Abstract
Borehole heat exchangers (BHEs) generally employ water-antifreeze solutions to allow working fluid temperatures to fall below 0 °C. However, some local regulations have forbidden antifreeze additives (even non-toxic ones) to avoid groundwater pollution in case of pipe leakage. This paper presents a techno-economic and environmental analysis of four different fluids: propylene glycol at 25% and 33% weight concentrations, calcium chloride at 20% weight concentration (CaCl2 20%), and pure water. Thermal loads from 36 case studies in six different climate zones are used to perform BHE sizing and compare the abovementioned fluids from the economic, operational, and environmental points of view. The economic analysis and the carbon footprint assessment are performed on a life cycle of 25 years considering the installation (BHE drilling, fluid) and operation (heat pump and ground-side circulation pump energy demand, fluid replacement) of the simulated GSHPs. Results highlight that using pure water as a heat carrier fluid is convenient for cooling-dominated buildings but, for heating-dominated buildings, this choice leads to a noticeable increase of the BHE needed length which is not compensated by the lower operational costs. On the other hand, avoiding the use of antifreeze additives generally leads to a reduction of the lifetime carbon footprint, with a few exceptions in very cold climates. CaCl2 20% proves to be a good choice in most cases, both from the economic and the environmental points of view, as it allows a strong reduction of the installed BHE length in cold climates with a low additional cost and carbon footprint.
Highlights
Ground source heat pumps (GSHPs) for heating and cooling of buildings are becoming more and more popular thanks to their low operational costs and to the increasing need to reduce anthropogenic greenhouse gas (GHG) emissions
The most diffused type of GSHP is equipped with Borehole Heat Exchangers (BHEs), i.e., boreholes with diameters of 15–20 cm and depths of 50–200 m, where a heat carrier fluid circulates through one (1U) or two (2U) U-pipe loops or, less frequently, into a loop composed of two coaxial pipes [3]
We present the results of the 144 simulations carried out, highlighting the energetic, economic, and environmental impacts of choosing among four different heat carrier fluids
Summary
Ground source heat pumps (GSHPs) for heating and cooling of buildings are becoming more and more popular thanks to their low operational costs and to the increasing need to reduce anthropogenic greenhouse gas (GHG) emissions. 1.9 million units, with a yearly increase of 6.5% between 2012 and 2018 [1,2]. The heat carrier fluids circulating into the BHEs are generally water solutions of antifreeze additives. This allows BHEs to operate below 0 ◦ C, increasing the heat extraction per unit length and reducing the BHE length needed to cover the building’s heating demand The most diffused type of GSHP is equipped with Borehole Heat Exchangers (BHEs), i.e., boreholes with diameters of 15–20 cm and depths of 50–200 m, where a heat carrier fluid circulates through one (1U) or two (2U) U-pipe loops or, less frequently, into a loop composed of two coaxial pipes [3].
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