Research paperThermal performance and pressure drop of spiral-tube ground heat exchangers for ground-source heat pump
Introduction
The increase of global warming problem has increased interest in using renewable energy sources. The ground-source heat pump (GSHP) system can be the desirable technologies in renewable energy markets. This system provides efficient space cooling and heating in residential and commercial buildings. The GSHP system is a heat pump coupled with a ground heat exchanger (GHE) that can be oriented vertically or horizontally. The GHE is used in the system to exchange heat with the ground. It is usually buried in 15–150 m depth in vertical type and laid in the bottom of 1–2 m deep horizontal trenches.
The design/simulation methods and programs of models and systems of vertical GSHPs were described in a detailed review [1]. Operating the GHEs with various conditions shows the different characteristic in their heat exchange rates [2], [3]. The performances of several types of GHEs applied to the pile foundations in actual buildings were studied by Hamada et al. [4]. Gao et al. [5] also studied several types of vertical pile-foundation heat exchangers.
The spiral tube GHE is gaining interest in recent years. In this type of GHE, a spiral pipe is installed in the borehole or building foundation pile. Modeling of heat transfer of the spiral tube GHE is important research areas. A cylindrical source model considering the radial dimension and the heat capacity of the borehole or pile was developed [6]. Cui et al. [7] developed the ring-coil source model taking into account the discontinuity of the heat source and the impact of the coil pitches. However, this model does not simulate the heat transfer of fluid circulating inside the spiral coil pipe. A spiral heat source model has been developed for better thermal analysis [8]. A spiral coil source model was developed to consider 3D shape and radial dimension effects [9]. Heat transfer around the helical GHE with varying helical pitch was presented [10]. Comparison study of helical GHE with triple U-tube [11] and double U-tube [12] GHEs were carried out. It is found that the helical GHE provided better thermal performance than others.
The effective borehole thermal resistance of spiral coil energy pile with considering coil pitch, pipe size, pile size and also, groundwater advection effect on the long-term ground temperatures were evaluated [13]. The groundwater flow enhances the heat transfer performance of spiral coil GHE [14]. Another work considering the groundwater advection effect on spiral coil energy piles are reported [15], [16], [17].
Various models of spiral-tube GHEs installed in a borehole and concrete pile were simulated [18]. Heat exchange rate and pressure drop are important parameter in design of the GSHP system. The present work investigate the heat exchange rate per meter borehole depth of the spiral tube GHE with various pitches of 0.05; 0.1 and 0.2 m respectively and their pressure drops along the pipe.
Section snippets
Ground heat exchangers and its simulation model
The GHE models such as U-tube and spiral-tube were simulated. A major feature is an analysis of heat exchange rate and pressure drop of water flow in the GHEs. The schematic diagrams of the U-tube and spiral-tube GHEs are shown in Fig. 1. The both GHEs were installed in the borehole of 20 m depth. Inlet and outlet pipes for the both GHEs, U-tube and spiral-tube, are made of Polyethylene pipes. In the spiral-tube GHE, a spiral pipe is used as the inlet tube of the GHE and a straight pipe is used
Heat exchange rate
Simulation of the U-tube and spiral-tube GHE models was performed. The thermal performances of the GHEs were investigated by calculating their heat exchange rates through the water flow. Heat exchange rates of the GHEs were evaluated in 72 h continuous operation because after 72 h, the heat exchange rate become nearly steady value, relative evaluation of the GHEs is available. The heat exchange rate of the U-tube GHE was used as a base of comparison. Outlet temperatures of water flow gradually
Pressure drop
Fig. 7 shows the pressure drop between inlet and outlet point of the GHEs. The pressure drop of water flow increased in the-spiral tube GHE due to increasing the length of pipe per meter borehole depth and its spiral geometry. The pressure drop also increased in the spiral pipe with decreasing the spiral pitch. Examples of pressure distribution along the pipe of the U-tube and spiral-tube GHEs are shown in Fig. 8. It shows that the pressure of the both GHE types declined along the flow path.
In
Conclusions
The heat exchange rate per meter borehole depth of the spiral-tube GHE with various pitches of 0.05; 0.1 and 0.2 m respectively and their pressure drops along the pipe were evaluated. Heat exchange rate per meter borehole of the spiral-tube GHE increased significantly compared with heat exchange rate of the U-tube GHE. The heat exchange rate of the spiral-tube GHE with p = 0.05 m increased about 69.2% in the laminar flow and 34.9% in the turbulent flow. In comparison with the straight pipe in
Acknowledgements
This study was sponsored by the project on “Renewable energy-heat utilization technology & development project” of New Energy and Industrial Technology Development Organization (NEDO), Japan.
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