Sweden innovates in the solar PV-geothermal system space

Sweden’s novel hybrid solar PV and geothermal system will open doors to even more innovation in the hybrid renewable energy space.
Published: Tue 13 Sep 2016

Hybrid power plants offer solutions to many problems and concerns faced by power producers, grid managers and consumers. In a hybrid system, clean but intermittent solar power can be paired with a reliable generator fuelled with low-priced natural gas. Waste wood from timbering can be co-fired with waste coal to lower emissions. Wind power can be coupled with compressed air to create a reliable source of power for remote areas with limited transmission. Some hybrid projects combine diesel generator sets with solar PV technology.

Hybrid power generation systems provide a source of power that is both clean and consistent. They can provide power on demand. What’s more, they can be an effective tool in integrating more renewable resources into the grid while providing firm capacity to back up those variable resources.

About five months ago, the world’s first triple renewable hybrid plant was opened by Enel Green Power in Fallon, Nevada, US. Called the Stillwater hybrid plant, the power plant uses geothermal power and two types of solar power – solar PV and solar thermal – to produce electricity. The project, which was officially inaugurated in March, is a cooperative system integrating a 33MW geothermal plant, a 26MW solar PV system and a 2MW solar thermal facility.

Coupling the solar thermal facility with the geothermal plant boosted the output at Stillwater by 3.6% versus output from the geothermal plant by itself. The finding was confirmed by researchers at the National Renewable Energy Laboratory and the Idaho National Laboratory.

Sweden has just become the next country to showcase innovation in this space.   

Innovation in the renewable hybrid market

A proof of concept for a novel hybrid renewable energy system featuring combined hybrid solar PV and geothermal power has been revealed by research carried out by SP Technical Research Institute of Sweden. This is one of the first demonstrations of hybrid solar combined with GSHP in Sweden, according to Pernilla Gervind, one of the lead researchers on the project.  

The new concept involves a system which integrates 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.

A significant 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 a form of cooling agent (water or air), which is circulated through PV panels. The aim 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.

As the water travels through the PV panels, it is heated to approximately 10-degrees Celsius. It then travels into the cold side GSHP system and used as heat source. The heat surplus is 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 cooled water is then cycled back up the system, and re-used in the cooling of PV panels in a closed-loop system.

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.

Monitoring system performance allowed the researchers to make mid-study adjustments. For instance, in the original system design, solar heat was first directed 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. This change resulted in more efficient heat pumps due to the increased temperature of the heat source.

Unique hybrid solar PV for recharging

Heating boreholes with direct heat, called ‘recharging’, is not a new concept. It is a well-known method for increasing the 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 can be used for the purposes of seasonal storage of thermal energy. In Sweden, seasonal temperatures vary significantly, presenting options for how the system can be used.  During the summer season, solar thermal energy is created and stored in boreholes for winter energy needs.

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% of the country’s buildings use GSHPs, according to the International Geothermal Association.

Owing to relative success of the pilot, the researchers are looking towards future studies. Jessica Benson, Gervind’s co-researcher says that they are already conducting a follow-up study to investigate key performance issues; for instance the effect of cooling on PV cell efficiency, and efficiency of GSHPs. She adds: “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.”

We expect there to be a growing interest in the space as more and more proof of concepts come out of the woodworks.

 

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