Bertsch and Groll (2008), however, argue that although the air-source heat pumps are advantageous due to their low costs of installation, they are challenged by several issues, particularly when operating during low outdoor temperatures. The researchers cite that in such conditions, the pumps decrease in heat output and coefficient of performance while increasing temperature discharge. In explanation, Wu et al. (2013) argue that the low efficiency during the low temperatures arises from the substantial frosting of its outdoor coils. As a result, the researchers argued that adding a solar hot water system to the air source pump, which was based on low-temperature operation helped alleviate the problem. Bertsch and Groll (2008) further proposed a two-stage air-source heat pump to overcome the challenge where they reported that the system was able to provide appropriate air conditioning during the cooling mode.
The second technology, ground-source heat pumps (GSHP) differs from the air-source variant in that they utilize the ground as a heat source to heat and cool buildings (Corberán et al., 2018). The researchers highlight that since the pumps utilize shallow geothermal energy as a source, they are considered one of the most efficient and reliable heating and cooling technologies available. In explanation, Mustafa Omer (2008) reveals that the earth has a relatively constant temperature at depth, which is warmer than winter air and cooler than summer air in Massachusetts. Using a ground-source heat pump, heat is easily transferred from the earth to the building during the cooler winter periods and likewise, transferred from out of the building during the hot summer conditions