Study of Combined Rice Husk Gasifier Thermoelectric Generator

Transcription

1 Available online at ScienceDirect Energy Procedia 5 (014 ) International Conference on Alternative Energy in Developing Countries and Emerging Economies Study of Combined Rice Husk Gasifier Termoelectric Generator C. Lertsatittanakorn 1*, J. Jamradloedluk and M. Rungsiyopas 3 1 Energy Tecnology Division, Scool of Energy, Environment and Materials, King Mongkut s University of Tecnology Tonburi, Bangmod, Tungkru, Bangkok 10140, Tailand Termal Processes Researc Laboratory, Faculty of Engineering, Maasarakam University, Kantarawicai, Maasarakam 44150, Tailand 3 Mecanical Engineering Department, Faculty of Engineering, Burapa University, 169 Long-Hard Bangsaen Road, Saensuk, Muang, Conburi, 0131, Tailand Abstract Te use of a rice usk gasifier as a cook stove is limited to te domestic sector of developing countries primarily because it needs electrical energy to drive a blower for te gasification process. To solve tis problem, we investigated te feasibility of attacing commercial termoelectric (TE) modules made of bismut telluride materials to te gasifier s side wall, tereby creating a TE generator system tat utilizes a proportion of te gasifier s waste eat. A rice usk gasifier TE generator (TE-RSG) aving an internal diameter of 16 cm was fabricated and tested. Te TE generator system consisted of two commercial TE modules, a metal seet wall wic acted as one side of te gasifier s structure and served as te ot side of te TE modules and a rectangular fin eat sink at te cold side of te TE modules. A blower was used to suck te ambient air to cool te eat sink and blow te air from te eat sink to te reactor of te gasifier. Gasification was conducted in a temperature range of C and gasification agent, air feeding rate of 18.6 m3/. Te results revealed tat te electrical power output and te conversion efficiency depend on te temperature difference between te cold and ot sides of te TE modules. At te temperature difference of approximately 60 C, te unit acieved a power output of 3.9 W and a conversion efficiency of.01%. Troug a comparison of results between te teoretic model and te experimental system, te reasonability of tis system model as been verified Publised by Elsevier by Elsevier Ltd. Tis Ltd. is an Selection open access and/or article under peer-review te CC BY-NC-ND under responsibility license of te Researc (ttp://creativecommons.org/licenses/by-nc-nd/3.0/). Center in Energy and Environment, Taksin University. Selection and peer-review under responsibility of te Organizing Committee of 013 AEDCEE Keywords: Conversion efficiency; Termal efficiency Reactor Publised by Elsevier Ltd. Tis is an open access article under te CC BY-NC-ND license (ttp://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of te Organizing Committee of 013 AEDCEE doi: /j.egypro

2 160 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) Introduction Te demand for renewable sources of energy is increasing due to an elevated concern about global warming, climate cange and te decline of fossil fuel reserves. Compared wit oter renewable energy resources, biomass is uge. Its annual production rate is ig and it is geograpically widespread trougout te world. In general, paddy, or rice, is one of te earts most prolific crops. Rice usk is a residue from rice farming and is considered an agricultural waste. In Tailand, were te average gross rice production is approximately 3.09 Mtons/yr., 6 Mtons of rice usk is produced and around 600 tousand tons of as is generated by burning te rice usk [1]. Currently, rice usk is widely used in stall mats, compost and fillers. However, due to increasing demands for utilization of waste-to-energy, many researces are actively involved in researcing ways to use rice usk as a fuel. Approximately.4 billion people depend on wood, dung, carcoal and oter biomass fuels for cooking. Most of tese people cook on open fires tat burn poorly leading to low termal efficiency and ig pollution emissions. Te current patterns of use cause significant negative impact of several types, including uman morbidity and mortality, outdoor air pollution, climate cange and deforestation. One interesting alternative to tese inefficient cooking metods is a rice usk gasifier; it is more efficient tan biomass cook stoves []. However, electrical power is needed to drive a blower tat is part of te rice usk gasifier system. Individual termoelectric (TE) power generators coupled wit rice usk gasifier offer an interesting option to provide electricity. In past years, TE generators ave been coupled wit biomass stoves. As te examples, Nuwayid et al. [3] considered te prospect of applying TE modules located on a wood stovetop to produce power. Tat system is well known and could be useful in regions wit unreliable electricity supply. Te stove-top TE system produces a maximum power output of.7 W per module. A eat sink composed of a termosyponic eat pipe as been adapted to furter improve te power output of a TE module [4]. Tese developments revealed tat a commercially available TE module could provide over 3 W of power wit a temperature difference between te ot and cold side of te TE module of C. Experiments ave also been conducted on te side-walls of cook stove. Stove wall temperatures are likely to be in te range of C. Lertsatittanakorn [5] investigated a combined biomass cook stove termoelectric (BITE) generator. Te results of tat investigation sowed tat te BITE produces a maximum power output of.4 W at a temperature difference of 150 C. Te conversion efficiency of 3.% was enoug to drive a low power incandescent ligt bulb or a small portable radio. Meanwile, te payback period of te BITE is 0.74 years if compared wit batteries supplying power to a 1.8 W load wit an annual operating time of 365 ours. Campier et al. [6] studied a TE generator incorporated in a multifunction wood stove to produce electrical power from te exaust gas of te wood stove. Onedimensional eat flow was used to predict te system s performance. Te TE module produced maximum power output of 9.5 W. An economic analysis sowed tat te price of TE modules varied wit order volume. By comparison between te cost per watt of te PV panels and te TE generator, it was found tat te cost per watt of te TE module is very competitive wit PV panels. Te objective of tis work was to study te feasibility of using TE modules coupled wit a rice usk gas gasifier in order to generate electricity. A more reasonable model of a TE generator as been adopted for system analysis. Testing results sowed promise of using a TE generator for waste eat recovery in a gasifier stove. Nomenclature A Seebeck coefficient area of termoelement

3 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) C pw H f I K l L l c m w specific eat of water calorific value of rice usk (iger eating value) input electric current termal conductivity of te TE module lengt (eigt) of termoelement latent eat of vaporization lengt (tickness) of solder/contact in module mass of water initially in te pot m w,evap mass of water evaporated m f n N r R T i T e T T c T m V Z mass of rice usk contact parameter number of termoelements per module contact parameter electric resistance of te TE module initial temperature of water in te pot temperature ofboiling water ot side temperatures of te TE module cold side temperatures of te TE module average temperature voltage of te TE modules figure of merit of te TE material(z= /K). Metods.1. Calculating Metods Te equations used to model te beavior of te TE power generator are based on te Seebeck, Fourier and Joule effects. Using te standard model [7] and assuming one dimensional conduction troug te module, te rate of eat supply (Q ) and eat removal (Q c ) can be estimated at te ot and cold junctions as ( ) - Tc ( - T ) Q = αit - 0.5I R + K T (1) c c Q = αit + 0.5I R + K T () c

4 16 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) Teoretically, te maximum power output (P) of a realistic TE modules takes into account te contact resistance as given by [8] ( ) α NA T - Tc P = (3) R rl ( ) c L + n 1+ L Typically, n = 0.1 mm, r = 0., l= 1. mm, lc = 0.8 mm, α =3.9 x 10-4 V/K, N= 17 couples, R = Ωm, K = 1.63 W/mK and A = 1.96 mm. Te electrical output of te TE modules (P) is also calculated from te measured data as follows: P=I V (4) Miller et al. [9] suggested tat te conversion efficiency is as follows M -1 η e = ηc (5) Tc M + T were M= 1+ZT m wic T m = 0.5(T + T c ) Note tat ZT m is a caracteristic parameter of te termoelectric element and essentially governs its internal conversion efficiency. It is well known tat te value of Z can ave strong variations in temperature. In tis study, in order to gain insigt into te optimal collector operating temperatures, te value of Z is assumed to be constant. Altoug tis may be an over simplification of te actual situation, it provides tractable solutions for te solar collector temperature and operating efficiency of te termoelectric element. T - Tc c is te Carnot efficiency; η c = (6) T Te termal efficiency of te gasifier is defined as te ratio of te energy entering te pot to te energy content of te fuel consumed. Te standard water boiling test (WBT) [10] is used for testing te efficiency of te gasifier. Te termal efficiency ( t ) can be calculated using te formula: ( T - T ) mwcpw e i + mw,evapl η t = (7) m H f f It is a given tat electrical energy is iger in grade tan termal energy. Terefore, Ji et al. [11] used te overall efficiency o for a TE-RSG as. η e η o = ηt + η (8) power were power, often assigned a value of 38%, is te electrical-power generation efficiency of a conventional termal power plant... Experimental Apparatus Te TE-RSG was a batc fed up-draft gasifier. It consists of two main parts: a termal part (gasifier) and an electrical part (TE). Te gasifier basically consists of a reactor, a burner and a blower. Te reactor

5 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) is a concentric cylinder in tube aving an inside tube diameter of 17 cm and a eigt of 70 cm. Te reactor is made of mm tick galvanized iron seet. Te outer cylinder as a diameter of 1 cm. An annular space between te inside and outside cylinder olds a ceramic fiber insulator to prevent eat loss from te reactor. Te inner cylinder is fixed to te top flange. Te flange, togeter wit te inner cylinder, can be made removable from te outer cylinder for easy cleaning and replacement wen worn out. At te lower end of te reactor inner cylinder is a fuel grate made of mild steel used to old te rice usk during gasification. A blower is used to supply te air needed for gasification of rice usks and to release eat from te cold side of te TE modules. Te inlet of te blower is connected wit an air duct to suck te ambient air troug a rectangular fin eat sink, wile te outlet of te blower is connected to te lower end of te reactor outer cylinder. Te scematic view of te TE-RSG is sown in Fig. 1. Metal steel was installed at te upper of te side of te reactor outer cylinder. Te ot side of two TE modules was fixed on te metal seet. Meanwile, te cold side was fixed on te rectangular fin eat sink. Te air-cool fin eat sink was made of aluminum wit 1 rectangular fins. Te fins were 1 mm tick, 85 mm long and ad a eigt of 35 mm from te base of te eat sink. Te space between te TE modules, ot plate and eat sink was insulated using a locally made ceramic fiber. Te gasifier was instrumented wit K-type (accuracy 0.5 C) termocouples tat measured te temperature of te ot side and te cold side of te TE modules and te gas. Te termocouple tat measured te ambient temperature was kept in a selter to protect it from direct sunligt. Te current and voltage were measured wit a multi-meter (Fluke model 189, accuracy VDC 0.05%, A 0.5%). Air velocitywas measured by a ot bulb velocity probe (accuracy 0.03 m/s) at te inlet of te air duct. A DC power supply was used to drive te blower. Te blower was supplied 18.6 m 3 / of te air. A data acquisition system was used to collect te data at regular intervals every 1 minute. Unit : cm Fig. 1.Scematic diagram of te TE-RSG

6 164 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) Results and Discussion Fig. presents te flaming temperature profile from te initiation until steady state gasification conditions were establised. Te rice usk was ignited at te top of te reduction zone. After about 13 min, eat propagated upwards and te flaming pyrolysis zone was formed. Indeed, te temperature at te eart of te fire reaces almost 700 C Temperature( C) Time (min) Fig..Rice usk gasification temperature profile Fig. 3 sows te ot and cold side temperatures and te corresponding electrical power for a alf our experiment. Te TE modules produced a maximum electrical power of 3.9 W at a temperature difference of 60 C. Temperature ( C) Tc T P Power (W) Time (min) Fig. 3.Te ot and cold side temperatures and power output of TE generator versus time

7 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) In order to verify te validity of te teoretical power model given by Eq. (3) of tis paper, te calculation and te experimental result are compared, as sown in Fig. 4, wit abscissa for temperature difference and ordinate as output electrical power. As can be seen in te temperature range sown in te figure, good agreement exists wit tose obtained in tis paper, so te model calculation results are reliable. Troug te adoption of blower can greatly promote system performance, te blower itself is driven by a power supply. In tis case, a blower need a 4.5 W power to drive, tat is to say, tis experimental setup is a system unable to run by itself. Fortunately, tere are still some metods, suc as increasing TE modules in series, expanding eat sink surface, wic could make te system work so effectively tat it would selfpower or even generate additional power Measured Predicted Fig. 4.Comparison between measurement and teoretical power modelof TE generator By using Eqs. (5), (7) and (8), te conversion, termal and overall efficiencies were.01%, 0.5% and 5.79%, respectively. Te conversion efficiency of tis TE generator is only a few percent (%). However, te conversion efficiency is not a prime consideration, wen te waste eat utilized ere is assumed to be no-cost. 4. Conclusion An experimental TE generator suitable for electricity production in a rice usk gasifier stove as been presented. Te comparison between a teoretic model and an experiment system proved te reasonability of tis model. Te results sowed tat te TE modules produced up to 3.9 W. Te termal and overall efficiencies of te TE-RSG were 0.5% and 5.79% respectively. Furter work is planned to conduct a system level optimization study. Areas of concern will be energy efficiency and system power capacity. Cooling tecnologies suc as liquid cooling and eat pipes operating in a more effective manner will also be examined for teir potential for TE generator waste eat power recovery.

8 166 C. Lertsatittanakorn et al. / Energy Procedia 5 ( 014 ) Acknowledgements Te elp provided by Mr. ParinyaNuwaboot and Miss BuangurnSnepim for setting up te experiment is deeply acknowledged. References [1] Tanpaiboonkul N., Asavapisit S., Sungwornpatansakul W.Effect of cemical and termal activations on te properties of rice usk as-based solidified wastes.j. Environmental Sciences 010; (1): [] Jain A.K., Goss J.R. Determination of reactor scaling factors for troatless rice usk gasifier. Biomass and Bioenergy 000;18: [3] Nuwayid R.Y., Siade A., Gaddar N.Development and testing of a domestic woodstove termoelectric generator wit natural convection cooling.energy Conversion and Management 005;46(9-10): [4] Nuwayid, R.Y., Hamade R.Design and testing of a locally made loop-type termosyponic eat sink for stove-top termoelectric generators.renewable Energy 005;30: [5] Lertsatittanakorn C. Electricalperformance analysis and economic evaluation of combined biomass cook stove termoelectric (BITE) generator.bioresource Tecnology 007;98(8): [6] Campier D., Bedecarrats J.P., Kousksou T., Rivaletto M., Strub F., Pignolet P.Study of a TE (termoelectric) generator incorporated in multifuncation wood stove. Energy 011;36: [7] Qiu K., Hayden A.C.S. A natural-gas-fired termoelectric power generation system.j. Electronic Materials 009;38(7): [8] Rowe D.M., Min G. Evaluation termoelectric modules for power generation.j. Power Sources 1998;73: [9] Miller W.E., Hendricks J.T., Peterson B.R. Modeling energy recovery using termoelectric conversion integrated wit an organic Rankine bottoming cycle. J. Electronic Materials 009;38: [10] Battacarya S.C., Albina D.O., Salam P.A.Emission factors of wood and carcoal=fired cookstoves.biomass and Bioenergy 00;3: [11]Ji J., Lu J.P., Cow T.T., He W., Pei G.A sensitivity study of a ybrid potovoltaic/termal water eating system wit natural convection.applied Energy 007;84:-37.