ARTICLE IN PRESSRenewable Energy 32 (2007) 2461–2478 www.elsevier.com/locate/renene Review Ground heat exchangers—A review of systems, models and applications Georgios Florides, Soteris Kalogirouà Higher Technical Institute, P.O. Box 20423, Nicosia 2152, Cyprus Received 17 October 2006; accepted 23 December 2006 Available online 26 March 2007 Abstract The temperature at a certain depth in the ground remains nearly constant throughout the year and the ground capacitance is regarded as a passive means of heating and cooling of buildings. To exploit effectively the heat capacity of the ground, a heat-exchanger system has to be constructed. This is usually an array of buried pipes running along the length of a building, a nearby field or buried vertically into the ground. A circulating medium (water or air) is used in summer to extract heat from the hot environment of the building and dump it to the ground and vice versa in winter. A heat pump may also be coupled to the ground heat exchanger to increase its efficiency. In the literature, several calculation models are found for ground heat exchangers. The main input data are the geometrical characteristics of the system, the thermal characteristics of the ground, the thermal characteristics of the pipe and the undisturbed ground temperature during the operation of the system. During the first stages of the geothermal systems study, one-dimensional models were devised which were replaced by two-dimensional models during the 1990s and three-dimensional systems during recent years. The present models are further refined and can accommodate for any type of grid geometry that may give greater detail of the temperature variation around the pipes and in the ground. Monitoring systems have been set up to test various prototype constructions with satisfactory results. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ground heat exchangers; Ground temperature; Heat exchanger models; Ground source heat pump ÃCorresponding author. Tel.: +357 22 406466; fax: +357 22 406480. E-mail address: [email protected] (S. Kalogirou). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2006.12.014 . . . Temperature variation with depth. . 4. . Deep geothermal systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . there is a time lag between the temperature fluctuations at the surface and in the ground. . . . . . . . . . . . . . Also. . . . . a heat pump may be used in winter to extract heat from the relatively warm ground and pump it into the conditioned space. . . . . . . . . . . . . . Ground thermal behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Also. . . . . . . . . . . . The graph shows actual ground temperatures as measured in a borehole drilled for this purpose in Nicosia. The temperature variation of the ground at various depths in summer (August) and winter (January) is shown in Fig. . . . . Conclusions . . . . . . The difference in temperature between the outside air and the ground can be utilised as a preheating means in winter and pre-cooling in summer by operating a ground heat exchanger. . . . . . . . . Kalogirou / Renewable Energy 32 (2007) 2461–2478 Contents 1. . . . . because of the higher efficiency of a heat pump than conventional natural gas or oil heating systems. . Vertical U-tube ground heat exchangers . In summer. . 2462 2463 2463 2464 2467 2467 2470 2473 2476 2476 2477 3. . . . . . . . . . .1. . . . . . the process 30 Temperature (°C) 26 22 18 14 10 6 0 5 10 15 20 25 30 Depth in ground (m) 35 40 45 50 25-Jan-05 20-Aug-05 Fig. . . . . . . . . . . . . . . . . . . . . . . . . . . . at a sufficient depth. . . . Therefore. 1. . . . . . . . . . . . . . . . . . . . . As can be seen. . . . . . . . . . . . . . . . . 6. . . . 2. . . . . . . . . Florides. . Calculation models and evaluated performance of ground heat exchangers . . . . . . . . . . the temperature is nearly constant below a depth of 5 m for the year round. . . . . . . . . . . . . . . . . . . . . 2. . . Introduction . . . .2. . . . . . . . . . . . . . . . . . . . This is due to the fact that the temperature fluctuations at the surface of the ground are diminished as the depth of the ground increases because of the high thermal inertia of the soil. . . . .ARTICLE IN PRESS 2462 G. . . . . . . . . . . 1. . the ground temperature is always higher than that of the outside air in winter and is lower in summer. . . . . . . . . .3. . . . . Types of ground heat exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. . . . . . . . . . . . . . . . 5. . . . . . . . . . References . . . Miscellaneous systems . . . . Closed systems . . 2. . . . . . Open systems . . . Cyprus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. . . . . . . . 1. . . . . . . . . . . . . Introduction Measurements show that the ground temperature below a certain depth remains relatively constant throughout the year. . Other types of heat exchangers used directly for heating and cooling utilise a copper piping placed underground. Open systems In open systems. The loop of the heat exchanger is made of a material that is extraordinarily durable but allows heat to pass through efficiently. heat is transferred directly through the copper to the earth.000 units installed annually. the house heating and air conditioning load. About 80% of the units installed worldwide are domestic [1]. As refrigerant is pumped through the loop. In an open system. These types of systems are examined separately below. The length of the loop depends upon a number of factors such as the type of loop configuration used. Ground source heat pumps are receiving increasing interest in North America and Europe and the technology is now well established with over 550. Fig. . ambient air passes through tubes buried in the ground for preheating or pre-cooling and then the air is heated or cooled by a conventional air conditioning unit before entering the building (Fig. local climate and many more. the ground may be used directly to heat or cool a medium that may itself be used for space heating or cooling. with heat fuse joints. soil conditions. Florides.000 units installed worldwide and with more than 66. 2). Basic principle of ground preheating or pre-cooling of air in an open system. The fluid in the loop is water or an environmentally safe antifreeze solution. This material is usually warranted for as much as 50 years. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2463 may be reversed and the heat pump may extract heat from the conditioned space and send it out to a ground heat exchanger that warms the relatively cool ground. the ground may be used indirectly with the aid of a heat carrier medium that is circulated in a closed system. Loop manufacturers typically use high-density polyethylene which is a tough plastic. 2. 2. Types of ground heat exchangers There are two general types of ground heat exchangers: open and closed [2].ARTICLE IN PRESS G. S. Also.1. 2. the ground water of a water-bearing layer may be used as a cooling carrier medium. In most cases two wells are required. shown in Fig. Ground water heat pump. 4 indicates the horizontal type which has a number of pipes connected together either in series or in parallel. In a similar way. In this way. 3. Kalogirou / Renewable Energy 32 (2007) 2461–2478 Fig. Horizontal ground loops are the easiest to install while a building is under construction. S. A typical horizontal loop is 35–60 m long per kW of heating or cooling capacity [3]. Vertical ground heat exchangers or borehole heat exchangers.2. 3.ARTICLE IN PRESS 2464 G. some special ground heat exchangers have been developed for heat pump systems. However. Fig. the main thermal recharge is provided by the solar radiation falling on the earth surface. and a heat carrier medium is circulated within the heat exchanger. 5). Therefore. For all horizontal systems in heating-only mode. 2. new types of digging equipment allow horizontal boring and thus it is possible to retrofit such systems into existing houses with minimal disturbance of the topsoil and even allow loops to be installed under existing buildings or driveways. either in horizontal. transferring the heat from the ground to a heat pump or vice versa. In USA. one for extracting the ground water and one for injecting it back into the water-bearing layer as indicated in Fig. it is important not to cover the surface above the ground heat collector. 6. it is possible to place more pipes into shorter trenches in order to reduce the amount of land space needed [3]. Closed systems In this case heat exchangers are located underground. Fluid runs through the pipes in a closed system. This configuration is usually the most cost-effective when adequate yard space is available and trenches are easy to dig. in which the pipe is curled into a slinky shape (Fig. vertical or oblique position. Florides. brought in direct contact with the heat pump coils. These collectors are best suited for heating and cooling in places where natural temperature recharge of the ground is not vital. The trenchers have a depth of 1–2 m in the ground and usually a series of parallel plastic pipes is used. are widely used when there is a need to install sufficient heat exchange capacity under . S. 4.ARTICLE IN PRESS G. a confined surface area such as when the earth is rocky close to the surface. Horizontal-type ground heat exchangers (redrawn from Ref. In a standard borehole. or where minimum disruption of the landscape is desired. plastic pipes (polyethylene or polypropylene) are installed. ‘‘Slinky’’-type ground heat exchanger. which in typical applications is 50–150 m deep. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2465 Fig. Fig. 1). [2]). Florides. and the space between the pipe and the hole is filled with an appropriate material to ensure good contact between the pipe and the undisturbed ground and reduce the thermal resistance [3]. . This is possible because the temperature below a certain depth remains constant over the year (see Fig. 5. S.ARTICLE IN PRESS 2466 G. connected with a U-turn at the bottom. Because of the low cost of the pipe material. 6. Vertical loops are generally more expensive to install. Fig. These are classified in two basic categories as shown in Fig. 7. U-pipes. two or even three of such U-pipes are usually installed in one hole. 7: (a). Common vertical ground heat exchanger designs. Kalogirou / Renewable Energy 32 (2007) 2461–2478 Fig. Vertical ground heat exchangers. but require less piping than horizontal loops because the earth deeper down is cooler in summer and warmer in winter. Several types of borehole heat exchangers were tested and are widely used. . consisting of a pair of straight pipes. Florides. compared to the ambient air temperature. Actually the ground temperature distribution is affected by the structure and physical properties of the ground. air humidity and rainfall.. solar radiation. 8. therefore they are very expensive [3]. 9) and need to be considered when designing a heat exchanger. The exiting water is percolated through gravel in the annulus of the well in order to absorb heat. S. The temperature distribution at any depth below the earth surface remains unchanged throughout the year with the temperature increasing with depth with an average gradient of about 30 1C/km. snow. This water has a steady temperature the whole year round and is easily accessible. the ground surface cover (e. Ground thermal behaviour The use of direct or indirect earth-coupling techniques for buildings and agricultural greenhouses requires knowledge of the ground temperature profile at the surface and at various depths. etc.3.e. Concentric or coaxial pipes. Standing column well. lawn. wind. Florides. The ambient climatic conditions affect the temperature profile below the ground surface (Fig. boundary conditions) determined by air temperature. Other sources of heat are the use of water in mines and tunnels. (b). Kalogirou / Renewable Energy 32 (2007) 2461–2478 2467 Fig.ARTICLE IN PRESS G. 8. Miscellaneous systems A number of ground systems cannot be categorised either as open or as closed. 3. Standing wells are typically 15 cm in diameter and may be as deep as 500 m. joint either in a very simple way with one straight pipe inside a bigger diameter pipe or joint in complex configurations. Such a system is the standing column well shown in Fig. where water is pumped from the bottom of the well to the heat pump. bare ground..g. 2.) and the climate interaction (i. The geothermal . is calculated by multiplying the geothermal gradient by the thermal conductivity of the ground. Mihalakakou et al. like sandstone. Rocks that are rich in clay or organic material. Heat flow. Florides.. indicating that heat readily passes through them. meaning that heat passes slower through these layers. Energy flows in ground. like shale and coal. water is not flowing up or down the hole). Kalogirou / Renewable Energy 32 (2007) 2461–2478 Fig. the latent heat flux due to evaporation at the ground surface as well as the . then it is obvious that low-conductivity shale layers will have a higher geothermal gradient compared to high-conductivity sandstone layers [4]. [5] present a complete model for the prediction of the daily and annual variation of ground surface temperature. gradient deviations from the average value are. Rocks that are rich in quartz. If the heat flow is constant throughout a drill hole (i.e. The model uses a transient heat conduction differential equation and an energy balance equation at the ground surface to predict the ground surface temperature. related to the type of rocks present in each section. The energy balance equation involves the convective energy exchange between air and soil. which is a gauge of the amount of thermal energy coming out of the earth. which is a measure of the ability of a material to conduct heat. S.ARTICLE IN PRESS 2468 G. 9. in part. the solar radiation absorbed by the ground surface. have a high thermal conductivity. Each rock type has a different thermal conductivity. have low thermal conductivity. S. As is the amplitude of the temperature wave at the ground surface and w is the frequency of the temperature wave. Therefore. In order to solve Eq. (1) where Tm is the mean annual ground surface temperature. Florides. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2469 long-wave radiation. the following energy balance equation at ground surface was used as a boundary condition equation at the ground surface [7]: .ARTICLE IN PRESS G. (1). the ground surface temperature can be estimated from [6]: T sur ðtÞ ¼ T m þ As Reðeiwt Þ. qT sur . . (2) qy . ÀK ¼ CE À LR þ SR À LE. . Shallow zone extending from the depth of about 1–8 m (for dry light soils) or 20 m (for moist heavy sandy soils). 3. From the point of view of the temperature distribution. Furthermore. in which the ground temperature is very sensitive to short time changes of weather conditions. The results are compared with the corresponding results of models using Fourier analysis.6 W/m2 was transferred. from the surface region at ground depth (below about 1. in this zone the ground temperature distributions depend mainly on the seasonal cycle weather conditions. The measurements also show that during the summer period the ground temperature under the bare surface below 1 m was about 4 1C higher in comparison to the temperature of the ground covered with short grass. where the ground temperature is almost constant and close to the average annual air temperature. It was found that the short-period temperature variations reached a depth of approximately 1 m. for two differently covered ground surfaces. a heat flux of 3.. in winter. a sensitivity investigation is performed to investigate the influence of various factors involved in the energy balance equation at the ground surface on the soil temperature profile. Popiel et al. Surface zone reaching a depth of about 1 m. the temperature distributions were almost the same. a bare surface and a surface covered with short grass. LR is the long-wave radiation emitted from the ground surface. However. Therefore. Deep zone (below about 8–20 m).5 to 2 m. for the ground ‘‘cold’’ source. Poland. for the air conditioning application the surface covered with short grass is recommended.g. the recommended depth for horizontal ground heat exchangers is from 1. From July to the end of September. [8] distinguish three ground zones: 1. where the ground temperature is practically constant (and very slowly rising with depth according to the geothermal gradient). A comparison of the Buggs’s formula for the ground temperature distribution adapted to the European region of Poznan shows a good agreement with the experimental data. Usually. The investigation was carried out in Poznan. Popiel et al. CE is the convective energy exchanged between air and ground surface. SR is the solar radiation absorbed from the ground surface and LE is the latent heat flux due to evaporation.y¼0 where K is the thermal conductivity of the soil. The model is validated against 10 years of hourly measured temperatures for bare and short-grass covered soil in Athens and Dublin. e. 2.5 m). [8] present the temperature distributions measured in the ground for the period between summer 1999 and spring 2001. Temperatures were measured with thermocouples distributed in the ground at a depth from 0 to 7 m (bare surface) and from 0 to 17 m (short grass). In this model the temperature and moisture profiles of the ground are included in the equations. Mihalakakou et al. Experimental results were also obtained for a PVC horizontal pipe. They concluded that the temperature distribution is important to the performance improvement of the GSHP. The authors show the importance of including the moisture content in the soil. The results showed that the heating potential of the system during winter is significantly important. there is an increase in effectiveness when the pipe is buried in greater depths (3 m instead of 1. Bi et al. Florides. [10] used a two-dimensional cylindrical coordinate system to model a vertical double spiral coil ground heat exchanger (GHx). The underground temperature distribution of the coil was solved numerically and the results were compared to measured temperature data. mainly because of the increased mass flow rate inside the pipe. buried at a depth of 1. a higher air velocity in the pipe (checked range: 5–15 m/s) leads to a reduction of the systems heating capacity. therefore. the thermal characteristics of the ground and the thermal characteristics of the pipe together with the undisturbed ground temperature during the operation of the system or only the temperature of the pipe surface. [12] investigated the heating potential of a single ground-to-air heat exchanger as well as the potential of a multiple parallel earth tube system. Input data include the geometrical characteristics of the system. By increasing the pipe diameter from 100 to 150 mm. Mihalakakou et al. The obtained results showed that the effectiveness of the ground-to-air heat exchanger increases with an increase in the pipe length (checked range: 30–70 m). The model is solved in the TRNSYS (a modular energy system simulation program) environment and validated with good results. [9] studied eight models to predict the performance of ground-to-air heat exchangers. The model had been successfully validated against an extensive set of experimental data. An accurate numerical model was used to investigate the dynamic thermal performance of the system during the winter period in Dublin. Also. S. Kalogirou / Renewable Energy 32 (2007) 2461–2478 4. [11] present a model in which the ground surrounding the pipe and the pipe itself are described in polar co-ordinates. Six of the eight models gave very close results to the actual values with an rms error in each case of about 3. the thermal capacity of the ground is not considered and therefore the influence of different pipes on each other and the temperature profiles in the ground cannot be studied. The algorithms of the eight models were introduced into computer programs to simulate the behaviour of the ground-to-air heat exchangers. The algorithms of the studied models either calculate the conductive heat transfer from the pipe to the ground mass or calculate the convective heat transfer from the circulating air to the pipe. it was shown that the heating capacity of the system was reduced.5%. . Finally. and especially for the GHx and that the analytical and experimental results prove that the GHx design is reasonable. providing a lower air temperature at the pipe outlet. The influence of the ground surface temperature is modelled by the superposition of the algebraic solution of the undisturbed temperature field caused by the surface air temperature and the temperature field caused by the pipe.1 m and compared to the calculated values.ARTICLE IN PRESS 2470 G.2 m). This is due to a reduction in the convective heat transfer coefficient and an increase in the pipe surface. Calculation models and evaluated performance of ground heat exchangers Several calculation models for ground-coupled heat exchangers are found in the literature. In the examined models. Tzaferis et al. This ground heat exchanger was designed by the authors for a ground source heat pump (GSHP) system. Early models generally used a one-dimensional description of the pipe to derive a relation between its inlet and outlet temperature. cp the heat capacity (J/kg K). The heat transfer by moisture gradients in the soil is neglected and the heat transfer in the pipe is dominated by convection. S. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2471 Bojic et al. Also an equation describing the heat flow between the airflow in the pipe and the neighbouring soil layer is used. At the ground surface the heat flux from the ambient air to the surface is calculated from: qT ¼ hsurr ðT soil À T surr Þ. and hsurr is the convection coefficient. but actually the ground may not be homogenous. Finally. All the pipes are placed in one layer at the same depth and parallel to each other. (5) qz where Tsurr is the temperature of the surrounding air and this can be a constant value or a time-dependent function.ARTICLE IN PRESS G. Florides. [14] approach and the model was used to study the performance of a ground-coupled air heat exchanger in the Belgian climate. All equations used for the soil layers in each time step are steady-state energy equations. the various parameters that affect the behaviour of the ground heat exchanger storage system are examined. [13] developed a model in which the soil is divided into horizontal layers with uniform temperature. [14] describe a fully three-dimensional model. the influence of different layers in the ground. The heat transported to the soil by convection from the air and the solar irradiation is calculated. De Paepe and Willems [15] further refined the above Gauthier et al. thus: rc p qT ¼ 0. In this way. (4) qn where n is the unit vector normal to the surface. A constant and uniform temperature for the horizontal plane deep underground is imposed. The model considers transient and fully three-dimensional conduction heat transfer in the soil and other materials. To make optimal use of the thermal l . The heat transfer caused by moisture gradients in the ground is assumed to be negligible with respect to that caused by temperature gradients. The thermophysical properties of the ground are considered constant and temperature independent. (3) qt where r is the density (kg/m3). The results show that the influence of the pipe on the temperature of the surrounding soil is limited to a distance of twice its diameter. A simple Cartesian coordinate system is used and the round pipes are replaced with square pipes of equivalent areas. Gauthier et al. Heat transfer in the pipes is dominated by convection in the axial direction but coupled with the temperature field in the ground via the boundary condition on the pipe surface. This model is a two-dimensional model therefore the influence that pipes have on each other may not be evaluated. The model is thoroughly validated with experimental data taken from a ground heat exchanger storage system installed in a commercial-type greenhouse. l the thermal conductivity (W/m K). The governing equation for the conduction in the soil may be stated as: qT ¼ lr T . The boundary conditions for the underground lateral external surfaces of the computational domain are assumed to be adiabatic. T the temperature (K) and t the time (s). A three-dimensional unstructured finite volume model was derived and the FLUENT solver was used to obtain the numerical solutions. concrete foundations and insulation can be evaluated. together with an (avoided) air conditioning system. depth of ducts 2 m partly below the foundation slab in dry gravel. capital costs and CFC gases. preheating of fresh air acts as a saving function on energy demand to which it is inherently linked by limitation of flow rate. It is concluded that the evaluation of a GAHx depends on project-specific criteria. Each of the evaluated GAHx is . The basic equations of the model describe the mass and energy exchanges between air and tube. S. buried pipe inertial cooling.3 m around the foundation slab in moist clay. Florides.2 W/m2 K. diverse border conditions. which it cannot substitute completely. a total duct surface area of 522 m2. The second GAHx is located at Freiburg designed with a pipe diameter of 250 mm.300 m3/h and an air speed of 2. which relates to surface temperature. the mean heat transfer coefficient hmean. the main characteristic describing energy performance is the overall heat transfer coefficient h. The simulation model of the air-to-ground heat exchanger used. The third GAHx is located at Weilheim designed with a pipe diameter of 350 mm. depth of ducts 2–4 m around the foundation slab in dry rock. The model additionally accounts for frictional losses and water infiltration and flow along the tubes. Buried pipe systems may be subject to water infiltration. The study concludes that in Central Europe there is a fundamental asymmetry between heating and cooling potentials with the ground used as a seasonal energy buffer. which in this case is 5. depth of ducts 2. The thermal performance of the GAHx is calculated using four different approaches. In winter. the tubes have to be buried below a depth of 2. the overall heat transfer coefficient is 5.5 m and the length of the tube can be optimised with the calculation model to obtain an efficient heat exchanger. The first GAHx is located at Hamm designed with pipe diameters of 200 and 300 mm.0 W/m2 K. In summer. As it is mentioned. In this case. Preliminary results in this climate show that for cooling purposes excavation should be kept to a minimum. Air preheating with buried pipes is more expensive than that with fuel (about twice the cost per kWh). which can lower winter performance and enhance summer performance. a mean airflow of 7000 m3/h and an air speed of 5. a dimensionless ratio of temperature variation RT. The overall heat transfer coefficient is 3. Pfafferott [17] presents a paper dealing with the dynamic temperature behaviour and energy performance of three ground-to-air heat exchangers (GAHx) for mid European office buildings located in Germany. Hollmuller and Lachal [16] examined the winter preheating and summer cooling potential of buried pipe systems under the central European climate. accounts for sensible and latent heat transfer.6 m/s. use of symmetries or pattern repetitions for run-time economy) and is adapted to TRNSYS. a mean airflow of 1100 m3/h and an air speed of 1.2 m/s. Kalogirou / Renewable Energy 32 (2007) 2461–2478 capacity of the soil and to eliminate the influence of the outside air.6 m/s. It further allows for control of airflow direction as well as for flexible geometry (inhomogenous soils.5 W/m2 K. the temperature ratio Y and the coefficient of performance COP. a mean airflow of 10. One of the economically important parameters to deal with is the pipe depth. inertial cooling (smoothening of ambient temperature below comfort threshold) increases along with flow rate and hence becomes an energy-producing service on its own. These problems can be avoided by replacing buried pipes with a closed water underground circuit coupled to the fresh air system via a water/air heat exchanger. but also raises sanitary problems related to stagnant water. On the contrary. a total duct surface area of 1650 m2. a total duct surface area of 198 m2.ARTICLE IN PRESS 2472 G. is competitive and allows simultaneous savings on electricity. the length of the tube and the number of tubes in parallel in the heat exchanger. with a large diameter. The second GAHx supplies the highest specific energy gain based on the total surface area. And the third GAHx has the highest COP. Pipe lengths up to 100 m and pipe diameters around 250 mm are profitable. The circulating fluid is usually water or a water–antifreeze mixture. Their analysis considers the air mass flow rate. A higher effectiveness is only achievable at the cost of a large increase in the tube length or in the number of tubes. lowering the diameter of the tube raises the effectiveness but on the contrary higher flow rates reduce the effectiveness. If the GAHx aims at a high temperature ratio. but often an effectiveness of 80% is considered to be an optimum value for a ground-air heat exchanger [19]. it is mentioned that all three GAHx supply more heating and cooling energy than the primary energy they use for the fan input.ARTICLE IN PRESS G. . De Paepe and Janssens [18] used a one-dimensional analytical method to examine the influence of the design parameters of the heat exchanger on the thermo-hydraulic performance and devise an easy graphical design method which determines the characteristic dimensions of the ground-air heat exchanger in such a way that optimal thermal effectiveness is reached with acceptable pressure loss. having a small flow rate per tube and a large diameter gives the least pressure loss. Florides. The effectiveness is dictated by the design requirements and climatic conditions. The first GAHx narrows the outlet air temperature close to the undisturbed earth temperature. The borehole. a high specific surface area should be reached using more pipes. after the insertion of the U-tube. is usually backfilled with grout in order to ensure good thermal contact with the ground. This would mean that it is better to use many tubes. This allows designers to choose the ground-air heat exchanger configuration with the best performance. the ground temperature and the geometric sizing parameters which are the diameter of the tube. the choice of the characteristic dimensions is independent of the soil and climatological conditions. with the possibility of having thermally enhanced additives in order to present a thermal conductivity significantly lower than the surrounding ground. S. a small specific surface area should be reached using fewer pipes. So it is better to have several tubes of small diameter over which the flow rate is divided. As they emphasise generally. The specific pressure drop is a measure for the pressure drop needed to achieve a given thermal performance. which is in conflict with the thermal demand of a small diameter. 5. a maximal specific pressure drop can be calculated when a value for the effectiveness of the ground-air heat exchanger is chosen. a water pump circulates fluid through pipes inserted into a borehole in the ground. the inlet air temperature. In both cases a large number of tubes is beneficial. The tube length and diameter combination have to be optimised and this is achieved by a graphical method by reducing the influencing parameters and introducing the specific pressure drop. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2473 shown to be the best from a certain point of view. the desired outlet air temperature. resulting in high fan energy. If the GAHx aims at a high specific energy performance. Long tubes with a small diameter are profitable for the heat transfer but at the same time the pressure drop in the tubes is raised. On the other hand. Therefore. Vertical U-tube ground heat exchangers In a vertical U-tube ground heat exchanger. The grout is often a bentonite clay mixture. In this way. Finally. borehole grouting.10–0.07–0. and data analysis is based on the theory describing the response of an infinite line source. a standard mixture of bentonite and cement with the addition of quartz sand or Table 1 Geology and results for some thermal response tests carried out in Germany since 1999 Geology Silt and clay (Quarternary/Tertiary) Mesozoic sediments Marl (‘‘Emschermergel’’. H is the length of borehole heat exchanger (m) and k is the inclination of the curve of temperature versus logarithmic time (K/s). leff (W/m K) 1. sandstone.12 0.0 3. .8 1. include the analytical line source model [20] and the cylindrical source model [21] and several numerical models [22–25]. The tested boreholes differ from one another by the filling material which may be a standard mixture of bentonite and cement. Rb.3 2. The test data consist of one or more curves showing the fluid temperature development against elapsed time.06a Filled with thermally enhanced grout (‘‘Stu ¨ watherm’’). For a thermal response test. limestone (Mesozoic) Silt. Pahud and Matthey [27] in their paper explain how the thermal performance of a borehole heat exchanger can be assessed with a response test. basically a specified heat load is applied into the hole and the resulting temperature changes of the circulating fluid are measured. marl (Cretaceous) Sand and clay (Quarternary/Tertiary) Sand and clay (Quarternary/Tertiary) Marl. Several models for calculating the thermal properties of a ground heat exchanger are available.08a 0.11 0. Kalogirou / Renewable Energy 32 (2007) 2461–2478 A borehole heat exchanger is usually drilled to a depth between 20 and 300 m with a diameter of 10–15 cm.11–0.0 2.12 0. which are based on Fourier’s law of heat conduction. A borehole system can be composed of a large number of individual boreholes.08a 0.6 2. This value gives the temperature drop between the natural ground and the fluid in the pipes.08a 0. Q is the heat injection/extraction (J). The easiest way to evaluate the test data makes use of the line source theory and the following formula is utilised to calculate the thermal conductivity: leff ¼ Q . A second value that can be determined by a response test is the borehole thermal resistance. S. Florides. These models. The authors present a table indicating the effective thermal conductivity and the thermal borehole resistance for various grounds in Germany (Table 1). In this way.2–2.18 0. Mands and Sanner [26] carried out the thermal response test on a number of boreholes. etc.3 2. the thermal conductivity of the borehole is measured on site allowing sizing of the boreholes based upon reliable underground data. (W/m K).4 Resistance.5–2. 4pHk (6) where leff is the effective thermal conductivity. sandy (Quarternary/Tertiary) a Thermal conductivity.7–2. Cretaceous) Sand/silt.5 4. The most widely used method at this time is the line source model. The model is a simplification of the actual experiment. clayey Marl. It is also possible to calculate Rb from the dimensions and materials used.8 2. including influence of groundwater flow.ARTICLE IN PRESS 2474 G. Rb (K/(W/m)) – 0. the temperature gain in a heat pump evaporator is +2 K. it is mentioned that for a typical residential house in Switzerland. borehole thermal resistance and undisturbed soil temperature). Kalogirou / Renewable Energy 32 (2007) 2461–2478 2475 only quartz sand. which were needed for designing a ground heat exchanger for underground thermal energy storage (UTES). the geothermal properties measurement (GPM) data evaluation software method and the two-variable parameter fitting method was performed in order to calculate the thermal conductivity and borehole thermal resistance. Roth et al. The determined value of thermal conductivity was 1. USA. (b). As they mention. England and Turkey. constant power supply. Netherlands. They also concluded the following: (a). the borehole length is sized for a heat extraction rate of 50 W per metre of borehole length. Florides. (e). For typical ground conditions and a single borehole heat exchanger. Norway. A comparison of results between the conventional slope determination method. (c).3 W/K was determined applying a model based on an energy balance between the hot parts of the system and the surrounding environment. [28] describe their in situ experiment for the determination of the thermal properties of the soil (thermal conductivity. about 10 countries in the world are dealing nowadays with this type of investigation including Germany. Important aspects are reliable temperature measurements.ARTICLE IN PRESS G. a borehole heat exchanger of 100–200 m is used with a diameter of 10–15 cm. converging for test duration over 120 h was the trend of the thermal conductivity when compared to the 1.5 W/m K. Sweden.8 W/m K and the borehole thermal resistance was 0. An overall heat loss coefficient of 5. The accuracy of the evaluation depends on the care taken when performing the test. a proper determination of undisturbed underground temperature and weatherproofing the system as much as possible. Using an average estimated ground thermal conductivity of 2. the tests show that the thermal resistance can be decreased by 30% when quartz sand is used instead of bentonite and when spacers are used to keep the plastic pipes in contact with the borehole wall. Zeng et al. Large relative error for short tests. The fluctuations exhibited by the experimental curve are mainly governed by corresponding fluctuations of ambient temperature and by fluctuations of electric power supply especially during night hours.3 m K/W. Also. and the use or omission of spacers to keep the plastic pipes apart from each other and close to the borehole wall. With a common heat extraction rate of 50 W per metre of borehole length. (d). The value of the thermal conductivity was quite insensitive to the operating hours before recording but exhibited an oscillatory behaviour regarding the duration of test condition. the ambient temperature and various other necessary parameters were measured. [29] in their paper present a new quasi-three-dimensional model for vertical ground heat exchangers (GHx) that accounts for the fluid axial convective heat transfer and thermal ‘‘short-circuiting’’ among U-tube legs. Canada.8 W/m K expected value. The application of the classical slope determination and/or two-variable parameter fitting can be used as a fast and reliable tool for data evaluation. Their test was carried out for over 9 days (24 June to 3 July 2003) while inlet and outlet fluid temperatures of the borehole heat exchanger. S. depending on the energy demand and the ground conditions. Analytical expressions of the borehole . These exchangers usually supply more heating and cooling energy than the primary energy they use for power input for the fan or pump. Published characteristics allow one to estimate the gained geothermal heat-energy flux as a function of the difference of temperatures of extracted as well as injected water at different volume fluxes of the geothermal water. various types of ground heat exchangers are described. In addition. A computational model is presented which estimates the temperature of the geothermal water extracted to the earth’s surface as well as the temperature of the water injected into a deposit level. One. As it can be concluded from the studies presented in this paper at a sufficient depth. This difference in temperature can be utilised as a preheating means in winter and pre-cooling in summer by operating a ground heat exchanger. many factors such as the capital cost of borehole fields and circulating pump energy consumption must also be taken into consideration when merits and weaknesses of different borehole configurations are to be assessed. In general. Kalogirou / Renewable Energy 32 (2007) 2461–2478 resistance have been derived for different configurations of single and double U-tube boreholes and analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. Calculations show that the double U-tube boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30–90%. the two-layer systems and two-hole systems are more advantageous than the one-hole system. Usually the recommended . discussions in this paper are limited to the thermal resistance inside the boreholes with U-tubes even though the thermal conduction outside the boreholes often plays an even more important role in the GHx heat transfer process. [30] studied the case of deep geothermal heat plants. Finally. borehole depths usually range from 40 to 200 m with diameters of 75–150 mm. Simulation models may be used successfully for sizing and predicting the thermal performance of ground heat exchangers. Conclusions In this paper. Calculations on typical GHx boreholes indicate that the U-tube shank spacing and the thermal conductivity of the grout are the prevailing factors in all the configurations considered in determining the borehole thermal resistance. made during crude-oil and or natural-gas exploration. 7. These plants operate with one or two-hole systems. As they mention. the hole is adapted to locate in it a vertical exchanger with a double-pipe heat exchanger in which the geothermal water is extracted via the inside pipe.ARTICLE IN PRESS 2476 G. The double U-tubes in parallel configuration provide better thermal performance than those in series. It is mentioned that the high expenditure incurred in drilling holes deters one from using this method in gaining thermal energy. Deep geothermal systems Kujawa et al. reduces the capital cost. Analyses have shown that the single U-tube boreholes yield considerably higher borehole resistance than double U-tube boreholes. the ground temperature is always higher than that of the outside air in winter and is lower in summer. Ground heat exchangers are used to exploit effectively the heat capacity of the soil and commonly they are coupled to heat pumps for increasing their efficiency. In one-hole systems. two and three-dimensional models can be found in the literature that simulate the heat transfer process. Florides. 6. S. The predicted characteristics do not take into account specific working conditions of the systems. The one-hole injection system or the use of existing single holes. Santamouris M. [13] Bojic M. Sanner B.pdfS. there is an increase in effectiveness when the pipe is buried in greater depths. /http://www. Performance degradation can occur if adjacent boreholes are too close. Claridge DE. Argiriou A. Jaeger JC. /http://www.60(3/4):181–90. Energy Buildings 1992. technical and economic evaluation of air-to-earth heat exchanger coupled to a building. Exp Therm Fluid Sci 2001. [2] Mands E. a separation distance of 30 cm between pipes is recommended and trenches should be at least 2 m apart. In order to minimise interference between multiple pipes. Energy Buildings 1996. no oversized or undersized equipment are installed. Analytical model to predict annual soil surface temperature variation. [4] Kelley S. Ground heat exchanger temperature distribution analysis and experimental verification. the effectiveness of ground-to-air heat exchangers increases with an increase in the pipe length.22:183–9. 2005.ubeg. Lewis O. Papadakis G. mainly because of the increased mass flow rate inside the pipe. [3] Geothermal Heat Pump Consortium. Trifunovic N.5 to 2 m. For a typical residential house in Switzerland. Analysis of the accuracy and sensitivity of eight models to predict the performance of earth-to-air heat exchangers. S.8(4):254–60. a borehole heat exchanger of 100–200 m is required with a diameter of 10–15 cm. [6] Carlslaw HS. and a separation distance of 5 m is usually considered adequate.geoexchange. Shallow geothermal energy. Asimakopoulos D. Renew Energy World 2005. Modelling the thermal performance of earth-to-air heat exchangers.htmlS. Appl Therm Eng 2002. On the heating potential of buried pipes techniques-application in Ireland. 2005. [9] Tzaferis A. therefore.117:91–9. A borehole system can be composed of a large number of individual boreholes. Temperatures in the Kiowa drill hole. Wojtkowiak J. For typical ground conditions and a single borehole heat exchanger. there is lower investment cost and the equipment function with the required output and performance. Energy 1997. Wu C.org/denverbasin/r_temp. . Conduction of heat in solids. 2nd ed. On the application of the energy balance equation to predict ground temperature profiles. Sol Energy 1994. Asimakopoulos D. A vertical borehole heat exchanger is usually drilled to a depth of 20–300 m with a diameter of 10–15 cm. [12] Mihalakakou G. 2005. Denver basin project—The Denver Museum of Nature Science (DMNS). Biernacka B. Measurements of temperature distribution in ground. Int J Energy Eng 1995.18:35–43. Lewis J. Kyritsis S. [11] Mihalakakou G. References [1] Lund J. the borehole length is sized for a heat extraction rate of 50 W/m of borehole length. Lopez-Alonso C.dmnh. [8] Popiel C. Kreider JF. [7] Krarti M.de/Downloads/ShallowGeothEngl. The line source model is an easy method of evaluating the characteristics of the borehole and does not need expensive equipment. Santamouris M.53(3):301–5. a higher air velocity in the pipe leads to a reduction of the systems heating capacity. Also. [10] Bi Y. Santamouris M.ARTICLE IN PRESS G. Liparakis D. Chen L. Ground heat worldwide utilization of geothermal energy. Oxford: Oxford Science Publishers. /http://www.22(12):1151–8. Kalogirou / Renewable Energy 32 (2007) 2461–2478 2477 depth for horizontal ground heat exchangers is from 1. The heating capacity of the system was reduced by increasing the pipe diameter. Calculations on typical GHx boreholes indicate that the U-tube shank spacing and the thermal conductivity of the grout are the prevailing factors in all the configurations considered in determining the borehole thermal resistance. [5] Mihalakakou G. Numerical simulation.orgS. Generally. Florides. A typical horizontal loop is 35–60 m long per kW of heating or cooling capacity. By evaluating the borehole characteristics. depending on the energy demand and the ground conditions. Sol Energy 1997. Finally. Santamouris M.24:19–25.25:301–9. 1980. 1997. Early design guidance for low energy cooling technologies.D. Stachel A. Determining effective soil formation thermal properties from field data using a parameter estimation technique. [23] Muraya NK. p. Georgiev A.D. Ph. ASHRAE Trans 1999. 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