An international research team has conducted a computational analysis on propane (R290) vapor compression heat pumps integrating a thermoelectric subcooler (TESC) to reduce the power consumption of vapor compression systems and has found this combination may increase the coefficient of performance of the heat pumps by 12.29%.
In the paper “Effect of thermoelectric subcooling on COP and energy consumption of a propane heat pump,” published in Applied Thermal Engineering, the scientists explained that the combination of these technologies was found to be very beneficial for refrigeration systems in previous research and said that propane heat pumps could also highly benefit from thermoelectric subcooling.
TESCs are composed of thermoelectric modules (TEMs) and solid-state heat engines that use electrons as the working fluid. TEMs are widely used for the electronic cooling of devices such as personal computer processors, as well as portable food and beverage storage systems.
The computational model was applied to a residential propane heat pump intended to provide space heating.
In the proposed system configuration, the TESC is located in between the condenser and the internal heat exchanger (IHX). The water that must be heated up passes through the TESC and subsequently through the condenser. Furthermore, the TESC absorbs heat from the propane to provide subcooling at the outlet of the condenser, which benefits the operation of the vapor compression system.
“The inlet water temperature defines the inlet to the TESC, while the outlet water temperature determines the inlet of the condenser,” the scientists explained. “Each of the blocks presents different hot and cold sink temperatures, as the propane flow cools down as it passes by the TESC system and the water flow heats up.”
The computational model is reportedly able to develop an iterative process that supposes the outlet temperatures of each block and solves each block until the supposition matches the operation of the block. It also calculates the outlet temperature of the propane and the outlet temperature of the water flow, as well as the COP and power consumption of the TESC.
“The computational model has been developed so the entire heat pump is able to provide a kW of heating capacity,” the group explained. “To that purpose, the compressor capacity is increased at each resolution to achieve the kW of heating capacity.”
In their simulations, the researchers considered ambient temperatures between −20 C and 15 C, TEM voltage levels from 0.5 V to 10 V, a thermoelectric system consisting of 1 to 8 blocks, and two water inlet temperatures of 40 C and 55 C. They also took into account mass flows, heating capacities, compression ratios, compressor capacities, cooling capacities, temperatures, COPs, and energy consumptions.
Their analysis showed that optimal TEM voltage levels are key to maximizing the operation of the heat pump, and that very high voltage supplies are counterproductive to the vapor compression. It also showed that the highest COP enhancement of 12.29% was obtained when the ambient temperature was −20°C and was achieved when the TESC was designed with 8 blocks consisting of 16 TEMs.
“However, if the TESC system is reduced to the half, being formed by just 4 blocks (8 TEMs), the COP enhancement is still 9.54%,” the group stressed. “When the heat pump is used for fan-coil space heating, the COP increments are 7.64 and 6.03 %, respectively.”
Further analysis also demonstrated that the TESC could also improve the seasonal coefficient of performance (SCOP) of the heat pump by 9.98 % if an 8-block TESC system per kW of heating capacity is integrated to heat water for conventional radiators.
The academics are currently planning to build a system prototype to confirm the results of the simulations. “The inclusion of thermoelectric subcooling into a propane vapor compression heat pump has been demonstrated to be very beneficial in terms of COP and energy consumption,” they concluded. “Additionally, thermoelectric systems are proved to be robust, simple, flexible, reliable, cheap and easy to control.”
The research team comprised academics from the Public University of Navarre and the Jaume I University in Spain, as well as the Silesian University of Technology in Poland.
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