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Alternative energy technologies

Materials UK Energy Review 2007Report 4 - Alternative Energy Technologies

Materials UK Energy Review 2007Alternative Energy Technologies

Contents1.0 Executive summary

2.0 Introduction

3.0 Wave and Tidal

4.0 Wind Power

5.0 Biomass Technologies

6.0 Hydrogen

7.0 Fuel Cells

8.0 Solar

AuthorsEdited by: John Oakey Cranfield UniversityChris Bagley TWI Ltd

1 Wave and TidalLead Author R Martin (MERL) with support from M Gower (NPL) and members ofSupergen Marine.

2 Wind PowerLead Author – M Gower (NPL) with support from R Martin (MERL), C Bagley(TWI) and S Court (NAMTEC).

3 Biomass TechnologiesLead Author – J Oakey (Cranfield) with support from P Thornley (ManchesterUniversity), M Kelly (Scottish Assoc. for Marine Science) and members ofSupergen Bioenergy.

4 HydrogenLead Author – Peter Edwards (University of Oxford), V.L. Kuznetsov (University ofOxford), M. Gersch (University of Oxford), S. Ellis (Johnson Matthey Plc), Z.X.Guo (Queen Mary, University of London), R Pargeter (TWI) and members of theUK Sustainable Hydrogen Energy Consortium.

5 Fuel CellsLead Author – J Kilner (Imperial College) with support from J McCarney (JohnsonMatthey Fuel Cells), G J Wright (Rolls-Royce Fuel Cell Systems Limited), A RChapman (Ceramic Fuel Cells (Europe) Limited) and members of Fuel cells UK .

6 SolarLead Author – S Irvine (University of Bangor), G Butler (Sharp UK), N Mason (BPSolar), P Stillman (ICP Solar), M Scott (IQE Europe), J Durant (Imperial College)and members of PV Supergen.

In addition, consultation has been held with many other UK groups including theUK Energy Research Centre and the Advanced Power Generation TechnologyForum.


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Executive summary1.0

The Materials UKAlternative EnergyTechnologies Grouphas undertaken aseries of reviewsaimed at identifyingmaterials challengesfacing renewableenergy technologies.

The technologies selected were thoseof most relevance to the UK andinclude those with close links tofossil fuel heat and powertechnologies and those that takeadvantage of naturally occurringprocesses - namely:

- Biomass / waste to energy,hydrogen and fuel cells and:

- Wind, wave/tidal and solar power

For each technology, the researchpriorities to support future materialsdevelopment in the UK have beenidentified.

The chosen technologies provide‘low carbon’ alternatives to fossilfuels, though for hydrogen powerthis depends on the source of thefeedstock. All can contribute toreducing CO2. No single technologypredominates but all play a part inmeeting future demand or displacingother equipment that is more‘carbon intensive’ when it comes tothe end of its commercial life.Maintaining a broad portfolio ofalternative energy options is criticalto the future energy security of theUK.

UK StrengthsUK strengths in Alternative Energyvary depending on the particulartechnology. In some cases, e.g. fuelcells, UK has leading-edgemanufacturers (e.g. Johnson MattheyFuel Cells & Rolls-Royce Fuel Cells),whereas in others UK strength lies inthe deployment of technologies fromoverseas, e.g. offshore wind farms.The UK is however able to supportfuture energy options via wellestablished manufacturing, servicingand integrated systems capabilitiesand through technical consultancies.UK strength also lies in the stronglinks between universities, Research& Technology Organisations (RTOs)and industry. These links are criticaland must be maintained and grown.

Research PrioritiesGeneric ‘alternative energy’ researchpriorities are identified below. Somepriorities can be met within UK’scurrent capabilities; some requireexpansion of UK research activities,while others are best addressed viaoverseas collaboration. High priorityshould be given to the followingtopics:

Structural Materials [Load-bearing]

• Light-weight/high strength FibreReinforced Polymer (FRP)composite wind and tidal turbineblades for large scale, low costdesigns that are able to resist highfatigue loads and harsh offshoreenvironments

• Continued development ofinnovative material designs andprocessing techniques forwind/wave/tidal turbines – such assandwich constructions, compositejoints, FRP pre-forming andinfusion techniques

• Sub-system componentdevelopment of fuel cells

• High strength, corrosion resistantheat exchanger alloys and coatingsfor biomass & waste systems

Functional Materials [Activesurfaces / electrical]

• New and improved materials forfuel cells, including electrolytes,electrodes and interconnectmaterials

• Separation membrane technology(e.g. for H2/CO2 separation),materials for hydrogen storage

Materials processing,fabrication and integration

• Advanced processing of silicon andthin film GaInP/GaAs forphotovoltaic cells to reduce costs

• Advanced processing of fuel cellmaterials for both high and lowtemperature applications

• Development of continuousmanufacturing processes formaterials that are currently batchproduced and require moreconsistent quality of manufacture

• Development of ‘in-process’inspection techniques to ensurethe supply of high qualitycomponents

• Development of repair techniquesthat can ideally be deployed ‘in-situ’.

Executive summary


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Environmental Resistanceand Protective Systems

• Paints – e.g. antifouling for wave /tidal components

• Coatings–e.g. corrosion protectionfor biomass/waste heat exchangers,oxidation/hot corrosion resistancein gas turbines/engines, hightemperature fuel cell components,antireflective coatings for PV cells,etc.

Life Cycle Modelling:Modelling offers a short cut toimproving component design and isan invaluable tool for management of the energy system inservice. Modelling areas include:

• Behavioural modelling of windand tidal turbine blades

• Thermo-mechanical behaviour ofheat exchangers in biomasscombustion and gasificationsystems

• Operating behaviour of low andhigh temperature fuel cellcomponents

• Improved reliability and reduceduncertainty through developmentof test and modelling methods forlifecycle analysis/fatigueperformance of constituentmaterials, sub-components andmajor structures such as blades

• Development of test andmodelling methods for materialscharacterisation in harshenvironments.

Generic MaterialRequirements:

Many technologies share commonmaterial development requirements:

• Condition monitoring and ‘smartsensors’ are also critical forassessing environmentalconditions and materialdegradation in extreme locations[such as offshore wind farms].Data from such sources providesinput for modelling/validatingservice behaviour. Monitoringtechnologies also facilitatescheduled maintenance and leadto reduced operating andmaintenance (O&M) costs.

• Development of ‘field deployable’non-destructive evaluation (NDE)techniques enables accurate andrapid defect detection ofdamage/defects [e.g. in windturbines or wave / tidal devices]

• Development of standards andcertification procedures

• Testing and characterisationprocedures

• Repair, rejuvenation and recyclingof expensive materials

• Investigation into fundamentalfailure mechanisms in advancedmaterials. Such studies shouldinclude effects of environment andmicro- / nano-scale flaws onmaterial durability.

In summary, the main challengesthat alternative energy systems haveto overcome to reach fullcommercialisation within the nextdecade are durability, reliabilityand cost.

Eco Considerations and Sustainability

As production capacity increases foreach alternative energy technology,greater attention has to be given tomaterial processing and potentialtoxic emissions/waste. Suchconsiderations are particularlyrelevant for newer, high performancematerials. It is not sufficient forfinished products to beenvironmentally safe and chemicallyinert. The environmentalimplications for new materials andmanufacturing technologies need tobe considered throughout the wholesupply chain.

High volume production will reducethe ready availability of manyspecialist materials. Limited access tocarbon fibre materials for windturbines is already being observed.Similar limitations are likely to affectexotic materials such as indium asused in some transparent conductingoxides for thin film PV productionand more ‘common’ materials suchas copper for generators. Readilyavailable alternative materials needto be developed.

Legislation and EnergySecurity

Alternative energy systems willrequire legislative incentives andfunding initiatives before theybecome readily available atcommercially attractive costs.

Environmental legislation isincreasingly concerned aboutproduct end-of-life management.Automotive and electrical/electronicssectors have already been setmandatory European recovery andrecycling targets. Introducinglegislation into the alternativeenergy sector may prove difficult,given the diversity of technologiesinvolved, but further constraints arelikely. It would be unacceptable if“green” sources of power were tocause environmental burdens duringlater decommissioning and disposal.Research into end-of-lifemanagement will be critical in thenext 20 years. In particular, researchis required into cost-effectiverecovery and recycling of materialsand into avoiding subsequentdisposal through landfill.


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Introduction2.0

Renewable energy isan integral part of theGovernment’s strategyfor tackling climatechange.

In its 2006 Energy WhitePaper [ref.1] the governmentproposed a range of measuresto enable 20% of UKelectricity to come fromrenewable sources by 2020.

That target will only be achieved bysignificant advances in all availabletechnologies and in associatedmaterials and processes. This report,carried out by specialists inconsultation with theircommunities, summarises theMaterial R&D priorities necessary toachieve full technical andmanufacturing readiness.

Renewable energy can be connectedinto the electricity network either atthe national transmission level [the‘grid’] or at the more localdistribution level. Technologies suchas large offshore windfarms are moresuitable for direct connection intothe transmission grid, whereas somesmall-scale offshore wind turbinesand most onshore facilities areconnected to local distributionnetworks. Other local renewableenergy sources [hydro, wasteincineration, photovoltaics etc.] arealmost exclusively connected intothe distribution network. Fig.1illustrates the range of energysources currently connected to theUK distribution network. Localdistribution is dominated by non-renewable gas and combined heatand power (CHP), with all combinedrenewables currently accounting forlittle more than 3,500 MW.

Fig.2 illustrates how current UKrenewable energy trends havedeveloped [ref.1]. Hydropower hasreduced at the expense of windpower and biofuels / waste. Theabove trends are historic and futureprojections should be treated withcaution. For example, despite thedecreasing trend for hydropower, ifthe proposed Severn Barrage were

built [at an estimated cost of £14bn]then that one hydropower facilityalone would be capable of generating5% of UK’s current energy needs by2020 [ref.1 p.100].

Very different technical designs andassociated materials developmentsare needed to exploit the differingenergy sources. Subsequent chaptersaddress the materials prioritiesneeded for the followingtechnologies:

• Section 3: Wave & Tidal Power

• Section 4: Wind Power

• Section 5: Biomass Technologies

Materials development for the threetechnologies identified above areessentially incremental in nature.The remaining energy sources wouldintroduce ‘step change’ technologyin which reliance on fossil fuels isreplaced by a ‘hydrogen economy’and/or one ‘powered by the sun’.Hydrogen power would require anextensive new production anddistribution infrastructure, whichwould be dependent on innovative

materials technology to store theportable ‘fuel of the future’ and toconvert it into power. The materialsR&D priorities for future hydrogenusage are covered in:

• Section 6: Hydrogen Power

• Section 7: [Hydrogen] Fuel Cells

Harnessing the power of the sun willrequire sophisticated functionalmaterials and processing techniquesto convert solar energy into electricalpower. Such energy conversion couldbe carried out locally with excesscapacity fed back into thedistribution network. The materialsissues associated with electricaldistribution are covered in anassociated report (ref.2) whileMaterials R&D priorities for solarpower are addressed here in:

• Section 8: Solar Energy

Fig.2. Contribution ofDifferent Technologies toUK’s Overall ElectricityGeneration fromRenewable Sources [ref: DTI Energy Challenge2006]

References:1 DTI Energy Review Report July 2006 “The EnergyChallenge” Cm 68872 Mat UK Energy Materials Review - R&D Prioritiesfor Energy Transmission, Distribution and Storage

Fig.1. Generation Plant Currently Connected to UK Distribution Network [ref.1]


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Contents

Introduction

3.1 State of the Art in Materials and Structures for Turbine Designs

3.2 State of the Art in Materials and Structures for Wave Device Designs

3.3 Structural and MaterialChallenges

3.4 RTD Needs for Materials and Structures

3.5 UK Capabilities


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Wave and tidal power3.0

Moreover, the movement ofearth, sun and moon arecyclical and thereforepredictable, aiding theplanning of energyproduction. Tidal power isessentially environmentallyfriendly, renewable andemission-free.

Similarly, as wind moves across thesurface of the sea it creates ripples,which grow into waves andultimately become great oceanswells. Once formed, ocean wavescan travel great distances withminimal loss of energy until theybreak on the shore. The Westerncoast of the British Isles is one of thehighest wave energy environmentson the planet with 60-70kW/mwaves in deep water off the WesternIsles. This energy levels falls 15-20kW/m at the shoreline due to theeffects of bottom friction and wavebreaking.

The World Offshore RenewableEnergy Report for 2004-2008 forecasta global capacity of 20.9MW tidalturbine power in 2004-2008 with84% of that power developed aroundthe UK. The Carbon Trust estimatedthat 2-5 GW of wave and tidal

energy could be installed acrossEurope by 2020 following capitalinvestments of the order £2-5bn. Thelonger-term potential for worldwidewave and tidal energy is open toquestion as estimates vary greatly. AEuropean Commission Joule projectestimated that more than 1000TWha year of global tidal power could beproduced with half being availablefor the EU. Wave and tidal powerrepresents a significant source ofrenewable energy. Production of tidalturbines is projected to double everythree years up to 2020. Up to 100tidal turbines could be manufacturedeach year by 2012 with some 600tidal/wave turbines in UK watersalone by 2020.

UK is in a strong technical positionto take a sizeable portion of theworld tidal turbine market. It leadsin turbine technology, has manycoastal locations for placement oftidal turbine farms and has a strongmarine and offshore industryheritage to support the industry.

Tidal power arisesfrom the gravitationalpull of the sun andmoon causing waterlevels to change. Thischange in level createscurrents and thatenergy can beharvested by the useof sub-surfaceturbines. Tidalvelocities as low as 0.5m/s can be used todrive a turbine,though velocities of 2-4m/s are moretypical. A tidal turbinecan produce morethan four times theenergy per squaremetre of a windturbine rotor.

Photo: courtesy of Marine Current Turbines



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3.1 State of the art inmaterials and structures forturbine design

There are two maintypes of sub-seaturbine - the rotaryturbine and thehydroplane.An example of a rotarysystem is the ‘state of the art’1.2MW twin rotor Seagentidal flow generator inNorthern Ireland that isplanned to come on line inthe latter part of 2007. TheSeagen turbine project hastwo 16m-diameter twin-bladed turbines that are liftedout of the water and turnedhorizontally formaintenance. AviationEnterprises Ltd. UKmanufacture the 7.5mcomposite material blades.The variable-pitch rotorsoptimise efficiency no matterin what direction the tide isflowing.

The supporting column is made ofrolled steel that is welded to formmonopiles that are 3m diameter atthe top and 4m diameter at thebottom. These are inserted intoholes or can be driven into theseabed. Cathode protection andpainting are used to protect themonopiles from corrosion. Use ofstainless steels or other corrosionresistant alloys are used in smallvolumes where the cost of increasedstrength and corrosion resistance canbe justified.

Carbon fibre composites are the onlypractical material for tidal turbineblades in order to reduce the weightof these moving parts, therebyimproving efficiency and reducingthe weight of the supportingstructure, gearboxes and hubs. Bladeconstruction generally uses a carbonfibre spar as the main load bearingstructure. The spars are mainlyunidirectional along the blade lengthbut can also contain +/-45 plies orwoven plies to resist blade pitchingand torque. Skins are manufacturedfrom GRP with aramid layers forimpact resistance.

Hydroplane turbines have also beeninstalled. The angle of attack ofthese devices is altered to create lift(and drag) as the hydroplane movesrepeatedly up and down. This linearmotion is used to drive hydrauliccylinders that in turn drive agenerator. Hydroplanes are generallycomposite structures with a steelsupporting structure with points forattachment to a gravity base. TheStingray tidal generator was the firstof its type to be installed off UKshores.

Other approaches to turbine designexist using floating rotating turbinesanchored to the seabed by chains.This arrangement could reduce thecosts associated with mountingturbines on anchored masts.Another design encloses the turbinein a cowling similar to an aircraft jetengine.

One of the key engineeringcomponents within a turbine is thegearbox. The planetary gears havecompliant mountings to avoidexcessive loads being transferred toany one gear. Natural water-coolingis used to maintain a low workingtemperature for the gearbox. Whilemost parts are steel, the fairings andhousings are GRP.

‘Seaflow’ tidal current turbine [Courtesy of M arine Current Turbines].


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3.2 State of the art inmaterials and structures forwave device design

Wave power can beharvested using arange of devices, suchas articulated rafts,‘nodding’ structures,compressible floatingbags, tethered buoys,bottom-standingoscillating watercolumns, over-spillingsystems andsubmerged pressurechambers.

There are more than 50different wave conversiontechnologies being developedworldwide. The devices canbe subdivided into ‘shallowwater’ systems that are builtnear-shore or into theshoreline and ‘offshoresystems’ designed for waterdepths of tens of metres.There are demonstrations atthe 500 -750 kW scale, butthe technologies have yet tobecome a commercial reality.

Ocean Power Delivery’s Pelamis, andthe Danish Wave Dragon are bothoffshore devices that serve toillustrate the current ‘state of theart’.

Pelamis is a freely floating hingedcontour device with 4 tubularsections connected by 3 hinges. The4 sections move relative to eachother in both pitch and yaw; thehinges then convert this motion bymeans of a digitally controlledhydraulic power system. The totaldevice length is 150m, with a tubediameter of 4.63m. Pelamis is of steelconstruction using standardfabrication techniques andequipment. The device is slack-moored and, within the mooringconstraints, is free to turn into thewave direction.

Wave Dragon combines a doublecurved overtopping ramp and tworeflector arms, which focus energyonto an overtopping basin where anumber of modified Kaplan-Turbinesare used to drive generators. Thisdevice is also slack-moored and ableto swivel in order to face the wavedirection. The structure is built usinga combination of steel andreinforced concrete, which can bebuilt using standard constructiontechniques at most shipyards. WaveDragon’s large size means that itrequires a large construction yard forassembly.

A number of designers of wavepower devices are considering use ofcomposites and face similar materialschallenges to those described fortidal turbines. The materials in wavedevice sub-systems, such asgearboxes, generators and electricalsystems are also subject to the sameaggressive conditions experienced bytidal turbines.

The Pelamis Wave Energy Converter (WEC) [Courtesy: Ocean Power Delivery].



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3.3 Structural and materialchallenges

Wave and tidal powerequipment operatesunder high fatigueloading in coldseawater conditionsfor periods in excessof 20 years. The main structural requirements arehigh static strength and stiffness.Tidal turbine blades require a highfatigue life to achieve long lifeexpectancy with minimuminspection and maintenance.Structural material challenges aresimilar for both rotary andhydroplane designs. Structures haveto withstand long-term underwateroperation, seabed erosion anddamage from flotsam, vessels andtrawler nets.

Tidal turbines structures tend to beover-designed to take account of saltcorrosion and heavy or turbulenttides. This results in performancepenalties. Methodologies to optimisewave/tidal power performance arestill required.

Cavitation can occur when pressuregradients form vapour bubbles onblade surfaces. Collapse of thesebubbles creates vibration andimparts local stresses that can causeblade damage and reduce efficiency.Blade designs and coatings can helpminimise cavitation damage.

The need to minimise biofouling is amajor design consideration.Preventative measures includepainting or inclusion of copperpowder in resin formulations as ablade skin. Many of the wave powerstructures [e.g. the tower, gears etc.]are fabricated from carbon steel thathas a finite life when operated inseawater. Corrosion protectioncorrosion is an ongoing task for thesteel parts and regular maintenanceand inspections are required. Suchinspection can be costly especiallywhen divers or robotic observationvehicles (ROVs) are required.

3.4 R&D Needs for Materialsand Structures

Materials are well established forcurrent turbine designs but future (5-10 year) developments will need newmaterials and manufacturingtechnologies:Resistance to seawater degradationand the ability to withstand highfatigue loadings are the two maindesign drivers for composite resinsystems used in wave turbine blades.Unfortunately, available resinsystems were not developedspecifically for such wave turbineapplications. Resins need to bedeveloped for low cost manufactureas well as in-service issues. Waveturbine blades tend to be relativelythick and are manufactured usingout-of-autoclave vacuum bagprocedures. Resins must be able toaccommodate the exotherms thatcan be generated in thick bladeinteriors and they must alsowithstand anti-fouling paint.

Wave overtopping on the Wave Dragon device[Courtesy: Wave Dragon. Photo: Earth-vision.biz]


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3.5 UK capabilities

UK is a world leader in marineenergy technology but commercialprogress has been slow. A growingnumber of small start-up companiesand academic institutes aredeveloping wave and tidal streamdevices. In addition, establishmentof the European Marine EnergyCentre (EMEC) in northern Scotlandallows pilot scale wave and tidalstream devices to be tested in anoffshore facility. EMEC is the onlymulti-berth purpose-built test facilityin the world for evaluating wave andtidal technologies. The New AndRenewable Energy Centre (NAREC) isanother testing facility at Blyth. UKcan claim a significant degree ofexpertise in wave/tidal powerdevelopment. However gaps in UKmaterials capability still exist:

There is substantial experience of theeffects of seawater on the long-termproperties of composites [from theboat-building industry] but littleexperience of fatigue behaviour inthick structures. Most compositefatigue data is from thin laminatestructures used in aerospace orrotorcraft applications. The designlife of a turbine structure is in excessof 20 years, while monopiles aredesigned for even greater lifetimes.Behaviour of welded and boltedjoints must also be considered.

Some turbine configurations are notremovable from the sea formaintenance and require the use ofROVs. Integrated inspectiontechniques combined with structuralhealth monitoring are needed toverify structural integrity. Use ofembedded fibre optics is increasingfor wind-turbine blade conditionmonitoring and for subsea oil andgas risers. Similar technology needsto be applied to wave turbine bladesand towers. Use of smartmaterials/structures that incorporateactuators and sensors would allowmaterials to respond to externalstimuli and achieve greater turbineblade or hydrofoil efficiency. Amulti-disciplinary approach isneeded to study actuation and fluid-structural interaction.

The effect of tidal turbines onmarine environment is thought tobe benign. Turbines rotate at 10-20rpm and although noise isgenerated it is not thought to affectmarine life. In addition, noenvironmentally damaging fluids areused within the subsea structure.

• Lack of a dedicated compositematerials R&D programme,addressing both fundamentalmaterials properties issues as wellas large-scale manufacturing.

• There is a need for a greaterunderstanding of the fatiguebehaviour, durability andmechanical performance of largecomposite structures in the marineenvironment, especially for sub-seastructures.

• The developing world market formarine energy devices demandshigh-volume manufacturingtechniques capable of producingconsistent quality items based onclearly understood design rules.

The Pelamis Wave Energy Converter (WEC) [Courtesy: Ocean Power Delivery].


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Contents

Introduction

4.1 Overview of the Wind PowerMarket

4.2 Wind Turbine Technology

4.3 Operating Environments

4.4 Priorities for Research and Development

4.5 UK Capability Gaps

4.6 References

Annex ANon-destructive InspectionStructural Health Monitoring


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Wind power generation4.0

Manufacture of windturbines on acommercial scalestarting in the 1980s.It is currently thefastest growing sectorof the powergeneration industry,with anticipatedindustry expansionrates consistentlybeing exceeded sincethe early 1990s.

As the wind energy industryhas grown, electricitygenerated by wind power hasshown a dramatic fall in cost.The average cost ofproduction from a coastalturbine has decreased bymore than 50% within 15years - from ~8.8€ cents/kWhin the mid-1980s to 4.1€cents/kWh for a modern 1MW turbine. Wind power isalready cost-competitive withsome new coal-fired powerstations and in some areascan challenge gas, currentlythe cheapest option. Despitethis progress, wind powerhas some way to go before itfulfills its potential as a large-scale supplier of electricity.

4.1 Overview of the Wind Power Market

Drivers

Key issues driving growth in windgenerated power include:

• Gradual depletion of fossil fuelreserves

• The need to ensure security ofpower supply via reduced energyimport dependence

• Electricity price rises

• Need to reduce emissions linked toglobal warming [CO2, green housegases (GHGs)]

• Government policy and direction

• Increased UK and global energydemand

Current and Future Market Sizes

UK

The UK Government set a target of a60% reduction in carbon emissionsby 2050 and introduced legislationto encourage uptake of renewableenergy. The Renewables Obligation(RO) was introduced in April 2002and requires all electricity suppliersto source 10% of their supply fromrenewables by 2010. In 2004 thattarget was revised to 15% of supplyby 2015.

Wind is currently the mosttechnically and economicallydeveloped renewable energy and isexpected to make up around 75% ofthe 10% target by 2010. This equatesto around 8,000 megawatts (MW) ofcapacity, which will be derived fromonshore and offshore farms. To meetthe 2010 target a further 1,500onshore turbines (assuming anaverage size of 2MW) need to bebuilt in addition to the 1,414turbines that have already beeninstalled. Offshore turbines aregenerally larger and more powerful(>3MW) so fewer (1,300) will beneeded.

The UK has recently broken the 2GW threshold of electricity fromwind power, around 1.5 % of thetotal UK electricity supply. The levelof electricity from wind power hasdoubled in little more than a yearand is now sufficient to providemore than 1 million homes withpower [ref. Renewable EnergyStatistics Databasewww.restats.org.uk/2010_target]

Europe

In 1997 the European Wind EnergyAssociation (EWEA) adopted targetsset by the European Commission’sWhite Paper [1] on renewablesources of energy for 40 GW of windenergy capacity by 2010.This targetwas revised 3 years later to 60 GWby 2010 [2]. However, even thisrevised forecast has proved to beconservative. Latest EC figures [3]indicate that wind power capacity inEurope could reach 69.9 GW by2010. The target now adopted by theEWEA for 2020 is for 180 GW ofpower, of which 70 GW would begenerated by turbines locatedoffshore. EWEA’s target for Europeanelectricity generation from wind isset at 5.5% in 2010 & 12.1% in2020 [4]. This is equivalent to theelectricity needs of more than 195million people.



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4.2 Wind Turbine Technology

The basic design of windturbines has not changedsignificantly since the early1980s. Most wind turbineshave upwind rotor bladesthat are actively yawed toensure alignment with thewind direction. Figure 1shows the components of aconventional wind turbine,together with an indicationof the percentage of theoverall wind turbine cost.

The three-bladed wind turbineconfiguration is the most common.It typically has a separate frontbearing with a low speed main shaftconnected to a gearbox that providesan output speed suitable for a four-pole generator. For the largest windturbines, the blade pitch is variedcontinuously under active control toregulate power under highoperational wind speeds. A nacellehousing encases the front bearingand pitch control system and theentire turbine drive train issupported on a mainframeconstruction mounted on the tower.

The drive train in Figure 1 shows therotor attached to the main shaftdriving the generator through thegearbox. Significant recentdevelopments in basic designconfiguration have seen the adventof direct drive generators, where thegearbox is removed and the rotor isused to drive the generator directly.Hybrid arrangements involving asingle stage gearbox and multi-polegenerators are also now appearing.

The main design drivers for currentwind turbine technology are:

• Improved technology for low andhigh wind sites

• Grid compatibility

• Aerodynamic and acousticperformance

• Low visual impact

• Offshore expansion

Although the proportion of offshorewind turbines is less than 1% of thetotal installed capacity, the latestwind turbine developments aredriven by the offshore market. Thismeans that the current technologydriver is to make wind turbines aslarge as possible with the followingkey issues:

• Low mass turbine and nacellearrangements

• Large blade technology andadvanced composite engineeringallowing enhanced exploitation oflow wind sites. For high wind sites,smaller blades, shorter towers andreinforced supporting structuresare used

• Design of offshore foundations,wind trubine erection andmaintenance

Using current wind turbinearchitectures, it will be extremelydifficult to build turbines with anoutput greater than 5 MW becausethe cost of the turbine will exceedthe value of the energy output. Tomeet this challenge lower costadvanced materials, new designs andmore cost-effectivemanufacturing/production processesare seen as critical. Suchdevelopments will require asignificant amount of R&D if larger,more reliable designs are to berealised.

The following sections provide detailon design issues, material choice andfuture technology/material trends forthe major component parts of windturbines. Two cross-cutting themes,applicable to all materials andstructures are outlined in Annex A.These are Non-DestructiveInspection (NDI) and StructuralHealth Monitoring (SHM).


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Figure 1: Components of a wind turbine and indicative percentage of overall cost

Note - Percentages contribution to overall cost based on a REpower MM92 turbine with 45.3 m length blades & 100m tower.[Source: Wind Directions - Jan./Feb. 2007 bulletin - “Focus on Supply Chain”, European Wind Energy Association]

1 ROTOR BLADES (22.2%)

Vary in length up to more than 50m,blades are manufactured in speciallydesigned moulds from composite materials,usually a combination of glass fibre andepoxy resin. Options includes polyesterinstead of epoxy and carbon fibre toincrease strength and stiffness.

2 PITCH SYSTEM (2.7%)

Adjusts the angle of the blades to makebest use of prevailing wind.

3 MAIN SHAFT (1.9%)

Transfers rotational force of the rotor tothe gearbox.

4 GEARBOX (12.9%)

Gearing to increase the low rotationalspeed of the rotor shaft in several stages tothe high speed needed to drive thegenerator.

5 BRAKE SYSTEM (1.3%)

Disc brakes to stop the turbine whenneeded.

6 TRANSFORMER (3.6%)

Converts the electricity from the turbine tohigher voltage required by the grid.

7 GENERATOR (3.3%)

Coverts mechanical energy into electricalenergy. Both synchronous andasynchronous generators are used.

8 POWER CONVERTER (5.0%)

Converts direct current from the generatorinto alternating curent to be exported tothe grid.

11 ROTOR HUB & BEARINGS (2.6%)

Typically made from cast iron although nowbeing made from glass reinforced plastic (GRP),the hub holds the blades in position, Bearingshave to withstand the varying loads generatedby the wind.

12 NACELLE HOUSING (1.4%)

Lightweight GRP box covering the turbine’sdrive train.

13 MAIN FRAME (2.8%)

Made from steel, must be strong enough tosupport the entire turbine drive train but nottoo heavy.

9 YAW SYSTEM (1.3%)

Mechanism that rotatesthe nacelle to face thechanging wind direction.

10 TOWER (26.3%)

Range in height from 40mto more than 100m.Usually manufactured insections from rolled steel;a lattice structure orconcrete are cheaperoptions. Glass fibre epoxyfilament wound orpultruded towers alsobeing developed.



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Blades

Wind turbine blades are complicatedstructures built from a large numberof materials and exposed to complexaerodynamic loading andenvironmental conditions.

Historically the vast majority ofwind turbine blades have beenconstructed from a combination of:

• Wood (e.g. birch, balsa) and:

• Glass fibre-reinforced plastic (GRP)composites

Initially glass fibre reinforcedpolyester systems were used but nowepoxy-based resin systemspredominate. Many of thecomponents are adhesively bonded.There has also been recent interest inthe use of natural fibre composites assustainable alternatives to syntheticpolymer matrix composites. Materialselection is heavily influenced by:

• Blade size

• Number of blades

• Turbine location and henceweather conditions and:

• The operating speeds of the rotor

With increased importance on massreduction, the overall relationshipbetween blade mass and diameter isbecoming slightly less than cubic.This trend will be hard to maintainas rotors continue to get larger.Increased size, without a radicalchange in design concept andmaterial properties, will lead tomuch higher internal material

stresses. This will necessitate higherspecific strength materials and/ormore compliant components. Ifcomponents bearing heavy dynamicloads are made of flexible materials,then the forces acting upon them arenot transferred to the supportingstructure. Hence dynamic forces areaveraged out, leading to significantlylower fatigue loadings.

The higher tip speeds of offshorewind turbines require a reduction inthe ratio of blade projected area torotor swept area [i.e. slimmer blades].Reduced blade area will only allowreduced blade mass if materials ofsufficiently high specific strength areavailable; hence the increased use ofcarbon fibre-reinforced plastics(CFRP) in modern, large blades.Increased automation in blademanufacture - from hand lay-up toResin Transfer Moulding (RTM) andpultrusion with accompanying costreduction - has led to wideracceptance of CFRP.

Innovative composite sandwichstructures are increasingly being usedfor primary blade structures. Theouter skins are made fromGRP/CFRP, while the core materialsare typically polyvinyl chloride(PVC) or bismaleimide (BMI), balsawood and less frequentlyhoneycombs (e.g. Nomex) for theirinherent high stiffness and strengthat low areal weights. However amajor disadvantage of sandwichconstructions compared tomonolithic materials is that they aremore prone to delamination/failurebecause of weak interfaces betweenadjacent materials with verydifferent stiffness and strengthproperties.

Blades contain a multitude ofadhesive and mechanical jointsbetween similar and dissimilarmaterials. Localised shearing andbending effects can lead to severeinduced through-thickness shear andnormal stress concentrations thatcan significantly affect the static andfatigue strength of joined sub-components. Key requirements arecomposite sandwich constructionsand joints/adhesives with improveddamage tolerance, as well as crack-stopping characteristics andimproved load introductiontechniques.

It is common practice to design largeFRP rotor blades according to designcodes and models based on linear-elastic fracture mechanics.Shortcomings in state-of-the-artnumerical tools and existingcomposite material models result inhigh levels of uncertainty that leadto conservatism in design and bladesof unnecessarily high weight andcost.

Wind turbine blades are subjected tosevere fatigue loads. The severity liesnot only in the magnitude of theforces but also the number of fatiguecycles the blade will see in its 20-year lifetime [typically 107 cycles].Although FRP materials haveexcellent fatigue properties, muchwork is still needed to understandhow these materials perform underfatigue loading and particularlycompression-dominated fatigue. It isalso crucial to be able to predict howthese materials and structures willbehave when they contain eitherimperfections introduced duringmanufacture/processing or damageinduced during the service life.Again improved fatigue modelling,design criteria and design proceduresare needed to optimise use ofcomposite materials in large windturbine blades.

Nacelle Housing

The nacelle housing provides anouter frame protecting the turbinemachinery from the externalenvironment. The nacelle is typicallymade of GRP with steelreinforcements and the housing ismounted through rubber dampers tothe main frame with steel supports.

Hub

The hub serves as a base for the rotorblades as well as a means of housingthe control systems for the pitchmechanism. The latest wind turbineshave threaded bushes adhesivelybonded into the root of the blade,which then allow the blade to bebolted to the hub. Hubs are typicallyfabricated from cast iron alloys dueto the complicated componentshape. The hub material must behighly resistant to metal fatigue,which therefore precludes the use ofwelded constructions. In order to


15

reduce hub weight and increasefatigue performance, the hub andshaft combination has recently beenmanufactured using preform-liquidGRP composite mouldingtechniques. Another benefit of acombined hub and shaft componentis that the bolted connectionbetween hub and shaft can beeliminated, again reducing weight,and manufacturing costs associatedwith drilling holes of tightgeometrical tolerances. The use ofcomposite materials allowsdirectional ‘tuning’ of materialproperties facilitating further weightreductions. Although prototypeshave been fabricated andproducts/techniques patented,commercial use of FRP moulded huband shaft components is some wayoff.

Main Shaft

The main shaft of a wind turbine istypically forged from hardened andtempered steel. Future developmentswill look to fabricate shaftcomponents from FRPs (Section 3.3)in order to reduce weight butwidespread use is likely to be longterm.

Gearbox

The gearbox is one of the mostimportant components in a windturbine. Situated between the mainshaft and the generator, its role is toincrease the slow rotational speed ofthe blades (in several stages) to thehigh speed needed to drive thegenerator. The trend toward largerwind turbines has led to expensivegearboxes. Multi-generator drivetrain configurations can reduce thecost for large wind turbines whileincreasing energy capture andreliability.

Gears are typically fabricated fromcarbonised steel alloys and the teethare case-hardened and polished toprovide enhanced surface strength.The fatigue and wear properties ofsteel alloys used in gearboxes arecritical.

Direct drive generators are beingintroduced into recent power trainarchitectures, thereby reducing costand maintenance problems. Futuredevelopment is anticipated in themedium to long term.

Tower and Foundations

Support towers are commonly madein tubular steel with a tapering wallthickness and diameter from thebase to the top of the tower.Concrete towers, concrete bases withsteel upper sections and latticestructures are less frequently used.GRP filament wound towers recentlybeen trialled, offering lighter weightfor ease of installation.

A key area of research is cost-effective foundation designs foroffshore turbines. The generalconsensus is that steel is a far morecompetitive material than concretefor larger offshore wind farms. Mostfoundation designs requireminimum attention until at least 15metres water depth. Corrosion is nota major concern with offshore steelstructures. Experience from offshoreoilrigs has shown that they can beadequately protected using cathodic(electrical) corrosion systems.Painted surface protection isroutinely delivered for offshore windturbines, with a higher protectionclass than onshore turbines.

Generators

Mention was made previously[section 3.5] to direct drivegenerators. Hybrid arrangementsinvolving a single stage gearbox andmulti-pole generators are also nowappearing.

Increasingly, High TemperatureSuperconducting (HTS) generatorsare being introduced in whichconventional copper rotor coils arereplaced by HTS materials. Thesewindings carry greater currentdensity and hence produce highmagnetic fields. Currentsuperconducting generators canmatch the power output of aconventional generator at aroundone-third of the size and half theweight. Transportation andinstallation costs are therefore less.Electrical losses in superconductingwires are also negligible and thenoise profile is reduced.



16

4.3 Operating Environments

Wind turbines are located on highground in remote areas in order tobenefit from strong winds. Windturbine components must be robustenough to withstand such harshconditions.

Low Temperatures

Temperatures as low as -40 °C can beexperienced by materials used inwind turbine construction. Steelcomponents can become brittle atsuch low temperatures. The matrixand fibres in composite materialshave different coefficients of thermalexpansion (CTE) resulting inthermally induced stresses that canlead to matrix microcracking whichreduce component stiffness andprovide paths for moisture ingress.

Thermal shock can damage electricalequipment including the generatorand yaw system. The viscosity ofgearbox lubricants and hydraulicfluids increases during long-termexposure to cold weather. Damage iscaused to gears on start-up afterextended stationary periods becausethe oil is too viscous. Seals andrubber mounts also perish over timein cold climates. Correct selection ofmaterials for turbine componentsexposed to cold climates is thereforeessential. Nickel and aluminiumalloys offer improved strength oversteel at low temperatures.Composites have yet to seewidespread applications in coldclimates. Mismatch in CTE can beaccommodated by careful selectionof fibre and resin combination.

Hot/Wet

In hot climates, turbine blades aremade from epoxy-based compositematerials that are resistant to heatand ultraviolet light. Althoughblades are sealed, moisture ingressvia damaged painted or gel-coatsurfaces can reduce the glasstransition temperature of the resin,impairing mechanical performance.Swelling may also lead todimensional changes.

Salt spray is particularly harsh onturbine components. Paint sealantsand nacelle designs that inhibit

penetration of salty air are used toprotect the turbine, generator, bladesand support tower from corrosion.Further work is needed tounderstand the mechanisms of long-term salt spray exposure for turbinematerials, especially composites.Measurement and prediction ofmaterial mechanical propertiessubjected to long-term exposureunder marine environments is ofparticular importance. Acceleratedageing techniques are also needed.

4.4 Priorities for Research andDevelopment

The European Wind EnergyAssociation’s (EWEA) StrategicResearch Agenda [5] identified anumber of ‘showstoppers’, barriersand bottlenecks to wind energydevelopment. Showstoppers areissues of such importance that, if notaddressed, they could halt windenergy progress altogether. Barriersare physical limitations in currenttechnology that should be overcomein the medium- to short-term andbottlenecks are issues that can beovercome through additional shortto medium term R&D. The followingis summary of the critical materialstechnologies:

• Showstoppers: Availability of robust, low-maintenance onshore turbines andlack of R&D into increasedreliability and availability ofoffshore turbines

• Barriers:(i) Integrated design tools forvery large wind turbines operatingin extreme climates, (ii) State-of-the-art laboratories foraccelerated testing of largecomponents under realistic climateconditions(iii) Standards and certification fordesign criteria, components andmaterials

• Bottlenecks:Development of componentdesign tools and rapid finalisationof ongoing standards

Bearing in mind the EWEA findingsthe following priority areas for windturbine materials have beenidentified: Timescales: Short (0-5yrs.); Medium (5-10 yrs); Long (10-20yrs.)

• Improved reliability and reduceduncertainty through developmentof test and modelling methods forlifecycle analysis/fatigueperformance of materials, sub-components and major structures[i.e. blades] - Medium term

• Reductions in operating andmaintenance (O&M) costs throughdevelopment of structural healthmonitoring technologies -Medium-to-long term

• Development and application ofNDI techniques for more accurateand rapid defect detection - Short-to-medium term

• Continued development ofinnovative material designs andprocessing techniques e.g.sandwich constructions, joints,FRP pre-forming and infusiontechniques - Short-to-mediumterm

• Development of test andmodelling methods for materialscharacterisation in harshenvironments - Short-to-mediumterm

• Development of standards andcertification procedures - Short-to-medium term

• Development and application ofcoatings to reduce drag andimprove environmentalperformance - Short-to- mediumterm


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4.5 UK Capability Gaps

UK has the skills and resources fordeveloping commercial wind farmsin a cost-effective manner. Howeverthere are gaps in UK capabilitycompared to other leading windpower nations. Some of those areasare highlighted below:

• Lack of large scale manufacturingcapability in the UK:

- There is currently one large blademanufacturer based in the UK(Vestas) who export blades mainlyinto the American and Europeanmarkets.

- No established UK turbinemanufacturers. The majority ofwind turbine components areimported.

* Lack of test facilities for acceleratedtesting of large components (e.g.blades) under realisticenvironmental conditions. Thislimitation is critical as windturbine blades continue to increasein size. The New and RenewableEnergy Centre (NaREC) in Blythhas a world-leading facility fortesting turbine blades up to 70min length. Further capability isneeded for testing under extremeenvironmental conditions.

* Lack of dedicated blade materialsR&D programme addressing bothfundamental materials propertyissues as well as manufacturingtechnology for wind turbines.

* There is a need for a greaterunderstanding of the fatiguebehaviour and mechanicalperformance of large compositestructures in a variety of serviceenvironments.

4.6 References

1 “Energy for the Future - RenewableSources of Energy”, EuropeanCommission White Paper for aCommunity Strategy and ActionPlan, COM (97) 599.

2 “On the Implementation of theCommunity Strategy and ActionPlan on Renewable Energy Sources(1998-2000)”, Communicationfrom the Commission to theCouncil, the European Parliament,the Economic and SocialCommittee and the Committee ofthe Regions, COM (2001) 69.

3 “Future Prospects for Wind PowerMarkets” European Wind EnergyAssociation, 2004

4 “Wind Force 12 - A Blueprint toAchieve 12% of the World’sElectricity From Wind Power by2020”, European Wind EnergyAssociation, May 2004.

5 “Prioritising Wind EnergyResearch: Strategic ResearchAgenda of the Wind EnergySector”, European Wind EnergyAssociation, July 2005.

Annex A

Non-Destructive Inspection(NDI)

With turbine components increasingin size, challenges exist indeveloping and adapting suitableNDI techniques for accurate defectdetection. For example, a majordisadvantage of sandwich bladeconstruction is that manufacturingdefects cannot readily be detectedusing conventional techniques.Rapid inspection is also an issue asblade size / volume of materialincreases. Visual inspection and taptesting have routinely been used butmore accurate techniques areneeded. The most promisingtechniques are X-ray, ultrasound andshearography. Specific challengesthat need to be overcome include:

• Development of techniques thatcan detect defects located deepwithin sandwich constructions

• Improved sensitivity and reliabilityof NDI techniques when appliedto composite materials to allowdetectability limits andprobabilities of detection to bedefined

Structural Health Monitoring(SHM)

Effective structural healthmonitoring of wind turbinecomponents is inhibited byaccessibility, due to the large /complex component geometry andto the often remote in-servicelocations. Development is requiredinto on-line, remote fibre optics,acoustic emission and accelerometersensor systems. Blade failures are rarebut many turbines and blades arestill relatively young and yet failureshave occurred. Long-termperformance attracts considerableinterest, especially when rapidlychanging blade sizes result infrequent ‘new’ designs. In order toestablish what critical performanceindicators need to bemeasured/monitored, it will benecessary to establish a fullerunderstanding of parent materialbehaviour and likely in-servicefailure modes.


18

Contents

Introduction

5.1 UK Strengths and Weaknesses

5.2 Combustion and Gasification

5.3 Biomass Characteristics

5.4 Materials Issues

5.5 Conclusions

5.6 References


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Biomass technologies5.0

Increased interest indevelopment ofrenewable energytechnologies has beenencouraged bylegislative measures inEurope to reduce CO2emissions from powergeneration in responseto the potential threatof global warming.

Of the available technologies,biomass firing has a high prioritybecause of the modest technologicalrisk involved and the readyavailability of waste biomass inmany countries [1]. Biomass fuel canbe supplied from a variety ofsources, and in a variety of forms.Sources include waste biomass, eitherfrom agriculture/forestry operations(e.g. wheat straw) or from the greenelement of municipal or industrialwaste and farmed biomass, ‘energycrops’ from land-based agriculture orfrom the sea. Biomass from the sea(seaweed) avoids the competitionwith food production for land andfresh water, but contains very highlevels of contaminants restrictingoptions for its use. Possible forms ofbiomass fuels are:

• as a solid fuel in either raw (ascollected/harvested and dried) orsemi-processed into pellets orsome other more handleable form

• as a liquid or gaseous fuel [e.g. byusing pyrolysis, gasification,anaerobic digestion, etc, withfurther processing]

5.1 UK Strengths and Weaknesses

UK has a growing industry in small-scale bioenergy systems for heat andpower applications. However, Europeis some way ahead in deployment ofthese systems, particularly incountries where large quantities ofwaste biomass are available from theforestry sector (e.g. Finland), orwhere Governmentpressures/initiatives have encouragedearly development. UK hasestablished strengths in heatexchangers, gas turbines, etc., and itsmajor energy companies are stronglyrepresented in the global market forbioenergy systems.

Prior to such initiatives as the Non-Fossil Fuel Obligation and theRenewables Obligation, biomasstechnologies were primarily focusedon heat applications, whichcontinue to be an economicallycompetitive niche market. Supportmeasures have encourageddevelopment of new heat and poweroptions but few of the installedplants would maintain their statuswithout continuing support.

Major research initiatives have beenestablished in recent years, such asthe Research Councils’ Supergen and‘Towards a Sustainable EnergyEconomy’ programmes. As a result,the UK has considerable strengthand capability in biomass energytechnology research.



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5.2 Combustion and Gasification

Combustion of waste andfarmed biomass has beenpractised for many yearsaround the world, mostly forheat production, but morerecently for the generation ofelectricity.

Highly efficient energyconversion from biomass hasnot been a priority wherebiomass was a waste productbut is of increasedimportance where biomass ispurchased as an energy crop.Efficiency is also importantin large-scale powergeneration, where bioenergyhas to compete with otherfuels. In such cases, bioenergysystem efficiencies havealways fallen well belowthose of equivalent fossil-fired systems.

The limited efficiencies of biomassconversion systems has been due toreduced steam conditions enforcedby severe fouling and corrosionproblems experienced as a result ofthe high contaminant levels frommany biomass fuels, which lead toreduced component lives. While coalplants aim for 650˚C/300bar steamand above, biomass plants arecurrently operating at less than540˚C/100bar steam with efficienciesof typically less than 30%. However,substituting biomass for coal in co-fired advanced coal plants bringsbiomass together with highefficiency plant. Issues associatedwith co-firing are reviewed in theFossil Energy Report.

In Denmark, government legislationhas driven the introduction of evermore efficient plants. The mostadvanced straw-fired biomass plantoperates at 540˚C/92bar with anelectrical efficiency of 29%.Experience from boilers in Swedenfiring 100% forest fuel indicates thatconventional superheater steels lastno longer than four years or 20,000hours before they must be replacedbecause of corrosion damage. Thisleads to higher operating costsmaking biomass combustion plantsuncompetitive with fossil plants,unless supported through subsidiesor grants. Using waste biomasssources, such as demolition wood,improves overall plant economicsbut leads to even more severecorrosion problems. In addition tothe impact of biomass fuels onoperating costs, capital costs ofbiomass plants are usually higherthan their fossil counterparts, due tomore complex fuel feedingarrangements, fuel drying, gascleaning, emissions monitoringrequirements, etc.

In order to improve systemefficiencies and economics ofbiomass plants, recent interest hasfocused on gasification systems.These include:

• Fixed or moving bed gasifiers(either updraft or downdraft) forsmall-scale use to:

• Circulating fluidised bed combinedcycle systems, which takeadvantage of the higherefficiencies of gas turbines andadvanced gas engines at medium-scale (up to 30MWe).

While different gasification optionsare possible [2], circulating fluidisedbed gasifiers have been developedfurthest because of their flexibilityand suitability for the largequantities of biomass feedstocks.Both atmospheric pressure andpressurised schemes have beendemonstrated using a small (4MWe)industrial gas turbine.

A variant of the TPS TermiskaProcesser AB circulating bedgasification system [3] from Swedenwas the world’s first ‘commercial’biomass gasification plant. It wasknown as the ARBRE project and wasdeveloped at Eggborough, Yorkshire.Unfortunately this plant has nowclosed. The plant aimed to usecoppiced willow and forestryresidues in chipped form to produce8MW of electricity with a cycleefficiency of ~31%. A genericflowsheet for the hot gas path of thisplant through to the gas turbine isshown in Figure 1.

Fig. 1. Simplified flowsheet for the hot gaspath of the ARBRE project


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Fig.1 shows the complexity of thehot gas path in such systems. Eventhough fluidised bed gasifiers lead tomoderate fuel gas tar levels, a hightemperature cracker is still needed toreduce energy losses and limit the tarremoval burden at the gaspurification/scrubbing stage. Tarswould otherwise cause substantialproblems in the gas compressor.Ammonia levels in the fuel gas are afurther concern as they lead toexcessive NOX levels in the gasturbine exhaust and exceed theallowable emissions limits. Scrubberscan reduce levels of ammonia in thefuel gas and post-combustioncatalytic systems can be used toreduce NOX emissions.

In a pressurised system, such as thatin the Varnamo project [4], there isno requirement for a fuel gascompressor (see Fig. 2). Hence tarscan be kept hot in the vapour phaseprovided they do not exceed the gasturbine entry limits and do not causeblinding problems in the hot gasfilter. Using high pressure and a hotgas cleaning approach reducescomplexity and raises cycleefficiency. The measured efficiencyin the Varnamo project was 32% butup to 38% could be expected fromapplication of the latest gas andsteam turbine technology. Thisscheme used a variety of biomassfuels to demonstrate its flexibilityand produced 5MW of electricityand 9MW of heat for districtheating. Figure 2 shows the hot gaspath through to the gas turbine.

Fig. 2. Flowsheet for the hot gas path of theVarnamo project

While gasification leads to higherefficiencies, it is more complex andexpensive to build. It is alsosusceptible to contaminationproblems, which have led tooperational restrictions in biomasscombustion plant. The next sectionof this report reviews the possibleeffects of contaminants in biomassgasification systems with particularemphasis on the durability of the gasturbine and the implications thismay have on gas cleaning.


22


5.3 Biomass Characteristics

Like coal, biomass contains awide range of elements thatmay react to form potentiallyharmful deposits ingasification systems and gasturbines.

The ‘mix’ of elements in fuelgases produced in gasificationis highly dependent on thebiomass fuel composition.Before biomass firing can beused with any confidence ingasification systems, it isnecessary to investigate theeffects of deposits and gasenvironments on gas turbinecomponents.

From such information, fuelspecifications for biomass-fired gasturbines can be derived, therebyensuring adequate component livesand the possibly of using state-of-the-art gas turbines.

To investigate the effects ofcontaminants in the hot gas path ofbiomass gasification plants it isnecessary to compare contaminantlevels in biomass fuels with those incoal-fired combined cyclegasification systems, for which thereis considerable experience. Althoughextensive composition informationhas been gathered on potentialEuropean biomass fuels [5] fewanalyses have been carried out for allminor and trace metal species.Average data for pinewood, wheatstraw, a range of grasses, sewagesludge and peat has been comparedwith coal and is given in Tables 1 &2. It should be noted that there aresignificant differences in the errorsassociated with each of the valuesdue to inherent fuel variations,variable sample sizes and differentanalytical methods.

Table 1 illustrates the majordifferences between biomass fuelsand coal. Biofuels have:

• Higher moisture content andtherefore use some heat for drying

• Lower ash (except sewage sludge)

• Lower sulphur (S) levels andsimilar or higher chlorine (Cl)levels

In addition, biofuels often givehigher NOX & ammonia (NH3) levelsin combustion and gasificationsystems respectively compared withfossil fuel counterparts.

Table 2 presents average ashcompositions derived using standardash analysis techniques. Note thedifferences in ash contents fromTable 1. It is generally true that fastgrowing biomass fuels containhigher alkali metals, in particularpotassium. Although the methodused to generate ash for analysis wasdesigned for coal combustion, thegeneral findings suggest that foulingand corrosion problems should beexpected in biomass systems.

Table 1. Average analyses of biomass and fossil fuels

Table 2. Ash analyses of biomass and fossil fuels

Wood Wheat ‘Grass’ Sewage CoalStraw Sludge

Moisture (wt%) 20.7 10.7 14.9 19.5 8.2

Ash (wt%) 1.7 5.9 5.2 43.4 12.7

S (wt%) 0.2 0.1 0.2 1.0 1.7

CI (wt%) 0.1 0.8 0.2 0.1 0.2

LHV, MJ/kg 18.6 17.3 18.3 10.7 26.2

AI2O3 SiO2 Na2O K2O MgO CaO Fe2O3 P2O5 SO3 TiO2

Wood 5.5 24.3 1.7 9.3 4.5 34.5 3.6 5.6 5.5 0.4

Wheat Straw 1.8 49.6 3.7 22.2 2.9 6.0 1.0 2.6 3.3 0.1

Grass 2.8 59.5 0.7 15.3 3.4 7.4 1.6 8.6 1.4 0.2

Sewage Sludge 15.0 34.6 1.0 1.4 3.1 17.3 10.6 10.0 1.3 1.0

Coal 18.1 40.8 3.5 2.4 3.8 10.3 12.3 6.2 6.2 0.8


23

There is a tendency for higherchlorine/lower sulphur levels tofavour formation of chlorides oversulphates, while the lower ashcontents provide less dilution of anydeposits formed on plantcomponents.

It is thought that the high potassiumcontent combined with chlorine isalso responsible for the formation oflow melting temperature compoundsduring combustion. The low sulphurcontent in many biomass fuels isanother contributing factor. Theselow melting point ash constituentsinduce widespread fouling andsevere corrosion problems.

In gasification systems there areseveral possible pathways that minorand trace elements, such as sodium,potassium and heavy metals, cantake, thereby influencing theamount that will reach the gasturbine or engine. Options include:

• No response to the gasificationprocess so the minor/traceelements will exit with theash/char/slag, to

• Formation of vapour species thatcan pass through the whole hotgas path and thereby will beemitted from the process

In between these two extremes it ispossible for reactions to take placethat lead to:

• Condensed particles or vapourspecies that can condense ontoentrained particles/plantcomponents within the hot gaspath [depending on specificoperating conditions]

The fate of the various traceelements is element-specific. It isalso influenced by:

• Relative and absolute levels ofother elements present in the fuels[e.g. S & Cl]

• The sorbents or catalysts used

• The material compositions of hotgas path components.


24

5.4 Materials Issues

In biomass-based combustionsystems the key materialschallenges are increasedlevels of fouling andcorrosion, which then limitoperating steamtemperatures. The aim is tobe ‘fuel flexible’, therebytaking advantage of cheaplocal sources of biomass fromeither waste or farmedsources.

Such versatility requires improveddurability of heat exchangermaterials or biomass pre-treatmentsto remove the aggressivecontaminants, or the use of additivesto reduce fouling/corrosion. Forexample use of sulphur-bearingadditives can reduce chlorine levelsin deposits. A combination of theseoptions is likely to deliver ‘bestpractice’. It is also possible to useadditives to reduce fouling/corrosionin certain circumstances.

Even the best available materials,such as Alloy 625, have problemsmeeting power plant liferequirements. Coatings have beeninvestigated at length but it isdifficult to achieve both effectivecorrosion resistance in the aggressiveconditions and economic viability.Further research is a priority for bothevaporative and superheating duties.

Major materials issues can beavoided in initial gasification-basedsystems, for both power generationand gaseous/liquid fuel production,by careful process and designchoices. The need to extract heatenergy from the hot gas is a lowerrequirement in biomass gasificationthan in much larger, coal-basedintegrated gasification combinedcycle power plants. However, futurebiomass systems may require greaterheat extraction [see the Gasificationreview for details] and so couldencounter significant materialschallenges. Cleaning the gas tosafeguard downstream componentsand reduce subsequent emissions arehowever critical issues.

Optimum performance of the gasturbine is vital in all biomasscombined cycle systems to ensureoverall plant efficiency andeconomic viability. Hot corrosionand/or erosion are likely to be lifelimiting for high temperaturecomponents such as gas turbinevanes and blades in the hot gas path,rather than creep and fatigueprocesses that limit life in naturalgas systems. This is due to highercontaminant levels in biomasssystems. As a result, process choicesand performance of gas cleaningcomponents are dominant factors.

Industrial gas turbines have beendeveloped to use a wide variety offuels - from natural gas to sour gasesand heavy fuel oils. Degradation ofmaterials in such systems has beenwidely investigated over the past 40years [6, 7]. Fuel gases derived frombiomass have the potential to causeboth erosion and corrosion damageto gas turbine hot gas pathcomponents. Fuel derived particlescan cause both erosion damage anddeposition depending on the particlesize and composition, as well asaerofoil design and operatingconditions. Corrosion can resultfrom the combined effects of gaseousspecies (e.g. SOX and HCl) anddeposits can form by condensationfrom the vapour phase (e.g. alkalisand other trace metal species) and/orfrom particle impact and sticking.

Corrosion damage is highlydependent on the local componentenvironments. The metal vapourspecies of most concern were alkalis[mainly sodium] in gas turbines firedon clean fuels or vanadium fromheavy fuel oils. In biomass firedsystems the levels of SOX and HClcan be similar or higher than thosefrom a coal gasifier (see Table 1).Also, the fuel gas may containsignificant levels of alkali metals [inparticular potassium] and heavymetals [e.g. lead and zinc (8)]depending on the biomass type(Table 2) and the effectiveness of thegas cleaning approach. If a water orchemical scrubber is included [as inthe ARBRE scheme for ammoniaremoval] levels of thesecontaminants will be significantlyreduced whereas a hot/dry cleaningapproach can lead to higher levels,

depending on the operatingtemperature of the filter.

Care is required in the control oftrace alkali and other contaminantsbecause of their potential to giveexcessive corrosion damage to hotgas path components in a gasturbine using a biomass-derived fuelgas [9]. The exact level of damagethat can be tolerated by the turbinedepends on specific process andoperating parameters [e.g. bladetemperatures, materials,repair/replacement strategy etc.] aswell as economic factors [10-15]. Itmay be possible to optimisematerials selection and operatingconditions in combination with amaintenance strategy that allowshigher than currently specifiedcontaminant levels, instead of usinga high cost elaborate gas cleaningsystem. One research option is tocontinue development of improvedprotective coatings that allowbiomass systems to take advantage ofadvanced high efficiency gas enginesand turbines, thus assistingeconomic viability.

For gaseous and liquid fuels preparedfrom biomass, whether for heat,power or transport, the issues revolvearound the levels of contaminantsremaining in the fuels. Similarmaterials issues to those describedabove can arise.



25

5.5 Conclusions

Materials challenges arising from useof biomass in energy processes havebeen evident for many years.Remaining impediments to increaseddeployment of these relatively lowrisk, renewable energy optionsinclude:

• Improved alloys and coatings forevaporator and superheater heatexchangers and gas turbine/gasengine hot gas path duties

• Life prediction modelling for heatexchangers to optimisemaintenance and repairprocedures

• Monitoring of corrosion/contaminants in order to provideearly warning of problems

• Improved repair/refurbishmentprocedures for heat exchangersand gas turbine parts

5.6 References

[1] Energy for the Future:Renewable Sources of Energy –Campaign for Take-off - aCommunity Strategy andAction Plan, EU CommissionPaper (1998).

[2] A.V. Bridgwater, ‘BiomassGasification for PowerGeneration’, Fuel, vol. 74 no. 5631-653 (1995).

[3] E. Rensfelt, ‘GasificationTechnology for Biomass andOther Solid Fuels’, BiosolidsEnergy Recovery TechnologySeminar, IWEX 2001,Birmingham UK, (Oct 2001).

[4] K. Stahl and M. Neergaard,‘IGCC Power Plant for BiomassUtilisation, Varnamo, Sweden’,Biomass and Bioenergy, vol. 15,no. 3 (1998).

[5] ‘Gas turbines in advanced co-fired energy systems’, ECSCProject 7220-PR/053

[6] C.T. Sims., N. S. Stoloff andW.C. Hagel, ‘Superalloys II’,Wiley (1987).

[7] ‘Hot Corrosion Standards, TestProcedures and Performance’,High Temperature Technologyvol. 7 (4) (1989).

[8] ‘Co-gasification of Coal/Biomassand Coal/Waste Mixtures’, FinalReport EC APAS ContractCOAL-CT92-0001, University ofStuttgart, Germany (1995).

[9] ASTM D2880, StandardSpecification for Gas TurbineFuel Oils (1990).

[10] M. Decorso, D. Anson, R. Newby, R. Wenglarz and I. GWright, Int. Gas Turbine andAeroengine Congress,Birmingham, UK, ASME Paper96-GT-76 (1996).

[11] I.G. Wright, C. Leyens and B.A. Pint, in Proc. ASMETURBOEXPO 2000, Munich,Germany, ASME Paper 2000-GT-0019 (2000).

[12] N J Simms, A Encinas-Oropesa,P J Kilgallon and J E Oakey,‘Performance of Gas TurbineMaterials in ‘Dirty Fuel’Environments’, to be publishedin Materials for PowerEngineering, Liege, (Oct. 2002).

[13] C Leyens, I G Wright and B A Pint, ‘Hot Corrosion ofNickel-based Alloys by Alkali-containing Sulfate Deposits’,5th International Symposiumon High Temperature Corrosionand Protection of Materials, LesEmbiez, France (May 2000).

[14] N.J. Simms, P.J. Smith, A. Encinas-Oropesa, S. Ryder,J.R. Nicholls and J.E. Oakey, inLifetime Modelling of HighTemperature CorrosionProcesses, Eds. M. Schütze et al,EFC No. 34 (Maney Publishing,London) 246-260 (2001).

[15] N. J. Simms, J. R. Nicholls andJ.E. Oakey, in Materials ScienceForum vol. 369-372, 833-840(2001)


26

Contents

Introduction:

6.1 Priority Research Areas for Hydrogen Energy

6.2 Hydrogen Production: Routes to Hydrogen

6.3 Hydrogen Storage

6.4 Cross-Cutting Materials Research Areas


27

Hydrogen6.0

The environmentaland energy securitybenefits of movingtowards a hydrogeneconomy have beenwidely discussed andaccepted in Europe,Japan and US.

The principal drivers behinda sustainable energy visionbased on a hydrogen-orientedenergy economy are:

• Reducing greenhouse gasemissions

• Improving our security of energysupply

• Strengthening the UK economy

• Improving local urban air quality.

Unlike fossil fuels, hydrogen is not aprimary energy source. Instead it isan “energy carrier” which is firstproduced from another energysource and then transported forfuture use, where its latent/storedchemical energy can be utilized.Fundamentally, hydrogen resembleselectricity as a carrier ofenergy/secondary energy sourcerather than a ‘prime mover’.

Hydrogen can be obtained from awide variety of both renewable andsustainable resources that includehydro, nuclear, wind, wave, solar,biomass and geothermal as well asnon-renewable resources, such ascoal and natural gas. Hydrogen canbe utilized in high-efficiencyelectrical power-generation systems,most notably in fuel cells for bothvehicular transportation anddistributed electricity generation.Fuel cells convert hydrogen (or ahydrogen-rich fuel) and an oxidant(usually pure oxygen or oxygen fromthe air) directly into electricity via anelectrochemical process. This closesynergy between hydrogen and fuelcells is a key facet of any future UKenergy scenario. Hydrogentechnologies are also set to play akey role in large-scale powergeneration, with pre-combustioncarbon capture and storage

technologies relying on initialconversion of fossil fuel into ahydrogen-rich synthesis gas stream.Hydrogen and CO2 are separated byphysical or chemical methods beforethe hydrogen is burnt in turbines or,in future scenarios, efficientlyconverted to electricity in solid oxidefuel cells.

Any transition from a carbon-based/fossil-fuel energy system to ahydrogen-based economy involvesovercoming significant scientific,technological and socio-economicbarriers before ultimateimplementation of hydrogen as theclean energy source of the future. Ifthese difficulties are overcome,energy supply and transformationcould be developed along twoprimary energy distribution networks- the electricity network and the

hydrogen network. Within such ascenario, oil may not be thedominant energy provider, but willbe utilised for the synthesis of keychemical products (Figure 1).

Research and development prioritiescan be divided into three over-arching clusters centred on hydrogenproduction, hydrogen storage andhydrogen utilization. In all of theseareas, materials research is a majorfactor in advancing a UK HydrogenEnergy Future. Particular emphasis isplaced here on the importance ofbuilding a strong base offundamental research activities inpriority areas. The importance ofcrosscutting activities is alsoemphasised, whereby progress in onearea will significantly impact onother areas, thereby acceleratingprogress.

Fig 1. A projectedhydrogen energyeconomy wheresustainable or renewabletechnologies areintensified and thecombination of hydrogenand fuel cells is extensivelyemployed. Note thatproduction of hydrogenfrom natural gas, or coalnecessitates CO2 captureand storage.


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Hydrogen6.0

6.1 Priority Research Areas forHydrogen Energy

Hydrogen Production: Routes to Hydrogen

Even though hydrogen is highlyabundant, it is invariably bound-upwith other elements in chemicalcompounds. Some 95% of hydrogenis currently produced from fossilfuels. Hydrogen production (Fig. 1) isthe key to developing a hydrogeneconomy and ultimately tosustainable or renewable energyroutes.

The UK has a world leading positionin production of hydrogen fromhydrocarbon feedstocks, withextensive catalysis, chemical andprocess engineering expertise. It isvital that the UK maintains andextends this position by expandingthis expertise into new areas, such aslarge-scale power generation anddistributed hydrogen technologies aswell as investing in next-generationhydrogen technologies centredaround renewable energy sources.

Bearing in mind regional variations,a broad portfolio of hydrogenproduction routes are nowenvisaged. A representation ofvarious process technologies isshown in Figure 2.

In the short and medium term,production options for hydrogen arestill based on reforming of naturalgas and coal and on distributedhydrogen production fromelectrolysis of water. Largercentralised production plants arelikely to be introduced at a laterstage, based on fossil fuels, againwith CO2 capture and storage, andbiomass. An increasing emphasis willbe placed on hydrogen from solarenergy (solar hydrogen) and onutilisation of thermo-chemicalenergy for massive-scale hydrogenproduction (via nuclear energy orsolar energy).

Hydrogen from Fossil Fuels:Reforming and gasification

90% of hydrogen is currentlyproduced by steam methanereforming. This “merchanthydrogen” is used mainly forammonia & methanol production, aswell as refinery upgrading.

Distributed-scale fuel processorsgenerate a hydrogen-rich reformatefeed locally for fuel cells by reactingreadily available hydrocarbons suchas natural gas, LPG and gasoline,with water (Steam Reforming), air(Partial Oxidation) or a combinationof the two (Auto-thermalReforming).

Materials challenges for next-generation hydrogen productioncatalysts at both industrial anddistributed scales arise from:

• The need to cope with heavierhydrocarbon fuels that areintrinsically more difficult toprocess

• Use of dirtier feedstocks (e.g.biomass and coal), which requirepre- and post-processing to removeimpurities, particularly sulphur,mercury (Hg) and arsenic (As)

• The harsher process environmentof small-scale hydrogen generationsystems, which demand catalyststhat are highly active and resistantto a wider range of operatingconditions, including repeatedcycles, reduced steam ratios andoxidative start-ups/shut-downs.

Hydrogen production and processingcatalysts need to be tailored to thespecific application (Figure 3). Largescale processes use pelleted basemetal catalysts (e.g. - nickel-based forreforming, FeCr or copper-based forwater gas shift), whereas distributedsystems can use supported preciousmetal catalysts that are coated ontoceramic or metallic substrates in amanner resembling automotivecatalytic converters. The higher costof these catalysts can be offset bytheir more effective intrinsicperformance and robustness.

Developing the next generation ofcatalysts and adsorbents will requirebasic research into nanoscalesynthesis, molecular modelling,kinetic and mechanistic reactionstudies. Advanced analyticaltechniques are also needed to rapidlycharacterise catalysts and theiractivities under realistic workingconditions. This will involvecharacterisation of structure anddynamics of active catalyst sites atthe atomic level.

Integration of catalysts into efficientand compact hydrogen generationsystems will require research intoadvanced reactor concepts, such asauto-thermal reforming, micro-channel reactors and mini-channelreactors. Many of these techniquesplace stringent demands on thematerials of construction andassociated manufacturing methods.

Fig 2. Possible hydrogen production pathways.


29

Hydrogen Purification

Fuel cells can directly use thehydrogen rich reformate gas streamthat comes from a fuel processor.However, such streams contain tracehydrocarbons, sulphur compoundsand carbon monoxide that can bedetrimental to the performance orlifetime of the fuel cell. The amountof gas clean-up necessary depends onthe fuel cell type. High temperaturefuel cells requiring less pre-processing than low temperature fuelcells, which may require severalstages of water gas shift, andselective oxidation to remove COfrom the feed.

Advanced catalyst and adsorbentmaterials need to be developed thatare appropriate to the fuel cellapplication. For example, deepdesulphurisation of liquidhydrocarbon feeds would allow for

Fig 3. The use of a variety of catalysts will be amajor feature of hydrogen production from small-to-large scale applications.

additional fuel flexibility, whereasremoval of sulphur from a hightemperature reducing stream couldoffer significant system benefits.Such feedstocks could also impact onlarger-scale hydrogen productionflowsheets, where catalysts andadsorbents tailored to operate in aparticular operating envelope couldimprove efficiency and reducecapital costs by simplifying heatmanagement.

Where hydrogen is stored as a fuelsource for fuel cells, it needs to befurther purified to remove alldiluents, particularly CO2.Technologies for achieving this at anindustrial scale are well establishedand include pressure swingadsorption and amine scrubbing.However, fuels such as coal, wasteand biofuels bring additionalmaterials challenges as the range andlevel of contaminants iscorrespondingly higher.

At the distributed scale, there issignificant scope to developadvanced gas separation techniques,particularly those based aroundmembranes and small scale PSA.Thin film Pd-alloy membranes are ofparticular interest as they have ahigh selectivity to hydrogen.Membrane technologies have thepotential to be applied to large-scalehydrogen plants, particularly thosewith associated carbon capture and

storage (CCS), where CO2 retentatewould remain at pressure. Membranesystems of interest include porousmaterials (e.g. - carbons, zeolites, andsilicas), as well as impermeablemetallic systems (e.g. vanadium andpalladium films). Materials scale-upissues, as with all membrane types,are significant. Chemical looping isan example of another advancedconcept for H2/CO2 separation. Atlarge scale this requires significantmaterials R&D.


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Hydrogen6.0

Hydrogen from Electrolysis

Water electrolysers have greatpotential to harness renewableenergy resources, such as wind andsolar to create hydrogen as theenergy carrier.

Electrolysers are electrochemicaldevices which can use electricalenergy to split water into hydrogenand oxygen. In electrolysis cells thehydrogen and oxygen are producedas pure gases. Where a solid polymeris used as electrolyte, this technologyis called Proton Exchange Membrane(PEM) electrolysis.

PEM electrolysers offer the possibilityof low-cost hydrogen and oxygengeneration in small, highly efficientunits that are suitable for distributed,as well as centralised, operations.

There are many similarities betweenthe materials challenges facing PEMelectrolysers and PEM fuel cells, [seeSection 7], particularly thedevelopment of lower costcomponents (catalysts, electrodes,membrane, bipolar plates) whichdemonstrate higher performance andimproved durability and can beintegrated into mass manufacturableprocesses.

Fig 4. Schematic diagram of Proton Exchange Membrane electrolysis.

Hydrogen from Solar Energy

For solar, photovoltaic, photo-electrochemical or photo-catalytichydrogen production to becomeeconomically viable, it will benecessary to achieve both a highlevel of efficiency and dramaticallyreduce the cost of the entire process.The need to create a “disruptivetechnology” here is great. Challengesinclude:

• Development of new electronic‘band-gap engineered’ ormolecular-level designed materialsand chemical processes that areintegrated for fabricatinginexpensive solar cells or photo-catalysts.

• Development of a fundamentalunderstanding of the mechanismof both electron and hole diffusionand transport in solids and acrossinterfaces to liquids. For example,the factors that influence themobility of charge carriers ininorganic, organic and hybridsystems need to be understood aswell as the role played by surfacestates, grain boundaries etc indictating the efficiency of PV andphoto-electrochemical cells.

• Developing synthetic routes toinorganic/organic/polymer hybridmaterials with controlled structure/ function in tandem withconsideration of mechanisms [asabove]. Synthesis of defect-free

nanoparticles with control ofshape, size and interfacialchemistry is also needed combinedwith an understanding of electronand hole transport and conductionin multi-component andmultilayer photo-catalyticmaterials.

• In situ characterisation ofcomplex, multi-componentchemical systems in thedevelopment of efficient solar cellsand photo-catalysts for hydrogenproduction. The aim is amolecular-level understanding ofboth structure and function formaterials optimisation.

• Theory and modelling of complexphoto-systems (e.g. femtoseconddynamics of charge carriers andtheir recombination processes).

• An understanding and control ofthe various processes leading todegradation of solar cells, mostnotably dye-sensitised andnanoparticle systems in order toachieve a minimum of 10 years ofuseful life.

• Enhanced understanding ofmolecular and heterogeneouscatalysts, particularly for theoxidation of water to oxygen.Photo-catalytic studies of directwater splitting and CO2 reductionwith complex redox catalysts areneeded for next-generation photo-catalysts.

• Exploration of hydrogenproduction involving electron andion transfer at catalyst/electrolyteinterfaces.


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Hydrogen from Thermal Energy

If the hydrogen economy is to cometo fruition, novel andthermodynamically efficient routesmust be discovered to manufacturehydrogen - from a sustainable non-carbon feedstock - at an industrialscale. The obvious feedstock is water.Thermodynamic studies have alreadydemonstrated the significantpotential for thermochemical cyclesto deliver mass-scale manufacture ofhydrogen.

A large number of thermochemicalcycle systems exist for watersplitting. Materials needs are urgentbecause of the requirement tooperate in an aggressive chemicalenvironment at elevatedtemperatures. Thermochemicalcycles using heat from either high-temperature solar or nuclear sourcesare possible.

Key high-level materialsdevelopment will include:

• Materials that are robust, costeffective and compatible withextremes of both chemical andthermal environments.

• Materials for improved separationprocesses that function at hightemperatures. Such processesinclude separation of SO2 and O2from SO3 and separation of H2Oand HI from H2O/I2/HI mixtures,and renewal of H2.

• Understanding the fundamentalelectrochemistry and materialschemistry leading to hydrogen andoxygen generation for improveddesigns of electrolyser andelectrode materials.

• Thermodynamic modelling aimedat estimating the achievableefficiency for all candidatethermochemical cycles. This willallow production of robust,experimentally-validated processmodels of the Hybrid SulphurProcess (HyS) [a variant of the so-called Westinghousethermochemical cycle]

• Improved next-generation catalystsfor these high temperatureprocesses, which would allowthermochemical cycles to operate

at lower maximum temperature,thereby offering more flexibility inreactor design and optimisation ofcycles.

• Production of alternative energycarriers beyond hydrogen, such asammonia (NH3) as a chemicalproduct synthesised using heatobtained from thermochemicalprocesses from both fission andfusion reactors.


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Hydrogen6.0

6.3 Hydrogen Storage

Main Requirements for Hydrogen Storage

Developing effective hydrogenstorage, particularly fortransportation, is a pivotal challengefor basic research and a key factor inenabling ultimate success of thehydrogen economy.

Hydrogen storage onboard vehiclesare considered central to achievingmarket success for fuel cell vehicles.

The necessary operatingrequirements for effective hydrogenstorage for transportation includethe following:

• Appropriate thermodynamics (i.e.favourable enthalpies of hydrogenabsorption and desorption)

• Fast kinetics (quick update andrelease)

• High storage capacity. Thiscapacity is determined by usage,e.g. transportation or stationaryapplications.

• Effective heat transfer

• High gravimetric and volumetricdensities (lightweight andconserving space)

• Long cycle lifetime for hydrogenabsorption / desorption

• High mechanical strength anddurability

• Safety under normal use andacceptable risk under abnormalconditions.

The major challenge is to storeenough hydrogen on board a vehiclefor an equivalent driving range ofsome 300 miles whilst meetingperformance (weight, volume,kinetics etc), safety and costrequirements, withoutcompromising passenger or cargospace.

Specific performance targets forhydrogen fuelled transportation aregiven in Table 1.

Recent systems studies suggest thatthe hydrogen storage system shouldhave a specific energy of 3.0 kWh kg-1 and a cost of $ 2 kWh-1 to meetthe Table I goals. No current

hydrogen storage technology meetsthese targets. Figure 5 shows thecurrent status of hydrogen storagesystems in terms of cost andvolumetric and gravimetric energycapacities. It is recognised worldwidethat new materials for hydrogenstorage represent an area where apotential step-change inperformance might be achievable.Current methods of hydrogenstorage are described below, togetherwith fundamental materials researchissues that are needed to achievehydrogen storage goals.

A representation of the range ofvolumetric and gravimetric densitiesfor storage systems is shown inFigure 6.

Table 1. Hydrogen storage system targets (US Department of Energy).

Targeted factor 2010 2015

Specific energy (MJ/kg) 7.2 10.8

Hydrogen (wt%) 6.0 9.0

Energy density (MJ/kg) 5.4 9.72

Delivery pressure (bar) 2.5 2.5

Operating temperature -20/50 -20/50

Absorption/desorption cycle lifetime 1000 1500

Flow rate (g H2/s) 4 5

System cost ($/kg/system) 6 3

Refuelling rate (kg H2/min) 1.5 2.0

Furthermore, whilst economic andenergy dense storage systems arerequired for the hydrogen fuelledvehicles, the requirements forhydrogen distribution should not beoverlooked. Performance limitationsand through life economic factorsmust be clearly understood forhydrogen tankers, pipelines andlarge underground / above groundstorage facilities.

Gaseous and LiquidHydrogen Storage

The most common and maturestorage technology is high pressurecontainment in steel tanks, althoughlight-weight composite tanks,designed to operate at very highpressures are becoming more and

Fig 5. The status of current hydrogen storage systems in terms of cost (kWh-1), gravimetric (kWh_ kg-1) and

volumetric (kWh L-1) densities. [Adapted from S.G. Chalk and J.F. Miller, J. Power Sources, 2006, vol. 159, p. 73.]


33

Fig 6. Gravimetric and volumetric densities of various hydrogen storage options (including weight and volume ofthe storage container). ‘DoE target’ represents the US Department of Energy target for 2015 set for an ‘ideal’hydrogen storage material.

more common. Although thistechnology advances at pace, thelong-term effect of hydrogen on thematerials under cyclic or coldconditions is still not fullyunderstood. Principal research needsfor improved compressed gas storagecentre on the development of newcontainer materials that are strong,reliable and low in cost. Theperformance of materials in highpressure gaseous and liquidhydrogen is also relevant to valves,regulators, and other parts of storageand handling systems. Materialsresearch areas can therefore beidentified specifically as:

• Research on materialembrittlement in high pressuregaseous hydrogen, centred on theatomic-scale mechanismsresponsible for materials failure.

• Development of lower-cost storageand construction materials(notably low cost stainless steelsand carbon fibres).

• Research on fatigue performance(particularly of welds) in highpressure gaseous hydrogen

• Development of ‘hybridapproaches’; for example, usinghigh pressure tanks which alsoinclude conventional metalhydride stores (e.g. TiVH1.5) toreduce operating pressures.

Storage of hydrogen in cryogeniccontainers offers a significantadvantage, since greater quantities ofhydrogen can be stored in a givenvolume as a liquid than can bestored in gaseous form. I.e. liquidhydrogen has a much better energydensity than pressurised gas options.BMW have developed a hydrogen-

fuel-cell vehicle with onboard liquidhydrogen storage. A major drawbackis the large quantity of energyrequired for hydrogen liquefaction -presently about one-third of theenergy value of the hydrogen stored.Similarly, loss of hydrogen throughevaporation is an issue, althoughquite major advances have takenplace in recent years in insulationtechnology.

Basic research issues related to liquidhydrogen storage include:

• Discovery of new lightweight, lowvolume and low cost materialsthat have very low heat transfercharacteristics.

• Development of systems andmaterials that automaticallycapture hydrogen gas boil-off. Forexample, use of metal hydrides /new materials for hydrogencapture and assistance insubsequent re-liquefaction ofhydrogen.


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Hydrogen6.0

Transition Metal Hydrides

Many transition metals and theiralloys can store hydrogeninterstitially in their crystalstructure, forming hydrides (e.g.LaNi5H6, FeTiH1.7, VH2 etc). Thoughthe hydrogen volumetric density insome of these materials is higherthan that of liquid hydrogen (Figure6), the weight of the materials isimpractical for onboard storage ofhydrogen in vehicles. However, sometransition metal hydrides are widelyused as the hydrogen-storageelectrodes in nickel hydriderechargeable batteries and could finduse in a variety of stationaryapplications.

Apart from rather modest hydrogenstorage capacity (around 1-3 wt%)and high cost, transition metalhydrides have a number of highlyattractive features such as:

• low enthalpies for hydrideformation

• good reversibility and cyclabilityof hydrogen adsorption /desorption

• the possibility to tune thehydrogen decompositiontemperature by alloying.

Another important benefit of thesematerials is that they provideextremely pure hydrogen, thereforeavoiding the cumulative build-up ofimpurities that could degrade theperformance of Proton ExchangeMembrane (PEM) fuel cells.

Basic materials research issues relatedto transition metal hydrides include:

• Understanding the effect ofincorporated hydrogen on theelectronic structure of the hostlattice and the dynamics andenergetics of hydrogen and itsinteraction with lattice defects intransition metal hydrides

• Development of new transitionmetal hydrides optimised for usein hybrid high-pressure hydrogentanks

• Cost reduction and improvementin gravimetric hydrogen densitywhilst still operating at lowtemperatures (40-100˚C) andpressures (4-10 bar).

Light Metal Hydrides

Storage of hydrogen in light metalhydrides offers a range ofpossibilities to meet transportationrequirements. Several classes of lightmetal hydrides have been studiedworldwide including non-interstitialmetal hydrides (e.g. MgH2), alanates(e.g. NaAlH4), borohydrides (e.g.LiBH4), amides (e.g. LiNH2) andrelated materials/destabilisedchemical systems. At present nosingle material investigated exhibitsall the necessary properties.

Research emphasis has focused onmeeting volumetric and gravimetrictargets but two other key targetslisted in Table 1 are fill time forhydrogen storage and the kinetics ofhydrogen update/release. System filltime is strongly dependent upon thehydride formation enthalpy and theefficiency of the thermalmanagement system. For example, inorder to fill a tank with 4kg ofhydrogen in 5 minutes a materialwith a heat of hydrogen adsorptionof 30 kJ/mol will generate 200 kW ofheat. To meet the power demands ofa fuel cell vehicle US Department ofEnergy set a 2015 hydrogen flow ratetarget of 5g/s, which is equivalent to50 litres of hydrogen /second.Ideally, this hydrogen release rateshould be achieved at around 100°Cto utilise waste heat from the fuelcells to desorb hydrogen, while ahydrogen storage material shouldstill hold hydrogen at ambienttemperatures and pressures of lessthan 10 bar.

Breakthroughs in solid statehydrogen storage will requirerevolutionary new materials to meetthe hydrogen storage requirements.Material research priorities shouldinclude:

• A fundamental understanding ofthe physical and chemicalproperties of light metal hydrides,including crystal structure,homogeneity ranges,thermodynamics and kinetics ofhydrogenation-dehydrogenationprocesses and electronic structure.

• An atomic- and molecular-levelunderstanding of the physical andchemical processes involved inhydrogen storage and release.

• Understanding the critical role ofdopants and the mechanisms ofdestabilisation reactions in alteringthe nature of hydrogen bondingand achieving reasonable kineticsand reversibility of complexhydrides at temperatures of 80-150K.

• New cheap synthetic processingroutes to avoid lengthy andexpensive solvent-based synthesisand regeneration of novel lightmetal complex hydrides.

• Studies on the benefits of nano-scale versions of hydride materialsrelative to their bulk counterparts,including shorter diffusiondistances, new phases with bettercapacity, reduced heats ofadsorption/desorption, fasterkinetics, as well as new surfacestates capable of catalysinghydrogen dissociation.

• A dramatic improvement inhydrogen cycling capacity and thechemical stability of reversiblelight metal hydrides to multiplecharging/discharging cycles.

• Development of cost-effective off-board regeneration processes andsystems for non-reversible lightmetal hydrides. A highthermodynamic and kinetic barrierinvariably exists for chemicallyrecalcitrant products (e.g. alkalimetal borates).

High Surface Area Materials

Hydrogen storage capacity has beeninvestigated in a range of activatedand nanoporous carbons, zeolites,metal organic framework andpolymer porous materials. Hydrogenmolecules usually form a physi-sorbed monolayer on the surface,which means that materials withvery high surface areas are requiredto achieve an attractive hydrogenstorage capacity. A combination ofrelatively high pressure and lowtemperature is required for effectivehydrogen storage in porousmaterials. Currently the maximumamount of hydrogen that can beabsorbed is around 6-7wt% at 77Kand 20 bar on activated carbons andnovel metal-organic frameworkmaterials with a surface area of up to3200m2/g. These materials areunlikely to meet the requirementsfor on-vehicle hydrogen storage butmay prove suitable for stationary


35

applications. Basic research issuesinto high surface area materialsinclude:

• An understanding of howhydrogen interacts with thesurface of materials and how thenature of hydrogen bonding withthe host framework can be tunedto optimise storage properties.

• A systematic investigation of theeffect of size and curvature ofnanomaterials on thethermodynamics of hydrogenstorage.

• A theoretical assessment andexperimental research to seewhether such materials can beengineered to reversibly store highlevels of hydrogen near roomtemperature.

Hydrogen Storage in Liquids

A range of liquid-phase organicmaterials with reversible hydrogenstorage capacities of >7 wt% at 200˚Chave been developed recently.Selective catalysts enable highlyreversible catalytic hydrogenationand dehydrogenation with nosignificant degradation of themolecules. Acceptable cost and lowvolatility would allow the use oforganic liquid-phase hydrogenstorage materials in onboardhydrogen storage systems.

Another potential liquid medium forhydrogen storage is ammonia (NH3)and its various chemical derivatives.NH3 has a gravimetric hydrogencontent of 17.6 wt% and atemperature of liquefaction of 240K,which corresponds to a pressure ofaround 8 bar at room temperature.Major materials research areasinclude the development of:

• Organic systems that can bedehydrogenated at lowtemperatures and producehydrogen at feasible pressures.

• Selective, highly reversiblehydrogenation anddehydrogenation catalysts,enabling multiple cycles of usewith no significant degradation ofthe organic molecule.

• Selective catalysts for low-temperature decomposition ofammonia.

• Novel liquid and solid ammonia-based storage systems

• Confirm the resistance of materialsto stress corrosion and corrosionfatigue in the storage media

6.4 Cross-Cutting MaterialsResearch Areas

Several common basic materialsresearch needs are apparent in anydiscussion of hydrogen production,storage and use. Such “cross-cutting”areas or themes are also a key sourceof synergy in hydrogen and fuel cellmaterials research and development.These cross-cutting themes areimportant since progress in one areawill significantly impact on otherareas, thereby accelerating progressacross a wide spectrum of activities.This also leads to an integratedapproach to the many and variedresearch challenges in HydrogenEnergy. Such cross-cutting areas arebriefly discussed below:

Current materials. Much of theinfrastructure for hydrogendistribution will employ currentlyavailable materials. Data to allow thesafe application must be generatedand novel means of ensuringintegrity cost effectively need to beexplored.

New materials. An over-archingtheme centred on the ability todesign and tailor advanced materialsfor particular applications. Thesenew materials synthesised throughthe entire spectrum of modernsynthetic methodologies andtechniques. New materials ormaterials combinations for structuralapplications need to be inspectable.

Catalysis. There is a need in all areasfor specifically designed catalystswith higher activity, higherspecificity, higher stability andreduced susceptibility to impurities.Similarly, catalysts are required thatlessen, or eliminate, the need fornoble metals thereby reducing thecost associated with catalysis. Theserequirements will involveexperimental programmesunderpinned by fundamentalunderstanding of catalysis sitestructure and dynamics at themolecular level.

Nanostructured materialsNanoscience and nanotechnologyhave the potential to address manymaterials performance issues bytaking advantage of the distinctivechemical and physical properties ofnanostructured systems.

Membranes Fundamental advances couldsignificantly reduce the cost ofmeeting the hydrogen purityrequirements of fuel cells and otherapplications. The combination ofsuperior permeable catalysts andmembranes constructed fromnanostructured hybridorganic/inorganic systems are ofparticularly importance.

Advanced Instrumentation andCharacterisation TechniquesKnowledge of chemical and physicalmaterial properties at the nanoscalelevel can have a significant impactacross all areas. Development of newanalytical tools to provide atomic-level information will support allaspects of materials programmes onhydrogen energy.

Theory, Modelling, and Simulationareas are essential for newdevelopments in all aspects ofhydrogen energy materials, fromtheoretical studies of the interactionof hydrogen with materials, throughcatalyst design, new materialsdesign, to the hydrogenembrittlement process. Similarly,understanding of electron transferprocesses in solids and interfaces iscrucial to enhanced photo-catalystsand photo-electrochemical processesin hydrogen production. Such theoryand modelling must be coupled inparallel with basic experimentalstudies.

Safety and Environmental IssuesHydrogen is characterised by highdiffusivity and inflammability.Hydrogen embrittlement in materialsand development of sensitive,selective sensor materials forhydrogen are key research areas.


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Contents

Introduction

7.1 Current Status of Technology

7.2 R&D Needs for Proton Exchange Membrane Fuel Cells (PEMFCs)

7.3 Intermediate and High Temperature Fuel Cells

7.4 Manufacturing and Technology Readiness Levels

7.5 Generic Challenges

7.6 Future Opportunities for Low and High Temperature Fuel Cells

7.7 Next Generation - Materials Development

7.8 Drivers for Fuel Cell Markets

7.9 Legislation and Energy Security

7.10 Conclusions


37

Fuel Cells7.0

Fuel cells areelectrochemicaldevices that convertthe chemical energyof a reaction directlyinto electrical energy.

The basic physicalstructure of a fuel cellis illustrated in Fig 1.It consists of anelectrolyte layer incontact with a porousanode and cathode oneither side.

The system is analogous to abattery, though a fuel cellpossesses the advantage ofbeing constantly rechargedwith fresh reactant. Unlikebatteries, fuel cell reactantsare stored outside the celland fed to the cell only whenpower generation is required.Hence, a fuel cell does notconsume materials that forman integral part of, or arestored within, its structure.

There are a number of different typesof fuel cells using varioustechnologies that are suited todifferent applications. The best-known fuel cell types are:

• Alkaline Fuel Cells (AFC)

• Proton Exchange Membrane FuelCell (PEMFC)

• Direct Methanol Fuel Cells(DMFC)

• Phosphoric Acid Fuel Cells (PAFC)

• Molten Carbonate Fuel Cell(MCFC) and:

• Solid Oxide Fuel Cell (SOFC).

Each device uses a different fuel andapplications range in scale from afew mW to MW.

Fuel cells offer a high efficiencyroute to generating energy that hasled to a high degree of global R&Dactivity. Each fuel cells uses adifferent range of materials:

Low temperature fuel cell stacks(AFC, PEM & DMFC) operate on ahydrogen or alcohol fuel and rely onmetal catalysis, particularly at thecathode, for viable reaction rates.PAFC and high temperature cells usea range of fuels including hydrogen,hydrocarbons and reformates. Table1 (overleaf) shows the six major fuelcell types. The first four can beclassified as low temperature cells,while the final two are hightemperature cells. This distinction isbecoming less clear as thetemperatures of operation converge.

Other fuel cell types such as formicacid fuel cells, boron hydride anddirect ethanol fuel cells presentfurther opportunities for newmaterials. Their power outputs arelikely to be exploited in specialist[military] applications in the next 20years. Each type of cell mentionedabove will produce a voltage oftypically 1 Volt and a power in therange of a few mW to a Watt. Inorder to obtain sufficient power forpractical applications [excludingsmall portable power] cells have tobe linked together in series or series-parallel to form a stack. Stacked cellsalso require an interconnect orbipolar plate. The interconnectortypically separates one cell from thenext and is highly conducting, stablein both oxidising and fuelatmospheres, has a low corrosionrate and a closely matched thermalexpansion coefficient to other cellcomponents. All cell and stackcomponents must also offercompetitive cost and ease ofmanufacture.

Fig 1. Schematic of a fuel cell showingthe principle of operation.


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Fuel Cells7.0

7.1 Current status of technology

US and Japan encourage strongacademic / industrial interaction andhave well funded / coordinatedprogrammes on fuel cell technology.They are well ahead of UK activities.For example, the US SECA (SolidState Energy Conversion Alliance)programme has successfullyaccelerated development of SOFCs2.SECA was established in 1999 tobring together government, industry,and the scientific community topromote development ofenvironmentally friendly SOFC for avariety of energy needs. SECArepresents a new model of jointgovernment/private industrytechnology research anddevelopment and effectivelychannels the use of fundingresources.

The SECA programme aims to createfuel cells that will meet Departmentof Energy (DOE) cost andperformance goals. Leadingresearchers in industry, academiaand at national laboratories supportindustry teams with R&D through aCore Technology Programme. As theteams identify research issues theybring these challenges to the CoreTechnology Programme for solutionsthat are shared across all teams. TheUS government has funded themajority of the technologydevelopment stage. As technology isreadied for commercialisation andpotential profits, industry willincrease its funding support of SECAto better reflect this shift in focus.The strength of the SECAprogramme is that it brings all USSOFC capabilities together to worktowards common goals.

Table 1 The six principal fuel cell types.

Fuel Cell type Electrolyte Mobile ion Operation Features Principal temp ºC application

Polymer Electrolyte CF(CF2)nOC Hydrated H+ 60 - 80 High power density,Membrane (PEMFC) F2SO3

2- Pt catalyst, must Mobile powerbe kept wet,poisoned by CO

Alkaline KOH OH- 50 - 200 High power density, Space flight(AFC) cannot tolerate CO2

Phosphoric Acid Phosphoric H+ ~ 220 Medium power density, Stationary(PAFC) Acid Pt catalyst sensitive power

to CO

Direct Methanol CF(CF2)nOC H+ and Medium power density, Electronics(DMFC) F2SO3

2- or OH-(H2O, 60 - 120 high Pt content Laptops,hydrocarbon CH3OH) mobile phonesmembrane

Molten Carbonate Alkali metal CO32- ~ 650 Low power density, Stationary

(MCFC) carbonates Ni Catalyst, needs powerCO2 recycle

Solid Oxide Ceramic YZG Medium power density, Stationary(SOFC) or O2- 500 - 1000 accepts CO as power

Doped Ceria a fuel


39

China and Korea are also investingheavily in academia and industry.Within Europe, German institutescontinue to ensure a strongcollaboration between universitiesand business. The EU Framework 7(FP7) Energy Programme runs from2007 to 2013. The first call forproposals was issued in December2006. As part of this scheme aHydrogen and Fuel Cells Platform(HFP) has been proposed as a publicprivate partnership (PPP), which is tobe known as the Joint TechnologyInitiative (JTI). The JTI is driven byindustry with major developmentactivities in fuel cells and hydrogen.Academic institutions and researchassociations are also represented.Materials are likely to constitute apart of the overall JTI.

There are more than 150 UKcompanies active in the fuel cellindustry, with the vast majorityoperating at component level.Increasingly, smaller specialist andUniversity ‘spin-out’ companies arebecoming involved. UK knowledgeand expertise spans the full supplychain, from materials suppliers tosystems integration, and alsoincludes finance and servicing. TheUK fuel cell industry has issued a

Fuel Cells UK Roadmap3. Many UKcapabilities have been developed inpartnership with companies /organisations from across the world.

The main UK fuel cell strengths atsystem level include PEMFC, DMFC& SOFC. These activities are theprimary focus for industrialinvestment. Many UK companieshave a rich history in fuel celltechnology and are among theleading experts in the design ofcomponents. The strengths of ‘spin-out’ companies lie in noveltechnology and innovativedevelopment toward a demonstratorand commercial products.

Market potential for UK companiesdeveloping materials andcomponents for fuel cells is global. Acomprehensive list of UK companiesinvolved in fuel cells can be foundon the Fuel Cells UK website .Materials developers are advised tolook toward other resources (e.g.DOE) for global fuel cell developers.

1 Fuel Cells UK is the trade association for fuel cells in the UK. http://www.fuelcellsuk.org2 http://www.netl.doe.gov/technologies/coalpower/fuelcells/seca/3 Fuel Cells UK Roadmap: http://www.fuelcellsuk.org/team/Library/Roadmap-Fuel_Cells_UK-final.pdf

4 Fuel Cells UK Capabilities Guides:http://www.fuelcellsuk.org/team/Library/CapabilityGuideWithCovers100903.pdfhttp://www.fuelcellsuk.org/team/Library/Fuel_Cells_UK_Research_Capability_Guide_2004.pdf


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5 http://www.supergenfuelcells.co.uk/

In order to reach mass marketstechnologies generally progressthrough a “hierarchy of needs”.Once proof of concept has beenestablished concern shiftssequentially, though withconsiderable overlap, to matters suchas durability, reliability and cost.Lead markets appear when durabilityand reliability problems have been atleast partially resolved. The genericpriorities for R&D in both low andhigh temperature fuel cells can besummarised by following thisapproach [Figure 2].

UK’s industrial strengths arecomplemented by a world-renownedscience and technology base that iscontinuously delivering innovationinto this dynamic industry. Over 35UK research organisations are highlyactive in fuel cell and hydrogenresearch. The Fuel Cells UK ResearchCapability Guide lists thoseorganisations4. The quality of theeffort is excellent, with highinternational standing and goodinteraction with UK industry. Forexample, the EPSRC Supergen5 FuelCell Consortium brings together anumber of major UK industrial andacademic research efforts in PEMFCand SOFC.

One of the main difficulties withdriving the development effort isthat advances in fuel cell technologyneeds a multidisciplinary approach.Figure 3 illustrates the foundation ofa fuel cell system and illustrates whyfuel cells do not sit easily within asingle area of expertise.

Sections 3 and 4 below presentexamples of specific R&D needs forPEMFC membrane electrodeassembly (MEA) and SOFCrespectively. The generic similaritiesbetween different types of fuel cellare summarised in Table 2 at the endof this document. This Table collatesthe materials-related industrial needsto specific property sets.

Figure 2. A schematic of the R&D priorities for fuel cell technology

7.2 R&D needs for PEMFCs

For PEMFCs applications, the globalsupply chain is well populated andwill see some consolidation as theindustry matures. Some majorglobal manufacturers are UK based,including component suppliers.Industry needs to strengthen links inthe supply chain that lead tomembrane electrode assembly andencourage innovative activity toproduce a suite of new and improvedmaterials and components that feedthe developing market. High qualityUK research has led to university‘spin-out’ companies exploiting

Figure 3 Interdisciplinary requirements for fuel cell development.

innovation in materials andcomponents for low temperature fuelcells. To strengthen this trend, moresupport from government is neededto initiate/focus R&D efforts innew/improved materials andcomponents. For low temperaturefuel cells, effort in materials needs tofocus on:

• Membranes

• Catalysts

• Supporting layers

• Bipolar plates

• End plates collectors

Fuel Cells7.0


41

Examples are given below of specificchallenges for membrane electrodeassembly (MEA):

The challenges for membranesinclude:

• Identification of rate controllingprocess in membrane degradation.

• New materials to reducedependence on water content forproton transport.

• Membranes that maintainconductivity at highertemperatures or dryer conditions.

• Improvement of OH- conductingmembranes.

• Membranes with lower gas andMeOH crossover.

The principal challenges for catalystsinclude:

• Mechanisms of O2 reduction fueloxidation, poisoning by CO, Pt/Rueffectiveness.

• Better cathode catalysts central toimproved performance forautomotive applications.

• Catalysts for higher alcohol fuels -more complex molecules.

• More CO tolerant anode catalysts.

• Characterisation of materials e.g.number & type of active sites &their de-activation.

• Simple non-MEA methods ofquantifying electro catalystactivity.

• Controlled synthesis methods toproduce particular types of activesite.

• Availability of lower cost, nonPGM catalysts.

• Reduction of catalyst content tobring cost down.

For porous layers, the challengesinclude:

• Rigorous application of porouselectrode theory to theelectrochemical reactions.

• Detailed work on understandingexisting porosity measurementsused in MEAs.

The main challenges forcharacterisation include:

• Measurement of the location andstate of water clusters withinporous materials.

• Tools to characterise ionomerproperties in the hydrated state.


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7.3 Intermediate and HighTemperature Fuel Cells

The main focus of activity in hightemperature stacks in the UK is onsolid oxide fuel cells (SOFCs). Theseoperate from IntermediateTemperatures of around 500°C (IT-SOFC) to High Temperatures up to1000°C (HT-SOFCs).

They are being developed forauxiliary power units (APUs) andstationary power applications in thekilowatt to Megawatt range. Solidoxide fuel cell development projectsexist in Europe, North America, Asiaand Australia.

Solid oxide fuel cells and SOFCsystems have two different sets ofchallenges:

• Materials for cell and stack;

• Materials in the components andsub-systems that constitute thebalance of plant.

Cell and stack challenges include awide range of ceramic and metalmaterials used for anodes, cathodes,electrolytes, inter-connectors etc.New materials with betterperformance than existing materialsare desirable and nano-scalematerials may also offer benefits.However all SOFC materials must beproduced in greater quantities, withreliable and consistent characteristicsand at lower cost.

Material challenges for componentsand sub-systems are less wellrecognised. Although not part of thecell and stack, these materials canimpact on the operation of cells dueto release of material. Thuschromium species released from avariety of stack components willaffect the performance of the cell.Alternative alloys and use ofprotective coatings are key areas forresearch.

Specific challenges for intermediateand high temperature fuel cells aresummarised here:

SOFC materials (electrolyte, anode, cathodeand interconnect)

Electrolyte materials: (see powdersand processing requirements below)

• Mainly yttria stabilised zirconia(YSZ), and rare earth doped-cerias.Depending on the technology,there will be R&D requirements forthinner, denser and morehomogeneous electrolytemicrostructures.

Electrode materials:

• The anode (fuel) side of the cell istypically a cermet of commerciallyavailable nickel and electrolyte.However the cermet is onlyavailable from specialist fuel cellmaterials suppliers at a high cost.

• Fuel (e.g. sulphur tolerant) anodesrequired (new materialsdevelopment).

• Anodes need to be resistant toredox cycles (e.g. fuel cut off) andto temperature cycles (fuel cellbeing switched on and off).

• The cathode (air) side is acommercially available perovskiteceramic, strontium-dopedlanthanum manganite (LSM) orcobalt ferrite (LSCF). The scale-upof material supply is requiredwhile maintaining consistency forthese materials.

• Cr tolerant cathodes required (newmaterials development).

• Fabrication and characterisation ofcontrolled electrodemicrostructures.

• High performance cathodes withimproved current collection.

Interconnect materials:

• Metallic materials are used atvolume within other technologyareas. Commercially availablealloys not tailored to SOFCrequirements.

• Need for specific alloys withproperties for fuel cellenvironment.

• Lanthanum chromate is aperovskite alternative with similarproperties and the same supplychain maturity as LSM.

Fuel Cells7.0

Powders and processing (electrolyte,anode, cathode and interconnect)

• Consistency of supply requirescharacterisation of ceramicpowders.- Controlling powder particle sizeand stoichiometry.

• Modelling of the sintering process.

• Characterisation of sinteredmaterials.

7.4 Manufacturing andTechnology Readiness Levels

Manufacturing ReadinessLevel (MRL)

The manufacturing readiness levelshows how commercialisation of fuelcell technology is being approachedby fuel cell developers. The currentmanufacturing readiness level forintermediate and high temperaturefuel cell materials is shown as anexample in Figure 4. Supply chaininvolvement will be criticallyimportant in the success of thisphase of fuel cell development.

Despite the maturity of some currentmaterials, there is a need to developand characterise next generationcomponents. Introduction of newmaterials will potentially reset theMRL status to levels 1-3 (basicprinciples) as shown in Figure 4.


43

Figure 4 Manufacturing readiness levels for individual SOFC materials.

7.5 Generic Challenges

Fuel cell technology requires thesolution of a number of genericchallenges for design, fabricationand characterisation. These include:

• Methods to measure and separateionic and electronic conduction.

• Techniques to create controlledstructure layers including pore sizedistribution, pore alignment, poreconnectivity and surfacechemistry.

• Control of the 3-phase interfaceand introduction of nanoscalematerials engineering to improvemass transport and catalystutilisation.

• In-situ techniques for monitoring& assigning causes of performanceand durability losses

• Ex-situ measurement of materialproperties that predict in-cellbehaviour.

• Quantitative 3D imaging ofnanoscopic structures containingmultiphase active components andspace (pores).

• Surface science to identify thenumber and nature of activecatalytic sites. This is particularlydifficult for real materials.

• High throughput, rapid screeningmethods for material inventionand design.

Practical work on materialsdevelopment should be supported byefforts to understand theoreticalprocesses by applying modellingtechniques in the followingapplications:

• Understanding of fundamentalchemical and physical processes.

• Atomistic modelling of catalyticsites to determine the relativeimportance of structural andelectronic properties.

• Support for experimental work onreaction mechanisms.

• Prediction of better materialstructures at every length scale.

• Effect of nano-confinement oncatalytic activity.

• Effect of solid electrolytes onelectron transfer and iongeneration processes.

• Construction of multi-scale modelsof components and cells.

• Prediction of fruitful areas formaterial discovery.

7.6 Future Opportunities forLow and High TemperatureFuel Cells

Supply Chain

Development of the supply chain forfuel cell commercialisation is vitalover the next two decades. Table 2lists examples of the range ofmaterials needed for development ofthe fuel cell industry within the UK.Materials processing andcharacterisation tools also need to bedeveloped to aid consistency of thefuel cell fabrication process.

Cost Targets

Cost is currently a major barrier /opportunity for fuel cellcommercialisation. Alternative orimproved materials help address thisin a variety of ways. For examplematerials that improve theperformance of the cells yieldincreased power density which inturn reduces costs. A major issuefacing fuel cell developers is thestacking of fuel cells. Currentstacking methods are too expensiveto meet cost-competitive targets.Further material cost and processimprovement must be aligned withhigh performance over the lifetimeof the fuel cell system. The overallcost of fuel cells is determined by thewhole life cycle costs, from rawmaterial extraction through to finaldisposal at end of life.

7.7 Next Generation -Materials Development

Alternative materials are needed totake fuel cell technology from nicheto mass markets in the energy sector.Developing an understanding ofelectrode materials throughmodelling and characterisation willlead to contaminant-tolerantelectrodes with improved lifetimes.Development efforts in alloys willfocus on lifetime and cost issuesacross the fuel cell types. Newcoating technologies will offerimportant opportunities.


44

Fuel Cells7.0

7.8 Drivers for Fuel Cell Markets

Below are listed some of the mainpotential applications for certaintypes of fuel cells:

• Replacement of batteries forportable electronics applications.

• Provide high efficiency distributedpower only and power and heatfor stationary applications.

• Replace low efficiency centralgeneration.

• Provide secure power supply tocritical applications.

• Provide power to remote areas.

• Military use; thermal footprint,portability, etc.

• Fuel flexibility; either directly orvia reforming.

In all applications fuel cells willprovide low-emission power forurban applications, including lowcarbon, low noise, low NOX andSOX.

7.9 Legislation and Energy Security

Environmental legislation isincreasingly concerned with end-of-life management of products. Theautomotive and electrical/electronicssectors have been set mandatoryrecovery and recycling targets byrecent European legislation.Although no legislation currentlyapplies to end-of-life management offuel cells, such legislation is likely infuture. Since fuel cells are promotedas a “green” source of powergeneration, it would be sociallyunacceptable if any aspect of thelifecycle were to present anunreasonable environmental burden.

Energy security is a vital part of thestrategy for meeting future powerrequirements but must be balancedagainst the need to reduce harmfulemissions from power generationsystems. Fuel cell systems canoperate independently of thenational grid and can use a range offuels that take advantage ofemerging social and political trendsin power generation.

The main challenges that fuel cellshave to overcome to reach fullcommercialisation within the nextdecade are durability, reliability, andcost. Further materialsimprovements, especially in lowercomponent, manufacturing andsystem costs will aidcommercialisation. Fuel celldevelopment will require legislativeincentives and further fundinginitiatives in order to penetrate leadmarkets. Forward commitmentprocurement will be critical inaccelerating adoption of fuel celltechnology.

7.10 Conclusions

Support for fuel cell materials R&D isneeded both to support the existingfledgling industry and to developnext generation technologies.

The majority of current applicationsare at a critical stage in their earlycommercialisation. The UK is wellplaced to capitalise on the needs ofthe industry but needs to increase itsmanufacturing readiness. Scale-up todemonstrator and production statuswill involve:

- More efficient processes leading tolower cost products

- New materials

- Understanding failure mechanismsand durability issues

- Increased durability and reliability

- Manufacturing consistency withinand between large-scale batchproduction.

- Fabrication techniques can then berun on a continuous flow basis.

- Inspection techniques to ensurethe supply of high qualitycomponents supplied is also vital,and these may need developmentto find the micro- or nano-scaleflaws that lead to failure.

Materials R&D is fundamental at allstages in the development process.UK’s strength in linking universitiesand industry must be maintainedand grown. Fuel cell challenges of:

- Increased conductivity for cell andstack components

- Environmentally stable thermaland chemical materials

The multi-disciplinaryelectrochemical nature of fuel celltechnology provides the idealcontext for future development ofmaterials knowledge and know-howin the UK.


45

Generic Low Temperature Intermediate & High Property set6 Materials involved UK involvementHeading (PEM/DMFC) Temperature (SOFC)

Cell Membranes/Electrolytes Electrolytes Density, ion conductivity Polymer films ResearchMaterials mechanical, life Ceramic films Material supply

permeability control Component productionelectronic insulator Mass production readyHydrothermal stability

Anode Anode Catalysis, fluid flow Carbon supports ResearchThree phase boundary Ceramic support Materials supplyoptimisation Catalysts Component productionPoisoning, watermanagementHydrothermal stability

Cathode Cathode Catalysis, fluid flow Carbon supports ResearchThee-phase boundary Catalysts Materials supplyoptimisation Component productionPoisoning, watermanagement

Gas diffusion layer Anode support Porosity control (for Carbon fibre, Research(GDLs) Ceramic support7 muti-phase fluid flow) polymer coatings, Materials supply

Metal support8 Mechanical integrity binders, Cermets Component prductionConformability (thermal,electrical, ionic)Water management (wetability),contact losses

Catalysts Catalysts Activity Precious metals ResearchStability Nickel oxides Materials supplyPoisoning Other non-precious Component productionQuantity reduction metals Mass production ready

Stack Bi-polar plates (BPPs) Interconnects Electric conductivity Carbons ResearchMaterials Gas tightness Conductive polymer Materials supply

Flow field formation composites Component productionThermal conductivity Stainless steelsCTE match

Current take off Current take off Electrical conductivity ResearchMaterials supplyComponent production

Metal BPP coatings Metal interconnect CTE match Researchcoatings Electrical conductivity Materials supply

CTE matchMechanical integrityChemical inert in useenvironment

Seals Seals Gas permeability Elastomers ResearchFluid compatibility Polymers Materials supply(corrosion/poisoning) Glass ceramics Component productionCTE matching Mineral compactsDisassembly WeldsCreep resistance

Clamping Clamping Physical performance Polymer composites ResearchCreep resistance Steels Materials supply

Component production

System – Thermal insulation Low thermal conductivity Ceramic fibre, foam, ResearchMaterials Low volume hollow spheres, etc Materials supply

Component production

Pump materials Pump materials Fluid compatibility Polymers, stainless Research(corrosion, poisoning etc) steels, Ceramic Materials supply

linings/coatings Component production

Pipe work Pipe work Fluid compatibility Polymers Research(corrosion, poisoning etc) Steels Materials supplyGas permeability Coatings Component production

Thermal management Thermal management Fluid compatibility Steels Research(corrosion, poisoning etc) Coatings Materials supplyConductivity Component production

Overall Cost and quantity All above All abovereduction to drivecommersial readiness andvolumetric power density

6 All property sets should be ‘under use environment; i.e. temperature, chemical, physical’7 RRFCS spinel tube support8 CERES Power porous metal support

Table 2. Generic commercial fuel cell materials activities in the UK


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Contents

Introduction

8.1 The Market for Photovoltaics

8.2 Barriers to Adoption of PV

8.3 Description of Different PV Material Technologies

8.4 PV Materials Opportunities

8.5 Solar Heating Materials Opportunities

8.6 Eco-friendliness

8.7 Recycling and Sustainability


47

Solar energy8.0

Capture of solarenergy can take one oftwo forms - eithersolar thermal orphotovoltaic (PV).The former entailssolar heating of water,which is passedthrough a heatexchanger to heatdomestic water. Incontrast PV systemsgenerate electricity byabsorption of solarenergy in a chargegenerating material,such as asemiconductor, lightabsorbing dye ororganic conductor. Inthe UK both forms ofsolar energycontribute to micro-generation.

Large, stand-alone PVinstallations exist inCalifornia, Spain, Germanyand elsewhere but areunlikely to be consideredseriously in the UK. The UKapproach is to integrate solarenergy generation intobuildings and connect theresulting excess power intothe distribution network. PVis the only renewable energythat can be used effectivelyin the urban environment.Around 20% of our electricityneeds might be supplied byPV, but at this level ofpenetration, considerationwould need to be given todistribution managementissues and energy storage.Solar heating systems havebeen available for 20 yearswhereas recent rapidexpansion in global PVmodule production to over2GW in 2006 has had littleimpact in the UK market.

There is a ready supply of solarradiation in the UK. The UK annualsolar insolation figures are in therange 1000-1300 kWh/m2. In Spainthis rises to 2000 kWh/m2.Although, more southern climesoffer a higher annual insolationrates, the available solar energy inNorthern Europe is sufficient to meeta substantial proportion of ourenergy needs. The challenge is toconvert a significant proportion ofthis into electricity and manage thediurnal and annual variations inelectricity production. In the UK wehave sufficient unused surfaces onbuilding roofs and façades togenerate more solar electricity thanwe could utilize. Issues associatedwith implementing this technologymainly involve PV module efficiencyand cost. Once installed, PV systemsrequire little maintenance but thecapital cost of PV modules and thebalance of systems for networkconnection are significant barriers towider adoption.

Photovoltaic materials

Photovoltaics includes a range ofcurrent technologies:

- Well established crystalline silicontechnology

- Numerous types of thin-filmtechnology including:

- amorphous silicon

- compound semiconductors(GaInP/GaAs)

- polycrystalline compoundsemiconductor (CIGS, CdTe)

- Dye-sensitised (Gratzel) cells

- Newer nanocrystalline andpolymer technologies

There is a range of potentialsolutions to match mostrequirements including theimportant issue of reducing the cost[per peak-Watt] of PV-generatedpower. Cost is dependent upon anumber of factors including:

- Conversion efficiency

- Materials costs and yields

- Energy costs and throughput

- Capital costs.

For example, crystalline silicontechnology achieves relatively highconversion efficiency in productionat around 18% (more than half ofthe theoretical maximum limit), butmaterial costs are high comparedwith thin-film technologies. Purifiedsilicon feedstock is currently alimiting factor due to rapid growthin PV technology. Thin-filmGaInP/GaAs multi-junction cellshave high manufacturing costs butthese are offset by higher conversionefficiencies of 35-40%. Thistechnology is finding increasedinterest for concentrators interrestrial applications.Polycrystalline thin-filmtechnologies have lower conversionefficiencies (typically around 10%)and potentially lower manufacturingcosts. Some newer technologies haveconversion efficiencies of only a fewpercent but are potentially extremelycheap to produce. R&D into PVmaterials and manufacturingtechnology is helping the industry toreduce the cost per Watt tocompetitive levels.


48

Solar energy8.0

8.1 The Market forPhotovoltaics

Global Context

In 2006 manufacturer shipments ofPV cells were 1,982MW, a 41%increase over the previous year. TotalPV cell production was 2,536MW,indicating increased productioncapacity to meet demand. In 2000PV cell production was only at287MW. Between 2000 & 2006 therewas an average growth of 40% p/a.

Over the last 10 years, theproportion of global grid-connectedPV has increased from 29% to 78%,largely due to government supportprojects. This period has seen adramatic increase in installation ofgrid connected PV from a mere 34.8MW in 1995. Japan, Germany andUSA accounted for 81% of the newglobal PV installations in 2006. Notsurprisingly it is in these 3 countrieswhere the PV module manufacturingindustry is most vigorous.

UK context

The current level of adoption of PVin the UK has been boosted by BERRinitiatives such as the BIPVdemonstrator programme and PVmodules for domestic roofs but therate of installation is small comparedwith our major competitors. UKindustry does however have thepotential to take advantage of astrong PV materials R&Dprogramme. Sharp, the world’slargest producer of PV modules hasits European production facility inNorth Wales and UK production isbeing increased to 220MW per year.In addition UK has a thin film PVmanufacture at ICP Solar in Bridgendand a new £60M investment to builda dye sensitised solar cellmanufacturing plant in Cardiff.Cardiff is also the headquarters ofIQE plc, the world’s foremostcompound semiconductor epi-wafermanufacturer, who haveconsiderable expertise in materialsfor use in concentrator PVapplications. In the North East,Romag manufacture innovativemodules and NaREC have a siliconcell development facility that iscapable of small-scale manufacture.PV Crystalox in Oxfordshire is oneof the world’s major suppliers ofmulti-crystalline silicon for cellmanufacture.

8.2 Barriers to Adoption of PV

The cost of buying and installing PVmodules is the major barrier to wideradoption of this technology but thisis mitigated in adopter countriesthrough strong incentive schemes.To the cost of the module must beadded the balance of systems coststhat include the mounting structure,cabling, DC-AC inverter and trackerelectronics that enable users toconnect to the grid and use commonAC appliances. The relative cost ofthe module and the balance ofsystems depend on the size ofinstallation but as a guide, fordomestic installations the modulecost is roughly half the totalinstalled cost. Once the system is inplace, it is maintenance-free and thefuel is free. However, placing all thecosts up-front acts as a significantbarrier to installation of PV andmakes the cost per energy yieldsomewhat artificial. Annual energyyields can be calculated withreasonable accuracy but the initialcapital cost has to be amortized overmany years for economiccomparisons against currentdomestic electricity prices. Analternative perspective is to assumethat the module will produceelectricity within 20% of its initialrating over 25 years and use this asthe basis for calculating the energyprice. Figure 1 provides an analysisof PV energy prices compared withdomestic electricity prices fordifferent parts of Europe to assesswhen price parity might occur. Withincreasing production capacity andimprovements in module efficiencythere has been a steady decrease insolar PV energy prices from over 1€ /kWh in 1990 to the currentposition of 0.20-0.44 € /kWh,depending on location. PV solarenergy will become competitive inall parts of Europe. Timing willdepend on fossil fuel electricityprices and the crossover in PVenergy price, beyond which PVactually becomes cheaper.Predictions are that PV energy pricesin Spain should be competitive nowand in Germany in 2020.

Fig 1: Projected Reduction in PV Energy costs inEurope. Reproduced from the EuropeanPhotovoltaic Industry Association (EPIA) Roadmap.

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49

Much of the reduction in cost of PVmodules has been due to anincreased production capacity. Thinfilm PV systems should be cheaperthan silicon cells because of the lessexpensive semiconductor materialper m2 of panel and the lowertemperature processing costs.However, efficiency of thin film PVmodules is well below predictedtheoretical values at around 8-10%.This compares with crystallinesilicon modules at over 15%. In thecase of the most efficient singlecrystal silicon modules efficiencies of19.7% are reported [for recentlyannounced Sun Power modules].Hence thin film PV modules willhave to be produced at less than halfthe system cost of crystalline siliconPV in order to be competitive. Thesuccess of silicon PV production hasled to supply constraints of siliconfeedstock, which is unlikely to causeany long-term impact on prices butin the short term has caused a slightprice rise. The cheapest singlecrystal silicon modules sell for 3.23€ /W whereas the lowest price thinfilm module is significantly lower at2.25 € /W. Such price comparisonsmay stimulate more rapid expansionof thin film PV production.

Cost reduction is not just aboutproduction capacity but isfundamentally linked to materialsand materials processing. Thefollowing factors can be consideredin the contribution of materialsscience to cost reduction:

• Improve efficiency of energyconversion at module level.

• Reduced amount of costlysemiconductor materials andefficient materials usage.

• Use of cheaper materials.

• Cheaper and lower energyprocessing combined with highthroughput.

• Improved durability and productlife

8.3 Description of DifferentPV Material Technologies

Crystalline Silicon

Crystalline silicon modules are madefrom individual cells consisting ofeither single crystal silicon or multi-crystalline silicon wafers. Currentwafer thickness is around 200µm.Silicon single crystal andpolycrystalline wafers account foraround 90% of the world productionof PV modules. The need is to meetglobal demand, increase materialsutilisation and reduce cost. Twoapproaches are being adopted tosilicon cost reduction:

- Reduced silicon consumption perWatt peak (g/Wp) and:

- Reduced silicon cost per kg.

Both approaches could be combined.Additional cost reduction exercisesinclude:

a low-cost solar grade siliconfeedstock

b high-quality, low-costcrystallization

c high-yield cutting of very thinwafers

d thin-film wafer equivalents

The above approaches would havethe desirable effect of shorteningenergy payback time for crystallinesilicon modules.

Thin film PV

Thin film PV involves deposition offunctional layers onto a low costsubstrate such as architectural glass.Materials currently used for thin filmPV include amorphous andmicrocrystalline silicon, cadmiumtelluride and copper gallium indiumdiselenide. Recent advances in thinfilm technology and in the use of α-Si have led to α-Si thin film PVmodules becoming cost-competitiveas part of a wider mix of thin filmPV technology.

The common elements to all thinfilm cells are barrier layers,transparent conducting oxides(TCOs) and back contact layers.

As these are less well developed thancrystalline silicon there is the needfor production equipment to includein-line diagnostics and enhancedproduction processes to increasequality and yield. Reproducibilitymust also be enhanced, togetherwith development of suitableencapsulation processes for bothrigid and flexible modules. There isneed for more in-depthunderstanding of the roles of thelayers and their properties in the celloperation. The limitations ofjunctions are also not wellunderstood.

The current drivers for thin film PVsare:

a) Improved efficiency of thin filmPV modules.

b) Improved production throughputand yield.

c) Implementation of in situmonitoring and process control

d) Increased production scale.

e) Better understanding of modulelifetime issues.

f) Increased materials utilisation.

g) Incorporation of innovativematerials.

h) Improved characterisationtechniques, in particular for thinfilm polycrystalline materials.


50

Solar energy8.0

Concentrators

Concentrators are an attractivemeans of using relatively expensivehigh efficiency (40%) cells such astriple junction GaInP/GaAs/Ge solarcells. By optical concentration ofthe solar radiation, the area of thecells can be reduced by a factor of1000 hence reducing material costs.These modules are now seriouslybeing considered for terrestrialapplications and have the potentialfor low cost solar energy conversion.For UK applications theconcentrators would have toaccommodate a high degree ofindirect solar irradiance. The need isto achieve high light concentrationwithout mechanical tracking of thesun. The requirements are:

a) Optical design of lenses fromcheap materials such as plastics.

b) Development of efficient photo-luminescent concentrators andlight guiding to the PV collectors.

c) Development of improvedmethods for characterising opticalconversion materials forconcentrators.

d) Materials integration.

Excitonic cells

Excitonic cells are organic absorbermaterials although some, such as thedye sensitised (Gratzel cell) arehybrids with porous titania as theelectron collector. They are thin filmPV cells and can be deposited ontoglass, plastic or metal substrates. Aswith inorganic thin film PV, the TCOis an important part of the cellstructure. Current drivers are:

• Understanding the chargeconduction (excitonic) conductionmechanisms

• Replacing the liquid redox couplewith a suitable polymer anddevelopment of new p-typepolymers

• Effective utilisation of the solarspectrum

• Development and evaluation ofnew materials

8.4 PV Materials R&DChallenges

Crystalline silicon

• Low energy conversion of quartz-to-silicon (e.g. electrolysis)

• Advanced crystal growthtechniques (e.g. continuous-feedcasting or multi-zone refining)

• Silicon ribbon growth

• High efficiency cell structures

• Rear contact cells, enabling low-cost high-volume automatedassembly

• Module materials (e.g. polymers)for improved stability andlongevity

Thin film PV

• Alternative TCOs (including p-typematerials, higher mobility n-typematerials, light trapping)

• Materials and photonic nano-structures for lighttrapping/harvesting, includingoptimisation for large areas

• Cell concepts for flexible substrates

• Large area module productionequipment, designed for lowenergy, low materials usage

• Roll to roll processing for flexiblesubstrates

• High throughput testing andquality control for roll to roll andin-line processing

• Research into improved thin filmdeposition technology such asplasma deposition, MOCVD andelectro-deposition

• Sustainable new materials.

Concentrator PV

• Use of low-cost plastic optics forconcentrators

• Routes to high volumemanufacture of very highefficiency multi-junction cells

• Innovative concentrator designs tocapture light over a wide angularrange.

Excitonic PV

• Low bandgap semiconductingpolymers and sensitiser dyes

• Low cost replacement for ITO astransparent conductor

• Nanostructured organic donor/acceptor heterojunctions

• Low cost environmentalencapsulation (oxygen/waterbarriers on plastic substrates)

• Development of materialsmanufacturing technology for dyesensitised and polymer solar cells,to include monitoring and in-linematerials characterisation


51

8.5 Solar Heating MaterialsR&D Challenges

* Antireflective coatings

* Cheap thermal conductors

* Life-time testing of materials

8.6 Eco-friendliness

PV installations can be incorporatedinto buildings or installed as stand-alone devices. Incorporation intoeither small domestic or largecommercial/public buildings offersminimum impact on the urbanenvironment. PV does not require afuel supply chain and at point of usethere are zero emissions. However, asproduction capacity increases, moreconsideration will have to be givento materials processing and thepotential for toxic emissions andwaste. It will not be sufficient forthe finished product to beenvironmentally safe and chemicallyinert; consideration will also have tobe given to the whole materialssupply chain and its impact. Thekey aspects of the eco-friendliness ofPV are as follows:

* Integration into buildings does notrequire land area dedicated to PV.

* No fuel supply chain

* Potential to be attractivelyincorporated into the fabric ofbuildings.

* Consideration needs to be given tomaterials supply and managementof toxic materials in PV cell andmodule manufacture.

8.7 Recycling andSustainability

High volume production wouldreduce availability of materials usedin thin film PV production such asindium in some transparentconducting oxides. More abundant,alternative materials need to besought to ensure materials supply asproduction volumes increase. Costeffective recovery and recycling ofmaterials will become increasinglyimportant to ensure long-termsustainability. Consideration mustalso be given to avoiding disposalthrough landfill. Copper pipes insolar-thermal installations areefficient heat conductors butrepresent a high materials cost.Similar issues of materials recovery atend of life and sustainability applyto solar-thermal systems.

The lifetime of a PV module dependson the effective lifetime forgenerating electricity and itseffectiveness as a roof or façadepanel. The module is normallyguaranteed for 25 years whereas itsrole as a covering has to be effectivefor at least 60 years to becompetitive with existing buildingmaterials. These considerations raisethe issue of testing and designinglonger lifetimes for PV functions tomatch the design lifetime of thebuilding. The recycle time for thematerials would be extended as anatural consequence of greater PVdurability. A short- term solutionwould be to design PV systems thatwere easily replaced by a moreefficient and durable PV product,whilst recycling the older product.


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Acknowledgements

The Energy Materials Task Group ofMaterials UK wishes to acknowledgethe considerable efforts of theindividuals and organisationsinvolved in the preparation of thisreport and assisted throughwidespread consultation.

The views and judgements expressed in this report reflect the concensus reached by the authors and contributors and do not necessarily reflect thoseof the organisations to which they are affiliated. Whilst every care has been taken in compiling the information in this report, neither the authors orMatUK can be held responsible for any errors, omissions, or subsequent use of this information.

Printed on material produced at a mill certified to the environmental management standard ISO14001, from well managed, sustainable forests



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