Thermal performance and economic evaluation of double U-tube borehole heat exchanger with three different borehole diameters
Introduction
Ground source heat pump (GSHP) system is one kind of renewable energy technologies providing space heating and cooling as well as domestic hot water. A number of GSHP systems have been used in residential and commercial buildings worldwide due to its noticeable high efficiency and environmental friendliness. It is reported that the installations of GSHP systems have grown continuously by 10–30% annually in the last decades [1], [2], [3]. GSHP systems are usually closed system with vertical borehole heat exchanger (BHE) or, less commonly, with horizontal loops. A BHE is commonly drilled to a depth between 20 and 300 m with a diameter of 100–200 mm. A closed single or double U-tube is often inserted inside the borehole and a heat carrier fluid is circulated in the U-tube to exchange the heat or cold with the surroundings. For safety and stability reasons, a bentonite-cement suspension or an enhanced-cement is used to backfill the space between U-tube and its surrounding soil/rock. To ensure the efficiency of such systems, appropriate dimensioning of the GSHP system is of crucial importance. The GSHP systems work efficiently and sustainably over its lifespan only if the amount of extracted energy is equal to or close to the amount of energy which can be replenished naturally [4].
Thermal performance analysis of GSHP systems is commonly conducted by numerical modeling or in situ monitoring of the heat transfer in BHE. In the last decades, many studies have been performed to evaluate the heat transfer in BHE [1], [2], [5], [6], [7], [8]. These studies discussed the impacts of material properties and geometrical configurations on heat transfer of BHE. Wagner et al. [4] performed a numerical sensitivity study of thermal response tests. The effects of pipe position, ground temperature distribution and thermal dispersivity on effective thermal resistance of BHE were discussed separately. Results indicated that borehole resistance is strongly dependent on the shank spacing. With increased shank spacing, the borehole resistance decreases, the thermal performance of BHE is then enhanced. Esen and Inalli [9] reported an in situ thermal response test for a GSHP system in Elazig, Turkey. This work pointed that increasing depth of the analyzed BHE yields a decrease of borehole resistance. Michopoulos et al. [8] demonstrated an experimental setup of a GSHP system in Northern Greece. Three years results were accumulated to analyze the temperature distribution and thermal performance of BHE. The results showed that the thermal load of the BHE is strongly affected by ambient temperature.
On the other hand, several studies [4], [10], [11] investigated the influence of convective heat flow on heat transfer of BHE. Wang et al. [10] reported that groundwater has an obvious influence on the temperature profile in the aquifer. Due to the strong groundwater advection, the thermal performance of the BHE is enhanced. Apart from the analysis of thermal performance, economic evaluation of the investments has also been considered. Charoenvisal [12] reported an example of economic assessment of a GSHP system in Virginia, USA. Cost recovery period and saving-to-investment ratio were used for studying the investments of this GSHP system. These two parameters were found to be effective in investments assessment.
However, the previous studies focus only on either thermal performance or economic performance. In other words, they analyzed thermal performance while neglecting the economic performance, or vice versa. In either case, a deviation tends to occur in the evaluation of the project investments. Consequently, the results from these studies may be not reliable for optimal design of future GSHP systems. In this paper, a comprehensive study is performed considering both thermal and economic performance. In order to investigate thermal performance of BHE, the experiments were conducted on a vertical double U-tube BHE installed in an office building in Nuremberg, Germany. Operating conditions of the GSHP system such as ground temperature, ambient temperature, fluid flow rate, and in-outlet temperature were continuously accumulated. Based on these measurements, the impacts of borehole diameter on thermal performance are analyzed. In addition, the economic evaluation of the investments is carried out considering the investigated thermal performance. This work regarding both the thermal and economic performance of BHE will provide helpful information for cost saving and optimal design of future GSHP systems.
Section snippets
System configuration
The GSHP system, in this study, was installed in a new office building constructed in 2008. The building has 3 floors and one basement with a total area of 1530 m2, as shown in Fig. 1. It is located in Nuremberg city in Southern Germany. Basic climate parameters of this area are listed in Table 1. These parameters, combined with the characteristics of energy utilization in the building, suggest both heating and cooling operations during those intermediate months. The main functional space of the
Investment evaluation
To obtain deeper insight into the project investment, capital costs and operational saving are estimated. These two parameters are the fundamentals for evaluation of the feasibility of investment. In this work, the capital costs are calculated by multiplying prices of the apparatus with their installations costs. The operational saving is assessed based on the investigated thermal loads of the BHEs.
Temperature profile
Fig. 3 shows the typical recording of a winter day regarding the GSHP system operation. Hourly mean in-outlet fluid temperature, subterranean temperature and ambient temperature are presented. It is observed that the fluid inlet temperature is lower than that of the fluid outlet, meaning that the ground is cooled around the heat exchanger. The difference between the inlet and outlet temperature varies from ∼0 °C to ∼5 °C. This abrupt variation can be attributed to the operation of the heat pump.
Conclusions
An experimental study was conducted to determine the thermal performance of double U-tube borehole heat exchanger (BHE) installed in an office building in Nuremberg, Germany. The system operating conditions such as temperature distribution and thermal load were investigated using data from monitoring the GSHP system. Furthermore, the economic performance of the GSHP system was analyzed by the estimation of cost recovery period and Saving-to-investment ration (SIR). Main conclusions are as
Acknowledgements
This work is financed by Bayerisches Staatsministerium für Umwelt und Gesundheit. The first author sincerely thanks 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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