Keywords: Radiative heat exchange, Electrical power, Waste heat recovery

Transcription

1 Advanced Materials Research Submitted: ISSN: , Vols , pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: Experimental Investigation of Thermoelectric Power Generation Using Radiative Heat Exchange Niran Watcharodom 1,a Withaya Puangsombut 1,b* Joseph Khedari 2,c Narong Vatcharasatien 3,d and Jongjit Hirunlabh 4,e 1 Sustainable Energy and Environment Technology and Management Program Rattanakosin College for Sustainable Energy and Environment, Nakhon Pathom,THAILAND 2 Faculty of Architecture, Kasetsart University, Bangkok, THAILAND 3 Electricity Generating Authority of Thailand, THAILAND 4 Faculty of Engineering, Bangkokthonburi University, Bangkok, THAILAND a n.watcharodom@gmail.com b,* wpuangsombut@gmail.com(corresponding Author), c joseph.khedari@hotmail.com, d labbvat@yahoo.com, e jongjit.hirunlabh@hotmail.com Keywords: Radiative heat exchange, Electrical power, Waste heat recovery Abstract. This paper reports experimental investigation of a new concept of waste heat recovery for Thermoelectric Power Generation using Radiative heat exchange principle (TERX). To this end a small scale experimental setup was considered; it was composed of a heated plate, an absorber plate, thermoelectric modules and water cooled heat sink. The dimensions of absorber and heated plates were 0.2 m width and 0.3 m length. The air gap space between the two plates could be adjusted. Ten thermoelectric modules were connected in series parallel (5x2). Tests were made for different air gap spaces and fixed water flow rate (2L/min). A constant electric current (200W) was supplied to the heater of hot plate. Data collected included temperature at various positions and the electrical power generated. Experimental investigation confirmed that using radiative heat exchange principle could be considered for TE waste heat power generation. Increasing air gap decreased the electrical power generated as less radiative heat is absorbed by the thermoelectric modules. Under test conditions, the maximum measured electrical power is W at 0.5 cm of air gap, the corresponding temperature difference between the hot and cool sides of thermoelectric modules was about 35oC. Due to its simplicity of installation as no there is no need for direct contact between the thermoelectric generation set and the source of heat, the proposed concept offers a new alternative for waste heat recovery. Introduction It is well known that industries consume huge amounts of energy and produce significant amount of waste heat at different levels. There are several sources of thermal energy for thermoelectric power generation system viz. waste heat from industries, waste heat from automobiles, biomass cook stoves, solar energy etc. Hsiao et al. [1-3] proposed a mathematical model of thermoelectric generation with application on waste heat from automobile engines. A one dimension mathematical model showed that a maximum power output of mw/cm2 at temperature difference 290 C. For the most applications studies consider a low cost low grade heat source [4-6] and aim to maximize the power output of commercially available thermoelectric modules by minimizing the pumping penalty of the heat sink. Bismuth Telluride (Bi2Te3) modules are common in thermal system studies [7-10]. Thermoelectric power generation from biomass cook stove was presented in [11]. Maneewan et al. [12] presented a new concept of roof design named The Thermoelectric Roof Solar Collector (TERSC), which was comprised of thermoelectric modules, a rectangular fin heat sink, a copper plate, a transparent acrylic sheet and air gap. Research results indicated that about 1.2 W power could be obtained by such generator with 10 thermoelectric modules in m2 surface area, under a solar radiation intensity of about 800 W/m2 at ambient temperature between 30 and 35oC. The electrical conversion efficiency of TE-RSC system was reported as low as 1-4%. N. Vatcharasathien et al. [13] designed and analyzed the performance of a solar thermoelectric power generation plant (STEPG). The system considers both truncated compound parabolic collectors (CPC) with a flat receiver and conventional flat plate collectors, thermoelectric (TE) cooling and power generator modules and appropriate connecting pipes and control devices. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,22:44:00)

2 1126 Advanced Materials, Structures and Mechanical Engineering To minimize system cost, seasonal adjustment of the slope angle between 0o and 30o was considered which can give relatively high power output under Bangkok ambient condition. Dan Dai et al. [14] demonstrated the feasibility of a new prototype of the liquid metal based TEG system which combined commercially available thermoelectric (TE) modules with the electromagnetic pump. It was shown that the maximum open-circuit voltage of 34.7 V was obtained when the temperature of the waste heat source was 195.9oC and the temperature gap between liquid metal heating plate and cooling-water plates was nearly 100oC. Various researchers [15-20] investigated flow channel thermoelectric systems for industrial waste-heat recovery applications. Many industrial liquid-to-liquid heat exchangers are commonly used for expelling excess heat from the working instruments and operating environment for production and security needs. Such heat exchangers offer a thermal system that could potentially maintain a thermal dipole for thermoelectric power production if the gain in power can offset the pumping penalty that results from the presence of the generator. Frédéric J. Lesagepresent et al. [22] study investigates thermoelectric power improvement due to heat transfer enhancement at the channel walls of a liquid-to-liquid. In Thailand, it was estimated that 25% of wasted heat could be converted into electricity using thermoelectric technology [23]. However Thermoelectric technology is still not well developed because of different problems. In fact apart from the low efficiency, technical issues have to be addressed when installing thermoelectric generators. Actually thermoelectric applications use conduction technique or circulating fluid to recover heat. This method cause relatively high investment cost and difficult practical issues that limited its general adaptation. This paper investigates on a new concept for based on radiate heat exchange principle to recover waste heat recovery from industries. Description of Experimental Setup The experimental setup is composed of a heated plate, an absorber plate, thermoelectric modules, and a water cooled heat sink. The dimensions of absorber and heated plates were 0.2 m width and 0.3 m length. The air gap space between the two plates could be adjusted. Ten thermoelectric modules were connected in series parallel (5x2). (Fig.1). Fig. 1 Schematic diagram showing lab scale thermoelectric power generation using radiative heat exchange concept The aluminum absorber plate was 200 mm wide, 300 mm height and 3 mm. thick. Ten thermoelectric cooling modules (TEC I-12706) were installed between the absorber plate and the aluminum heat sink. Thermoelectric modules were connected in series and parallel (5X2). And load of resistance load 1 W was connected as shown in Fig.(2). The design of aluminum heat sink (Fig. 3) included sixteen fins, 38.1 mm. height, 5 mm. wide and 300 mm. length; the spacing between fins is 5 mm. The base of heat sink is 40 mm. thick, 200 mm. wide and 300 mm. length with five hollow tubes of 20 mm. diameter for circulating cooling water. The heat source (heater plate) was made from a strip heater. The strip heater was 2 mm. thick, 180 mm wide, and 280 cm long. The

3 Advanced Materials Research Vols power supplied to the heater was adjusted by variable transformer at 200 W. Figure (3) shows the experimental setup of TERX system. Fig. 2 Thermoelectric modules arrangement Fig. 3The Experimental set up of TERX system. Eight thermocouples type K were installed at different positions as shown in fig.1 and 3. The thermocouples were connected to the data logger (Testo: model 177.T4) to record the temperature of each point, the data loggers measured temperatures were in the range of -200 to 2000oC, and accuracy was ±0. 3oC. Two digital multi meter (UNI-T: model UT 33B) were used to measure the generated voltage and ampere from thermoelectric modules. The accuracy of voltage ± (0.5%) and DC current ± (1% ). A flow rate meter measured the water flow through the heat sink, the range of flow rate meter was 1-30 L/min; the accuracy was 5%. Eight thermocouples type K were installed at different positions as shown in fig.1 and 3. The thermocouples were connected to the data logger (Testo: model 177.T4) to record the temperature of each point, the data loggers measured temperatures were in the range of -200 to 2000oC, and accuracy was ±0. 3oC. Two digital multi meter (UNI-T: model UT 33B) were used to measure the generated voltage and ampere from thermoelectric modules. The accuracy of voltage ± (0.5%) and DC current ± (1% ). A flow rate meter measured the water flow through the heat sink, the range of flow rate meter was 1-30 L/min; the accuracy was 5%. Results and Discussion As mentioned earlier this paper focuses mainly on investigating the effect of air gap space on thermoelectric power generated. Six air gap spaces namely 0.5, 1, 2, 3, 4 and 5 cm. were considered. The corresponding calculated view factor defined as the ratio between the air gap space and height is given in Table 1. Tests were made with a constant electric current supplied to the heater (200W) and fixed water flow rate for cooling the heat sink at 2 l/min.

4 1128 Advanced Materials, Structures and Mechanical Engineering Table 1 View factor between the two aligned parallel heated and absorber plates. D = Air gap space L1, L = Length and wide of rectangular plates (heater, absorber). Temperature Difference Figure 4 shows the measured variations of temperature diffence between heated and absorber plates for the different air gap spaces considered. As expected, increasing air gap decreased the temperature difference. The temperature difference reached a relatively steady state condition within min. The smaller the air gap, the longer is the time needed to reach equilibrium. Fig. (5) shows the corresponding measured temperature differnce between the TE hot and cold sides for the different gaps. It can be obseved that, about min was necessary to attend a relative steady temperature difference. This could be explained due the heat capacitance of the absorber and heat sink used. Power Output Figures 6, 7 and 8 show the variations of measured voltage, ampere and electrical power generated by our experimental setup for the different air gaps considered respectively. It could be observed that measured current voltage (Fig.6) continued to increase steadily with time. However current ampere (Fig.7) reached constant conditions with 30 to 40 min following the variations of measured temperature difference across the TE sides. The variations of generated power (Voltage x Ampere), (Fig.8) increased continuously with time. The smaller is the air gap the higher the temperature difference and more power is generated.

5 Advanced Materials Research Vols Under test conditions, the nearest air gap considered 0.5 cm produced the highest voltage 1.08 V whereas a 5.0 cm air gap produced the lowest voltage 0.47 V (see Fig.5).Fig.7 shows that the air gap thickness and time affect generated electrical current considerably. The smallest air gap 0.5 cm. produced the maximum electrical current (0.29 Amp). While the largest one (5 cm.) produced only 0.07 Amp. The measured data and the corresponding decrease of power output with the increase of air gap are summarized in table (2) after 60 minutes of operation. Table 2 Summary of maximum measured V, A, Watt and percent of power decrease with increasing air gap after 60 minutes. Air Gap and Radiative Heat Exchange Analysis To better understand the effect of air gap on power output under the test conditions, figures 9, 10 and 11 show the variations of the voltage, ampere and power generated by our experimental set up as a function of air gap at different time intervals (10 min) each during 60 minutes. It can be seen that the air gap between the heated plate and absorber affects the generated electrical current significantly. For a specific power generation demand, the smaller is the air gap, the shorter the time needed to produce the required power.

6 1130 Advanced Materials, Structures and Mechanical Engineering TERX Performance and System Efficiency To a better assessment of the TERX system performance, figure 12 shows the variations of generated power as a function of temperature difference between the TE hot and cold sides for the different gaps considered during 60 minutes of operation. It can be seen that for every air gap considered, a relatively constant temperature difference between the hot and cold sides of TE modules is observed after a certain time of operation. For 0.5 cm air gap, the temperature difference is 35.7oC while for 5 cm air gap, it is 10.5oC. Maintaining this temperature difference allows to increase the power generated by TE module continuously. This depends on the specifications of TE modules used. As supplied heat from waste energy might fluctuate in practical application, increasing or decreasing the air gap could be used to stabilize the generated power as required. Fig. 12 Variation of generated power as a function of temperature difference between the hot and cold sides of TE modules for the different air gaps considered The system efficiency of our system is defined as the ratio of power generated (P) to the heat transferred from the heat source to the generation system as given below.

7 Advanced Materials Research Vols Where =System efficiency, P =Power (W), Q 12=The radiative heat transferred from the heat source to the absorber (W), Q 12 could be expressed mathematically in the equation below[2] 4 4 ( T1 T2 ) Q12 (2) A1 1 A1 F12 A2 2 Where =Stefan-Boltzmann constant (5.67x10-8 W.m -2. K -4 ), T 1 =Heat source temperature(k) T 2 =Aluminum absorber temperature (K), 1 =Emissivity of Stainless steel (0.16) [23] 2 =Emissivity of Aluminum (0.03) [23], A 1 =Area of heat source m 2 A 2 = Area of Aluminum absorber plate m 2, F 12 = View factor between the two aligned heated and absorber plates. To generalize our results, the ratio between the temperature difference between the hot and cool sides of TE modules (DT) and the heat supplied to the system (HS) is introduced. Fig. 13 Shows the variation of system efficiency as a function of the calculated radiative heat relationship between DT/HS and TERX efficiency for the different air gap spaces considered. As discussed earlier the highest efficiency is obtained at the smallest gaps. Conclusion This paper investigated the use of the concept of radiative heat exchange for thermoelectric power generation. An experimental setup was built and various air gap spaces were tested namely 0.5,1,2,3,4 and 5 cm. The dimensions of heated and absorber plates were 0.2 m. wide and 0.3 m. height. The calculated view factor of radiative heat exchange between the two surfaces between defined as the ratio between the air gap space and height varied. Measured data confirmed that the proposed concept could be adopted for waste heat power generation using thermoelectric modules. The small the air gap, the higher is the generated electrical current. Despite its low power efficiency, its main advantage concerns its simplicity as TE generation set is not attached directly to the heat source. Further investigation is recommended with

8 1132 Advanced Materials, Structures and Mechanical Engineering a more sophisticated experimental set up that could adjust the air space gap and maintain electrical output at desired value. References [1] Y.Hsiao, A mathematic model of thermoelectric module with applications on waste heat recovery from automobile engine, Energy, Vol. 35 (2010), [2] W.H. Lai et al., Experimental simulation on the integration of solid oxide fuel cell and micro-turbine generation system, Power Sources, Vol. 171 (2007), [3] Basel I. Ismail and Wael H. Ahmed, Thermoelectric Power Generation Using Waste-Heat Energy as an Alternative Green Technology Recent pantents on electrical engineering,.vol. 2 (2009), [4] Gou X, Xiao H. and Yang S. Modeling, Experimental study and optimization on low temperature waste heat thermoelectric generator system, Apply Energy, Vol. 87 (2010), [5] Wee D., Analysis of thermoelectric energy conversion efficiency with linear and nonlinear temperature dependence in material properties, Energy Conversion and Management, Vol. 52 (2011), [6] Bélanger S. and Gosselin L. Thermoelectric generator sandwiched in a crossflow heat exchanger with optimal connectivity between modules. Energy Conversion and Management, Vol. 52 (2012), [7] Crane DT. and Jackson GS. Optimization of cross flow heat exchangers for thermoelectric waste heat recovery, Energy Conversion and Management, Vol. 45 (2004), [8] Rodriguez A., Vian JG., Astrain D. and Martinez A., Study of thermoelectric systems applied to electric power generation, Energy Conversion and Management, Vol. 50 (2009), [9] Whalena SA. and Dykhuizenb RC., Thermoelectric energy harvesting from diurnal heat flow in the upper soil layer, Energy Conversion and Management, Vol. 50 (2012), [10] Karabetoglu S., Sisman A., Faith Ozturk Z. and Sahin T. Characterization of a Thermoelectric generator at low temperatures, Energy Conversion and Management, Vol. 62 (2012), [11] D. Champier, J.P. Bedecarrats, M. Rivaletto and F. Strub, Thermoelectric power generation from biomass cook stoves, Energy, Vol. 35 (2010), [12] Maneewan S., Khedari J., Zeghmati B., Hirunlabh J. and Eakburanawat J. Investigation on generated power of thermoelectric roof solar collector, Renewable Energy, Vol. 29 (2004), [13] Vatcharasathien N., Hirunlabh J., Khedari J., and Daguenet M., Design and Analysis of Solar Thermoelectric Power Generation System, International Journal of Sustainable Energy, Volume 24 (2005), [14] Dan D., Yixin Z., Jing L., Liquid metal based thermoelectric generation system for waste heat recovery, Renewable Energy, 36 (2011), [15] Stevens JW., Optimal design of small DT thermoelectric generation systems, Energy Conversion Management, Volume 42 (2001), [16] Yu C., Chau KT., Thermoelectric automotive waste heat energy recovery using maximum power point tracking, Energy Conversion Management, Volume 50 (2009), [17] Miller EW., Hendricks TJ. and Peterson RB., Modeling energy recovery using thermoelectric conversion integrated with an organic rankine bottoming cycle, Journal of Electronic Materials, Volume 38 (2009), [18] Kristiansen NR., Snyder GJ., Nielsen HK. and Rosendahl L., Waste heat recovery from a marine waste incinerator using a thermoelectric generator, Journal of Electronic Materials, Volume 41 (2012),

9 Advanced Materials Research Vols [19] Hendricks TJ., Karri NK., Hogan TP., Cauchy CJ. and New perspectives in thermoelectric energy recovery system design optimization, Journal of Electronic Materials, Volume 42 (2013), [20] Lesage FJ. and Pagé-Potvin N., Experimental analysis of peak power output of a thermoelectric liquid-to-liquid generator under an increasing electrical load resistance, Energy Conversion Management, Vol. 66 (2013), [21] Frédéric J. Lesage, Éric V. Sempels and Nathaniel Lalande-Bertrand, A study on heat transfer enhancement using flow channel inserts for thermoelectric power generation, Energy Conversion and Management, Vol.75 (2013), [22] Yodovard, P. et al., The potential of waste heat thermoelectric power generation from diesel cycle and gas turbine cogeneration plants, Energy sources, Vol. 23 (2001), [23] Incropera F.P. and De WITT, D.P., Fundamentals of heat and mass transfer, Jonh Wiley & Son, 1990,

10 Advanced Materials, Structures and Mechanical Engineering / Experimental Investigation of Thermoelectric Power Generation Using Radiative Heat Exchange / DOI References [14] Dan D., Yixin Z., Jing L., Liquid metal based thermoelectric generation system for waste heat recovery, Renewable Energy, 36 (2011), /j.renene