The technology of aquifers as energy storage systems is in many respects closely related to hydrothermal-geothermal energy. The storage medium is the natural substratum, i. e. the rock layers and the deep water they contain; the latter also serves as heat-transport medium. Aquifer storage systems are as a rule accessed via two boreholes or groups of boreholes. These are placed at a certain distance from each other, in order to avoid mutual thermal influences. The systems are open below ground and closed above ground. Above ground is a heat exchange apparatus, so that only energy transport – and no matter transport – occurs.
Both boreholes are fitted out with pumps and injection piping, which allows the flow of the heat-transport medium in the above-ground part of the plant to pass through in either direction. For charging, that is for storing thermal energy in the reservoir, water is taken from the cooler boreholes, warmed by the above-ground apparatus, and injected into the warm borehole. For discharging, the direction of flow is reversed: The pump in the warm borehole extracts water up to ground level, where it can give up heat to the heat exchanger system. The heat transferred is proportional to the mass flow rate of the thermal water in each case.
Charging and discharging of the aquifer storage system takes place horizontally (Figure 1). Around the warm borehole, a heated volume of water forms, which is pumped out again on discharging. Since the thermal water serves both as heat-transport medium and as storage medium, the temperature present in the reservoir is also available above ground, as long as the thermal losses within the borehole can be neglected. The discharging temperature will, however, always be lower than the charging temperature, since the warm water volume cools at its outer surface. This heat transport to the cooler surroundings takes place through
heat conduction and natural convection, as the warm water mixes with the cooler water from the surroundings, and through groundwater flows.
From this, we can derive the initial requirements for aquifers which are to be used for seasonal energy storage:
• The geological formation should be closed above and below, and the natural flow rate of the ground water as low as possible (preferably zero), to prevent the warmed or cooled water from flowing away.
• The temperature below ground increases as a rule with increasing depth. Heat storage reservoirs will thus be more readily found in deep strata than cooling reservoirs, since then the natural temperature of the aquifer lies nearer to the desired mean storage temperature, and storage losses are reduced. The requirement that the natural aquifer temperature be in the range of the mean storage temperature cannot always be fulfilled in an economically feasible way, in particular for high-temperature storage. A rough calculation shows: In order to obtain a temperature of 70 °C below ground, given an average temperature increase with depth of 30 K per kilometer, and 10 °C external temperature, boreholes of two kilometers depth would be required.
• To keep the required pumping power low, which is desirable, a high water throughput in the horizontal direction within the rock layers is desirable.
The maximum storage temperature and the storage volume, which determine the total storage capacity, are given by the natural properties of the aquifer. An advantage of aquifer storage is its relatively low investment cost. It is about 25 €/m3 (including planning, excluding taxes, for storage volumes of more than 100,000 m3) .
– 30 m
– 60 m
Heating reservoir 20 °C
Fig. 2 An aquifer reservoir for heating and cooling
(source: GTN, BBG).
Charging and discharging temperatures of the thermal storage medium. Red and dark blue lines: measurements; light blue: model calculations.