Heated and cooled water is moved back ad forth between wells.

Heated and cooled water is moved back ad forth between wells.

Technology for storing heat — and even cold — in underground wells has been in use since the 1970s. Boreholes for geothermal energy may be 60 to 200 metres deep, depending on energy needs and geologic conditions. You might be tempted to believe that heat comes from the earth’s deep, hot magma, but you would be wrong. Rather, it comes from a source as pedestrian as rainwater, essentially lakes and streams, warmed by the sun.

In Sweden, the groundwater temperature stays a fairly constant 6 – 8°C throughout the year. This temperature stability provides good conditions for extracting energy via heat pumps for use in homes and offices.

Subterranean cooling

The idea of extracting heat from groundwater is fairly well known, but most of us are probably less familiar with the possibility of using the same source to generate a cooling effect. During the winter, water is pumped from a “warm well” by a heat pump and used to buildings. After the heat is extracted, colder water is led to a “cold well”, where it can be extracted during the summer for cooling.

In practice, instead of single wells for heat and cold, a large number of wells are drilled and connected with one another. Hoses fed into the boreholes are filled with a liquid refrigerant, which is either heated or cooled as it circulates from the surface, down the well and back up again. The system of warm and cold wells allows for seasonal storage of energy in an effective, environmentally sound technology with powerful economic advantages. In effect, building owners pay for heating and get nearly free cooling in the bargain.

The Swedish company MalmbergGruppen was one of the first to commercialise heating and cooling systems using groundwater storage. Malmberg claims a number of reference installations in its home market:

Akademiska Hus in Lund

Malmberg constructed one of northern Europe’s largest underground energy storage systems for Akademiska Hus, Sweden’s largest property management company and a leader in creative environments for higher education and research. The installation comprises 153 boreholes reaching 230 metres underground, delivering heating and cooling to several buildings on the campus of the Lund University.

Kristianstad Central Hospital

Four wells drilled to a depth of 100 metres supply heating and cooling to the hospital. The wells are placed in pairs about 500 metres apart. During the summer, water is pumped up from one pair of wells at a temperature of some 8°C and used to cool a heat exchanger feeding ventilation air to the building. Having reached a temperature of about 20°C, the water is pumped back to the other pair and stored until ambient temperatures fall in the autumn.

The system reverses direction for the winter, providing heat. Cooled water is then pumped back underground until the weather changes and more cooling than heating is required. During a season, several hundred thousand cubic metres of water will move back and forth between the wells, generating enough energy to replace heating oil for the equivalent of 300 single-family homes.

The world’s largest groundwater storage system is beneath Stockholm’s Arlanda Airport.

The world’s largest groundwater storage system is beneath Stockholm’s Arlanda Airport.

World’s largest groundwater energy deposit

Sweden’s state airport operations company, Luftfartsverket, has set an ambitious goal of making Stockholm’s Arlanda Airport climate neutral. To that end, the aquifer under the airport, which is naturally sealed against leakage to surrounding water, is used for heat storage. Circulation pumps feeding a number of boreholes bring heated or cooled water to the surface as needed. Malmberg won the contract to drill the wells and install the piping. Annual energy savings are calculated at 4 GWh of electricity and 15 GWh of heat, equivalent to the needs of 2000 single-family homes. The installation is both profitable and environmentally advantageous, with a reduction in carbon dioxide releases of 80 to 100 per cent. It’s the world’s largest groundwater energy storage system.

Published in January 2009