Coupled Renewable Energy Systems for Seasonal High Temperature Heat Storage Via Medium Deep Borehole Heat Exchangers

Abstract

Heating of buildings requires more than 50% of the overall energy consumption in Germany (German Ministry of Economics and Technology, 2012). Worldwide many people live in a comparable climate. Therefore, especially in this sector, new techniques are needed to save energy and reduce greenhouse gas emissions. Shallow geothermal systems for indirect use as well as shallow geothermal heat storage systems like aquifer thermal energy storage (ATES) or borehole thermal energy storage (BTES) typically provide low temperatures only. The temperature levels and ranges usually require a coupling with heat pumps. Based on a case study of the Institute of Applied Geosciences at the University of Darmstadt (Germany) the feasibility and design criteria of a coupled renewable energy system, designed to store and supply high temperature heat, which is needed for a conventional heating system, are assessed. By storing hot water, which is provided from solar panels installed on the building’s roof and from the combined heat and power station (CHP) of the campus with temperatures of 90 °C to 110 °C, a medium deep high temperature heat storage (MDHTS) can be operated on high temperature levels of 50 °C to 90 °C. Thus, it is possible to avoid the use of heat pumps or at least to significantly reduce their energy costs for the operation of conventional radiator based high temperature heating systems. Furthermore, target depths of more than 400 m below the surface typically avoid conflicts with groundwater use. In addition, the temperature difference between the storage system and the surrounding rocks decreases with increasing depth, which results in a reduction of thermal losses. Recently a demonstration project is realized. A more than 200 m deep exploration borehole into the crystalline basement is drilled with a hydraulic down the hole hammer (DTH), which promises high verticality of the borehole and low drilling costs. A coax BHE will be installed and used for testing different operation modes. One aim is to calibrate the existing numerical models with these operational data. Especially the coupling of different renewable energy sources – solar thermal and geothermal – with already existing district heating systems – e.g. combined heat and power stations (CHP) – as presented here, proves to be a very promising approach to cover the heating demand of renovated or old buildings at higher temperature levels with renewable energies.