New model to predict defrost cycles, behavior of air-sourced heat pumps in cold weather

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A group of scientists from Nottingham Trent University in the UK has developed a simulation model to predict the behavior of air-sourced heat pumps in cold weather. Their method utilizes a calculation model for frost growth rate, providing an in-depth investigation of heat pump defrost cycles.

The technique considers a hot-gas bypass method for defrosting. “When the temperature drops below freezing, frost could accumulate on the outdoor unit of the air-source heat pump and cause a drop in performance; hence a mode is triggered to remove the frost build-up,” the researchers explained.

With the hot gas bypass method, the refrigerant completely skips the condenser and expansion valve lines and directly flows from the compressor to the evaporator. “This would lead to lower heat output affecting the dwelling demand,” they stressed.

In order to foresee energy consumption and heat output by the heat pump, the scientists have created a computerized model that considers, among other things, heat pump properties, thermal characteristics of the building in which it is used, and storage in the form of heated water. To calculate the energy needed for the defrost cycle, the researchers developed a new model to estimate the frost build-up on the outdoor unit.

“The standard model does not take into consideration the relative humidity and the heat exchanger size,” they added. “With the novel suggested numerical model, the humidity and size of the outdoor heat exchanger are taken into consideration, and the results are more dynamic and change according to the temperature and relative humidity.”

To validate the model, the academics then constructed a test rig. Setting up the model with the exact parameters of the rig, they then validated their estimates with the model’s output. They found that the percentage error for total energy consumption was 0.63%, for total heat output 5.2%, and 1.6% for water content after the defrost cycle.

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Following the validation, the group input data from 10 cities from around the world into the model. That data included average housing parameters such as area and insulation levels and data for three months from December 1 to February 28.

The selected cities were: Amsterdam (Netherlands); Copenhagen (Denmark); Helsinki (Finland); Stockholm (Sweden); Vancouver (Canada); Ruse (Bulgaria); Moscow (Russia); Reykjavik (Iceland); Harbin (China); and Nottingham (United Kingdom). Vancouver had the lowest number of defrost cycles in the cold season – 56 – while Harbin had the highest – 1,070.

“The results indicate that there is a strong relationship between the number of defrost cycles of heat pumps and the ambient temperature, with a correlation coefficient of – 0.94,” the academics said. “The lower the ambient temperature, the more frost removal operations are done by heat pump.”

“On global level, governments should consider gradual support to renewable energy for a better management of the grid and have a better control on grid demand,” the academics concluded. However, they emphasized that “taking into consideration the climate in the UK, it is highly unlikely the demand of energy would suddenly increase dramatically as the temperature will not likely plummet below freezing, hence defrost cycles will rarely be required by heat pumps.”

They presented the results of the simulation in the study “Investigating the effect of the defrost cycles of air-source heat pumps on their electricity demand in residential buildings,” published in Energy & Buildings.

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