Efficiency of buried cables depending on hydrogeological and geothermal properties of trench fill materials

Abstract

Small power plants using geothermal and other forms of renewable energy will cause a decentralization of electric power production and this requires extensive changes of the electric supply network. Considerable amounts of heat are emitted by underground power cables under electrical current flow. The maximum current rating of the cables is limited by the maximum conductor temperature. The dimensioning of the distances between cables to avoid mutual heating, as well as the maximum current load relies on standardized values. These values are considered as partly conservative limits, because any thermal overload can significantly reduce the lifespan of the cables and increase operation costs. Each form of grid optimization is of considerable economic interest. Medium and low voltage cables are usually laid at a depth of 60 to 80 cm. In order to optimize the maximum load, precise knowledge of the thermophysical and hydraulic properties and their interrelation in the partially saturated surrounding unconsolidated rocks is required. Within the research project several parts can be distinguished. Laboratory tests are conducted on unconsolidated rocks from the project area in Bavaria (Germany), determining the relevant hydrogeological and thermophysical properties and compiling the data in a geographic information system (GIS). In addition, field investigations on an underground cable test field in which the cables are embedded in representative (well-defined) soils and thermally enhanced bedding materials are conducted, as well as monitoring of real cable routes. The results of all tests are used for the calculation of heat flow using the finite elemente method and analytical methods. The results can be used to optimize the design of electrical cable routes as well as the design of shallow geothermal installations.