Ground source heat pumps, which take advantage of borehole thermal storages (BTES), are some of the most flexible heat pumps which provide high energy efficiency. When designing a new BTES, care must be taken to avoid the dropping of temperatures below freezing in the boreholes. The lifetime of the underground storage is typically designed for 25 years. In practice, this is controlled on-site by dumping excess building heat in free-cooling mode during spring/early summer and excess heat from the hot side into the borehole in warm summer months during active cooling. Thermal energy meters can be placed around the site to estimate the heat power going into the boreholes, but they are expensive and in most applications, the energy meters are therefore limited to only measuring the building interface (heating/cooling side).
This article presents a case study of an Energy Machines installation located in Sweden, where the total heating/cooling production from the installation, as well as the imbalance between heat storage and heat recovery from the BTES connected to this site, have been investigated. The purpose of this study was to gain more insight into the performance of the EM installation, in particular with a focus on the total heat and cooling production vs the measured heating and cooling from the energy meters and the imbalance imposed on the borehole storage. This insight can improve the control of the system to prevent long-term freezing of the boreholes, as well as help, understand the actual system COP.
The system examined in this case study consists of three heat pumps, two of which are in parallel and one which has its heat source connected to the outlet of the two parallel ones. There are also electrical energy meters measuring the electrical power used for each of the three compressor pairs (two circuits with two compressors/circuit each per heat pump). With the Energy Machines Verification tool (EMV), the heat power to the evaporator can be estimated as well as the power released in the condensers. The EMV algorithm requires input measurements of pressure and temperature throughout the heating cycle, which are logged with 1-minute resolution and are available using the energymachines.cloud platform, which logs and monitors all EnergyMachines installations. Many more outputs can be derived, such as COP, refrigerant flow and compressor efficiency, which given the right data analysis, may help diagnose/predict potential performance issues.
Tobias Dokkedal Elmøe, Denmark
This text has been shortened by the HPC team
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