Elsevier

Energy and Buildings

Volume 105, 15 October 2015, Pages 100-105
Energy and Buildings

Evaluation of thermal efficiency in different types of horizontal ground heat exchangers

https://doi.org/10.1016/j.enbuild.2015.07.054Get rights and content

Highlights

  • We performed indoor TRT with horizontal slinky, spiral-coil and U-type GHEs installed in a steel box.

  • We evaluated heat exchange rates from TRT results.

  • Cost analysis with TRT results was conducted in order to evaluate optimal thermal efficiency of each type horizontal GHEs.

Abstract

The utilization of geothermal energy is constantly increasing for economic and environmental advantages that this brings. Use of horizontal ground-heat exchangers (GHEs) can reduce installation cost and compromise between efficiency and cost. Among many kinds of horizontal GHEs, slinky and spiral-coil-type GHEs show higher thermal efficiency. This paper presents the results of experiments on the heat exchange rates of horizontal slinky, spiral-coil and U-type GHEs installed in a steel box (5 m × 1 m × 1 m). A commercial dry sand was used to fill the steel box, and thermal response tests (TRTs) were conducted for 30 h to evaluate heat-exchange rates according to various GHE-types. The U-type GHE showed the highest heat exchange rate per pipe length, about two and two and half times higher thermal efficiency than that for the horizontal slinky and spiral-coil-type GHEs, respectively. Furthermore, the heat exchange rates per pipe length with a relatively long pitch interval (pitch/diameter = 1) were 100–150% higher than those with a relatively short pitch interval (pitch/diameter = 0.2), in both spiral-coil and horizontal slinky-type GHEs. A cost-efficiency analysis was also performed, and it revealed that the U-type GHE was most economical under conditions of providing equivalent thermal performance.

Introduction

Among various renewable energy resources, geothermal energy has been regarded as the most efficient for space heating and cooling [1], [2], [3], [4], [5]. Geothermal energy has great potential as a directly usable type of energy, especially in connection with ground-source heat pump (GSHP) systems. Hence, GSHP systems combined with various types of ground-heat exchangers (GHEs) have been widely used since the early 20th century [6], [7], [8].

The main elements of a GSHP system are the geothermal heat pump and a GHE. The GHE extracts heat from, or injects it into a circulation fluid (e.g., water or anti-freeze solution) flowing through a heat exchanger installed in the ground. Since the ground provides a relatively uniform temperature year-round, the circulation fluid is able to release heat to the ground in summer and absorb heat from it in winter. The GHE is an important element that determines the performance and initial installation cost for the entire system. The most widely used types involve 150–200 m-deep vertical, closed loops. Considering their high initial cost of construction, there have been many studies [9], [10], [11], [12] aimed at obtaining higher thermal efficiency and lower construction cost of closed-loop, vertical, ground-heat exchangers. Recently, a closed-loop vertical-type GSHP system with an energy-pile foundation was used, in which the GHEs were embedded in cast-in-place grout piles [13], [14], [15], [16].

Although there has been substantial research covering closed-loop vertical-type GHEs, there has been little about closed-loop horizontal-type GHEs (Fig. 1). Furthermore, there is only one commercial design program which is called GLD (ground loop design) for the horizontal-type GHEs in contrast with many design program for the vertical-type GHEs [17], [18]. Even so, the use of horizontal GHEs can reduce installation cost and minimize the compromise between increase in efficiency and cost [19], [20], [21]. Horizontal GHEs are usually installed in a trench approximately 1.5–3 m deep, and their thermal efficiency is affected by pipe configuration, type of pipe, trench depth and ground thermal properties [22], [23], [24], [25]. Among them, Congedo et al. [23] analyzed the thermal efficiency of different types of horizontal GHEs using numerical analysis method. Their calculation suggested the thermal superiority of spiral-coil-type GHE in comparison with line and slinky type GHEs. Li et al. [26] considered thermal performance of spiral-coil-type GHE under the existence of the groundwater flow effect. However, there are a few researches for thermal efficiency evaluation among different kinds of horizontal GHEs with experimental results, and a few researches for relation between cost analysis and thermal efficiency results.

Therefore, this paper presents the results from an experimental study by comparing the heat exchange rates of horizontal slinky, spiral-coil and U-type GHEs installed in a steel box. In situ TRTs (thermal response tests) were conducted for these three kinds of horizontal GHEs so as to evaluate heat exchange rate. In addition to the experimental approach to calculate the heat exchange rate, a cost-efficiency analysis considering actual whole construction procedure using horizontal ground heat exchangers was conducted in order to evaluate optimal thermal efficiency of each type GHEs and suggested optimal horizontal GHE type.

Section snippets

Mockup of steel box

Equipment was installed in order to measure the heat exchange rate of each GHE. The setup included a heater, pump, flow meter, water tank and mockup steel box. The set-up was multi-functional; it was able to measure heat exchange and ground thermal conductivity because it was equipped with controllers for both temperature and heater. Soils were compacted to a certain density within the steel box (5 m × 1 m × 1 m) and the GHEs were installed. The steel box was insulated with double layers of 10 mm

Experimental results

The TRTs were conducted for 30 h continuously to measure the heat exchange rates for the five different GHE cases. The temperature of the circulating water reached a near steady state after 20 h in the TRT. The initial temperature of the sand was 17–18 °C, and the average flow rate of the circulating water was 4–5.55 lpm. Fig. 4, Fig. 5 show the heat-exchange rate per pipe length, and the average fluid-temperature distribution of short-pitch-interval (pitch/diameter = 0.2) horizontal slinky and

Conclusion

In this paper, in order to measure the heat-exchange rate of horizontal GHEs per type, several (i.e., spiral-coil, horizontal slinky, and U) types of GHE were installed inside a steel-box mockup (5 m × 1 m × 1 m). TRTs were conducted for 30 h continuously, and the heat-exchange rates were calculated. Based on these heat-exchange rates, a cost-efficiency analysis was conducted. In consideration of the GHE type and the pitch interval, heat-exchange rates were measured for five combinations, and the

Acknowledgements

This research was supported by a basic research project (2013R1A2A2A01067898) of the National Research Foundation of Korea and by a Regional Development Research Program (15RDRP-B076564-02) of the Ministry of Land, Infrastructure and Transport of the Korean Government.

References (35)

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Current address: Department of Civil and Environmental Engineering, KAIST, 291, Gwahakro, Yuseong-gu, Daejeon 305-701, South Korea.

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