Conceptual Design of a Thermoelectric Edu-Kitchen System

Akshaya Srivastava, Daryl Duran, Mark Pinder, Vrishank Raghav, Narayanan KomerathDaniel Guggenheim School of Aerospace Engineering

Georgia Institute of TechnologyAtlanta, GA, USA

Email: komerath@gatech.edu

Abstract—Woodburning kitchen stoves provide a potentialsource of power to bring electric lighting, pollution controland water sterilization to underprivileged communities aroundthe world. The conceptual design of a thermoelectric powergeneration system is described. A DC fan powered by arechargeable battery drives air into the stove, optimizing thefuel-air ratio to improve heat release and reduce smoke, sootand other pollutants. A 5-watt light emitting diode floodlampprovides steady lighting so that a child may learn in the kitchen.A milliwatt LED is used to sterilize drinking water. A 13-watt thermoelectric converter module operating at 225 degreesCelsius recharges the battery using the heat from the fire. Fanpower is regulated using a thermocouple sensor to maximizethe heat release in the stove. Measurements and calculationsshow that the design closes with the selected parameters, andthat enough air flow variation is available from the fan toensure optimal stoichiometry in the flame. While unit costs ofthe components are high, wholesale prices are much lower.

I. INTRODUCTION

Many families around the world must do their cookingusing rudimentary wood-burning stoves made of threestones or bricks, burning whatever wood scraps they cangather. These stoves are inefficient and with no more thanbuoyant natural convection for exhaust removal, generatehigh levels of pollution, leading to a high incidence ofhealth problems. With mothers having to attend to cooking,their children must do their homework sitting in thesame kitchen, with poor lighting and air quality. A highpossibility of bacterial infection from drinking water is alsoa reality. The Edukitchen system described in this paperuses a thermoelectric module from spacecraft technologyas the centerpiece of a low-cost electric power generationto bring ventilation, pollution control, fuel efficiency, cleanwater and lighting to kitchens. The paper defines therequirements for the system, and presents an initial versionof our solution, as a testbed for research and developmenttowards a mass-producible system.

Gordon [1] used the example of a thermoelectricgenerator to derive generalized characteristics for heatengines. Thermoelectric power generation has been usedin spacecraft power applications for several years. Bennettet al. [2] describe the General-Purpose Heat SourceRadioisotope Thermoelectric Generator (GPHS- RTG) that

was used to provide power for the Galileo mission to Jupiterand the Ulysses mission to study the polar regions of theSun. While radioisotope decay generated little heat andthus only a relatively low temperature, the near-absolutezero sink temperature of Space enabled spacecraft designersto exploit the large available temperature difference. Interrestrial applications, such a temperature differencecan be obtained from exhaust systems of heat enginesand automobiles, as well as from domestic stoves. A1-kilowatt thermoelectric generator for diesel engines isdescribed by Bass et al. [3]. Ikoma et al [4] reported athermoelectric module and generator for vehicles withgasoline engines. Snyder and Ursell [5] discuss theefficiency and compatibility of thernmoelectric conversionsystems, developing a compatibility factor, and followedthat up [6] to the design of systems with segmented andcascaded generators.

The application of thermoelectric generation to powermicro-devices has been explored beyond the Space powerapplication. Schaevitz [7] described a combustion-drivenmicro electromechanical (MEMS) thermoelectric powergenerator as a solid-state actuator power source. Stordeurand Stark [8] describe a low power generator as a self-sufficient energy supply for micro systems.

The idea of using thermoelectric converters with domesticstoves as standalone sources of electric power has beenused by some researchers. Nuwayhid et al. [9] discusseda low-cost stove-top thermoelectric generator for usewhere the electric power grid was unreliable. Clearly oneconsideration in the researchers’ minds was the high costper unit power of a thermoelectric converter, making itharder for poor families to justify the cost of such devices,whereas middle-class users in regions with unreliable gridsmight serve as customers at least at the initial market stages.In a 2005 paper they described development and test resultsfrom a domestic woodstove incorporating thermoelectricgeneration. Amrose et al. [10] describes a student teamproject to develop and test the Tara fuel-efficient BerkeleyDarfur Stove prototype intended for the people in theDarfur region of Sudan. While this device did not includethermoelectric generation, it provides a good window

to the thinking of people trying to help the refugees inthe Darfur region. Reducing fuel needs, and improvingperformance of the stove during high winds, were seen tohave literally life-saving implications for the refugees asit reduced the need to go foraging for scrap wood fuel ina region where there were roving bands of armed banditswho would murder the refugees. Other well-known effortsalso sponsored by the United Nations and corporations,include the Phillipps stove, and the Rocket Stove. Theseefforts, however, were to develop portable stoves thatcould be set up in the refugee communities, rather thandevices to be incorporated into existing mud and stonewoodstoves. The former type is suitable in refugee campsand other locations where the device can be imported andsold or given to the locals. The latter, meaning the deviceto be incorporated into existing stoves, may work better inseveral independent communities that are either too pooror too difficult to convert to entirely new ways of cooking.Among the cited experiences is that of South America,where modern low-emission stoves were rejected by themarket because of the feeling that food cooked using thenew stoves lacked the smoky flavor of the previous stoves.

The present work was motivated by a televisioncommercial advertisement from the Hutch mobile telephonecompany, that used to air a few years ago in India.This advertisement showed a young mother in an Indianhome kitchen, clearly in an economically underprivilegedcommunity. The mother is cooking chappatis in a panheated by a wood fire, on a stove that consists of threemud bricks, a scene that should be instantly familiar tomillions of people from many regions of the world. Herson is seated not far to the side holding a slate pad,clearly under disciplinary supervision and required to dohis school homework. The pan is very hot, and the womanhas difficulty holding the bread to turn it over. Her fingersare clearly feeling the heat. The son gets up and walksaway, prompting a sharp reprimand from the anxious andobviously distracted mother. The boy returns in a momentwith a short length of stiff wire, which he proceeds to bendinto an effective pair of tongs, for his mother to hold thebread comfortably. Her smile is as much in gratitude as inpride in the ingenuity of her son.

A touching and inspiring scene and one that we sincerelyhope, increased sales of Hutch products and of the adver-tisement producers. However, the well-made-up, photogenic,smartly-attired and accomplished actress, the bright youngschoolboy and the clear, well-lit scene in high-resolutioncinematography, mask some harsh realities that apply tohundreds of millions of families every day. We enumeratesome below:

1) The mud brick stove is the family’s only way to cooktheir meal. If they could afford a nice clean-burning

gas stove they would get one.2) The wood fuel for the stove is probably what the

family could gather, and forms a mixture of branchesand pieces of logs, and perhaps leaves, not of anyuniformly predictable or homogeneous composition.

3) The fire usually generates a great deal of smoke, andthe combustion is quite inefficient, with only naturalconvection to bring air into the flame.

4) The kitchen has at best an open slot in the wall abovefor ventilation.

5) The family has no access to electric power. The childmust study in the dark, smoky kitchen, by whateverlight is available through doorways or from the fire.

6) Anyone familiar with the environs of such homesteadsalso realizes that pure drinking water is a luxury thatmost such families do not get.

7) The sum total is an environment that puts hundredsof millions of growing citizens at severe risk ofpulmonary, optical and gastro-intestinal disease, andat a severe disadvantage in education. The cost to thenation is immense, and therein lies the answer to howimprovements can be brought to these families.

The Edu-Kitchen project was conceptualized at theMicro Renewable Energy Systems Laboratory at GeorgiaInstitute of Technology. This laboratory is set up to exploitopportunities at the interface between high-end aerospaceresearch and development, and mass-market needs offamilies. We reason that bottom-up empowerment (nopun intended) of people to develop their own stand-alonerenewable energy solutions is the best way to rapid adoptionof renewable energy, and reduction of fossil fuel usage.The Edu-Kitchen is one of five family-sized testbeds thatwe are developing.

The system as presently conceived serves multiplepurposes with one thermoelectric converter module. Themodule is sized to fit at the edge of a wood-burning stovesuch as that described above. Power from the modulere-charges a small battery. A thermocouple attached to thecasing of the converter module senses temperature in orderto provide guidance to a programmed controller chip onimproving heat release rate. The battery, guided by thecontroller chip, provides power to a DC fan, which drivesair past the cool side of the thermoelectric module and intothe stove, trying to achieve an optimal fuel-to-oxidizer ratofor the best heat release and minimized pollution. In moresophisticated versions it may also power a ventilator toremove exhaust gases.

A DC lamp consisting of an array of light-emitting diodes(LEDs) provides steady white illumination, sufficient fora child to read and write under. Another LED is usedto illuminate a drinking-water container to sterilize it bykilling the most harmful known species of bacteria that are

typically present in water. Thus the EduKitchen system isintended to revolutionize the family’s living environment,providing clean-burning and fuel-efficient cooking stoves,efficient electric lighting and clean, safe drinking water.The technical and socio-economic challenges in perfectingsuch a device are formidable and span several disciplines.It opens a long-term research portfolio with immediateapplication to the tesbed.

The inspiration for the water purification aspect comesfrom the UV Waterworks system [11] developed by AshokGadgil and Vikas Garud at Lawrence Livermore nationallabs, which is credited with saving hundreds of thousandsof lives by sterilizing drinking water and eliminating severalharmful bacteria including e-coli. Their system used a 40-watt fluorescent black light, whose broadband UV contentwas sufficient for effective sterilization. This was based onstudies showing that 254 nanometer far-ultraviolet radiationeffectively killed well over 99 percent of bacteria present inwater, quickly enough to permit installation of the blacklightover slowly-flowing water. Luckiesh [12] described exper-iments at General Electric Corporation on the effects of abroad spectrum of radiation wavelengths from the infraredto the ultraviolet, on human skin. This book also describedgermicidal lamps and their spectral range of effectiveness.They later showed that a dosage of over 200 microwatt-minutes per square centimeter was adequate to achieve 100percent destruction of e-coli in water even if the strain hadevolved through previous doses of the same radiation. Morerecent studies have suggested that the range of radiationbetween 260 and 270 nm, centered probably at 264 nm,is much more effective, since this includes the resonancewavelengths of the DNA of these bacteria. LEDs with outputover this narrow range can thus achieve the same results asthe earlier fluorescents, at power levels of milliwatts.

II. REQUIREMENTS

It should be stated at the outset that we do not plan toconvince families such as the one pictured in the Hutchcommercial, to buy the resulting device at full cost, whichwould be far too expensive for them. The economicviability is in the tradeoff between the very real economicand human costs of disease and lost opportunities on theone hand, and the cost to taxpayers or other sponsorsof buying and subsidizing empowerment of citizens toirmprove their lifestyles and opportunities on the other.Most probably, there can be exotic versions intended forrecreational customers, that can be economically viable ontheir own. In this paper we will consider the unit cost ofeach component in small quantities, and the projected perunit cost when mass-produced. The former is quite steep,since LED and thermoelectric converter markets are stillnascent, and batteries are expensive. The latter, as we see,can come close to the levels at which even poor families

may be in a position to afford nearly the full cost.

From the above considerations, a set of requirements maybe defined for the system, as enumerated below.

1) The system must capture enough electric power duringan average evening cooking cycle of a family of 4using a wood fire in a mud-and-brick domestic stove,to recharge a battery to compensate for the demandson the battery.

2) The battery must drive a DC fan to deliver enoughair flow rate and dynamic pressure to enable fuel-lean combustion and pollutant removal from the woodfire, while keeping the cold side of the thermoelectricmodule at an optimal temperature.

3) The battery must power a lamp that is bright enoughto allow one student to read and write comfortably for5 hours continuously.

4) The battery must provide enough power to operate acontrol system.

5) The battery must provide enough power for a UVwater purifier system that adequately meets all thedaily drinking water needs of the family.

6) The resulting system must be compatible with thepresent lifestyle of families in the customer area, andrequire only minimal instruction to operate safely.

7) The system must be robust enough to survive normalwear and tear as well as the occasional spills andmechanical shocks that may be expected in a crampedand poorly lit family kitchen.

8) The system mass produced cost per unit must be wellbelow the annual health care cost to the nation perfamily in the relevant localities.

III. TESTBED CONCEPT DESCRIPTION

The initial system testbed model consists of a bismuthtelluride alloy thermoelectric module manufactured by Hi-Z corporation, enclosed in a flattened metallic cone suit-able for placing among firewood pieces in a stone stove.The flattened cone is made out of the aluminum sheetobtained by unwrapping an empty 12-oz soft-drink (Coca-Cola) can. A separate Type J thermocouple sensor monitorsthe temperature, while the thermoelectric power is usedto charge a battery. The output from the battery, and thetemperature signal, will go through a microcontroller (notyet built), which controls power to a small DC computerfan that drives air through the conical insert, optimizing thestoichiometry of the combustion and powering the exhaustout of the kitchen. A separate power line from the batterygoes to an LED flood lamp. Another power line goes toa small ultraviolet LED mounted in the lid of a drinkingwater container. At this writing we are testing the fan andthermoelectric module systems, as the primary power userand generator in the system. Fig. 1 shows the conceptgraphically.

Figure 1. Concept Map

A. Components

Figure 2 shows the HZ-14 thermoelectric module. The .Figure 3 shows a picture of the initial mock up of the system.The DC LED floodlamp acquired for testing is visible. Thewater purifier module is at present only at a conceptualdesign stage, but will use a UV light in the deep ultravioletrange of 250 - 270 nm. Figure 4 shows a second prototype,with a DC Cyclone Blower fan being run. This fan comesintegrated with a 90-degree turn of the flow, compatible withbuilding the insert and permitting a substantial reduction insize from the initial prototype. Table I shows the pricesof the various parts of the design. Two components (therechargeable battery and the UV light) have yet to be bought,and as such, are not included in the table.

Figure 2. HZ-14 ThermoElectric Module

IV. SAMPLE CALCULATIONS USING BLACK SPRUCEWOOD

To assess the air flow requirement, Black Spruce wood,while not a common fuel found in rural areas, waschosen because information on its chemical makeup and

Component PriceThermoelectric Module $100

Computer Fan $8LED Light $25

270nm LED, single unit $200

Table IPRICE PER PART

Figure 3. Mock Up of System

Figure 4. Fan and Nozzle

properties is available. The calculations shown below setup a simple procedure to find the amount of air needed forstoichiometric combustion. The assumptions are also stated.

Black Spruce contains 23.7% lignin, 42.1% cellulose, and12.1% pentosan [13]. One cord (85 cubic feet) of blackspruce wood with approximately 20% moisture contentproduces 15.9 million BTU’s of usable heat

The heating value of wood and the stoichiometric fuel-airratio are used to compute the air flow rate needed forstoichiometry. We then estimate much heat must be releasedduring a typical evening’s cooking for the family, and hencehow much fuel and how much air flow rate are needed. Not

all of this air flow need go through the fan. There is alreadynatural convection due to the heat release, sufficientlybelow the rich limit for combustion to enable such stovesto operate, however inefficiently. In addition, the conicalinsert jet nozzle will entrain flow around it, into the flameso that the net mass flow addition will be higher than thatflowing through the insert.The issue then is whether the fancan make a substantial difference to this amount, so that theequivalence ratio (ratio of fuel-air ratio to the stoichiometricfuel-air ratio) can be varied enough to control heat releaserate and pollution. We assume that no more than 50 percentof the air sent into the stove can help in reaction, due toimperfect mixing. The selection of an optimal fan size isthus an iterative issue to be investigated later. At this stageit is enough to lay out the amount of fan mass flow ratethat can be generated with an inexpensive DC computer fan.

V. TESTING OF THE FAN AND THE THERMOELECTRICMODULE

A TSI Velocicalc probe is being used to measure time-averaged velocity at several points across the exit plane ofthe conical nozzle with the fan operated at various powerlevels. The fan was driven using a DC power supply withvoltage varied and current measured. Parabolic curve fitshave been generated across the jet, and used to estimatemass flow rate and jet kinetic energy. These initial resultsshow considerable spreading of the jet beyond the exitdiameter, validating the idea that the jet exiting the nozzlewill entrain substantial air flow around it. The mass flowrate within the exit diameter is plotted in Figure ?? asa function of the measured electric power input goinginto the fan. The mass flow rate was also plotted againstthe input voltage (not shown). At 12 volts, the fan couldgenerate roughly .3 grams per second of core air flow,consuming roughly 7 watts. As can be seen from Figure5, the mass flow rate generated by the fan is enough tooperate the stove with a heat release rate of 1kW at anyfuel-air ratio richer than roughly 0.4, which is very lean.Inpractice, due to poor mixing, it may take an overall fuel-airratio of 0.5 to ensure that enough air gets to the fuel forstoichiometric burning. Thus the flame equivalence ratiois well within the the control of the fan. If a temperaturedifference of 170 degrees Celsius can be maintainedacross the thermoelectric module, roughly 8 watts can begenerated, adequate to power the fan and leave about 5watts for the DC LED lamp and the UV LED water sanitizer.

Economic and societal viability of the system remainsto be studied. At single-unit retail prices that a researchuniversity can receive, the thermoelectric module and theLED for the UV sterilizer cost hundreds of dollars each,while the DC LED lamp costs several tens of dollars.However, the DC fan is now down to about 1 dollar per

Figure 5. Airflow capacity of fan and flame equivalence ratio for 1kWheat release

fan in quantity, showing the effect of mass production. Itis quite possible that fans can be acquired from computerjunkyards and junk prices. The idea of fabricating the unit’shousing from discarded aluminum soft drink cans is viableat the small and large scales of production, if combined withthe interests of a recycling company. In mass purchases,the UV LED is also down to low per unit prices. The costof DC LED lamps is coming down much more slowly. Thethermoelectric module is also fundamentally a low-costitem to manufacture in quantity. Thus except for the LEDlighting, all other components appear to be coming downto cost levels that should be affordable for governmentprograms to purchase in bulk, and they are already down tolevels where recreational customers can well afford to buythe system.

VI. CONCLUSIONS

In this first paper on the system, a conceptual designof the EduKitchen concept is presented along with itsmotivation and rationale. The paper goes far enough toshow that the basic concept of integrating devices forthermoelectric generation, fuel efficiency improvement andair quality improvement, and basic lighting and drinkingwater sterilization, is technically viable. The conceptualdesign is shown to close, with the power levels that canbe obtained at very modest heating rates and temperaturesfrom a domestic wood stove being sufficient to generateenough power using a single thermoelectric module tooperate a DC fan to cool the sink side of the module, andoperate a a DC LED floodlamp generating the equivalent ofa 40-watt incandescent lamp in illumination. The fan-drivenair reaches a speed of several meters per second, adequateto greatly augment natural convection. An estimationprocedure is laid out to estimate stoichiometry and to

generate design curves for various levels of heating neededfrom the stove.

The paper is also intended to convey the multidisciplinarynature of this research project. Although the issue ofmaking a home stove burn better, appears quite mundane,the exploration of issues above shows that it challengesleading-dge research capabilities in several areas. Computingthe heating from a stove burning assorted scrap wood,enhancing mixing inside a cluster of wood pieces, modelingand improving entrainment by a low-Reynolds number fannozzle, designing the EduKitchen insert, protecting thethermoelectric module and cooling it enough for optimalpower generation and safety, all demand innovation. Currentwork includes a more extensive characterization of thejet showing the effective boundaries of the jet with theentrained flow. This will permit a better estimation of theair flow forced by the fan, which would add to the naturalconvective flow into a stove. Figure ?? shows the expectedperformance of the HZ-14 thermoelectric module as afunction of the temperature difference across the module.This plot indicates that a single such module is sufficientto provide the needed wattage. However we have notsucceeded in optimizing power extraction from the moduleto reach such values, and this issue is being investigated.

Figure 6. Power Output of Hz-14 versus Temperature Difference

ACKNOWLEDGMENT

This study was enabled by NASA Grant NNX09AF67GS01, the EXTROVERT initiative to develop resources forcross-disciplinary innovation. Mr. Tony Springer is the tech-nical monitor.

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