Underground thermal energy storage The ability to store energy in an economical way can be achieved through the use of underground energy storage systems, which provide efficient, clean and discrete storage units. The alternatives, involve the use of power plants, which not only require large areas of land, but are also a source of pollution. Energy © PhotoDisc Through project TESSAS or high temperature Thermal Energy Storage in Saturated Sand layers with vertical heat exchangers, the use of underground borehole storage was examined. This involves inserting vertical heat exchangers underground, which ensure the transfer of thermal energy towards and from the ground. Factors, which must be considered in such systems, are the thermal characteristics of the ground such as borehole resistance and thermal conductivity. These characteristics are strongly influenced by the ground composition with factors such as homogeneity, sand, silt or clay content, water saturation, and ground water level all playing an important role. In the past, laboratory experiments were used to assess thermal characteristics. This resulted in a recommended value within a large range of possible values. Therefore there was larger scope for errors. To improve estimations, TESSAS performed measurements in situ, which monitor the actual situation and all influencing parameters. The characteristics of a homogeneous water saturated layer were measures by logging temperatures and flows while supplying heat to a vertical heat exchanger. This test, known as the Thermal Response Test, was also used to collect information on three different types of borehole refilling material. The borehole filling with drilled out sand provided better results than fillings with a sorted sand mixture. Such results are of great significance in creating optimal borehole design. Ground coupled heat pump systems, which use vertical heat exchangers can also benefit from these results. Any party wishing an in situ assessment of an underground system with regard to its thermal characteristics is invited to participate in the research of this area. With continuing research in this area, underground storage should become the best means of energy containment in a clean and efficient manner.
