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Thermal capacity effects in borehole ground heat exchangers
Highlights We propose a new geothermal borehole model to account for thermal capacity. An equivalent diameter is used and one-dimensional heat transfer is assumed. The numerical borehole model is coupled to the CHS method for ground heat transfer. We use the model to examine the outlet temperature for varying inlet conditions. Simulations indicate that the annual COP can be underestimated by 4–5%.
Abstract A one-dimensional transient ground heat exchanger model is proposed to account for fluid and grout thermal capacities in borehole ground heat exchangers with the objective of predicting the outlet fluid temperature for varying inlet temperature and flow rate. The standard two-pipe configuration is replaced with an equivalent geometry consisting of a single pipe and a cylinder core filled with grout. Transient radial heat transfer in the grout is solved numerically while the ground outside the borehole is treated analytically using the cylindrical heat source method. The proposed model is validated successfully against analytical solutions, experimental data, a three-dimensional transient numerical model, and TRNSYS's Type 451. For a typical two-pipe configuration, it is shown that the fluid outlet temperature predicted with and without borehole thermal capacity differs by 1.4, 0.35, and 0.23°C after 0.1, 0.2 and 1h, respectively. Annual simulations are also performed over an entire heating season (5600h) with a 6min time step. Results show that the outlet fluid temperature is always higher when borehole thermal capacity is included. Furthermore, the difference in fluid outlet temperature prediction with and without borehole thermal capacity increases when the heat pump operates infrequently. The end result is that the annual COP predicted is approximately 4.5% higher when borehole thermal capacity is included.
Thermal capacity effects in borehole ground heat exchangers
Highlights We propose a new geothermal borehole model to account for thermal capacity. An equivalent diameter is used and one-dimensional heat transfer is assumed. The numerical borehole model is coupled to the CHS method for ground heat transfer. We use the model to examine the outlet temperature for varying inlet conditions. Simulations indicate that the annual COP can be underestimated by 4–5%.
Abstract A one-dimensional transient ground heat exchanger model is proposed to account for fluid and grout thermal capacities in borehole ground heat exchangers with the objective of predicting the outlet fluid temperature for varying inlet temperature and flow rate. The standard two-pipe configuration is replaced with an equivalent geometry consisting of a single pipe and a cylinder core filled with grout. Transient radial heat transfer in the grout is solved numerically while the ground outside the borehole is treated analytically using the cylindrical heat source method. The proposed model is validated successfully against analytical solutions, experimental data, a three-dimensional transient numerical model, and TRNSYS's Type 451. For a typical two-pipe configuration, it is shown that the fluid outlet temperature predicted with and without borehole thermal capacity differs by 1.4, 0.35, and 0.23°C after 0.1, 0.2 and 1h, respectively. Annual simulations are also performed over an entire heating season (5600h) with a 6min time step. Results show that the outlet fluid temperature is always higher when borehole thermal capacity is included. Furthermore, the difference in fluid outlet temperature prediction with and without borehole thermal capacity increases when the heat pump operates infrequently. The end result is that the annual COP predicted is approximately 4.5% higher when borehole thermal capacity is included.
Thermal capacity effects in borehole ground heat exchangers
Shirazi, Ali Salim (author) / Bernier, Michel (author)
Energy and Buildings ; 67 ; 352-364
2013-08-14
13 pages
Article (Journal)
Electronic Resource
English
Thermal capacity effects in borehole ground heat exchangers
| Online Contents | 2013
Thermal response testing of compromised borehole heat exchangers
| Oxford University Press | 2013