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HomeMy WebLinkAboutThermo PV 1995A AUG-21-95 HON 14:53oOMr Peter Crime |Wa UA F SCHOOL OF ENGINEER FAX NO.9074746087 P,O1 'fo IEEE Proceedings -'First World Conference on Photovoltaic Energy Conversion',12/5-12/9/84,Waikoloa,Hawaii A THERMOPHOTOVOLTAIC ELECTRIC GENERATOR USING GASB CELLS WITH A HYDROCARBON BURNER Lewis Fraas,Russell Ballantyne,John Samaras,&Michael Seal* JX Crystals Inc.,lssaquah,WA *Western Washington University,Bellingham,WA ABSTRACT New GaSb photovoltaic cells with infrared response extended out to 1.8 microns are well matched to hydrocarbon combustion heated silicon carbide infrared emitters operating at 1500 €.These continuous combustion thermophotovoltaic units run quietly and cleanly.Applications include small scale distributed cogeneration of heat and electricity and power units for clean electric vehicles.Herein,we report the fabricationof1em2GaSbcellsoperatingat6Ampsand2.6 Watts, 10 cell circuits operating at over 22 Watts,and a small demonstration generator producing enough power to operate a television. INTRODUCTION Silicon photovoltaic cells are used to convert sunlight to electricity.However,three obstacles have. stood in the way of the widespread use of these light sensitive cells.The first obstacle is the low power densityofonly0.1 Watts/em2 associated with sunfight,the second obstacle fs that sunlight is only available for approximately &hours in each 24 hour day,and the third obstacle Is thet sunlight is not available on demand with the result that battery storage for several days of cloud cover Is expensive. Combining #photovoltaic cell with radiant energy from a hydrocarbon bumer eliminates ail of theseobstacles.Power densities of aver 10 Watts/em2 are available because the radiation emitter will be much closer to the cells than the sun,the bumer can operate 24hoursperday,and energy will be available on demand. Unfortunately,silicon cells are not useful in this application because the bummer cannot be as hot as thesun.A practical emitter temperature limit of 2000 K willproduceblackbodyradiantenergypeakingat1.5 micronswhichisbeyondthebandedgeofsiliconat1.1 microns. Thus,@ new infrared photovoltaic cell is required to makethermophotovoltalegeneratorspractical.: In 1989,a significant jump in solar energy conversion efficiency:resulted from the invention of the GaSb cell [1]with Its extended infrared (IR)response.A GaSb cell located behind a visible light sensitive GaAs cell captured infrared energy from the sun out to 1.8 microns and boosted the conversion efficiency by 10 percentage points from 25%for a GaAs cell by itself to35%for the tandem cell, This new infrared sensitive GaSb cell now promises a different path to practical electric power production using photovoltaic devices with man made radiant heat sources operating at between 1700 K and 2000 K.This temperature range can be obtained by buming a hydrocarbon fuel inside a silicon carbide tube. JX Crystals is now developing a new patented [2] thermophotovoitaie generator with the trademark name "Midnight Sun",in which natural gas is burned Inside a ceramic tube,the tube glows red hot,and the IR ceils are mounted around the glowing tube to generate electricity. The resultant generator shown conceptually in figure 1 is a quiet fightweight electric generator with no moving parts. eedfotorenesRaa poe Deco nesapn 23 Saree eaePeetotceesebocesheateakagarA pes Figure 1:Stand-alone natural-gas-fired TPV electric generator AUG-21-95 MON 14:55 Wr:U A F SCHOOL OF ENGINEER JX Crystals'thermophotovoltaic generator has threaimportantadvantagesoverpriorsolartoelectricpower supplies.First,the radiation energy source is close to the cells,whereas the sun is 93 million miles away.This leads to one thousand times higher power densities for the cells which makes the cells and resultant electric power much more economical.Second,unlike solar electric generators,the "Midnight Sun*operates on demand day of night,not simply when the sun Is shining. Finally,the thermophotovoltaic generator can supply heat as weil as electricity for mountain cabins, recreational vehicles,marine vehicles,of off-grid homes. When compared to traditional gasoline fired AC generators,besides being much quieter,the thermophotevoltaic generator retains the environmentally friendly feature of solar electric generators.This occurs because combustion in the hydrocarbon burner is continuous,producing a clean burn.Combustion in an internal combustion engine is dirty because of the periodic short duration explosions with high peaktemperature,Short duration explosions lead to incomplete combustion with resultant carbon monoxide and hydrocarbon emissions while high peak temperatures lead to oxides of nitrogen. DEMONSTRATION PROTOTYPE Figure 2 presents the results of power density calculations [3]for GaSb and silicon cells operated in conjunction with a blackbody for emitter temperatures ranging from 1700 K to 2000 K assuming perfect optical coupling.Nete that high power densities are predicted and thet GaSb cell power densities are at least 4 times higher than silicon cell power densities.The conversion efficiency for the IR absorbed by a GaSb cell below 1.8 microns is also plotted.Note that although this efficiency is over 35%,this is not the system efficiency.Losses associated with longer wavelength IR and exhaust heat losses must also be considered. - N g 12 3 >Las =103 fry=GaSb [39 § "3 -[383ceid 5 GaSb 37 5 a.6 = 3 & =4 4 Silicon -_<7 _-ri oO 1700 1800 1900 2000 Temperature (K) Figure 2:Cell power density and efficiency vs.blackbody emitter temperature FAX NO,9074746087 P,02 In order to demonstrate the promise of the "Midnight Sun”concept,we have built the generator shown in figure 3.This novel thermophotovoitaic generator uses galliumantimonide(GaSb)cells.This demonstration generator uses eight water cooled GaSb circuits each of which produces 3 Volts at maximum power,The eight receiversarearrayedaroundthecentralIRemitter/burner.The IRemittedpassesthrougheightlanternwindowstotheeightreceivers.IR filters are located in the lantern windows. These {R filters allow useful shorter wavelength IR to pass through to the cell receivers while reflecting longerwavelengthIRbacktotheemittertorecycletheheat. Each 3 V receiver contains nine GaSb cells originally designed for point focus solar applications.Theactiveereaofeachcellis0.2 cm2.After fabrication,each of the eight receivers was tested.and produced between2.5 and 3.5 Watts at maximum power.Two groups of four receivers were series connected to produce 12 Volts and over 20 Watts.The resultant generator then produced enough power to operate a small television. The result was the world's first thermophotovoitaic generator successfully integrating IR sensitive GaSb photovoitaic cells with a natural gas fired IR emitter. Figure 3.Photograph of demonstration TPV generator The power density produced by the receivers in thisunitisapproximately1.57 Watts /cm2.With an activeareaof14.4 cm2,the unit's total power is 22.6 Watts. Given that the bumer in this generator was fabricated using stainless steel and that its operating ternperature fs limited to 1500 K or 1227 C,this power density is then quite reasonable and consistent with extrapolation of the calculations presented in figure 2. AUG-21-95 MON 14:56 Wer UA F SCHOOL OF ENGINEER FAX NO,9074746087 PERFORMANCE IMPROVEMENTS We are now in the process of increasing the powerfromthe"Midnight Sun”unit by using (1)cells with five times the active area,(2)a higher temperature silicon carbide emitter,and (3)a heat exchanger to coupfe the exhaust heat into the supply air. As of this writing,we have fabricated and tested new GaSb cells specifically designed for TPV applications.These new cells are 1 cm in area and have electroplated thicker front grid metal coatings so that they can handle the high current densities associated with a TPV generator.Figure 4 shows the illuminated current vs. voltage data for one of these new TPV GaSb ceils.At an illumination level corresponding to a short circuit current of 5.95 Amps,the open circuit voltage is 0.52 Volts,the fill factor is 0.64,and the cell maximum power is 2.59 Watts.The open circuit voltage and fil)factor are spectacular and correspond to theoretical limit values for a material with a bandgap of 0.72 eV.This cell was the best from a lot of 36 cells from three 2°diameter wafers. All of the cells in the lot had open circuit voltages over 0.49 Volts and fill factors over 0.79, GaSb Cell -1 ¢m? 64 s+FF ©°0.%4 at Voo *052V E Ise 60 =5.95A 37 Imax =5.63A 2+Vimax =0.46V Pmax =259W 1+ 0 +t t ++-| 0 O01 02 03 04 65 06 Volts Figure 4:Illuminated current vs.voltage data for a 1 em2 GaSb TPV cell. Several TPV circuits were then fabricated by series connecting GaSb cells from our first lot of electroplated cells.Figure §shows a photograph of a TPV receiver incorporating #10 cell clrouit and figure 6 shows the iltuminated current vs.voltage data for one such ten cell circuit.For an fiumination level corresponding to a short circuit current of 6.33 Amps,the circuit operated with an open circuit voltage of 4.91 Voits,a fill factor of 0.72,and produced 22.35 Watts of electrical power. We have not yet incorporated these new more powerful cells and circuits into a TPV generator. However,we have now demonstrated a natural gas heated silicon carbide bummer /emitter that operates continuously at 1450 C.This bumer is equipped with a P,03 counterflow heat exchanger to reduce the exhaust heat losses and to increase the emitter temperature.In thenearfuture,we plan to surround this burner with 12 circuits and based on the calculations in figure 2,weexpecttoobtainaTPVgeneratorwhichwillproduceover 200 Watts. $3 €.iv. Figure 5:Photograph of JX Crystals'10 call TPV receiver GaSb Ten Cell Circuit -10 cm? 7 o st . FF 0.72 57 Voo =491V R47 Ise 633A 2 5 Imax =5.77A 2 Vmax =3.88 V Pmax=22.35 W 1+ C')++++7 '1 2 3 4 5 Volts Figure 6:Illuminated current vs.voltage data for a ten cell TPV GaSb circuit AUG-21-95 MON 14:57 Wes UA F SCHOOL OF ENGINEER FAX NO.9074746087 P,04 Table 1 Applications Summary for Thermophotovoltaic Generators Market Sector Electric Power Heating AdvantagesperUnitCapacity Small 200 W 2 kW Quiet,Clean,&Cagenerator Portable home 2kW 8 kW Quiet,Clean,&Cogenerator Efficient Electric Vehicle 20 KW Ciean,Light Weight,& Efficient APPLICATIONS The two major application areas for TPV generatorsare(1)for cogeneration of heat and electricity in homes and (2)as electric power supplies In electric vehicles.In homes,a TPV generator can replace a fummace and the electric power line operating with a total system efficiency near 100%.In an electric vehicle,a TPV generator will decrease the battery size and weight requirement and extend the vehicle range.A nearer term market for TPV Is In remote cold off-grid applications like mountain cabins,Table 1 summarizes these potential markets, with small cogenerators representing the remote location market.It is interesting to note that TPV generators can impact both major sectors of oil consumption,Le. transportation and space heating. Approximately half of the oil consumed in the US is consumed in the Intemal combustion engine in the transportation sector.If the current gasoline powered automobile fleet were to be converted to TPV electric powered vehicles,oil consumption in the US could be halved since TPV powered electric vehicles would consume more plentiful natural gas.However,net only would TPV switch the primary transportation fuel from oil to natural gas,fuel efficiency could be increased by @ factor of two from 10%to 20%.The following argumentindicateshowthiswouldcomeaboutTheintemal combustion engine is generally sized to provide for maximum power demand rather than average power demand.Thus,an Internal combustion engine In today's car should provide for additianal acceleration at 100 mph. However,the average driving speed in a city may be only 30 mph.So the intemal combustion engine is oversized and designed for a broad range of driving conditions.As a result,its efficiency for an average driving cycie is only10%.A TPV generator,on the other hand,will be designed to continuously provide for the average power .required.Thus et 100 mph or during acceleration,the additional current requirement for the electric rotors would come from the batteries.A TPV generator might be designed to maintain the vehicle speed at 45 mph without additional current from the batteries.However, when the vehicle is stopped at a stop light,the TPV generator would keep providing current to recharge the batteries.[In fact,when a commuter arrives at his destination,the motors would be switched off but the TPV generator could keep on operating until a sensor indicatesthatthebatteriesarerecharged.In summary,a TPV generator could be smaller in power and designed for maximum efficiency at that power level.This battery storage capability inherent in electric vehicles will lead ta dramatically improved fuel efficiency. The second major use of oil is for space heating.A natural gas fired TPV generator can provide bothelectricityandheatfeadingnotonlytooverallefficiency improvement but also to the energy independence desired by solar advocates. CONCLUSIONS The GaSb cell first developed as @ booster cell for solar energy conversion can also be used to make practical thermophotovoltaic generators.Using these new infrared sensitive cells with infrared from @ natural gas fired bumer,we have demonstrated a small generator which produced enough electric power to operate @ television.New GaSb TPV cells now operateatapowerdensityof2.6 Watts/em2.With reasonably likely power and efficiency Increases,high volume markets should develop for this class of thermophotovoltaic generators in the transportation and space heating (cogeneration)sectors eventually significantly decreasing oil Imports. REFERENCES [1]LM.Fraas,.J.M.Gee,KA Emery,et.al.21st /EEE Photovottaie S,Conference,pp.190-195 (1990).[2}U.S.Patent 5,312,521:"Compact DC Electric Power Generator Using Low Bandgap Thermophotovoltaic Cell Stings with a Hydrocarbon Gas Sumer Fitted with a Regenerator”assigned to JX Crystals Inc. {3}O.L.Doeliner,Ph.O.thesis,University of Arizona, (1991).BreOets