Ground-source Heat Engine Prototype Design and Test

Figure 1. Generalized Schematic

A company wished to explore feasibility and performance of ground-source heat engines designed to generate a tiny amount of electrical power from the daily fluctuating temperature difference between the air and the ground using solid-state thermoelectric generators. 

 Figure 1 shows the general structure of the device. Multiple thermoelectric modules were sandwiched between air-side and ground-side heat exchangers. The thermoelectric modules provided the electrical power generation from heat that flowed as a result of the air-ground temperature difference. The design consisted of appropriate geometric design for both heat exchangers, selection and interface configuration of the thermoelectric modules, and thermal resistance matching between the heat exchangers and the thermoelectric modules. 

Figure 2. Instrumented Prototype

 Prototype devices were designed and built. Figure 2 shows one of the prototypes situated in the test area and instrumented for performance evaluation. This prototype included fins on the air-side heat exchanger visible in the figure. Extended test results indicated peak power of 5 milliwatts and average power of 1 milliwatt could be obtained from the prototype configurations. It was determined that approximately 50% of the power generated in extended tests could be attributed to direct solar insolation. 

 Figure 3 shows a sample of measured power generation rates over a short part of the test period, and Figure 4 shows the total energy generated by hour of the day for the entire test period.

Figure 3. Measured Power

Figure 4. Total Energy