Comparison of the Air-to-Water and Water-to-Water (Ground-to-Water) Heat Pumps
In general, the idea that the water-to-water (ground-to-water) heat pumps are more efficient in terms of their performance and operating costs compared to the air-to-water heat pumps has become deeply enrooted. But the practical experience shows that systems with the same rated output values have similar operating costs in long term. Conditions for different applications may differ from case to case, but the reasons why the operating costs are becoming even, will be outlined below.
The figure on the left shows an air-to-water heat pump that extracts energy from the ambient air, and the figure on the right shows a ground-to-water heat pump that takes energy from the ground or ground water.
Temperature of the underground collector of the water-to-water heat pump will stabilize slightly below the freezing point, while the average temperature of the air, which is taken by the air-to-water heat pump, is higher during the heating season, and lies at around +4°C.
Heat extracted by the air-water heat pump from the ambient air in the summer months is much greater than the heat taken from the ground by the water-to-water pump. Hot water preparation is highly cost-effective with the air-to-water heat pump during the summer months.
Due to ongoing thermal insulation of properties and the use of underfloor heating systems, it is not necessary to use additional (bivalent) heat source during short-time drops in temperature below -15°C, which was common in the past.
The input power of the circulating pump in the glycol circuit, which has half the transfer properties and higher density than water, is usually much greater than the input power of the fan motor. The difference can easily compensate the loss caused by the cyclic de-icing of the evaporator of the air-to-water heat pump.
Of course, we cannot omit one essential factor when making choice in favour of the air-to-water version, namely the investment costs. The purchase price of the complete air-to-water system by REVEL reaches the price of the ground collector for the water-to-water (ground-to-water) option, and the payback period is near to 2 years.
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