Ground Energy Storage can be implemented in different ways depending on the ground conditions. If there is a natural ground water flow so called Aquifer Thermal Energy Storage can be implemented. However mostly the ground conditions is not suitable for ATES and so called BTES (Borehole Thermal Energy Storage is the only viable way to go when a ground energy storage is going to be implemented. Already during the oil crises during the 70’s different project started to investigate the possibilities to realize seasonal energy as storage for solar energy. In Sweden some projects ended up with some promising results but when the oil crises ceased the interest disappeared. Later on during the 80’s and the 90’s ground couple heat pump emerged as an attractive technology in Sweden because electricity prices was pretty low and oil and other heating technology started to be expensive again. However, ground couple heat pump only makes use of the natural ground temperature and upgrade the temperature to desired temperature with the compressor. In some big installation several ground heat exchangers are used and the heat waves from the different boreholes start to interfere with each other, which is preconditions when implementing a BTES if we are going to increase temperature in the ground above the natural temperature. However, when configure many different borehole together with a certain distance between the bore hole it is necessary to charge and discharge the ground with the same amount of heat, otherwise the ground temperature will change during the coming years.  The ultimate installation requires the same amount of heat during winter and cool during the summer, that is, a so-called balanced BTES. There are reports of such a BTES but very little real seasonal energy storage has been implemented despite many people call some big installation as BTES even if they only use the ground as a heat source or heat sink (for cooling).

It seems there is a lack of knowhow regarding BTES in the commercial community but also in the academic world. UPONOR has studied the old reports from the 70’s and according to these reports there is a need for an improved GHEX (Ground Heat Exchanger) to really exploit the potential to use the ground as a seasonal BTES. Uponor has developed one ground heat exchanger called G12 based on this research report results. The G12 has been installed in two bore holes during the summer 2010 and measured by fiber optics along the length of the G12 to get temperature profile along the vertical bore hole.  The results are all found to be in line with theoretical assumptions and have been presented in scientific papers as;

Acuna et al, 2011. Distributed Temperature Measurements on a Multi-pipe Coaxial Borehole Heat Exchanger. 10^th IEA Heat Pump Conference. Tokyo, Japan.

Acuna et al, 2011. First Experiences with Coaxial Borehole Heat Exchangers. IIR Conf. on Sources/Sinks alternative to the outside Air for HPs and AC techniques. April 5^th-7^th. Padua, Italy.

  The G12 offers higher heat transfer per length compared to conventional U-loop. The performance is about 80-100 % better than U-loops depending on operation conditions. With the characteristics that G12 offer it is expected to obtain seasonal ground energy storage with lower heat losses and hence, make it more efficient to store heat from summer to winter. If there is no need for space cooling during summer the BTES has to be charged with other green energy. Such a green energy can be solar energy from cheap low temperature solar collectors. It might even be interesting to store surplus electricity as low grade heat in the ground. Normally it must be considered as waste to store high Exergy (quality) energy as electricity as low grade (20-40 °C) thermal energy. However, the more the intermittent renewables are introduced into the large electric grid the more periods of surplus electricity that cannot be utilized immediately. For instance a wind power unit can produce electricity during summer when there is no need for the produced electricity. The electricity can be stored in electric batteries for later use but electric batteries have a limited life time and have to be replaced, which implies high capital cost. The surplus wind electricity can also be charged in hot water accumulators for shorter periods. But if neither one of the alternatives with electric batteries or the hot water accumulator is not a viable choice it could be justified to charge a BTES with surplus electricity rather than loose it away, even if it means conversion from electricity to low grade heat.