Vertical-borehole ground-coupled heat pumps: A review of models and systems
Introduction
Ground source heat pump (GSHP) systems use the ground as a heat source/sink to provide space heating and cooling as well as domestic hot water. The GSHP technology can offer higher energy efficiency for air-conditioning compared to conventional air conditioning (A/C) systems because the underground environment provides lower temperature for cooling and higher temperature for heating and experiences less temperature fluctuation than ambient air temperature change.
The first known record of the concept of using the ground as heat source for a heat pump was found in a Swiss patent issued in 1912 [1]. Thus, the research associated with the GSHP systems has been undertaken for nearly a century. The first surge of interest in the GSHP technology began in both North America and Europe after World War Two and lasted until the early 1950s when gas and oil became widely used as heating fuels. At that time, the basic analytical theory for the heat conduction of the GSHP system was proposed by Ingersoll and Plass [2], which served as a basis for development of some of the later design programs.
The next period of intense activity on the GSHPs started in North America and Europe in 1970s after the first oil crisis, with an emphasis on experimental investigation. During this time period, the research was focused on the development of the vertical-borehole system due to the advantage of less land area requirement for borehole installation. In the ensuing two decades, considerable efforts were made to establish the installation standard and develop design methods [3], [4], [5], [6].
To date, the GSHP systems have been widely used in both residential and commercial buildings. It is estimated that the GSHP system installations have grown continuously on a global basis with the range from 10% to 30% annually in recent years [7].
The GSHPs comprise a wide variety of systems that may use ground water, ground, or surface water as heat sources or sinks. These systems have been basically grouped into three categories by ASHRAE [8], i.e. (1) ground water heat pump (GWHP) systems, (2) surface water heat pump (SWHP) systems and (3) ground-coupled heat pump (GCHP) systems. The schematics of these different systems are shown in Fig. 1. The GWHP system, which utilizes ground water as heat source or heat sink, has some marked advantages including low initial cost and minimal requirement for ground surface area over other GSHP systems [9]. However, a number of factors seriously restrict the wide application of the GWHP systems, such as the limited availability of ground water and the high maintenance cost due to fouling corrosion in pipelines and equipment. In addition, many legal issues have arisen over ground water withdrawal and re-injection in some regions, which also restrict the GWHP applications to a large extent. In a SWHP system, heat rejection/extraction is accomplished by the circulating working fluid through high-density polyethylene (HDPE) pipes positioned at an adequate depth within a lake, pond, reservoir, or other suitable open channels. Natural convection becomes the primary role in the heat exchangers of the SWHP system rather than heat conduction in the heat transfer process in a GCHP system, which tends to have higher heat exchange capability than a GCHP system. The major disadvantage of the system is that the surface water temperature is more affected by weather condition, especially in winter.
In a GCHP system, heat is extracted from or rejected to the ground via a closed loop, i.e. ground heat exchanger (GHE), through which pure water or antifreeze fluid circulates. The GHEs commonly used in the GCHP systems typically consist of HDPE pipes which are installed in either vertical boreholes (called vertical GHE) or horizontal trenches (horizontal GHE). In the horizontal GCHP systems, the GHEs typically consist of a series of parallel pipe arrangements laid out in dug trenches approximately 1–2 m below the ground surface. A major disadvantage is that the horizontal systems are more affected by ambient air temperature fluctuations because of their proximity to the ground surface. Another disadvantage is that the installation of the horizontal systems needs much more ground area than vertical systems.
In the vertical GCHP systems, the GHE configurations may include one, tens, or even hundreds of boreholes, each containing one or double U-tubes through which heat exchange fluid is circulated. Typical U-tubes have a diameter in the range of 19–38 mm and each borehole is normally 20–200 m deep with a diameter ranging from 100 mm to 200 mm. The borehole annulus is generally backfilled with some special material (named as grout) that can prevent contamination of ground water. A typical borehole with a single U-tube is illustrated in Fig. 2.
The worldwide growing energy shortage and increasing energy demand have recently driven a great incentive of the GSHP applications in air conditioning field. Among the various GSHP systems, the vertical GCHP system has attracted the greatest interest in research field and practical engineering as well, owing to its advantages of less land area requirement and wide range of applicability. During the past few decades, a considerable number of studies have been carried out to investigate the development and applications of the GCHP systems with various types of GHEs and addressed their individual advantages and disadvantages in detail. Furthermore, various hybrid GCHP systems which couple the conventional GCHP equipment with a supplemental heat rejection/generation device have been recently developed in order to improve the economics of the GCHP systems for unbalanced climates. Several literature reviews on the GCHP technology have been reported [7], [10], [11], [12]. This paper mainly presents a detailed literature review of the vertical-borehole GCHP systems, primarily related to the typical heat transfer models of the GHEs and the representative design/simulation programs as well as advanced engineering applications of hybrid GCHP systems.
Section snippets
Simulation models of vertical GHEs
The major difference between the GCHP system and a conventional A/C system is the use of a special heat exchanger (i.e. GHE) instead of a cooling tower. The construction costs of the GHEs are critical for the economical competitiveness of a GCHP system for a heating or an A/C system. On the other hand, the GHE size also plays a decisive role on the operation performance of the GCHP system. Thus, it is of great importance to work out sophisticated and validated tools by which the thermal
Computer programs for GCHP design/simulation
The reliability and stability of a GHE design mainly depends on its ability to reject or extract heat to/from ground over a long-term period and avoidance of excessive heat buildup or heat loss in the ground. A good design program for the GCHPs should have high computational efficiency, which allows the calculation of the transient effects over long time periods. Actually, there are numerous uncertain factors which affect to some extent the final sizing of a GHE, such as the employed
Hybrid GCHP systems
It is well known that the GCHP systems can achieve better energy performance in specific locations where building heating and cooling loads are well balanced all the year round because of the long-term transient heat transfer in the GHEs. However, most buildings in warm-climate or cold-climate areas have unbalanced loads, dominated by either cooling loads or heating loads. When the GCHP systems are used in the cooling-dominated buildings in warm climates, more heat will be rejected to the
Conclusions and recommendations for future work
During the past few decades, a large number of GCHP systems have been widely applied in various buildings around the world due to the attractive advantages of high efficiency and environmental friendliness. Most typical heat transfer simulation models currently available for vertical GHEs have been described in detail in this work, which include two separate regions: one is the heat transfer process outside the borehole and the other is the heat transfer inside the borehole. A comparison of
Acknowledgements
The work described in this paper is supported by a grant from the Sun Hung Kai Properties Group (Project No. ZZ1T) and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5332/08E).
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