Chapter 4 Test setup and procedure, experimental results, and
5.6 Results and discussion
The thermoelectric generator with the specifications mentioned in Table 4 was used for the investigation in this research. Various heat fluxes were applied across the thermoelectric generator while maintaining the hot side temperature of the TEG at or below 300°C.
Table 4 Properties of the thermoelectric generator under study
Properties Value (unit)
Number of thermoelectric couples 127
Length(l) x width(w) x height (h) 40 x 40 x 3 (mm) Hot side maximum temperature (Th) 300°C
Maximum power output @∆T=200°C 5.7W
Maximum current (ISC) 1.3 A
Maximum Open Circuit Voltage (VOC) 4.2 V Thermal resistance (RTEG) 1.5 °C/W
Ceramic material Alumina (Al2 O3)
The cold side temperature of the thermoelectric generator is dictated by the amount of heat flux supplied and the number of heat pipes used to transfer the heat to the water tank. Water temperature cannot go above 100°C since the water tank was maintained at atmospheric pressure.
The amount of heat supplied to the setup was varied from 20W to 80W with steps of 20W. Figure 93 shows the predicted water temperature for all the 5 heat input conditions.
Figure 93 Transient behaviour of water temperature under various heat inputs and fixed “ ” for cooling of water reservoir
In the above graph it is observed that the predicted thermal equilibrium temperature of water is 35°C when the heat input is 20W, 55°C when the heat input is 40W, 75°C when the heat input is 60W and 95°C when the heat input is 80W with fixed rate of water cooling from the tank.
However in actual setup the value of will vary with the rate of heat input and the water temperature. The value of is measured for various heat inputs and water temperatures for the theoretical prediction of the temperature of water. Figure 94 illustrates the comparison between the transient behaviour of the predicted and experimental water temperature. Estimated steady state temperature for a heat input rate
of 20W is 53°C while the experimental steady state water temperature is 49°C. Similarly for heat input rate of 40W, 60W and 80W the predicted steady state temperatures were 68°C, 75°C and 83°C respectively while the measured steady state temperature were 65°C, 71°C and 79°C respectively.
All the graphs show good agreement between the predicted and actual values of transient temperature rise as well as steady state water temperature.
(b)
(d)
Figure 94 Comparison between predicted and experimental results for water temperature at different power inputs (a) 20W (12.5kW/m2) (b) 40W (25kW/m2) (c) 60W (37.5kW/m2)
(d) 80W (50kW/m2)
By knowing the rate of heat loss from the water storage tank we can predict the transient and steady state behaviour of the water temperature very closely.
Figure 95 Experimental results for 72W heat input and 450ml of water in the tank with type A thermoelectric generator (maximum hot side temperature limited to 150°C)
Figure 95 presents the experimental results for the test setup with 72W heat input to the single thermoelectric generator with 4 heat pipes to transfer the heat from the cold side of the TEG to the water tank containing 450 grams of water. The graph presents experimental results for transient behaviour of TEG hot side temperature, TEG cold side temperature, TEG open circuit voltage and water temperature. The temperature difference between the hot and cold side of the thermoelectric generator is calculated and presented on the same graph for comparison. For 72W heat input maximum hot side temperature at steady state reaches 154°C while the cold side steady state temperature reaches 94°C. It takes approximately 65 minutes for the TEG hot side, TEG cold side and water temperature to reach steady state. However it can be clearly seen that the temperature difference between the hot side and the cold side of the TEG reaches steady state at 54°C in approximately 12 minutes. Open circuit voltage of the thermoelectric generator is a direct function of temperature difference and therefore its trend is similar to that of the temperature difference.
Figure 96 Experimental results for 160W heat input and 450ml of water in the tank with type B thermoelectric generator (maximum hot side temperature limited to 250°C)
Figure 96 illustrates the experimental results for type B thermoelectric generator with heat input of 160W. The limiting hot side temperature of type B TEG is 250°C. For 160W of heat input it can be can be observed that the temperature of the water in the tank reaches to 97°C and the open circuit voltage of 4.5V can be achieved. Similar to type A TEG with 72W of heat input, the temperature difference between hot side and cold side reaches steady state in approximately 11 minutes.
The total thermal mass ( ) of the thermoelectric cell, aluminium heat spreader and the heat pipes together is significantly smaller (1/28th) than the thermal mass of the water in the system. Due to this reason the temperature difference between the hot side and the cold side of the thermoelectric generator reaches steady state more quickly than the steady state of the hot and cold side temperatures itself. We can assume the whole system to be in the steady state at any time after the temperature difference
reaches its steady state due to the negligible thermal mass of the system components compared to the thermal mass of water.
Figure 97 Limiting heat flux for type A and type B thermoelectric generator with non- conventional passive cooling method
Limiting heat flux for type A and type B thermoelectric generators with non- conventional heat sink is illustrates in Figure 97. Type A TEG with allowable hot side temperature of 150°C has a limiting heat flux of 45kW/m2 and type B TEG with allowable hot side temperature of 250°C has a limiting heat flux of 101.25kW/m2.
Figure 98 Comparison of Thermoelectric power for two different temperature differences between the hot and cold sides
Figure 98 shows the power generated by thermoelectric generator for ∆T of 60°C and 150°C. Maximum power generated by the thermoelectric generator in the lab setup for 72W heat input and ∆T of 60°C is 1.27 W, whereas that for the heat input of 160W and ∆T of 150°C is 3.5W.