as demand for food and energy grows , innovative ways to meet demand while enhancing environmental quality will be needed .
anaerobic digesters ( ads ) can produce renewable energy from livestock manure , prevent the release of methane , and reduce air and water pollution , and digested manure can be applied to crops as a fertilizer.(1 ) most ads in the u.s .
sell electricity and digested manure , but the net present value of most systems is insufficient to promote widespread adoption . placing an economic value on the climate , energy , and environmental benefits that ads provide can help to accelerate their deployment .
deployment of renewable energy technologies grows under climate policy compared to business - as - usual.(4 ) although support for ads in the u.s .
has been limited,(5 ) countries such as china,(6 ) india,(7 ) and germany ( 8) have higher rates of ad adoption , mostly due to government support and financial incentives .
the incentives currently available at the local , state , and federal levels in the u.s . have stimulated some ad projects .
comprehensive inclusion of the ghg mitigation benefits and low - carbon energy generation of ad projects within a federal climate and energy policy would further enhance prospects for new projects .
although economic and environmental models have tested the integration of many renewable energy technologies , a rigorous evaluation of ads within a computable general equilibrium model has yet to be completed .
we used an economic model to test the effects of a representative climate stabilization policy on the penetration of ads as a ghg mitigation and low - carbon energy generation technology in the u.s .
engineering and life - cycle data were used to calculate the cost of electricity from a typical ad system .
spatially explicit livestock density maps(13 ) and state - level methane emissions data(14 ) were used to estimate potential electricity generation capacity and emissions reductions from livestock manure .
the climate policy scenarios simulated in the economic model included a reference case and an emissions reduction of 50% below 2005 levels by 2050.(4 ) as carbon dioxide equivalent ( co2e ) emissions prices increased under more stringent caps , ad systems became competitive , in part , because of additional credits for methane mitigation .
unlike most other low - carbon energy sources , ads deliver additional nonmarket environmental benefits .
the mit emissions prediction and policy analysis ( eppa ) model was used to test a range of scenarios to quantify the economic and environmental responses to the introduction of ads .
eppa is a recursive dynamic , multiregional , multisector computable general equilibrium model that simulates the world economy.(30 ) the model has been applied to a range of policy - relevant topics including energy legislation , health,(32 ) biofuels,(9 ) agriculture,(33 ) and alternative energy technologies.(34 ) in this study , we compared the impacts of three scenarios on the use of electricity from ads as a substitute for more carbon - intensive sources .
anaerobic digesters are introduced into the model as a low - carbon alternative technology , in which the electricity produced competes with traditional electricity sources based on the levelized cost of electricity ( lcoe ) across sources with additional consideration of intermittency and experience with technology.(35 ) the lcoe takes into consideration the capital , operations , and fuel costs of electricity produced over the lifetime of the plant.(12 ) with no climate policy in place , alternative electricity generation technologies such as solar and wind power are one to four times more expensive than fossil - fuel - based generation.(12 ) we compare three scenarios in eppa to gauge the impact of ads under climate legislation . the first , or reference scenario , assumed no climate policy .
the policy scenarios described in refs ( 4 ) and ( 31 ) cover the range of recent congressional proposals and are referred to by the cumulative number of ghg emission allowances each policy issues between 2012 and 2050 .
the policy specified an economy - wide emissions cap on all ghgs beginning in 2010 .
the 2010 cap was 95% of 2005 emissions in 2010 and was progressively lowered to 50% of 2005 emissions by 2050 .
version 5 of eppa disaggregates the agricultural sector into separate crop , livestock , forestry , and biofuels production structures as described by ref ( 36 ) .
we modified the model to include livestock manure output and separate livestock production into traditional livestock , for which manure is treated as a byproduct , and livestock for which manure can potentially be used in ads .
detailed changes to the model are described in the supporting information ( si ) methods and figure s1 .
livestock within the new production function is eligible for offsets from reduced emissions of methane , and income from the sale of electricity .
the ad production structure employs capital , labor , and intermediate inputs from other industries to produce electricity ( figure s1 ) .
ads enter endogenously in eppa when they become economically competitive with other forms of generation .
similar to other technologies within eppa , ads are parameterized using bottom - up engineering , life cycle and fuel cost data.(11 ) there are several types of ads currently in use that range in size from 50 to 2800 kw ( n = 55 , mean = 573).(37 ) acknowledging that there are several digester designs that operate best with certain feedstocks or in certain geographies , we based our analysis on capital cost data from horizontal plug flow ads , as the most data were available from this technology.(37 ) the lcoe from ads is determined by two factors : capital costs and transportation costs .
ads exhibit capital cost trends similar to other energy generation technologies : larger , centralized units are less expensive to operate per unit of energy produced.(38 ) data collected by epa agstar on generator capacity and capital costs exhibit a power law relationship ( r = 0.911).(39 ) we assumed that each system had a postdigestion solids separation system and hydrogen sulfide ( h2s ) treatment at an added cost of 9.5% of capital costs.(39 ) although centralized ads might be less expensive to operate , there are additional logistical and coordination factors that need to be considered for optimal day - to - day management .
there are currently more centralized ads outside the u.s . as ad size increases , the amount of manure needed to supply the generator increases proportionally .
large ad systems often require manure inputs from several farms in order to take advantage of lower capital costs per kwh for larger generator systems .
we assumed that the manure input from multiple sources was optimized for total solid content , ph , and other physical characteristics important to the digestion process .
the cost of hauling large amounts of manure can be a significant portion of the final lcoe . in this study
, we represent the trade - off between low capital cost with high transportation cost of larger systems , and high capital cost with low transportation cost of smaller systems , by including 1000 , 500 , and 250 kw ads and spatially grouping manure resources according to system size . for each system
, we assumed that 50% of the manure was available on - site , while the other half was transported via truck.(42 ) we further assumed that biogas was combusted on - site at 40% thermodynamic efficiency , and the electricity generated was sold to a utility at market prices averaged across users and states .
waste heat collected from the generator was used to maintain the digester within the mesophilic temperature range ( 38 c ) .
we assumed that digested manure was used as a fertilizer substitute , but not given an economic value .
lcoe values for each digester size were computed using the methods described in ref ( 12 ) with operations and maintenance assumed to be 3% of capital costs ( table s2 ) .
manure availability was estimated using spatially explicit maps of livestock density , manure production and management parameters , and identification of areas with high manure densities .
gridded densities of cattle , pigs , and poultry available at 0.05 spatial resolution ( 5 km ) adjusted to match faostat 2005 national totals for the u.s .
were used to estimate livestock populations.(13 ) reference ( 14 ) provided state - level parameters on the excretion rate of volatile solids , maximum methane producing capacity , and typical animal mass needed to calculate methane production for dairy cattle , beef cattle , swine , and poultry in each state . the u.s .
department of agriculture national agricultural statistics service quickstats database(47 ) provided a breakdown of state swine and poultry data by animal type , while the cattle enteric fermentation model(48 ) provided the distribution of cattle types .
it was assumed that all manure was available for digestion , except manure from animals managed in pastured systems , as manure collection would be uneconomical under current conditions .
given these data , statewide coefficients for methane production potential were computed for each livestock group over the contiguous u.s .
to determine the manure input for a typical ad , the proportion of potential methane for each livestock type was used to compute the percentage of manure input into a typical digester . using ref ( 14 ) manure management data , average methane emissions from livestock manure
not diverted to an ad were calculated as the potential project offset value . to assess the full lcoe of ads , costs for digesters that transport manure from off - site
given the gridded methane production potential , clusters were identified that met the minimum amount of methane needed for a given digester size and were contained in the smallest number of contiguous grid cells . the arcgis spatial order tool constructed a peano curve over the input data set to quantify the proximity of a given cell to its neighbors .
next , the arcgis collocate tool grouped points based on the spatial order value until a specified threshold of methane production potential was met .
clusters of grid cells less than 900 km in area were identified as areas compact enough to support an ad without excessive manure hauling costs .
remaining clusters were separated into groupings less than 225 , 400 , 625 , and 900 km , and it was assumed that each cluster was square and manure densities were higher in cells closer to the central cell ( where the ad would be located ) .
transport distances were calculated by summing the distance of every cell to the central cell .
transport costs for each cluster size were computed with distance - cost hauling relationships from refs ( 40 and 49 ) .
we first identified clusters of grid cells that met the biogas requirements of a 1000 kw ad and were within a reasonable transportation distance ( 900 km ) , and the remaining cells were recursively analyzed to identify clusters that met the biogas production potential threshold for 500 and 250 kw ads . for ads of each size , we determined the lcoe by calculating the weighted average of ad clusters from each of the four transportation distance categories .
we assumed that manure located near an ad of a particular size could not be used in an ad of a different size .
this approach is suitable for determining the potential methane production potential across a region , but has limitations for siting a specific ad .
over two billion cattle , swine and poultry in the u.s . produce manure that can be diverted to ads to produce energy and then used as fertilizer .
our estimates show that manure collected and deposited in lagoons or pits currently has the potential to produce 11  000 megawatts ( mw ) of electricity , while manure from pastured animals could produce an additional 7000mw with modified collection practices . in our core scenarios ,
only manure collected and stored in lagoons or pits , and not pasture manure , is available for use in ads .
the greatest density of manure available for ads is located in the southeast , midwest , and western regions , and 14% of electricity demand in iowa and nebraska could potentially be met by ads .
readily available manure resources can contribute over 11  000 mw of electricity generation potential . each colored grid cell
is included in a cluster less than 900 km that can support an ad of a given capacity .
electricity cost for each cluster is based on ad capital costs and manure transportation costs .
ad electricity generation is initially uncompetitive with conventional electricity but enters as the cost of conventional electricity rises .
economies of scale for ads , and variable distances between manure sources and ads , result in a range of generation costs for electricity from manure .
we first identified three potential ad sizes based on manure density : 1000 , 500 , and 250 kw .
we estimate that , ignoring transport costs , a 1000 kw ad is able to produce electricity at $ 0.086/kwh , while a 250 kw ad is 58% more expensive at $ 0.136/kwh .
the cost to transport manure ranges from 30 to 53% of total electricity cost ( capital + transportation cost ) , based on digester size and transportation distance .
transportation costs are $ 0.060/kwh for the smallest ( 225 km ) and $ 0.096 for the largest ( 900 km ) clusters .
total electricity costs range from $ 0.128/kwh to $ 0.204/kwh , which is 1.52 to 2.44 times the cost of conventional electricity in the base year ( 2004 ) of our modeling framework .
electricity from ads competes with electricity from traditional sources based on generation costs . under a climate policy that includes emissions from all sectors , electricity from fossil fuels
becomes more expensive , and renewable and low - carbon electricity sources become more competitive .
we consider a policy where between 2010 and 2050 the emissions cap is progressively reduced . under the cap
, the price per tonne of emissions increases to $ 316/tco2e ( figure 3a , table s3 ) by 2050 .
this co2e price is much higher than prices currently observed in the e.u . , but is consistent with other studies that consider emission limits that decrease over time.(50 ) co2e prices increase faster in the later years of the scenario , as more costly emission reductions are put into place .
there is a sharp increase in the co2e price after 2045 , as prior to this date the cap is largely met by switching electricity generation from coal to gas , but more radical measures are required to meet the cap after this date .
the availability of ads reduces co2e prices by $ 42 in 2050 relative to when ads are not available , since ads are able to produce energy less expensively than other low - carbon energy technologies and reduce agricultural methane emissions . by 2050 ,
relative to a scenario with climate policy without ads , ads displace electricity from natural gas combined cycle ( 0.1 petawatt - hours , pwh ) and wind ( 0.03 pwh ) in 2050 . simulated u.s .
note : advanced fossil includes natural gas combined cycle ( ngcc ) , ngcc with sequestration , integrated gasification with combined cycle and sequestration , and wind with gas backup .
changes in reference and policy scenarios until 2050 for ( a ) carbon prices , ( b ) economic welfare , ( c ) livestock greenhouse gas emissions , and ( d ) greenhouse gas mitigation ( there is a net increase in ghgs from electricity production between 2020 and 2025 when ads displace expensive , low - carbon ) . under the climate policy ,
ads are first introduced in 2025 when the price of co2e is $ 76/tonne and electricity is $ 0.15/kwh . in the first year ads
are economically available , assuming that potential ad electricity generation is maximized , they produce 0.1 pwh of electricity , which is 2.6% of national electricity generation . in 2050 ,
ads contribute 0.24 pwh of electricity , or 5.5% of national generation ( figure 2 , figure s2 ) .
this increase is mainly driven by the expansion of the livestock sector , but the introduction of more costly ad electricity generation as the price of electricity increases also plays a role .
compared to the climate policy scenario without ads , the livestock sector grows faster when ads are available , as increased profits from electricity sales and methane mitigation credits are realized .
as carbon prices rise , the cost to produce electricity from ads becomes competitive with other electricity generating technologies and ad market penetration increases .
the least expensive electricity is available from 1000 kw ads , which enter in 2025 .
electricity production from 500 kw and 250 kw begins in , respectively , 2035 and 2040 .
changes in consumer welfare , measured as equivalent variation changes in annual income , are often used as an indicator to measure the economic effects of a policy.(51 ) not accounting for climate benefits , welfare under climate policy ( without ads ) decreased by 3.5% relative to the reference scenario in 2050 ( figure 3b ) .
when ads were included , welfare increased by 0.2% ( $ 33 billion ) , as they provided an additional mitigation option .
this indicator of consumer welfare measures only changes due to the cost of ghg mitigation , and does not take into account potential social and environmental benefits of implementing this technology .
manure collected and managed under anaerobic conditions releases methane , a potent ghg . by diverting the manure to ads , an opportunity to capture and combust
enables livestock operations to sell emissions permits , thereby increasing the economic viability of the projects . by 2050 , ads are able to mitigate 151 million metric tons ( mt ) of co2e , mostly from methane abatement ( figure 3c , table s3 ) . in the reference scenario ,
the livestock sector emits 477 mt co2e of methane in 2050 , which is reduced to 250 mt co2e under a climate policy without ads as technologies are used to mitigate livestock emissions .
if electricity from ads displaces an electricity generation technology with higher emissions intensity per unit of electricity , then additional ghgs are mitigated . in 2050 , 31
this was mainly due to a decrease in electricity generation from natural gas - combined cycle ( ngcc ) and under the emissions cap , economy - wide emissions remained constant .
interestingly , ads do not necessarily displace high - carbon electricity production , such as coal . in our framework electricity generation sources
the electricity mix is determined endogenously so as to minimize the cost of meeting the emissions cap .
when ads are available ( and are profitable ) , ads reduce the co2 price , which reduces the costs of electricity from high - carbon sources , relative to when ads are not available .
our results demonstrate the potential for climate policy to hasten the use of ads , both to reduce ghg emissions from livestock and to produce renewable energy . by including ads within an economic modeling framework
, we illustrated the opportunity for a win  win scenario where , by providing incentives for the ghg benefits of digester operation , there are additional nonmarket benefits , even though they were not explicitly incentivized .
although capital costs are a major barrier to further introduction of ads , there are opportunities to improve the efficiency of manure collection , processing , and subsequent biogas combustion that would increase the economic competitiveness of the technology .
most ad systems are currently installed at livestock operations with existing manure management strategies that may not be optimal for biogas extraction .
further research , development , and innovation is needed to design manure collection systems that simultaneously maximize biogas production and animal well - being , while minimizing the release of nutrients and ghgs .
siting ads near energy - intensive industries would allow for better utilization of the waste heat from the combustion process .
although livestock that spend a majority of their time on pasture were excluded from our core scenarios , financial incentives to produce biogas may spur development of pasture - based manure collection systems that allow for both grazing and manure collection .
these systems would realize both the environmental and animal welfare benefits of pasturing animals , and the economic benefits of biogas production .
the assumptions and core data that are the backbone of eppa are routinely updated to the latest state of the science ( si and refs ( 4 and 31 ) .
while we parameterized the model with values from the literature , and conducted sensitivity analyses , many social , economic , and environmental trends can not be modeled with certainty far into the future .
we assumed that there were no major changes in consumer preferences , but as we move into an increasingly energy and resource constrained future , these assumptions may be optimistic .
therefore , less manure may be available for ads in the future than in our estimates .
additionally , concern over environmental and health impacts of meat consumption may also reduce future livestock production .
although we only considered livestock manure as an input to ads , they can also break down many other forms of organic wastes to produce biogas , often at higher rates of biogas production per unit input than manure , as manure has already been digested by the animal.(54 ) co - digesting other organic materials with manure can relieve pressure on other waste processing facilities and increase biogas production without greatly increasing the size and capital costs of the digester.(55 ) several municipalities already collect household food scraps and waste grease , and digestion of these materials could increase ad profitability and further reduce the environmental impact of waste disposal.(56 ) we derived model parameters for ad ghg mitigation and electricity generation from published sources , but we acknowledge that there remains uncertainty about methane emission rates from livestock under different management practices.(57 ) even if methane credits increased by 30% , which we considered in a sensitivity analysis , electricity generated by ads only increased by 0.003 pwh in this scenario , and 500 kw ads became economical five years earlier ( table s6 ) .
improved methods to measure ghg emissions from livestock , e.g. ref ( 58 ) , will be needed to improve upon currently used generalized emissions models .
life cycle assessment is one tool that can be used to assess the release of ghgs and nutrients from a farm that can lead to implementing the most effective mitigation options.(58 ) even in the absence of a broad climate policy that prices carbon , there are other mechanisms to encourage installation of additional ad capacity .
several states have implemented renewable portfolio standards that have driven the adoption of alternative energy sources.(59 ) germany uses a feed - in - tariff to guarantee competitive prices for energy produced from ads , and is a global leader in biogas production.(54 ) california s low carbon fuel standard ( lcfs ) ranks transportation fuels by their life - cycle carbon intensity . for illustration ,
biogas from dairy ads can be used as a transportation fuel if it undergoes upgrading and compression .
it is then comparable to traditional compressed natural gas with one - fifth the carbon intensity because a credit is applied to the biogas owing to decreased methane emissions compared to traditional manure management techniques.(60 ) some ad projects are intended to reduce other environmental impacts such as nutrient runoff , and ghg emissions may be a secondary concern .
ads can provide energy for a single household , as seen in india and china , or up to several thousand households such as in toronto , canada.(61 ) while the technology is scalable , decisions regarding sites , operations , and sources of digestable material are outside the context of this study .
our approach matched digester sizes ( 1000 , 500 , 250 kw ) to resource density in order to minimize the capital investment and transportation costs per unit of energy generation . while this approximation is useful at a national level , each potential ad project will need to survey the availability and cost of manure and organic materials for codigestion to maximize the environmental and economic efficiency of the project .
using a computable general equilibrium model in this context allows us to investigate the interactions among sectors , illustrated here in the novel linkages between agriculture and energy production .
while economic welfare decreased across all scenarios relative to the reference , the climatic benefits were excluded from these values , mostly because such calculation will suffer from much greater uncertainty and lack of information than on the cost side .
additionally , there are few metrics to quantify nonclimatic environmental benefits from ads and thus these were excluded from the analysis.(62 ) caution should be used when applying the results of this study to a specific project , since they are estimated across the entire economy and the projected changes in welfare do not include all costs and benefits to society .
many of the fuel sources used today have social and environmental impacts that are not accounted for in standard economic transactions .
similar externalities exist within the agricultural sector , which will increase as livestock operations expand .
implementing a climate policy that places a value on carbon will ease the transition from diverting livestock manure to ads to provide energy .
as the external costs of fossil fuel energy are realized throughout the economy , the environmental cobenefits of ad further increase the societal value of avoiding traditional manure management .
 livestock husbandry in the u.s . significantly contributes to many environmental problems , including the release of methane , a potent greenhouse gas ( ghg ) . 
 anaerobic digesters ( ads ) break down organic wastes using bacteria that produce methane , which can be collected and combusted to generate electricity . 
 ads also reduce odors and pathogens that are common with manure storage and the digested manure can be used as a fertilizer . 
 there are relatively few ads in the u.s . , mainly due to their high capital costs . 
 we use the mit emissions prediction and policy analysis ( eppa ) model to test the effects of a representative u.s . 
 climate stabilization policy on the adoption of ads which sell electricity and generate methane mitigation credits . under such policy , ads become competitive at producing electricity in 2025 , when they receive methane reduction credits and electricity from fossil fuels becomes more expensive . 
 we find that ads have the potential to generate 5.5% of u.s . electricity . 
Introduction
Methods
Results
Discussion