Geothermal Heat Pumps

Transcription

1 Geothermal Heat Pumps Ping Cui 1,2,YiMan 1,2, and Zhaohong Fang 1,3 1 Shandong Jianzhu University, Jinan, P. R. China 2 Shandong Key Laboratory of Building Energy-saving Technique, Jinan, P. R. China 3 Shandong Zhongrui New Energy Technology Co. Ltd., Jinan, P. R. China 1 INTRODUCTION 1.1 Categories of geothermal heat pump Geothermal heat pump (GHP) technologies utilize the underground environment as a heat source/sink to provide space cooling and heating. From a thermodynamic perspective, the operating cost of the GHP systems is much lower than that of the air source heat pumps because the underground environment experiences less temperature fluctuation compared to the ambient air temperature swing. Besides the advantage of high efficiency, GHP systems offer the following attractive benefits over conventional heating/cooling systems. 1. Low maintenance cost and low noise Eliminate the need for a cooling tower or any other outdoor equipment, which can significantly reduce the maintenance cost and noise. 2. Environmental friendliness Produce less carbon dioxide and other pollutants than the conventional alternatives, thus reducing global warming and other environmental impacts. 3. Building esthetics and less required mechanical room Without boilers and cooling towers, the building esthetics is improved by fewer external penetrations of the building This article was published in the Handbook of Clean Energy Systems in 2015 by John Wiley & Sons, Ltd. envelope, and the system requires less mechanical room, allowing for more profitable space use. As one of the fastest growing renewable energy applications, GHP technology has seen several revivals during the over 100 year s development. 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 (Ball, Fischer, and Hodgett, 1983). However, the first surge of interest in the GHP technology began in both North America and Europe only after World War II 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 GHP system was proposed by Ingersoll and Plass (1948), which served as a basis for development of some of the later design programs. The next period of intense activity on GHPs started in North America and Europe in the 1970s after the first oil crisis, with an emphasis on experimental investigation. During this period, the research was focused on the development of the GHP system with vertical boreholes because of 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 (IGSHPA, 1988; Kavanaugh and Rafferty, 1997; Bose, Parker, and McQuiston, 1985; Eskilson, 1987). To date, GHP systems have been widely used in both residential and commercial buildings because of the worldwide growing energy shortage. It is estimated that the GHP system installations have grown continuously on a global basis with the range from 10% to 30% annually in recent years (Yang et al., 2010). Geothermal heat pumps that may use various underground sources have been basically grouped into three categories, that is, (i) groundwater heat pump (GWHP) systems, (ii)

2 2 Geothermal Energy Pond Groundwater heat pump Surface water heat pump a b c Ground-coupled heat pump (a) Vertical GHE; (b) Horizontal straight GHE; (c) Horizontal slinky GHE Figure 1. Schematics of different geothermal heat pumps. surface water heat pump (SWHP) systems, and (iii) groundcoupled heat pump (GCHP) systems, as shown in Figure 1. The GWHP system, also referred to as open-loop systems, is the original type of the GHP systems, which utilizes groundwater as heat source or heat sink. Groundwater is supplied directly or indirectly to the heat pump units from a well or wells equipped with submersible pumps, and it can be discharged on either the subsurface through another well or the surface (ASHRAE, 2003). In a SWHP system, heat rejection/extraction is accomplished by 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 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. In a GCHP system, heat is extracted from or rejected to the ground via a closed loop through which pure water or an antifreeze solution circulates. The ground heat exchangers (GHEs) used in the closed-loop systems typically consist of pipes installed in vertical boreholes or horizontal trenches, which are called vertical or horizontal GHE systems, respectively. 1.2 Determining a GHP for heating and cooling applications Determining what type of GHP system is most suitable for a given building in a particular location is actually a complex design process that should comprehensively consider many factors including the geology and hydrogeology condition, the building configuration and loads, the capital and operating costs, and the local regulations for groundwater withdrawal. Among these factors, the geology and hydrogeology condition is the primary determining factor for the system type. Therefore, a general site survey should be first conducted to secure the basic site characteristics that may include the presence or absence of groundwater/surface water, depth of water table, underground temperature, soil/rock type, and other geology and hydrogeology information. The site information helps determine what type of GHP system is geologically feasible for the building. For example, available surface water in a moderate climate may guarantee an SWHP system with a high operating efficiency. The GWHP system could be preferred in the locations with sufficient water resources if the groundwater withdrawal for space heating/cooling applications is legally permitted by local government. After the GHP type has been decided, some specific parameters, such as the underground thermal properties, water quality, and water well static and pumping levels, are necessary to design the system. The methods for obtaining these parameters are described in other parts of this article. 1.3 Objectives of this article The primary objectives of this article are to describe the basic concept and various configurations of the three categories of GHP systems, to address the current status of the GHP applications and to provide an advanced review of the three categories of GSPs in terms of the recent research and developments. Some design methods or guidelines will be suggested for engineering applications.

3 Geothermal Heat Pumps 3 2 GROUNDWATER HEAT PUMP 2.1 Basic concept of GWHP system As a branch of GHP systems, the GWHP, which is also called a simple open-loop system, pumps groundwater from developed wells or depleted oil wells and delivers it to a heat pump or an intermediate heat exchanger to serve as a heat source or sink. Then, the groundwater will be discharged into the injection wells (Nam and Ooka, 2010). The groundwater is a kind of thermally stable heat exchange medium that can provide heat source with high temperature in winter and heat sink with low temperature in summer, resulting in excellent coefficients of the GWHP system performance. A literature review shows that groundwater has been selected as a medium for space heating and cooling for more than 60 years. The main form to utilize the thermal energy of groundwater is the GWHP system, which employs large quantities of groundwater as heat source/sink using relatively inexpensive wells and small land areas. The use of the GWHP system for the air conditioning of the Equitable Building built in 1948 is the pioneering achievement in the Western world (Hatten and Morisson, 1995; Hatten, 1992). The moderate and relatively constant temperature of groundwater guarantees the GWHP system a higher energy efficiency in both commercial and residential buildings, causing the system to have been the most widely used type of GHP until the emerge of GCHP system (ASHRAE, 2003). The GWHP systems have some marked advantages and limitations over other GHP systems, as given in Table 1. Table 1. Advantages and disadvantages of GWHP. Advantages Operation cost is much lower as the groundwater temperature keeps nearly constant throughout the year. The capital cost and land area requirement to dig groundwater wells is much lower than the GCHP system. Properly designed groundwater loops with correctly developed water wells require little maintenance. When groundwater is injected back into the aquifer, net water use is zero. Disadvantages Groundwater availability is limited for many locations and may be restricted by local environmental regulations. Energy consumption of water pumps may be high when the system is poorly designed or draws groundwater from a deep aquifer. Fouling, corrosion, and blockage may occur when groundwater is used directly in heat pumps and water quality is poor. The precious groundwater will be consumed unless the total extracted groundwater can be properly injected. 2.2 Application types of GWHP system The GWHP system can be classified by two criterions. According to the first criterion, the GWHP system can be divided into unitary plant and central plant system. Another criterion to classify the GWHP system is considered from the viewpoint of the usage of the groundwater side, that is, direct system and indirect system (ASHRAE, 2003). For the central plant system, only one or a small number of largecapacity heat pumps are utilized to supply hot and chilled water to a water distribution system inside a building, as shown in Figure 2. In the unitary plant GWHP system, a large number of small heat pumps are distributed throughout the building, as shown in Figure 3. Compared with the central type, the unitary approach is more common and tends to be more energy-efficient. For the direct GWHP system, the groundwater is pumped directly to the heat pump without an intermediate heat exchanger, as shown in Figure 2. This type is not recommended except small installations because of the possible blockage and corrosion occurred inside heat pump units. Thus, the most widely used applications for commercial/industrial-scale buildings are designed as the indirect system to isolate groundwater from the building system with a heat exchanger, as shown in Figure 3. The water-to-water heat exchanger is commonly used to connect the groundwater loop and a closed water loop, which is Production well Fan coil unit Heat pump Injection well Figure 2. Schematic diagram of central plant GWHP system (direct system).

4 4 Geothermal Energy Groundwater production well Production well Indoor terminal unit Heat pump Heat pump Heat pump Heat exchanger Injection well Figure 3. Schematic diagram of unitary plant GWHP system (indirect system). connected to heat pumps located in the building. It should be noticed that groundwater below 15 or above 40 C can be circulated directly through coils, which may be buried inside floor, ceiling, or wall to form the radiant cooling or heating systems. This type of the direct thermal utilization of groundwater can save a large amount of energy that would otherwise have to be generated by mechanical refrigeration. 2.3 Production/injection wells of GWHP system As shown in Figure 3, a typical GWHP system may consist of four primary components: (i) groundwater wells (production and injection), (ii) well pumps, (iii) groundwater heat exchanger, and (iv) water-to-water heat pump unit. It should be noticed that the production/injection wells are the critical component that is very distinctive from other GHP systems, whereas the other three components are quite similar to those in conventional systems. The operating efficiency of the production wells can influence the system performance to a large extent. The groundwater is extracted from the aquifer, which is a geologic unit that is capable of yielding groundwater to a well in sufficient quantities to be of practical use (UOP, 1975). Aquifers can exist in areas where water is present in conjunction with pore spaces in the subsurface materials sufficient to allow the water to move laterally. There are four configurations of groundwater production, which are the tube well, the large opening well, the infiltration galleries, and the spring chamber. When the local aquifer with thickness larger than 5 m and buried at least 15 m below the ground surface, groundwater needs to be collected with the tube well. For the local aquifer with thinner thickness buried at most 15 m below the ground surface, the large opening well should be selected. If the local aquifer with thinner thickness and shallower bury depth less than 5 m, the infiltration galleries is the optimal configuration of the groundwater production well. Unusually, the spring chamber is utilized to collect the spring water, which is an especial form of groundwater. Generally, the tube well is the most frequent choice for the groundwater production well of GWHP projects. The component of the tube well is shown in Figure 4a, which mainly includes the well wall pipe, and the screen perforated casing or open hole, the bowl assembly, the grout seal, and the gravel-filled layer. The groundwater in the aquifer can permeate the gravel-filled layer and flow into the well wall pipe. In most situations, a single tube well cannot satisfy the groundwater amount requirement of the GWHP system. The siphon tube well group comprised by series of single tube wells as shown in Figure 4b is the common configuration of the groundwater production well for the practical GWHP projects. The well-casing diameter depends on the diameter of the pump (bowl assembly) necessary to produce the required flow rate Groundwater injection well In order to protect groundwater resource from destruction, most GWHP systems are required to install an injection well in addition to the production well to dispose the groundwater after it has passed through the heat exchanger. On the other hand, groundwater injection stabilizes the aquifer by reducing or eliminating long-term drawdown, and helps to ensure long-term productivity of the groundwater production well. Generally, construction of the groundwater injection well differs from the groundwater production well primarily in the recommended screen velocity and well sealing design. The screen velocity of the injection well is usually selected to be m/s, or 1/2 that of production wells (ASHRAE, 2003). Most injection well walls are subjected to positive injection

5 Geothermal Heat Pumps 5 Ground level Ground level Pump Pump Grout seal Well pipe wall Siphon pipe Siphon pipe Bowl assembly Gravel filled layer Aquifer Tube well Collection well Tube well (a) (b) Figure 4. (a, b) Groundwater tube well and siphon tube well group. pressure, so they should be fully cased and sealed from the top of the injection zone to the surface. A variation on the injection well configuration is the standing column well, which is a tradeoff between groundwater systems and ground coupled systems. For the standing column GWHP system, the majority of the return groundwater is reinjected back into the production well, so the capital cost of injection well and the amount of surface discharge water can be cut down. 2.4 Design strategy of GWHP system In order to obtain accurate and optimal design strategy, the insitu groundwater well testing should be completed initially to collect the actual flow test data and water chemical analysis information. Groundwater well tests can be divided into three different types: rig, short-term, and long-term (Stiger, Renner, and Culver, 1989). The key and distinguishing part of the GWHP system design is to project the groundwater loop. Energy consumptions of the groundwater pumps and the heat pumps of the GWHP system must be well balanced for proper design. The effect of increasing the groundwater flow rates on the system energy consumption lies in two inverse aspects: firstly, the energy consumption of well pumps will increase; on the contrary, the energy consumption of heat pumps will decrease as more favorable average temperatures are produced with the increase of the groundwater flow rate. In order to minimize the energy consumption of the whole GWHP system, the key design strategy is to identify the optimal groundwater flow rate to obtain the maximum system performance with respect to heat pumps and well pumps power requirements. Then the optimization design process involves evaluating the performance of the heat pumps and well pumps over a range of groundwater flow rates. Corresponding data needed to make this calculation include well performance, which is generally derived from well pump test results, and heat pump performance, which is available from the manufacturer. Generally, an optimum groundwater flow rate is less than the building loop flow rate. According to the studies by Mustafa Omer (2008), the recommended flow rate of the groundwater typically lies in the range of and l per second per system cooling capacity (L/(s kw)). 3 SURFACE WATER HEAT PUMP 3.1 Basic concept of SWHP system Owing to its big heat capacity, surface water bodies such as river, lake, and ocean can be the excellent heat source and sink if they are properly utilized. The SWHP system is a subset of the GHP systems, which utilizes the thermal energy of the surface water to provide space heating and cooling. A pioneer study on the feasibility of using shallow ponds for dissipation of building heat was carried out by Cantrell and Wepfer in north Ohio (Cantrell and Wepfer, 1984). After that, a number of researchers focused on the design considerations and its practical operation performance of the SWHP

6 6 Geothermal Energy Table 2. Advantages and disadvantages of SWHP. Advantages Capital cost and maintenance requirement is lower than the GWHP system and GCHP system. Energy consumption of circulation pump is low for the closed-loop system because there is no elevation head from the water surface to heat pumps. The closed-loop system can be used for heating provision in cold climate because the antifreeze can be added into the circulation fluid loop. Fouling, corrosion, and blockage can be reduced in closed-loop system. Disadvantages Operation cost is higher than GWHP system as the surface water possesses less stable thermal characteristics. A large body of water is required, and the submerged pipes may restrict the use as well as ecology of the water body. Coils submerged in the surface water body are subject to damage especially for those submerged in public waters. Fouling may occur on the outside of the surface water coil, particularly in murky lakes or where coils are located on or near the water body bottom. system for heating and cooling in cold and hot climates separately (Kavanaugh and Pezent, 1990; Aittomäki, 2003; Büyükalaca, Ekinci, and Yilmaz, 2003; Tim and Joyce, 2002). As for its configurations, the SWHP system can be either open-loop systems similar to GWHP system or closed-loop systems similar to GCHP system. However, the thermal characteristics of surface water are quite different from those of the groundwater or ground-coupled systems. The major disadvantage of the system is that the surface water temperature is more affected by weather conditions, especially in winter. Table 2 summarizes the advantages and disadvantages of SWHP systems. 3.2 Application types of SWHP system In the closed-loop system, heat is rejected into or extracted from the surface water by the fluid (usually a water/antifreeze mixture) circulating inside the coil submerged inside the surface water bodies. The esthetics of closed-loop system is high, and it requires adequate surface area and depth of surface water to function adequately in response to heating or cooling requirements under local weather conditions. In the open-loop system, water is pumped from the surface water through a heat exchanger and returned to the surface water some distance from the point where it is extracted. The pump can be located either slightly above or submerged below the surface water level. Water surface Fan coil unit Heat pump Pipe network Water body bottom Water body Figure 5. Schematic diagram of closed-loop SWHP system. As the open-loop system circulates the surface water directly inside the heat pump unit, and no antifreeze can be added into the circulation water, the temperature of surface water must remain above 5 C to prevent freezing for heating provision in winter. Therefore, the application of the openloop SWHP system is restricted to warmer climates. Usually, the open-loop SWHP system tends to be smaller with only a few heat pump units. It should be noticed that there is often enough thermal stratification in surface water bodies deep than 12 m throughout the year that direct cooling or precooling is possible. Large-scale cooling-only systems have been deployed successfully in some locations, including Cornell University and the city of Toronto (Cornell University, 2006; Enwave). As shown in Figure 5, a closed-loop SWHP system consists of a heat pump unit connected to a pipe network submerged in a surface water body. The pump circulates water or a water/antifreeze solution through the waterto-refrigerant heat exchanger in the heat pump and the submerged piping loop. The recommended pipe material is thermally fused HDPE. All connections must be either thermally socket-fused or butt-fused. These HDPE pipes should have ultraviolet (UV) radiation protection, especially when near the water surface. Polyvinyl chloride (PVC) pipe and plastic pipe with band-clamped joints are not recommended. 3.3 Design strategy of SWHP system The distinguishing part of the SWHP system design is to project the surface water circulation loop. Energy consumptions of the circulation pumps and the heat pumps of the

7 Geothermal Heat Pumps 7 SWHP system must be well balanced for proper design. For the open-loop SWHP system, the design project is similar to that of the GWHP system. Its key design strategy is to identify the optimal surface water flow rate to obtain the maximum system performance with respect to heat pumps and circulation pumps power consumptions. For the closedloop SWHP system, the design of the surface water circulation loop is the key part. The pipe networks of closed-loop SWHP systems are similar to those used in GCHP systems. Both a largediameter header between the heat pump and lake coil and several parallel pipe loops submerged inside water body are required. Loops are spread out to limit thermal interference, hot spots, and cold pockets. One of the practical compensation methods for thermal interference is to make bundled coils longer than the spread coils. Generally, the submerged piping system is installed in loops attached to concrete anchors. Typical installations recommended by ASHRAE (2003) require around 26 m of heat-transfer pipes per system cooling capacity (26 m/kw) and around 79 m 2 of water surface area per system cooling capacity (79 m 2 /kw). In order to ensure the good convective heat transfer between pipes and their surrounding water, the concrete anchors are utilized to hold the pipes from movement and keep the pipes cm above the floor of the surface water body. In order to maintain adequate thermal mass in times of extended drought or other low water conditions, it is also recommended that the pipes should be at least m below the water surface. Rivers are not widely used in SWHP system because they are subject to drought and flooding, both of which may damage the system. 4.1 Vertical borehole ground-coupled heat pump Basic concept of vertical system In vertical GCHP systems, the GHE configurations may include one, tens, or even hundreds of boreholes, each containing one or double U-tubes through which the heat exchange fluid is circulated. Typical U-tubes have a diameter in the range of mm and each borehole is normally m deep with a diameter ranging from 100 to 200 mm. The borehole annulus is generally backfilled with some special material (named as grout) that can prevent the contamination of groundwater. A typical borehole with one or double U-tubes is illustrated in Figure 6. The main advantage of the vertical GCHPs is that they require smaller land areas. Besides, the system can be installed at any location where drilling or earth trenching is feasible. Therefore, the vertical GCHP system has been recognized as the most widely used application among all the GHP systems Heat transfer models The main objective of the GHE thermal analysis is to determine the temperature of the heat carrier fluid, which is circulated in the U-tubes and the heat pump, under certain operating conditions. A design goal is then to control the temperature rise/drop of the ground and the circulating fluid within acceptable limits over the system lifespan. There are roughly three categories of approaches in dealing with the thermal analysis and design of the GHEs. 4 GROUND-COUPLED HEAT PUMP (GCHP) Among the various GHP systems, the 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 GHE configurations 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. U-tube U-tube Grout Grout (a) Double U-tube (b) Single U-tube Figure 6. (a, b) Schematic of a grouted borehole.

8 8 Geothermal Energy Empirical or semi-empirical formulations are recommended in textbooks and monographs for GHE design purposes (Bose, Parker, and McQuiston, 1985; Kavanaugh and Rafferty, 1997). These approaches are relatively simple, and may be manipulated easily by design engineers. However, they do not reveal in detail the impacts of complicated factors on the GHE performance. The second kind of approaches involves numerical simulation of the heat transfer in the GHEs (Mei and Baxter, 1986; Yavuzturk and Spitler, 1999). The third method is the analytical approach, which was first presented by Eskilson (1987) and further improved by Zeng, Diao, and Fang (2002). The numerical models using polar or cylindrical grids may be computationally inefficient because of a large number of complex grids. Besides, they can hardly be incorporated directly into a design and energy analysis program, unless the simulated data are precomputed and stored in programs as a massive database with some parameters. The analytical models make a number of assumptions and simplifications in order to solve the complicated mathematical algorithms; therefore, the accuracy of analytical results will be slightly reduced but required computation time is much less. Another advantage is that the straightforward algorithm deduced from analytical models can be readily integrated into a design/simulation program. Therefore, only the typical analytical models that are widely used in GHE design are presented in this section. Actually the heat transfer process in a GHE involves a number of uncertain factors, such as the ground thermal properties and the groundwater and building loads, over a long lifespan of several or even tens of years. In this case, the heat transfer process is rather complicated and must be treated, on the whole, as a transient one. In view of the complication of this problem and its long time scale, the heat transfer process may usually be analyzed in two separated regions. One is the solid soil/rock outside the borehole, where the heat conduction must be treated as a transient process. With the knowledge of the temperature response in the ground, the temperature on the borehole wall can then be determined for any instant on specified operational conditions. Another sector often segregated for analysis is the region inside the borehole, including the grout, the U- tube pipes and the circulating fluid inside the pipes. This region is sometimes analyzed as being quasi-steady-state and sometimes analyzed as being transient. The analyses on the two spatial regions are interlinked on the borehole wall Heat conduction outside borehole. The earliest approach to calculating the thermal transport around a heat exchange pipe in the ground is the Kelvin line-source theory, that is, the infinite line source (Ingersoll et al., 1950). The cylindrical source solution for a constant heat transfer rate was first developed by Carslaw and Jaeger (1946). Both the one-dimensional model of the Kelvin s theory and the cylindrical source model neglect the axial heat flow along the borehole depth; therefore they are inadequate for the longterm operation of the GCHP systems. A major progress was made by Eskilson (1987) to account for the finite length of the borehole. Based on the Eskilson s model, an analytical solution to the finite line source has been developed by a research group, which considers the influences of the finite length of the borehole and the ground surface as a boundary (Zeng, Diao, and Fang, 2002). This analytical model approximates the borehole with the U-tube as a finite line source with radial heat flow. The computed results from the analytical solution were compared with the data from numerical solutions in references (Eskilson, 1987; Zeng, Diao, and Fang, 2002), and they agreed with each other perfectly when aτ r 2 b 5. Temperature rises that occur at any time τ on the wall of the borehole can then be calculated in the following manner (Zeng, Diao, and Fang, 2002): t b t 0 = q l 4kπ 0 erfc H ( ) r b2 + (0.5H h) erfc 2 2 aτ rb2 +(0.5H h) 2 ) ( r b2 + (0.5H + h) 2 2 aτ rb2 +(0.5H + h) 2 dh (1) where t 0 is the initial temperature of the soil, that is, the annual mean temperature of the soil; k and a denote the thermal conductivity and thermal diffusivity of the soil, respectively; H and r b the borehole length and the radius, respectively; and q l the heating rate per length of the line source Heat transfer inside the borehole. The thermal resistance inside the borehole, which is primarily determined by the thermal properties of the grouting materials and the arrangement of flow channels of the borehole, has a significant impact on the GHE performance. The main objective of this analysis is to determine the entering and leaving temperatures of the circulating fluid in the borehole according to the borehole wall temperature, its heat flow, and the thermal resistance. A few models with varying degrees of complexity have been established to describe the heat transfer inside the GHE boreholes. A simplified one-dimensional model has been recommended for GHE design, which considers the

9 Geothermal Heat Pumps 9 U-tube as a single equivalent pipe (Gu and O Neal, 1998). Hellstrom (1991) derived the analytical two-dimensional solutions of the thermal resistances among pipes in the cross-section perpendicular to the borehole axis, which is superior to empirical expressions and one-dimensional model. On the basis of the two-dimensional model aforementioned, a quasi-three-dimensional model was proposed by Zeng, Diao, and Fang (2003), which considers the fluid temperature variation along the borehole depth. Being minor in the order, the conductive heat flow in the grout and ground in axial direction, however, is still neglected so as to keep the model concise and analytically manageable. The energy equilibrium equations for a single U-tube can be written for up-flow and down-flow of the circulating fluid: Mc dt f1 dz Mc dt f2 dz = (t f1 t b) R Δ 1 = (t f2 t b ) R Δ 2 + (t f1 t f2 ) R Δ 12 + (t f2 t f1 ) R Δ 12 (0 z H) (2) Two conditions are necessary to complete the solution: } z = 0, t f1 = t f (3) z = H, t f1 = t f2 where R Δ 1 and RΔ are the relative thermal resistances between 2 the circulating fluids and the borehole wall, respectively, and R Δ the resistance between the pipes. The detailed expressions can be found in reference (Zeng, Diao, and Fang, 2003). 12 M and c are the mass flow rate and specific heat of the circulating fluid. The general solution of this problem is derived by Laplace transformation, which is slightly complicated in form. At the instance of the symmetric placement of the U-tube inside the borehole, the temperature profiles in the two pipes were illustrated by Diao, Zeng, and Fang (2004). For the purpose of practical applications an alternative parameter ε =(t f t f ) (t f t b) is derived from the temperature profiles, which is named as the heat transfer efficiency of the borehole. It should be noticed that t and t are f f the entering/exiting fluid temperatures to /from the U-tube. From the derived temperature profile the more accurate heat conduction resistance between the fluid inside the U-tube and the borehole wall can be calculated by R b = H Mc ( 1 ε 1 2) The authors validated that the quasi-3-d model was more accurate than the other current models and recommended it for design and thermal analysis of GHEs. A summary of the characteristics of the numerical and analytical models of the GHEs reviewed is given in Table 3 (Yang et al., 2010). Combined the heat transfer models outside and inside the boreholes, the temperatures of the circulating fluid to/from the heat pump can be determined. Finally, the modeling procedure uses spatial superimposition for multiple boreholes and sequential temporal superimposition to determine the arbitrary heating or cooling loads of the systems, as proposed by Eskilson (1987). This approach can be easily incorporated into computer programs for thermal analysis and sizing of the GHEs while providing better insight into influences of various factors on the GHE performance (Yu et al., 2002; Cui, Yang, and Fang, 2007). (4) Table 3. Comparisons of the current models of vertical GHEs. Model Method Thermal Interference Between Bore Holes Outside borehole Kelvin s line source Infinite line source Yes No Cylindrical source Infinite cylindrical source Yes No Eskilion s model Combination of numerical Yes Yes and analytical methods Finite line-source solution Analytical method Yes Yes Short time-step model Numerical Yes Yes Inside borehole Model Method Thermal Interference between U-tube Pipes One-dimensional model No No (equivalent pipe) Two-dimensional model Yes No Quasi-three-dimensional model Yes Yes Boundary Effects Heat Flux along Depth

10 10 Geothermal Energy Design methods Building and heat pump models. Building load characteristic including peak loads and annual hourly loads is another important determining factor for the GCHP system design. While the conventional air conditioning systems can be adequately designed only according to a peak heating load and cooling load at a peak day, the GCHP systems require consideration of a whole year, at a minimum, and the GHE needs an annual simulation. As the heat transfer of GHEs is assumed to be transient across the lifespan of a GCHP system, the building loads can be calculated on an hourly, daily, or monthly basis according to the design program. Therefore, an annual heating and cooling load profile must be pre-analyzed in the design and decision process, which can result in an accurate and reliable vertical GHE system. The heat pump unit is a key component to connect the vertical GHE loop and building loop. It has been shown that the performance of a heat pump unit is a function of the entering fluid temperature (EFT) of the water that is transported to the water-loop heat pump from the GHE side. The coefficient of performance (COP) for the heating mode can be correlated to the EFT as follows: COP = a + b EFT + c EFT 2 (5) The energy efficiency ratio (EER) for the cooling mode is derived as: In-situ thermal response test. Thermal properties of the underground soil/rock are important parameters for designing GHE system. In-situ test combined with parameters estimation algorithm is an accepted method to determine the ground thermal properties. The in-situ test system with measuring apparatus is illustrated in Figure 7. Figure 8 shows the inner configuration of measuring apparatus, which includes an electrical heater, a circulating pump, two thermocouples, a flow meter, a data logger, and data transition equipment. The measuring apparatus is connected to the buried loop of the GHE at worksite. To reduce heat loss, all exposed pipes should be coated with insulation materials. During tests, being heated by the electrical heater and driven by the circulating pump, water is circulated in the buried loop and releases heat to the ambient grout and soil. The U-tube inlet and outlet water temperatures and flow rate are measured and transmitted to a data processor. The thermal response test should be performed for about h. All Measuring apparatus Ground Borehole & grout EER = d + e EFT + f EFT 2 (6) In these expressions, the coefficients (a, b, c, d, e, f) can be obtained by fitting the performance data that are provided by heat pump manufacturers. By using the definition of the cooling EER, the required compressor power consumption for the cooling mode, which is rejected to the ground together with the building cooling loads, can be obtained. The cooling load for the GHE Q gc or the heat that is rejected to the ground is given as follows: Figure 7. Schematics of in-situ thermal response test. To ground loop Thermometer Electrical heater Q gc = Q bc (1 EER + 1) (7) Circulating pump where Q bc is the cooling capacity of the heat pump, which is approximately equal to the building cooling load. Similarly, the heating load of the GHE Q gh, or the heat that is extracted from the ground, is obtained from the heating COP: Q gh = Q bh (1 1 COP) (8) From ground loop Thermometer Flow meter where Q bh denotes the heating capacity of the heat pump. Figure 8. Components of the measuring apparatus.

11 Geothermal Heat Pumps 11 Table 4. Thermal properties of selected soils, rocks, and bore grouts/fills. Dry Density, Conductivity, Diffusivity, Ib/ft 3 Btu/h ft F ft 2 /day Soils Heavy clay (15% water) Heavy clay (5% water) Light clay (15% water) Light clay (5% water) Heavy sand (15% water) Heavy sand (5% water) Light sand (15% water) Light sand (5% water) Rocks Granite Limestone Sand tone Wet shale Dry shale Source: Reproduced with permission from Kavanaugh and Rafferty, ASHRAE. the data should be collected at least once every 10 min (Yu et al., 2004). Based on the measured data, soil conductivity k s, borehole heat resistance R o, and volumetric specific heat capacity ρ s c s can be found by inverse heat transfer analysis. Table 4 lists the thermal properties for some typical soils/rock Simplified Design Method. The International Ground-Source Heat Pump Association (IGSHPA) is one of the earliest groups that are involved in the development of GHE design methods (Bose, Parker, and McQuiston, 1985). The IGSHPA modeling procedure is based on the Kelvin s line-source theory with a number of simplifying assumptions. It can only estimate the GHE length for the coldest and the hottest month of a year using the following two simple formulas. The required length for heating is: ( ) Q COP 1 bh (R COP p + R s F h ) L h = (9) T s,m T min and for cooling, L c = ( ) Q EER+1 bc (R EER p + R s F c ) (10) T max T s,m where R s is the soil resistance of a single vertical heat exchanger obtained by the Kelvin s line-source theory; R p the thermal resistance of the U-tube, which is assumed to be an equivalent diameter pipe; F the run fraction; T s,m the mean soil temperature; and T min and T max the design heat pump minimum and maximum entering fluid temperatures, respectively. The required bore length is the larger of the two lengths L c,andl h found from Equations (9) and (10). The efficiency benefits of an oversized bore length could be used to compensate for the higher capital cost. If the designer expects to install the smaller length to reduce the capital cost, a supplemental heating or cooling source should be installed to compensate for the undersized coil. Obviously, the simplified method does not account for the transient effects of the long-term operation and the variations of building loads, which may cause a significant deviation from practical conditions Design and simulation programs for GHEs. As mentioned earlier, the heat transfer process in a GHE involves a large number of factors. It is necessary to further develop an accurate, reliable, and convenient program for GHE design and simulation. In the past decade, a number of GHE models have been developed and they have been combined, directly or indirectly, with models of the building, heat pumps, and other components in various modeling environments such as TRNSYS, EnergyPlus, equest, HVACSIM+, and Geostar. The GHE model used in TRNSYS (Hellström, 1989) is called the Duct Ground Heat Storage model, originally intended for underground thermal storage systems. The model uses numerical solutions for the global heat transfer between the storage volume and the far-field, and for the local problem of the heat transfer around the boreholes. An analytical method is employed to solve the steady-flux problem around the nearest pipe. The three models implemented in HVACSIM + (Xu and Spitler, 2006), EnergyPlus (Fisher et al., 2006) and equest (Liu, 2008) have a common heritage, which are based on extensions of Eskilson s model (1987). A software package named GeoStar, which is based on the quasi-3-d model and the finite line source model, has been developed and spread for the design and simulation of the GHEs mainly in China (Fang, Diao, and Cui, 2002). This program is able to size GHEs to meet a user-specified minimum and maximum heat pump EFTs for a given set of design conditions, such as building loads, ground thermal properties, borehole configuration, and heat pump operating characteristics. Another function of the program is to simulate the system performance and predict the GHE heat transfer rates for an existing GCHP project. The flow chart of the computing procedure for the model implementation is described in Figure 9. The design process is actually a simulation-based process by means of the trial-and-error method. When completing the calculation, the program will produce a detailed report to describe the whole design process and the results.

12 12 Geothermal Energy Begin Input heat pump data and building loads Design Simulate Set the max and min temp of EFT Input the GHE size Assume the GHE size Cal resistance of borehole Adjust GHE size Cal the resistance of borehole Cal the predicted EFT, ExFT Cal the predicted EFT, ExFT Cal the heat transfer rate of GHE and power consumption of HP No <ε EFT set EFT Cal Yes Output End Figure 9. The flowchart of the GeoStar program. 4.2 Horizontal ground-coupled heat pumps Basic concept of horizontal system The horizontal ground heat exchanger (HGHE) usually consists of straight or spiral/slinky coiled loops, which are buried in a trench at a depth of approximately m. An antifreeze solution is recommended to be employed in heat-dominated regions because the ground temperature may fluctuate as much as 10 C at a depth of 1.5 m. For the straight pipe configuration, multiple pipes are usually placed in different depths in a single trench because this installation can reduce the land area needed for horizontal system, as shown in Figure 10. The pipes can be connected in parallel or in series in trenches, as shown in Figure 11. The most common straight pipe application is the (a) Two pipes per trench (b) Four pipes per trench (c) Six pipes per trench Figure 10. (a c) Typical configurations with straight pipes.

13 Geothermal Heat Pumps 13 Figure 11. Parallel and series horizontal GHEs. Table 5. Advantages and disadvantages of horizontal GHE. Advantages Loop installation is less expensive compared to the drilling cost No potential for aquifer contamination because of the shallow depth of the trench The effect from unbalanced annual loads can be ignored because more heat is transferred through the ground surface The construction of trenches is convenient Disadvantages Large land area is needed, not feasible for most urban buildings Heat transfer efficiency is more affected by the ambient temperature fluctuations Pipe buried relatively near the surface is more susceptible to being cut during excavations for other utilities An antifreeze solution must be used in most heating dominated regions (a) Figure 12. (a) Overlapping slinky loops and (b) vertical spiral loops. two-pipe arrangement in parallel. Rarely are more than two layers of pipes used in a single trench because of the extra time needed for the partial backfilling (ASHRAE, 2003). The parallel loops are recommended to be used compared to the series loops because of the lower pumping power consumption. Compared to the series loops, the parallel loops use pipes of smaller diameter, and thus require smaller volumes of antifreeze (if needed) though they may need slightly more pipes. The overlapping slinky loop configurations have also been used with some success because of the advantages of less land area requirement and higher pipe installation density compared to conventional straight HGHEs, as shown in Figure 12a. As for the overlapping slinky loops, special attention should be paid to backfilling process to guarantee soil fills all the gaps formed by the overlapping pipe. Otherwise, additional air thermal resistance may be produced in the gaps between the soil and the pipes, and thus reduce the heat transfer performance. Recently, a novel configuration of vertical spiral loops has attracted great interest in research field because the thermal interference between the vertical coils is much insignificant when compared to the overlapping slinky coils (Congedo1, Colangelo1, and Starace, 2012), as shown in Figure 12b. However, more installation cost and time may be required for the vertical spiral coils than the horizontal slinky coils. Table 5 summarizes the advantages and disadvantages of a horizontal GHE system when compared to a vertical GHE (from ASHRAE 2003 Handbook HAVC Application). In summary, the HGHE offers a cost-effective alternative for (b) GCHP systems in some specific locations where the land area is sufficient for the pipe installations, such as the residential buildings in suburbs or buildings near large playground or golf course Thermal response test for horizontal GHE It should be noticed that the ground thermal properties and the ground temperature are important parameters for any ground loop design. As for horizontal GHE systems, the soil thermal properties can be obtained either by in-situ thermal response test or by lab test for the sample soil excavated from the site Design methods The design objective of horizontal applications is to have enough buried pipe length within the available land area to serve the space heating and cooling. Compared to the advanced heat transfer models for the vertical GHE, the horizontal GHE system is still in its early stage of numerical and experimental investigations. Mei first proposed a numerical model suitable for horizontal GHEs with straight pipes in 1986 (Mei, 1986). In recent years, research into the horizontal GHE systems has been carried out on thermal performance, thermal interference, and optimal geometric design methods for different operation conditions and loop arrangements. Little work has been carried out to analytically model horizontal systems for multiyear hourly energy calculations, where interaction with the aboveground environment is important (Spitler, 2005). The deficiency of an accurate design procedure can be attributed to the inherent difficulty of the thermal analysis of the horizontal systems as the temperature fluctuations around a year

14 14 Geothermal Energy Table 6. Land area needed for straight pipes per ton (ft 2 /ton = m 2 /kw). Numbers of Buried Straight Pipe North South Two pipe loops per trench Four pipe loops per trench Six pipe loops per trench in the shallow ground and on the boundary (the ground surface) cannot be neglected as in the cases of the vertical borehole GHEs. The rule of thumb approximations have been in vogue for the horizontal system design. Rules of thumb can serve well for specific localities where soil and weather conditions are fairly uniform because design specifications are based on the experience with related installations. However, some systems designed by this method may suffer from the inability of the rule of thumb designer to properly assess the effect of varied design parameters, such as burial depth, pipe spacing, and different ground surface conditions. In addition to the rule-of-thumb method, some numerical models with different complexity have been developed for the design and performance prediction of horizontal GHEs in engineering applications (Demir et al., 2009; Wu et al., 2010). The detailed design guidelines for residential horizontal GHE installations can be found in OSU (1988). The Commercial/Institutional Ground-Source Heat Pump Engineering Manual published by ASHARE has recommended some land area indexes required for per ton for the straight pipe loop design according to the different climate characteristics in north and south area in America (Table 6). However, the values provided in the table are suitable for the preliminary design and can be only used in the regions of America. More accurate and detailed design method should be consulted to the relative professionals or be completed by using design/simulation programs. 4.3 Ground-coupled heat pumps with pile GHEs Basic concept of pile GHE A conventional GHP with vertical or horizontal GHEs requires a large plot of land and high installation cost, which has significantly hindered the wide applications of the GHP technology. Recently a so-called pile ground heat exchanger (PGHE), which utilizes the building foundation piles as part of the GHE, becomes promising in the GHP industry. Besides supporting the building, the piles can act as heat exchangers. Each pile carries a closed-loop pipe connected to a heat pump on the ground for heating, cooling, and hot water supply as well. The pile grout material, mainly concrete, provides good tight contact between the buried pipes and the piles, and between the piles and the surrounding soil, which can significantly reduce the thermal contact resistance, and hence improve the heat transfer efficiency. The most competitive advantage of such a system is the considerable reduction of the cost and the plot of land for the borehole field (Morino and Oka, 1994). In the PGHE technology, pipes may be buried in concrete piles in configurations of U-tubes or spiral coils, as shown in Figure 13 (Cui et al., 2011) Heat transfer models A literature review has shown that most of existing studies of pile GHE were based on either experiments or numerical simulations (Morino and Oka, 1994; Pahud, Fromentin, and Hadorn, 1996; Pahud, Fromentin, and Hubbuch, 1999; Laloui, Nuth, and Vulliet, 2006; Hamada et al., 2007; Sekine et al., 2007). Besides, pipes buried in concrete piles are in configurations of U-tubes in most of such applications. In these U-tube configurations, the effective heat transfer area in a certain pile is limited, and air choking may occur in the turning tips of the tubes connected in series. In order to overcome these drawbacks, a novel configuration of the foundation pile GHE with a spiral coil has been proposed (Man et al., 2010a). The distinct advantage of (a) Single U-tube (b) W-tube (c) Double U-tube (d) Triple U-tube (e) Spiral coil Figure 13. (a e) Typical configurations for pile GHE.