These-days environmental pollution is one of the main concerns among governments and energy consumption of the buildings has a significant portion in it. One of the efficient ways in supplying heating and cooling demands of the buildings is using potential energy stored in ground which is clean and sustainable. These types of systems are called ground source heat pump (GSHP) systems, which are well established in Western and European countries for space heating and cooling applications.The initial cost and efficiency of GSHP systems can be influenced by optimal design of ground heat exchangers (GHEs). One of the most important GHEs that is used these days is spiral GHEs. These types of GHEs are cheaper than others and also have some advantages. Three parameters affecting the spiral GHE’s performance are GHEs spacing, GHE major diameter and helical configuration of the pipe. In large scale GSHP applications, more than one GHE is needed, therefore determining the distance between GHEs becomes as an important issue. This article investigates the effects of distance between vertical spiral GHEs on the heat transfer ratio (HTR) as well as other important parameters. The heat transfer from/to the pipe wall of the heat exchanger to/from the ground depends on the turns number, location, material and configuration of the pipes.
Introduction

Air pollution is becoming a significant environmental concern in the most countries. Reaching solutions to environmental problems that we face today needs long term activities for sustainable development. Renewable energy sources seem to be one of the best and effective solutions.1. In recent decades, energy consumption for building sector has increased around the world. Efforts are being done to develop alternative energy sources for supplying the demand of building heating and cooling loads. One of the best alternate ways is the use of ground energy, which is clean, green and sustainable. This energy can be utilized through GSHP system, which is well established in most of the European countries for space heating applications2. GSHP systems contain two main parts, GHEs and heat pump. GHEs use underground soil as a heat sink or source. When water flows through pipes, heat is transferred from the water to the earth or from earth-to-water depending on the temperature of water relative to temperature of earth that remains nearly constant at the annual mean temperature of that place3. In some cases, the physical and thermal properties of fluid/water coming out from the pipelines is such that it can be directly supplied to the space where it is to be used, whereas under extreme weather conditions, it needs another stage of processing before becoming acceptable for supplying to the connected space.3 In large-scale GSHP applications, more than one GHE is needed. Optimizing distances between these GHEs plays an important role in GSHP performance. In addition, different parameters highly affect the efficiency of GSHP. One of the most important GHEs are spiral ones. The purpose of this article is to show the different parameters that are affecting spiral GHE performance such as distance between GHEs, pitch lengh and major diameter of GHEs. As it was discussed GSHP systems consists of two main parts, heat pump and GHEs. Here are some details about these two parts3;
Heat pump

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Heat is transferred from high-temperature media to low-temperature media without requiring any device. However, to perform the reverse process, some devices are needed. Heat pumps and refrigerators are devices that receive heat from a lower temperature reservoir and reject it to a higher temperature reservoir. Heat pump has the same cycle with a refrigerator that is commonly used in residences. The difference in between a refrigerator and a heat pump is only the purpose. Refrigerators are intended for cooling a selected space by extracting heat at lower temperature. If desired effect is to heat the space, heat is discharged at higher temperature in heat pumps4. A heat pump can either extract heat from a heat source and reject heat to air and water at the higher temperature for heating, or provide refrigeration at a lower temperature and reject condensing heat at a higher temperature for cooling. During summer, the heat extraction, or refrigeration effect, is the useful effect for cooling in a heat pump. In winter, the rejected heat provides heating in a heat pump. The heat pump uses the concepts of the vapor compression cycle to transfer heat from one source to another. Heat pumps exchange energy between the conditioned interior space and either the ground or the air5.
Ground Source Heat Pump Systems

Ground source heat pumps, often referred to as geothermal heat pumps, are recognized to be heating, cooling, and water-heating systems. They provide high levels of comfort, offer significant reductions of electrical energy use and demand, have very low, levels of maintenance requirements, and are environment friendly. A ground source heat pump is a heating and cooling system that transfers heat to or from the ground, using the ground as a heat sink in the summer and heat source in the winter. A ground source heat pump can be significantly more energy efficient than conventional air source heat pump. The heat source of a GSHP is the ground. The heat is taken from the ground by a borehole heat exchanger (there is a lot of different kind of BHEs: U-pipe, helical shaped…). The heat is then transported from the ground to the evaporator of the heat pump and supply the evaporator energy6.
Ground heat exchanger

Ground heat exchangers use underground soil as a heat sink or source. When water flows through pipes, heat is transferred from the water to the earth or from earth-to-water depending upon the temperature of water relative to temperature of earth that remains nearly constant at the annual mean temperature of that place. In some cases, the thermal condition of water coming out from the pipes is such that it can be directly supplied to the space where it is to used, whereas in extreme weather conditions, it needs another stage of processing before becoming acceptable for supplying to the connected space. Ground temperature below a certain depth remains relatively constant throughout the year because 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. Therefore ground temperature is always higher than that of the outside environment in winter and is lower in summer at a sufficient depth. The difference in temperature between the outside environment and the ground can be utilized as a preheating means in winter and pre-cooling in summer by operating a ground heat exchanger. Efficiency of a heat pump is higher than conventional natural gas or oil heating systems, a heat pump may be used in winter to extract heat from the ground and pump it into the conditioned space. In summer, the process 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. A ground source heat pump extracts heat from the ground – whose temperature will be warmer than the air in winter (and cooler than the air in summer). Therefore ground heat exchangers are more efficient than air source heat pumps, especially in the coldest weather when they are most needed. Ground heat exchangers generate very little noise and should last for many years with minimal servicing. Ground heat exchanger are the system that is simple to use and easy to maintain. In addition, since the system takes care of both heating and cooling. Geothermal energy is a form of clean energy because using it doesn’t emit any type of pollutions.3 Ground heat exchangers use underground soil as a heat sink or source. Ground temperature below a certain depth remains relatively constant throughout the year because 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. When water flows through pipes, heat is transferred from the earth to the water or from the water to the earth depending upon the temperature of water relative to temperature of earth that remains nearly constant at the annual mean temperature of that place7. Heat is transferred in the ground by two ways, convection and conduction.3
Fig.1 Spiral GHE

As it was discussed several types of GHEs such as shallow and deep ones are existed. Deep GHEs are generally U-tube GHEs with common ranges of 50-200 meter. Shallow GHEs are divided to spiral , slinky, pond/lake and snails types. Among different types of shallow GHEs spiral ones show the best performance in the same application unit area which means that more amount of heat can be extracted or injected into the ground. This also means that more space area of the building could be heated or cooled by spiral GHEs. Spiral GHEs are favorable due to their high efficiencies and low initial costs. When spiral GHEs are connected to the heat pump system and total system (GSHP) is integrated to the buildings, heating/cooling demands of that specific residential building could be supplied easily by spiral GHEs. Many researches show that when nine numbers of spiral GHEs are connected to the heat pump system, heating demands of at least 200 square meter space area of a building could be supplied. Based on different parameters of spiral GHEs this value could be changed between 200-800 square meter space area. In Fig. 1 schematic view of series of spiral GHEs are shown.3
Integration of spiral GHEs to the building
Fig.2:Integration of GSHP system and building

Fig. 2 shows integration of a GSHP system to the building. As it is shown in this figure, heat is extracted from ground by spiral GHEs. After extraction process, fluid enters a controller to control inlet and out fluid temperature of the machine. After that fluid will enter heat pump system and supply required heating/cooling demands of the building. To calculate effective amount of space area that can be heated or cooled by GSHP system different parameters are needed. These parameters are amount of extracted/injected heat from/to ground by spiral GHEs, coefficient of performance (COP) of heat pump, heating and cooling loads of the building8 Different researches show that distance between spiral GHEs are highly affecting performance of GSHP system. It means that distance between them should be optimized well. By choosing low distance between them (less that 5 meter) there will be high performance loss in system and suitable amount of heat cannot be delivered or rejected from building9.
Optimization of distance between spiral GHEs

As it is known, concept of extracting/injecting heat from/to ground is temperature different between two mediums. When there are more than one spiral GHEs embedded in ground it means that we have more thermal interactions in ground. When distance between heat exchangers is low (lower than 5 meter) thermal interactions between them is also too high. By increasing distance between them thermal interactions will be reduced and therefore we have low performance loss in system. Different researches show that at least 6/7 meter distance must be chosen between GHEs. For example when number of spiral GHEs is nine we have 15%, 18%, 22% performance loss for 7, 6 and 5 meter distance between them respectively. In ground source heat pump applications, optimization of distance between GHEs plays an important role in total performance of the system. This value also should be chosen based the availability of application area and also investors needs. In Fig. 3 thermal interactions between nine mumbers of spiral GHEs are shown. As it is demonstrated as the time passes thermal interactions between spiral GHEs are increased10.
Fig.3: Thermal interaction
The Influences of the Pitch Distance (Lp) and Major Diameter (D) of a Single Vertical spiral GHE on it’s Performance

The pitch between the turns of the spiral and GHE major radius play a crucial role in the design process. These parameters are geometric parameters that directly affect the HTR and initial cost of the installation for the system. These properties are directly related to the vertical lengths required to construct the heat exchanger as well as amount of excavation.10 It is seen that by increasing the Lp and D, HTR value of a GHE is improved. On the other hand, the total cost of embedding the GHE into ground becomes higher and higher by increasing the values of Lp and D. Therefore, it is important for designers and investors that they know the exact requirements of consumers. The improvement of HTR value with longer and bigger exchangers results in increment of installation costs, which are the main drawback of GSHPs. During the designing of GHE fields, maximum heat load of the consumer is considered, and the minimum required borehole sizes (Lp and D) are chosen. On the other hand, trying to minimize the installation cost by decreasing the sizes of Lp and D alone, causes higher number of GHE which also increases the initial cost. Therefore, initial cost should be minimized by considering the whole system. Furthermore, not only the initial cost but also the operational costs should be considered during the optimization of the system overall costs. This requires the long term predictions of HTR value of a GHE.Results show that the average HTR values linearly increase with the value of D and Lp. These linear dependencies are due to the increment of peripheral surface area and heat capacity of ground per turn.10
A Typical Example for Vertical Spiral GHE Analysis

In this part, a building with 120 m2 heating/cooling area is considered. For heating and cooling of this building, GSHP system with vertical spiral GHE is chosen. As it is stated in different references, general required heat load for a standard building is approximately 80 W/m2. Therefore total heat load of this building is 9600 W. Furthermore, COP value of a heat pump is assumed as 4.0. In this case, 7200 W heat is needed to pump from ground to building. To predict the number of GHEs that are needed for heating and cooling applications, first of all long term performance of a single vertical spiral GHE should be estimated. Again by using COMSOL and based on previous validated data, HTR value of a single vertical spiral GHE is numerically predicted. The averaged HTR value is around 870 W for 1-6 months time interval. In GHE designing procedure generally the most critical conditions are considered. The system is assumed to work 6 months non-stop in heating or cooling mode. For 6 month operation, COMSOL results show that averaged HTR of a single vertical spiral GHE is 870 W. By taking all the above arguments into consideration it is concluded that 9 vertical spiral GHE is needed for this building. For more than 5 GHEs, we assume that five GHEs graph can be used for performance loss prediction with an acceptable error. By looking relevent figures in literature, ten meter distances between GHEs are chosen. For this distance, performance loss is less than 5%. Based on consumers utilization and designers goals this amount can of course be vary. The suggested configuration is shown in Fig. 4 for most critical conditions (6 months non-stop operation). In this configuration, performance loss is predicted less than 5%.10
Fig.4: Configuration of spiral GHEs

Sometimes the application area is limited and there is no enough space for placing GHEs. In that case, more performance losses can be accepted to minimize the application area. For nine vertical spiral GHEs, the following configuration can also be used. In corners and sides we can use three GHEs performance loss graph. Figure 5 demonstrates other configuration of nine spiral GHEs that can be used.10
Fig.5: Circular configuration of spiral GHEs
Beneficial use of some environmental waste energy through spiral GHEs

Nowadays keeping our environment clean is one the most important issues. There are still many waste heat and energies which are wasted into the environment such as, waste heat of gas turbines exhaust gasses or waste gasses in biomass systems. These waste heats can be used beneficially for supplying heating demands of the buildings. One way to use this energy usefully is to store it in ground and then re-extract it through spiral GHEs for providing heating demands of the buildings. Recent researches show that spiral GHEs could be one of the best choices. First waste heat could be stored in ground by spiral GHEs and then extracted by them. Ground has ability to store this energy in ground for a long period. Also results of different researches show that amount of space area that can be heated by GSHP system is multiplied by four.11
General discussion about spiral GHEs

This short article presents the analyses of different parameters on the performance of vertical spiral GHE for GSHP systems. The most important parameters, which influence the performance of GSHPs, have been thoroughly analyzed, running long-term simulations and estimating the performance losses for each GHE configuration. The results prove that Lp and D are one of the important parameters in the design of a GSHP. Although increasing these parameters can improve the efficiency of GSHP, a larger investment is needed for installation. Therefore an optimum length should be found, which minimizes the total cost over the plant lifetime for an acceptable performance value. In the large number of GHE, one of the most essential parameters that affect the system performance is distance between GHEs. The performance losses are less than 5% if the distance between GHE is more than 5 m, 7 m and 9 m for 2, 3 and 5 GHEs configuration.

Also spiral GHEs could be one good choice for storing environmental waste energy in the ground and then re-utilize it.

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