Economic and environmental evaluation of compressed-air cars Felix Creutzig 1,2, Andrew Papson 3, Lee Schipper 4,5 and Daniel M Kammen 1,2,6

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1 Envirn. Res. Lett. 4 (Octber-December 2009) di: / /4/4/ Ecnmic and envirnmental evaluatin f cmpressed-air cars Felix Creutzig 1,2, Andrew Papsn 3, Lee Schipper 4,5 and Daniel M Kammen 1,2,6 1 Berkeley Institute f the Envirnment, University f Califrnia, Berkeley, USA 2 Renewable and Apprpriate Energy Labratry, University f Califrnia, Berkeley, USA 3 ICF Internatinal, 620 Flsm Ave, Suite 200, San Francisc, CA 94107, USA 4 Precurt Energy Efficiency Center, Stanfrd University, USA 5 Glbal Metrplitan Studies, University f Califrnia, Berkeley, USA 6 Energy and Resurces Grup, University f Califrnia, Berkeley, USA creutzig@nature.berkeley.edu Received 29 June 2009 Accepted 3 Nvember 2009 Published 17 Nvember 2009 Abstract. Climate change and energy security require a reductin in travel demand, a mdal shift, and technlgical innvatin in the transprt sectr. Thrugh a series f press releases and demnstratins, a car using energy stred in cmpressed air prduced by a cmpressr has been suggested as an envirnmentally friendly vehicle f the future. We analyze the thermdynamic efficiency f a cmpressed-air car pwered by a pneumatic engine and cnsider the merits f cmpressed air versus chemical strage f ptential energy. Even under highly ptimistic assumptins the cmpressed-air car is significantly less efficient than a battery electric vehicle and prduces mre greenhuse gas emissins than a cnventinal gas-pwered car with a cal intensive pwer mix. Hwever, a pneumatic cmbustin hybrid is technlgically feasible, inexpensive and culd eventually cmpete with hybrid electric vehicles. Keywrds: cmpressed-air car, life-cycle analysis, greenhuse gas emissins, innvatin, electric mbility Cntents 1. Intrductin 2. Thermdynamic efficiency 2.1. Cmpressin 2.2. Expansin 3. Mdel cnsideratins 3.1. Ttal energy requirement 4. Perfrmance 4.1. Fuel strage weight and vlume

2 4.2. Primary energy required 4.3. Greenhuse gas emissins 4.4. Cnsumer savings, break-even csts 4.5. Capital csts 4.6. Summary f perfrmance cmparisn 5. Discussin f assumptins and csts 6. Hybrid slutins 6.1. The pneumatic-cmbustin hybrid 6.2. Air engine hybrid 7. Cnclusin Acknwledgments References 1. Intrductin There is an urgent need fr mbility technlgies and infrastructures that are based n a technlgy ther than il and that have acceptable csts. Cnsumers are affected by high and fluctuating il prices, and in 2008 ttal vehicle miles decreased fr the first time in decades in the United States. Manufacturers must address plummeting car sales. The US gvernment spends billins n natinal il security while wrldwide greenhuse gas emissins are still increasing at a rapid pace. The cmpressed-air car has been prmted by cmpanies such as Mtr Develpment Internatinal, France, and Energine, Krea as an envirnmentally friendly car f the future. As a frm f strage, cmpressed air is nthing new. Indeed, cmpressed-air engines were utilized in pwer mining lcmtives in the United States and Eurpe at the beginning f the 20th century [1]. It is als reprted that cmpressed-air engines have pwered tramways in Bern, Switzerland and Nantes, France [2]. Hwever, a tramcar kilmeter needed mre than 7 kg cal because the lading statins were run by steam engines. This crrespnds t 13 kg CO 2 km 1. T get a feeling fr the magnitude: a fully ccupied tram with 31 passengers has a wrse carbn ftprint per persn km (410 g CO 2 pkm 1 ) than a Prsche Cayenne (358 g CO 2 pkm 1 ). Thus, a cmpressed-air tram had ten times mre emissins than a mdern electric tram per passenger km. In fact, tramways and lcmtives were sn pwered by mre efficient electricity. This time, cmpressed air is prpsed as a prpellant fr autmbiles. D high il prices ffer a secnd chance fr the cmpressed-air engine? Here we reprt n the thermdynamic limits, the verall efficiency, the envirnmental impact, the prpellant vlume and the cst-benefit balance f the cmpressed-air car (CAC) and cmpare it with a battery electric vehicle (BEV). We discuss sme f the advantages and disadvantages f this methd f energy strage and prpulsin mechanism while pinting ut the range f uncertainty n real perfrmance. We cnclude that the main drawback is the thermdynamic efficiency lss and uncertainties abut required technlgies. Hence, in cntrast t the BEV, the CAC is nt likely t successfully cmpete against gasline cars. The cmpressed-air technlgy, hwever, might be feasible in a hybrid cnfiguratin. All calculatins, graphs and cnsideratin are als specified in a spreadsheet mdel. The mdel, cmpressed-air car analysis meta-mdel (CACAMM) can be dwnladed at stacks.ip.rg/erl/4/044011/mmedia where users can verify the influence f different assumptins and variable values. 2. Thermdynamic efficiency We first investigate the thermdynamic efficiency f cmpressed-air strage. The different stages f efficiency lss are depicted in figure 1. We fcus n air cmpressin and air expansin, tw stages that are specific t the

3 cmpressed-air car. Tank leakage lss is negligible cmpared t the lss f air cmpressin and air expansin. A similar analysis was cnducted by Bssel [3]. Our investigatin differs frm that study in assuming cling at cnstant pressure [4, 5]. Furthermre, we specify that cmpressin is plytrpic and expansin adiabatic Cmpressin Our reference scenari crrespnds t data published by Zer Pllutin Mtrs, a subsidiary f MDI [6]. A vlume V 1 = l is cmpressed frm nrmal pressure p 1 = bar t p 2 = 310 bar (= 4500 psi), resulting in a final vlume V 2 = 327 l. The technical wrk required fr filling the tank under isthermal cnditins is Isthermal cnditins are nt achievable in practice. Real prcesses deviate frm the isthermal ptimum. A lwer bund is given by an adiabatic prcess, i.e. when there is n heat exchange and n rapid cmpressin. The adiabatic cefficient is n = 1.4. The prcess is then characterized by Then the verall efficiency f the prcess is the energy in the tank divided by the wrk dne, There are tw practical measures t increase efficiency: (A) multi-stage prcessing with inter-cling at cnstant pressure and (B) slw cmpressing allwing fr cncurrent heat exchange [4, 5]. Cmpressrs wrking in 4 stages are cmmercially available. MDI indicated that they use 2-stage cmpressin. In this case, tw subsequent cmpressin prcesses have vlume rati ~ 17.5 instead f ne cmpressin with vlume rati ~ 310. Furthermre, using a 5.5 kwh cmpressr charging takes 4 5 h allwing fr heat exchange, by this increasing energy efficiency. Taking this diabatic prcess int accunt, we assume n = 1.2. Hence, the verall wrk is calculated by repeatedly applying and updating equatin (2), resulting in W cm ~ 61.9 MJ and E cm ~ 93.6%.

4 Figure 1. The different stages f efficiency lss fr transprtatin with the cmpressed-air car. The tw stages that are specific fr cmpressed-air strage are marked in gray Expansin The expansin wrk can be retrieved similar t cmpressin wrk by equatin (2). In this case, we cnservatively assume the prcess t be adiabatic, n = 1.4, as the expansin is instantaneus. If there is heat exchange, expansin wuld be mre efficient. The pressure in the strage chamber can be kept cnstant by an adjusting valve. It is assumed that all expansin wrk is used t drive the car with cnstant trque a nrmal driving cycle wuld be much mre inefficient (see sectin 5). A single-stage expansin results in expansin wrk W exp ~ 22.8 MJ, i.e., As fr cmpressin, a multistep expansin can increase efficiency, in this case E exp = 49% fr 2 steps as used by MDI. The verall efficiency f the cmpressed-air strage nly is E air = E cme exp ~ 45.7%. 3. Mdel cnsideratins Hw des the cmpressed car perfrm cmpared t the cnventinal internal cmbustin engine and battery electric vehicle? In particular, we want t measure the verall energetic perfrmance and the ttal fuel vlume required t drive a reasnable distance. Furthermre, there are tw majr cst drivers fr a switch in mbility technlgy: envirnmental externalities in terms f greenhuse gas emissins and increasing gasline prices and price fluctuatins. Hence, we cmpare different car technlgies with respect t (A) required energy per km (B) greenhuse gas emissins per km (C) fuel vlume and (D) cst per km. The real energy perfrmance f CAC has nt been verified in independent tests, and, like virtually all ther vehicles, will depend n bth technlgies and behavirs, such as speeds and driving cycles. As a reference vehicle with internal cmbustin engine (ICE), we rely n an established mini-car, the Smart frtw [7, 8]. This car drives 5.2 l/100 km (45 mpg) in the Eurpean drive cycle, weighs (including driver) ca 900 kg and has a maximum speed f 145 km h 1. This chice is apprpriate as the same mdel will be available as an battery electric vehicle, the Smart frtw ed. We cmpare the cnventinal internal cmbustin engine pwered by gasline with the cmpressed-air car (CAC) and the battery electric car (BEV), i.e. the Smart frtw ed which drives 13.7 kwh/100 km (as measured frm the grid), weighs 100 kg mre than the Smart frtw, has maximum speed 100 km h 1, and a range f 115 km. Hence we limit ur analysis t urban transprtatin. BEVs and CACs are cmmnly classified as electric cars as bth types f car btain their energy frm the grid. T achieve ambitius climate change mitigatin gals, nt nly must fuel ecnmy must be imprved but als ttal energy requirements (and travel demand) must be reduced. T elucidate the weight factr, we cmpare the Smart frtw and its electric cusins with a hypthetical super-light car, r glf cart, that weighs 300 kg and is assumed t drive 1.7 l/100 km. The imprved fuel ecnmy is achieved mainly by weight reductin, as well as

5 ther feasible technlgical imprvements, and reduced mtr pwer. Such a vehicle is suited nly fr urban transprt but nt fr highways and has a maximum velcity f arund 60 km h 1. As such it is mre suitable fr relatively dense Eurpean r Asian cities that have high accessibility and shrt distances Ttal energy requirement First, the efficiencies f the three different strage technlgies is calculated. The grid-t-wheel efficiency f BEVs is 77.5% [9 11]. Cmparing energy cntent f fuels, an apprximate fuel-t-wheel efficiency f 21.2% fr gasline can be inferred. Furthermre, ne sixth f all carbn emissins f gasline are upstream in the supply chain, resulting in a well-t-wheel efficiency f ca 17.7%. Fr CAC, we cnservatively assume that additinal t 45.7% strage efficiency, 10% are lst due t mechanical and flw lsses. Furthermre, anther 29% are lst due t shaft energy requirements [10, 12]. Fr the electric mdes, an average grid transmissin lss f 9.5% must be included [13]. If electricity is generated by thermal pwer plants, plant efficiency lss must be included, e.g. 0.4 fr a relatively efficient cal pwer plant. The resulting efficiency values are summarized in table 1. Table 1. Efficiencies fr prpellants. Fr the electric mdes, specific pwer plant efficiencies must be added. Smart CAC Smart ed Cal/well-t-wheel (%) Wind/well-t-wheel (%) Grid/pump-t-wheel (%) Prpellant-t-wheel (κ) (%) Perfrmance 4.1. Fuel strage weight and vlume The prpellant weight is nt significant as a fractin f ttal weight fr cnventinal cars, but the weight f batteries can be cnsiderable fr BEVs. Here, we als cmpute the fuel weight fr cmpressed-air cars. The weight f prpellant needed is a functin f the range required, the efficiency f cnversin f stred energy t wrk at the wheels and the average strage capacity in MJ. The cruising range frm here n is r = 115 km, as has been specified fr the electric Smart. In a CAC r gasline vehicle but nt in a BEV, the weight f the fuel changes ver time. T take this int accunt, average filling is a = 0.6 fr gasline and CAC, and a = 1 fr BEV. The relatinship between fuel weight and vehicle prperties can be stated as fllws.

6 where w f is the prpellant weight, e f the energy per weight fuel (gasline: 45 MJ kg 1, CAC: 1.94 MJ kg 1, BEV: 0.40 MJ kg 1 ) and κ the fuel-t-wheel efficiency. Defining the range-specific energy required per weight as, the required prpellant weight is The fuel weight fr different strage technlgies and car sizes is summarized in table 2. Cmpressed-air weighs ten times mre than gasline with similar energy cntent, but three times less than batteries with similar energy cntent. Table 2. Fuel weight. Car weight (kg) Gasline (kg) CAC (kg) BEV (kg) One f the main bjectins against electric-nly vehicles is their limited range. Abve, we required that all vehicles have a 115 km range. This requires a significant amunt f cmpressed air, adding mre than 50 kg. Hwever, the real issue is the lw energy density: a large strage vlume is required. Fr the 900 kg CAC, 780 l strage is required mre than duble the trunk vlume f the Smart and hence psing a serius challenge fr car design. In cntrast, fr gasline less than 4 l is required. Li-in batteries have values f 400 whr l 1 and 150 whr kg 1 [14]. This implies kg l 1 density. In table 2 we specify kg battery pack fr the Smart, and 46.8 kg battery pack fr the super-light car. Using the density abve, this crrespnds t 374 l fr the nrmal car, a little abve the trunk vlume f the Smart, and 125 l fr the small ca. Nte that battery densities are defined in tw different ways: either just the active material, r fr the active material plus packaging. The numbers abve refer t packaging, which is apprpriate fr this use since were lking at the vlume f battery packs. Figure 2 summarizes the vlume requirement fr the different energy strage technlgies. Whereas battery vlume is already at the upper limits f what can be deplyed fr small cars, the vlume f cmpressed-air cars pses a very serius hurdle fr vehicles with suitable range.

7 Figure 2. Cmpressed-air stres energy nly at lw density, and a cmbinatin f large tank vlume, high pressure and lw vehicle weight is required t prvide an acceptable range. Frm here nwards nrmal car refers t the 900 kg mdel whereas light car refers t the 300 kg city car Primary energy required Frm the efficiency table 1, the primary energy per distance can be calculated, given the car plus fuel weight. The results are summarized in figure 3. The primary energy requirement is crucially dependent n the pwer plant. If electric cars are pwered by renewable energies, less primary energy is required than fr the gasline car. If electric cars are pwered by cal pwer plants, mre primary energy is required fr CACs but still less fr BEVs. Frm the verall efficiency perspective battery cars are much mre efficient.

8 Figure 3. Primary energy required. When pwered by renewable energies, the cmpressed air needs less energy per km than the gasline car but significantly mre than the battery car. The CAC needs 75% mre primary energy than the gasline car when the CAC is pwered by a cnventinal cal plant Greenhuse gas emissins Envirnmental perfrmance as zer lcal emissins is ne f the primary arguments fr the cmpressed-air car. Indeed, nxius matter is nt emitted lcally when driving, thereby ne f the main cntributrs t urban air pllutin is eliminated. This benefit is shared by battery cars Nte7. Hwever, cmpressed-air tanks can be dispsed f r recycled with less txic waste pllutin than batteries, depending n the precise recycling requirements. One imprtant envirnmental cncern relating t car use is the impact f cars n climate change. Greenhuse gas emissins themselves depend critically n the surce f electricity used fr charging batteries r running the CAC cmpressr. Whereas a cmpressed-air car r the BEV d nt emit greenhuse gases (GHG) when perated, emissins are shifted t pwer plants. Emissin levels then depend n the pwer plant characteristics. In general there is a great deal f uncertainty assciated with mdeling the effect f electr-mbility n the grid. It is agreed that even huge penetratin rate f electric cars can be serviced using the present pwer plant capacity if cars are mstly plugged-in at ff-peak hurs [15]. When advanced scenari-based simulatins f verall grid grwth, electricity dispatch and gegraphic generatin distributin are perfrmed, results are regin specific. Usually emissin reductins are larger than when assuming the current pwer mix [15]. Here, we calculate the emissins prduced by electricity purchase frm tw different German utilities, RWE and EWS Schönau. Prviders can be chsen in the German electricity market. Nte that RWE prduces the charging

9 statins fr the Smart ed in the Berlin trial. The pwer plant mix f RWE is dminated by cal plants, prducing 887 g CO 2 kwh 1 [16], whereas EWS Schönau relies nearly exclusively n renewables and hydr-energy, prducing 17.3 g kwh 1 [17]. The future pwer plant mix f RWE crucially depends n plicy decisin, such as permissins fr new cal plants. Results are summarized in figure 4. The chice f pwer plant mix has significant impact n greenhuse gas emissins. The cmpressed-air car indirectly emits mre than twice the greenhuse gases than its cnventinal cunterpart when pwered by RWE. The pr envirnmental perfrmance f the CAC is due t its thermdynamic inefficiencies, see sectin 4.2. Even the BEV perfrms nly similar t the gasline car. In cntrast, the CAC and the BEV mitigate greenhuse gas emissins cnsiderably when a renewable energy prvider is chsen. PG&E, a majr Califrnian energy prvider, has medium GHG emissin (238 g CO 2 kwh 1 ). As a result, the deplyment f bth CAC and BEV prduces sme envirnmental benefits in Califrnia. Figure 4. Greenhuse gas emissins per km f the cmpressed-air car are even higher than thse f the cnventinal gasline car. Ttal emissins are crucially dependent n the chice f prvider in Germany. The BEV has lwer emissins fr typical Califrnian cnditins Cnsumer savings, break-even csts One f the dminant cncerns fr cnsumers is high fuel prices. What fuel csts d cnsumers save when driving the cmpressed-air car? Tw gasline price scenaris and tw regins are cnsidered. The regins are Califrnia and Germany. Califrnia has a lw gasline price scenari with 2$/gal and high gas price scenari with 4$/gal. The price fr electricity is taken as 0.128$ kwh 1, the Califrnian average retail price in 2007 [18]. Rad maintenance csts are usually recvered via fuel taxes, e.g. $0.37/gal in Califrnia, crrespnding t

10 rughly Nte8 t $ km 1 which must be added t the marginal cst f using cars that are run by electricity. Assuming that this charge is levied as tax n electricity fr cars, the price wuld increase by anther $0.046 kwh 1 fr the average 900 kg electric vehicle and $0.017 kwh 1 fr the mre inefficient cmpressed-air car (taxes are prprtinal t km kwh 1 ). Results as marginal csts per km driven are presented in table 3. Table 3. Califrnia: marginal price (c km 1 ) with $4/gal ($2/gal). Gasline CAC BEV Nrmal fuel 5.5 (2.7) Light fuel 1.8 (0.9) Nrmal grid Light grid Break-even nrmal $ 300 ( 2700) 3500 (400) Break-even light $ 100 ( 900) 1200 (100) Fr Germany, the lw gasline price scenari is ($6.4/gal), the high price scenari ($8.4/gal). The higher gasline prices favr electric cars. Hwever, bth electricity prices and fuel taxes are significantly higher in Germany than in Califrnia, and respectively [19]. This translates int very high electricity charges fr electric cars, with rad taxatin amunting t mre f 50% f the ttal price f 0.57$ kwh 1. Results are displayed in table 4. Table 4. Germany: marginal price (c km 1 ) with $8.4/gal ($6.4/gal). Gasline CAC BEV Nrmal fuel 11.5 (8.8) Light fuel 3.8 (2.9)

11 Nrmal grid Light grid Break-even nrmal $ 2400 ( 5400) 4300 (1300) Break-even light $ 800 ( 1800) 1400 (400) In the fllwing, we fcus n the high fuel csts scenaris which are mre ptimistic fr the cmpressed-air cars. Cnsumer have fr a variety f reasns [20] a high discunt rate f ca 16% [21]. Taking the respective gasline car (Smart frtw) as a benchmark, in Califrnia the CAC has ttal usage savings f nly $300 whereas the Smart ed accumulates savings f $3500 ver its lifetime. In this calculatin, a user fee r rad charge fr electric vehicles substitutes fr fuel taxes. If there are n rad charges, and electric vehicles are subsidized, the CAC saves $1000, and the BEV $4200. In Germany, the CAC prduces additinal usage csts f $2400 whereas the Smart ed accumulates savings f $4300 ver its lifetime. This is due t cmparatively high fuel taxes (r equivalently rad charges) in Germany. If there are n rad charges, the CAC saves $2000, and the BEV $8700. All results are summarized in figure 5. Figure 5(a) displays the annual usage csts fr the 2 different strage technlgies in Califrnia and Germany with and withut user fees and cmpares them with varying gasline prices. In figure 5(b), the ttal savings fr each scenari are displayed as a functin f the gasline price. Only the usage f battery electric vehicles allws significant savings in each regin. Furthermre, the plitical chices f hw and when t intrduce user fees fr electric vehicles has a significant influence n the csts and benefits f electric cars. These numbers can be interpreted as break-even csts, i.e. the ttal csts that the strage technlgy can csts, including fr the BEV battery csts, battery maintenance, battery substitutin, and fr the CAC cmpressed-air tanks plus cmpressr. Only if the strage technlgy prduces fewer additinal csts than ttal usage savings, can the respective technlgy becmes ecnmically reasnable. Hence, if csts fr batteries fall belw $290 kwh 1 in Califrnia r belw $360 in Germany, it becmes ecnmical t drive the BEV, assuming all ther csts remain the same. The CAC prduces lifetime savings nly in Califrnia. It becmes nly ecnmical t drive the CAC, if the engine cnstructin saves csts cmpared t the cmbustin engine.

12 Figure 5. Csts and savings f different prpellants as a functin f the gas price. (a) Annual csts f each prpellant fr driving km. (b) Ttal savings fr CAC and BEV. Ttal savings are cnsiderably higher fr BEV drivers. The CAC des nt prvide saving fr gas prices belw $4/gal. Nte that we included a rad charge in the electricity price, equivalent t tday's gasline tax. Withut the rad charge, bth mdes are mre prfitable Capital csts It is instructive t cmpare the break-even csts with knwn capital csts fr the strage technlgy. Nte that the strage technlgy csts are nt necessarily equivalent t the additinal full vehicle csts. The frnt-up cst fr the CAC are mstly (a) the n-bard cmpressr and (b) the carbn-fiber tank. A cmmercially available 4700 PSI cmpressr csts ca $3000 [22]. Carbn-fiber tanks with ca 300 l ttal vlume cst $3500 [23]. Hwever, whlesale prices shuld already be much lwer. Nnetheless, the lw savings achieved by driving the CAC (see table 4) d nt allw fr csts abve $300 fr the strage technlgy; the CAC technlgy is nt cmpetitive. The break-even csts fr batteries abve crrespnds t a battery price f ca $ kwh 1. Current prices are in excess f $1000 kwh 1 [24], and there are additinal csts fr ancillary cling and electrnics. That is, ttal csts fr batteries must cme dwn by a factr f at least 4.

13 4.6. Summary f perfrmance cmparisn Frm the analysis abve, it is clear that the CAC is utperfrmed by the BEV and even the gasline car in mst dimensins. The nly exceptin is primary energy required the gasline car has higher ttal energy needs. The results are summarized in a radar plt (figure 6, left). 5. Discussin f assumptins and csts There is reasn t believe that currently prjected CAC csts are ptimistic because current mdels have much lwer real perfrmance. We specified tw expansin steps. Cmmercially available air mtrs wrk with ne expansin step. There is n physical reasn why bth expansin steps cannt be used, althugh this may be technlgically very challenging. T circumvent this challenge, ne culd use the secnd expansin step alne t pwer the vehicle. Fr example, assuming zer technlgical innvatin and using a cmmercial air mtr that is driven by input pressure f 9 bar fr the secnd expansin step nly, wuld reduce expansin efficiency frm 68% t maximal 16% and ttal efficiency frm 34% t nt mre than 8%. In this scenari, we still assume strage at 300 bar and the first expansin step is nly used t bring dwn the pressure t the apprpriate input pressure fr the cmmercial air mtr. Of curse, such lw efficiency translates int pr perfrmance f the cmpressed-air car, e.g. mre than fur times higher GHG emissins than a cnventinal gasline-pwered car. A cmparisn f this mdificatin in cmparisn t the default mdel is drawn in figure 6, right panel. As a limitatin, we did nt carry ut a drive-cycle analysis here. Instead, we assumed that the air mtr can perate at equal efficiency acrss different utput pwer regimes. Hwever, at least with cnventinal cmmercially available air mtrs, such as pistn air mtrs, this is nt true. Mre specifically, the air flw rate translates int rtatinal speed and trque f the air mtr and frm their int utput pwer. Hence, the ptimal efficiency regime is a prperty f the air mtr [25]. A cmplete drive-cycle analysis wuld certainly lead t inferir results. 6. Hybrid slutins Internal cmbustin engines are usually driven usually at lw lad and hence, in a lw efficiency regime. Als their thermdynamic cycle cannt be reversed and in braking situatins kinetic energy dissipates as heat. A hybrid pwertrain is ne slutin t these prblems, requiring a battery as the strage system. The dwnside f this slutin is the additinal weight and csts f the battery, electric mtr and generatr. The bvius questin is whether hybrid pneumatic cmbustin engines ffer better perfrmance. Tw different hybrid appraches have been prpsed, ne based n the cmbustin engine, the ther n the air engine. We explre bth appraches cnceptually The pneumatic cmbustin hybrid This apprach takes the cnventinal internal cmbustin engine and adds an additinal valve that cnnects the cmbustin chamber t an air tank. The tank is charged by the cmbustin engine when perfrming belw peak efficiency and adds pwer t the engine in supercharged mde, i.e. when additinal pwer is required.

14 Additinally, the tank can be charged when the engine peratin is reversed upn regenerative braking. One theretical study fund that ptimizing a hybrid air tank t 16 kpa and 80 l with cmbined engine dwnsizing can imprve fuel efficiency by 31% [26]. Fuel ecnmy imprvements f 64% in the city and f 12% n the highway have been reprted in anther mdel [27]. Experimental wrk demnstrated the feasibility f this cncept, recvering up t half f the energy cntent f the cmpressed air; the expansin efficiency is > 48% [28]. In cntrast t the battery hybrid, the pneumatic hybrid apprach des nt require a secnd prpulsin system nr des it increase the car's weight cnsiderably. The main csts f pneumatic hybrid arise frm the variable valve actuatin system. As n batteries are required, csts are cnsidered t be lwer than in the hybrid electric vehicles. A switch frm a tw-strke t a fur-strke pneumatic engine wuld further reduce csts fr actuated valves [29] Air engine hybrid The ther hybrid cncept is fcused n the air engine. Here, a cmbustin engine wuld be used t recharge the air tank. Here the internal cmbustin engine can cnstantly wrk in the maximum efficiency regime. Energy is lst in the cmpressin and expansin stages, cmparably t sectin 4.2. Hwever, waste heat f the cmbustin engine can be used t heat up the expanded air and, hence, increase expansin efficiency. Mdeling studies claim that such an air engine hybrid can reach ttal vehicle efficiency > 33% [30, 31], cmpared t vehicle efficiency f 20% f the cnventinal car in ur study. These results have t be independently verified. Figure 6. Perfrmance summary (Germany, electric cars pwered by RWE pwer mix). Left: verview n the perfrmance f the CAC, BEV and gasline car. GHG emissins, primary energy requirement, strage vlume and marginal csts are pltted against each ther. Perfrmance is better fr pints further inwards. With the exemptin f primary energy requirement, the CAC perfrms wrse than the ther vehicles. Right: cmparisn f tw CAC scenaris. The scenari with tw expansin steps crrespnds t the

15 scenari used n the left side. The 1-step expansin scenari is mst likely used in existing air cars. 7. Cnclusin The cmpressed-air car shuld be regarded as a car similar t the cmmn BEV, pwered by electricity frm the grid but different in strage technlgy. In principle, cmpressed-air cars culd cmpete with BEVs in substituting fr gasline cars. The life-cycle analysis f the cmpressed-air car, hwever, shwed that the CAC fared wrse than the BEV in primary energy required, GHG emissins, and life-cycle csts, even under ur very ptimistic assumptins abut perfrmance. Cmpressed-air energy strage is a relatively inefficient technlgy at the scale f individual cars and wuld add additinal greenhuse gas emissins with the current electricity mix. In fact, the BEV utperfrms the cmpressed-air car in every categry. Uncertainty in technlgy specificatins is cnsiderably higher fr CACs than fr BEVs, adding a risk premium. We prvide a transparent spreadsheet mdel that can be used t replicate results r experiment with ther values. A hybrid cncept, where the air tank is recharged with an internal cmbustin engine, is mre efficient but has yet nt been experimentally verified. Hwever, a pneumatic cmbustin hybrid is similar t the hybrid electric vehicle in cncept and efficiency gain, ffers ptential cst and weight advantages and is clsest t implementatin. Overall, the CAC des nt appear t ffer any advantage ver purely electrical means f string energy n bard a vehicle. Batteries are cmmn and imprving almst daily, while the cmpressed-air cycle has n present rle in any ppular autmbile platfrm. Since there are great pressures n battery perfrmance frm ther applicatins such as cell phnes, it is hard t imagine that CAC will gain an advantage ver BEV in the freseeable future. Autmbiles must becme lighter and mre efficient if even the best batteries are t prvide lnger autnmus ranges. At the same time, cmbustin technlgy itself is evlving rapidly in the face f cncerns abut il and climate change. As lng as there are n substantial innvatins in cmpressed-air technlgy and its deplyment, the real prgress in this sectr may be the emphasis n light materials and small car design, fr which the cmpetitin between batteries and fuel will just intensify. Acknwledgments We thank Emilie C Mathieu and fur annymus reviewers fr helpful cmments n the manuscript. The wrk was supprted by the Eurpean Recvery Prgram and by a grant frm Vulcan, Inc. We thank Jim Byden fr discussins and assistance. References [1] Gairns J F 1904 Industrial lcmtives fr mining, factry, and allied uses. Part II. Cmpressed air and internal cmbustin lcmtives Cassier's Mag [2] accessed 23 June 2009 [3] Bssel U 2005 Thermdynamic Analysis f Cmpressed Air Vehicle Prpulsin Eurpean Fuel Cell Frum [4] Barber A 1997 Pneumatic Handbk (Amsterdam: Elsevier) [5] Rajput R K 2006 Thermal Engineering (New Delhi: Laxmi) ISBN:

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17 [31] Huang K D, Quang K V and Tseng K-T 2009 Study f recycling exhaust gas energy f hybrid pneumatic pwer system with cfd Energy Cnvers. Manag CrssRef Ntes Nte7 Mdern superlw emissin vehicles, cmprising standard mdels such as the Tyta Prius, als emit substantially lwer levels f hydrcarbns, carbn mnxide, nitrus xides and particulate matter than cnventinal vehicles; hence, fr addressing air pllutin cncerns neither a BEV r CAC is required. Nte8 Fr the Califrnian value, the US Envirnmental Prtectin Agency rating f 36 mpg is assumed fr the Smart frtw. Yur last 10 viewed 1. Ecnmic and envirnmental evaluatin f cmpressed-air cars 2. Felix Creutzig et al 2009 Envirn. Res. Lett