Evaluation of thermal efficiency in different types of horizontal ground heat exchangers
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)
- et al.
Greenhouse gas emission savings of ground source heat pump systems in Europe: a review
Renew. Sustain. Energy Rev.
(2012) Effect of multiple ground layers on thermal response test analysis and ground-source heat pump simulation
Appl. Energy
(2011)- et al.
A review on thermal response test of ground-coupled heat pump systems
Renew. Sustain. Energy Rev.
(2014) - et al.
Performance study of a ground heat exchanger based on the multipole theory heat transfer model
Energy Build.
(2013) - et al.
Geometric arrangement and operation mode adjustment in low-enthalpy geothermal borehole fields for heating
Energy
(2013) - et al.
Evaluation of heat exchange rate of GHE in geothermal heat pump systems
Renew. Energy
(2009) - et al.
Comparison of the thermal performance of double U-pipe borehole heat exchangers measured in situ
Energy Build.
(2001) - et al.
An experimental and numerical approach to derive ground thermal conductivity in spiral coil type ground heat exchanger
J. Energy Inst.
(2015) - et al.
Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests
Renew. Energy
(2012) - et al.
Heating performance characteristics of the ground source heat pump system with energy-piles and energy-slabs
Energy
(2015)
Thermal performance and ground temperature of vertical pile-foundation heat exchanger: a case study
Appl. Therm. Eng.
Field performance of an energy pile system for space heating
Energy Build.
Heat transfer of horizontal parallel pipe ground heat exchanger and experimental verification
Appl. Therm. Eng.
In-field performance analysis of ground source cooling system with horizontal ground heat exchanger in Tunisia
Energy
Horizontal ground coupled heat pump: thermal-economic modeling and optimization
Energy Convers. Manag.
Numerical investigation of horizontal ground coupled heat exchanger
Energy Procedia
Numerical simulation and sensitivity study of double-layer slinky-coil horizontal ground heat exchangers
Geothermics
Cited by (0)
- 1
Current address: Department of Civil and Environmental Engineering, KAIST, 291, Gwahakro, Yuseong-gu, Daejeon 305-701, South Korea.