L4.6 FUEL CEllS1. ,wf&4c:~~~~8zp/2~ (Del ~ Electrical block diagram of a battery-based micro-hydro system.

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1 206 DSTRBTEDGENERATON Figure 4.24 Generatr c:=:j c:=:j Charge Cntrller (Del Electrical blck diagram f a battery-based micr-hydr system. On the ther end f the scale, hme-size micr-hydr systems usually generate de, which is used t charge batteries. An exceptin wuld be the case in which utility pwer is cnveniently available, in which case a grid-cnnected system in which the meter spins in ne directin when demand is less than the hydr system prvides, and the ther way when it desn't, wuld be simpler and cheaper than the battery-strage apprach. The electrical details f grid-cnnected systems as well as stand-alne systems with battery strage are cvered in sme detail in the Phtvltaic Systems chapter f this text (Chapter 9). The battery bank in a stand-alne micr-hydr system allws the hydr system, including pipes, valves, turbine, and generatr, t be designed t meet just the average daily pwer demand, rather than the peak, which means that everything can be smaller and cheaper. Lads vary thrughut the day, f curse, as appliances are turned n and ff, but the real peaks in demand are assciated with the surges f current needed t start the mtrs in majr appliances and pwer tls. Batteries handle that with ease. Since daily variatins in water flw are mdest, micr-hydr battery strage systems can be sized t cver much shrter utages than weather-dependent PV systems must handle. Tw days f strage is cnsidered reasnable. A diagram f the principal electrical cmpnents in a typical battery-based micr-hydr system is given in Fig T keep the batteries frm being damaged by vercharging, the system shwn includes a charge cntrller that diverts excess pwer frm the generatr t a shunt lad, which culd be, fr example, the heating element in an electric water heater tank. Other cntrl schemes are pssible, including use f regulatrs that either (a) adjust the flw f water thrugh the turbine r (b) mdulate the generatr utput by adjusting the current t its field windings. As shwn, batteries can prvide de pwer directly t sme lads, while ther lads receive ac frm an inverter. L4.6 FEL CEllS1 believe that water will ne day be emplyed as a fuel, that hydrgen and xygen which cnstitute it, used singly r tgether, will furnish an inexhaustible surce f heat and light. -Jules Verne, Mysterius sland, 1874 ff/.<,'jv 'l"'tjv''""/ljc/',wf&4c:8zp/2 /' (\C::fLl. :v,v',",", -rc.:r:pe-f-{.;l (d-oiyf) ' Y" FEL CELLS 207 The prtin f the abve qute in which Jules Verne describes the jining f hydrgen and xygen t prvide a surce f heat and light is a remarkably accurate descriptin f ne f the mst prmising new technlgies nw nearing cmmercial reality-the fuel cell. Hwever, he didn't get it quite right since mre energy is needed t dissciate water int hydrgen and xygen than can be recvered s water itself cannt be cnsidered a fuel. Fuel cells cnvert chemical energy cntained in a fuel (hydrgen, natural gas, methanl, gasline, etc.) directly int electrical pwer. By aviding the intermediate step f cnverting fuel energy first int heat, which is then used t create mechanical mtin and finally electrical pwer, fuel cell efficiency is nt cnstrained by the Camt limits f heat engines (Fig. 4.25). Fuel-t-electric pwerefficiencies as high as 65% are likely, which gives fuel cells the ptential t be rughly twice as efficient as the average central pwer statin perating tday. Fuel cells have ther prperties besides high efficiency that make them especially appealing. The usual cmbustin prducts (SOx, particulates, CO, and varius unburned r partially burned hydrcarbns) are nt emitted, althugh there may be sme thermal NO x when fuel cells perate at high temperatures. They are vibratin-free and almst silent, which, when cupled with their lack f emissins, means they can be lcated very clse t their lads-fr example, in the basement f a building. Being clse t their lads, they nt nly avid transmissin and distributin system lsses, but their waste heat can be used t cgenerate electricity and useful heat fr applicatins such as space heating, air-cnditining, and ht water. Fuel-cell cgeneratin systems can have verall efficiencies frm fuel t electricity and heat f ver 80%. High verall efficiency nt nly saves fuel but als, if that fuel is a hydrcarbn such as natural gas, emissins f the principal greenhuse gas, CO 2, are reduced as well. n fact, CONVENTONAL COMBSTON FEL CELL Figure 4.25 Cnversin f chemical energy t electricity in a fuel cell is nt limited by the Carnt efficiency cnstraints f heat engines.

2 FEL CEllS 209 Plug Pwer, Analytic Pwer, General Mtrs, H-Pwer, Allisn Chalmers, Siemens, ELENCO (Belgium), nin Carbide, ExxnJAshthm, Tyta, Mazda, Hnda, Tshiba, Hitachi Ltd., shikawajima-harima Heavy ndustries, Deutsche Aerspace, Fuji Electric, Mitsubishi Electric Crp. (MELCO), Daimler Chrysler, Frd, Energy Research Crpratin, M-C Pwer Crp., Siemens-Westinghuse, CGE, DenNra, and Ansald. Clearly, there is an explsin f activity n the fuel cell frnt Basic Operatin f Fuel Cells There are many variatins n the basic fuel cell cncept, but a cmmn cnfiguratin lks smething like Fig As shwn there, a single cell cnsists f tw prus gas diffusin electrdes separated by an electrlyte. t is the chice f electrlyte that distinguishes ne fuel cell type frm anther. The electrlyte in Fig cnsists f a thin membrane that is capable f cnducting psitive ins but nt electrns r neutral gases. Guided by the flw field plates, fuel (hydrgen) is intrduced n ne side f the cell while an xidizer (xygen) enters frm the ppsite side. The entering hydrgen gas has a slight tendency t dissciate int prtns and electrns as fllws: H2 B 2H+ + 2e (4.17) This dissciatin can be encuraged by cating the electrdes r membrane with catalysts t help drive the reactin t the right. Since the hydrgen gas releases Electrical Lad (40% - 60% Efficiency) Electrns 2e-i i Current + Heat (85 0C) nused Fuel Recirculates _ H 2 0 Air + Water Vapr Flw Field Plate Flw Field Plate Gas Diffusin Electrde (Ande) H 2 - > 2W+2e- Catalyst Gas Diffusin Electrde (Cathde) 1/2 O 2 + 2W + 2e- -> H20 Figure 4.26 Basic cnfiguratin f a prtn-exchange membrane (PEM) fuel cell. 208 DSTRBTED GENERATON if fuel cells are pwered by hydrgen btained by electrlysis f water using renewable energy surces such as wind, hydrelectric, r phtvltaics, they have n greenhuse gas emissins at all. Fuel cells are easily mdulated t track shrt-term changes in electrical demand, and they d s with mdest cmprmises in efficiency. Finally, they are inherently mdular in nature, s that small amunts f generatin capacity can be added as lads grw rather than the cnventinal apprach f building large, central pwer statins in anticipatin f lad grwth Histrical Develpment While fuel cells are nw seen as a ptentially dminant distributed generatin technlgy fr the twenty-first century, it is wrth nting that they were first develped mre than 160 years ag. Sir William Grve, the English scientist credited with the inventin f the riginal galvanic cell battery, published his riginal experiments n what he called a "gaseus vltaic battery" in 1839 (Grve, 1839). He described the effects caused by his battery as fllws: "A shck was given which culd be felt by five persns jining hands, and which when taken by a single persn was painful." nterestingly, this same phenmenn is respnsible fr the way that the rgans and muscles f an electric eel supply their electric shck. Grve's battery depended upn a cntinuus supply f rare and expensive gases, and crrsin prblems were expected t result in a shrt cell lifetime, s the cncept was nt pursued. Fifty years later, Mnd and Langer picked up n Grve's wrk and develped a 1.5-W cell with 50% efficiency, which they named a "fuel cell" (Mnd and Langer, 1890). After anther half century f little prgress, Francis T. Bacn, a descendent f the famus seventeenth-century scientist, began wrk in 1932 that eventually resulted in what is usually thught t be the first practical fuel cell. By 1952, Bacn was able t demnstrate a 5 kw alkaline fuel cell (AFe) that pwered, amng ther things, a 2-tn capacity frk-lift truck. n the same year, Allis Chalmers demnstrated a 20-hrsepwer fuel-cell-pwered tractr. Fuel cell develpment was greatly stimulated by NASA's need fr n-bard electrical pwer fr spacecraft. The Gemini series f earth-rbiting missins used fuel cells that relied n permeable membrane technlgy, while the later Apll manned lunar explratins and subsequent Space Shuttle flights have used advanced versins f the alkaline fuel cells riginally develped by Bacn. Fuel cells nt nly prvide electrical pwer, their byprduct is pure water, which is used by astrnauts as a drinking water supply. Fr lnger missins, hwever, phtvltaic arrays, which cnvert sunlight int electric pwer, have becme the preferred technlgy. Fuel cells fr cars, buildings, central pwer statins, and spacecraft were the subject f intense develpment effrts in the last part f the twentieth century. Cmpanies with majr effrts in these applicatins include: Ballard Pwer Systems, nc. (Canada), General Electric Cmpany, the nternatinal Fuel Cells divisin f nited Technlgies Crp. and its ONS subsidiary,

3 A single cell, such as that shwn in Fig. 4.26, typically prduces n the rder f V r less under pen circuit cnditins and prduces abut 5 V under nrmal perating cnditins. T build up the vltage, cells can be stacked in series. T d s, the gas flw plates inside the stack are designed t be biplar; that is, they carry bth the xygen and hydrgen used by adjacent cells as is When cmbined, (4. 8) and (4. 19) result in the same equatin that we write fr rdinary cmbustin f hydrgen: H2 + i H20 '---v---- (4. 8) (4.19) wuld (4.20) FEL CELLS 211 The reactin described in (4.20) is exthermic; that is, it liberates heat (as ppsed t endthermic reactins, which need heat t be added t make them ccur). Since (4.20) is exthermic, it will ccur spntaneusly-the hydrgen and xygen want t cmbine t frm water. Their eagerness t d s prvides the energy that the fuel cell uses t deliver electrical energy t its lad. The questins, f curse, are hw much energy is liberated in reactin (4.20) and hw much f that can be cnverted t electrical energy. T answer thse questins, we need t describe three quantities frm thermdynamics: enthalpy.free energy, and entrpy. nfrtunately, the precise definitins f each f these quantities tend nt t lend themselves t easy, intuitive interpretatin. Mrever, they have very subtle prperties that are beynd ur scpe here, and there are risks in trying t present a simplified intrductin. The enthalpy f a substance is defined as the sum f its internal energy and the prduct f its vlume V and pressure P: suggested in Fig Enthalpy H = + PV (4.21) The internal energy f a substance refers t its micrscpic prperties, including the kinetic energies f mlecules and the energies assciated with the frces acting between mlecules, between atms within mlecules, and within atms The ttal energy f that substance is the sum f its internal energy plus the bservable, macrscpic frms such as its kinetic and ptential energies. ThE units f enthalpy are usually kj f energy per mle f substance. Mlecules in a system pssess energy in varius frms such as sensible am latent energy, which depends n temperature and state (slid, liquid, gaseus) chemical energy (assciated with the mlecular structure), and nuclear energ; (assciated with the atmic structure). Fr a discussin f fuel cells, it is change in chemical energy that are f interest, and thse changes are best described ij terms f enthalpy changes. Fr example, we can talk abut the ptential energy 0 an bject as being its weight times its height abve sme reference elevatin. Ou chice f reference elevatin des nt matter as lng as we are nly intereste in the change in ptential energy as an bject is raised against gravity frm n elevatin t anther. The same cncept applies fr enthalpy. We need t describ it with respect t sme arbitrary reference cnditins. n the case f enthalpy a reference temperature f 25 C and a reference pres sure f atmsphere (standard temperature and pressure, STP) are assumed. is als assumed that the reference cnditin fr the chemically stable frm c an element at 1 atmsphere and 25 C is zer. Fr example, the stable frm c xygen at STP is gaseus O2, s the enthalpy fr 02(g) is zer, where (g) jus means it is in the gaseus state. On the ther hand, since atmic xygen (0) j nt stable, its enthalpy is nt zer (it is, in fact, kj/ml). Ntice that th state f a substance at STP matters. Mercury, fr example, at 1 atmsphere an 25 C is a liquid, s the standard enthalpy fr Hg(l) is zer, where (l) means th liquid state. One way t think abut enthalpy is that it is a measure f the energy th: it takes t frm that substance ut f its cnstituent elements. The differenc prtns in the vicinity f the electrde n the left (the ande), there will be a cncentratin gradient acrss the membrane between the tw electrdes. This gradient will cause prtns t diffuse thrugh the membrane leaving electrns behind. As a result, the cathde takes n a psitive charge with respect t the ande. Thse electrns that had been left behind are drawn tward the psitively charged cathde; but since they can't pass thrugh the membrane, they must find sme ther rute. f an external circuit is created between the electrdes, the electrns will take that path t get t the cathde. The resulting flw f electrns thrugh the external circuit delivers energy t the lad (remember that cnventinal current flw is ppsite t the directin that electrns mve, s current "flws" frm cathde t ande) Fuel Cell Thermdynamics: Enthalpy The fuel cell shwn in Fig is described by the fllwing pair f halfcell reactins: Ande: H H+ + 2e- Cathde: i02 + 2H+ + 2e- -+ H 2 0 Mlti-Ceil Stack 210 DSTRBTED GENERATON Electrlyte Figure 4.27 A multicell stack made up f multiple cells increases the vltage. After Srinivasan et al. (1999).

4 212 DSTRBTED GENERATON between the enthalpy f the substance and the enthalpies f its elements is called the enthalpy ffnnatin. t is in essence the energy stred in that substance due t its chemical cmpsitin. A shrt list f enthalpies f frmatin at STP cnditins appears in Table 4.6. T remind us that a particular value f enthalpy (r ther thermdynamic prperties such as entrpy and free energy) is at STP cnditins, a superscript "0" is used (e.g. H O ). Table 4.6 als includes tw ther quantities, the abslute entrpy S and the Gibbs free energy GO, which will be useful when we try t determine the maximum pssible fuel cell efficiency. Ntice when a substance has negative enthalpy f frmatin, it means that the chemical energy in that substance is less than that f the cnstituents frm which it was frmed. That is, during its frmatin, sme f the energy in the reactants didn't end up as chemical energy in the final substance. n chemical reactins, the difference between the enthalpy f the prducts and the enthalpy f the reactants tells us hw much energy is released r absrbed in the reactin. When there is less enthalpy in the final prducts than in the reactants, heat is liberated-that is, the reactin is exthermic. When it is the ther way arund, heat is absrbed and the reactin is endthermic. f we analyze the reactin in (4.20), the enthalpies f Hz and Oz are zer s the enthalpy f frmatin is simply the enthalpy f the resulting HzO. Ntice in Table 4.6 that the enthalpy f H 2 0 depends n whether it is liquid water r gaseus water vapr. When the result is liquid water: Hz + Oz HzO(l) When the resulting prduct is water vapr: Hz + Oz HzO(g) H = kl H = kl TABLE 4.6 Enthalpy f Frmatin H 0, Abslute Entrpy S and Gibbs Free Energy GO at 1 atm, 25 C fr Selected Substances Substance State W (kl./ml) S (kllml-k) GO (kllm!) H Gas H 2 Gas Gas Oz Gas H 2O Liquid H 2O Gas C Slid C-i+ Gas CO Gas CO 2 Gas CH 30H Liquid (4.22) (4.23) The negative signs fr the enthalpy changes in (4.22) and (4.23) tell us these reactins are exthermic; that is, heat is released. The difference between the enthalpy f liquid water and gaseus water vapr is 44.0 kl/ml, Therefre, that amunt is the familiar latent heat f vaprizatin f water. Recall that latent heat is what distinguishes the higher heating valuez (HHV) f a hydrgen-cntaining fuel and the lwer heating value LHV. The HHV includes the 44.0 kl/ml f latent heat in the water vapr frmed during cmbustin, while the LHV des nt. Example 4.8 The High Heating Value (HHV) fr Methane. Find the HHV f methane CH 4 in kllml and kl/kg when it is xidized t COz and liquid HzO. Slutin. The reactin is written belw, and beneath it are enthalpies taken frm Table 4.6. Ntice thatwe must balance the equatin s that we knw hw many mles f each cnstituent are invlved. CH 4 (g ) (g ) (-74.9) 2 x (0) COz(g) + (-393.5) 2HzO(l) 2 x (-285.8) Ntice, t, that we have used the enthalpy f liquid water t find the HHV. The difference between the ttal enthalpy f the reactin prducts and the reactants is H = [(-393.5) + 2 x (-285.8)] - [(-74.9) + 2 x (0)] = -892 kl /ml f CH4 HHV = 892 kl/ml glml x 1000 g/kg = 55,490 kl/kg FEL CELLS 213 Since the result is negative, heat is released during cmbustin; that is, it is exthermic. The HHV is the abslute value f H, which is 892 kllm!. Since there are x = glml f CH 4, the HHV can als be written as Entrpy and the Theretical Efficiency f Fuel Cells While the enthalpy change tells us hw much energy is released in a fuel cell, it desn't tell us hw much f that energy can be cnverted directly int electricity. T figure that ut, we need t review anther thermdynamic quantity, entrpy. Entrpy has already been intrduced in the cntext f heat engines in Sectin 3.4.2, where it was used t help develp the Carnt efficiency limit. n a similar fashin, the cncept f entrpy will help us develp the maximum efficiency f a fuel cel!.

5 <:.,.. ;, hlb t: l:>t:nt:ha tn f-::lcl::lls ::> T begin, let us nte that all energy is nt created equal. That is, fr example, 1 jule f energy in the frm f electricity r mechanical wrk is much mre useful than a jule f heat. We can cnvert that jule f electricity r wrk int heat with 100% cnversin efficiency, but we cannt get back the jule f electricity r wrk frm just a single jule f heat. What this suggests is that there is a hierarchy f energy frms, with sme being "better" than thers. Electricity and mechanical energy (ding wrk) are f the highest quality. n thery we culd g back and frth between electricity and mechanical wrk with 100% cnversin efficiency. Heat energy is f much lwer quality, with lwtemperature heat being f lwer quality than high-temperature heat. S, where des chemical energy fit in this scheme? t is better than thermal, but wrse than mechanical and electrical. Entrpy will help us figure ut just where it stands. Recall that when an amunt f heat Q is remved frm a thermal reservir large enugh that its temperature T des nt change during the prcess (i.e., the prcess is isthermal), the lss f entrpy t1s frm the reservir is defined t be t1s = With Q measured in kiljules (kj) and T in kelvins (K = c ), the units f entrpy are kj/k. Recall, t, that entrpy is nly assciated with heat transfer and that electrical r mechanical wrk is perfect s that these frms have zer entrpy. And, finally, remember that in any real system, if we carefully add up all f the entrpy changes, the secnd law f thermdynamics requires that there be an verall increase in entrpy. Nw let us apply these ideas t a fuel cell. Cnsider Fig. 4.28, which shws a fuel cell cnverting chemical energy int electricity and waste heat. The fuel cell reactins (4.22) and (4.23) are exthermic, which means that their enthalpy changes t1h are negative. Wrking with negative quantities leads t awkward nmenclature, which we can avid by saying that thse reactins act as a surce f enthalpy H that can be cnverted t heat and wrk as Fig implies. Q T (4.24) The cell generates an amunt felectricity We and rejects an amunt f thermal energy Q t its envirnment. Since there is heat transfer and it is a real system, there must be an increase in entrpy. We can use that requirement t determine the minimum amunt f rejected heat and therefre the maximum amunt f electric pwer that the fuel cell will generate. T d s, we need t carefully tabulate the entrpy changes ccurring in the cell: H 2 + i02 -+ H 20+ Q t1s = Q T (4.25) where we have included the fact that heat Q will be released. The entrpy f the reactants H 2 and O 2 will disappear, but new entrpy will appear in the H 2 0 that is frmed plus the entrpy that appears in the frm f heat Q. As lng as the prcess is isthermal, which is a reasnable assumptin fr a fuel cell, we can write the entrpy appearing in the rejected heat as (4.26) What abut the entrpy assciated with the wrk dne, We? Since there is n heat transfer in electrical (r mechanical) wrk, that entrpy is zer. T make the necessary tabulatin, we need values f the entrpy f the reactants and prducts. And, as usual, we need t define reference cnditins. t is cnventinal practice t declare that the entrpy f a pure crystalline substance at zer abslute temperature is zer (the "third law f thermdynamics"). The entrpy f a substance under ther cnditins, referenced t the zer base cnditins, is called the abslute entrpy f that substance, and thse values are tabulated in a number f places. Table 4.6 gives abslute entrpy values, S, fr several substances under STP cnditins (25 C, atm). The secnd law f thermdynamics requires that in a real fuel cell there be a net increase in entrpy (an ideal cell will release just enugh heat t make the increase in entrpy be zer). Therefre, we can write that the entrpy that shws up in the rejected heat and the prduct water (liquid water) must be greater than the entrpy cntained in the reactants (H 2 and O 2 ) : Enthalpy in H Entrpy gain Entrpy lss ; + L Sprducts > L Sreactants (4.27) (4.28) Enthalpy utput We which leads t Q > T (L Sreactants - L SprductS) (4.29) Figure 4.28 Rejected heat Q The energy balance fr a fuel cell. Equatin (4.29) tells us the minimum amunt f heat that must appear in the fuel cell. That is, we cannt cnvert all f the fuel's energy int electricity-we are stuck with sme thermal lsses. At least ur thermal lsses are ging t be less than if we tried t generate electricity with a heat engine.

6 216 DSTRBTED GENERATON We can nw easily determine the maximum efficiency f the fuel cell. Frm Fig. 4.28, the enthalpy supplied by the chemical reactin H equals the electricity prduced We plus the heat rejected Q: H = We + Q (4.30) Since it is the electrical utput that we want, we can write the fuel cell's efficiencyas T find the maximum efficiency, all we need t d is plug in the theretical minimum amunt f heat Q frm (4.29). a. Find the minimum amunt f heat rejected per mle f H 2. b. What is the maximum efficiency f the fuel cell? Slutin We H - Q Q '7=-=--=1-- H H H (4.31) Example 4.9 Minimum Heat Released frm a Fuel Cell. Suppse a fuel cell that perates at 25 C (298 K) and atm frms liquid water (that is, we are wrking with the HHV f the hydrgen fuel): H :> H 20(l) t1h = kl/rnl f H 2 a. Frm the reactin, mle f H 2 reacts with l/2 mle f O 2 t prduce mle f liquid H2 The lss f entrpy by the reactants per mle f H 2 is fund using values given in Table 4.6: FEL CELLS b. Frm (4.22), the enthalpy made available during the frmatin f liquid water frm H 2 and O 2 is H = kllml f H 2. The maximum efficiency pssible ccurs when Q is a minimum; thus frm (4.31) Qrnin = % f]rnax = - H Gibbs Free Energy and Fuel Cell Efficiency The chemical energy released in a reactin can be thught f as cnsisting f tw parts: an entrpy-free part, called/ree energy t1g, that can be cnverted directly int electrical r mechanical wrk, plus a part that must appear as heat Q. The "G" in free energy is in hnr f Jsiah Willard Gibbs ( ), wh first described its usefulness, and the quantity is usually referred t as Gibbs free energy. The free energy G is the enthalpy H created by the chemical reactin, minus the heat that must be liberated, Q = T t1s, t satisfy the secnd law. The Gibbs free energy t1g crrespnds t the maximum pssible, entrpyfree, electrical (r mechanical) utput frm a chemical reactin. t can be fund at STP using Table 4.6 by taking the difference between the sum f the Gibbs energies f the reactants and the prducts: t1g = L Gprducts - L Greactants 217 (4.32) This means that the maximum pssible efficiency is just the rati f the Gibbs free energy t the enthalpy change t1h in the chemical reactin t1g f]rnax = t1h (4.33) L Sreactants = 130 kjml-k x ml H kj/ml-k x 5 ml O 2 = 2325 k1lk The gain in entrpy in the prduct water is L Sprduct = 0699 kl/rnl-k x ml H 20(l) = 0699 kjk Frm (4.29), the rrurumum amunt f heat released during the reactin is therefre Qrnin = T (L Sreactants - L SprduClS) Example 4.10 Maximum Fuel Cell Efficiency sing Gibbs Free Energy. What is the maximum efficiency at STP f a prtn-exchange-membrane (PEM) fuel cell based n the higher heating value (HHV) f hydrgen? Slutin. The HHV crrespnds t liberated water in the liquid state s that the apprpriate reactin is H :> H 20(l) t1h = kl/ml f H 2 Frm Table 4.6 the Gibbs free energy f the reactants H 2 and f O 2 are bth zer, and that f the prduct, liquid water, is kj. Therefre, frm 4.32) = 298 K ( ) kj/k = kj per mle H 2 t1g = (0 + 0) = kl/rnl

7 This is the same answer that we fund in Example 4.9 using entrpy. (4.34) T use (4.34) we just have t adjust the units s that the electrical utput We will have the cnventinal electrical units f vlts, amps, and watts. T d s, let us intrduce the fllwing nmenclature alng with apprpriate physical cnstants: Fr each mle f H2 int an ideal fuel cell, tw electrns will pass thrugh the electrical lad (see Fig. 4.26). We can therefre write that the current flwing thrugh the lad will be ( (4.35) t-bt, G;:LL::> c; ':J sing (4.34), the ideal pwer (watts) delivered t the lad will be the kl/rnl f Hz times the rate f hydrgen use: lw pew) = 237.2(kJ/ml) x n(ml/s) x 1000(J/kJ). -1- = 237,200n (4.36) And the reversible vltage prduced acrss the terminals f this ideal fuel cell will be pew) 237,200n Electrical Characteristics f Real Fuel Cells J s V =-- = = V R (A) 192,945n (4.37) Ntice the vltage des nt depend n the input rate f hydrgen. t shuld als be nted that the ideal vltage drps with increasing temperature, s that at the mre realistic perating temperature f a PEM cell f abut 80 C, V R is clser t 1.18 V. We can nw easily find the hydrgen that needs t be supplied t this ideal fuel cell per kwh f electricity generated. Hydrgen rate = n(ml/s) x 2(g/ml) x 3600 slh 237,200n(W) x 1O-3(kWW) = 335 gh2/kwh (4.38) Just as real heat engines dn't perfrm nearly as well as a perfect Carnt engine, real fuel cells dn't deliver the full Gibbs free energy either. Activatin lsses result frm the energy required by the catalysts t initiate the reactins. The relatively slw speed f reactins at the cathde, where xygen cmbines with prtns and electrns t frm water, tends t limit fuel cell pwer. Ohmic lsses result frm current passing thrugh the internal resistance psed by the electrlyte membrane, electrdes, and varius intercnnectins in the cell. Anther lss, referred t as fuel crssver, results frm fuel passing thrugh the electrlyte withut releasing its electrns t the external circuit. And finally, mass transprt lsses result when hydrgen and xygen gases have difficulty reaching the electrdes. This is especially true at the cathde if water is allwed t build up, clgging the catalyst. Fr these and ther reasns, real fuel cells, in general, generate nly abut 60-70% f the theretical maximum. Figure 4.29 shws the relatinship between current and vltage fr a typical fuel cell (phtvltaic V curves bear a striking resemblance t thse fr a fuel cell). Ntice that the vltage at zer current, called the pen-circuit vltage, is a little less than 1 V, which is abut 25% lwer than the theretical value f V. Als shwn is the prduct f vltage and current, which is pwer. Since pwer at zer current, r at zer vltage, is zer, there must be a pint smewhere in between at which pwer is a maximum. As shwn in the figure, that maximum crrespnds t peratin f the fuel cell at between 4 The Gibbs free energy f:,.g is the maximum pssible amunt f wrk r electricity that a fuel cell can deliver. Since wrk and electricity can be cnverted back and frth withut lss, they are referred t as reversible frms f energy. Fr an ideal hydrgen fuel cell, the maximum pssible electrical utput is therefre equal t the magnitude f f:,.g. Fr a fuel cell prducing liquid water, this makes the maximum electrical utput at STP equal t 218 DSTRBTED GENERATON S, frm (4.33), f:,.g kllml = 830 = 83.0% ]max = f:,.h - c c Q 1.T/_u, Electrical Output f an deal Cell We = f:,.g = kj per ml f H 2 q = charge n an electrn = x culmbs N = Avgadr's number = x mlecules/ml v = vlume f 1 mle f ideal gas at STP = 22.4 liter/ml n = rate f flw f hydrgen int the cell (mlls) = current (A), where 1 A = 1 culmb/s V R = ideal (reversible) vltage acrss the tw electrdes (vlts) p = electrical pwer delivered (W) mol) n (molecles HJ) 2 electrns s ml mlecule H 2 (A) = n x x (COlOmbS) (A) = 192, 945n electrn

8 220 DSTRBTED GENERATON FELCELLS Activatin 1.2 Lss Regin (J) OJ 21 0 > 6 ill () "[ 4 CD ;;: 3 e 2 1 Slutin. With 6-V cells all wired in series, 48/6 = 80 cells wuldbe needed t generate 48 V dc. The current that needs t flw thrugh each cell is sing (4.39) t find the area f each cell yields P = V = Types f Fuel Cells 25 6 = 85 A x W/80 cells = 283 A 6 V/cell A = 283 crrr' Current Density (Alcm 2 ) V = = Figure 4.29 The vltage-current curve fr a typical fuel cell. Als shwn is the pwer delivered, which is the prduct f vltage and current. and 5 V per cell. The three regins shwn n the graph pint ut the ranges f currents in which activatin, hmic, and mass-transprt lsses are individually mst imprtant. Over mst f the length f the fuel cell - V graph, vltage drps linearly as current increases. This suggests a simple equivalent circuit cnsisting f a vltage surce in series with sme internal resistance. Fitting the _ V curve in the hmic regin fr the fuel cell shwn in Fig yields the fllwing apprximate relatinship: 25 A (4.39) where A is cell area (crrr'), is current (amps), and is current density (Azcm-). Example 4.11 Rugh Parameters f a Hme-Scale Fuel Cell Stack. A 1-kW fuel cell perating n a cntinuus basis wuld prvide all f the electrical needs f a typical.s. huse. f such a fuel cell stack generates 48 V dc with cells perating at 6 Veach, hw many cells f the type described by (4.39) wuld be needed and what shuld be the membrane area f each cell? T this pint in this chapter, the fuel cell reactins and explanatins have been based n the assumptin that hydrgen H 2 is the fuel, Eq. (4.18) and (4.19) describe the reactins, and the electrlyte passes prtns frm ande t cathde thrugh a membrane. While it is true that these are the mst likely candidates fr vehicles and small, statinary pwer systems, there are cmpeting technlgies that use ther electrlytes and which have ther distinctive characteristics that may make them mre suitable in sme applicatins. Prtn Exchange Membrane Fuel Cells (PEMFC) Originally knwn as Slid Plymer Electrlyte (SPE) fuel cells, and smetimes nw called plymer electrlyte membrane fuel cells, these are the furthest alng in their develpment, in part because f the early stimulus prvided by the Gemini space prgram, and especially nw since they are the leading candidates fr use in hybrid electric vehicles (HEVs). Their efficiencies are the highest available at arund 45% (HHV). Currently perating units range in size frm 30 W t 250 kw. PEM cells generate ver 5 W/cm 2 f membrane at arund 65 V per cell and a current density f 1 Azcrrr'. T cntrl water evapratin frm the membranes, these cells require active cling t keep temperatures in the desired perating range f 50 C t 80 C. With such lw temperatures, waste heat cgeneratin is restricted t simple water r space heating applicatins, which is fine fr residential pwer systems. One limitatin f PEM cells is their need fr very pure hydrgen as their fuel surce. Hydrgen refrmed frm hydrcarbn fuels such as methanl (CH 30H) r methane CH 4 ften cntains carbn mnxide (CO), which can lead t CO pisning f the catalyst. When CO adsrbs nt the surface f the ande catalyst, it diminishes the availability f sites where the hydrgen reactins need t take place. Minimizing CO pisning, managing water and heat in the cell stack, and develping lwer-cst materials and manufacturing techniques are current challenges fr PEM cells.

9 FELCELLS 223 The majr prblem with alkaline fuel cells is their intlerance fr expsure t C02, even at the lw levels fund in the atmsphere. Since air is the surce f O 2 fr the cathdic reactins, it is unlikely that these will be used in terrestrial applicatins. Mlten-Carbnate Fuel Cells (MCFC) These fuel cells perate at very high temperatures, n the rder f 650 C, which means that the waste heat is f high enugh quality that it can be used t generate additinal pwer in accmpanying steam r gas turbines. At this high temperature, there is the ptential fr the fuel cell waste heat t be used t directly cnvert, r refrm, a hydrcarbn fuel, such as methane, int hydrgen by the fuel cell itself. Mrever, the usual accmpanying CO in fuel refrming des nt pisn the catalyst and, in fact, becmes part f the fuel. Efficiencies f 50-55% are prjected fr internalrefrming MCFCs. With cmbined-cycle peratin, electrical efficiencies f 65% are prjected, and cgeneratin efficiencies f ver 80% are pssible. n an MCFC the cnducting in is carbnate C032- rather than H+, and the electrlyte is mlten lithium, ptassium, r sdium carbnate. At the cathde, C 02 and 02 cmbine t frm carbnate ins, which are cnducted thrugh the electrlyte t the ande where they cmbine with hydrgen t frm water and carbn dixide as shwn in the fllwing electrchemical reactins: H2 + C H20 + C02 + 2e- (Ande) 1 : CO 2 + 2e C03 2- (Cathde) (4.45) (4.46) Ntice the verall reactin is the same as that described earlier fr a "generic" fuel cell H2+ : H20 (Overall) (4.47) MCFCs perate in a very crrsive envirnment, and the challenges assciated with devising apprpriate materials that will perate with suitably lng lifetimes are significant. Slid Oxide Fuel Cells (SOFC) SOFCs and MCFCs are cmpeting fr the future large pwer statin market. Bth perate at such high temperatures (MCFC, 650 C; SOFC, C) that their waste heat can be used fr cmbinedcycle steam r cmbined cycle gas turbines, and bth can take advantage f thse temperatures t d internal fuel refrming. The SOFC is physically smaller than an MCFC fr the same pwer, and it may ultimately have greater lngevity. The electrlyte in an SOFC is a slid ceramic material made f zircnia and yttria, which is very unlike the liquids and slid plymers used in every ther type f fuel cell. The charge carrier that is transprted acrss the electrlyte is the xide 0 2- in, which is frmed at the cathde when xygen cmbines with 222 DSTRBTED GENERATON Direct Methanl Fuel Cells (DMFC) These cells use the same plymer electrlytes as PEM cells d, but they ffer the significant advantage f being able t utilize a liquid fuel, methanl (CH30H), instead f gaseus hydrgen. Liquid fuels are much mre cnvenient fr prtable applicatins such as mtr vehicles as well as small, prtable pwer surces fr everything frm cell phnes and lap-tp cmputers t replacements fr diesel-engine generatrs. The chemical reactins taking place at the ande and cathde are as fllws: CH30H + H CO2 + 6H+ + 6e- (Ande) 1, H' + 2e H20 (Cathde) 2 (4.40) (4.41) fr an verall reactin f 3 CH30 H + : CO2 + 2H20 (Overall) (4.42) Significant technical challenges remain, including cntrl f excessive fuel crssver thrugh the membrane cncern fr methanl txicity, and reducing catalyst pisning by CO and ther methanl reactin prducts. The advantages f prtability and simplified fuel handling, hwever, almst guarantee that these will be cmmercially available in the very near future. Phsphric Acid Fuel Cells (PAFC) These fuel cells were intrduced int the marketplace in the 1990s, and there are hundreds f 200-kW units built by the ONS divisin f FC currently in peratin. Their perating temperature is higher than that f PEMFCs (clse t 200 C), which makes the waste heat mre usable fr absrptin air cnditining as well as water and space heating in buildings. The electrchemical reactins taking place in a PAFC are the same nes that ccur in a PEM cell, but the electrlyte in this case is phsphric acid rather than a prtn exchange membrane. These cells tlerate CO better than PEM cells, but they are quite sensitive t H2S. Althugh there are already a number f PAFCs in use, their future will be clsely tied t whether higher prductin levels will be able t reduce manufacturing csts t the pint where they will be cmpetitive with ther cgeneratin technlgies. Alkaline Fuel Cells (AFC) These highly efficient and reliable fuel cells were develped fr the Apll and Space Shuttle prgrams. Their electrlyte is ptassium hydrxide (KOH), and the charge carrier is OH- rather than H+ ins. The electrchemical reactins are as fllws: H2+ 2 OH H20 + 2e HO + 2e H- 2 (Ande) (Cathde) (4.43) (4.44)

10 224 DSTRBTED GENERATON electrns frm the ande. At the ande, the xide in cmbines with hydrgen t frm water and electrns, as shwn belw: H H20 + 2e- (Ande) e (Cathde) 2 (4.48) (4.49) Efficiencies fr SOFCs f 60% fr electric pwer and greater than 80% fr cgeneratin are prjected. When cmbined with a gas turbine, such as is suggested in Fig. 4.30, electrical efficiencies appraching 70% (LHV) may be achievable. A summary f the main characteristics f these varius categries f fuel cells is presented in Table 4.7. u u, tr: u, :2 "'.;: :::ll >< N ;g f/)!j) cd BZ u b f) - trl t> ;g. 0::'::: C::.. '" -<...:-.:a8d :2 -e B '" c:: c :QO"Cl '" '" -e c:: c a 5"Cl 8 ti " c:: <8::r:: A e:: c f/) OZ a 0 "2 trl a ci Z"ClB"A.- c::... ::r:: (' u uz::r:: ::r:: c 0 c::.- Ol.- -< -< Cti '" bl) '" '" e 0 c, C 0 c. '.c c.- -< Cj t -< C '" '" -< C 01) '" e '" 0 '" c.. [i u ;2; 0:; 0:;.a "0 Ow '" u c.c0- '".c 0- [i g; Hydrgen Prductin With the exceptin f DMFCs, fuel cells require a surce f hydrgen H 2 fr the andic reactins. Fr thse that perate at higher temperatures (MCFCs and SOFCs), methane may be refrmed t yield hydrgen as part f the fuel cell system itself; but in general, btaining a supply f hydrgen f sufficient purity and at a reasnable cst is a majr hurdle that must be dealt with befre largescale cmmercializatin f fuel cells will be achieved. Hydrgen as a fuel has many desirable attributes. When burned, it yields nly small amunts f NO x created when cmbustin temperatures are high enugh t cause the nitrgen and xygen in air t cmbine, and when used in fuel cells, the nly end prduct is water. Given its lw density, it readily escapes frm cnfined envirnments s that it is less likely t cncentrate int dangerus pls the way that gasline fumes, fr example, d. t is, hwever, nt an energy surce. t is, Exhaust.- pwer ACDC +., nverter Air Recuperatr SOFC Cathde Blwer Cmpressr Figure 4.30 Gas turbines witb pressurized slid-xide fuel cells may be capable f nearly 70% LHV efficiency. CH 4 s, "0 u 0) :::l u, "-.:!i 1: 0 u 1: ::l ;;..... e '" <l>?'... : <l>... is... e ::: '"' ' : '" <l>... '"' ell... ell.c: -; ::: : <l> "l '" r- -.i "l...:l '"?' u, :2 Q "Cl 'u ;;;- c< : "'0 8'" 0-..c:: 0 <8 N f) "'00 0..c:: N c, - ::r::c..::r:: u, 'C 0 b 05 n.j '" V gf -< G ::r:: c'sb 0 c "'B Qj. ("'"', 2x=_+::r:: c.. 2"'-;;:; ::r::c..u '" bl)'" :g 15 -B a b ><i\l0\ 2 '" '" c.. f) 0 '"S b '" "Cl '" E.;< z 0 u..:2 8 0 < "'"Cl trl >-'N Z.....c:'-'l ::r:: N 0 c.. f) O:;::r:: [i.g!f.; 'C B c a..c:: 5 8 :::l N 1-< ::r::c..::r:: C) 1-< t). c-. :: <;; gr 8 =:..v r01)-< '" ;a..c:: _ :::l r.r.lu. E < :O _trl A A f/) u 0' 6 E A '"B c 'B ] -< '" "Cl '".- c:: <t: c '0'" c, c._ g ',c -E c '" 0 -< C -< 1.) C) Zl '" C en c:: 5 8 E c, '" f/) C.9 Ol 1::,,,is....- en..0 C ::'d -< -< c, c::l c:: ".g 1::,,,is '" c:: CO CO -< B a, '" C.S.. < 0:; 0:;.;::.S "'" [i <t: 0:; 2 '0.,s '"E t) e '6 0= ;2;':: '"'" '" " =" "'''0._ ::l ' 4-. "0 0,);'::: c: 0 '" '".Eu E u, r. "'00\ El8:: = :: c Q) C'j u r.c u '" '" 2 c: 25 e * "2.1: -c 01'll '"'" '.. ;2; i3 i:! u.;-'=::: 225

11 226 DSTRBTED GENERATON like electricity, a high-quality energy carrier that is nt naturally available in the envirnment. t must be manufactured, which means an energy investment must be made t create the desired hydrgen fuel. The main technlgies currently in use fr hydrgen prductin are steam refrming f methane (SMR), partial xidatin (POX), and electrlysis f water. Mre extic methds f prductin in the future may include phtcatalytic, phtelectrchemical, r bilgical prductin f hydrgen using sunlight as the energy surce. Methane Steam Refrming (MSR) Abut 5% f.s. natural gas is already cnverted t hydrgen fr use in ammnia prductin, il refining, and a variety f ther chemical prcesses. Almst all f that is dne with steam methane refrmers. After sme gas cleanup, especially t remve sulfur, a mixture f natural gas and steam is passed thrugh a catalyst at very high temperature ( C), prducing a synthesis gas, r syngas, cnsisting f CO and H 2: CH4 + H CO + 3H2 The abve reactin is endthermic; that is, heat must be added, which may be prvided in part by burning sme f the methane as fuel. The hydrgen cncentratin in the syngas is then increased using a water-gas shift reactin: CO + H CO2 + H2 (4.50) (4.51) This reactin is exthermic, which means sme f the heat released can be used t drive (4.50). The resulting syngas in (4.51) is 70-80% H 2, with mst f the remainder being CO2 plus small quantities f CO, H 20, and CH. 4 Final prcessing includes remval f CO2 and cnversin f remaining CO int methane in a reverse f reactin (4.50). The verall energy efficiency f SMR hydrgen prductin is typically 75-80%, but higher levels are achievable. Partial Oxidatin (POX) These systems are based n methane (r ther hydrcarbn fuels) being partially xidized in the fllwing exthermic reactin: FEL CELLS 227 the primary methd f hydrgen prductin befre natural gas becme s widely available. With the likelihd f relatively inexpensive technlgy t remve CO 2 frm the resulting syngas, there is grwing interest in cal gasificatin fr hydrgen prductin, fllwed by capture and sequestratin f CO2 in deep saline aquifers r depleted gas fields. Sme researchers hpe such carbn sequestratin may prvide a way t cntinue t explit the earth's large cal resurces with minimal carbn emissins. Electrlysis f Water n reactins that are the reverse f cnventinal fuel cells, electrical current frced thrugh an electrlyte can be used t break apart water mlecules, releasing hydrgen and xygen gases: 2H H2 + O2 (4.53) n fact, the same membranes that are used in PEM cells can be used in lwtemperature electrlyzers. Similarly, slid-xide electrlytes can be used fr hightemperature electrlysis. A sketch f an electrlysis cell that uses a prtn exchange membrane is shwn in Fig De-inized water intrduced int the xygen side f the cell dissciates int prtns, electrns, and xygen. The xygen is liberated, the prtns pass thrugh the membrane, and the electrns take the external path thrugh the pwer surce t reach the cathde where they reunite with prtns t frm hydrgen gas. Overall efficiency can be as high as 85%. Hydrgen prduced by electrlysis has the advantage f being highly purified, s the prblems f catalytic CO pisning that sme fuel cells are subject t is nt a cncern. When the electricity fr electrlysis is generated using a renewable energy system, such as wind, hydr, r phtvltaic pwer, hydrgen is prduced withut emissin f any greenhuse gases. And, as Fig suggests, when the resulting hydrgen is subsequently cnverted back t electricity using fuel cells, Pwer surce CH4 + " CO + 2H2 (4.52) Since (4.52) is exthermic, it prduces its wn heat, which makes it ptentially simpler than the MSR prcess since it can eliminate the MSR heat exchanger required t transfer heat frm (4.51) t (4.50). After the partial xidatin step, a cnventinal shift reactin can be used t cncentrate the H2 in the resulting syngas. H 20 O 2 Ande Water 2H 20 -> 4H e- 4e- Cathde 4e- + 4H+ -> 2H 2 2H 2 Gasificatin f Bimass, Cal, r Wastes As mentined in Sectin 4.4, gasificatin f bimass r ther slid fuels such as cal r municipal wastes by high-temperature pyrlysis can be used t prduce hydrgen. n fact, that was Figure 4.31 Prtn exchange membrane (PEM) A prtn exchange membrane used t electrlyze water.

12 228 DSTRBTED GENERATON PROBLEMS 229 Phtvltaics Oxygen Oxygen Srinivasan, S., R. Msdale, P. Stevens, and C. Yang (1999). Fuel Cells: Reaching the Era f Clean and Efficient Pwer Generatin in the Twenty-First Century. Annual Review f Energy and Envirnment, pp PROBLEMS Figure 4.32 Renewable energy Surces cupled with fuel cells can prvide electric pwer where and when it is required, cleanly, and sustainably. the ultimate gals f carbn-free electricity, available whenever it is needed, whether r nt the sun is shining r the wind is blwing, withut depleting scarce nnrenewable resurces, can becme an achievable reality. REFERENCES Fuel Cell Pwer Babcck and Wilcx (1992). Steam, 40th ed., Babcck & Wilcx, Barbertn, OH. Cler, G. L, and M. Shepard (1996). Distributed Generatin: Gd Things Are Cming in Small Packages, Esurce Tech pdate, T-96-12, Esurce, Bulder, CO. Davenprt, R. L., B. Butler, R. Taylr, R. Frristall, S. Jhanssn, J. lrich, C. VanAusdal, and J. Mayette (2002). Operatin f Secnd-Generatin Dish/Stirling Pwer Systems, Prceedings fthe 31st ASES Annual Cnference, June 15-2 Grve, W. R. (1839). On Vltaic Series in Cmbinatins f Gases by Platinum, Philsphical Magazine, vl. 14, pp nversin, A. R. (1986). Micr-;!ydrpwer Surcebk, Natinal Rural Electrificatin Cperative Assciatin, Arlingtn, VA. McNeely, M. (2003). ARES-Gas Engines fr Tday and Beynd, Diesel and Gas Turbine Wrldwide, May, pp Mnd, L. L., and C. Langer (1890). Prceedings f the Ryal Sciety (Lndn), Services, A, vl. 46, pp Perchers, N. (2002). Cmbined Heating, Cling and Pwer: Technlgies and Applicatins, The Fairmnt Press, Lilburn, GA. Price, H., E. Lupfert, D. Kearney, E. Zarra, G. Chen, R. Gee, and R. Mahney (2002). Advances in Parablic Trugh Slar Pwer Technlgy, Jurnal fslar Engineering, May,pp A natural-gas-fired micrturbine has an verall efficiency f 26% when expressed n an LHV basis. sing data frm Table 4.2, find the efficiency expressed n an HHV basis. 4.2 On an HHV basis, a 600-MW cal-fired pwer plant has a heat rate f 9700 Btu/kWh. The particular cal being burned has an LHV f 5957 Btu/ bm and an HHV f 6440 Btu/lbm. a. What is its HHV efficiency? b. What is its LHV efficiency? c. At what rate will cal have t be supplied t the plant (tnslhr)? 4.3 A natural-gas fueled, 250-kW, slid-xide fuel cell with a heat rate f 7260 Btu/kWh csts $1.5 millin. n its cgeneratin mde, 300,000 Btu/hr f exhaust heat is recvered, displacing the need fr heat that wuld have been prvided frm a 75% efficient gas-fired biler. Natural gas csts $5 per millin Btu and electricity purchased frm the utility csts $O.lO/kWh. The system perates with a capacity factr f 80%. a. What is the value f the fuel saved by the waste heat ($/yr)? b. What is the savings assciated with nt having t purchase utility electricity ($/yr)? c. What is the annual cst f natural gas fr the CHP system? d. With annual O&M csts equal t 2% f the capital cst, what is the net annual savings f the CHP system? e. What is the simple payback (rati f initial investment t annual savings)? 4.4 Suppse 200 gpm f water is taken frm a creek and delivered thrugh 800 ft f 3-inch diameter PVC pipe t a turbineloo ft lwer than the surce. f the turbine/generatr has an efficiency f 40%, find the electrical pwer that wuld be delivered. n a 30-day mnth, hw much energy wuld be prvided? 4.5 The site in Prblem 4.4 has a flw rate f 200 gpm, 100-ft elevatin change, and 800-ft length f pipe, but there is excessive frictin lss in the pipe. a. What internal pipe diameter wuld keep flw t less than a recmmended speed f 5 ft/sec. b. Assuming lcally available PVC pipe cmes in l-in diameter increments (2-in, 3-in, etc), pick a pipe size clsest t the recmmended diameter. c. Find the kwh/mnth delivered by the 40% efficient turbine/generatr. d. With a 4-nzzle peltn wheel, what diameter jets wuld be apprpriate?