Researchers in the United Kingdom have presented a new heat pump system that may avoid reducing heat supply during defrosting operations. The proposed concept can reportedly execute defrosting operations while ensuring continuous heating by condensing the refrigerant in the frosted evaporator.
A group of researchers from the University of Glasgow and the University of Liverpool in the United Kingdom have presented an experimental flexible heat pump concept that can carry out a defrosting operation while avoiding a reduction of heat supply when the refrigerant condenses in the frosted evaporator.
The novelty of the proposed approach consists of the evaporator being defrosted by condensing the refrigerant inside. “Our work presents a theoretical analysis under ideal conditions,” the research’s lead author, Zhibin Yu, told pv magazine. “We plan to conduct experimental research to demonstrate the concept.”
“One of the most challenging issues for air source heat pumps is the requirement of defrosting the evaporator under low ambient temperatures and high humidity conditions,” the researchers explained in the study “A multi-valve flexible heat pump system with latent thermal energy storage for defrosting operation,” which was recently published in Energy and Buildings. “Ice build-up reduces the airflow in the evaporator and increases the thermal resistance of the coils leading to a decreased heat transfer performance. This results in a drop in the heating capacity and the coefficient of performance (COP) and can potentially cause a complete shutdown of the air source heat pump unit.”
The proposed flexible heat pump system is integrated with heat storage based on the conventional Evans-Perkins vapor compression cycle, which is the most widely used method for air conditioners and automobiles and enables the system to recover and store part of the heat that exits the condenser during defrosting operations, with the stored heat being reused to power defrosting itself.
“It can execute a defrosting operation while ensuring continuous heating by condensing the refrigerant in the frosted evaporator,” the academics further explained. “Using the heat storage as the heat source during the defrosting process allows for an increase in the evaporating temperature leading to a drop in the electrical consumption and improved efficiency.”
The proposed system consists of a compressor, a condenser, a refrigerant storage tank, a heat storage system, an evaporator, two expansion devices, and six ball valves. It works in four operation modes: Heating and charging of the heat storage; discharging of the heat storage and power saving; discharging of the heat storage, power saving and defrosting; and charging of the heat storage only.
In the first mode, the systems recover subcooled heat and charge it into storage for later use, while the second mode is intended to reduce power consumption in the compressor by increasing the evaporating temperature. In the third mode, the heat pump system can maintain the same heating capacity indoors and save compressor power while defrosting, while the fourth mode enables the charging of heat faster by directly condensing the refrigerant inside.
Switching between these modes should be ensured by a microcontroller that opens the valves according to the required operations and with different monitoring strategies to initiate and terminate the modes at the right times.
Through thermodynamic analysis, the team analyzed the potential performance of the system and found it can efficiently carry out the defrosting cycle while extracting heat stored in the thermal storage during the charging cycle, which they said results in significant compressor power saving and increased COP.
“Depending on the storage temperature, we observed a COP improvement varying from 7.5% to 11.2% for R410a and from 7.5% to 10.8% for R134a, compared to a conventional heat pump using a reverse cycle defrosting method,” the academics emphasized. “Their low global warming potential (GWP) substitutes R1234yf and R32 have also been studied, and R1234yf has been concluded to be the best-performing refrigerant with this system with an improvement of up to 13.2%.”
The researchers also estimated that the recovery phase after defrosting could be implemented in 1.8 minutes for R134a and R410a at their optimal storage temperature, without significantly affecting the heat pump performance. They also found that R32 would require 2 minutes and R1234yf 1.7 minutes.
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