In the European Union, 40% of the overall final energy consumption is attributable to the buildings sector. A reason for such data may be found considering that the great majority of the building stock is more than 40 years old. According to the European Commission, an interesting potential lies in the refurbishment of the building sector, and heat pump technology has been recognized as one of the most cost-effective solutions to tackle the environmental issue of this sector. Regarding heat pump technology, ground-source heat pumps (GSHPs) have been proven to be the most efficient solution on equal boundary conditions. Despite this, in most EU states’ markets, GSHPs hold only a small market share with respect to air-source heat pumps. In this paper, the state of art and possible future developments of GSHP technology have been reviewed together with a focus on the potential of such technology, most of all on the refurbishment of existing buildings, and on the obstacles to its spread. The state of art of borehole heat exchangers has been studied, focusing on the parameters characterizing the outside pipe and the pipe itself, i.e., pipe and grout materials. Moreover, an overview on the last developments involving refrigerants and secondary fluids is given. Finally, the design and control strategies of GSHPs have been reviewed.
In the last decades, the industry of HVAC&R has faced continuous changes trying to identify environmentally friendly refrigerants for the numerous applications of the sector. However, the amount of low GWP fluids still available as potential refrigerants is limited to some natural fluids and, among synthetic chemicals, to hydrofluoroolefins (HFO). The knowledge of the thermophysical properties of these compounds and the evaluation of their energy efficiency in experimental apparatuses is essential to properly address the selection of the most suitable fluids. However, regarding the wide majority of HFOs, the information on the thermophysical properties, especially for the blends, are still scarce and require further research. In this work, an analysis of the possible substitutes and the available experimental data sets on their thermophysical properties was carried out to find out for which fluids further studies are needed to obtain an accurate representation of their thermophysical properties. Specifically, for 21 pure refrigerants, an overview of the thermodynamic (critical point, psat, PVT, heat capacity and speed of sound) and transport properties (λ, μ, σ) data published in the peer reviewed literature was provided. In addition, a more comprehensive analysis was carried out for four fluids (R1243zf, R1233zd(E), R1336mzz(Z), and R1224yd(Z)), for which major efforts have been made in the last 4 years to investigate the above thermophysical properties. Although an increasing amount of data sets on thermophysical properties have been compiled in recent years, the present study indicates that research efforts are still needed, especially on transport properties, as only 4 of the fluids of interest for the present research have been fully investigated (R1234yf, R1234ze(E), R1233zd(E), R1243zf), while other 4 (R1234ze(Z), R1336mzz(Z), R1224yd(Z), R1336mzz(E)) have been almost completely characterised.
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