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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We have installed a switch on our heat pump systems to prevent the back up heat strips from coming on during the minutes of reverse cycle defrosting. That creates a significant electrical savings. It also allows our emergency generator to power the heat pump without the heavy load of the heat strips. For another of our systems, we have removed the heat strips, and installed a water coil in the air handler. When either defrost cycle or emergency heat are called, a small pump sends hot water through the coil.
I know this is not as involved in the British design, but it serves two functions without the added valving complications. 1. Rapid excellent heat during the call, 2. In case of compressor failure, or demand greater than generator capacity, the gas powered water heater is more than adequate to supply needed heat.
A continuation of previous comment. Since we are always going to have hot water in storage within our house, simply using a bit of that heat keeps the system quite simple. While I applaud the valving/refrigerant heat storage system for it’s 2 minutes of defrost efficiency improvement, I do not see the advantage vs complexity in a household environment where hot water storage is always available.
Houses have many design changes available that are being ignored in the field of energy conservation.
I have built a house that incorporates many.
Some examples: we pull hot filtered air from above our attic radiant barrier to supply our laundry room with 125 , degree f heated air.
We can do the same for supplying warm air to our basement.
We have isolated our laundry room from house conditioned air.
No duct work is external to the house environment, ie, no duct work in attic or crawl space.
All water lines are insulated throughout the house.
Attic radiant barrier reduces A/C costs, and helps trap heat in winter months.
All external facing walls and ceilings are strapped with 3/4″ horizontal firing with foam board placed between on 16″ centers. This greatly reduces the thermal transfer through the studs, adds strength to the structure, and increases the overall wall R- value.
All ductwork and boots are silicone sealed and have added insulation.
Laundry room is insulated both for heat and sound containment.
Basement floor slab has radiant heat piping to allow hot water circulation. In times of pending ice storms,large amounts of nuts can be stored by heating the concrete slab.
Heat pump water heater.
All band board chambers are foam filled.
Just a few energy savings built into the house.
Cost to operate the total electric supplied house is 3 cents per sq ft per month ave per year. $120/ month, 4000 sq ft.
This article about a new defrosting technology for air-source heat pumps sounds very promising. The idea of a system that can continue supplying heat during defrosting cycles is a major improvement.
Traditionally, defrosting cycles can significantly reduce the efficiency of air-source heat pumps, so this new technology could be a game-changer. The use of a multi-valve flexible heat pump layout and heat storage seem like clever solutions to address this issue.
I’m curious to learn more about the practical applications of this technology. The article mentions it may improve COP by up to 11.2%, but are there any estimates on how this might translate to real-world energy savings for homeowners? Additionally, is this technology expected to be significantly more expensive than traditional air-source heat pumps?