Chapter 1

INTRODUCTIONGround Source Heat Pump (GSHP) systems are the effective technology for heating as well as cooling purposes. Using the ground as a thermal energy source and/or a heat sink for heat pumps has long been recognized to have a number of advantages over the similar use of ambient air. Ground temperatures at about 3-ft depth or lower are much less variable than ambient air temperatures. Further, soil or rock at these depths is usually warmer than ambient air during the coldest winter months and cooler than ambient air during the summer months. This fact leads directly to cooler condensation temperatures (during cooling operation) and warmer evaporating temperatures (during heating) for a heat pump with consequent improved energy efficiency. It also results in increased heating and cooling capacity at extreme temperatures, thereby reducing or eliminating the need for auxiliary heat.

Heat pump systems that make use of the ground in this way are called ground source or geothermal heat pumps (GHPs). GSHPs are also known by a variety of other names: geoexchange heat pumps, ground coupled heat pumps, earth-coupled heat pumps, ground-source systems, ground-water source heat pumps, well water heat pumps, solar energy heat pumps, and a few other variations. Some names are used to describe more accurately the specific application; however, most are the result of marketing efforts and the need to associate (or disassociate) the heat pump systems from other systems.1.1 Background of Ground Source Heat Pump systemsMaintaining a comfortable temperature inside a building can require a significant amount of energy. Separate heating and cooling systems are often used to maintain the desired air temperature, and the energy required to operate these systems generally comes from electricity, fossil fuels, or biomass. Considering that 46% of suns energy is absorbed by the earth, another option is to use this abundant energy to heat and cool a building. In contrast to many other sources of heating and cooling energy which need to be transported over long distances, Earth Energy is available on-site, and in massive quantities.

Because the ground transports heat slowly and has a high heat storage capacity, its temperature changes slowlyon the order of months or even years, depending on the depth of the measurement. As a consequence of this low thermal conductivity, the soil can transfer some heat from the cooling season to the heating season, heat absorbed by the earth during the summer effectively gets used in the winter.Becuase of yearly, continuous cycle between the air and the soil temperature results in a thermal energy potential that can be harnessed to help heat or cool a building.Another thermal characteristic of the ground is that a few meters of surface soil insulate the earth and groundwater below, minimizing the amplitude of the variation in soil temperature in comparison with the temperature in the air above the ground. This thermal resistivity fluctuations further helps in shifting the heating or cooling load to the season where it is needed. The earth is warmer than the ambient air in the winter and cooler than the ambient air in the summer.This warm earth and groundwater below the surface provides a free renewable source of energy that can easily provide enough energy year-round to heat and cool an average suburban residential home, for example. A Ground-Source Heat Pump (GSHP) transforms this Earth Energy into useful energy to heat and cool buildings. It provides low temperature heat by extracting it from the ground or a body of water and provides cooling by reversing this process. Its principal application is space heating and cooling, though many also supply hot water, such as for domestic use. It can even be used to maintain the integrity of building foundations in permafrost conditions, by keeping them frozen through the summer.

A heat pump is used to concentrate or upgrade this free heat energy from the ground before distributing it in a building through conventional ducts. It operates much as a refrigerator or conventional air conditioning system in that it relies on an external source of energy - typically electricity - to concentrate the heat and shift the temperature. Typically, each kilowatt (kW) of electricity used to operate a GSHP system draws more than 3 kW of renewable energy from the ground. Heat pumps typically range from 3.5 to 35 kW in cooling capacity (about 1 to 10 refrigeration tons), and a single unit is generally sufficient for a house or a small commercial building. For larger commercial, institutional or industrial buildings, multiple heat pumps units will often be employed [20].

Since a GSHP system does not directly create any combustion products and because it draws additional free energy from the ground, it can actually produce more energy than it uses. Because of this, GSHP efficiencies routinely average 200 to 500% over a season. GSHP systems are more efficient than air-source heat pumps, which exchange heat with the outside air, due to the stable, moderate temperature of the ground. They are also more efficient than conventional heating and air-conditioning technologies, and typically have lower maintenance costs. They require less space, especially when a liquid building loop replaces voluminous air ducts, and are not prone to vandalism like conventional rooftop units. Peak electricity consumption during cooling season is lower than with conventional air-conditioning, so utility demand charges may be also reduced. For the above reasons, significant energy savings can be achieved through the use of GSHPs in place of conventional air-conditioning systems and air-source heat pumps. Reductions in energy consumption of 30% to 70% in the heating mode and 20% to 50% in the cooling mode can be obtained [20]. Energy savings are even higher when compared with combustion or electrical resistance heating systems. This potential for significant energy savings has led to the use of GSHPs in a variety of applications.1.2 History of Ground Source Heat Pump systemsThe concept of a heat pump has been known since the 1850s, but it was 1940 before Robert Webber was credited with using the technology for heating a home with heat stored in the ground. The first commercial demonstration of a GSHP was in the Commonwealth Building in Portland, Oregon, in 1946. Heat pumps experienced a rise in popularity during the Arab oil embargo of the 1970s. The market then leveled off before expanding again in recent years. Annual worldwide growth rates for GSHP installations have exceeded 10% over the past 10 years, and the industrys support organizations, led by the International Ground-Source Heat Pump Association (IGSHPA) are mature and robust.

Figure 1.1 First geothermal power plants, 1904, Larderello, Italy [17]1.3 Principle of Ground Source Heat Pump systemsGeothermal technology relies on the fact that the Earth (beneath the surface) remains at a relatively constant temperature throughout the year, warmer than the air above it during the winter and cooler in the summer, very much like a cave. The ground source heat pump takes advantage of this by transferring heat stored in the Earth or in ground water into a building during the winter, and transferring it out of the building and back into the ground during the summer. The ground, in other words, acts as a heat source in winter and a heat sink in summer. Heat pumps can be used to extract low grade heat from water within the ground or from the ground itself to provide space heating. Conversely, a heat pump can be operated in reverse to extract heat from buildings and provide cooling.

Figure 1.2 Schematic diagram of a heat pump system 1.4 Importance of Ground Source Heat Pump systemsIt is renewable energy, having low cost. With increasing in pollution and increasing in the prices of fossils fuels, it is the best alternative to replace them, because of high availability for heating and cooling purposes. It doesnt cause pollution.Geothermal systems are quieter, more reliable, more efficient, and more compact compared to regular heating and cooling systems. The earth under our feet stays the same temperature year round, whether its blazing hot in summer or freezing cold in winter. In summer the earth is cooler than the air, and in winter its warmer. Geothermal heat pumps cleverly put that fact to good use. They use the earth to warm buildings in the winter and keep them cool in the summer. There are so many uses of geothermal heat pump in direct uses as well as indirect use.1.5 Current Status International and nationalCurrently, over 3 million GSHP units are installed worldwide in 43 countries. Of the total worldwide capacity, 37%are installed in the United States and Canada, 47% in Europe and 16% in Asia. Sweden leads Europe in number of GSHP installations, and markets in China, Japan, and South Korea represent the largest growth within Asia. In the USA alone, over 50,000 GSHP units are sold each year, with a majority of these for residential applications. It is estimated that a half million units are installed, with 85% closed-loop earth connections (46% vertical, 38% horizontal) and 15% open loop systems (groundwater) [20].

Nearly 400 low to medium enthalpy thermal springs exists in India. These are distributed in seven geothermal provinces. The surface temperatures of these thermal springs vary from 47 to 98oC. Total power generating capacity of these provinces is estimated to be of the order of 10,000 MW. The reservoir temperatures estimated based on water and gas geothermometers vary from 120 to little over 150oC.There are many plans made by the government across the country, mainly in the state of Jharkhand, HP, and Chhattisgarh.

1.6 Objective of the studyGround Source Heat Pump (GSHP) systems are the effective technology for heating as well as cooling purposes. It is renewable energy, having low cost. With increasing in pollution and increasing in the prices of fossils fuels, it is the best alternative to replace them, because of high availability for heating and cooling purposes.

These are the objectives of the study: To carry out details literature survey. Identify various types of ground source heat pump systems for heating and cooling.

Identify various uses of GSHP, direct or indirect. Study hybrid systems of ground source heat pumps.

Study the role of solar energy when combined with ground source heat pumps. Study the benefits of ground source heat pump.

Chapter-2

LITERATURE SURVEYThe Ground source heat pump (GSHP) has evolved to become a mature technology over the past two decades. However, it is not applied as widely as it should or could be. Initial costs, system design and integration remain to be challenging problems. Efficient use of energy in such energy-intensive operations as district cooling/ heating, drying and cogeneration is crucial to the reduction of net energy consumption and hence emissions of greenhouse gases. With the eventual acceptance of a carbon/energy tax around the world energy, energy conservation will become a key concern in many industrial operations.2.1 Theoretical studies on Ground Source Heat PumpWith raising cost of fuel and global warming at the forefront of world attention, the interest in GSHP as a means of energy recovery appears to have been resurrected. Ground source heat pumps offer one of the most practicable solutions to the greenhouse effect. It is the only known process that recirculates environmental and waste heat back into a heat production process; offering energy efficient and environmentally friendly heating and cooling in applications ranging from domestic and commercial buildings to process industries [1].

The ground source heat pumps are classified according to their connection from the ground. They are broadly classified as closed and open loops systems. Hybrid systems are also introduced today to have greater benefits of solar energy as well as other resource to increase the efficiency of GSHP. The GSHP has significant benefits to the end user, to utility companies and to the local and national economies [1].

There are many direct utilization of geothermal energy. The utilization consists of various forms for heating and cooling instead of converting the energy for electric power generation. The major areas of direct utilization are swimming, bathing and baineology, space heating and cooling including district heating, agriculture applications, aquaculture applications, industrial processes, and heat pumps. The world-wide thermal energy used is estimated to be at least 108,100 TJ/yr (30,000 GWh/yr) - saving 3.65 million TOE/yr. The majority of this energy use is for space heating (33%), and swimming and bathing (19%). In the USA the installed thermal power is 1874 MWt, and the annual energy use is 13,890 TJ (3,860 GWh). The majority of the use (59 %) is for ground source heat pumps (both ground coupled and water source), with space heating, bathing and swimming, and fish and animal farming each supplying about 10% [2].Practical studies have shown the potential of heat pumps to drastically reduce greenhouse gases, in particular CO2 emissions, in space heating and heat generation. The positive impact on environment depends on the type of heat pump and the energy-mix and efficiency of driving power used [3].2.2 Applications Based studies on Ground Source Heat PumpThere are many world wide applications of GSHP systems. GSHP systems are also used in combined with other units. A detailed analysis of the heating and cooling performance of environmental heat sources and sinks is done for 12 low-energy buildings in Germany. In particular, the analysis focuses on the given temperature levels and the efficiency performance of the environmental heat sources and sinks in summer and winter. The annual efficiency performance of the geothermal heat sources and sinks results in a seasonal performance factor of 8-10 kWhtherm/kWhend, where the end energy use is electricity [4].

The GSHP system is also used in agriculture sectors. The heat from ground is very useful in the cultivation of plants. A studied was done on the possibility of using a constant temperature underground geothermal water source, which flows naturally, as an economic option to solve the problem of plant freezing and plant-growth inhibition in greenhouse cultivation in the central part of Argentina. A system of heating by means of geothermal energy, with energy-conservation measures, was designed and evaluated for typical production greenhouses in the southern part of Cordoba, Argentina. The results of tests carried out during 3 years are presented. These results are really promising, taking into account the high benefit/cost relation of the design and the availability of similar geothermal resources in many farms of this region [5].

Mainly GSHP are used for district heating. In cold climate areas, the heating provided by GSHP system is very useful and efficient. It is a very attractive alternate of fossil fuels. A studied was done on the use of geothermal energy for the district heating in Frederikshavn. Results shown, that the use of geothermal energy in combination with an absorption heat pump shows promise in a situation where natural gas supply to conventional cogeneration of heat and power (CHP) plants decreases radically [8].

Today hybrid GSHP systems are very efficient in use. Solar energy GSHP systems, Separate storage GSHP systems etc are the various combinations for Hybrid systems. Various models of solar assist GSHP system are introduced in the world. An investigate is done on the performance characteristics of a solar assisted ground-source heat pump greenhouse heating system (SAGSHPGHS) with a 50m vertical 1 1/4 in. nominal diameter U-bend ground heat exchanger using exergy analysis method. The heating coefficient of performances of the ground-source heat pump unit and the overall system are obtained to be 2.64 and 2.38, respectively, while the exergetic efficiency of the overall system is found to be 67.7% [12].

Also analyze was done on the performance of underground thermal storage in a solar-ground coupled heat pump system (SGCHPS) for residential building. Based on the experimental results, the system performance during a longer period is simulated by the unit modeling, and its parametric effects are discussed. The results show that the performance of underground thermal storage of SGCHPS depends strongly on the intensity of solar radiation and the matching between the water tank volume and the area of solar collectors [13].

The performance of hybrid GSHP systems on cooling as well as heating demand was analyzed. GSHP systems work on both heating and cooling load. A Monitored and analyzed three buildings employing HyGSHP systems (two cooling-dominated, one heating-dominated) to demonstrate the performance of the hybrid approach. One innovation to ground-source heat pump (GSHP, or geothermal) systems is the hybrid GSHP (HyGSHP) system. A HyGSHP system can dramatically decrease the first cost of GSHP systems by using conventional technology (such as a cooling tower or a boiler) to meet a portion of the peak heating or cooling load. The buildings were monitored for a year and the measured data was used to validate models of each system [10]. HVAC systems are commonly used in many industries for district cooling, chillers etc. There is huge potential of energy saving in HVAC systems. HVAC systems combined with GSHP reduced the electricity consumption of HVAC. Investigation of a desiccant assisted air conditioning system was done. For this a demonstration plant was built in an office building in Hamburg, Germany. The HVAC system consists of a small CHP-plant, a desiccant assisted ventilation system and an earth energy system (borehole heat exchangers) for cooling instead of an electric driven compression chiller. The radiant floor heating system of the building is used for cooling. It was found that considerable primary energy savings can be achieved (70%) using desiccant air conditioning with borehole heat exchangers. Starting costs for the demonstration plant were not higher than for a conventional system, but running costs could be reduced drastically [9].

Another example of Hybrid system is using GSHP system with air handling unit. In this author analyzed the possible synergies provided by the combination of underground thermal energy storage (UTES) system with a desiccant based air handling unit (AHU). Differently from the conventional solutions, the summer humidity control is obtained here by chemical dehumidification of the ventilation airstream performed by liquid desiccants in a packed column. In winter, the higher temperature level at which the UTES works and to the AHU configuration allowing sensible and latent heat recovery. The required UTES size is sensibly smaller, reducing in this way not only the operation but above all the investment costs. The UTES system competitiveness is then increased. The described solution is investigated by a computer simulation referring to a modern office building in the climate of northern Italy and its performance has been compared to a traditional HVAC plant and to a traditional ground source heat pump (GSHP) system [7]. 2.3 Recent developments in GSHPThe GSHP becomes a critical heat system as it possesses the capacity to recover thermal energy. As heat pump continues to find new novel applications in various energy-related industries, research efforts have been expanding to make it more energy efficient while evolving new hybrid systems that improve overall system efficiency. Recent progresses in heat pump systems have centered upon advanced cycle designs for both heat- and work-actuated systems, improved cycle components (including choice of working fluid), and exploiting utilization in a wider range of applications, the incorporation of a heat-driven ejector to the heat pump has improved system efficiency by more than 20%. Additionally, the development of better compressor technology has the potential to reduce energy consumption of heat pump systems by as much as 80%. The evolution of new hybrid systems has also enabled the heat pump to perform efficiently with wider applications [14].All these papers describe various uses of geothermal heat pumps .Direct uses and indirect uses, both are explained. The hybrid system of geothermal heat pump is also described. The solar assist geothermal systems are explained. The design consideration and the economic analysis of geothermal heat pumps are described. GSHP systems are widely used in the world. GSHP systems have many applications such as district heating; cooling etc.Some application project papers are also shown. Chapter-3DESIGNS OF GROUND SOURCE HEAT PUMPS Ground Source Heat Pump connected to the earth may be categorized as open loop or closed loop systems. Many factors affect the design of the loop section of the earth connection of a geothermal system.

Design factors include: geologic conditions (the thermal and hydraulic characteristics of the underground), technical parameters (length and type of ground heat exchanger, type and quality of grouting), other technical factors include the heating/cooling load, the type of space to be heated/cooled, and the supply temperature from underground. Generally speaking, the advantage of closed loop systems is the independence from aquifers and water chemistry. An advantage of open systems is the higher heat transfer capacity of the wells compared to a borehole. Designing the system calls for professional expertise. Designs choices open to the geothermal system installer of the earth connection are: Open loop earth connection systems where ground Heat is used directly and may include single well or multiple wells drilled to depths up to 1500 feet deep. Types of open loop systems are shown below.

Closed loop earth connection systems where air, water or a water antifreeze mixture is circulated through polyethylene tubing. Closed loop systems come in many configurations which are shown below.

Hybrid closed loop earth connection systems which incorporate features into their design that are not normally part of the geothermal system, but which add to the systems efficiency. Hybrid systems are shown below.

Direct Exchange (DX) systems do not use an intermediate working fluid or heat exchanger. Instead, DX systems employ closed loops of soft copper tubing to directly transfer heat between the ground and the refrigerant -- the heat pump's refrigerant loop is buried in the ground. Direct exchange systems are shown below.

3.1 Open loop earth connection systemsWater from a surface water source (oceans, lakes, rivers) or groundwater or air is pumped through the heat exchanger and then discharged to the same or a different body. Open-loop systems can be cheaper than closed-loop systems, because their installation involves less work; they also can have an efficiency that is comparable to or higher than a closed-loop system. However, local codes and regulations regarding groundwater discharge must be met. Consistency of the water supply in terms of quantity and quality are crucial to ensure uninterrupted heat pump operation and long service life. In cold climates, open-loop systems are further limited due to freezing temperatures that can make the source unavailable or cause pipes to freeze. The various types of Open loop earth connection systems are described below3.1.1 Single open loop earth connection systems Open loop systems are the simplest. Used successfully for decades, ground water is drawn from an aquifer, passes through the heat pumps heat exchanger, and is discharged. Also air is circulated in the underground pipes and use that energy into useful work .After it leaves the building, water may be disposed of by one of three methods. Note that local codes and regulations may restrict which discharge method is allowed

1. Surface drainage to a low area such as a pond, river, lake or stream, etc.

2. Sub surface to a dedicated drain field sized to the required volume of water of the heat pump (shown below).

3. Re-injection - water is pumped back into the same aquifer

Figure 3.1 Open loop earth connection systems [15]3.1.2 Multiple well open loop earth connection systemsOpen loop systems typically include one or more supply wells and one or more diffusion, discharge, recharge, return or injection wells (each of the preceding five terms diffusion, discharge, recharge, return, injection means the same thing as the terms are used here).

Figure 3.2 Multiple well open loop earth connection systems An open loop geothermal well system, groundwater is withdrawn from an aquifer through the supply/production well and pumped to the heat pump where it acts as a heat source or sinks in the heating or cooling process. Once the groundwater passes through the heat pump it is returned to the aquifer through an injection well. The only difference between the supplies and return water is the temperature.

Generally, two to three gallons per minute per ton of capacity are necessary for effective heat exchange. Since the temperature of ground water is nearly constant throughout the year, open loops are a popular option in areas where they are permitted.

Open-loop systems are used less frequently than closed loop systems, but may be employed cost-effectively if ground water is plentiful. Local environmental officials should be consulted whenever an open-loop system is being considered. In some localities, all or parts of the installation may be subject to local ordinances, codes, covenants or licensing requirements. Poor water quality will cause serious problems in open-loop applications. Water should be tested for hardness, acidity and iron content before the heat pump is installed. Poor water quality can cause mineral deposits to build up inside the heat pump heat exchanger and periodic cleaning will be required.

No environmental damage is created by open loop wells since the only difference between the water being removed by the supply well and the water being reinjected through the discharge well is a slight increase in temperature. The distance between the production and injection wells is an important design consideration. It is not necessary to completely prevent flow from the injection well to the production well, but simply to make sure that any flow between the wells is sufficiently low that discharged water arrives at the production well at a temperature at nearly the same temperature as the aquifer.

Well spacing typically will be in the range of 200 to 600 feet, depending on the maximum system cooling or heating load, the typical duration of the maximum load, and the thickness and natural flow rate of the aquifer. If proper attention is not given to this important design factor undesired temperature increases in the aquifer can lead to the growth of undesirable organisms which can cause increased befouling and incrustation.3.1.3 Standing water column open loop earth connection Another type of open loop system is the standing water column system.

Figure 3.3 Standing Water Column open loop earth connection systems A standing water column system is generally a single deep well drilled into bedrock. A casing is set from grade down to bedrock and from there the well is essentially an open rock well. The standing water column method works best with non-corrosive and non-scaling water, as the water is used directly in the heat pumps.

The geothermal water in this case is circulated within the same well. Here, if the water is withdrawn from the bottom of the well. The water will be returned at the top and allowed to heat or cool as it traverses down the well to where it is being withdrawn. This vertical movement of water and heat exchange is called a standing column well and provides a convenient and effective heat transfer method. Based on experience by the Water and Energy Systems Corporation, 50 to 60 feet of water column is needed per ton (a nominal 12,000 Btu/hr of cooling) of building load.

Standing column wells, also called turbulent wells have become an established technology in some regions, especially the northeastern United States. Standing wells are typically six inches in diameter and may be as deep as 1500 feet.

Ground water must be plentiful for a standing well system to operate effectively. If the standing well is installed where the water table is too deep, pumping would be prohibitively costly. Under normal circumstances, the water diverted for building (potable) use is replaced by constant- temperature ground water, which makes the system act like a true open- loop system. If the well-water temperature climbs too high or drops too low, water can be bled" from the system to allow ground water to restore the well-water temperature to the normal operating range. Permitting conditions for discharging the bleed water vary from locality to locality, but are eased by the fact that the quantities are small and the water is never treated with chemicals.

3.1.4 Surface water systemsSurface water systems use a large body of water such as an ocean bay or inland lake for a water supply, as well as discharge. Conceptually, surface water systems are similar to the Standing Water Column Systems described above.

A leading example of a surface water open-loop system is shown below. Water from Cayuga Lake is used as the source for geothermal exchange to provide cooling at Cornell University. This cooling-only application uses no heat pumps.

Figure3.4 Surface water system 3.2 Closed loop earth connection systemsThere are several types of closed loop systems. All types use a continuous loop where the heat transfer fluid is circulated. The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently.

When installers connect sections of pipe, they heat fuse the joints, making the connections stronger than the pipe itself. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.

3.2.1 Horizontal Closed Loop Earth Connection

A horizontal loop is usually the most cost effective when adequate yard space is available and trenches are easy to dig. Using trenchers or backhoes digging trenches three to six feet below the ground, you then lay a series of parallel plastic pipes.

The trench is then back filled, taking care not to allow sharp rocks or debris to damage the pipe. A typical horizontal loop will have 400-600 feet of pipe per ton of heating and cooling capacity. The land area required for horizontal ground loops will range from 1500-3000 square feet per ton of heating/cooling depending on soil properties and earth temperatures.

Figure 3.5 Closed loop earth connection systems Where there are restrictions is available land the individual pipes may be laid in a relatively dense pattern and connected in series or parallel as show below.

Figure 3.6 Closed loop earth connection systems in series and parallel collectors [15]Slinky loops are used to reduce the heat exchanger per foot trench requirements but require more pipes per ton of capacity. This pipe is coiled like a slinky, overlapped and laid in a trench. Two-pipe systems may require 200-300 feet of more pipe per ton of nominal heat exchange capacity. The trench length decreases as the number of pipes in the trench increases or as slinky overlap increases.

Figure 3.7 Closed loop earth connection systems with Slinky and Svec spiral collectors

Figure 3.8 Closed loop earth connection systems with Slinky collectors installationTo save surface area some special ground heat exchangers have been developed. They use a smaller surface area and a deeper trench for the installation of a number of circuits of narrow diameter pipe as shown.

Figure 3.9 Closed loop earth connection systems with Trench Collector3.2.2 Vertical Closed Loop Earth Connection

This type of loop is used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible. Vertical holes 150 to 450 feet deep - much like wells - are bored in the ground, and single or multiple loops of pipe with a U-bend at the bottom is/are inserted before the hole is backfilled. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit.

Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface. Typical piping requirements range from 400-600 feet of borehole per system ton of heating/cooling, depending (as always) on the soil properties and ground temperature conditions. This requirement usually results in 1-2 boreholes per ton of system load, again, the exact requirement being dictated by the thermal properties of the soil.

An important design factor is the spacing between boreholes. A rule of thumb is that boreholes should be 15-20 feet apart to avoid having the thermal conductivity of the boreholes conflicting with each other. Vertical ground loops typically require 150-300 square feet of land area per system ton of heating/cooling capacity.

Figure 3.10 Vertical Closed loop earth connection system Several types of borehole heat exchangers have been used or tested. The geothermal industry has developed simultaneously in several countries and a variety of techniques are used. In Europe, where land is generally at a higher premium than in the U.S. techniques to minimize surface land requirements have driven the development. For example, placing a single U-tube per borehole as opposed to placing 2-3 U-tubes per borehole.

Likewise several types of borehole heat exchanger designs are used in different countries, although the single U-tube or U-pipe design seems most common.

Figure 3.11 Different types of borehole heat exchanger 3.2.3 Energy Pilings

Foundation pilings may be equipped with ground loop heat exchanger piping. This system may be used with pre-fabricated or cast on-site pilings and in piling sizes from 16 to 3.

Figure 3.12 Energy Pilings 3.2.4 Submerged closed loop earth connections

If a large river or moderately sized pond or lake is available, the closed- loop piping system can be submerged. Some commercial and institutional buildings have artificial ponds for aesthetic reasons, and these may have adequate surface area and depth for fully immersing a closed-loop heat exchanger.

Submerged-loop systems typically require about 300 linear feet of piping per system ton. Depending on the pond depth ponds can support GSHP systems ranging from 15 to 85 tons per acre of pond surface area. This range corresponds to a unit area requirement of 500 to 3,000 square feet per system ton. The minimum acceptable pond depth for submerged ground loops is 10 feet. Concrete anchors are used to secure the piping coils, preventing their movement and holding the coils 9 to 18 inches above the pond floor, to allow good convective circulation of water around the piping. It also is recommended that the coils be submerged at least 6 to 8 feet below the pond surface , preferably deeper, in order to maintain adequate thermal mass in times of extended drought or other low-water conditions. Pond loops are a special kind of closed loop system. Geothermal transfer fluid is pumped just as a closed loop ground system. First cost economics are very attractive and there is no aquatic enviournment impact.

Figure 3.13 Submerged closed loop earth connections [15]3.3 Hybrid closed loop geothermal systems The advantages of GSHPs over conventional alternatives make them a very attractive choice for space conditioning and water heating for both residential and commercial/institutional buildings. However, GSHPs often have higher first costs than conventional systems making short-term economics unattractive. An alternative, lower cost approach for such applications can be use of a hybrid GSHP design. In hybrid GSHPs, the ground heat exchanger size is reduced and an auxiliary heat rejecter

(e.g., a cooling tower or some other option) is used to handle the excess heat rejection loads during building cooling operation. The extent to which the ground heat exchanger size can be reduced in a hybrid GSHP system will vary with location and climate, but it must be at least large enough to handle the building heating requirements. Hybrid GSHPs can also be used for sites where the geological conditions or the available ground surface will not allow a ground heat exchanger large enough for the building cooling loads to be installed. Various options of Hybrid systems are:3.3.1 Hybrid Closed Loop Earth Connection with Supplemental Cooling PondPond loops can also be utilized as part of a hybrid GSHP (ground source heat pump) system where an additional heat sink is needed to add to the efficiency of the system.

GSHP systems for school buildings in the southern areas of the U.S. can be expected to reject significantly more heat to the ground loop than they extract from it during the course of a year. To avoid overheating the ground loop area and thus decreasing the systems cooling efficiency, this seasonal imbalance can be accommodated a supplemental means of heat rejection such as a pond.

Research at Oklahoma State University suggests that it is less costly to build artificial ponds than to install cooling towers, particularly if the pond is built on school property and additional land does not have to be purchased for pond construction.

Figure 3.14 Hybrid GSHP Systems with Supplemental Cooling Pond [15]3.3.2 Hybrid closed loop earth connection with cooling tower

A cooling tower is an alternative to consider in cases where an artificial pond cannot be built for supplemental heat rejection. In this type of hybrid GSHP configuration, the cooling tower can be connected directly to the ground loop, or it may require an isolation heat exchanger, depending on the type of cooling tower.

Figure 3.15 Hybrid GSHP Systems with Supplemental Cooling Tower 3.3.3 Hybrid loop with solar collector

For GSHP systems being designed for colder weather parts of the U.S. eating load might be the driving ground loop design factor. In such cases, supplementing a GSHP system with solar thermal collectors will reduce the required size of the ground loop and increase heat pump efficiency by providing significantly higher building loop temperatures than could be attained by the ground heat exchanger alone. In most cases, the solar thermal collector can be connected directly to the ground loop, as shown.

A liquid/liquid isolation heat exchanger would be required, however, if the solar recalculating loop needs a different level of antifreeze protection than the ground loop or uses a different antifreeze additive. Solar thermal collectors almost always use propylene glycol for both antifreeze and anti-boiling protection, whereas methanol is the preferred antifreeze additive for closed ground loops, where environmental and health regulations permit its use

Figure 3.16 Hybrid GSHP System with Supplemental Solar thermal collector 3.4 Direct exchange (dx) closed loop systems The closed ground-loops described above use water or a water-antifreeze solution as an intermediate working fluid to move heat energy between the ground (or water body) and the building, with a liquid/refrigerant heat exchanger in each heat pump unit. Direct-exchange (DX) systems do not use an intermediate working fluid or heat exchanger. Instead, DX systems employ closed loops of soft copper tubing to directly transfer heat between the ground and the refrigerant - the heat pump's refrigerant loop is buried in the ground. By eliminating the intermediate heat exchanger, the refrigerant's temperature is closer to the ground's temperature, which lowers the heat pump's required compression ratio, reducing its size and energy consumption. Also a shorter ground loop can be used, because copper tubing is six (6) times more efficient at transferring heat than the polyethylene pipe used in conventional closed loops; the thermal conductivity of copper is about 19 Btu/sq.ft-hr-F per inch of wall thickness, whereas that of HDPE pipe is only 2.7 Btu/sq.ft-hr-F per inch. DX ground loops can be installed in a horizontal trenched configuration or a vertical U-tube configuration. Horizontal-loop DX systems require about 350 feet of copper tubing per system ton, as opposed to 450 to 500 feet per ton for polyethylene ground loops. Similarly, vertical DX systems require only a 3-inch diameter bores to a depth of 120 feet per ton, as opposed to 4- to 6-inch diameter bores to a depth of 200 to 300 feet per ton for polyethylene U-tubes in conventional vertical closed loops [15]. Today DX GHPs have been installed only in residential and small commercial applications, where a blower forces air through a refrigerant/air heat exchanger and a duct system distributes the warmed or chilled air throughout the building. In larger building applications a refrigerant/water heat exchanger is used to transfer the heat to a pipe system that can distribute warmed or chilled water to hydraulic terminal systems such as radiant floor slabs or fan-coil units. Because of their shorter length, horizontal DX ground loops need only about 500 square feet of land area per system ton, considerably less than the 1,500 to 3,000 square feet needed for conventional horizontal closed-loops. Vertical DX loops, on the other hand, need at least the same land area as their conventional counterparts, or even somewhat more. Vertical DX boreholes should be spaced at least 20 feet apart to minimize the possibility of ground freezing and buckling in the heating mode or excessive warming and drying of the soil in the cooling mode.

Heat from DX ground loops can bake fine-grained soils, reducing their thermal conductivity and thus the performance of the system. DX ground loops perform best in moist sandy soils or sand bed installations. Because DX ground loops are copper, they are subject to corrosion in acidic soils and should be installed in soils with a pH between 5.5 and 10. Chapter-4 CASE STUDIES OF GROUND SOURCE HEAT PUMP SYSTEMS 4.1 CASE STUDY 1- For Public Law building in Greece

The European Centre for Public Law is a building complex of a main building and a hostel located at Legraina, ca 65 km southeast of Athens on the Saronic Gulf coastline. The heating and cooling needs of the buildings are covered by a combined system of geothermal heat pumps and solar air collectors. Solar air collectors seem to play an important role in the energy savings of the preheating of the fresh air, as well as of the heating of the air mixture

4.1.1 The heating and cooling systemThe overall configuration of the geothermal part of the building heating and cooling system is shown in Fig. 4.1. In order to minimize the required water flow from the well, two heat pump units have been installed. The two units are connected in cascade in order to maximize the temperature difference (DT) of the ground water, and as a result the energy extracted from a given water flow rate. In order to facilitate this configuration within the building heating and cooling system, the thermal and cooling load of the building is split into two parts. In addition, the DT is monitored and controlled by the control system in order to avoid freezing and overheating conditions.

Figure4.1 Layout of geothermal heat pumps system and measuring points [18]The two heat pump units, HP1 (of 70 kW nominal capacity) and HP2 (of 100 kW nominal capacity), are both water-to-water type, electrically driven. The first unit (HP1) serves the auditorium and the classrooms of the ground floor of the main building through an all-air system (air handling units). The second unit (HP2) serves the offices and the library facility of the main building, as well as the guesthouse (hostel), with the aid of a hydraulic system (fan coils).

The air handling units comprise a return fan section, a double mixing box, a diverting solar mixing box, a coil section with a dual purpose heating/cooling coil, a spray humidifier for winter application, bag filters, a supply fan section, as well as an air to air heat recovery section. Both fans have been designed for two speed operation because winter load is much less than the summer one and winter mode is operated with half air flow rate (the supply air temperature then can take values in the comfort zone).

The diverting solar mixing box is connected to 45 m2 solar air collectors through a 350 mm insulated air duct for solar energy utilization as well. The solar energy input to the air handling units is presented schematically in Fig. 4.2.

Figure 4.2 Layout of air handling units (solar energy input) and measuring points [18]This mixing box diverts the air mixture flow to bypass the collector during summertime. Then relief dampers have been foreseen for the protection of the collector against overheating. During winter, the same diverting solar mixing box regulates the diverting airflow so that the airflow through the collectors achieves positive DT. The source/rejection sides of the two heat pumps (Fig. 4.1) are connected in series upon a single water loop, in which a plate heat exchanger represents the source of the required amount of thermal energy. An open loop animated by one pump and fed with water from an open concrete and insulated storage tank of 70 m3 volume, receives the thermal energy of the previous loop for rejection. The tank is also constantly fed by another open loop driven by a submerged stainless steel pump, inside the geothermal well. Both last open loops circulate saline groundwater through the titanium plate heat exchanger. The storage tank is needed for back up reasons. This autonomy rises up to 6 h (at peak load conditions). For water saving reasons, an inverter driven control system (IDCS) reduces the pumping energy consumption at partial load conditions of the system. This same control protects the heat pumps against freezing and overheating, in case wellhead temperature rises above its present value of 24 oC after long term production of groundwater.

During wintertime, both heat pumps operate in the heating operation mode, absorbing heat from the source (rejection in summer) closed loop. In order to maximize energy efficiency, the water pump feeding the heat pumps through this water loop, feeds the unit HP2 in priority. As a result, HP2, which is larger and operates more hours yearly, operates with higher COP. Therefore, the HP1 operates with colder evaporator, while HP2 in priority, operates with warmer evaporator and higher COP. Nevertheless, instead of 24 oC, the temperature of the water circulating within the closed loop is controlled and kept at a lower temperature. The maximum value of this lower temperature is controlled at 18 oC; the water is then supplied to the HP2 entry (scroll compressor technologies of both heat pumps cannot afford higher evaporating temperatures). For energy efficiency purposes, the temperature set point during wintertime has been set at the maximum allowable value, which is 18 oC.

During middle-seasons it can happen that both heating is needed in the HP2 system and cooling in the HP1 system, because of the high latent load inside the classrooms and the auditorium. In this case, the closed loop, assisted by the inverter open loop, can integrate opposite thermal loads. The hostel of the building requires hot water supply, which is provided by solar water heaters.

4.1.2 Results of the energy performance of the systemIn order to evaluate energy performance, the system operation was monitored under the following conditions:

One day with heating load was selected among several heating days of continuous monitoring. Both HP2 and HP1 operated in the heating mode. The HP1 supplied the auditorium operating with the AHU2 facility only. The fresh air (AHU2) supply was fixed to