Necessary measures and calculation for dimension of coaxial heat exchanger for deep boreholes

Abstract

This paper aims to illustrate the complexity of the approach to modeling a potential heat exchanger envisaged for deep drilling. The problem was presented in one example of the installation study for the heating and cooling of the University building in the city of Aachen, Germany, with a deep coaxial heat exchanger, located in a depth well of 2500m (BHE). Prior to any modelling of the heat exchanger, the geological analysis of the wall layers along the well was to be approached; to conduct complex field and laboratory geophysical measurements and ultimately to approximate the modeling of heat processes in the heat exchanger. Also, this study was aimed at assessing the feasibility of such an installation for heating and cooling space, as well as clarifying operational characteristics and performance (in the long run). Direct heating of the building in the winter period requires a temperature of 40°C. In the summer period, for the cooling of the university building, an adsorption cooling unit is used, which requires a temperature of at least 55°C. Dredged walls up to a depth of 2500m have extremely low permeability and porosity, less than 1%. Their thermal conductivity varies between 2.2W/(mK) and 8.9W/(mK), more values are associated with quartz sandstone. The maximum temperature in the well is 850C at a depth of 2500m, corresponding to a mean specific heat flow of 85mW/m2-90mW/m2. The experience of twenty years of work has indicated that only in a short period of time, the borehole can supply the desired temperature for the operation of the absorption refrigerant. In winter, the heat exchanger can also supply a building with sufficient heat.