The new concept is based around a system integrating hybrid solar PV, ground-source heat pump (GSHP) and borehole thermal energy storage (BTES) technologies. The result is a system in which outputs of each technology are highly complementary to one another, and carry the potential to increase energy efficiency and cost effectiveness of individual components.
“It’s one of the first demonstrations of hybrid solar combined with GSHP in Sweden,” Pernilla Gervind, one of the lead researchers on the project, told Renewable Energy World.
A critical feature of the system is the role played by the hybrid solar PV. Unlike conventional solar PV, hybrid solar PV (sometimes referred to as hybrid solar photovoltaic/thermal (PV/T)) is a popular, well-established method for cooling PV cells. Hybrid solar PV modules consist of conventional PV cells with embedded systems containing some form of cooling agent, typically water or air, which is circulated through PV panels. The intention here is to reduce PV cell temperatures, as it is known that overheating — through either solar radiation or ambient temperatures — reduces PV cell efficiency significantly.
The new system advances the hybrid solar PV concept by making use of the output water within a vertical loop GSHP system to which it flows.
Pernilla explained: “Having passed through the PV panels, water is heated to around 10-degrees Celsius; it is then directed into the cold side GSHP system and used as heat source; if there is a surplus of heat, this is then directed down into boreholes. Here, the thermal energy of water is absorbed by the surrounding ground as a result of a temperature differential that arises from the ambient temperature of the ground being between 2 to 3 degrees Celsius. The now-cooled water is then cycled back up the system, and re-used in the cooling of PV panels in a closed-loop system.”
Heating boreholes with direct heat is not new — it’s a process referred to as ‘recharging,’ and is a common method for increasing efficiency of heat pumps in response to temperatures surrounding boreholes declining over time, in part through absorption of thermal energy. Commonly, however, direct heat is generated through more conventional means, or through concentrated solar power (CSP). Using hybrid solar PV in this recharging context is unique.
The system may be used for the purposes of seasonal storage of thermal energy, as Gervind explains: “In Sweden, seasonal temperatures vary greatly, providing options for how the system can be used accordingly. In the summer we can generate solar thermal energy, but it’s not required for anything — so we can use boreholes to store this excess energy for use during the winter when it is required.”
The system stands to be especially useful in Sweden, where geothermal energy is dominated by low temperature, shallow systems featuring GSHPs used for space heating and domestic hot water heating. About 20 percent of the Swedish buildings use GSHPs, according to the International Geothermal Association.
In the study, which was supported by the Swedish Energy Agency together with Energiförbättring Väst, the system was piloted through 2015 on the west coast of Sweden over 70 terraced houses.
“Our focus was on evaluating the system, and ensuring it worked,” Jessica Benson, Gervind’s co-researcher, told Renewable Energy World. “On this level, we’re very confident in the potential of the system.”
Monitoring system performance allowed the researchers to make mid-study adjustments. Benson explained an example of those adjustments: “In the original system design, solar heat was first directed down to the boreholes and then to the heat pumps. This was adjusted so that the heat is now first directed to the GSHP and only the surplus is directed to the boreholes.”
The changes in directing the solar heating first to the GSHP, Benson said, increased the efficiency of the heat pumps due to the increased temperature of the heat source.
Owing to relative success of the pilot, the researchers are looking towards future studies, as Benson explained: “We have already begun conducting a follow-up study to investigate key performance issues; for instance the effect of cooling on PV cell efficiency, and efficiency of GSHPs. There’s much work to do in studying the dynamics of heat transfer from boreholes to ground and how best to ensure added thermal is retained in a manner optimal for thermal storage solutions.”
Images credit: Pernilla Gervind and Jessica Benson