Generating Light from Stoves using a Thermoelectric Generator

Transcription

1 Generating Light from Stoves using a Thermoeletri Generator Dan Mastbergen, dmast@engr.olostate.edu Dr. Bryan Willson, Bryan.Willson@olostate.edu Engines and Energy Conversion Laboratory Department of Mehanial Engineering Colorado State University One of the largest obstales in the aeptane of lean stoves is that they do not give off the light that the traditional fire does. This downside of lean stoves annot be overlooked if stove programs are to be suessful. Many people would rather deal with smoke than omplete darkness, and may rejet a lean stove for this reason. Developing a means to reate light from stoves will solve this dilemma, and ould have additional benefits. It is believed that the ability to generate lean, white light will be so highly valued by users that they will be motivated to invest money and effort towards a lean stove. One of the most promising methods for generating light from stoves is through thermoeletri power generation. A prototype generator has been developed that produes approximately five watts of power, and is projeted to ost around $30 in large quantity. An in depth analysis was performed on eah omponent to maximize the system effiieny and redue ost. The seletion proess for the thermoeletri module, the heating and ooling fins, and the fan is outlined in detail. Computational and analytial models have been developed to predit the performane of the omponents individually, and as a system. Initial testing and alulations show that the thermoeletri generator is a feasible and relatively heap solution to a large problem. Introdution In the proess of designing a system for generating light from stoves, several options were onsidered. All the tehnologies onsidered are external ombustion devies that onvert heat into eletriity. Devies onsidered were the Stirling engine, the Rankine yle engine, a Brayton yle engine, and a thermoeletri generator. In seleting a tehnology to suessfully address this problem, there are many important onsiderations. The most ritial of these is ost. Most potential users of this tehnology live at or near a subsistene level, and have very little extra inome to invest. The solution must also be quiet. If the generator is loud enough to be distrating, the users will simply be trading one annoyane for another. The generator should also require minimal maintenane, and have an aeptable lifetime. Previous experiene has shown that hanging the habits of stove users to think of maintenane is diffiult. However, it is believed that the ability to generate quality light will motivate the users to take more responsibility. Finally, the generator should not require a battery to store power, sine lead aid batteries are prohibited in some regions, and add additional ost. A thermoeletri generator has many advantages over the various heat engines onsidered. Thermoeletri modules make no noise when they run, the loudest omponent of the system would be a small fan used for ooling. Similarly, the only

2 moving part in the system would be the ooling fan, whih typially runs for thousands of hours without failure on most personal omputers. The thermoeletri generator onverts heat diretly into eletriity, eliminating the need for an eletri generator as would be needed by the engines onsidered. Thermoeletri generators are also very modular. By the seletion of the proper module, any inrement of power an be produed from half a watt, to a hundred watts. All of the omponents neessary for the system an be purhased, making ost estimating and prototyping very straightforward. For these reasons, a thermoeletri generator was seleted as the most appropriate tehnology for generating light from stoves. Design of a Thermoeletri Generator In designing a thermoeletri generator, there are many omplexities that must be onsidered. Eah omponent must be evaluated on how it will perform with the rest of the system, rather than individually. The omponents of the system are the thermoeletri module, the heat exhangers, the ooling fan, the power eletronis, and the load. This is a very dynami system that requires a thorough effort in design to maximize performane. The various omponents of the system are shown in Figure 1. Module Seletion Figure 1: Generator Components fan, old sink, module, hot sink. There are several onsiderations in seleting the appropriate module. The most important are the material and module onstrution. There are many materials apable of produing power from a temperature differene. These materials vary in ost, effiieny, and operation temperature. Module onstrution also affets these three ategories, as well as the maximum power of a module, and the voltage/urrent harateristis of the system. In seleting a module it is important to evaluate eah one as it will perform in the entire system. In many ases, espeially when using air for ooling, the power output listed by the manufaturer is very diffiult to ahieve. Initially, it was believed that a low ost, low effiieny material may be the best solution for this appliation sine the heat soure is abundant and ould be onsidered free. However, this is not neessarily the ase. In general, thermoeletri materials have very low effiienies, typially less than 10%. A very heap material may have an effiieny of around 1%. This means to generate 5 watts of power, 500 watts must be moved through the system. This would require a very large heat exhanger, inluding a powerful ooling fan. These omponents have additional monetary and power osts, negating any advantage of using a low ost material. Bismuth telluride (Bi Te 3 ) is the material with the highest effiieny in the range of temperatures that ould be seen in a

3 stove [1]. Bismuth telluride is also the most ommon material used in Peltier oolers, making it relatively heap. High temperature bismuth telluride modules ahieve around 4% effiieny at max power. For these reasons, a Be Te 3 module was seleted for this appliation. There are also many deisions to be made in module onstrution. The module onstrution onsists of the number and geometry of the thermo-elements, as well as the method for onneting the elements. Figure shows the onstrution of a thermoelement. Modules are typially omposed of around a hundred elements. Modules an be onstruted as Peltier oolers or as high temperature generators. Both of these use the same materials and an be used to produe power, but they differ in how the thermoelements are soldered to the onduting strip. The solder on a Peltier ooler is typially BiSn, whih melts at approximately 138 C [1]. Most high temperature generator modules an withstand intermittent temperatures up to 400 C. This inreased performane omes at a ost though, typially twie as muh as a low temperature Peltier module, but is ompensated by a dramati inrease in power. The reason for this is explained below. Cerami insulator (isolates module from heat sink eletrially) Conduting strips (joins elements eletrially in series) p Figure : Constrution of a thermoelement. n Thermoelements (P and N type semiondutor) Element geometry is important to onsider when seleting a module, however, most designers will have to pik from what is being produed unless the volume is high enough to warrant ustom module design. The geometry of the thermoelements affets the power of the module, the effiieny, and the voltage ahieved [1,,3]. The effets of element geometry and the number of elements an be seen in the following equation []. P = α N A ρ ( L + ρ ρ )(1 + ( λ λ )( L / L)) ( T H T C ) (equation 1) Where : P N T L ρ λ H is the module power is the number of is the module hot side temperature is the element length elements is the eletrial resistivity is the thermal resistivity of the module α A T L ρ λ C is the Seebek oeffiient is the area of elements is the module old side temperature is the thikness of the insulating erami is the ontat eletrial resistivity is the ontat thermal resistivity

4 The previous equation is very useful in estimating the power output of a module. Unfortunately, getting all this information from a manufaturer an be diffiult. Typial values for some of these onstants an be found in the referenes. For the ideal module, negleting ontat resistanes, the following equation an be applied [4]. α T P = ρ N A L (equation ) One important observation that an be made from this equation is that power is proportional to the temperature differene squared. This is extremely useful when given a power output at a ertain temperature differene from a manufaturer. The power at other temperatures an be estimated by: P P = (equation 3) T ref T ref Another interesting observation from equation 1 is that the power inreases as the thermoelement leg length dereases. This means in some ases a higher power module requires less material than a lower power module. Beause of this, modules of very different power outputs may ost the same if they have the same footprint. This is true for the.7 W module and 5.9 W module made by the Thermonami Eletronis Co. Another useful equation desribing thermoeletri performane relates the output voltage to the operating temperature []. V m α N ( TH TC ) = (equation 4) 1+ ( λ λ )( L L) For the ideal module, the denominator of equation 4 an be omitted. From this equation, it an be seen that voltage is proportional to the number of elements sine they are ombined eletrially in series. Voltage is also proportional to the temperature differene. This is important sine many eletrial loads may require higher voltages than the module provides, therefore a module operating at a higher temperature differene will require less of a boost. This is another reason why high temperature modules are preferred over heaper Peltier modules. Finally, the effet of temperature on module effiieny an be desribed by: T T H C φ (equation 5) TH Heat Exhangers In order to maintain a large temperature differene aross the module, heat exhangers are required on eah side. Sine the fluid on both sides of the module in this

5 ase will be air, large finned heat exhangers are neessary. Proper use and seletion of these heat exhangers is ritial to a suessful design. In order to predit the performane of the generator, the harateristis of the heat exhanger must be well understood. On the hot side of the module, the heat exhanger is less ritial. Most high temperature modules an operate with hot side temperatures up to 50 C. Temperatures inside a stove an be up to 600 C. This allows the designer to use this high temperature differene as leverage. Even if the hot side heat exhanger is ineffiient, the large temperature differential an ompensate for this. The hot side heat exhanger has larger gaps within the fins for better performane using natural onvetion. It is also a solid extruded piee so there are no bonded joints that ould melt. The old side heat exhanger must be muh more effiient. The air used to ool the old side of the module an only be as old as the ambient air. Beause of the quadrati dependene of temperature on power, the old side should be kept as old as possible. There is a trade off though, sine power must be put into the ooling fan to keep the module old. For this reason, an in depth analysis of the old side heat exhanger has been undertaken. Thermoeletri generators are traditionally analyzed using a thermal resistane model. All omponents in the system are given a resistane in C/W. This value tells the temperature drop through any omponent of the system for eah watt passed through. For the heat exhanger, the resistane is used to alulate the old side temperature of the module given the ambient temperature. Most heat exhangers require fored air to be effetive. Beause of this, manufaturers may list a thermal resistane at a given flow rate. Some may have plots of thermal resistane at various flow rates. Figure 3 shows the thermal iruit used in analyzing a generator. In this work, the hot side heat exhanger has been omitted. Figure 3: Thermal resistane iruit. The thermal resistanes at the interfaes between the module and heat sinks are also important to keep to a minimum. This requires very flat surfaes (within.001 ), high temperature thermal grease, and uniform lamping pressure (up to 00 psi). Belleville spring washers are also inluded in the assembly to ompensate for thermal expansion. The thermal resistane seen in this work was around.1 C/W. The heat sinks hosen for this work are the HX6-0 and HX6-0 sold by Melor. These are bonded fin heat sinks, so they an only be used on the old side. Melor gives the thermal resistane for these sinks as. C/W and.18 C/W at 45 CFM, respetively. Unfortunately, no additional data is available for the heat sinks at different flow rates. A omputational fluid dynamis (CFD) model was reated to model these heat sinks as well as other possible geometries. From the CFD model, urves of thermal resistane vs. flow rate were generated for various onfigurations. In the model, the flow

6 is parallel to the fins as it flows through a dut from the fan. This onfiguration was hosen so that the ooling air ould be duted into the stove to improve ombustion. The fins were modeled with the dut tight against the fins (mm gap above), and with a larger gap above the fins (10mm gap). The results from the HX6-0 analysis are shown in Figure 4. 1 HX6-0 mm gap HX6-0 10mm gap resistane (C/W) flow (mh) Figure 4: Thermal resistane of heat sink vs. flow rate. Figure 4 shows the effet of flow rate on thermal resistane. In this figure the non-linear nature of thermal resistane is illustrated. It is important to know where on this urve the system is operating. If the system is operating on the flat part of the urve at high veloities, it may be possible to ahieve almost the same amount of ooling with less power into the fan. If the system is operating on the steep part of the urve at low flows, signifiant improvements may be made by a slight inrease in fan power. In most ases, a heat sink is well mathed to the system if it operates near the knee of the urve. Fan Seletion The seletion of the appropriate fan is as important as the heat sink and module. The purpose of the fan is to inrease the power output of the system. If the fan is ineffiient, or poorly mathed to the system, it may onsume more power than it generates. In order to hoose the best fan, it is one again neessary to evaluate eah one as it would perform with the rest of the system. As a starting point, it is important to look at the no load flow rate and the power onsumption of a fan. This gives an indiation of the fan effiieny, but is not the flow rate that will be seen in a real appliation. Typially, a fan that generates over 30 CFM/watt is fairly effiient. It has also been found that the larger the fan, the more effiient it is. Larger fans typially generate higher flows at lower powers than their smaller ounterparts. For example, the 60 mm fan by Panaflo reates 14 CFM at 1.3 W, while the 9 mm fan produes 4 CFM at 1.3 W. The smaller fan does generate a higher pressure, but it is not enough to ompensate for the differene in flow rates. Of all the

7 fans evaluated, the Vante 10 mm Stealth fan is the most effiient, and very quiet. Unfortunately, the fan is larger than the heat sink, whih is 10 mm, so a dut is required to diret the air. Of the 9 mm fans, the Panaflo low power fan is one of the most effiient. In order to know how the fan will perform as a part of the system, the stati pressure vs. flow rate must be known at all points. These urves an then be ompared to the heat sink harateristis to find the operating point of the system. Figure 5 shows the manufatures fan data, along with the data for the heat sink generated by the CFD model. For the Panaflo fan, data was given for 1 V, 10 V, and 7 V operation. Note that the point of intersetion is the operating point. HX6-0 mm gap HX6-0 10mm gap Vante 10mm fan 1V total pressure drop (Pa) flow (mh) Figure 5: Mathing soure(fan) to load (heat sink). One the flow rate is known, the heat sink thermal resistane an be determined from Figure 4. Finally, the system an be analyzed as a whole, and omponent seletions an be made. System Integration In order to determine the system performane, only a few parameters are required for a good approximation. These are a referene power and referene temperature differene for the module as used in equation 3. In addition, the thermal resistane of the module is required. This may be alulated if a heat flux is given with the referene temperature differential. For the heat sink, the thermal resistane and power onsumed by the fan is required. In addition to the heat sink resistane, a ontat resistane should be inluded. First, the heat flow (Q) though the module must be determined. Given a fixed hot side temperature (T_hi), the following equation an be used based on the thermal model in Figure. T T Q = (equation 6) R H amb mod + Rint + Rsin k _ old

8 The temperature on the old side of the module (T_i) an be alulated from: T C = TH Q R mod (equation 7) Finally, the power an be alulated from: P P = T ref TH TC ) ref ( P (equation 8) fan From this analysis, the total system power an be determined for various module, heat sink, and fan ombinations. These equations are only approximations, and are most aurate when the atual temperatures are lose to the referene temperatures. For this work, a power output of at least 3.8 W is desired to power a high intensity light emitting diode (LED). The module seleted is the Thermonami TEP This module has a maximum ontinuous temperature of 50 C. As a referene, it generates 5.9W at Th = 30 C and T = 50 C. The thermal resistane is 1.8 C/W. From the tests onduted for the HX6-0 module, and the Vante Stealth fan, the heat sink resistane is.4 C/W. At a hot side temperature of 50 C and ambient temperature of 0 C, the net power output is predited to be 4 W. Experimental Results In order to validate the CFD models and determine the optimum onfiguration, a series of experiments were performed. In these experiments, the HX6-0 and HX8-0 heat sinks were used. Eah heat sink was tested in several onfigurations. Both heat sinks were tested with a mm gap and a 10mm gap above the fins, as well as with the fan impinging on the fins (flow perpendiular to the fins). The Vante 10 mm fan and the Panaflo 9 mm fan were used on both. During these experiments, flow rate ould not be measured, but stagnation pressure behind the fins was. At eah point, the temperature of the hot side and old side heat exhangers was measured. The atual temperature at the module was determined by measuring the power dissipated in a 3 ohm resistor, and omparing to a referene value as in equation 3. A omparison of the predited heat sink properties to the experimental values is presented in Table1. Table1: Experimental results vs. model preditions. HX6-0 P drop model (Pa) P drop experiment (Pa) Thermal resistane model (C/W) Thermal resistane experiment (C/W) mm gap mm gap Considering the many simplifiations in the CFD model, the results are reasonably lose to the experimental values. Some of the largest soures of air ould be

9 the resistane where the fins are bonded to the base plate, whih was not inluded in the model, and variability in the fan output. From these results, further improvements will be made to the model to improve its auray. Through these experiments, and further modeling based on the results, some interesting phenomena an be illustrated. First of these is the effet of the gap height on the thermal resistane of the heat sink. As the gap is inreased, the total flow rate inreases due to the redued fluid resistane, however, the thermal resistane stays nearly onstant. In both ases there is nearly the same amount of flow passing though the fins. If the air is to be used for spae heating or to aid ombustion, the large gap would be preferred to keep the flow rate high. Also seen was the fat that the impinging onfiguration outperforms the parallel flow onfiguration. This is believed to be due to the fat that most of the ooling in the parallel onfiguration ours at the front of the fin where the thermal boundary layer is small. As the flow nears the end of the fin the air has been heated signifiantly so that little ooling ours. In the impinging onfiguration, ool air is ontating the tips of all the fins. The fat that there is a stagnation zone at in the middle of the fin does not seem to be signifiant. Also, the pressure drop through the air box used to rediret the flow in the parallel onfiguration is about 1/3 of the total pressure drop. This results in a lower flow rate. These experiments also illustrated how improvements in system performane an be ahieved by running the fan at redued power, espeially at low hot side temperatures. Figure 6 is of the net power out of the module with different fan voltages. This data is for the Panaflo 9 mm fan in the impinging onfiguration, with the HX8-0 heat sink. net power (W) fan voltage (V) Th = 50 C Th = 00 C Th = 140C Figure 6: Effet of fan voltage on net power at various hot side temperatures.

10 Figure 7: Benh testing the generator Power Management In order to power the LED and the fan from the module, a power eletronis iruit will be required. Due to the nature of the thermoeletri module, the voltage and urrent both vary as the module temperature differene hanges. For maximum power, the load resistane must math the module internal resistane, whih also hanges slightly with temperature. Unfortunately, the voltage and urrent at maximum power are typially different than the voltage required by the light or the fan. In order to keep the module operating at maximum power, the voltage from the module must be boosted to power the fan, and may need to be inreased or dereased to power the light. This iruit will need to sense the onditions of the module, and make the appropriate hanges to the power distribution. An example load urve is shown below for a module made by the Tellurex Corporation. Figure 7: V-I harateristis of a module with a 15 degree temperature differential.

11 Cost Analysis Table : Projeted osts Component Single unit ost High volume ost (10,000+) Module $100 $10 $10 Hot Sink $10 $3 $3 Cold Sink $16 $8 $1 Fan $7 $3 $0 Hardware $ $0.50 $0.50 Eletronis? $4 $1 LED $7 $4 $4 Total $14 $3.50 $7.50 Natural onvetion (10,000+) Further redutions in ost may be aomplished in several ways. First, if the old heat sink ould be made loally, its ost may be redued. Also, if the old heat sink is large enough, it may be possible to operate without a fan. This would make the heat sink more expensive, but would eliminate the ost of the fan and greatly simplify the eletronis required. If the hot side an be plaed near the ombustion hamber where the temperature is high enough, it may be as simple as a steel blok to distribute the heat, further reduing osts. In order to get enough ooling using natural onvetion only, a dut has been proposed that would use the heat from the himney to inrease the draft at the heat sink. After the air passes through the heat sink it would travel upward along the himney. As it is heated further, its added buoyany will inrease the draft and improve the performane of the old sink. The proposed design is shown in Figure 8. Stove himney Heated air Dut parallel to himney Hot Sink Cold sink Cold air in Figure 8: Setup for natural onvetion operation.

12 Conlusions and Future Work Through this work a thermoeletri generator has been designed and tested that produes enough power for a small fan and a high intensity LED. An in depth analysis has been performed in seleting eah omponent of the system to maximize power. A high temperature Be Te 3 module made by Thermonami has been seleted as the most ost effetive module for this appliation. System osts in large quantity are estimated to be around $30. Through CFD modeling and benh testing, the most effiient heat sink/fan onfiguration has also been identified. The best performane was ahieved when the fan was in the impinging onfiguration. Also, it was seen that at lower operating temperatures the greatest power is ahieved with a redued fan voltage. Further work will fous on improving the heat sink effetiveness through the use of the CFD model. Alternative fin onfigurations suh as a staggered geometry will be modeled and tested. Work will also fous on the design of the power eletronis iruit. Referenes 1. Buist, Rihard, Lau, Paul, Thermoeletri Power Design and Seletion from TE Cooling Module Speifiations, 16th International Conferene on Thermoeletris (1997), Aug , Dresden, Germany, Rowe, D. M., Min, G. Design Theory of Thermoeletri Modules for Eletrial Power Generation, IEE Proeedings: Siene, Measurement and Tehnology, Vol. 143, No 6, November 1996, Nuwayhid, R. Y., Rowe, D. M., Min, G., Low Cost Stove-Top Thermoeletri Generator for Regions with Unreliable Eletriity Supply, Renewable Energy, Vol. 8, 003, Min, G., Rowe, D. M., Optimization of Thermoeletri Module Geometry for Waste Heat Eletri Power Generation, Journal of Power Soures, Vol. 38, 199, 53-59