Modeling and experiments on energy loss in horizontal connecting pipe of vertical ground source heat pump system
Introduction
Recently, the ground source heat pump (GSHP) system has attracted more and more attention due to its superiority of high energy-efficiency and environmental friendliness [1], [2], [3], [4]. A vertical GSHP system mainly consists of borehole heat exchanger (BHE), heat pump and indoor units. Heat pump is a device that transfers heat energy from the ground to a building, and vice versa. The thermal efficiency of a heat pump depends heavily on the temperature of heat carrier fluid. Therefore, thermal behavior of heat carrier fluid circulating through U-pipe as well as horizontal connecting pipe plays a vital role in performance of GSHP systems.
In general, the temperature of heat carrier fluid can be measured on site or estimated by numerical modeling. In the last decades, several studies [2], [5], [6] have been performed for evaluation of fluid temperature regarding operation of GSHP systems. Choi et al. [7] reported a study in which the fluid temperature drops of BHE with different configurations was examined using a two-dimensional finite-elemental model. The impacts of groundwater flow on dropping in mean fluid temperature were compared and discussed for three different BHE arrays. Wang et al. [8] investigated the effects of groundwater flow on fluid temperature distribution. The results showed that groundwater flow has noticeable influence on the fluid temperature profile of BHE. The fluid temperature was increased and the thermal performance of BHE was enhanced due to strong groundwater advection.
In the realm of numerical studies, Eskilson and Claesson [9] developed an analytical approach considering the local steady-state conditions. In this approach, a thermal equilibrium is formulated between inlet and outlet pipes by given a solid temperature at the borehole wall. Al-Khoury and Bonnier [10] proposed a 1D finite-element model in the context of geothermal heating systems. By resorting the 1D model to FEFLOW's finite-element matrix system, this approach was shown to be effective in modeling fluid circulation in BHE [11]. However, most of the previous studies are limited to fluid temperature of BHE in vertical GSHP systems. Thereby, the thermal performance of GSHP system tends to be overestimated due to the ignorance of energy loss in the horizontal pipes.
In this paper, the impacts of pipe burial depth and pipe insulation on energy loss in horizontal connecting pipe of GSHP systems are investigated via experimental measurement as well as numerical simulation. The measurement is conducted on a GSHP system installed in an office building in Nuremberg, Germany. More specifically, parameters such as flow rate, fluid temperature and ambient temperature have been monitored and the collected data are stored by an acquisition system. Based on the measurements, a 3-demensional finite-element model is developed to examine the heat loss in the horizontal pipe. The numerical model is firstly validated with the measurement results. The model is then applied to simulate heat transfer process of horizontal connecting pipe with five different pipe burial depths as well as different ambient temperature. Furthermore, the energy loss is also examined and compared between uninsulated and insulated pipes.
Section snippets
System configuration and test setup
The measurements are conducted on a GSHP system installed in an office building in Nuremberg, Germany. The building, built in 2008, has 3 floors and one basement with a total area of 1530 m2. Functional regions of the building (offices, meeting rooms, corridors) are located in the 3 over-ground floors, covering a total air-conditioned area of 1150 m2. Auxiliaries (server room, store rooms, heat pumps) are installed in the basement that has an area of 380 m2.
Fig. 1 shows major components of the
Model description
In order to investigate the coupled heat and flow transfer, a numerical model is needed [5], [12]. FEFLOW is a suitable commercial finite-element program for modeling the coupled heat and flow transfer in porous medium [13]. In this work, FEFLOW is used to model the heat loss in the horizontal connecting pipes. In the release of the FEFLOW version 6.0×, a 1D discrete element is embedded in the finite-element matrix system similar to fracture elements. Forced heat and flow transfer phenomenon
Temperature profile
Fig. 7 shows the measured ambient temperature in 2010. It is observed that the ambient temperature changes abruptly during that whole year. The lowest temperature is about −10 °C in winter. In summer, the highest ambient temperature can reach up to around 40 °C. The main periods for space heating lasts from December to March, whereas the major cooling period is July when the ambient temperature can reach up to 30 °C. On average, the heating period is 1800 h per year and the cooling period is
Conclusions
In this paper, the impacts of pipe burial depth and pipe insulation on the energy loss for horizontal connecting pipe of a GSHP system are investigated via experimental measurements and numerical modeling. The experimental measurement has been conducted on the GSHP system to measure the fluid temperature distribution in the horizontal connecting pipes. Based on the measurement results, a coupled heat conduction-advection numerical model is developed to simulate the heat transfer process in the
Acknowledgements
This work is financed by Bayerisches Staatsministerium für Umwelt und Gesundheit. Sincere thanks to the Bayerische Forschungsstiftung for its generosity and providing a scholarship. The authors would also like to thank for the support provided by Ochs Company (Nuremberg, Germany).
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