By Jillian Kunze
The European Union has placed a major emphasis on developing new energy strategies and investing in novel energy technologies, particularly to improve the energy efficiency of buildings. One particular renewable energy technology that is highly energy efficient and environmentally friendly is the ground source heat pump (GSHP), a geothermal system for heating and cooling buildings. GSHPs use ground heat exchangers (GHEs) that can cover the building’s energy demand in both the summer and winter, and are controllable by a remote building management system (BMS).
During a contributed presentation at the 2024 SIAM Conference on Mathematics of Planet Earth—which took place last week in Portland, Ore., concurrently with the 2024 SIAM Conference on the Life Sciences—Paul Christodoulides of the Cyprus University of Technology assessed the performance of a GSHP system at the University Municipal Library of Limassol in Cyprus, which serves as a research and education laboratory. Theoretical and experimental investigations can evaluate the system’s performance and help create recommendations to improve its efficiency. “It’s not only studied for the purposes of this talk — it’s an ongoing project,” Christodoulides said.
The ground temperature in Cyprus remains constant at depths of about 50 to 100 meters. In the winter, the ground is warmer than the environment, so the building absorbs heat from the ground; the opposite occurs during the summer. The GSHP system at the University Municipal Library of Limassol, which was installed in 2018, includes several different GHEs for the sake of comparison: vertical GHE boreholes with a range of specifications, helicoidal coils in a well, and a complex of open wells. There are also two chillers that operate on alternating days.
Figure 1. The amount of power that the ground source heat pump absorbed due to the cooling load of the building (yellow) compared with the electric power in the chiller (blue) over June 2019. Figure courtesy of Paul Christodoulides.
A digital program monitors the GSHP system’s energy, flow, volume, and incoming and outgoing temperature at each heat pump. A computational model that is based on the three-dimensional convection-diffusion equation represents each individual GHE; while this type of investigation is most often performed when building a system and finding its optimal size, in this case it serves an important experimental role as well. Researchers use the data to estimate and compare the electrical power that goes into the system, the power that is absorbed or rejected by the system, and the system’s coefficient of performance (COP). “By studying the results, we can discuss several suggestions regarding the operation and optimization of the GSHP as well as the BMS,” Christodoulides said.
Figure 1 displays the data from June 2019—at which time the GSHP system was cooling the building—for the GSHP system’s power absorption due to the cooling load and the electric power consumed by the chiller. “The important thing to note here is the scale difference between the two,” Christodoulides said. This difference can be monitored with the COP, which is calculated as the power absorbed or rejected by the building divided by the total electrical power into the system. The COP for the system generally hovered above five during this time period, meaning that the electrical power consumed by the chillers and other system components was more than five times lower than the amount of power that the geothermal system put into the building.
Figure 2. The amount of power that the three different types of ground heat exchangers—boreholes, open well, and helicoidal configuration in a well—rejected to the ground while in cooling mode during June 2019, as well as the total power rejection. Figure courtesy of Paul Christodoulides.
“We showed that the COP is really high, so now we want to check the three types of GHEs,” Christodoulides continued. Figure 2 compares the amount of power that was rejected to the ground in June 2019 by the different GHEs while in cooling mode. The open well exhibited the best performance out of the three, though not by a huge margin. “We get a very good COP for heating performance as well,” Christodoulides added.
Overall, the operation of the chillers consumes the most electricity within the system, though they were more efficient in cooling mode than in heating mode. “It is crucial to recognize that the ranges for COP can fluctuate due to variables such as the specific ground temperature and the geological characteristics of the region, the level of installation quality, the design temperatures of the system, the type of ground loop implementation, and the indoor and outdoor air temperature and humidity,” Christodoulides said. “Still, the good results in terms of energy performance confirm that GSHP systems located in the Mediterranean climate zone are highly efficient solutions in terms of primary energy savings.”
Acknowledgements: Paul Christodoulides acknowledged coauthors Lazaros Aresti, Christakis Christou, Iosifina Stylianou, and Georgios Florides of the Cyprus University of Technology. The work was funded by the Cyprus Research and Innovation Foundation and the European Regional Development Fund.
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Jillian Kunze is the associate editor of SIAM News. |