Insulating Piles for the Cost-effective Construction of Very Large-scale High Temperature Thermal Energy Storage

Large-scale thermal energy storage (TES) represents a key component in renewables-based district heating (DH) networks. However, the storage of water at high temperature (< 100 °C) for long periods can lead to a significant amount of thermal losses to the surroundings and to unwanted increase of groundwater temperature. Insulating the side walls is consequently required, but it is associated with large investment costs. Installation represents a high share of the total investment costs of the insulation, which has to be temperature and pressure resistant and resistant against humid environments. Hence, costs-effective insulation installation methods and processes supported by a proper envelope design are crucial. The new approach proposed in this work is based on the use of overlapping bore piles and considers the use of piles filled with foam glass gravel (FGG) as insulation. The advantages of this solution rely on the possibility to reduce the installation costs and on the thermal characteristics of the adopted material. FGG is a frequently used insulation material in underground constructions due to its low thermal conductivity, pressure resistance and draining properties and relatively low cost. FGG can be used with a loose, compacted or bounded configuration. While the loose material outperforms the bounded in terms of thermal conductivity, the second one presents improved structural properties. One approach is to alternate bounded and loose piles where the bounded represent the primary piles and these are overdrilled producing the loosely filled secondary piles. An alternative is to use different degree of compaction in the primary and secondary piles. A compromise between required strength and thermal performance has to be found. The paper reports results of material tests, mock-ups and simulation results.