Modeling Integrated Thermoelectric Generator-Photovoltaic Thermal (TEG-PVT) System

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1 Modeling Integrated Thermoelectric Generator-hotovoltaic Thermal (TEG-VT) System R. Kiflemariam *, M. Almas, and C. Lin Deartment of Mechanical and Materials Engineering Florida International University, Miami, FL * Abstract: A D steady state heat conductionelectric current model was created in COMSOL Multihysics to study the erformance of thermoelectric generator-hotovoltaic-thermal (TEG-VT) system. Four different cases were studied in the aer. In case, V cells without concentrator was simulated while in case, concentrator ratio range from to 5 was utilized In case 3, the convection heat transfer coefficient was varied between 6. and. W/m K. The value of thermal insulation between V cells and TEG was decreased for case. The results indicate that having higher concentration ration results in more ower generation while increases convection heat transfer coefficient between outside surfaces and atmoshere and lower thermal resistance between V cells and TEG hel kee the V temerature at otimum level. Keywords: Thermoelectric generator, hotovoltaic thermal (VT), air cooling, COMSOL. Introduction There is a demand for efficient and clean energy due to the rising cost of energy and globally increasing environmental awareness. Thermoelectric generation is a romising technology which cleanly converts waste heat into electricity. They have been alied in aerosace alications and waste heat recovery from cars and industries [-3]. hotovoltaic (V) based technology is also another clean source of energy which roduces electricity from sunlight. However, the efficiency of V decreases with V cell temerature and they need to be cooled to maintain their efficiency by removing waste heat from the anels. The waste heat from the V anels could be ut into useful energy by using TEG. Thus, there has been researches on integrating hotovoltaic cells and thermoelectric system into one hybrid generation system [-8]. One of the roblems cited in the studies was the low temerature across the TEG which minimizes the ower generated by TEGs. Therefore, the erformance of a hybrid hotovoltaic thermalthermoelectric generation with finned air cooling has been studied numerically using COMSOL and the results are resented. The basic design consists of TEG modules attached to the base of V modules as shown in Figure. The cold side of the TEG module is also connected to finned heat sink. The heat sink hels cool both the V and the TEG. The arrangement is deicted in Fig.. Four different cases were studied. In the first case (case ), V cell with no concentrator is simulated. In the second case (case ) the concentration ratio and heat transfer coefficient between was varied between to 5. For the third case (case 3), convection heat transfer coefficient between the V cells and atmoshere and at cold heat sink attached to the TEG module is varied between 6. and.. In the fourth case (case ), the thermal insulation conductivity was changed. 5 Glass Cover V cells 3 Insulator TEG module 5 Finned heat sink Figure. Schematic of VT-TEG system 3

2 y (mm) 3.6. Governing equations The heat absorbed by the V anels is given by: Q net Q abs Q rad r Q, conv where Q abs is the roduct of incident insolation on the V surface (G), the concentration ratio (C r), efficiency of V (η v) and area of the V (A ).It is reresented in the comutational model as a heat source in V domain. Q ( )C GA abs V r Q rad,r is the heat reflected back to the surrounding air and is described as: 3 rad, r ( TV Q T ) A amb where ε is the emissivity of the V surface, σ is the Stefan-Boltzmann constant and T v and T amb are the temeratures of V anel and the surrounding air resectively. It is reresented as surface boundary condition (BC in Fig. ). 0 BC BC3 BC Figure. Comutational model of integrated system 0 x (mm) BC in the V surface and BC3 on the surfaces of fins resectively. Q h( T T ) A conv where h is the convection heat transfer coefficient A TEG module generates electric otential (V sb) when a temerature difference is maintained between the hot and cold sides of the TEG, and its magnitude is roortional to the difference in Seebeck coefficient (α) of the two airs of TEG elements. The airs of the TEG elements are designated as and n elements. The ower generated from TEG module can be exressed as Eq.5: n ( )( T T ) 5 n n hs cs gen VsbReq [ ] Re, Re, Re, m where T hs and T cs are the cold and hot side temerature of the TEG module, α and α N are the Seebeck coefficients of the and n elements, resectively. n n is the number of and n elements in a TEG module. R eq, R e, and R e, m are the equivalent total electrical resistance, external electric resistance and the internal TEG module electric resistance resectively. The heat flow in the TEG as described in Eq. 6 is mainly due to the temerature difference between the hot side and cold side of the thermoelectric generator but the heat flow due to eltier effect (the first term on the left hand side of Eq. 6) and joule heating (the last term in Eq.6) also contribute to the total heat flow in the TEG. The total heat flow in the TEG module is given as: Q NIT ( ) K( T T ) 6 mod I R V amb N N hs cs Q conv reresents the convection heat loss to the surrounding atmoshere and is deicted as surface boundary condition and 3 which are

3 Table. Thermal roerties Name Seebeck coefficient_bite3 electric conductivity_bite3 thermal conductivity_bite3 secific heat caacity_bite3 density_bite3 thermal conductivity coer electric conductivity cu thermal conductivity_v thermal conductivity glass thermal conductivity insulation 3. Use of COMSOL Multihysics Value 05e-6[V/K].e5[S/m].6[W/(m*K)] 5.[J/(kg*K)] 770[kg/m^3] 00[W/(m*K)] 5.998e7[S/m] 39[W/(m*K)].[W/(m*K)] 0.035[W/(m*K)] A D model of thermoelectric generator, heat source V anel and the microchannel heat sink is constructed and numerical simulations were conducted using the FEM solver COMSOL (Version.). The heat transfer module (ht) is used to study heat transfer and fluid flow arameters and results of temerature, velocity, and ressure field are obtained. The electric current module (ec) is also used to estimate electric otential and ower generation from TEG module. The modules are linked using weak form equation which reresents the seebeck effect. The simulation arameters are concentration ratio is varied from C= to C=5 and the convection heat transfer which is defined corresonding to the wind velocity U= m/s to 3 m/s. The ambient temerature is ket at 98 K. Mesh grid indeendence study were carried out and fine mesh with 53 elements were used. Segregated grou solvers were used for solving Electric otential (V), Temerature field (T), Velocity field (U), and ressure. Steady state simulations were carried out with convergence criteria of Results Table. Geometrical roerties Name ellet width ellet height electrode thickness ga between ellets height ceramic fin base height fin base width fin height fin width ga between fins number of fins 0 height V width V height glass cover width glass cover height back insulation width back insulation Value.5[mm] 0.38[mm].[mm] 0.9[mm] 0[mm] 5[mm] 5[mm] 0.3[mm] 0[mm] 3[mm] 0[mm] 0.5[mm] 0[mm] The ower roduced by TEG modules deends on the temerature difference between the hot side and cold side of the module (ΔT teg). Thus, an increase in net het inut in V cells results in higher hot side temerature at the TEG module. Case refers to V cells without concentrator (C r=). Simulation results indicate T V=3 K and ΔT teg=.5 K. This suggests that for a viable ower roduction, an increase sin concentration ratio is necessary to achieve more favorable ΔT teg and thus more ower. For Case, C r is varied between and 5. As Fig 3 shows, with an increase in concentration ratio, T V increased from 330 K to 377 K as C r is raised from to 5. With high concentration ration, more irradiation is absorbed at the surface which increases the temerature of the V cells.

4 a) b) For case 3 and as shown in Fig, the effect of the cold heat sink convection heat transfer was simulated for heat transfer coefficient between 6.6 and. W/m.K For the constant C r=3, ΔT teg remained at the same value, but the temerature of V cells (T V) decreased by around 8 K. Therefore, having high heat transfer at the cold heat sink hels decrease T V without affecting ower roduction from TEG modules. The electric otential was simulated to be around 0. V er module (Fig 5). As there is a thermal and electrical insulation between the V cells and TEG modules, it could be inferred that there is high T V but the hot side temerature could be less by as much as 30 K due to the thermal insulation. c) a) d) b) T (K) Figure 3. Temerature distribution for C equal to a) b) 3 c) d) 5 and h=0. W/m K c) Likewise, the hot side temerature of the TEG module also increased by 6.5 K for a.5 fold increase in C r. For a constant ambient temerature at 98 K and convection heat transfer coefficient of 0. W/m.K, ΔT teg increases from 3 K to about 7.3 K. T (K) Figure. Temerature distribution for C=3 and wind velocity (U) of a) m/s b) m/s c) 3 m/s

5 For case the thermal conductivity of the insulation material between the V cells and TEG module was increased. This resulted in a decrease in T V from 3 K to 38 K for C r=3 and h=0. W/m. K which suggest that reducing thermal resistance between V cells and TEG module could hel kee V cells t otimum temerature for higher C r. 5. Conclusions Electric otential [V] Figure 5. Electric otential for C=3 and U=3 m/s In this aer, the integrated design of thermoelectric generator and VT cells is studied using COMSOL. Temerature and electric distribution in the integrated system and electric otential and current at TEG module has been simulated. The results indicate that having a higher concentration ratio results in higher ower roduction from TEG module due to increased absorbed heat flux. It is also observed that decreasing thermal resistance between the V cells and TEG module also assists in keeing the V cells temerature at otimum value. With an increased efficiency of TEG modules, the integrated system could roduce useful ower from the TEG module augmenting ower roduction from V cells. [3] Kiflemariam, R., Lin, C.X., Moosavi, R., Numerical simulation, arametric study and otimization of thermoelectric generators for selfcooling of devices, in: th AIAA/ASME Joint Thermohysics and Heat Transfer Conference, American Institute of Aeronautics and Astronautics, 0. [] He, W. Su, Y., Riffat, S.B. Hou, J., and Ji, J. arametrical analysis of the design and erformance of a solar heat ie thermoelectric generator unit, Al. Energy. 88, (0). [5] Muhtaroglu, A., Yokochi, A., Jouanne, A., Integration of thermoelectrics and hotovoltaics as auxiliary ower sources in mobile comuting alications, J. ower Sources. 77, 39 6(008). [6] Deng, Y., Zhu, W., Wang, Y., Shi, Y., Enhanced erformance of solar-driven hotovoltaic thermoelectric hybrid system in an integrated design, Sol. Energy. 88, 8 9(03). [7] Zhang, M., Miao, L., Kang, Y.., Tanemura, S., Fisher, C.A.J., Xu, G. et al., Efficient, low-cost solar thermoelectric cogenerators comrising evacuated tubular solar collectors and thermoelectric modules, Al. Energy. 09, 5 59(03). [8] Xi, H., Luo, L., Fraisse, G. Develoment and alications of solar-based thermoelectric technologies, Renew. Sustain. Energy Rev., (007). References [] Wang, Y., Dai, C., Wang, S. Theoretical analysis of a thermoelectric generator using exhaust gas of vehicles as heat source, Al. Energy (03). [] Gou, X. Xiao, H., Yang, S., Modeling, exerimental study and otimization on lowtemerature waste heat thermoelectric generator system, Al. Energy. 87, (00).