Sustainability constraints on UK bioenergy development

Author's personal copy

Energy Policy 37 (2009)5623–5635

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Sustainability constraints on UK bioenergy developmentPatricia Thornley a,*, Paul Upham b, Julia Tomei ca Tyndall Centre Manchester and the School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester M60 1QD, United Kingdomb Tyndall Centre Manchester and Manchester Business School, University of Manchester, United Kingdomc Department of Geography, King’s College London, United Kingdom

a r t i c l e i n f o

Article history:Received 13 May 2009Accepted 12 August 2009Available online 16 September 2009

Keywords: Bioenergy Constraints Sustainability

a b s t ra c t

Use of bioenergy as a renewable resource is increasing in manyparts of the world and can generate significant environmental,economic and social bene tsfi if managed with due regard tosustainability constraints. This work reviews the environmental,social and economic constraints on key feedstocks for UK heat, powerand transport fuel. Key sustainability constraints includegreenhouse gas savings achieved for different fuels, landavailability, air quality impacts and facility siting. Applyingthose constraints, we estimate that existing technologies wouldfacilitate a sustainability constrained level of medium-termbioenergy/biofuel supply to the UK of 4.9% of total energy demand,broken down into4.3% of heat demands, 4.3% of electricity, and 5.8% of transportfuel. This suggests that attempts to increase the supply abovethese levels could have counterproductive sustainability impacts inthe absence of compensating technology developments oridenti cfi ation of additional resources. The barriers thatcurrently prevent this level of supply being achieved have beenanalysed and classi ed. fi This suggests that the biggest policyimpacts would be in stimulating the market for heat demand inrural areas, supporting feedstock prices in a manner thatincentivised ef cifi ent use/maximum greenhouse gas savings andtargeting investment capital that improves yield and reduces land-take.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Biomass is a renewable resource that isincreasingly being used to provide lowcarbon energy in the UK, Europe andworldwide. One of its key advantages overother renewable energy sources is that itcan be stored to provide energy whenit is required. Biomass is also flexible,

having the potential to service heat,electricity and transport fuel demands, withappropriate choice of feedstock andconversion technology. The mainsources of biomass are biodegradablewastes, forestry-derived material, purpose-grown energy crops, and agricultural by-products. It is important to ensure thatthese are used ‘sustainably’ and this

Author's personal copyinvolves consideration of the entire

bioenergy system (production, transport,processing and use) across a full range oftechnical, economic, environmental andsocial principles.

Given the different ways in whichsustainability can be construed and thepowerful role of stakeholders in shapingpolicy, resource assessment should not beonly a technical or economic exercise. Thispaper considers the main sustainabledevelopment issues associated with a broadrange of potential biomass feedstocks (bothindigenous and imported) that may be usedin the UK in the near future. Itparticularly highlights key technical,

* Corresponding author. Tel.: +44 (0) 161 306 3257; fax: +44 (0) 161 306 3255.E-mail address: [email protected] (P. Thornley).

economic, social and environmental areas of concern, and is intended to facilitateidenti cationfi of:

• fundamental constraints on the sustainably accessible UKbiomass potential;

• barriers to accessing the UK’s sustainable potential;• potential feedstock quantities availablefor UK consumption.

In this context, a constraint refers toa factor that limits the potentialavailability of a sustainably producedbiomass resource, and which we treat asinsurmountable from a sustainabilityperspective. We de nefi a barrier as anobstacle to accessing the potentialresource that, if overcome, will presentno signi cant fi problem in terms ofsustainability. Signi cantfi error margins anda degree of subjectivity are inherent inthis process and the values provided shouldonly be considered as illustrativeestimates. Nonetheless, the results providea policy-relevant basis for discussion.

2. Methodology

A list of feedstocks relevant to supplyingthe UK with biomass and biofuels wascollated through consultation withindustrial and academic partners in theEPSRC Supergen Biomass and BioenergyConsortium (www.supergen-bioenergy.net),who in-

0301-4215/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enpol.2009.08.028


5624 P. Thornley et al. / Energy Policy 37 (2009) 5623–5635

clude growers, technology providers andindustrial users with an understanding ofcurrent and future feedstock options. Thesemay not be fully comprehensive, butinclude the main options for UK futuresupply. The feedstocks chosen are shown inTable 1.

Production and sustainability literaturerelating to each feedstock was reviewedand input obtained from partners withcorresponding expertise. From these reviews,key environmental, technical, economic andsocial issues were identi edfi and expertjudgement used to further identifythe principal supply constraints likelyto be rstfi encountered from asustainability perspective (taking intoaccount ecological, social and economicaspects).

The key constraint for each feedstockwas then applied to estimate sustainableresource potential and an appropriateconversion ef ciencyfi used to convert theresource to the delivered energy potentialin the most likely demand sector based ondevelopment to date. Detail on theassumptions used to estimate quantitiesare appended as /Electronic appendixAS with principles discussed in the text.The accuracy of this is limited byavailable data and some subjectivityregarding different views of how the sectorwill develop. However, details of thecalcula- tions are provided in Appendix B, sothat readers can adjust them to re ectfltheir own perspectives or priorities.

For each feedstock, the key barriers toaccessing the sustainability constrainedpotential were then identi ed.fi Thefeedstocks were grouped by barrier andmapped so that the potential supply impactin different demand sectors ofaddressing different barriers could beidenti ed.fi

While the work was carried out from a UKperspective and the resource quantities arespeci cfi to the UK, the sustainabilityconstraints, particularly with respect toimported feedstocks, are equally applicableto other European member states and many ofthe barriers are relevant in widersettings.

Table1List of feedstocks consideredin the analysis.

Feedstock

3. Feedstockreview

3.1. Indigenous waste materialsand by-products

3.1.1. Mechanically and biologicallytreated waste (MBT)

MBT stabilises biodegradable organicmatter in municipal solid waste (MSW) tolimit methane emissions, reduceland ll fi leachates and, to reduce the volumeof waste deposited (Leikam and Stegmann,1995; Rieger and Bidlingmaier, 1995; Soyezand Plickert, 2002). MBT often incorporatessorting and separation, followed by aerobicor anaerobic biological processes thatconvert the biodegradable waste fractioninto a compost-like output and sometimesbiogas (Archer et al., 2005). A non-compostable fraction remains as a refuse-derived fuel (RDF). Energy recovery from theRDF fraction generally involvesthermochemical decom- position (combustion,gasi cationfi or pyrolysis) and recovery ofenergy via a steam-turbine, gas-turbine orgas-engine.

3.1.1.1. Sustainability issues. Recovery ofenergy from waste comes below other wastemanagement practices in the traditionalwaste management hierarchy. Its practicetherefore can only be con- sideredsustainable when nested within a wastemanagement framework with strongrecycling objectives. A signi cantfi en-vironmental issue is that recovering energyfrom MBT via thermal processing results inemissions of airborne pollutants, which arecontrolled under the Waste IncinerationDirective (WID) for larger systems or localauthority pollution control for smallsystems. Work by the Environment Agency(e.g., Environment Agency Wales, 2007)has concluded that waste managementoptions consisting of high levels ofrecycling and composting, followed by eitherenergy from waste approaches or MBT (orboth) perform well environmentallycompared to other waste treatment/dis-posal options over the full life-cycle.

3.1.1.2. Key constraints, potential andbarriers. Currently, UK energy from wastedevelopments are only economicallysustainable where the costs of alternativedisposal options (land llfi charges includingtaxes) are comparable to the gate feeneeded to make a

Author's personal copyIndigenous waste

materials and by-products

1. Mechanically and biologically treated municipal waste

2. Used cooking oil3. Waste wood4. Poultry litter5. Straw

plant viable. It is widely believed thatwaste disposal costs will continue to risein future and that this will make newplants more likely to be economicallysustainable. Social constraints on ac-ceptable sites for new developments willthen become the single most importantbarrier—in many areas this is already thecase. It is dif cultfi to estimate theextent to which MBT would then de-velop its potential and the approachadopted here is based onForestry-related material 6. Forestry residues

7. Arboricultural arisings and sawmill co- product

8. Wood pellets (indigenous and imported)

Indigenous energy crops 9. Short rotation coppice10. Short rotation forestry

11.Miscanthus

12.Switchgrass13. Reed canary grass

14. Oilseed rape15.

Wheat

Imported feedstocks 16. Jatropha17.

Corn/maize18. Palm

oil19. Palm kernel expeller

20.Soya21. Sugar

cane22. Olive

cake

projections of historic development rates.Given the current permitting framework and

social constraints on development, thepotential of this resource is estimatedat62.5 PJpa. The main barrier to achieving this is increasing the public acceptability of new facilities.

3.1.2. Used cooking oilUsed cooking oil is cooking oil that has

been used in food production and is nolonger viable for its intended use. It isideally suited to conversion to biodiesel viatransesteri cation,fi for which there areseveral plants existing in the UK. In2000, 1,461,409 tonnes of cooking oil wereused in the UK for human consumption inhousehold, industrial food processing andcatering (EUROSTAT in (SEPA, 2005)), 80,000tonnes per annum of which wereseparately collected and identi edfi fromcommercial establish- ments (AngliaPolytechnic University, 2003 in SEPA, SEPA,2005) after use; the remainder is consumedin cooking or ends up in the mixed wastestream.


P. Thornley et al. / Energy Policy 37 (2009) 5623–5635 5625

3.1.2.1. Sustainability issues. Thegreenhouse gas balance of biodiesel madefrom used cooking oil is very good, with70–75% lower greenhouse gas impact and 50%less total environmental impact than lowsulphur petrol (Zah et al., 2007), but onlywhen the used cooking oil is a genuine wastematerial and the economic sustainability ofthis feedstock is highly subject to thepolicy environment.

3.1.2.2. Key constraint(s), potential andbarriers. The main constraint for UCOis availability of a suitable andsustainably col- lectable waste resource. Anational free commercial collection servicealready exists, so expansion would involveextension to domestic collection, which hasbeen successfully trialled in some areas,but quantities are dif cultfi to estimate(Stagecoach Group plc, 2007). If domesticcollection optimistically matched com-mercial collection, then the resource is 6.3PJpa. The main barrier to achieving thispotential would be the development of acost- effective, domestic collectioninfrastructure.

3.1.3. Wastewood

Waste wood is routinely separated fromthe main municipal waste stream either atsource or by post-collection processes(Ryu et al., 2007). It can be used forgeneration of electricity and/ or heat,although pollution control may restrict itsuse at small scales.

3.1.3.1. Sustainability issues. Wood has arelatively low embodied energy compared toits calori cfi value, so recovery as a fuelcan confer greater greenhouse bene tsfi thanrecycling the material resource (ERM, 2006).Restricting airborne emissions, which canarise from heavy metals and halogens in thewood, is best managed by conversion insuitably engineered (usually larger) facil-ities. To maintain economic sustainabilitythe cost of separating out waste woodshould not exceed that of virgin material.

3.1.3.2. Key constraint(s), potential andbarriers. The cost of separating out thewaste wood stream is £20–47/tonne (Excelarltd.,2002); while virgin wood can be sourced for£30/tonne or less. This constrains theeconomically sustainable resource, whilepublic acceptability of new facilitiesconstrains the socially acceptable

resource.Our estimate of the socially

acceptable resource, based on numbers offacilities and rates of development, giveswaste wood supply level of 18.8 PJpa,similar to the economically sustainableresource calculated (Excelar ltd., 2002).The main barrier to developing thispotential is the high cost of developingenvir- onmentally suitable processingfacilities.

3.1.4.Poultrylitter

Poultry litter is the animal beddingmaterial (e.g., wood shavings, straw,shredded paper) and manure produced duringchicken production. Moisture content variesfrom 35% to 60% and there are signi cantfiquantities of important fertilisernutrients: N, P, K (Nicholson et al., 1996).

3.1.4.1. Sustainability issues. Poultryrearing takes place in large, dedicatedfacilities in rural areas, with particularconcentrations in Suffolk, Powys, NorthLincolnshire and East Yorkshire. The littermust be disposed of in an environmentallyresponsible manner, and is frequentlyspread on land to return nutrients to thesoil. However, this risks nitrateleaching, particularly in nitrate vul-nerable zones, and the release of nitrousoxides after land- spreading contributesto greenhouse gas emissions. Energy re-covery and incorporation of the residue inthe soil is more en- vironmentallysustainable. Increased energy recoveryis only

Author's personal copyeconomically sustainable for litter that

cannot be land-spread in the immediatevicinity (most likely because ofregulatory re- strictions) and for whichsome transport or disposal cost is in-evitable. Social concerns relate primarilyto siting, transport, and environmentalimpacts.

3.1.4.2. Key constraint(s), potential andbarriers. There is no signi cantfienvironmental constraint on theavailability of poultry litter, insteadavailability is dictated by constraints onthe economically viable quantity ofarisings, given dispersed locations andsocial acceptability of new plants. Thesetwo factors both originate in the patternof arisings density and are linked and bothpoint to new capacity only being acceptablein areas of suf ciently fi high productiondensity.

The dispersed nature of the resourceconstrains the viability of new facilitiesand we estimate the potential as 7.5 PJpa.Barriers to achieving this are that itis not economically feasible undercurrent policy with low levels of supportfor utilisation of wastes for energy and lowcosts for competing alternatives.

3.1.5.Straw

Straw is the naturally occurring residuefrom crop production, most commonly cerealsor oil seeds, which may be ploughed in tosoil, returning nutrients, or baled forstorage and further use.

3.1.5.1. Sustainability issues. Ploughingstraw into land facilitates the return ofnutrients and minerals to the soil. Whilethis could be compensated for by returningash to the soil after combustion, thiswould not contribute organic matter, whichin uencesfl soil organic carbon (SOC) content.Modelling work has shown that in a scenariowhere straw is removed from a eld,fi long-termdecreases in SOC would range from 2.5% to10.9% of initial SOC after 50 years. Thevariation depends on a number ofparameters, including soil, climate andproductivity (Saf h-Hdadifi and Mary, 2008).Baled wheat straw is typically sold at£20–25/tonne in the UK (Nix, 2006), butits use for energy is curtailed by thecost of transporting the low bulk densitystraw over long distances.

3.1.5.2. Key constraint(s), potential andbarriers. Environmentally, maintaining SOCcontent constrains supply to less thanhalf of that annually produced, if the SOCreduction above is considered acceptable.Where this is acceptable, it is only

economically viable in regions where largequantities are produced and competinguses are limited, e.g. areas where cerealfarming dominates over livestock farming.

With the above assumption, we estimatethat 18 PJpa may be sustainable, dependingon the variability and acceptability of SOClosses through reduced levels of ploughingstraw back into land. However, whilst thetechnology is relatively well-proven, therisks associated with the development arenot suf cientfi ly offset by nancialfi rewardsin the current regulatory market and thisis the main barrier.

3.2. Indigenous forestry-related materials

3.2.1. Forestryresidues

Most of the UK’s 2,800,000 ha ofwoodland is traditionally managed toprovide timber for a wide range of uses(Defra, 2006). Residues from this have nocurrent economic use and are usually lefton the forest oorfl to decompose.

3.2.1.1. Sustainability issues. As forestresidues decompose they return bothnutrients and organic matter to the soiland there is concern (Stupak et al., 2007)that removing these residues may impact onfuture soil quality and soil organic carbonsequestra-



tion, as well as biodiversity andsediment transport to water courses.Forestry residue processing is not adeveloped industry so economicsustainability is dif cultfi to assess, butestimates for Belgium have suggested 58–120euro/odt (Van Belle et al., 2003), which wasbroadly competitive with UK wood-fuel pricesat the time. Socially, the lack of an end-user market to match the scale andlocation of the resource pre-emptssustainable development.

3.2.1.2. Key constraints, potential andbarriers. The key constraint is therequirement to preserve ecologicalstability and then to con- sider economicsustainability with respect to existingmarkets in estimating the sustainablepotential. A total of 800,000 tpa or7.9 PJpa is considered economically andenvironmentally sustainable, but thereis no existing wood-fuel market for thisma- terial. Barriers to implementing thisare the need for substantialinfrastructure development for residuecollection and processing, although thesecould be addressed through suf cientfimarket demand, which is presentlycurtailed by the high capital cost ofheating systems (particularly domestic).

3.2.2. Arboricultural arisings andsawmill co-product

Management of trees in urban settingsproduces 341 ktpa arboricultural arisings,similar to forestry residues, and upto86 ktpa sawmill co-product could beavailable, taking into account competinguses (McKay et al., 2003).

3.2.2.1. Sustainability issues. Arisings arecurrently waste materials processedthrough the waste managementinfrastructure; the main environmentalissue would relate to airborne emissionsat the point of conversion, particularlyif feedstock contamination was likely. Thenature of the waste would requirecentralised, li- censed disposal facilities,and con ictsfl over siting could arise at thelocal level.

3.2.2.2. Key constraint(s), potential andbarriers. The key constraint is theavailability, siting and/or development ofsuitable recovery facilities to deal withthe waste resource. However, it shouldstill be feasible to develop the

relatively small 427 ktpa capacity re-quired. There is potential for up to 427ktpa, or 3.4 PJpa, to be used, distributedaccording to local availability. Thebarrier to the im- plementation iscollection and processing of the resourceeco- nomically competitively.

3.2.3.Pellets

Pellets are compressed, densi edfi pieces ofbiomass, which can be derived from avariety of sources, includingagricultural residues and energy crops, butare most commonly made from virgin wood. Ingeneral pellets are produced by compressingne fi sawdust in a die, so that the heatand pressure generated melts the ligninin the pellets and binds theparticles together. Sometimes binders maybe added, although not generally fordomestic use (Stewart, 2006). Wefocus here only on the additional(internationally sourced) resource thatcould be available to the UK, asindigenous material has already beencovered in the other categories presented.

3.2.3.1. Sustainability issues. Environmentalsustainability at point of export is onlyassured if the wood is sustainably sourced.Pre- sently most internationally tradedpellets are manufactured from sawdust fromtimber operations that are certi edfi as‘sustainable’. Substantial capacity existsin Canada and North America to re- spondto increased demand, which would mostlikely be met by accessing forestryresidues. This raises similar issues tothose discussed in Section 3.2.1, but isnot expected by experts to constrain avery substantial growth in European (andUK) pellet im-

Author's personal copyports (Junginger et al., 2009). The social

response to increased pellet utilisationis likely to be positive, as it has been inSweden and other countries, where it isseen as a convenient, accessible fuel. Thisis unlikely to constrain UK utilisation.Large-scale switching to imported pellets asa heating fuel would introduce new securityof supply risks. The extent to which thesebecome material would likely depend on theUK demand as a proportion of globalproduction. However, a key issue withdomestic use is local air quality impactsand these are likely to constrain devel-opment in urban areas.

3.2.3.2. Key constraint(s), potential andbarriers. The impact of airborne emissionsis a key geographical constraint, which islikely to constrain national consumption atindividual domestic level to rural areaswith existing oil/coal heating.

Assuming that half of all UK domestic oiland coal heating is replaced by pellets fromsustainable forests, the potential could be5 Mtpa or 72.0 PJpa. The main barrierto this is the lack of economicincentives, particularly with respect toinitial capital costs.

3.2.4. Importedwood chips

While not yet a major contributor to UKenergy demand there is increasing interestin the utilisation of internationallytraded wood chips, often from forestryoperations. The sustainability constraintsrelated to these will depend on what typeof material is being imported and itsorigin. The sort of issues likely to ariseare net greenhouse gas balance aftertransportation, sustainability of forestryoperations and ecological impacts onforestland and biodiversity. These can onlybe assessed for speci cfi feedstocks/countries of origin and so are not dealtwith in this paper. It is acknowledgedthat they represent an additional resourceto those discussed herein, but one whichmust be carefully assessed from asustainability perspective.

3.3. Indigenousenergy crops

All energy crops involve devoting land toproduction of fuel rather than food andmany of the crops below will be competingfor the same land area. Hence, the crops areconsidered separately below, to highlighttheir particular sustainabilityconsiderations, but the potential isconsidered for all energy crops combined.

3.3.1. Short rotation

coppice (SRC)Short rotation coppices are fast-growing

trees, whose growth is accelerated by severecutting back (coppicing) after 1–2 years.The whole trees are then regularlyharvested on a rotation every 2–5 yearsduring a lifetime of 20 years or more.

3.3.1.1. Sustainability issues. SRC canoffer sustainable environmental bene tsficompared to arable land, but fewercompared to grassland, provided potentiallynegative hydrological effects are managedthrough appropriate site selection (Rowe etal., 2009). SRC production can assistdiversi cationfi in rural areas, with de-velopment of transport and conversion jobsand associated infrastructure, but willnot create agricultural jobs at a levelcommensurate with arable farming.Economic constraints are governed by theextent of the perceived pro tfi marginrelative to crop growth. High cereal pricesact against this and the signi cant fi up-front establishment costs and long-termcommitment required deter even whenthere is an economic case.

3.3.2. Shortrotation forestry

Short rotation forestry involves plantingtrees, which are harvested for their stemwood only after 8–20 years (Hardcastle,



2006), compared to conventional forestryrotations of 40–150 years (Biomass EnergyCentre, 2007).

3.3.2.1. Sustainability issues. A study onthe potential for short rotation forestryin the UK by the Forestry Commission in2006 (Hardcastle, 2006) concluded thatthere were no serious issues relating tobiodiversity, soils, hydrology, pests,disease and land- scape that would ruleout short rotation forestry as apotential land use. However, SRF isecologically inferior to existing nativeforests, so development should beadditional, and should not displaceexisting native forests. Negative impactsare more likely with higher yieldingspecies although good practice, includinglimiting planting to hydrologicallysuitable areas, can mitigate these. Thelong-term investment involved makes this anunlikely prospect for farmers and privatelandowners.

3.3.3.Miscanthus

Miscanthus is a perennial rhizomatousgrass, similar in appearance to bamboo orsugar cane. It has tall, thin, woodystems, which grow rapidly and can maintainproductivity with annual harvest (El Bassam,1998).

3.3.3.1. Sustainability issues. Thegreenhouse gas balance for miscanthus isgenerally found to be quite positiveunder realistic conditions in the BritishIsles (Styles and Jones, 2007; Le-wandowski et al., 1995). Fertiliserrequirements and potential for leaching arelow and there are few negative impactson soil quality. Studies of thebiodiversity impacts of miscanthus com-pared to other crops have raised fewconcerns (Semere and Slater,2007a, 2007b) but the European MiscanthusImprovement Pro- gramme (2000) recommendedthat sterile varieties be cultivated on anon-going basis to prevent them becomingweeds, which results in high propagationcosts. The crop is not familiar in the UK,giving rise to visual impact issues.

3.3.4.Switchgrass

Switchgrass is a perennial, cool-seasongrass, growing up to 2.5 m tall, which isnative to the United States, commonly in wetareas.

3.3.4.1. Sustainability issues. The energyratios for switchgrass production aregenerally very positive (Bullard andMetcalfe,2001) and there are particular biodiversitybene tsfi for pheasants, quail and rabbits.Agro-chemical inputs are low (Anon., 2004)and nitrogen leaching rates and soilerosion rates are both low compared toarable crops (Bullard and Metcalfe, 2001).The main issue with switchgrass is that itis not well-known or well-proven in the UK.Preliminary estimates of production cost are62 euro/tonne in northern Europe (Elbersenet al., 2004), which is competitive withcurrent UK wood-fuel prices.

3.3.5. Reedcanary grass

Reed canary grass (RCG) is a robust,coarse perennial, widely distributed acrosstemperate regions of Europe, Asia andNorth America, frequently in wet places, e.g.along river beds.

3.3.5.1. Sustainability issues. Nutrientrequirements for RCG are

Author's personal copyappears to be less than miscanthus and

switchgrass and therefore costs ofproduction are higher (Riche, 2005.)

3.3.6. Key constraints and consolidated constrained potential for energy crops

For all the above energy crops thekey constraint is the availability ofsuitable land. This is assessed indetail in the electronic annex and takesinto account the different types of landrequired and competing pressures. Thepotential considered‘sustainable’ issummarized in Table 2.

3.3.7. Barriers forenergy crops

The main barrier to SRC uptake is thatfarmers will not make the initialinvestment when the balance of long-term costs remains uncertain compared toarable farming. The main barrier to shortrotation forestry is also a lack ofeconomic incentives. Forestry Commissionstudies (Hardcastle, 2006) show that areasonable internal rate of return oninvestment can be obtained only withsubstantial grant funding of £2000 or more.Similarly, for miscanthus the main barrieris the availability of nancial fi supportfor initial costs. The main barrier forswitchgrass is its long-term economicviability, coupled with the less provennature of this dif cultfi to establish crop(Riche, 2005). The main barrier for RCG isthat its lower energy ratios make it lesssustainable than other energy grassesexcept where it is being used to treatwaste water.

3.5. Indigenous energycrops—other

3.5.1. Oilseed rape

Oil seed rape (or canola) is a brassica,which can be cultivated in cooleragricultural regions. UK oilseed areauctuatedfl between

400,000 and 500,000 ha from 1993 to 2003(Booth et al., 2005) and is currentlyaround 600,000 ha of oilseed rape, mainlyused for food products.

3.5.1.1. Sustainability issues. Oilseed rape(OSR) is a nitrogen-de- manding crop, whichimpacts signi cantfi ly on its greenhouse gasbalance for biodiesel production. Italso has a higher nitrate leaching riskthan cereal crops. Pesticide use and itsimpacts are moderate compared to other foodcrops, though higher than for other energycrops, but winter OSR supports morebiodiversity than winter cereals. OSR area inthe UK has uctuafl ted in line with economicincentives in recent years, providing someindication that UK production is noteconomically sustainable without some degreeof intervention.

3.5.1.2. Key constraint(s), potential andbarriers. The overall carbon balance forbiodiesel production from OSR usingexisting technology is quite poor and anypreference for better performing feedstocksin terms of carbon reduction potentialwill constrain OSR contribution tobioenergy.

Accordingly, we assume no newinvestment in UK crush capacity beyondthe current 27.5 PJpa and treat this as anupper

Table2UK energy crop sustainablepotential.

higher than other energy grasses discussedhere and the crop has been used to catchnutrients in land treatments of waste water(Ge- ber, 2002). Little is known aboutimpacts on biodiversity, but there are

Energy resource at source (TJ)

Energy resource(Mtpa)

concerns about invasiveness. Recorded yields in the UK to date (Riche,

Crops on contaminated orpublic land

4160 0.21

2005) have been poor, which would result ina poor energy balancecompared to other options on the sameland. RCG is cheap and reli- able toestablish but requires nitrogen fertilizerfor full yield potential to be reached andalso can suffer from pest attack. Full yieldpotential

Crops on agricultural land 54,520 3.46Crops for sewage treatment 940 0.05

Total 59,620 3.7



limit on supply. The barrier to achievingthis will be the extent to which productioncosts can be kept low.

3.5.2.Wheat

In the UK climate, wheat has beenpreferred as a bioethanol feedstock, sinceit produces more harvestable starch thanany other UK crop, and UK wheat yields areamongst the highest in the world (Smith etal., 2005). In 2005/2006 nearly 1.9million hectares of wheat were grown inthe UK, producing nearly 15 million tonnesof wheat grain; 2.4 million tonnes ofthis were surplus and available for export.

3.5.2.1. Sustainability issues. The keyenvironmental issue is that greenhouse gassavings from wheat-ethanol production arehighly variable, depending on nitrous oxideemissions during cultivation, the fuel usedin the ethanol production plant, the fateof by-products, and the type of land used(Bo¨ rjesson, 2009). Land-use ef- ciencyfi isquite low, with current benchmarks at 3220l/hectare (Smith et al., 2005). Ecologicalperformance and environmental ef ciencyfi arealso low relative to other perennialenergy crops used for electricitygeneration.

3.5.2.2. Key constraint(s), potential andbarriers. The primary constraint is theavailability of suitable arable land for fuelwheat, which is likely to result in a lossof other food production capacity. A theo-retical potential of 15.8 PJpa isestimated. The main barrier is eco- nomicwhen wheat prices are high as they have beenin recent times.

3.6. Importedfeedstocks

3.6.1.Jatropha curcas

Jatropha is a tropical plant that can begrown in areas of low or high rainfall,that may also be grown on marginallands, as a hedge or a commercial crop(Openshaw, 2000). Inedible oil can beextracted from the seed, with propertiessimilar to those of palm oil, which can beused as biodiesel.

3.6.1.2. Sustainability issues. Littleexperience of cropping Jatropha exists,making its sustainability dif cult fi toassess. It has been welcomed as a crop that

can be cultivated in arid conditions, buthigher yields almost certainly requireirrigation. Biofuelwatch et al. (2007)express concerns about potentialdeforestation and biodiversity impacts inIndia and the Himalayas. However, Ja-tropha is being grown by small-scale farmerson a contract basis and as such may help toreduce rural poverty (Fairless, 2007).

3.6.1.2. Key constraint(s), potential andbarriers. Land-take and land-use changewill be the most signi cantfi constraints,particularly if reasonable yields arenot obtainable on the arid land beingallocated in many national programmes.

Assuming that the UK obtains a proportionof the global total of Jatropha-derivedbiodiesel that re ecfl ts its oil consumptionas a percentage of global oil consumption,62.2 PJpa may be available. If the crop candemonstrate a justi ablefi yield, then theavailability of investment capital is thebiggest barrier. However, although Jatrophais touted for its relatively positiveenvironmental and social performance, it istoo early to assume this and the abovevolume should be treated as optimistic.

3.6.2.Corn/maize

There are hundreds of varieties ofcorn/maize, which is a tall annual cerealgrass bearing kernels on large ears,grown for human and animal consumption. Itis the single largest US crop and amenableto monoculture (Finke et al., 1999) but isnot at

Author's personal copypresent commercially grown on a large scale

in the UK owing to climate suitability.

3.6.2.1. Sustainability issues. Maize maycause soil erosion on hill- slopes and canrequire irrigation, especially in lowrainfall areas. Life-cycle analyses ofgreenhouse gas savings from corn ethanolare variable, and it is fair to say thatthe greenhouse gas bene ts fi are morecontested than for many other crop-fuelcombinations (Shapouri et al., 2002). Aswith other crops, any clearance ofgrassland for cropland will be unsustainableboth ecologically and in terms of GHGrelease.

3.6.2.2. Key constraint(s), potential andbarriers. The relevance of maize as a UKfeedstock will be from imported material,most likely corn-derived ethanol, since itis not well-suited to the UK climate.However, this supply will be constrained bythe extent to which corn- derived ethanol(which meets the minimum RTFO greenhouse gassaving requirements) is available viainternational trade. To date no corn-derived ethanol has been submitted underthe RTFO and, as the threshold forgreenhouse gas compliance is set to increaseto a level where this feedstock-technologycombination will not necessarily becompliant, it is dif cfi ult to envisagesignificant quantities even when or if USexport capacity increases.

3.6.3.Palm oil

The African oil palm is cultivatedthroughout the tropics but predominantly inMalaysia and Indonesia. Ten percent of thetotal dry biomass is in the form of twoglobally important vegetable oils: crudepalm oil and palm kernel oil. Productionand trade in both has increased from 3 to44 million tonnes in the last 30 years (FAO,2008).

3.6.3.1. Sustainability issues. Productionincreases have required rapid expansion ofthe area planted under oil palm (Carter etal., 2007), giving rise to serious concernsover deforestation and habitat loss as wellas the drainage of peatlands, (e.g.,Wakker, 2004; Greenpeace,2007). Oil palm has also been asignificant source of land tenure con ifl ct,particularly in Indonesia (Colchester et al.,2006).

3.6.3.2. Key constraint(s), potential andbarriers. Palm oil demand is dominated bythe food and cosmetic industries;biodiesel is a small but growing proportionof global demand. Although there is some

scope for increased yield, any substantialdiversion from traditional markets tobiodiesel will reduce reserves and increaseprices (Murphy, 2007); moreover, it willdrive land-take and forest clearance forincreased plantation areas. In thesecircumstances, a sustainable increase insupply for biodiesel is practically im-possible. In theory, palm oil forbiodiesel could be taken from existingproduction quantities and new capacityachieved by development on previouslycultivated land and/or via yield im-provements. However, even if limited tothe above in-principle sources, anyincentivisation of palm oil-derivedbiodiesel for the UK market could provide anindirect stimulus for land clearance.

We estimate a theoretical potential of2.5 PJpa for the UK on the basis of yieldimprovements and a small percentageincrease in use of existing production.However, for the above reasons, we do notadvocate incentivisation of palm oil-derived biodiesel. Even with site-speci cfiveri cationfi of the previous carbon storeand the future sequestration potential ofthe plantation to ensure ecologicalsustainability, indirect displacement ofproduction is possible. The main barrier toobtaining the theoretical potential iscompetition with other uses.

3.6.4. Palm kernelexpeller

Palm kernel expeller (PKE) is a by-productof the production of palm kernel oil. Onehectare of oil palm typically producesaround



4 tonnes of crude palm oil and 0.4 tonnes ofPKE per year (Carter et al., 2007). PKE hastraditionally been used in the formulationof animal feed, but an emerging marketis as a solid biomass feedstock.

3.6.4.1. Sustainability issues. Thesustainability issues are similar to thosefor palm oil, except that this material isnot in demand as an edible or industrialoil, so availability may be higher andprice lower.

3.6.4.2. Key constraint(s), potential andbarriers. As for palm oil (Section 3.6.3),this feedstock is only sustainable to theextent that it can be procured withoutincurring additional land-take thatresults in loss of natural habitats,biodiversity or existing carbon sinks.

On the basis of yield improvements, weestimate a potential increase in PKE forthe UK market at 7.5 PJpa, with the mainbarrier to obtaining this competingmarkets, mainly animal feed production.Again, we do not advocate such anexpansion for reasons of indirect impact.

3.6.5.Soya

Soybean is one of the world’s mostimportant sources of edible oil and high-quality plant protein for both humans andanimals. Demand for soymeal for animal feedhas increased dramatically since the1970s, reaching 130 million tonnes in2002, and has replaced soy oil as theprincipal driver of soybean production(FAO, 2008). By 2010, Brazil and Argentinacombined are projected to surpass the US tobecome the world’s leading producers of soy(Mathews and Goldsztein, 2009).

3.6.5.1. Sustainability concerns. Agriculturalexpansion, dominated by soy, is a keydriver of deforestation and habitat lossin Latin America (Grau et al., 2005).Agricultural intensi cfi ation, which hasincreased yields, has favoured large-scaleproduction, intensive use of pesticides andwidespread adoption of GM soy. Large-scaleproduction offers fewer employmentopportunities, and con- centrates landownership and income. The intensive use ofagrochemicals is associated with negativehealth impacts on local communities (GRR,2009). In addition, the greenhouse gasbene- ts fi are uncertain where land changeis involved. DfT (2008) gives worst-casevalues for CO2e payback periods for

grassland cleared for soy as 132 years inArgentina and 609 years in Brazil.

3.6.5.2. Key constraint(s), potential andbarriers. Achieving worth- while greenhousegas reductions without additional land-usechange is extremely challenging andconstrains development to within existingagricultural land. Despite this there arehugely ambitious expansion plans in manyLatin American countries.

Dros (2004) provides a scenario in whichglobal demand for soy in 2020 could beachieved with limited land-use change,under a soy-cattle rotation and no/low tillsystem. However, Dros acknowledges that manysocial issues related to soy cultivationand expansion are not addressed by thismodel and therefore it may not besocially sustainable. In recognition ofthis the sustainable potential has beenlimited to the existing supply level of 21.1PJpa. The main barrier will be meetingsustainability requirements.

3.6.6. Sugarcane

Sugarcane is widely considered to be oneof the world’s most important economicplants (Husz, 1989 in El Bassam, 1998).Sugarcane is produced in 103 countries, and15 countries devote25% or more of their land to its production(Clay, 2007). It grows

Author's personal copybest in the tropics and sub-tropics and

within this region speci c fi cultivars arebred to suit climatic variations (El Bassam,1998).

3.6.6.1. Sustainability issues. Currentproduction of ethanol in Brazil is veryef cientfi in terms of greenhouse gasreductions; however, further expansion intothe savannah would incur land- use changecarbon debts resulting in payback times ofup to 88 years. Other environmentalimpacts of sugar cultivation includeintensive water and agro-chemical use,and air pollution from eldfi burning(Fritsche et al., 2006). Social impactsinclude health impacts from the use ofagrochemicals, poor working conditions andland ownership concentration (Ferm,2007). Ethanol from Brazilian sugarcane isby far the cheapest biofuel availabletoday; yet, more than half of theproduction cost is determined by the priceof the feedstock, which has risen inBrazil in response to increased demand(Doornbosch and Steenblik, 2007).

3.6.6.2. Key constraint(s), potential andbarriers. The key constraint is the extentto which additional quantities can besupplied with effective greenhouse gassavings, i.e. without incurring signi cantfiland-use change. In Brazil (the biggestproducer) the government- run EnergyResearch Corporation believes that thecurrent production area of 7 Mha could bedoubled by converting degraded pastureland that would not threaten foodproduction or un- converted ecosystems(Hirsch, 2008).

We have estimated a potential of 15.6PJpa, based on Brazilian governmentstrategy, but the carbon stocks of thesesoils will determine the actual extent towhich this is sustainable. The main barrierto achieving development of the sustainableproportion is the extent to which the fuelcan be produced cost-effectively on thisnew land.

3.6.7.Olive cake

Olive residues are a by-product ofolive and olive oil production, whichmay be combusted to provide renewableenergy (Zabaniotou et al., 2000).Their physical properties facilitate easyco-milling and they are one of the mostcommon fuels for biomass co- ringfi in the UK(Woods et al., 2006).

3.6.7.1. Sustainability issues. The disposal

of olive residues has long been anenvironmental concern for the industry,which is alle- viated by their use as anenergy resource. However, their removal mayresult in loss of soil nutrients inplantations that currently use residues asfertiliser. In the UK, any social concernsabout the use of olive residues willlikely relate to the potential negativehealth impacts from the incineration ofwaste and transport-related emissionsand nuisance. The economic pro tabilityfi ofcurrent processes for the disposal ofolive residues is uncertain and, as aresult, there is growing interest in addingvalue to residues by using them to produceenergy (Celma et al., 2007).

3.6.7.2. Key constraint(s), potential andbarriers. As a by-product, the greatestconstraint on the supply of residues isolive production and the requirement toretain some residues to preserve soilquality. We assume 50% of residues are notavailable and that the UK secures a sharein line with its proportion of Europeanelec- tricity consumption. This gives apotential of 4.0 PJpa at source, or1.5 PJpa if converted to electricity inco- ringfi plant. The main barrier toachieving this potential is economic:suppliers in the UK being in a position topay the required price to producers,parti-



cularly in light of changes to the Renewable Obligation for co- ring.fi

4. Discussion

4.1. Sustainability constraints

Fig. 1 shows a mapping of the diversesustainability issues that constrain UKbiomass resources allocated to the principaldimensions of sustainable development(economic, ecological and social). These arebased on existing technology, marketconditions, etc., and represent afundamental constraint on sustainabledevelopment. The main options formanipulating these to increase thesustainable resource base are discussedbelow.

The most common ecological constraint islimitations on greenhouse gas savings(particularly when land-use change isinvolved). This indicates a signi cantfiresearch priority to enhance greenhouse gassavings and to improve the understandingand accuracy of accounting for land-usechanges. It also points to the need forpolicy instruments, designed to increasebioenergy utilisation, to ensure thatlong-term incentives to maximize carbonsavings are put in place. These measuresare essential to increasing the long-termsustainable contribution of bioenergy.

Two further ecological constraints relateto maintaining soil fertility and qualitywhile utilising straw and forestryresidues. This is an area for which littleexperience exists and monitoring would beneeded to establish the maximum offtakethat can be sustained in practice if thisfeedstock utilisation increasedsubstantially. There is also a need tosafeguard against increased fertiliser andsoil improver use to compensate for residueremoval.

The constraint related to air qualityis one that has already been raised inrelation to bioenergy developments in someurban areas. The key to expanding thisconstraint to maximize supply levels is tofocus bioenergy development (particularlysmall-scale heating) on existing coaland oil users. Some targeted policysupport measures could help here.

Land availability and land-take is asocial/ecological constraint for a number ofimported and indigenous energy crops.Maximiz- ing the bioenergy output withinthis constraint condition requiresprioritisation of high yielding feedstocks inappropriate agro- ecological zones.Traceability of this is a key issue. However,it also involves complex issues of landallocation, for which many producercountries have no enforceable strategy, e.g.relative prioritisation of food crops,energy crops, grazing, forestry, recreationalland, etc.

The public acceptability and siting ofnew facilities is a key constraint if weare to expand capacity of wasteresource utilisation. This may be adif cultfi problem to resolve.

Ecological

Straw – need to return organic matter to soil

OSR – need to improve GHG balance on existing land

Corn – need to improve GHG balance on existing land

Imported pellets –local air quality

Forestry residues – maintaining forest ecology

Sugar cane – GHGbalance on new land

Jatropha – land take &land-use change

Energy crops – availability of

Author's personal copysuitable land

Economic

PKE – demand from competing uses

Soy – land availability foracceptable greenhouse gas savings

Wheat – competition with food production

Palm oil – demandfrom competing uses

Olive residues –continued demand forand production of olive oil

Waste wood –cost ofseparaiingwaste woodstream

UCO – resource availability

Poultry litter –arisings density

Arboricultural arisings– availability/siting of facilities

Waste wood – public acceptability of new facilities

MBT waste – facility siting

Social

Fig. 1. Sustainability constraints for UK biomass resources, grouped by the most relevant sustainability dimension (economic, ecological, social).



Table 3Sustainability constrained UK biomass potential.

Feedstock Energetic content of resource (PJpa)

Energetic content delivered as electricity (PJpa)

Energetic content delivered as heat (PJpa)

Energetic content delivered as transport fuel (PJpa)

MBT 62.5 14.9Used cooking oil 6.3 5.9Waste wood 18.8 4.5Poultry litter 7.5 1.8Straw 18.0 5.0

Forestry residues 7.9 7.1Arboricultural arisings 3.4 3.0Sawmill co-product 1.5 1.4Wood pellets 81.3 73.2

Short rotation forestry—contaminated or public land

Miscanthus, switchgrass, SRC onagricultural land

4.2 3.7

38.9 34.9

Reed canary grass 0.9 0.8Oil seed rape 27.5 26.0Wheat 15.8 14.9

Jatropha 62.2 58.8Palm oil 2.5 2.3PKE 7.5 2.6Soya 21.1 19.9Sugar cane 15.6 14.7Olive cake 4.0 1.5

Total constrained bioenergy supplied

407.1 30.4 124.2 142.5

Total sector demand 705 2914 2,472

Percentage of sector serviced by bioenergy 4.3% 4.3% 5.8%

Energetic content ofresource (PJpa)

Olive cake, 4

Sugar cane, 15.6

Soya, 21.1

PKE, 7.5

Palm oil,2.5

MBT, 62.5

Used cooking oil, 6.3

Jatropha, 62.2

Wheat, 15.8

Waste wood,18.8

Poultry litter, 7.5

Straw, 18

Forestry residues, 7.9

Arboricultural arisings, 3.4

Sawmill co-product, 1.5

Oil seed rape, 27.5

Author's personal copyReed canary grass, 0.9

Miscanthus, switchgrass, SRCon agricultural

land, 38.9

Short rotation forestry, 4.2

Wood pellets, 81.3

Fig. 2. Breakdown of constrained UK biomass resource potential by feedstock origin.

Electricit

y deli

vered



Resource availability is the keyconstraint for UCO and the arisingspattern for poultry litter. Little canbe done about the former, but the lattercould be addressed by support fordemonstration of the feasibility ofsmaller-scale technologies, includinganaerobic digestion.

Only one feedstock (waste wood) isdirectly constrained by cost. Given thatthis cost is equally applicable to recyclingof that waste stream, targeted supportwould again seem sensible.

The internationally sourced feedstocksof palm oil, PKE and olive residues areall constrained by competing markets andin some cases this will be reflected in highfeedstock costs. There is therefore a needfor caution in targeting economic supportto expand these resources, taking intoaccount the impact on these markets.

4.2. Constrained UKbiomass resource

The total raw biomass resource availableto the UK when the constraints in Fig. 1are applied is summarized in Table 3 as407.1 PJpa or 113 TWh, which equates to6.7% of total UK energy demand (DTI/BERR,2007). The UK Environment Agency (2009)recently estimated a 2020 bioenergy usage inthe UK as 80 TWh, a level that we estimatecould be met through sustainable sources.

Fig. 2 shows the contribution ofdifferent feedstocks to this constrainedpotential. It shows a diverse picture,with the most signi cantfi contributions fromwood pellets, MBT waste, Jatropha, UKperennial energy crops, UK oil seed rape andsoya oil. Each of these diverse resources isconstrained by very different issues andavoiding over-exploitation of sectorswill require careful

160.00

140.00

120.00

100.00

80.00

60.00

40.00

20.00

0.00Transport - energetic

at point of delivery

Heat - energetic at point of delivery

Electricity - energetic at point of delivery

Olive

cake

Sugar

cane Soya

PKE Palm oil Jatropha Wheat

Oil seed rape

Reed canary grass

Author's personal copyShort rotation forestry - contaminated or public

land

W

o

o

d

p

e

llets Sawmill

co-product

Arboricultural

arisings

Forestry

residues Straw

Poultry litter

Waste wood

UCO

Miscanthus/switchgrass & short rotation coppice - agricultural land

MBT waste

Fig. 3. Constrained biomass energy delivered disaggregated by demand sector and feedstock.



management and control, which is not provided by current market-focused UK policy incentives.

4.3. Bioenergy delivered in end-use sector

Fig. 3 shows the resultant bioenergydemand that can be serviced in each ofthe transport, electricity and heatsectors by utilizing the constrainedresource with existing technologies andthese gufi res are also presented in Table3. For simplicity it has generally beenassumed that a particular feedstock willpredominantly serve one demand sectorbased on market patterns that haveemerged to date. However, these patternsmay change and this would affect thequantity of energy delivered.

Using this approach provides estimates of5.8% of current UK transport fuelrequirements, 4.3% of heat demand and 4.3% ofelectricity demand. Using thesustainability-related judgements madehere, which could be made even moreprecautious, increasing the level of biomassutilisation above this level would only beachievable with a signi cantfi change inmaterial impacts, technology development orscienti cfi knowledge. For example improvedyields, development of second generationtechnologies, or changes in ourunderstanding of soil carbon balances couldall make a difference to this resource.

4.4. Barriers to accessing the constrained resource

While there is potential to sustainablyservice the demand levels referred toabove, there remain substantial barriers todoing so, and it is important thatgovernment policy objectives are focused onthese in order to achieve sustainableindustry expansion. The most signi cantfibarrier for each feedstock is identi edfi inthe relevant section above. All feedstocksto which the same barrier is assigned arecollated and assigned a weighting that isproportional to the sustainable resourcepotential that could be released byaddressing that barrier. This issubdivided according to the sector inwhich the resource would be applied (heat,electricity, transport fuel). The result isa quantitative indication of thesustainable potential in each of the 3demand sectors that is linked to theidenti edfi barrier.

Previous work on bioenergy barriers hadused literature review and workshops withEuropean experts to map barriers tobioenergy in Europe (Thornley and Prins,2009) according to their nature asstructural, market, interaction orperformance barriers. For this work the sameframework and classi cationfi was used to mapthe barriers identi edfi in this work asillustrated in Fig. 4. The area of eachcircle in Fig. 4 is proportional to thedelivered energy potential linked to thatbarrier and the colour of the circle iscoded according to whether the barrierrelates to transport, electricity or heat.This gives a clear indication of whichbarriers are most

Development of cost-effective collection infrastructure

Structural

Market

Insufficientoperational costsupport

Insufficient financial start-up support

Lack of heatmarket demand

Social issues associated with soy-cattle rotation

Investment risks not outweighed byrewards

High cost of

processing f

Author's personal copya

ci

lit

ies

Performance

High feedstock market prices

Need forinvestment capital

IimprovingGHG savings

Competitionwith other uses

Public acceptability of new facilities

Developing synergies with waste water treatment industry

Transpo

rt

Electri

city

Heat

InteractionFig. 4. Barriers to achieving constrained bioenergy supply levels, mapped according to barrier classi cationfi (market, structural, performance, interaction), weighted by the size of their potential supply impact and colour-coded to indicate the most relevant demand sectors.



signi cantfi and gives a guide to howaddressing different barriers haspotential to unlock differentmagnitudes of delivered bioenergy andwhere efforts can most effectively befocused.

The most signi cantfi barrier on this basisis judged to be lack of market heat demand.This is an issue which could be addressed bygovernment policy incentives, but shouldonly be done so within the constraints inSections 3.2.1 and 3.2.3—namely targeting(largely rural) areas where existingheating is by oil or solid fuel andmonitoring impacts on soil quality.The barrier of high feedstock marketprices also has signi cantfi leveragepotential, but for the relevant feedstocks(oilseed rape, wheat and sugar cane) thesigni cantfi constraints are greenhouse gasbalances and land use. Care must be takenthat any attempts to address the barrier ofmarket prices do not inadvertently stretchthese limits. It would therefore beinappropriate to attempt to subsidize marketprice if this had potential to encourageinef cientfi land use or failed toincentivise higher greenhouse gas savings.The need for investment capital isalso highlighted as signi cant fi andthis is particularly in relation toJatropha, where the constraint is land-take and land-use change, linked toyield/ef ciencfi y. This suggests that effortsto address the barrier of investmentcapital should focus on measures that donot violate but expand the constraint, e.g.targeted investment in programmes toimprove yield.

5.Conclusions

Sustainability constraints and resourceallocation rules have been applied tofuture UK bioenergy supply with the resultthat, even under a relatively pessimisticscenario there is scope for bioenergy tosustainably provide 4.3% of electricitydemand, 4.3% of heat demand and 5.8% oftransport fuel demand. Further expansion ofbioenergy supply will require long-termimprove- ments in underlying technologies,agricultural investment, etc. This paperhas also identi edfi a number ofconstraining factors. These factors must beconsidered when developing policyinstruments to address the barriers thatcurrently prevent these levels of

deployment being achieved. Finally, itshould be noted that these estimatesrelate to 2020; we have not takeninto account the potential longer-termimpacts of climate change on biomass andbiofuel production.

Acknowledgements

Funding for this work was provided bythe Engineering and Physical SciencesResearch Council. Other colleagues in theSupergen bioenergy consortium havecontributed data and other insightsthroughout the work, including Ian Shield,Andrew Riche and Nicola Yates fromRothamstead Research Institute, DavidBown from AMEC and Bob Saunders from BP.

Appendix A. SupportingInformation

Supplementary data associated with thisarticle can be found in the online versionat doi:10.1016/j.enpol.2009.08.028.

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Sustainability constraints on UK bioenergy development