Fuel Cell Chp

8/9/2019 Fuel Cell Chp

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4.4 Fuel Cells

Fuel cells are an entirely different approach to the production of electricity than traditional prime mover technologies, and are currently in the early stages of development. Fuel cell stacks available and under development are silent, produce no pollutants, have no moving parts, and have potential fuel efficiencies

far beyond the most advanced reciprocating engine or gas turbine power generation systems. Fuel cellsystems with their support ancillary pumps, blowers, and reformers maintain most of these advantages. Aschematic of a fuel-cell-based CHP system is shown in Figure -!.

Figure 4-8: Fuel Cell System

As with most new technologies, fuel cell systems have several disadvantages, such as productimmaturity, over-engineered system comple"ities, and unproven product durability and reliability. #hesetranslate into high capital cost, lack of support infrastructure, and technical risk for early adopters, whichin turn cause market resistances that propagate the disadvantages. However, the many advantages of fuel cells over other prime movers suggest that they could well become the prime mover of choice for many applications and products in the future.

Fuel cells produce power electrochemically from hydrogen delivered to the negative pole $anode% of thecell and o"ygen delivered to the positive pole $cathode%. #he hydrogen can come from a variety of sources, but the most economic method is by reforming of natural gas or li&uid fuels. #here are several

different li&uid and solid media that support these electrochemical reactions - phosphoric acid $PAFC%,molten carbonate $'CFC%, solid o"ide $()FC%, and proton e"change membrane $P*'% are the mostcommon systems. *ach of these media comprises a distinct fuel cell technology with its own performancecharacteristics and development schedule. PAFCs are the most widely deployed fuel cells in commercialservice now with + k units delivered to over + customers. P*' fuel cells are now entering themarket and ()FC, and 'CFC technologies are in field test or demonstration stages. irectelectrochemical reactions are generally more efficient than using fuel to drive a heat engine to produceelectricity. Fuel cell efficiencies range from /0- percent for the PAFC to upwards of 1 percent for developing systems. Fuel cells are inherently &uiet and have e"tremely low emissions levels as only a


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small part of the fuel is combusted. 2ike a battery, fuel cells produce direct current $C% that must be runthrough an inverter to get 1 H3 AC. #hese power electronics components can be integrated with other power &uality components as part of a power &uality control strategy for sensitive customers. 4ecause of current high costs, fuel cells are best suited to environmentally sensitive areas and customers with power &uality concerns.

4.4.1 Technology Description

#his section describes five types of fuel cell under development for a variety of distributed generation$5% applications. #hese are6 7% phosphoric acid $PAFC%, +% proton e"change membrane $P*'FC%, /%molten carbonate $'CFC%, % solid o"ide $()FC%, and 0% alkaline $AFC%. *ach type is distinguished bythe electrolyte used and by operating temperatures. #hese fuel cells operate from near-ambienttemperature through 7,8 C. As a result, they can have very different performance characteristics,advantages and limitations, and therefore will be suited to 5 applications in a wide variety of approaches.

#he different fuel cell types share certain important characteristics. First, fuel cells are not Carnot cycle

$thermal energy-based% engines. 9nstead, they use an electrochemical or battery-like process to convertthe chemical energy of hydrogen into water and electricity. As simple cycle devices, they can achievee"tremely high electrical efficiencies. #he second shared feature is that they use hydrogen as their fuel,which is typically derived from a hydrocarbon fuel such as natural gas. #hird, each fuel cell system iscomposed of three primary subsystems6 7% the fuel cell stack that generates direct current electricity: +%the fuel processor that converts the natural gas into a hydrogen-rich feed stream: and /% the power conditioner that processes the electric energy into alternating current or regulated direct current. Finally,all types of fuel cells emit minimum emissions. #his is because the only combustion process is theburning of a low energy hydrogen e"haust stream that is used to provide heat to the fuel processor.

Fuel cells produce direct current electricity through an electrochemical process, much like a standardbattery. ;nlike a standard battery, the fuel cell is chemically replenished on a continuous basis. #hereactants, most typically hydrogen and o"ygen gas, are fed into the fuel cell reactor, and power is

generated as long as these reactants are supplied. #he hydrogen $H + % is typically generated from ahydrocarbon fuel such as natural gas or 2P5, and the o"ygen $) + % is from ambient air.

Basic Processes and Components

Fuel cell systems designed for 5 applications are primarily natural gas or 2P5 fueled systems. *achfuel cell system is composed of three primary subsystems6 7% the fuel cell stack that generates directcurrent electricity: +% the fuel processor that converts the natural gas into a hydrogen rich feed stream:and /% the power conditioner that processes the electric energy into alternating current or regulated directcurrent.

9n a fuel cell it is the change in the 5ibbs free energy of formation that results in energy release. #hischange is the difference between the 5ibbs free energy of the product and the 5ibbs free energy of thereactants. ater $H + )% is the chemical product of this process. #he overall reaction in a fuel cell ischaracteri3ed as follows6

+H + $gas% < ) + $gas% => +H + ) $vapor% < *nergy


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#he hydrogen and o"ygen gases are not directly mi"ed and combustion does not occur. 9nstead, thehydrogen is o"idi3ed one molecule at a time, in the presence of a catalyst. 4ecause the reaction iscontrolled at the molecular level, there is no opportunity for the formation of ?) " and other pollutants asthere is inside piston and turbine engines.

Figure -@ illustrates the electrochemical process in a typical single cell, acid-type fuel cell. #he hydrogenand o"ygen are fed to the anode and cathode, respectively. #he anode and cathode are electricallyconductive, porous electrodes separated by an ionic conductive $non-electrically conductive% media,known as the membrane, ionic matri" or electrolyte layer. At the anode the hydrogen gas iselectrochemically dissociated into hydrogen ions $H < % and free electrons $e - %. #he half-cell reaction isthe following

+H + $gas% => H < < e -* o . volts

Figure 4-9: Fuel Cell Electrochemical Process

#he electrons flow out of the anode through an e"ternal electrical circuit. #he hydrogen ions flow into theelectrolyte layer and eventually to the cathode, driven by both concentration and potential forces. At thecathode the o"ygen gas is electrochemically combined with the hydrogen ions and free electrons togenerate water. #he half-cell reaction is the following6

) + $gas% < H < < e - => +H + ) $vapor% * o 7.+/ volts $2HB% 1!, or

) + $gas% < H < < e - => +H + ) $li&uid% * o 7.! volts $HHB% /


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As power is generated, electrons flow through the e"ternal circuit, ions flow through the electrolyte layer and chemicals flow into and out of the electrodes. *ach process has natural resistances that must beovercome, which reduces the operational cell voltage below the theoretical potential. #herefore, some of the chemical potential energy is converted into heat. #he electrical power generated by the fuel cell is theproduct of the current measured in amps and the operational voltage. 4ased on the application andeconomics, a typical operating fuel cell will have an operating voltage of between .00 volts and .!volts. #he ratio of the operating voltage and 7.!volts represents a simplified estimate of the stackelectrical efficiency on a HHB basis. As stated, resistance heat is also generated along with the power.(ince the electric power is the product of the operating voltage and the current, the &uantity of heat thatmust be removed from the fuel cell is the product of the current and the difference between the 5ibbs freeenergy potential and the operating voltage. 9n most cases the water produced by the fuel cell reactionse"its the fuel cell as vapor, and therefore, the 7.+/-volt 2HB free energy potential should be used toestimate sensible heat generated by the fuel cell electrochemical process.

Practical fuel cell systems re&uire voltages higher than .00 to .!. #his is achieved by combiningseveral cells in electrical series into a fuel cell stack. #ypically, several hundred cells are use in a singlecell stack. #he current is managed by increasing the active area of individual cells. #ypically, cell area canrange from 7 cm + to over one m + depending on the type of fuel cell and application power re&uirements.

Fuel Processors

9n distributed generation applications, the most viable fuel cell technologies use natural gas as the stockfuel. #o operate on natural gas or other fuels, fuel cells re&uire a device that converts the stock fuel intothe hydrogen-rich gas stream. #his device is known as a fuel processor or reformer. hile adding fuelfle"ibility to the system, the reformer adds significant cost, comple"ity, and the potential for undesiredemissions. #here are three primary types of reformers6 steam reformers, autothermal reformers andpartial o"idation reformers. #he fundamental differences are the source of o"ygen used to combine withthe carbon within the fuel to release the H+ gases and the thermal balance of the chemical process.(team reformers use steam: while partial o"idation units use o"ygen gas and autothermal reformers useboth steam and o"ygen. Partial o"idation units combust a portion of the fuel $i.e. partially o"idi3e it%, thethermal balance is e"othermic and releases heat.

(team reforming is e"tremely endothermic and re&uires a substantial amount of heat input. Autothermalreformers are typically operated at or near the thermal neutral point, and therefore, do not generate or consume thermal energy. hen integrated into a fuel cell system that allows the use of anode-off gas, atypical natural gas reformer can achieve efficiencies in the 0 to @D 2HB range with !/ to !0D being ane"pected characteristic. #hese efficiencies are defined as the 2HB of hydrogen generated divided by the2HB of the natural gas consumed by the reformer.

(ome fuel cells can be constructed and operated as internally steam reforming fuel cells. (ince thereformer is an endothermic catalytic converter and the fuel cell is an e"othermic catalytic o"idi3er, the twoare combined into one with mutual benefits from the thermal processes. 'ore comple" than a purehydrogen fuel cell, these types of fuel cells are more difficult to design and operate. hile combining twocatalytic processes is difficult to arrange and control, these internally reforming fuel cells are e"pected to

account for a significant market share as fuel cell-based 5 becomes more common.

Power Conditioning Subsystem

#he fuel cell generates direct current electricity, which must be conditioned before being used within a 5application. epending on the cell area and number of cells, this direct current electricity is appro"imately+ to volts per stack. 9f the system is large, enough, stacks can be operated in series to double or triple individual stack voltages. (ince the voltage of each individual cell decreases with increasing load or power, the output is considered an unregulated voltage source. 9n the power conditioning subsystem, the


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output voltage can be boosted to provide a regulated higher voltage input source to an electronic inverter.#he inverter then uses a pulse width modulation techni&ue at high fre&uencies to generate simulatedalternating current output. #he fre&uency of the output is controlled by the inverter and can be adEusted toenhance power factor characteristics. 4ecause the alternating current is generated within the inverter theoutput power is considered to be a very clean, reliable source. #his characteristic can be important tosensitive electronic e&uipment in premium power applications. #he efficiency of the power conditioningprocess is typically @+ to @1D, and is dependent on system capacity and input voltage-currentcharacteristic.

Types of Fuel Cells

#here are five basic types of fuel cell under consideration for 5 applications. #he fuel cells electrolyte or ion conduction material defines the basic type. #wo of these fuel cell types, polymer electrolyte membrane$P*'% and phosphoric acid fuel cell $PAFC% have acidic electrolytes and rely on the transport of H < ions.#wo others, alkaline fuel cell $AFC% and carbonate fuel cell $'CFC% have basic electrolytes that rely onthe transport of )H - and C) / ions, respectively. #he fifth type, solid o"ide fuel cell $()FC%, is basedon a solid-state ceramic electrolyte in which o"ygen ions $) % are the conductive transport ion. *ach fuelcell type has been designed to operate at optimum temperatures, which are a balance between the ionicconductivity and component stability. #hese temperatures differ significantly among the five basic types,

ranging from near-ambient to as high as 7!8 F. #he proton conducting fuel cell type generates water atthe cathode and the anion conducting fuel cell type generates water at the anode. #able -0 presentsfundamental characteristics for each fuel cell type.

Ta"le 4-#: Characteristics o$ %a&or Fuel Cell Types

PE%FC $Proton *"change 'embrane Fuel Cell or Polymer *lectrolyte 'embrane%

#his type of fuel cell was initially developed in the 7@1s for the first ?A(A manned spacecraft. #heP*'FC uses a solid polymer electrolyte and operates at low temperatures, about that of boiling water.)ver the past ten years, the P*'FC has received significant media coverage due to the large investmentthe auto industry has made in the technology. ue to their modularity and apparent ease of manufacturing, much has been made of the reformerGP*'FC system for residential 5 applications.


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'FC 'l(aline Fuel Cell!

F.#. 4acon in Cambridge, *ngland first demonstrated AFC as a viable power unit during the 7@s and7@0s. 9t was later developed for ?A(A, and was used on the Apollo moon mission spacecraft and on thespace shuttles. #he primary advantages of AFC technology are improved performance, use of non-precious metal electrodes, and the fact that no unusual materials are needed. #he primary disadvantage

is the tendency to absorb carbon dio"ide, converting the alkaline electrolyte to an a&ueous carbonateelectrolyte that is less conductive. #he attractiveness of AFC has declined substantially with the interestand improvements in P*'FC technology.

P'FC Phosphoric 'ci) Fuel Cell!

PAFC is generally considered to be the most established fuel cell technology. #he first PAFC 5 systemwas designed and demonstrated in the early 7@s. PAFCs are generally considered commerciallyavailable since the early 7@@s and are considered to be viable in niche markets. PAFC-based systemshave been installed in the ;.(., apan, and *urope, and are commercially available in the + k si3e.hen operated at elevated pressures, PAFCs are capable of fuel-to-electricity efficiencies of better than/1D. #he technology on which this system is based was developed as the result of agovernmentGindustry research, development and demonstration $IJ% effort starting in the 7@s and

continuing through the early 7@@s. uring the 7@@s, over + commercial units were manufactured,delivered and operated. #he current + k product is reported to have a stack lifetime of over ,hours, units with nearly eight years of operation, and commercially based reliabilities in the @ to @0Drange. #he maEor market barrier has been the initial installed cost that has not yet fallen below the K/,0to ,Gk range.

%CFC %olten Car"onate Fuel Cell!

#he 'CFC has a developmental history that dates back to the early part of the twentieth century. ue toits operating temperature range of 1 to 08 C, the 'CFC holds promise in both CHP and 5applications. #his type of fuel cell can be built with internal reforming, run at high efficiencies $0D HHB%,and is relatively tolerant of fuel impurities. 5overnmentGindustry IJ programs during the 7@!s and7@@s resulted in several individual pre-prototype system demonstrations. )f these developers, Fuel Cell

*nergy is approaching an initial stage of market introduction. #he primary technical issue with 'CFCtechnologies is the degradation of cell components due to the corrosive nature of the electrolyte operatingtemperature combination.

S*FC Soli) *+i)e Fuel Cell!

(everal ()FC units up to 7 k in si3e and based on a concentric tubular design have been built andtested. ,! 9n addition, there are many companies developing planar ()FC designs, which are believed tooffer higher power densities and lower costs than the tubular design, but these have yet to achieve thereliability of the tubular design. #he ()FC is generally considered to be much less mature in itsdevelopment compared with the 'CFC and PAFC. espite relative immaturity, the ()FC has severaladvantages that Eustify its further development. High efficiency, stability and reliability, and high internaltemperatures, distinguish the ()FC from other fuel cell technologies. #he ()FC has proEected service

electric efficiencies of 0 to 1D and higher, for larger combined cycle plants. *fficiencies for smaller ()FC 5 units are e"pected to be in the 0D range. (tability and reliability of the ()FC are high due toan all-solid-state ceramic construction. #est units have operated in e"cess of 7 years with acceptableperformance. #he high internal temperatures of the ()FC are both an asset and a liability. As an asset,high temperatures make internal reforming possible. As a liability, these high temperatures add tomaterials and mechanical design difficulties, which reduces lifetime and increases cost. hile ()FCresearch has been ongoing for / years, costs of these stacks are still comparatively high.


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4.4., Per$ormance

#able -1 summari3es performance characteristics for representative developmental natural gas fuel cellCHP systems over the 7 k to + ' si3e range. #his si3e range covers the maEority of the marketapplications for fuel cell CHP systems.

Ta"le 4-: Fuel Cell CP - Typical Per$ormance Parameters


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#his group of systems was selected as representing the most likely units to be commercially introducedwithin the ne"t five years. *ach fuel cell technology would ultimately be e"pected to have applicationsover a broad range of capacities from under 7 k to over several '.

Heat rates and efficiencies shown were taken from manufacturers specifications and industry publicationsor are based on the best available data for developing technologies. Available thermal energy was

calculated from estimated overall efficiency for these systems. CHP thermal recovery estimates arebased on producing low &uality heat for domestic hot water process or space heating needs. #his featureis generally acceptable for commercialGinstitutional applications where it is more common to have hotwater thermal loads.

#he data in the table show that electrical efficiency increases as the operating temperature of the fuel cellincreases. Also illustrated is an increase as system si3e becomes larger. As electrical efficiencyincreases, the absolute &uantity of thermal energy available to produce useful thermal energy decreasesper unit of power output, and the ratio of power to heat for the CHP system generally increases. Achanging ratio of power to heat impacts proEect economics and may affect the decisions that customersmake in terms of CHP acceptance, si3ing, and the desirability of selling power.

4.4./ Emissions

As the primary power generation process in fuel cell systems does not involve combustion, very fewemissions are generated. 9n fact, the fuel processing subsystem is the only source of emissions. #heanode-off gas that typically consists of ! to 70D hydrogen is combusted in a catalytic or surface burner element to provide heat to the reforming process. #he temperature of this very lean combustion can bemaintained at less than 7,!8 F, which also prevents the formation of o"ides of nitrogen $?) " % but issufficiently high to ensure o"idation of carbon mono"ide $C)% and volatile organic compounds $B)Cs -


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unburned, non-methane hydrocarbons%. )ther pollutants such as o"ides of sulfur $() " % are eliminatedbecause they are typically removed in an absorbed bed before the fuel is processed.

Fuel cell systems do not re&uire any emissions control devices to meet current and proEected regulations.

4./.4 CP 'pplications

CHP applications are onsite power generation in combination with the recovery and use of by-productheat. Although less dependent on effective thermal utili3ation, the heat available from fuel cell systemsgenerally represents + to 0D of the inlet fuel energy. As an e"ample, the PAFC system achieves /1DHHB electric efficiency and + to !D overall efficiency, which means that it has a /1D thermal efficiencyor power to heat ratio of one. )f the available heat, +0 to 0D is recovered from the stack-cooling loop.#he balance of heat is derived from the e"haust gas-cooling loop that serves two functions. #he first iscondensation of product water, making the system self-sufficient, and the second is the recovery of by-product heat. #his tends to limit the application of this heat to domestic hot water applications becausethe heat is recovered over a range from 78 F to 78 F.

(ystem capacities and thermal loads most amenable to PAFC and P*' fuel cell CHP systems are incommercialGinstitutional buildings with space heating and hot water re&uirements. #he simplest thermalload to supply is hot water. Primary applications for CHP in the commercialGinstitutional sectors are thosebuilding types with relatively high and coincident electric and hot water demand such as colleges anduniversities, hospitals and nursing homes, and lodging. )ffice buildings and certain warehousing andmercantileGservice applications can be economic applications for the high efficiency fuel cell CHP systembecause their economics are much less sensitive to heat utili3ation. #echnology development efforts for engine-driven systems targeted at heat activated coolingGrefrigeration and thermally regenerateddesiccants will also enhance fuel cell CHP applications by increasing the thermal energy loads in certainbuilding types. ;se of these advanced technologies in applications such as restaurants, supermarketsand refrigerated warehouses provides a base thermal load that opens these applications to CHP.

'CFC and ()FC have a much broader potential for CHP including steam production and direct use of

heat in industrial applications.

4./.# Thermal Energy 0eneration

#here are four primary potential sources of usable waste heat from a fuel cell system6 e"haust gasincluding water condensation, stack cooling, anode-off gas combustion, and reformer heat. #he PAFCsystem achieves /1D electric efficiency and + to !D overall efficiency, which means that it has a /1Dthermal efficiency or power to heat ratio of one. )f the available heat, +0 to 0D is recovered from thestack-cooling loop that operates at appro"imately 8 F and can deliver low- to medium-pressure steam.#he balance of heat is derived from the e"haust gas-cooling loop that serves two functions. #he first is

condensation of product water, thus rendering the system water self-sufficient, and the second is therecovery of by-product heat. (ince its primary function is water recovery, the balance of the heat availablefrom the PAFC fuel cell is recoverable with 7+8 F return and /8 F supply temperatures. #his tends tolimit the application of this heat to domestic hot water applications. #he other aspect to note is that all of the available anode-off gas heat and internal reformer heat is used internally to ma"imi3e systemefficiency.

Heat can generally be recovered in the form of hot water or low-pressure steam $L/ psig%, but the &ualityof heat is very dependent on the type of fuel cell and its operating temperature. #he one e"ception to thisis the P*' fuel cell, which operates at temperatures below +8 F, and therefore has only low &uality


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heat. 5enerally, the heat recovered from fuel cell CHP systems is appropriate for low temperatureprocess needs, space heating and potable water heating.

'edium pressure steam $up to about 70 psig% can be generated from the fuel cells high temperaturee"haust gas $in the case of ()FC and 'CFC technologies%, but the primary use of this hot e"haust gas isin recuperative heat e"change with the inlet process gases. 2ike engine and turbine systems, the fuel cell

e"haust gas can be used directly for process drying. Any available higher &uality heat can be used todrive absorption chillers providing cold water, air conditioning or refrigeration. #he high electric efficiencyof fuel cell systems tends to imply that this heat is only a small fraction of the inlet fuels heating value,and therefore, the economics of the reduced capacity absorption chiller may impact CHP-coolingeconomics unless direct-fired chillers are economically feasible and the recovered heat has only asupplemental role.

4.4. Current %ar(et 'pplications

Fuel cell systems are envisioned to serve a variety of distributed generation applications and markets.(ince all fuel cells are in an early stage of development, there is limited e"perience that can be used to

validate those applications considered most competitive for fuel cells in comparison to alternate primemovers such as reciprocating engines and gas turbines. #his early stage of development and commercialuse causes fuel cells to be higher in capital cost and to have a higher proEect risk due to unprovendurability and reliability. #hese two characteristics will force introductory fuel cell models into specificmarkets and applications that are most tolerant of risk due to other market or operational drivers.

9n 5 markets, the primary characteristic driving early market acceptance has been the ability of fuel cellsystems to provide premium power and high environmental benefits. 9nterest has been driven by their ability to achieve high efficiencies over a broad load profile and near-3ero emission signatures withoutadditional controls. 'ost of the +-plus commercially operational fuel cells have been installed ascombined heat and power units to improve overall economics.

Potential 5 applications for fuel cell systems include combined heat and power $CHP%, premium power,

remote power, specialty applications, grid support, peaking power and microgrid applications.

#oday, the most deployed and commercially available fuel cell is a + k PAFC unit /!. ith over +units sold, this fleet has achieved over million operating hours in a variety of distributed generationapplications. #hese range from a ?ew Mork City police station to a maEor postal facility in Alaska and acredit card processing system in ?ebraska. 2ocated in over 70 countries, this initial commercial fuel cellproduct has made valuable contributions in introducing the capabilities and features of fuel cells to thedistributed generation marketplace.

4.4. CP Potential

Fuel cells can achieve overall efficiencies in the 10 to !0D range. aste heat is used for domestic hotwater applications and space heating. 4ased on the operating temperature of the fuel cell, some higher &uality heat can be recovered, typically up to 0D of the available heat. 'edium pressure steam $up toabout 70 psig% can be generated from the fuel cells high temperature e"haust gas $in the case of ()FCand 'CFC technologies%, but the primary use of this hot e"haust gas is in recuperative heat e"changewith the inlet process gases. 2ike engine and turbine systems, the fuel cell e"haust gas can be useddirectly for process drying. Any available higher &uality heat can be used to drive absorption chillersproviding cold water, air conditioning or refrigeration. #he high electric efficiency of fuel cell systems tendsto imply that this heat is only a small fraction of the inlet fuels heating value.


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7. 2ower heating value $2HB% and higher heating value $HHB%. 'ost of the efficiencies &uoted in thisreport are based on higher heating value $HHB%, which includes the heat of condensation of thewater vapor in the products. 9n engineering and scientific literature the lower heating value $2HB -which does not include the heat of condensation of the water vapor in the products% is often used.#he HHB is greater than the 2HB by appro"imately 7D with natural gas as the fuel $i.e., 0D2HB is e&uivalent to 0D HHB%. HHB efficiencies are about 70D for hydrogen, !D greater for oil$li&uid petroleum products% and 0D for coal.

+. 4y (iemensGestinghouse *lectric Corp.

/. (old by ;#C Fuel Cells as the PC+0.

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