Fjell 2020 High Temperature Borehole Energy Storage - System Control for Various Operation Modes

Abstract

Energy storage is defined one of the most promising solutions for cutting peak demands. It enables for smaller installed power to cover the same demand and allows larger flexibility. Among the solutions of storage, borehole thermal energy storage (BTES) is gaining interest. Most existing BTES systems use ground source heat pump (GSHP) with storage temperatures in the range of 3 – 30 °C. However, recent solutions have encouraged the development towards high-temperature BTES (HT-BTES) with storage temperature in the range 60 - 100 °C, which enables optimizing the seasonal use of excess heat or other sources. In Norway, the research project Rockstore, focus its interest towards this topic. Fjell2020 is one of the pilot installations in this project. Fjell2020 in Drammen, Norway, is designed to use high-temperature geothermal energy storage (50 – 60 °C). The system attempts to cover the space heating needs of this school via direct heat extraction from the HT-BTES (without using heat pump). The BTES stores heat from solar thermal panels and excess heat from the CO2 heat pump. The 100 boreholes ca. 50 meters deep are placed relatively close to each other, with maximum 4 m between them. They are monitored by means of Distributed Temperature Sensing. The energy wells are placed concentrically. The space heating (low temperature floor heating) and the ventilation heating demands are covered with heat extracted from the innermost wells. The two outer shells have a lower temperature and can be used indirectly via heat pump. The domestic hot water to this school is covered via a CO2 heat pump. The goal of this publication is to analyze the control strategies and to discuss the functioning of the solution in Norway and how the control of the systems can be optimized in order to be the most energy-efficient.