounting for energy losses in ancillaries the efficiency is somewhat lowered but is in any case higher than conventional systems.

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Reduced Emissions

Because fuel cells are electrochemical systems and do not rely on combustion, they are the cleanest fuel‐consuming energy technology, with near‐zero smog‐causing emissions. Theyproduce benefits in all applications: power generation, industrial equipment, transportation, military power and consumer electronics.

The emissions produced by a fuel cell system strongly depend on the fuel used and its origin. For example, a FC vehicle produces only water if it is fed by compressed hydrogen, some CO, CO2 and CH4 if it is fed with ethanol, and additional SO2 if it is fed with gasoline. Under fuel cell operation, undesirable products such as carbon monoxide CO, sulfur oxides SOx or nitrogen oxides NOx, ashes and carbon particulate emissions are virtually zero.produced by water electrolysis from renewable electricity. Emissions of pollutants are increased for electricity from the grid, i.e. a mixture of thermic, nuclear and renewable sources.

• Reliability, low maintenance and quietness Fuel cells can help provide stability and continuity to the electric grid since they can maintain a continuous base power in parallel with or independent of the grid. Fuel cells provide high quality power without any risk of power outage. They have more predictable performance over wider operating temperature ranges than lead acid batteries.

Fuel cells can be recharged everywhere within a few minutes by refuelling while batteries have to be plugged in for time‐consuming recharge (and eventually replaced). They operate at constant peak performance from fuel replenishment to depletion. Therefore operation time is well‐known and directly proportional to the amount of fuel supplied.

Fuel cells systems have practically no rotating or even moving parts. Certain types of fuel cells (PEMFC, SOFC) are all solid state thus close to mechanically ideal. This means less noise and potentially reduced maintenance work (and related costs) besides refueling. Stationary fuel cells require only minimal maintenance (once every one to three years) compared to monthly or quarterly site visits to lead acid battery‐based installations.

Fuel cells are relatively silent systems making them suitable for residential areas. The only parts that are liable to cause moderate noise are the pieces of ancillary equipment like fans, compressors and pumps. Noise levels measured on stationary systems are typically as low as 50‐60 dB.

• Sustainability Fuel cells are powered by hydrogen, the most abundant element in the Universe. Hydrogen can be produced from a variety of sources including fossil fuels, natural gas, methanol, and various renewable energy sources: wind, photovoltaic, geothermic, waves, etc. This is a key point asset from the perspective of greenhouse gas reduction and follow‐on process of the Kyoto Conference.Fuel cells are essential to achieving carbon reduction goals, with CO2 reduction ranging from 40% or better using conventional fuels to nearly 100% using renewable derived hydrogen, as compared to conventional power sources. Fuel cells can contribute to the world’s end of dependence on hydrocarbons. They can greatly simplify the sequestration of CO2 from

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hydrocarbon fuels, enabling the use of domestically‐produced fuels including coal, biomass and hydrogen.Due to their low environmental footprint, fuel cells are a realistic option in several fields concerned by the climate change debate: automotive, residential, industrial.

• Compactness

Fuel cells offer higher energy density and higher storage capacity compared to batteries, and thus good compactness, which is an interesting feature especially for portable applications.

• Modularity and flexibility

Fuel cells allow independent scaling between power (determined by the fuel cell size) and capacity (determined by the fuel reservoir size). The fuel cell size can be adapted by simply changing the number of elementary cells and the active area. Scale up is therefore very easy, from the W range of a cell phone to the MW range of a power utility plant. For miniaturized systems techniques derived from microelectronics are being developed.

Fuel cells are the ideal solution when space is limited or weight is a concern, offering clean and quiet operation in a wide range of installation conditions. For example, the reduced footprint requirements for normal rooftop loading limits, and zero‐emission combined with silent operation make them highly suitable for indoor/outdoor, urban/rural applications.

In addition, they can be fueled by a variety of fuels including intermittent renewable energy.

4.5 Issues Of Hydrogen Fuel CellThere are three main barriers remaining to widespread adoption of the fuel cell technology:

- Cost

- Durability

- Lack of hydrogen infrastructure

The lack of hydrogen infrastructure has long been considered the biggest obstacle in particular for introduction of fuel cell vehicles, although this question resembles the classic chicken‐and‐egg problem (there are no FCVs because there are no hydrogen fuelling stations, but there are no hydrogen fuelling stations because there is no demand for hydrogen as fuel for FCVs…). Establishing the necessary infrastructure for hydrogen production, transport and distribution would require significant capital investment, but there is no absolute impediment, since hundreds of hydrogen refuelling stations already exist in the U.S.A., Japan, and Europe. Obviously, legislation and standards are however still missing. In a recent paper published by General Motors, it is projected that consumers will not have to pay significantlymore for hydrogen than gasoline in the longer term and also that the key challenge remains matching scale and timing of hydrogen investment with actual hydrogen demand

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We should add to the list the public acceptance of a daily use of hydrogen.

4.6 Application Of Fuel Cell

4.6.1 Automotive applications (50‐250 kW)

This section is limited to the application of fuel cells for light‐duty vehicle and bus propulsion. The other related application as an auxiliary power unit onboard the vehicle will be treated in the next section named “Niche transport applications”.

a. Light‐duty vehicles (50 kW)

Almost all major car manufacturers have demonstrated prototype fuel cell vehicles and have announced plans for production and commercialization in the near to midterm future (5‐10 years). The race to develop a viable fuel cell vehicle and bring it to market began during the 1990s and continues today. The big drivers for the development of automotive fuel cell technology are its efficiency, low or zero emissions, and fuel that could be produced from local sources rather than imported. The main obstacles for fuel cell commercialization in automobiles are the cost of components and the availability of hydrogen.

The only fuel cell technology satisfying to both temperature and time response criteria for vehicle propulsion is the PEMFC. The low operating temperature combined with good durability and range makes it ideal for use in light duty vehicle. Power range is about 50 kW.

PEMFCs meet the 4000 h lifetime target for automotive applications at the laboratory scale. The effect of real‐life conditions on the fuel cell system (repeated startups and shutdowns, impurities in fuel and air, low and high temperatures) has to be assessed more thoroughly.Startup and steady operation in extremely cold climates (‐40°C) require specific water management controls, whereas the heat rejection system must be sized for hot weather conditions (+40°C). Water balance is a prerequisite in a PEMFC for optimal operation; this results also in an additional cooling system.

Four configurations are possible in a Fuel Cell Vehicle (FCV):

The fuel cell is sized to provide all the power needed. Due to the slower response of fuel processors (reformers), this configuration only applies for fast dynamic hydrogen‐fed vehicles. A small battery may be present but for start-up only.

In the parallel hybrid configuration, the fuel cell is sized to provide the base load, but the peak power for start-up and acceleration is provided by a battery. The battery allows rapid start-up without preheating of the fuel processor and recapturing of the braking energy, resulting in a more efficient system.

In the serial hybrid configuration, the fuel cell is sized to recharge the battery and the battery drives the electric motor. The relative sizes of the battery and the fuel cell are tied up: a smaller battery will have to be recharged faster and will result in a larger fuel cell.

Fuel cell serves only as an auxiliary power unit, that is, not for propulsion. This configuration is attractive for idling trucks requiring operation of air‐

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conditioning or refrigeration systems.

The difficulty of storing hydrogen onboard a vehicle, as well as lack of hydrogen infrastructure has forced car manufacturers to consider other, more conveniently supplied fuels. In that case the fuel cell must be integrated with a fuel processor that produces hydrogen from gasoline or methanol. However, apart from being a non zero‐emission process, onboard reforming is not easy and raises numerous engineering issues:

Onboard reforming reduces the overall efficiency of the propulsion system, which leads to upgrade the fuel cell size;

Onboard reforming enhances complexity, size, weight, and cost of the propulsion system;

Start-up time of fuel processors is too long in practice: this issue may be avoided in hybrid configurations;

Durability issues of the PEMFC due to impurities in the reformate hydrogen have been evidenced.

Many car manufacturers are perfecting their proprietary PEM units for use in their vehicles, e.g. Honda with the FCX Clarity, General Motors with fuel cell‐powered Chevrolet Volt and Equinox models, and Volkswagen with fuel cell‐powered Touran and Tiguan models. WhileA hydrogen fuel cell vehicle does not generate any pollution and is qualified as Zero Emission Vehicle (ZEV). If another fuel is used and reformed onboard, the propulsion system has some emissions generated during the reforming process, but those emissions are in general still much lower than the emissions from an internal combustion engine (ICE); therefore the fuel cell vehicles using a fuel reformer are typically qualified as Ultra Low Emission Vehicles (ULEV). Fuel cell‐powered vehicles also generate significantly less greenhouses gases than the comparable gasoline‐, diesel‐, or methanol‐powered ICEs.

Hydrogen is the only fuel that results in a zero‐emission vehicle, particularly if hydrogen is produced from renewable sources. Use of hydrogen as transportation fuel could reduce dependency on imported oil. A fuel cell system that runs on pure hydrogen is relatively simple, has the best performance, runs more efficiently, and has the longest stack life. Hydrogen is nontoxic and, despite its reputation, has some very safe features. One of the biggest problems related to hydrogen use in passenger cars is its onboard storage. Hydrogen can be stored as compressed gas, as cryogenic liquid, or in metal hydrides. Tanks for gaseous hydrogen are bulky, and the amount to be stored depends on the fuel efficiency and the required range (typically 300 miles or 500 km). In order to achieve a better match between the storage capacity of the tank, the fuel efficiency of the car and its range, further improvements in vehicle design, introduction of new lightweight composite materials, and compression of hydrogen at 700 bar are mandatory.the first one is based on a specific fuel cell design, all the others are derived from standard ICEs, with mere replacement of the propulsion engine by a fuel cell system. Alone amongst the major automakers, BMW is developing an SOFC‐based auxiliary power unit for its 7‐ series luxury car model.

b. Buses (250 kW)

Buses for city and regional transport are considered the most likely type of vehicles for an early market introduction of the PEMFC technology. Most of the issues discussed in the previous section, Light Duty Vehicles, also apply for the fuel cell applications in buses. The major differences are in power requirements, operating conditions and resulting lifetime, space available for hydrogen storage, and refuelling sites.

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Buses require typically 250 kW under high demanding, intermittent conditions, with frequent starts and stops. Compared to their diesel engine equivalent the efficiency gain is about 15%.

Buses are almost always operated as a fleet and refuelled in a central facility. Storage of large quantities of hydrogen onboard (the roof location is very safe for a gas lighter than air) is not a concern. These two characteristics make use of hydrogen much easier.

Thanks to use of hydrogen, fuel cell buses are Zero Emission Vehicles (ZEVs), which is a big advantage over diesel buses in densely populated regions. Demonstration programs funded from local to international level have seen several fleets of fuel cell buses deployed in European cities (Clean Urban Transport for Europe program), in the U.S.A. (Sun line Transit Authority in Palm Springs), and in large cities worldwide (United Nations Development Program, Global Environment Facility).

The main obstacles for commercialization of fuel cell buses are their cost and durability. Because the production series for buses are smaller than for passenger cars, their cost per kW is somewhat higher, as is the expected lifetime. Together with the intermittent operating regime, this could eventually challenge the current fuel cell technology.

4.6.2 Niche transport applications (1‐10 kW)

Small mobile fuel cell systems are designed to produce 1 to 10 kW of electrical power with low to zero emissions. This application is not as demanding as passenger cars or buses. The possible applications are very diverse and include:

- Utility and leisure vehicles, material handling industrial vehicles, e.g. forklifts, tow trucks, bicycles, scooters, motorbikes, golf carts; and wheelchairs for mobility assistance;

- Aircraft and aerospace applications; - Marine and submarine applications; - Auxiliary power units (APUs) for on‐board power supply.

An auxiliary power unit is composed of a small fuel cell system (a stack in the KW power range and a balance of plant part, with or without a reformer), which is associated to a prime driver engine i.e., internal combustion engine or electric motor, in order to supply additional power not related to the propulsion of the vehicle: air‐conditioning, multimedia playing or other comfort features. The fuel cell technology allows power generation without engine operation and enhances the run time of batteries. This is especially a good point at the time where anti‐idling regulations are setting place in a number of countries. Hence fuel cell‐based APUs improve the power flexibility of the vehicle without a complete replacement of the existing technology, which could foster an early market uptake of these “secondary” power sources. Moreover, the continuous increase in electrical demand for leisure vehicles and equipments is now accompanied by a desire for environmentally friendly on‐board

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conveniences. The growth in this sector is being driven by the need for clean, quiet, efficient power with extended run‐time particularly in the high end of the campervan and luxury boating markets. In a market largely insulated against recession, consumers are willing to pay a premium for the advantages that fuel cells have over batteries and generators. Campervan manufacturers have understood this very well and are now offering fuel cell‐ based APUs at least as optional extra and even standard equipment.Fuel cells for these applications are of the PEMFC or DMFC type, with a small number of SOFC‐based units essentially for APU applications. PEMFC‐based units are largely dominant in the aerospace, aircraft and materials handling markets while DMFC‐based units are more often found for leisure, marine and mobility assistance vehicles. DMFC‐based APUs run on methanol without a reformer. SOFC‐ and PEM‐based APUs usually incorporate a fuel reformer built into the unit so that the system can run on alternatives to hydrogen. In a peculiar market approach, the selection of fuel determines the type of fuel cell stack and the reformer technology. The reason for this fuel diversification is the desire to design the fuel cell APU to run on a fuel that is readily available to the end user. The choice of the same fuel for the fuel cell‐based APU as the main engine is a specific requirement of this application. Today this means gasoline or diesel, and development efforts are currently geared towards efficient reformers in order to make this eco‐friendly option available for use in commercial trucks shortly.

Like the portable sector, the materials handling sector is one where a real value proposition is now available to warehouse professionals because fuel cell‐powered materials handling vehicles operate near silently, with no or few emissions, and offer faster refuelling (1‐3 minutes) as well as longer run‐time than lead acid batteries and conventional internalcombustions engines. Compared with battery‐powered equipment, fuel cell systems have also the advantage of not requiring lengthy and floor space‐hungry recharges. Moreover, capital investment is less since a single fuel cell will operate continuously while from a logistic point of view, two or even three batteries are needed per battery‐powered vehicle. Fuel cell‐ based two‐ and three‐wheeled vehicles basically combine clean and efficient indoor operation with lower downtime, rapid refuelling, extended range and no operational degradation over time: since power provided by the fuel cell is constant throughout each shift, there is no performance loss of the vehicle.

Altogether, lower lifetime running costs are expected from fuel cell‐based systems than from their equivalents. This explains why an increasing number of manufacturers are developing and selling fuel cell‐powered bikes and three‐wheelers. A worldwide potential market exists for example in national postal services that use thousands of bikes and delivery trolleys. In the materials handling market, fuel cells seem to be still some way off being a serious competitor to long‐used ICEs and acid‐lead batteries, but some early niche markets are being actively explored and experience is currently gained in warehouse environments before large scale deployments. Ballard, Plug Power, Nuvera Fuel Cells and Hydrogenise are main players in the materials handling sector who are currently testing products on‐site. Finally, mobility assistance vehicles are an interesting niche market for light fuel cell systems, which offer an extended range but none of the inconveniences of battery recharging. SFC Smart Fuel Cells from Germany and Ajusa from Spain have shipped tens of units for impaired customers in 2008 In the aerospace and aircraft sectors, successful test flights have been reported in recent years, as well as a continued development of fuel cells for auxiliary power units on board larger aircrafts. Due to their silentness and long run time, unmanned aerial vehicles (UAVs) are especially attractive in the defence and aerospace fields for handling military

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reconnaissance, surveillance missions, or remote communications in strict secrecy. Other civilian applications are studied like remote scientific data collection under harsh conditions, disaster relief missions… It is unlikely that fuel cells will be used as a primary source of power for commercial aircrafts any time soon, but demonstration is being made that they are able to operate under extreme conditions: low temperatures and pressures, and unusual spatial orientations; hence they could provide efficient energy for on‐board electrical systems in‐ flight or under ground operation: heating/cooling, entertainment devices, and even essential controls in the aircraft, thereby reducing fuel consumption. Here a fuel cell APU may offer better efficiency than turbine APUs used today in spite of the necessary kerosene reformer. As a further advantage, in‐flight production of water is under investigation by several aircraft companies, e.g. Airbus. In the marine sector, legislation is likely to act as a key driver for the adoption of fuel cells. New restricting policies requiring low or zero emission for vessels in certain rivers, lakes and inland waterways in China and Europe, as well as growing pressure on regulating pollutant emissions in harbours, in coastal waters and on the high seas, are a favourable ground for the uptake of fuel cells as APUs on board vessels to reduce overall emissions and also for development of fuel cells as main means of propulsion. This has already caused a doubling of unit shipments in 2008 (mainly in Europe) and the trend will supposedly accelerate in the forthcoming years. Proton Motor’s Zero Emission Ship is the first fuel cell repulsed boat: as a demonstration project it has carried passengers since July 2008 in Hamburg’s harbour. Other proof‐of‐concept projects of low‐emission and low‐noise fuel cell‐powered or battery hybrid systems are under development for integration in canal barges, tug or river boats. Silent operation is of utmost importance for certain applications like scientific studies of sea animals. National governments and the International Maritime Organisation are in the process of voting further reductions of pollutant and noise emissions and new laws will certainly follow. Clearly, there is a great opportunity for fuel cells at the time of regulation in the marine environment.

4.6.3 Portable applications (0.1‐100 W)

In the portable sector, industrial interest for fuel cells in the W power range is great because of recurrent issues inherent to battery technologies (Nickel‐Metal Hydrure, Lithium‐Ion or Lithium‐Polymer). Significative improvements are possibly brought by fuel cells in this field:

Fuel cells have a higher energy density than batteries, i.e. they provide more energy per unit of weight, up to 5 times more. This allows longer run time before refuelling.

Portable fuel cell systems including the fuel storage container can be designed smaller and lighter than a battery of equivalent power.

Continuous operation of fuel cells (as long as fuel is supplied) means also longer standby time (depending on the fuel reservoir volume), no time‐ consuming recharge and associated logistics (e.g., need for several units for battery exchange), and less degradation of the components.

Micro power applications of fuel cells are typically the same as batteries, i.e. all electronic devices for nomad use like mobile phones, laptop computers, personal digital assistants, cameras, and music or multimedia players. Other applications are found in portable military, healthcare or camping tools. In these applications fuel cells are expected to replace batteries thanks mainly to their higher storage capacity. Conversion efficiencies are of less importance as long as they do not restrict autonomy.

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Micro fuel cells are of the DMFC or the PEMFC type; they run at low temperatures on liquid methanol, formic acid, or hydrogen stored in low pressure hydride containers. The operation temperature should not exceed 50 to 60°C, which excludes the use of a reformer. Further, it is important that the fuel storage must achieve a high level of security. Use of liquid high‐ pressure hydrogen is of course excluded as well. In this field PEMFCs with chemical hydrogen storage are competing with DMFCs. While PEMFCs have a higher power density than DMFCs, chemical hydride solutions are not ready for market yet. Portable DMFC‐based micro fuel cells have been first demonstrated by Toshiba, Smart Fuel Cell and MTI Micro Fuel Cells. An increasing number of companies are also developing DMFC‐based fuel cell cartridges, either as stand‐alone products (BIC, Gillette; Neah Power) or for powering their own consumer electronics portfolio (Motorola, Hitachi, Panasonic, Sony). Motorola expects its fuel cells to run about 10 times longer than today’s batteries before needing to be recharged.

Smart Fuel Cell is selling a wide range of DMFC‐based products; from 50 W‐units targeted to the recreational market, to portable docking stations for laptop computers.The real challenge is the miniaturization of the system, which may consist of either scaling down the different components of larger existing stacks or developing a specific architecture based for instance on silicone‐supported thin films derived from microelectronics. Each of these solutions implies a specific management of the various fluxes in the fuel cell: flux of reactant gases, flux of products water and heat. The crucial point for the micro fuel cell is to handle a power surge upon switching the device from idle to active.

The current trend for portable devices is an ever growing power demand in conjunction with their increasing number of Internet functionalities. Therefore, the advantages of fuel cells of storing more power in the same volume, for longer time while being able to refuel the product quickly instead of recharging will be hopefully seen by most consumers as another area for freedom and lead to a progressive decay of the battery‐based today technology. Furthermore, standard batteries on use today like lithium‐ion batteries are quite expensive themselves so that the cost barrier for the introduction to fuel cells to the portable market is lower than in other applications.

4.6.4. Wireless applications (0.1‐1 kW)

Portable soldier power Wireless tools

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CONCLUSION

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied.

There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety of sizes. Individual fuel cells produce very small amounts of electricity, about 0.7 volts, so cells are "stacked", or placed in series or parallel circuits, to increase the voltage and current output to meet an application’s power generation requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.

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REFRENCES

www.google.com

www.wikipedia.com

http://www1.eere.energy.gov/hydrogenandfuelcells/production/

natural_gas.html.

http://www.eia.gov/oiaf/servicerpt/hydro/appendixc.html.

https://www.hfpeurope.org/infotools/energyinfos__e/hydrogen/

main03.html

http://www.eere.energy.gov/hydrogenfuel/

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[61]

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