Evaluation of a Counterflow Thermoelectric Heat Exchanger in a Solar Domestic Hot Water System

Abstract

Solar assisted heat pumps are a method of improving the energy efficiency of a domestic hot water system while reducing the impact on the environment. Traditional vapour-compression heat pumps use harmful refrigerants that are susceptible to leaks and have moving parts that require maintenance. Thermoelectric devices have recently shown an increase in performance and a decrease in unit cost, allowing them to be a viable, maintenance-free alternative to traditional heat pump cycles. A thermoelectric model was developed using EES to determine the operating performances of an indirect solar assisted heat pump system (ISHAP), as well as a solar-side ISAHP. The model was verified using a twisted-tape liquid-liquid heat exchanger which incorporated 21 modules for a total heat pumping capacity of 1.62 kW. The thermoelectric heat exchanger was tested across a range of inlet conditions and applied currents and was shown to be accurate to within 7.9 % of the model. A custom TYPE was created in TRNSYS to allow for the optimization of the two systems over a simulated year using directly-coupled PV/T hybrid panels. Using the designed heat exchanger the optimized ISAHP produced a solar fraction of 70.2 % using 50 modules and 6 m2 of collector area. The solar-side ISAHP operated at a higher efficiency with a solar fraction of 73 % using just 21 modules and 6 m2 of collector area. A control scheme was recommended that showed an improvement of 10.3 % in solar fraction over the solar-side ISAHP, while operating at an average COP of 2.5. It was shown that in improvement in the heat transfer rate of the liquid-liquid heat exchanger could further improve the solar fraction of the standard ISAHP system by 12 %. Additional system optimization is required to aid in matching the electrical and thermal requirements of the thermoelectrics, through the decoupling of the electrical production of the panels to allow for more accurate control, or the addition of photovoltaic modules to supplement the electrical output of the PV/T panels.