Thermal Energy Storage

The two main types of thermal energy storage (TES) are Aquifer Thermal Energy Storage (ATES) and Borehole Thermal Energy Storage (BTES). With ATES the groundwater is pumped up, cooled or heated and then re-injected. With BTES a piping system is fitted in the subsoil in which a coolant circulates and – depending on the temperature difference – the soil warms up or cools down. Because the heating and cooling alternate depending on the season, excess heat in summer can reduce primary energy usage for heating needs in winter and vice versa for cooling in summer. The relatively small temperature differences between the soil or groundwater and the circuit of a building often require the use of a heat pump.

Both techniques can be applied when the possibility exists for drilling boreholes or wells. In areas with thick aquifers ATES will have the advantage but where the soil has less potential for attracting and injection of groundwater, BTES will have the advantage. Both systems have advantages and disadvantages and the choice between one of these systems is not only dominated by the subsurface properties. The availability of knowledge on the two techniques means that each country may differ in a striking preference for one or the other.

In The Netherlands the preference now lies with ATES; over 1,000 systems are in use, of which hundreds have been deployed in the last few years. Nevertheless, there are also already 20,000 boreholes made for BTES. In The Netherlands, the attention is focused on open systems because the power generation per system often exceeds the BTES systems. Also, BTES systems do not require a registration, meaning that the number is an estimate. Elsewhere in Western Europe energy storage systems are also widely used. In Sweden and Germany particularly BTES systems are deployed. Moreover, the design of these systems differs greatly from the Dutch designs.