Thermal Challenges on Solar Concentrated Thermoelectric CHP Systems and Thermal Battery Systems
Engineering
EE280Q
This abstract is a two-part report analyzing our work in renewable power generation and efficient energy storage through two related research projects;
1. Thermal Challenges on Solar Concentrated Thermoelectric CHP Systems, and
2. Thermal Battery Systems.
In project 1., we report thermal challenges based on our experiments on a solar concentrated combined heat and power (CHP) generation system. The system is designed to harvest solar
radiation as a renewable energy source. Electrical power is generated by a thermoelectric (TE) module and waste heat produced by the TE is recovered with a water cooling heat exchanger placed behind the module. The goal is to achieve a water temperature that is useful for hot water residential applications while generating some electricity. This CHP system includes optical solar concentration to obtain a lower cost per performance [$/W] applying the higher heat flux to the thermoelectric generator, which was theoretically calculated in our previous work. The results are compared with the generic analytical optimization model.
For the experimental apparatus, we used a Fresnel lens for the optical concentration. Previous analysis showed that matching of both electrical and thermal resistances of the TE module and the load resistance are key factors. We optimized the load resistor to get the maximum electrical power from the TE generator. However, the thermal resistance match appeared to be quite challenging, due to the non-uniform profile of the energy flux on the TE module. This yielded inhomogeneous power
generation for each thermoelement in the TE module and resulted in significant degradation to power output. This can be improved with appropriate heat spreading at the TE hot plate
and/or with optical optimization of the Fresnel lens. The harvested combined power was more than 53% of the received solar with the thermally non-optimized module, while 0.44- 0.46 W of electricity was generated by the TE module with a center temperature difference of 143-152oK between the hot
side and the cold side of the TE module. An extensive calculation for the thermally matched design (optimum) of the TE module suggests that the same system can produce 10 times as much electrical power.
In project 2., we report the details of the ongoing Thermal Battery Systems project, whose aim is the design, development, and experimental testing of an electrical-to-thermal energy storage system. Residential homes equipped with renewable energy systems, such as solar panels or wind turbines, spend a substantial amount of generated electricity for water heating and cooling. However, when energy demand is relatively low, these homes generate unused, excess electricity. In order for renewable energy systems to prevail, they must employ a mechanism that will effectively make use of the full amount of generated energy, and thus maintain a healthy return on the investment for renewable energy harvesting. As a solution, we propose a heat pump system which utilizes this generated excess electricity to produce hot and cold water.
We built and tested a table-top apparatus based on a thermo-electric-heat-pump (TE-HP) design to investigate the system’s electrical performance and response to a stepped power input. We carried out a multi-point transient temperature characterization to observe the system’s heat transfer and thermal performance. Starting with room temperature water at 22 °C, preliminary data showed the system generated hot and cold water at temperatures of 67 °C and 14 °C, respectively, with a system COP of 0.68 and time constant of 40-50 seconds for the TE-HP alone (without considering the system’s water reservoirs heat capacities). We observed considerable heat losses in the preliminary apparatus setup, thus with careful treatment for the observed heat leaks, we expect to achieve a COP greater than 1 with greater temperature differences for both the hot and cold water supplies.
The TE-HP COP values are low compared to conventional compression heat pumps. However, the time constant of the system’s TE-HP is far quicker than common frequency-driven mechanical heat pumps that typically have 10x more inertia than the TE-HP. Since this system is scalable, this technology allows 10 kW~MW scale conventional, mechanical heat pump systems to work more effectively in conjunction with the proposed TE-HP system.
