Two commercially available thermoelectric generators were tested for their thermal and electrical properties during this experiment. Each had a different limiting hot side temperature. Different properties of thermoelectric generators are illustrated in this section for varying heat flux. The heat flux is varied keeping the hot side temperature of the thermoelectric generator within the maximum limit. The rate of heat leaving the thermoelectric generator is calculated using the temperature of the water inlet and outlet and the measured mass flow rate of the water through the micro channel cold plate.
Figure 35 Behavior of various temperatures with change in heat flux for type A thermoelectric generator with limiting hot side temperature of 150°C
Figure 35 shows the behavior of hot side and cold side temperature of the type A thermoelectric generator with change in the heat flux. A micro channel cooling plate removes the heat form the cold side. It is observed that the hot side and cold side temperature increases almost linearly with increase in the heat transfer. It can also be observed that the temperature difference between the hot and cold side of the
thermoelectric generators increases with almost similar slope as that for the hot side temperature than that for cold side temperature. This behavior is attributes to high heat carrying capacity of the water cooled heat sink.
Figure 36 Behavior of various temperatures with change in heat flux for type B thermoelectric generator with limiting hot side temperature of 250°C
Type B thermoelectric generator demonstrates similar behavior to the type A thermoelectric generator for the hot and cold side temperature with change in heat flux. A maximum temperature difference between the hot and the cold side of 196°C is achieved for the heat of 177W while maintaining the hot side temperature of 250°C using the same micro channel heat sink.
Figure 37 Power generated from type A and type B TEG for different heat flux
Figure 37 compares the maximum power output at various heat transfer rates for type A and type B thermoelectric generators. With the limiting hot side temperature of 150°C type A thermoelectric generator can produce 2.15W at heat flux of 101W and the temperature difference across the hot and cold side is 95°C. Type B thermoelectric generator has a limiting hot side temperature of 250°C and produce 3.87W for a heat flow rate of 177W and the temperature difference between the hot and cold side is 195°C.
Figure 38 Thermal resistance of type A and type B thermoelectric generator with respect to applied heat flux
Figure 38 shows the thermal resistance of type A and type B thermoelectric generators with change in the applied heat flux. Thermal resistance of thermoelectric generator is calculated from the measured values of temperatures and rate of heat output. Temperatures are measured using the K type thermocouples and rate of heat output is determined using the heat extracted by cooling water jacket. Whilst thermal resistance was being determined no power was being generated and hence heat input is assumed to be equal to measured heat output.
Thermal resistance of thermoelectric generator is given by
̇
⁄ 5.2
As specified in the manufacturers specifications (refer Appendix) the thermal resistance of the thermoelectric generator stays fairly constant with varied applied heat flux. For the type A thermoelectric generator the thermal resistance was determined by the
measurements and it vary between 1.007 -1.03°C/W over the heat input variation of approximately 25-91W. Whereas for type B thermoelectric generator with the limiting hot side temperature is 250°C, the thermal resistance varies from 1.10-1.16 °C/W over the heat flux variation of 22-177W. These values of thermal resistances for type A and type B thermoelectric generator are used in the further section for numerical and theoretical analysis to predict the performance of the system. Thermal efficiency of the TEG was determined by the equation
̇ ⁄ 5.3 Where ̇ ̇ ̇ 5.4
Figure 39 Thermal efficiency of type A thermoelectric generator
Figure 39 illustrates the thermal efficiency of the type A thermoelectric generator with respect to temperature difference between the hot and colds side of the
thermoelectric generator. The open circuit voltage of thermoelectric generators is directly proportional to the temperature difference between the hot and cold side of TEG. It can be observed in Figure 39 that the thermal efficiency increases with increase in the temperature difference across hot and cold side of TEG. Some nonlinear fluctuations in the graph are attributed to the unstable mass flow rate of cooling water that is supplied from the mains water supply line. Thermal efficiency of the type A thermoelectric generator varies in the range of 0.8% for temperature difference of 25°C to 2.1% for temperature difference of 95°C.
Figure 40 Thermal efficiency of type B thermoelectric generator
Figure 40 shows the comparison between the thermal efficiency of type B thermoelectric generator with temperature difference across hot and cold side of TEG similar to the type A thermoelectric generator as shown in Figure 39. A similar trend of increase in the thermal efficiency is seen for the type B thermoelectric generator. Thermal efficiency of the type B thermoelectric generator is 0.8% at ∆T of 25°C which
rises to 2.4 at temperature difference of 180°C. The observed fluctuations in the temperature difference are again attributed to the variation in the mass flow rate of cooling water.