Capabilities of a heat exchanger systems

The second evaluation process is to simulate the heat exchanger system and find the corresponding relationship between the performance of the heat exchanger system and the whole TEG system. Similar to evaluate the TE module, four different situations have been considered respectively and the corresponding experimental tests have been carried out to validate the accuracy of the model. Figure 4.15 a, b, c, and d show the results of these four situations. With respect to the material being used in TEG construction, a good thermal conductor is required in order to transfer the heat from the heat source to TE module to achieve high system efficiency. Three kinds of hot side heat exchangers are compared in the model, as shown in Figure 4.15 a. The most efficient one is copper plate, because the thermal behaviour of copper is better than the other two types of metal plates. The corresponding experimental test, which uses an aluminium plate, has been tested in the laboratory with the test setup to show the

accuracy of the model. The results show the accuracy of the proposed model is around 89.7% in the worst case.

As the energy conversion efficiency of the TEG is highly relative to the heat dissipation capability of the heat sink, three types of heat sinks have been simulated by the model, respectively. The simulation results are depicted in Figure 4.15 b. The energy generation of electric power of the TEG is increased by around 50% when the heat dissipation capability has been improved 40%. In order to examine the model, a real experiment, using a 1K/W aluminium heat sink, has been tested in the laboratory and the result shows that the accuracy of the model is around 89.4%.

Furthermore, in order to show that the ambient temperature can affect the system performance, the model has been simulated with five different air temperatures. The result is illustrated in Figure 4.15 c. By examining the curves, the generated electrical power is sharply increased by reducing the air temperature. Figure 4.15 d shows that system performance can be affected by the thermal contact resistance of the TEG system. Two different contact resistance values have been integrated into the model to see the difference. According to the curves, the lower thermal contact resistance the system has, the higher generated power the system can achieve. By analysing the contact resistance in Equation 4.34, several critical parameters such as surface roughness of two touching surfaces, contact pressure, density of interstitial gas, heat capacity, thermal and mechanical properties of the filling thermal grease, could be used to determine the value of the contact resistance when two surfaces are attached together. In conclusion, the electric power of the TEG increases considerably when these heat exchangers’ thermal resistances, surrounding air temperature, and contact resistance decrease. These results illustrate that how critical of designing a high efficient heat exchanger system (hot side heat exchanger and heat sink), reducing temperature of the surrounding air and reducing the thermal contact resistance in TEG applications. The properly design of the heat exchanger system can easily generated more than double power than the improperly design. This is critical in low temperature TEG applications, which the harvesting efficiency is quite low.

Figure ‎4.15 (a) Power generated with different hot side heat exchangers (b) Power generated with different heat sinks (c) Power generated with different ambient air (d) power generated with different contact resistance

Whilst keeping the heat source and sink at a stable temperature, change the status of the circuit with different load resistors. The current, voltage and power characteristics of the TEG for different ΔT are plotted in Figure 4.16. As shown in the figure, variations in temperature difference result in variations in output power of the TEG.

The same as solar panels, there is a unique MPP for the system at a fixed temperature difference. For instance, the output voltage of the TEG is in the range of 0-0.16V when there is a temperature difference between the TE module and at the MPP, in which maximum power output is around 0.023W when the system is working at 0.0806V. If the design requires a TEG system working at the MPP, the output voltage of the TEG should always satisfy the half of its open circuit voltage . This can be shown in Equation 4.53

(4.53) where .

Figure ‎4.16 (a) power output with different load resistor (b) maximum power point at different

4.4 Summary

In this chapter, two computational models of a solar panel and a TEG system have been developed, respectively. The feature of these two models is that they are designed based on manufacturers’ specification datasheets. This allows the proposed model to be quickly constructed. For the model of a solar panel, the simulation model of a solar cell developed by Petreus et al (2009) is adapted in this chapter because of the fast way to extract five parameters from a solar panel. The test results show that the model is well related with the solar cell placed in the real environment.

For a thermal energy harvesting system, a new computational model, based on manufacturers’ specification datasheets, has been developed based on an equivalent circuit which several non-electrical processes are emulated by electrical analogies. The numerical model is used at a system level optimization to determine the optimal design of both the heat exchangers system design and the TE module selection. Two main components, which are a TE module model and the heat exchanger system model, assembly together to emulate the performance of the TEG. The proposed model has been evaluated by the experiments tests held in the laboratory. The accuracy of the model is around 89.4% by showing in the comparison results. Then the model has been simulated in three different ways at the infinite heat situation to show the performance of the TEG is highly dependent on the TE material, the heat exchanger system and load. Based on these findings, the system designers can improve the TEG’s

energy conversion efficiency by selecting a proper TE module, designing a high efficient heat exchanger system and tracking MPP of TEG.

By examining the accuracy of the two computation models, the results show that there is a good agreement between the real measurements and simulations. Hence, these two models, which can be used to express the harvested energy in a known environment, will be adopted in the later chapter as the models of energy harvesters.

Chapter 5. Modeling micro-energy