Proceedings of the IGSHPA Technical/Research Conference and Expo 2017
https://hdl.handle.net/11244/49192
2024-03-28T21:13:37ZThermal response of helix ground heat exchangers
https://hdl.handle.net/11244/49345
Thermal response of helix ground heat exchangers
Fossa, Marco; Stutz, Benoit; Priarone, Antonella; Coperey, Antoine
This paper is devoted to the thermal analysis of shallow ground heat exchangers with pipes arranged in a helix configuration. The pipes where the carrier fluid is circulated typically embrace a cylindrical volume that is filled by ground or concrete, the latter being the case of the so called geopiles. Other pipes dispositions include conic helices that can be easily inserted in proper excavations. The analysis of the transient thermal behavior of a helix/ground assembly is here carried out according to different approaches, including the exploitation of superposition techniques, the finite element modelling and experiments in a reduced scale mock up. Different geometrical configurations have been taken into account and also the variability of ground and concrete thermal properties have been considered. A detailed description of the experimental set up is provided and the model results have been processed in order to develop suitable temperature response factors (or g -functions) to be employed for predicting the ground heat exchanger behavior in different operating conditions.
2017-01-01T00:00:00ZTransient thermal resistance of borehole heat exchangers for hourly simulations of geothermal heat pumps systems
https://hdl.handle.net/11244/49346
Transient thermal resistance of borehole heat exchangers for hourly simulations of geothermal heat pumps systems
Priarone, Antonella; Fossa, Marco
The correct design of borehole fields requires the correct evaluation of the transient ground thermal response in time, but also the accurate estimation of the borehole (BHE) thermal resistance, expecially the grout contribution. Generally, the borehole thermal resistance is considered as steady-state; however, when considering the borefield hourly response to the building variable thermal loads, also the transient behavior of the grout thermal resistance plays an important role, which is quite often neglected. This study analyzes, with a dimensionless approach, the transient grout thermal resistance, with particular attention devoted to the effect of the boundary condition imposed to the internal tubes, namely imposed heat flux, imposed temperature and imposed convective coefficient, the last being the real operating conditions. In addition, the effects of grout to ground thermophysical properties and of shank spacing are analysed. The steady state numerical results are also compared with literature correlations. Finally, numerical evidences are given to demonstrate that the usual approach of calculating the overall BHE resistance just summing the grout resistance, numerical obtained by imposing a temperature on the tube surface, to the convective one can lead to meaningful errors at low Biot numbers.
2017-01-01T00:00:00ZInvestigation on the effects of different time resolutions in the design and simulation of BHE fields
https://hdl.handle.net/11244/49344
Investigation on the effects of different time resolutions in the design and simulation of BHE fields
Fossa, Marco; Rolando, Davide; Priarone, Antonella
The correct design of a field of Borehole Heat Exch angers (BHE) requires the knowledge of ground thermal properties, heat pump performance and building heating and cooling demand. The sequence o f heat pulses from (to) the ground by the heat pump can be described according to different time steps , from hours to months and even years. The monthly time step approach is often the preferred design choice which involves recursive calculations (temporal superposition techniques) and the availability of precalculated temperature response factors (or g-functions) for given BHE field geometries. Such a complex computing task is usually performed thanks to commercial codes in order to fulfil a carrier fluid temperature at the end of a given time horizon, typically 10 or 25 years. In this paper the monthly design approach (EED code and TecGeo proprietary code) is compared with the three thermal pulse approach (modified ASHRAE Method Tp8) and it is demonstrated that for a representative series of case studies the three pulse calculation, easy to be performed at engineering level, is able to provide the correct BHE field overall length with 8% accuracy with respect to the reference monthly calculations.
2017-01-01T00:00:00ZInfluence of ground heat exchanger zoning operation on the GSHP system long-term operation performance
https://hdl.handle.net/11244/49343
Influence of ground heat exchanger zoning operation on the GSHP system long-term operation performance
Yu, Mingzhi; Rang, Hongmei; Zhao, Jinbao; Zhang, Kai; Fang, Zhaohong
To alleviate the ground heat accumulation after long term running of ground source heat pumps (GSHP), ground heat exchanger (GHE) zoning operation can be adopted. Two GHE operation modes - zoning operation and full running - are compared in a case where heat release to the ground in summer is larger than the heat extraction from the ground in winter. In this study the soil thermal conductivity, volumetric specific heat capacity, borehole depth and spacing are 2.0 W(mK)-1, 5.0x106 J(m3K)-1, 100m and 5m respectively with the boreholes arranged in a square 20x20 array. Under the given conditions the simulation results show that GHE zoning operation depresses the increase in amplitude of GHE outlet water temperature and so that the GSHP systems operate normally throughout the whole service life. By adopting GHE zoning operation, the energy consumption of the GSHP system is found to be reduced compared with that of a GHE operated without zoning. Operation without zoning shows that the GHE summer outlet water temperature increases faster than that with zoning operation and power demands are increased for the given GSHP load. Furthermore, in this case, the GSHP would not be able to run normally in the last several years due to the condensing temperature exceeding its upper limit.
2017-01-01T00:00:00Z