Topical Article: Heat Pump System Improved High-Temperature Borehole Thermal Energy Storage Efficiency


A cooling tower to get rid of excess heat and bought energy for space heating. That is the reality in many factory buildings today. At the Xylem facility in Emmaboda, Sweden, a solution to this was sought for. A high-temperature borehole thermal energy storage was installed in 2010, with the idea to store large quantities of waste heat (or cold) on a seasonal basis in the geological environment.

The storage consists of 140 boreholes, each 150 meters deep and a 4-meter separation between them. This covers an area of 2400 m2. Initially, the storage functioned properly and was gradually heated to approximately 40°C in 2014. However, the storage temperature eventually stabilized. It became evident that instead of an increase in temperature, there was an unwanted lateral growth of the storage; theoretical calculations and empirical observations suggest that seasonal thermal recovery factors over 50% are difficult to obtain. With such low storage temperatures, it was not possible to extract as much heat as wanted through the direct heat exchange system that was installed.

The heat extraction being much lower than expected had also already been noted. Two measures were recommended and implemented: installing a heat pump system and reducing the working temperature to 40/20°C. With these measures, the storage could also be used for cooling.

This system proved to work. Now, it covers the heat demand down to an outdoor temperature of about -5°C. The storage temperature has been gradually lowered and was down to 28°C in April 2021. The increased cooling potential from the system has resulted in a significant reduction in the use of the cooling tower and a reduction in heat losses.

Both the initial thermal storage investment and the subsequent heat pump system investment have been profitable. The savings in thermal energy outweighed the additional costs for system electricity. Also, the need for water treatment and maintenance has decreased. The experiences from Xylem also demonstrate that heat pumps are needed for reaching the full potential in such a system.

Olof Andersson, Geostrata HB Sweden, Leif Rydell, Reikab AB Sweden, Niklas Håkansson, Xylem Sweden

 

This text has been shortened by the HPC team.

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