Study on the hybrid ground coupled heat pump systems with borehole and foundation pile ground heat exchangers for cooling load dominated buildings

Abstract

The ground coupled heat pump (GCHP) technology is becoming an attractive choice for space air conditioning due to its high energy efficiency. However, when the GCHP system is utilized for cooling load dominated buildings, the heat rejected into ground will accumulate around the ground heat exchangers (GHE), which results in degradation of system energy performance. One of the available options to resolve this problem is to utilize hybrid ground coupled heat pump (HGCHP) systems to reject the accumulated heat by supplemental heat rejecters. Due to insufficient research toward this topic, this thesis is to study how to reduce the initial cost as well as land requirement and how to improve the energy efficiency of the HGCHP systems to make the systems more attractive for cooling load dominated buildings. Firstly, an analytical simulation model of a conventional HGCHP system with cooling tower serving as a supplemental heat rejecter is developed on short time step basis. A computer program based on this analytical model to predict the energy performance of the conventional HGCHP system operated with different control strategies are independently compiled. The accuracy of the proposed simulation model and computer program is verified by detailed on-site experiments on a HGCHP system test rig operated with four control strategies. The parameter deviations between experimental data and simulative results are less than 10%. Secondly, some case studies of the conventional HGCHP systems for different types of cooling load dominated buildings are carried out. The optimal design and operation control strategy of the conventional HGCHP systems for different types of buildings are investigated based on the verified simulation model. Valuable recommendations for engineering applications of the conventional HGCHP system are proposed. It is demonstrated that the conventional HGCHP system with optimal design possesses considerable low life cycle cost. For a medium sized heavy cooling load dominated hotel building, the life cycle cost consumed by an optimum conventional HGCHP system just accounts for 56.8% and 44.6% compared with that of a traditional GCHP system and an ASHP system, respectively. Thirdly, in order to further reduce the initial cost and the land area requirement of the HGCHP system for cooling load dominated buildings, a novel pile foundation GHE with spiral coils is proposed. The two-dimensional solid-cylindrical source model, three-dimensional ring coil source model and three-dimensional spiral source model with their respective analytical solutions are established for analyzing the heat transfer process of the buried coil in a semi-infinite medium. The spiral source model is then utilized to simulate the proposed novel foundation pile GHE. The temperature responds of the ground, the coil pipe wall as well as the circulating water and the heat exchange capacity of the novel foundation pile GHE are predicted by the computer program based on the developed analytical models. The results show that the heat exchange capacity of the novel foundation pile GHE could be as high as 212W/m for a normal foundation pile configuration. The novel HGCHP system with foundation pile GHE can save 52.90% initial cost and 225 m² land area compared with the conventional HGCHP system with borehole GHE for a heavy cooling load dominated office building. Finally, as cooling towers are not suitable to some buildings as supplemental heat rejecters of the HGCHP system, a novel HGCHP system with nocturnal cooling radiator serving as a supplemental heat rejecter is proposed to provide a new valuable choice for cooling load dominated buildings. The practical analytical models of the nocturnal cooling radiator and the novel HGCHP system are established. Compared with a traditional GCHP system, the novel HGCHP system can save 10.2% of the total cost during 10-years' operation for a sample building located in humid subtropical climate, which provides poor working condition for the nocturnal cooling radiator. It is believed that this novel HGCHP system is more attractive for buildings located in arid areas. In summary, the experimentally verified short time step simulation model and computer program for the conventional HGCHP systems can provide a reliable tool for system analysis and optimal design. The proposed novel foundation pile GHE is a new contribution to decrease the initial cost and land requirement of the HGCHP system. The established analytical model for a buried spiral coil heat source can be utilized for thermal analysis of the novel foundation pile GHE and other similar engineering problems. The developed novel HGCHP systems provide new valuable choices for developers and engineers. The outcomes of this project can effectively prompt the wide application of the HGCHP technology for cooling load dominated buildings.