We have seen that heat can be extracted from the ground. We can also dump “waste” heat from a building into the ground - in other words, we can also provide cooling from the ground. A modern, large building might require heat from the ground in the winter and cooling in the summer. Thus, we can “dump” surplus heat from the summer into the ground (for example, via a borehole heat exchanger), store it there and then utilize it for heating purposes in the winter. We can store surplus heat in the long term (seasons or years), but can also store in for short periods (days or hours; e.g. between morning heating requirements and afternoon cooling). Seasonal storage of heat is relevant to many types of larger-scale buildings, such as hospitals, nursing homes, all types of office buildings and shopping centres.
We can provide cooling with the aid of energy storage via two mechanisms:
- “active cooling”, where we use a reversed-cycle heat pump as a cooling machine,
- “free cooling”, where no heat pump is used.
Free-cooling is virtually gratis, as it involves the use of just one circulation pump to circulate a low temperature carrier fluid from the chilled ground to the building’s cooling system. The fluid absorbs heat from the building and returns it to the ground. Compared with ordinary ground source heat systems that are used for heating only, a UTES scheme can cover the same energy requirements with fewer energy wells, at a closer spacing, thereby saving on investment costs.
Energy storage (plan view of temperatures around array of borehole heat exchangers). Illustration: Göran HellströmOther variants of seasonal storage could involve coupling an underground borehole array to solar panels, snow melting on roads/runways or industrial waste heat - all of these provide surplus heat to recharge the temperatures in the ground. By using some forms of industrial waste heat, we can recharge the underground thermal energy store to such high temperatures that we do not even need a heat pump to provide space-heating in winter. In addition to providing the industry that generates the surplus heat with the possibility of low-cost winter space-heating, the surplus heat could also be used to satisfy space heating requirements in neighbouring properties or urban areas. In effect, a large enough UTES scheme could become a form of district heating, with the industry being able to “sell” the stored heat that it previously regarded as “waste”.
Energy storage on a diurnal basis involves more specialized systems, and could be of particular relevance to the horticultural industry, particularly for greenhouses that use CO2 -enhanced atmospheres to promote growth. Combined heat and power systems that produce the CO2 may generate a surplus of heat during the daytime and the greenhouse may also require dehumidification. The surplus heat from these processes can be stored in the ground and released again at night to provide heating. There are currently no known diurnal energy storage facilities in Norway.
Both bedrock and groundwater represent excellent mediums for energy storage. As regards examples from Norway, the Nydalen complex in Oslo uses borehole heat exchangers in bedrock to provide underground thermal energy storage. Gardermoen International Airport, on the other hand, uses groundwater in unconsolidated sand and gravel deposits as a storage medium. In addition, an energy storage facility is under construction at Ahus in Lørenskog. When completed, this facility will be the largest in Europe and consist of 300 energy wells in bedrock to a depth of 200 m.