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Geothermal heat pump systems: Status review and comparison with other heating options
Applied Energy 101 (2013) 341–348
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Geothermal heat pump systems: Status review and comparison with other heating options Stuart J. Self ⇑, Bale V. Reddy, Marc A. Rosen Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, Canada L1H 7K4
a r t i c l e
i n f o
Article history: Received 6 July 2011 Received in revised form 12 January 2012 Accepted 17 January 2012 Available online 12 February 2012 Keywords: Heating Geothermal energy Heat pump Thermal energy storage Efficiency Economics
a b s t r a c t Heating is a major requirement in many regions, and growing energy demands and pollutant emissions have allowed unconventional heating technologies to be considered, including geothermal. Geothermal heat pumps are reviewed, including heat pump technology, earth connections, current world status and recent developments. Geothermal heat pump technology and conventional heating systems are compared in terms of costs, CO2 emissions and other parameters. Geothermal heat pump use is economically advantageous when the price of electricity is low. Alternatively geothermal heat pump units have the lowest emissions depending when electricity is produced from a low emitting source. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A large portion of the global energy supply is used for electricity generation and space heating, with the majority derived from fossil fuels. Fossil fuels are finite resources and their combustion is harmful to the environment, through the emission of greenhouse gases, which contribute to climate change, and other pollutants. Demand for energy is increasing and future fossil fuel shortages are predicted [1]. Hammond [2] argues that fossil fuel depletion along with pollutant emissions and global warming are important factors for sustainable and environmentally benign energy systems. Such concerns have motivated efforts to reduce society’s dependence on fossil fuels, by reducing demand and substituting alternative energy sources. Alternative energy resources are sought that are more environmentally benign and economic than conventional fossil fuels. Beyond fossil fuels, the earth’s crust stores an abundant amount of thermal energy. Geothermal energy systems are relatively benign environmentally, with the emissions much lower than for conventional fossil fueled systems [3,4]. Geothermal energy is used in three main ways: electricity generation, direct heating, and indirect heating and cooling via geothermal heat pumps [4]. These three processes use high, medium, and low temperature resources, respectively. High and ⇑ Corresponding author. Tel.: +1 905 441 6697. E-mail addresses: [email protected] (S.J. Self), [email protected] (B.V. Reddy), [email protected] (M.A. Rosen). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.048
medium temperature resources are usually the product of thermal flows produced by the molten core of the earth, which collects in areas of water or rock. Low temperature resources are near ambient temperature and are mostly attributable to the solar energy incident on the ground and ambient air. High and medium temperature thermal resources are often deep within the earth [5], and the depth affects whether they can be exploited economically as drilling and other extraction costs can become high at great depths. Low temperature geothermal resources are abundant and can be extracted and utilized in most locations around the world. Extracting such thermal energy is relatively simple because the depths involved are normally small. Heat pumps extract low temperature thermal energy and raise the temperature to that required for practical use [4]. Geothermal heat pumps can provide an environmentally and economically advantageous option for space heating, and can also be utilized for space cooling. In this article, we review geothermal heat pump systems and recent developments and compare them with other heating options, with the objective of improving understanding of geothermal heat pump systems and increasing their utilization in appropriate applications. 2. Geothermal heat pumps Heat pumps can provide heat efficiently and economically with low emissions [6]. The concept of heat pumps has been recognized since the 1800s, and commercial applications have existed for
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about 60 years. Heat pumps move thermal energy from a lower to higher temperature medium similar to refrigerators [7]. The product of a heat pump is useable heat, usually at a temperature suitable to maintain a comfortable environment within a space. One of the most attractive characteristics of heat pumps is that they transport more thermal energy than the energy required to operate [4,8]. Geothermal heat pumps (GHPs), also referred to as ground source heat pumps (GSHPs), earth energy systems, GeoExchange heat pumps, ground-coupled heat pumps, earth-coupled heat pumps and ground-source systems [9,10], are comprised of three main systems: Geothermal heat pump: Moves heat between building and ground and modifies its temperature [11]. Earth connection: Facilitates heat extraction from the ground via a heat exchanger loop for use in the heat pump unit [11]. Interior heat distribution system: Conditions and distributes heat throughout the space [11,12]. 2.1. Heat pump systems Heat pumps operate using electricity to drive compressors that provide the necessary work for the concentration and transport of thermal energy [4,8]. Basic heat pumps operate on the vapor-compression refrigeration cycle. The working fluid within the heat pump is usually a refrigerant, with the selection dependent on the overall characteristics and requirements of the GHP system [6,13]. A GHP moves thermal energy between the earth and the heated space by controlling pressure and temperature by means of compression and expansion [4,8,11]. Five major components are incorporated in a heat pump (Fig. 1) [10,11,14]: compressor, expansion valve, reversing valve, and two heat exchangers. There are also various other minor components and accessories such as fans, piping and controls that assist in operation. GHPs for heating operate as follows [12]: 1. Thermal energy is extracted from the earth and transported to the evaporator. 2. Inside the heat pump unit cold refrigerant, in a liquid dominated liquid/vapor state, enters the evaporator. Heat is transferred from the earth connection to the refrigerant and causes the refrigerant to boil and become a low pressure vapor; the temperature increases slightly.
DESUPERHEATER
3. The vapor enters an electrically-driven compressor, where the pressure is increased, resulting in a high temperature and high pressure vapor. 4. High temperature vapor enters the condenser. The refrigerant is at a higher temperature than the space, inducing heat transfer from the refrigerant to the building. The refrigerant cools and condenses, yielding a high pressure, high temperature liquid. 5. Hot liquid passes through an expansion valve that reduces its pressure, resulting in a temperature decrease. The refrigerant enters the evaporator to begin another cycle. Many systems include a cooling mode that removes thermal energy from a space and rejects it to the ground. In the cooling mode, a reversing valve is used to move the fluid in the opposite direction in the cycle. The heat exchangers are reversed, with the earth connection heat exchanger becoming the condenser and the building heat exchanger the evaporator [8,12]. Some systems include a desuperheater (Fig. 1), which is an auxiliary heat exchanger that supplies heat to a hot water tank. Located at the compressor exit it transfers heat from the compressed vapor to water circulating through a hot water tank, reducing or eliminating the energy required for water heating [12]. Merit is usually evaluated in terms of energy efficiency, the ratio of product energy output to driving energy input, in percent. Heat pumps deliver more product heat than the input driving energy, so this definition yields an energy efficiency greater than 100%. To avoid this awkwardness, the term coefficient of performance (COP) is used for heat pumps, defined as the ratio of product thermal energy to input driving energy [9]. COPs for geothermal heat pumps usually range from 3 to 6, with the value dependent on the earth connection setups, system sizes, earth characteristics, installation depths, local climate and other characteristics [10,15].
2.2. Heat distribution systems The heat distribution system of a GHP system moves heat supplied by the heat pump throughout the space. Two main types of distribution systems exist: water to air and water to water. Water to air systems transfer thermal energy from the ground to air, which is used as the transport medium within the space, while water to water systems use water or another fluid as the heat transfer medium. The most common GHP system in North America is water to air, where an air coil, heated by the heat pump
COMPRESSOR
(not always present)
REVERSING VALVE
REFRIGERANTBUILDING SPACE HEAT EXCHANGER
GROUND LOOP HEAT EXCHANGER
REFRIGERANTGROUND LOOP HEAT EXCHANGER
EXPANSION VALVE GROUND LOOP CIRCULATION PUMP Fig. 1. Basic layout of geothermal heat pump system including desuperheater.
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condenser, warms air that passes over it. The air is moved throughout the building using HVAC ducts and air vents [12,16]. In water to water systems, also commonly known as hydronic systems, thermal energy is extracted from the ground loop, processed by the heat pump and distributed throughout the building using water as the carrier. The system pumps water through the condenser of the heat pump unit, extracting heat. Water is then pumped around the building delivering the heat to the space using in-floor radiant heaters, radiators or localized air coils. These systems heat using relatively low temperatures compared to conventional forced air systems. The warmest air in rooms that are heated with a forced air furnace ascends towards the ceiling, leaving a lot of living space at a cooler temperature. In order to have living space at a desired temperature the temperature of the air entering a space must be higher than that of the space. A room with in-floor radiant heaters will have a more uniform temperature from the ceiling to the floor and require lower operating temperatures to provide suitable conditions within living spaces [8,15,16]. Hybrid systems also exist, which combine both types of distribution methods to provide system flexibility and enhanced control of space temperature.
2.3. Earth connections Whereas air source heat pumps use the ambient air as a heat source, geothermal heat pumps use surrounding ground. Ambient air exhibits a wide variation in temperature throughout the year and on a daily basis, compared to ground [17]. On a daily basis the ground temperature fluctuates at depths shallower than 0.8 m; at greater depths the temperature variation decreases [18]. The variations are more pronounced on a seasonal rather than daily basis [19]. Fig. 2 shows the annual change in ground temperature with increasing depth for Ottawa, Canada. As the depth increases the extreme warm and cold temperatures converge. The depth where the ground temperature becomes constant depends on factors such as incoming solar radiation, snow cover, air temperature, precipitation and thermal properties of the ground. In Canada constant annual temperatures are usually observed at depths beyond 10 m [18]. For example, Fig. 3 illustrates seasonal ground temperature variations for Ottawa at different depths.
Ambient Air Temperature Range
Annual Mean Ground Temperature
Ground Temperature Range
Temperature ( ºC) Fig. 2. Variation in ground temperature with depth for Ottawa, Canada. Modified from Ref. [12].
Temperature (ºC)
S.J. Self et al. / Applied Energy 101 (2013) 341–348
5.0 m 2.0 m
0.3 m Winter
Spring
Summer
Fall
Winter
Fig. 3. Annual ground temperature range for different depths for Ottawa, Canada. Modified from Ref. [12].
GHPs exploit the relatively constant temperature in the ground, which is warmer than the ambient air during winter and cooler in summer [17]. The ground temperature remains nearer to the desired temperature inside a building. When there is a large variation between inside and outside temperatures, as is the case for air source heat pumps, more work is required to provide the same degree of heating, which reduces the COP [14]. If an excessive temperature difference exists, heat pump systems do not operate as intended. Earth connection or ground loop heat exchangers are comprised of a collection of pipes that transfer fluid between the heat pump unit and the ground. Two main ground loop designs exist: double loop and single loop configurations. 2.3.1. Double loop configuration A double loop configuration is the most common system configuration and involves an earth connection that is separate from the heat pump. Heat is transferred to the refrigerant via a heat exchanger from water or a water/antifreeze mixture, which is circulated through piping from the heat pump unit to the ground. The standard pipe currently is composed of polyethylene or polypropylene and has an inner diameter of 19 mm (3/4 in.) for small and medium size applications [7]. Two types of double loop configurations exist: closed and open. 2.3.1.1. Closed loop systems. In closed loop systems, which are commonly utilized, the heat transfer fluid is enclosed in a circulating loop and has no direct contact with the ground; heat transfer with the ground occurs through the piping material [20]. There are four classes of closed loop heat exchange systems: vertical, horizontal, spiral, and pond. 2.3.1.1.1. Vertical closed loop. A vertical closed loop system includes a loop field consisting of vertically oriented heat exchange pipes. A hole is bored into the ground, typically ranging 45–75 m deep for residential and over 150 m for larger industrial applications. Pairs of pipes, connected at the bottom by a U-shaped connector, are fed into the hole (Fig. 4) [21]. To enhance heat transfer, the gap between the pipes and the borehole wall are filled with a pumpable grout material [20,22]. The borehole diameter is approximately 102 mm for a typical residential home. For a typical residential application the spacing between boreholes is around 5–6 m in order to prevent adjacent boreholes from affecting one another and changing ground conditions [21,23]. To assure equal flows for multiple borehole systems a manifold system is used, which can be located in the building or buried in the loop field [23]. An advantage of the vertical loop configuration is reduced installation area [22], making them advantageous where land is limited. Another incentive for these systems is low landscape disturbance, since drilling
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Fig. 7. Horizontal loop with piping in parallel for a geothermal heat pump.
Fig. 4. Vertical closed loop heat exchange system for a geothermal heat pump.
has a reduced impact compared to trenching [17,23]. Also, locating the piping deep in the ground, where the temperature is constant year round, allows consistent heat pump performance and reduces overall loop length [20,23]. The main disadvantage of using a vertical system is the installation costs, since drilling is normally more costly than horizontal trenching. Consequently, vertical loop systems are normally more economic for larger applications [9]. 2.3.1.1.2. Horizontal closed loop. In horizontal closed loop systems, which are common where ample ground area is available, the ground loop is laid out horizontally slightly below the earth’s surface in backfilled trenches. The arrangement of the loops can vary depending on heat transfer requirements and land availability. The three most common configurations are basic loop (Fig. 5), series loop (Fig. 6), and parallel (Fig. 7). The basic layout usually requires a substantially larger land surface area than the series and parallel setups. The series layout is common for its reduced land require-
Fig. 5. Basic horizontal loop configuration for a geothermal heat pump.
Fig. 6. Horizontal loop piping in series for a geothermal heat pump.
ment and simplicity [9]. Series and parallel loops can also be combined, increasing the flexibility horizontal installations. Horizontal heat exchange systems are normally more cost effective than vertical for residential installations, through lower costs involved with trenching compared to drilling [9]. The trenches that house the piping usually do not exceed more than a couple meters below the surface, but in areas of frost they are located below the frost line. With shallow depths, there is increased interaction between the soil and the ambient environment, which results in daily and annual variation in ground temperatures, which affects the heat transfer and system performance. Other factors that affect heat transfer characteristics include rain, snow, vegetation growth and shade [9]. These factors usually cause horizontal systems to require more piping than vertical arrangements. Horizontal arrangements require a water/antifreeze mix to protect against freezing during winter in cold climates [9]. 2.3.1.1.3. Closed spiral loop. Spiral loop arrangements are similar to conventional horizontal loops, as they are typically horizontally oriented within shallow trenches. But the piping is laid out in circular loops within the trench. The end of each spiral has a straight return pipe to the heat pump [9,24]. Spiral loops require less area than conventional horizontal loops and have lower trenching requirements, but they need greater piping lengths for a fixed load [20]. A variation of the spiral-loop system involves placing the loops upright in narrow vertical trenches. The main advantage of this vertical-loop layout is reduced horizontal area requirements, which allow different trenching equipment to be used, sometimes yielding advantageous economics [17]. Note that spiral loops can reduce initial costs when trenching constitutes a substantial portion of the GHP system cost, but not necessarily when materials costs are high [21]. Other disadvantages of spiral loops are as for horizontal setups, including poorer heat transfer and greater area requirements. Spiral loops have greater pumping requirements than other horizontal systems due to the added pipe length, lowering system COP. 2.3.1.1.4. Closed pond loop. Closed pond loops, which are the least common of the closed loop heat exchange systems, are essentially a spiral loop systems submerged in a water body. The coiled piping is attached to framework and submerged using concrete anchors. The framework is typically supported 23–48 cm above the pond bottom to allow for convective flow around the piping [21]. The loop is normally at least 1.8 m below the water surface. It is necessary to assure sufficient thermal mass is maintained during low water conditions and prolonged draughts, and to ensure the temperature of the water immediately surrounding the loop never drops below the freezing point of water during cold seasons. Rivers are not ideal for this application due to their unpredictable behaviors, including flooding and draughts that can damage systems as well as hazards due to moving debris [9,24]. Pond loops are gaining popularity partly because they potentially require less piping than other closed loop systems, due to
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their superior heat transfer characteristics, and they require neither drilling nor trenching [9]. The main disadvantages of this system include the requirement of a sufficiently large body of water and the limitations on its use for other purposes such as boating. 2.3.1.2. Open loop systems. Open loop heat exchange systems interact directly with the ground. These systems use local groundwater or surface water, such as lakes and ponds, as a direct heat transfer medium. The water is extracted and passed through the heat exchanger of the heat pump, and then discharged back to the source or on the ground for irrigation [9]. Currently the use of water in abandoned mines is being explored since water filled mines may be able to heat towns inexpensively using heat pump technology. Open systems tend to be used for large installations. The largest GHP currently in operation utilizes an open loop system and supplies about 10 MW of heating to a hotel and connected offices [9]. There are three common open loop configurations: extraction wells, extraction and reinjection wells, and surface water systems. The most common type has extraction and reinjection wells (Fig. 8). Water is extracted from a drilled production well reaching the water table and, after passing through the heat pump heat exchanger, is injected back into the water table a distance from the production well, which is sufficient to allow adequate heat transfer from the ground to the water between the wells [9]. Reinjection can be excluded; open drainage is inexpensive but requires the water source, supplying the heat pump, to have a high capacity with little draw down in order to provide prolonged use [14]. The water flow rate to the heat pump unit is generally between 5.7 and 11.4 litres per minute per ton of heating capacity. An advantage of open loop setups is that the source water temperatures remain nearly constant. Also the losses associated with the extra heat exchanger needed for closed loop systems are avoided, increasing the ground heat pump COP [18]. Depending on the extraction method used, open loop can have high pumping loads but their overall COPs are usually high, reducing operating costs [9]. Further, open loop arrangements require less drilling than vertical closed loops and have simpler ground-connection designs and lower associated initial costs. The amount of water that can be extracted for open loop GHP systems is sometimes limited by local water resource regulations [9]. The main disadvantage of open loops is the need to protect water quality, usually by following clean water and surface water regulations; sometimes open loop systems not allowed [18]. The
heat exchanger between the heat exchange loop and the heat pump unit is subject to corrosion, fouling and scaling, so the water should have a fairly neutral chemistry and a low amount of minerals such as iron [24]. If the water chemistry is not neutral wells may require maintenance, increasing user involvement [9]. 2.3.2. Single loop configuration In single loop configurations, also known as direct exchange systems, the heat pump working fluid flows through the ground heat exchanger, which avoids the need for a ground loop to heat pump heat exchanger. Essentially the ground loop becomes the heat pump evaporator during heating applications [9]. Single loop arrangements also exclude the ground loop circulation pump, instead relying on a slightly larger compressor. These measures increase the overall ground heat pump COP [18]. Copper piping is commonly used in these systems due to its superior heat transfer characteristics, reducing the required pipe area. Direct exchange systems are pressurized, necessitating good engineering [18] since the probability of rupture is increased due to substantial above ground forces or abnormal operation. If piping is damaged the entire system may need to be dug up for repairs. Another disadvantage involves the requirement for increased refrigerant to accommodate the volume of the ground loop, which increases cost [9]. Nonetheless, single loop GHP systems are gaining popularity globally due to their higher COPs, and some countries (France and Austria) are exploring direct exchange units with direct evaporators coupled with direct condensers through the implementation of floor heating [9]. 2.4. Global status The main advantage of geothermal heat pumps is their ability to utilize soil and ground water temperatures between 5 °C and 30 °C, which is common at reasonable depths around the world [15]. As of 2004 about 30 countries were utilizing GHP systems, with the leading countries including USA, Sweden, Germany, Switzerland, Canada and Austria. Table 1 outlines the installed GHP capacity for several countries. As of 2004 the worldwide installed GHP thermal capacity was around 12 GW which required an annual energy usage of 20 TWh. The technology is gaining popularity in France, Netherlands, China, Japan, Russia, UK, Norway, Denmark, Ireland, Australia, Poland, Romania Turkey, Korea, Italy, Argentina, Chile, Iran, UK and Norway [15]. Since 1994 the annual growth rate for geothermal heat pumps has been about 10%, which is approximately 1.7 million applications currently [12]. The continental United States and Europe are presently the leaders in terms of growth of the technology. The growth of GHP technology has been slower than for some other renewable and conventional energy technologies. Limited growth can be attributed to many factors including non-standardized system designs, significant capital cost compared to other systems, limited individuals knowledgeable in the installation of GHPs, limitations placed on use through government policies,
Injection well
Pump
Production well
Table 1 Leading countries using geothermal heat pumps as of 2004 [9].
Water table
Fig. 8. Open loop heat exchange system with production and injection wells for a geothermal heat pump.
Country
Installed thermal capacity (MW)
Annual energy use (GWh)
Number of GHP installations
US Sweden Germany Switzerland Canada Australia
6300 2000 560 440 435 275
6300 8000 840 660 300 370
600,000 200,000 40,000 25,000 36,000 23,000
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and economies of scale and scope are rarely exploited [6,18]. Even though these issues exist they are being resolved on an ongoing basis, which is increasing the acceptance of the technology [15].
3. Recent developments Various developments in GHP systems have been reported recently.
4.1. Efficiency Geothermal heat pumps have high efficiencies, as reflected in their COPs. Typical equivalent COPs for various heating systems follow: ground source heat pumps: 3–5, air source heat pumps: 2.3–3.5, electric baseboard heaters: 1, mid-efficiency natural gas furnaces: 0.78–0.82, and high-efficiency natural gas furnaces: 0.88–0.97. 4.2. Economics
3.1. Auxiliary component cooling Since compressors and pumps are not 100% efficient, they release waste heat during their operation. Compressor and pump waste heat can be used to preheat the refrigerant within the heat pump cycle. This could be accomplished by having cool refrigerant enter a sealed hermetic shell enclosing the compressor or pump along with their electric motor. Preheating increases the performance of the components, increasing the COP of the overall GHP system, as well as reducing the ground loop heat exchanger load requirements [8]. 3.2. Ground frost loop GHP systems have started to be utilized in areas of permafrost. Heat transfer from building foundations can melt the permafrost and compromise structural integrity. Through installing a ground loop close to the foundation, permafrost melt can be reduced or eliminated. Heat is extracted from the area around the foundation ensuring that the building does not affect the ground temperature drastically. The heat extracted is used to supply supplemental heat to the building, typically 20–50% of the total building heating load [12]. The system should not keep the ground frozen outside of its natural annual cycle [12], and should not disturb the local ecology. The heat exchange loop should be reliable to prevent failures, which could impact a building’s structural stability [12]. 3.3. Standing column well heat exchange systems Standing column wells combine aspects of open and closed water heat exchange systems. They are essentially groundwater heat pump systems that use groundwater drawn from wells in a semi-open loop arrangement. In such a system, a vertically drilled borehole draws warm water from the bottom of a deep rock well using a submersible pump and feeds it to the heat pump unit. The cool return water is injected near the top of the original well. The cold water moves down towards the extraction pipe and is heated by the surrounding rock [9], eliminating the need for a separate injection well. Standing column wells are recently receiving increased attention because of their good overall performance in suitable regions. These systems are installed in locations having bedrock within 45–60 m of the surface. Domestic wells that have been used for drinking water can be retrofitted easily to accommodate this system. This system can also be applied to water filled mines and tunnels [9].
4. Analysis and comparison of heating systems A comparison is presented of the following heating systems: geothermal heat pumps, air source heat pumps, electric baseboard heaters, and natural gas furnaces (mid and high efficiency). Three Canadian provinces (Alberta, Ontario and Nova Scotia) are considered, and efficiencies, costs and emissions are assessed. The results are presented in Tables 2 and 3. European trends are also explored.
Geothermal heat pumps have substantially higher initial costs than conventional heating systems, mainly because of the capital costs of the heat pump unit and the ground connection (including drilling or trenching). But, geothermal heat pumps can have low operating costs due to their high efficiencies. 4.2.1. Economics: Canadian trends For the analysis of the situation in Canada, the initial cost of each system is assumed the same in all locations. The annual heating cost is based on the cost of electricity and natural gas in the specific province. A life span of 20 years and an average COP of 4 for the geothermal system is assumed. Typically geothermal heat pumps have warranties of 20–25 years, but systems exist that have been in operation in excess of 30 years. It is assumed that the installation does not require the installation of new duct work. The evaluated costs are summarized in Table 2. It is demonstrated that the economic feasibility of geothermal heat pumps depends strongly on location. The prices of electricity, natural gas and other heating fuels vary regionally. In Alberta and Nova Scotia the GHP is the most economically competitive option. In Ontario the air source heat pump is determined to have a slightly lower cost after 20 years. This result is influenced by the fact that Alberta and Nova Scotia have higher electricity prices than Ontario. High electricity prices allow for greater economic savings over air source heat pumps and electric baseboards. It is also found that when the cost of natural gas is low the difference narrowed between the cost of natural gas furnaces and GHPs. Where natural gas or other heating fuels are inexpensive, geothermal heat pumps may not be the most economic option [18]. For locations requiring air conditioning GHP systems exhibit an increased economic advantage because heat pumps work in reverse, allowing them to remove heat from a building and reject it into the ground. While conventional heating systems require a separate air conditioner for space cooling, GHP systems avoid this initial cost [24]. The payback period for a GHP system is typically between 6 and 20 years, depending on capital costs, energy prices and energy price increases [18]. Another advantage not quantified in the study relates to property value. GHPs tend to increase property value, which allows for high return on investment in the building and land and promotes more desirable mortgage assessments [18]. Note that GHP systems are most cost effective if installed during construction of the building or when an old heating system needs to be replaced. The purchase and installation of a GHP, as a replacement of a working system, is seldom worthwhile from energy and economic perspectives [14]. 4.2.2. Economics: European trends Table 4 illustrates the natural gas and electricity prices for various countries within the European Union. The analysis assumes a static heating load for all countries and a 20 year lifespan, and compares costs (including initial costs) for geothermal heat pumps, air source heat pumps, electric baseboard heaters, and natural gas furnaces (mid and high efficiency). For simplicity, the initial costs are assumed identical to those used for the Canadian comparison.
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S.J. Self et al. / Applied Energy 101 (2013) 341–348 Table 2 Comparison of economic parameters for various heating systems in several locations. Heating system
Geothermal HP Air source HP Electric baseboard Natural gas furnace a Natural gas furnace b
Capital cost ($)
Alberta
Ontario
Nova Scotia
Annual heating cost ($)
Present worth ($)
Annual heating cost ($)
Present worth ($)
Annual heating cost ($)
Present worth ($)
9000 4900 1550 1500
601 813 2257 1276
21,020 21,160 46,690 27,020
328 444 1231 2344
15,560 13,780 26,170 48,380
649 877 2432 1885
27,230 27,940 50,190 44,750
1900
1109
24,080
1049
22,880
1653
40,460
Monetary units are in 2009 Canadian dollars. Present worth values are for a 20 year period. a Represents mid efficiency. b Represents high efficiency. Table 3 Comparison of CO2 emissions for various heating systems in several locations. Heating system
Geothermal HP Air source HP Electric baseboard Natural gas furnacea Natural gas furnaceb a b
Annual fuel use (kWh)
Alberta Emission intensity (kgCO2/kWh)
CO2 emission (kg)
Emission intensity (kgCO2/kWh)
Ontario CO2 emission (kg)
Emission intensity (kgCO2/kWh)
Nova Scotia CO2 emission (kg)
6080 8214 22,280
1.12 1.12 1.12
6826 9222 25,015
0.188 0.188 0.188
1143 1544 4188
1.04 1.04 1.04
6346 8573 23,255
28,475
0.190
5410
0.190
5410
0.190
5410
24,655
0.190
4684
0.190
4684
0.190
4684
Represents mid efficiency. Represents high efficiency.
Table 4 Natural gas prices, electricity prices, and CO2 emission associated with electricity production for several countries in the European Union (EU) [27,28]. Country
Natural gas price ($/kWh)
Electricity price ($/kWh)
Electricity emission intensity (kg CO2/kWh)
Austria Belgium Cyprus Czech Rep. Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Netherlands Norway Poland Portugal Slovak Rep. Slovenia Spain Sweden Switzerland UK EU average
0.08 0.08 N/A 0.07 0.15 0.05 N/A 0.08 0.08 N/A 0.07 0.07 0.10 0.05 0.06 0.07 0.10 N/A 0.07 0.09 0.06 0.09 0.07 0.12 N/A 0.06 0.08
0.27 0.28 0.27 0.19 0.39 0.14 0.20 0.18 0.35 0.17 0.22 0.27 0.27 0.15 0.17 0.25 0.25 N/A 0.20 0.24 0.23 0.20 0.26 0.26 N/A 0.21 0.23
0.239 0.311 0.974 0.922 0.680 1.015 0.403 0.108 0.626 0.882 0.695 0.706 0.565 0.443 0.367 0.367 0.619 0.015 1.108 0.630 0.382 0.392 0.493 0.076 0.041 0.558 0.486
Monetary units are in 2009 Canadian dollars.
Europe has high natural gas and electricity costs but it appears that the increases in cost over the Canadian situations are relative. For the majority of the European countries considered, geothermal
heat pumps are economically advantageous compared to conventional heating methods where the overall cost of installing and operating the geothermal heat pump is considerably lower over a 20 year lifespan. Within Germany, Ireland, Luxembourg, Slovokia Rep., Spain and the United Kingdom it is found that the use of high efficiency natural gas furnaces is more economical, which is a result of high prices for electricity compared to natural gas. This study provides a general overview of heat pump implementation within European countries. Heating loads vary across the countries, introducing some inaccuracies in this study. Implementation of geothermal heat pumps in locations with low heating requirements might not be as economical, since the capital cost of geothermal heat pump units are significant. Alternatively the capital cost of heat pump units in warmer climates would be reduced through the implementation of smaller units. More in-depth analysis considering local climate is required for a detailed understanding of the outcomes of heat pump implementation for specific European countries.
4.3. Carbon dioxide emissions In this assessment, we compare carbon dioxide (CO2) emissions for heating systems. Although other pollutants are significant, we focus here on CO2 since it is the most common greenhouse gas and is considered the main contributor to climate change [18]. Geothermal heat pumps do not directly emit CO2; rather the emissions originate in the power plants that produce the electricity [16]. When electricity is produced in high-emission power plants, the CO2 emissions of GHP systems are correspondingly high. The threshold at which GHP systems become environmentally advantageous is related to the CO2 associated with producing the electricity used by the heat pump, its COP, and the efficiency of conventional heating systems [25].
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4.3.1. Carbon dioxide emissions: Canadian trends The CO2 emissions are determined for the situation in Canada considering the amount of electricity or natural gas consumed by a device and the fuel emission intensity (mass of CO2 emitted per kWh energy produced). We again examine the three provinces considered earlier. Assuming the natural gas composition is identical for Alberta, Ontario and Nova Scotia, the emissions per unit gas consumed is fixed. The average emission intensity for each province is used, taken from the Carbon Monitoring for Action (CARMA) [26] online database. The CO2 emissions of various heating systems, for each province, are listed in Table 3. Since Ontario has low emitting power generation, with over 50% of its electricity production from nuclear energy and the remainder mainly divided between natural gas fired plants and hydroelectric plants, the application of GHPs is advantageous environmentally. Over 80% of the electricity production in Alberta and Nova Scotia is from fossil fuels, including coal and natural gas plants [16]. Relative to a high efficiency (95%) natural gas furnace, GHP systems offer emission reductions when the emission intensity of the electricity produced is less than 0.76 kg/ kWh [18]. In general GHP systems provide the largest emissions reductions relative to conventional electrical heating devices and natural gas fired systems when the electricity used by heat pumps is derived from environmentally benign power plants. In jurisdictions with high CO2 emitting power plants, geothermal heat pumps have less CO2 emissions than other systems utilizing electricity. When renewable energy is used to produce electricity, GHP technology uses only renewable energy in operating, with little or no carbon emissions. Overall, geothermal heat pumps normally provide the greatest (or almost the greatest) emissions reductions. 4.3.2. Carbon dioxide emissions: European trends Carbon intensities associated with electricity production within various European Union countries are listed in Table 4. Using the carbon emission threshold associated with electricity production, determined by Dowlatabadi and Hanova [18] to be 0.76 kg/kW, it can be seen that the majority of the countries listed would encounter CO2 emission reductions with the implementation of geothermal heat pump units instead of conventional heating units. Using geothermal heat pump units within a country could significantly reduce the overall CO2 emissions of the country. Coupling ground source heat pumps to the current United Kingdom electricity grid, for example, can lead to reductions in CO2 emissions of over 50% compared to conventional space heating technologies based on fossil fuels, considering the current generation mix on the UK grid [15]. 5. Conclusions Geothermal heat pumps are highly efficient heating technologies that allow for reductions in CO2 emissions, the potential avoidance of fossil fuel usage and economic advantages. Heat pumps utilize significantly less energy to heat a building than alternative heating systems. Many variations of geothermal systems for heating exist, with different configurations suitable in different situations and most locations around the world. In deciding among heating options, it is important to determine the benefits for different ground heat pump options, typically in terms of efficiency, emissions and economics.
Acknowledgments The authors acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. References [1] Ediger VS, Hosgor E, Surmeli AN, Tatlidil H. Fossil fuel sustainability index: an application of resource management. Energy Policy 2007;35:2969–77. [2] Hammond GP. Energy, environment and sustainable development: a UK perspective. Trans Inst Chem Eng, Part B: Process Safety Environ Prot 2000;78:304–23. [3] Office of Energy Efficiency and Renewable Energy. Geothermal today: 2003 geothermal technologies program highlights. US Department of Energy 2004; Washington. [4] Wu R. Energy efficiency technologies – air source heat pump vs. ground source heat pump. J Sust Dev 2009;2:14–23. [5] Shengjun Z, Huaixin W, Tao G. Performance comparison and parametric optimization of subcritical organic rankine cycle (ORC) and transcritical power cycle system for low-temperature geothermal power generation. Appl Energy 2011;88:2740–54. [6] Chua KJ, Chou SK, Yang WM. Advances in heat pump systems: a review. Appl Energy 2010;87:3611–24. [7] Ozyurt O, Ekinci DA. Experimental study of vertical ground-source heat pump performance evaluation for cold climate in Turkey. Appl Energy 2011;88:1257–65. [8] Hepbasli A, Kalinci Y. A review of heat pump water heating systems. Renew Sust Energy Rev 2009;13:1211–29. [9] Omer AM. Ground-source heat pumps systems and applications. Renew Sust Energy Rev 2008;12:344–71. [10] Office of Energy Efficiency. Heating and cooling with a heat pump – groundsource heat pumps (earth-energy systems). Ottawa, CA: Natural Resources Canada; 2009. [11] Bi Y, Wang X, Liu Y, Zhang H, Chen L. Comprehensive exergy analysis of a ground-source heat pump system for both building heating and cooling modes. Appl Energy 2009;86:2560–5. [12] RETScreen International. Clean energy project analysis: RETScreen engineering and cases textbook. Ottawa, CA: Natural Resources Canada; 2005. [13] Zhang S, Wang H, Guo T. Experimental investigation of moderately high temperature water source heat pump with non-azeotropic refrigerant mixtures. Appl Energy 2010;87:1554–61. [14] Healy PF, Ugursal VI. Performance and economic feasibility of ground source heat pumps in cold climates. Int J Energy Res 1997;21:857–70. [15] Curtis R, Lund J, Sanner B, Rybach L, Hellström G. Ground source heat pumps – geothermal energy for anyone, anywhere: current worldwide activity. In: Proceedings of world geothermal congress, Antalya, Turkey; April 24–29, 2005. [16] Bloomquist RG. Geothermal space heating. Geothermics 2003;32:513–26. [17] Lee CK. Effects of multiple ground layers on thermal response test analysis and ground-source heat pump simulation. Appl Energy 2011;88:4405–10. [18] Dowlatabadi H, Hanova J. Strategic GHG reduction through the use of ground source heat pump technology. Environ Res Lett 2007;2:1–8. [19] Michopoulos A, Papakostas KT, Kyriakis N. Potential of autonomous groundcoupled heat pump system installations in Greece. Appl Energy 2011;88:2122–9. [20] Cui P, Li X, Manc Y, Fang Z. Heat transfer analysis of pile geothermal heat exchangers with spiral coils. Appl Energy 2011;88:4113–9. [21] Office of Energy Efficiency and Renewable Energy. Types of geothermal heat pump systems. Washington, DC: US Department of Energy; 2010. [22] Yang W, Shi M, Liu G, Chen Z. A two-region simulation model of vertical Utube ground heat exchanger and its experimental verification. Appl Energy 2009;86:2005–12. [23] Yang H, Cui P, Fang Z. Vertical-borehole ground-coupled heat pumps: a review of models and systems. Appl Energy 2010;87:16–27. [24] Ma GY, Chai QH. Characteristics of an improved heat-pump cycle for cold regions. Appl Energy 2004;77:235–47. [25] Blum P, Campillo G, Munch M, Kolbel T. CO2 savings of ground source heat pump systems – a regional analysis. Renew Energy 2010;35:122–7. [26] Center for Global Development. CARMA-carbon monitoring for action; 2007.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Geothermal heat pump systems: Status review and comparison with other heating options Stuart J. Self ⇑, Bale V. Reddy, Marc A. Rosen Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, Canada L1H 7K4
a r t i c l e
i n f o
Article history: Received 6 July 2011 Received in revised form 12 January 2012 Accepted 17 January 2012 Available online 12 February 2012 Keywords: Heating Geothermal energy Heat pump Thermal energy storage Efficiency Economics
a b s t r a c t Heating is a major requirement in many regions, and growing energy demands and pollutant emissions have allowed unconventional heating technologies to be considered, including geothermal. Geothermal heat pumps are reviewed, including heat pump technology, earth connections, current world status and recent developments. Geothermal heat pump technology and conventional heating systems are compared in terms of costs, CO2 emissions and other parameters. Geothermal heat pump use is economically advantageous when the price of electricity is low. Alternatively geothermal heat pump units have the lowest emissions depending when electricity is produced from a low emitting source. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A large portion of the global energy supply is used for electricity generation and space heating, with the majority derived from fossil fuels. Fossil fuels are finite resources and their combustion is harmful to the environment, through the emission of greenhouse gases, which contribute to climate change, and other pollutants. Demand for energy is increasing and future fossil fuel shortages are predicted [1]. Hammond [2] argues that fossil fuel depletion along with pollutant emissions and global warming are important factors for sustainable and environmentally benign energy systems. Such concerns have motivated efforts to reduce society’s dependence on fossil fuels, by reducing demand and substituting alternative energy sources. Alternative energy resources are sought that are more environmentally benign and economic than conventional fossil fuels. Beyond fossil fuels, the earth’s crust stores an abundant amount of thermal energy. Geothermal energy systems are relatively benign environmentally, with the emissions much lower than for conventional fossil fueled systems [3,4]. Geothermal energy is used in three main ways: electricity generation, direct heating, and indirect heating and cooling via geothermal heat pumps [4]. These three processes use high, medium, and low temperature resources, respectively. High and ⇑ Corresponding author. Tel.: +1 905 441 6697. E-mail addresses: [email protected] (S.J. Self), [email protected] (B.V. Reddy), [email protected] (M.A. Rosen). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.048
medium temperature resources are usually the product of thermal flows produced by the molten core of the earth, which collects in areas of water or rock. Low temperature resources are near ambient temperature and are mostly attributable to the solar energy incident on the ground and ambient air. High and medium temperature thermal resources are often deep within the earth [5], and the depth affects whether they can be exploited economically as drilling and other extraction costs can become high at great depths. Low temperature geothermal resources are abundant and can be extracted and utilized in most locations around the world. Extracting such thermal energy is relatively simple because the depths involved are normally small. Heat pumps extract low temperature thermal energy and raise the temperature to that required for practical use [4]. Geothermal heat pumps can provide an environmentally and economically advantageous option for space heating, and can also be utilized for space cooling. In this article, we review geothermal heat pump systems and recent developments and compare them with other heating options, with the objective of improving understanding of geothermal heat pump systems and increasing their utilization in appropriate applications. 2. Geothermal heat pumps Heat pumps can provide heat efficiently and economically with low emissions [6]. The concept of heat pumps has been recognized since the 1800s, and commercial applications have existed for
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about 60 years. Heat pumps move thermal energy from a lower to higher temperature medium similar to refrigerators [7]. The product of a heat pump is useable heat, usually at a temperature suitable to maintain a comfortable environment within a space. One of the most attractive characteristics of heat pumps is that they transport more thermal energy than the energy required to operate [4,8]. Geothermal heat pumps (GHPs), also referred to as ground source heat pumps (GSHPs), earth energy systems, GeoExchange heat pumps, ground-coupled heat pumps, earth-coupled heat pumps and ground-source systems [9,10], are comprised of three main systems: Geothermal heat pump: Moves heat between building and ground and modifies its temperature [11]. Earth connection: Facilitates heat extraction from the ground via a heat exchanger loop for use in the heat pump unit [11]. Interior heat distribution system: Conditions and distributes heat throughout the space [11,12]. 2.1. Heat pump systems Heat pumps operate using electricity to drive compressors that provide the necessary work for the concentration and transport of thermal energy [4,8]. Basic heat pumps operate on the vapor-compression refrigeration cycle. The working fluid within the heat pump is usually a refrigerant, with the selection dependent on the overall characteristics and requirements of the GHP system [6,13]. A GHP moves thermal energy between the earth and the heated space by controlling pressure and temperature by means of compression and expansion [4,8,11]. Five major components are incorporated in a heat pump (Fig. 1) [10,11,14]: compressor, expansion valve, reversing valve, and two heat exchangers. There are also various other minor components and accessories such as fans, piping and controls that assist in operation. GHPs for heating operate as follows [12]: 1. Thermal energy is extracted from the earth and transported to the evaporator. 2. Inside the heat pump unit cold refrigerant, in a liquid dominated liquid/vapor state, enters the evaporator. Heat is transferred from the earth connection to the refrigerant and causes the refrigerant to boil and become a low pressure vapor; the temperature increases slightly.
DESUPERHEATER
3. The vapor enters an electrically-driven compressor, where the pressure is increased, resulting in a high temperature and high pressure vapor. 4. High temperature vapor enters the condenser. The refrigerant is at a higher temperature than the space, inducing heat transfer from the refrigerant to the building. The refrigerant cools and condenses, yielding a high pressure, high temperature liquid. 5. Hot liquid passes through an expansion valve that reduces its pressure, resulting in a temperature decrease. The refrigerant enters the evaporator to begin another cycle. Many systems include a cooling mode that removes thermal energy from a space and rejects it to the ground. In the cooling mode, a reversing valve is used to move the fluid in the opposite direction in the cycle. The heat exchangers are reversed, with the earth connection heat exchanger becoming the condenser and the building heat exchanger the evaporator [8,12]. Some systems include a desuperheater (Fig. 1), which is an auxiliary heat exchanger that supplies heat to a hot water tank. Located at the compressor exit it transfers heat from the compressed vapor to water circulating through a hot water tank, reducing or eliminating the energy required for water heating [12]. Merit is usually evaluated in terms of energy efficiency, the ratio of product energy output to driving energy input, in percent. Heat pumps deliver more product heat than the input driving energy, so this definition yields an energy efficiency greater than 100%. To avoid this awkwardness, the term coefficient of performance (COP) is used for heat pumps, defined as the ratio of product thermal energy to input driving energy [9]. COPs for geothermal heat pumps usually range from 3 to 6, with the value dependent on the earth connection setups, system sizes, earth characteristics, installation depths, local climate and other characteristics [10,15].
2.2. Heat distribution systems The heat distribution system of a GHP system moves heat supplied by the heat pump throughout the space. Two main types of distribution systems exist: water to air and water to water. Water to air systems transfer thermal energy from the ground to air, which is used as the transport medium within the space, while water to water systems use water or another fluid as the heat transfer medium. The most common GHP system in North America is water to air, where an air coil, heated by the heat pump
COMPRESSOR
(not always present)
REVERSING VALVE
REFRIGERANTBUILDING SPACE HEAT EXCHANGER
GROUND LOOP HEAT EXCHANGER
REFRIGERANTGROUND LOOP HEAT EXCHANGER
EXPANSION VALVE GROUND LOOP CIRCULATION PUMP Fig. 1. Basic layout of geothermal heat pump system including desuperheater.
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condenser, warms air that passes over it. The air is moved throughout the building using HVAC ducts and air vents [12,16]. In water to water systems, also commonly known as hydronic systems, thermal energy is extracted from the ground loop, processed by the heat pump and distributed throughout the building using water as the carrier. The system pumps water through the condenser of the heat pump unit, extracting heat. Water is then pumped around the building delivering the heat to the space using in-floor radiant heaters, radiators or localized air coils. These systems heat using relatively low temperatures compared to conventional forced air systems. The warmest air in rooms that are heated with a forced air furnace ascends towards the ceiling, leaving a lot of living space at a cooler temperature. In order to have living space at a desired temperature the temperature of the air entering a space must be higher than that of the space. A room with in-floor radiant heaters will have a more uniform temperature from the ceiling to the floor and require lower operating temperatures to provide suitable conditions within living spaces [8,15,16]. Hybrid systems also exist, which combine both types of distribution methods to provide system flexibility and enhanced control of space temperature.
2.3. Earth connections Whereas air source heat pumps use the ambient air as a heat source, geothermal heat pumps use surrounding ground. Ambient air exhibits a wide variation in temperature throughout the year and on a daily basis, compared to ground [17]. On a daily basis the ground temperature fluctuates at depths shallower than 0.8 m; at greater depths the temperature variation decreases [18]. The variations are more pronounced on a seasonal rather than daily basis [19]. Fig. 2 shows the annual change in ground temperature with increasing depth for Ottawa, Canada. As the depth increases the extreme warm and cold temperatures converge. The depth where the ground temperature becomes constant depends on factors such as incoming solar radiation, snow cover, air temperature, precipitation and thermal properties of the ground. In Canada constant annual temperatures are usually observed at depths beyond 10 m [18]. For example, Fig. 3 illustrates seasonal ground temperature variations for Ottawa at different depths.
Ambient Air Temperature Range
Annual Mean Ground Temperature
Ground Temperature Range
Temperature ( ºC) Fig. 2. Variation in ground temperature with depth for Ottawa, Canada. Modified from Ref. [12].
Temperature (ºC)
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5.0 m 2.0 m
0.3 m Winter
Spring
Summer
Fall
Winter
Fig. 3. Annual ground temperature range for different depths for Ottawa, Canada. Modified from Ref. [12].
GHPs exploit the relatively constant temperature in the ground, which is warmer than the ambient air during winter and cooler in summer [17]. The ground temperature remains nearer to the desired temperature inside a building. When there is a large variation between inside and outside temperatures, as is the case for air source heat pumps, more work is required to provide the same degree of heating, which reduces the COP [14]. If an excessive temperature difference exists, heat pump systems do not operate as intended. Earth connection or ground loop heat exchangers are comprised of a collection of pipes that transfer fluid between the heat pump unit and the ground. Two main ground loop designs exist: double loop and single loop configurations. 2.3.1. Double loop configuration A double loop configuration is the most common system configuration and involves an earth connection that is separate from the heat pump. Heat is transferred to the refrigerant via a heat exchanger from water or a water/antifreeze mixture, which is circulated through piping from the heat pump unit to the ground. The standard pipe currently is composed of polyethylene or polypropylene and has an inner diameter of 19 mm (3/4 in.) for small and medium size applications [7]. Two types of double loop configurations exist: closed and open. 2.3.1.1. Closed loop systems. In closed loop systems, which are commonly utilized, the heat transfer fluid is enclosed in a circulating loop and has no direct contact with the ground; heat transfer with the ground occurs through the piping material [20]. There are four classes of closed loop heat exchange systems: vertical, horizontal, spiral, and pond. 2.3.1.1.1. Vertical closed loop. A vertical closed loop system includes a loop field consisting of vertically oriented heat exchange pipes. A hole is bored into the ground, typically ranging 45–75 m deep for residential and over 150 m for larger industrial applications. Pairs of pipes, connected at the bottom by a U-shaped connector, are fed into the hole (Fig. 4) [21]. To enhance heat transfer, the gap between the pipes and the borehole wall are filled with a pumpable grout material [20,22]. The borehole diameter is approximately 102 mm for a typical residential home. For a typical residential application the spacing between boreholes is around 5–6 m in order to prevent adjacent boreholes from affecting one another and changing ground conditions [21,23]. To assure equal flows for multiple borehole systems a manifold system is used, which can be located in the building or buried in the loop field [23]. An advantage of the vertical loop configuration is reduced installation area [22], making them advantageous where land is limited. Another incentive for these systems is low landscape disturbance, since drilling
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Fig. 7. Horizontal loop with piping in parallel for a geothermal heat pump.
Fig. 4. Vertical closed loop heat exchange system for a geothermal heat pump.
has a reduced impact compared to trenching [17,23]. Also, locating the piping deep in the ground, where the temperature is constant year round, allows consistent heat pump performance and reduces overall loop length [20,23]. The main disadvantage of using a vertical system is the installation costs, since drilling is normally more costly than horizontal trenching. Consequently, vertical loop systems are normally more economic for larger applications [9]. 2.3.1.1.2. Horizontal closed loop. In horizontal closed loop systems, which are common where ample ground area is available, the ground loop is laid out horizontally slightly below the earth’s surface in backfilled trenches. The arrangement of the loops can vary depending on heat transfer requirements and land availability. The three most common configurations are basic loop (Fig. 5), series loop (Fig. 6), and parallel (Fig. 7). The basic layout usually requires a substantially larger land surface area than the series and parallel setups. The series layout is common for its reduced land require-
Fig. 5. Basic horizontal loop configuration for a geothermal heat pump.
Fig. 6. Horizontal loop piping in series for a geothermal heat pump.
ment and simplicity [9]. Series and parallel loops can also be combined, increasing the flexibility horizontal installations. Horizontal heat exchange systems are normally more cost effective than vertical for residential installations, through lower costs involved with trenching compared to drilling [9]. The trenches that house the piping usually do not exceed more than a couple meters below the surface, but in areas of frost they are located below the frost line. With shallow depths, there is increased interaction between the soil and the ambient environment, which results in daily and annual variation in ground temperatures, which affects the heat transfer and system performance. Other factors that affect heat transfer characteristics include rain, snow, vegetation growth and shade [9]. These factors usually cause horizontal systems to require more piping than vertical arrangements. Horizontal arrangements require a water/antifreeze mix to protect against freezing during winter in cold climates [9]. 2.3.1.1.3. Closed spiral loop. Spiral loop arrangements are similar to conventional horizontal loops, as they are typically horizontally oriented within shallow trenches. But the piping is laid out in circular loops within the trench. The end of each spiral has a straight return pipe to the heat pump [9,24]. Spiral loops require less area than conventional horizontal loops and have lower trenching requirements, but they need greater piping lengths for a fixed load [20]. A variation of the spiral-loop system involves placing the loops upright in narrow vertical trenches. The main advantage of this vertical-loop layout is reduced horizontal area requirements, which allow different trenching equipment to be used, sometimes yielding advantageous economics [17]. Note that spiral loops can reduce initial costs when trenching constitutes a substantial portion of the GHP system cost, but not necessarily when materials costs are high [21]. Other disadvantages of spiral loops are as for horizontal setups, including poorer heat transfer and greater area requirements. Spiral loops have greater pumping requirements than other horizontal systems due to the added pipe length, lowering system COP. 2.3.1.1.4. Closed pond loop. Closed pond loops, which are the least common of the closed loop heat exchange systems, are essentially a spiral loop systems submerged in a water body. The coiled piping is attached to framework and submerged using concrete anchors. The framework is typically supported 23–48 cm above the pond bottom to allow for convective flow around the piping [21]. The loop is normally at least 1.8 m below the water surface. It is necessary to assure sufficient thermal mass is maintained during low water conditions and prolonged draughts, and to ensure the temperature of the water immediately surrounding the loop never drops below the freezing point of water during cold seasons. Rivers are not ideal for this application due to their unpredictable behaviors, including flooding and draughts that can damage systems as well as hazards due to moving debris [9,24]. Pond loops are gaining popularity partly because they potentially require less piping than other closed loop systems, due to
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their superior heat transfer characteristics, and they require neither drilling nor trenching [9]. The main disadvantages of this system include the requirement of a sufficiently large body of water and the limitations on its use for other purposes such as boating. 2.3.1.2. Open loop systems. Open loop heat exchange systems interact directly with the ground. These systems use local groundwater or surface water, such as lakes and ponds, as a direct heat transfer medium. The water is extracted and passed through the heat exchanger of the heat pump, and then discharged back to the source or on the ground for irrigation [9]. Currently the use of water in abandoned mines is being explored since water filled mines may be able to heat towns inexpensively using heat pump technology. Open systems tend to be used for large installations. The largest GHP currently in operation utilizes an open loop system and supplies about 10 MW of heating to a hotel and connected offices [9]. There are three common open loop configurations: extraction wells, extraction and reinjection wells, and surface water systems. The most common type has extraction and reinjection wells (Fig. 8). Water is extracted from a drilled production well reaching the water table and, after passing through the heat pump heat exchanger, is injected back into the water table a distance from the production well, which is sufficient to allow adequate heat transfer from the ground to the water between the wells [9]. Reinjection can be excluded; open drainage is inexpensive but requires the water source, supplying the heat pump, to have a high capacity with little draw down in order to provide prolonged use [14]. The water flow rate to the heat pump unit is generally between 5.7 and 11.4 litres per minute per ton of heating capacity. An advantage of open loop setups is that the source water temperatures remain nearly constant. Also the losses associated with the extra heat exchanger needed for closed loop systems are avoided, increasing the ground heat pump COP [18]. Depending on the extraction method used, open loop can have high pumping loads but their overall COPs are usually high, reducing operating costs [9]. Further, open loop arrangements require less drilling than vertical closed loops and have simpler ground-connection designs and lower associated initial costs. The amount of water that can be extracted for open loop GHP systems is sometimes limited by local water resource regulations [9]. The main disadvantage of open loops is the need to protect water quality, usually by following clean water and surface water regulations; sometimes open loop systems not allowed [18]. The
heat exchanger between the heat exchange loop and the heat pump unit is subject to corrosion, fouling and scaling, so the water should have a fairly neutral chemistry and a low amount of minerals such as iron [24]. If the water chemistry is not neutral wells may require maintenance, increasing user involvement [9]. 2.3.2. Single loop configuration In single loop configurations, also known as direct exchange systems, the heat pump working fluid flows through the ground heat exchanger, which avoids the need for a ground loop to heat pump heat exchanger. Essentially the ground loop becomes the heat pump evaporator during heating applications [9]. Single loop arrangements also exclude the ground loop circulation pump, instead relying on a slightly larger compressor. These measures increase the overall ground heat pump COP [18]. Copper piping is commonly used in these systems due to its superior heat transfer characteristics, reducing the required pipe area. Direct exchange systems are pressurized, necessitating good engineering [18] since the probability of rupture is increased due to substantial above ground forces or abnormal operation. If piping is damaged the entire system may need to be dug up for repairs. Another disadvantage involves the requirement for increased refrigerant to accommodate the volume of the ground loop, which increases cost [9]. Nonetheless, single loop GHP systems are gaining popularity globally due to their higher COPs, and some countries (France and Austria) are exploring direct exchange units with direct evaporators coupled with direct condensers through the implementation of floor heating [9]. 2.4. Global status The main advantage of geothermal heat pumps is their ability to utilize soil and ground water temperatures between 5 °C and 30 °C, which is common at reasonable depths around the world [15]. As of 2004 about 30 countries were utilizing GHP systems, with the leading countries including USA, Sweden, Germany, Switzerland, Canada and Austria. Table 1 outlines the installed GHP capacity for several countries. As of 2004 the worldwide installed GHP thermal capacity was around 12 GW which required an annual energy usage of 20 TWh. The technology is gaining popularity in France, Netherlands, China, Japan, Russia, UK, Norway, Denmark, Ireland, Australia, Poland, Romania Turkey, Korea, Italy, Argentina, Chile, Iran, UK and Norway [15]. Since 1994 the annual growth rate for geothermal heat pumps has been about 10%, which is approximately 1.7 million applications currently [12]. The continental United States and Europe are presently the leaders in terms of growth of the technology. The growth of GHP technology has been slower than for some other renewable and conventional energy technologies. Limited growth can be attributed to many factors including non-standardized system designs, significant capital cost compared to other systems, limited individuals knowledgeable in the installation of GHPs, limitations placed on use through government policies,
Injection well
Pump
Production well
Table 1 Leading countries using geothermal heat pumps as of 2004 [9].
Water table
Fig. 8. Open loop heat exchange system with production and injection wells for a geothermal heat pump.
Country
Installed thermal capacity (MW)
Annual energy use (GWh)
Number of GHP installations
US Sweden Germany Switzerland Canada Australia
6300 2000 560 440 435 275
6300 8000 840 660 300 370
600,000 200,000 40,000 25,000 36,000 23,000
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and economies of scale and scope are rarely exploited [6,18]. Even though these issues exist they are being resolved on an ongoing basis, which is increasing the acceptance of the technology [15].
3. Recent developments Various developments in GHP systems have been reported recently.
4.1. Efficiency Geothermal heat pumps have high efficiencies, as reflected in their COPs. Typical equivalent COPs for various heating systems follow: ground source heat pumps: 3–5, air source heat pumps: 2.3–3.5, electric baseboard heaters: 1, mid-efficiency natural gas furnaces: 0.78–0.82, and high-efficiency natural gas furnaces: 0.88–0.97. 4.2. Economics
3.1. Auxiliary component cooling Since compressors and pumps are not 100% efficient, they release waste heat during their operation. Compressor and pump waste heat can be used to preheat the refrigerant within the heat pump cycle. This could be accomplished by having cool refrigerant enter a sealed hermetic shell enclosing the compressor or pump along with their electric motor. Preheating increases the performance of the components, increasing the COP of the overall GHP system, as well as reducing the ground loop heat exchanger load requirements [8]. 3.2. Ground frost loop GHP systems have started to be utilized in areas of permafrost. Heat transfer from building foundations can melt the permafrost and compromise structural integrity. Through installing a ground loop close to the foundation, permafrost melt can be reduced or eliminated. Heat is extracted from the area around the foundation ensuring that the building does not affect the ground temperature drastically. The heat extracted is used to supply supplemental heat to the building, typically 20–50% of the total building heating load [12]. The system should not keep the ground frozen outside of its natural annual cycle [12], and should not disturb the local ecology. The heat exchange loop should be reliable to prevent failures, which could impact a building’s structural stability [12]. 3.3. Standing column well heat exchange systems Standing column wells combine aspects of open and closed water heat exchange systems. They are essentially groundwater heat pump systems that use groundwater drawn from wells in a semi-open loop arrangement. In such a system, a vertically drilled borehole draws warm water from the bottom of a deep rock well using a submersible pump and feeds it to the heat pump unit. The cool return water is injected near the top of the original well. The cold water moves down towards the extraction pipe and is heated by the surrounding rock [9], eliminating the need for a separate injection well. Standing column wells are recently receiving increased attention because of their good overall performance in suitable regions. These systems are installed in locations having bedrock within 45–60 m of the surface. Domestic wells that have been used for drinking water can be retrofitted easily to accommodate this system. This system can also be applied to water filled mines and tunnels [9].
4. Analysis and comparison of heating systems A comparison is presented of the following heating systems: geothermal heat pumps, air source heat pumps, electric baseboard heaters, and natural gas furnaces (mid and high efficiency). Three Canadian provinces (Alberta, Ontario and Nova Scotia) are considered, and efficiencies, costs and emissions are assessed. The results are presented in Tables 2 and 3. European trends are also explored.
Geothermal heat pumps have substantially higher initial costs than conventional heating systems, mainly because of the capital costs of the heat pump unit and the ground connection (including drilling or trenching). But, geothermal heat pumps can have low operating costs due to their high efficiencies. 4.2.1. Economics: Canadian trends For the analysis of the situation in Canada, the initial cost of each system is assumed the same in all locations. The annual heating cost is based on the cost of electricity and natural gas in the specific province. A life span of 20 years and an average COP of 4 for the geothermal system is assumed. Typically geothermal heat pumps have warranties of 20–25 years, but systems exist that have been in operation in excess of 30 years. It is assumed that the installation does not require the installation of new duct work. The evaluated costs are summarized in Table 2. It is demonstrated that the economic feasibility of geothermal heat pumps depends strongly on location. The prices of electricity, natural gas and other heating fuels vary regionally. In Alberta and Nova Scotia the GHP is the most economically competitive option. In Ontario the air source heat pump is determined to have a slightly lower cost after 20 years. This result is influenced by the fact that Alberta and Nova Scotia have higher electricity prices than Ontario. High electricity prices allow for greater economic savings over air source heat pumps and electric baseboards. It is also found that when the cost of natural gas is low the difference narrowed between the cost of natural gas furnaces and GHPs. Where natural gas or other heating fuels are inexpensive, geothermal heat pumps may not be the most economic option [18]. For locations requiring air conditioning GHP systems exhibit an increased economic advantage because heat pumps work in reverse, allowing them to remove heat from a building and reject it into the ground. While conventional heating systems require a separate air conditioner for space cooling, GHP systems avoid this initial cost [24]. The payback period for a GHP system is typically between 6 and 20 years, depending on capital costs, energy prices and energy price increases [18]. Another advantage not quantified in the study relates to property value. GHPs tend to increase property value, which allows for high return on investment in the building and land and promotes more desirable mortgage assessments [18]. Note that GHP systems are most cost effective if installed during construction of the building or when an old heating system needs to be replaced. The purchase and installation of a GHP, as a replacement of a working system, is seldom worthwhile from energy and economic perspectives [14]. 4.2.2. Economics: European trends Table 4 illustrates the natural gas and electricity prices for various countries within the European Union. The analysis assumes a static heating load for all countries and a 20 year lifespan, and compares costs (including initial costs) for geothermal heat pumps, air source heat pumps, electric baseboard heaters, and natural gas furnaces (mid and high efficiency). For simplicity, the initial costs are assumed identical to those used for the Canadian comparison.
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S.J. Self et al. / Applied Energy 101 (2013) 341–348 Table 2 Comparison of economic parameters for various heating systems in several locations. Heating system
Geothermal HP Air source HP Electric baseboard Natural gas furnace a Natural gas furnace b
Capital cost ($)
Alberta
Ontario
Nova Scotia
Annual heating cost ($)
Present worth ($)
Annual heating cost ($)
Present worth ($)
Annual heating cost ($)
Present worth ($)
9000 4900 1550 1500
601 813 2257 1276
21,020 21,160 46,690 27,020
328 444 1231 2344
15,560 13,780 26,170 48,380
649 877 2432 1885
27,230 27,940 50,190 44,750
1900
1109
24,080
1049
22,880
1653
40,460
Monetary units are in 2009 Canadian dollars. Present worth values are for a 20 year period. a Represents mid efficiency. b Represents high efficiency. Table 3 Comparison of CO2 emissions for various heating systems in several locations. Heating system
Geothermal HP Air source HP Electric baseboard Natural gas furnacea Natural gas furnaceb a b
Annual fuel use (kWh)
Alberta Emission intensity (kgCO2/kWh)
CO2 emission (kg)
Emission intensity (kgCO2/kWh)
Ontario CO2 emission (kg)
Emission intensity (kgCO2/kWh)
Nova Scotia CO2 emission (kg)
6080 8214 22,280
1.12 1.12 1.12
6826 9222 25,015
0.188 0.188 0.188
1143 1544 4188
1.04 1.04 1.04
6346 8573 23,255
28,475
0.190
5410
0.190
5410
0.190
5410
24,655
0.190
4684
0.190
4684
0.190
4684
Represents mid efficiency. Represents high efficiency.
Table 4 Natural gas prices, electricity prices, and CO2 emission associated with electricity production for several countries in the European Union (EU) [27,28]. Country
Natural gas price ($/kWh)
Electricity price ($/kWh)
Electricity emission intensity (kg CO2/kWh)
Austria Belgium Cyprus Czech Rep. Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Netherlands Norway Poland Portugal Slovak Rep. Slovenia Spain Sweden Switzerland UK EU average
0.08 0.08 N/A 0.07 0.15 0.05 N/A 0.08 0.08 N/A 0.07 0.07 0.10 0.05 0.06 0.07 0.10 N/A 0.07 0.09 0.06 0.09 0.07 0.12 N/A 0.06 0.08
0.27 0.28 0.27 0.19 0.39 0.14 0.20 0.18 0.35 0.17 0.22 0.27 0.27 0.15 0.17 0.25 0.25 N/A 0.20 0.24 0.23 0.20 0.26 0.26 N/A 0.21 0.23
0.239 0.311 0.974 0.922 0.680 1.015 0.403 0.108 0.626 0.882 0.695 0.706 0.565 0.443 0.367 0.367 0.619 0.015 1.108 0.630 0.382 0.392 0.493 0.076 0.041 0.558 0.486
Monetary units are in 2009 Canadian dollars.
Europe has high natural gas and electricity costs but it appears that the increases in cost over the Canadian situations are relative. For the majority of the European countries considered, geothermal
heat pumps are economically advantageous compared to conventional heating methods where the overall cost of installing and operating the geothermal heat pump is considerably lower over a 20 year lifespan. Within Germany, Ireland, Luxembourg, Slovokia Rep., Spain and the United Kingdom it is found that the use of high efficiency natural gas furnaces is more economical, which is a result of high prices for electricity compared to natural gas. This study provides a general overview of heat pump implementation within European countries. Heating loads vary across the countries, introducing some inaccuracies in this study. Implementation of geothermal heat pumps in locations with low heating requirements might not be as economical, since the capital cost of geothermal heat pump units are significant. Alternatively the capital cost of heat pump units in warmer climates would be reduced through the implementation of smaller units. More in-depth analysis considering local climate is required for a detailed understanding of the outcomes of heat pump implementation for specific European countries.
4.3. Carbon dioxide emissions In this assessment, we compare carbon dioxide (CO2) emissions for heating systems. Although other pollutants are significant, we focus here on CO2 since it is the most common greenhouse gas and is considered the main contributor to climate change [18]. Geothermal heat pumps do not directly emit CO2; rather the emissions originate in the power plants that produce the electricity [16]. When electricity is produced in high-emission power plants, the CO2 emissions of GHP systems are correspondingly high. The threshold at which GHP systems become environmentally advantageous is related to the CO2 associated with producing the electricity used by the heat pump, its COP, and the efficiency of conventional heating systems [25].
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4.3.1. Carbon dioxide emissions: Canadian trends The CO2 emissions are determined for the situation in Canada considering the amount of electricity or natural gas consumed by a device and the fuel emission intensity (mass of CO2 emitted per kWh energy produced). We again examine the three provinces considered earlier. Assuming the natural gas composition is identical for Alberta, Ontario and Nova Scotia, the emissions per unit gas consumed is fixed. The average emission intensity for each province is used, taken from the Carbon Monitoring for Action (CARMA) [26] online database. The CO2 emissions of various heating systems, for each province, are listed in Table 3. Since Ontario has low emitting power generation, with over 50% of its electricity production from nuclear energy and the remainder mainly divided between natural gas fired plants and hydroelectric plants, the application of GHPs is advantageous environmentally. Over 80% of the electricity production in Alberta and Nova Scotia is from fossil fuels, including coal and natural gas plants [16]. Relative to a high efficiency (95%) natural gas furnace, GHP systems offer emission reductions when the emission intensity of the electricity produced is less than 0.76 kg/ kWh [18]. In general GHP systems provide the largest emissions reductions relative to conventional electrical heating devices and natural gas fired systems when the electricity used by heat pumps is derived from environmentally benign power plants. In jurisdictions with high CO2 emitting power plants, geothermal heat pumps have less CO2 emissions than other systems utilizing electricity. When renewable energy is used to produce electricity, GHP technology uses only renewable energy in operating, with little or no carbon emissions. Overall, geothermal heat pumps normally provide the greatest (or almost the greatest) emissions reductions. 4.3.2. Carbon dioxide emissions: European trends Carbon intensities associated with electricity production within various European Union countries are listed in Table 4. Using the carbon emission threshold associated with electricity production, determined by Dowlatabadi and Hanova [18] to be 0.76 kg/kW, it can be seen that the majority of the countries listed would encounter CO2 emission reductions with the implementation of geothermal heat pump units instead of conventional heating units. Using geothermal heat pump units within a country could significantly reduce the overall CO2 emissions of the country. Coupling ground source heat pumps to the current United Kingdom electricity grid, for example, can lead to reductions in CO2 emissions of over 50% compared to conventional space heating technologies based on fossil fuels, considering the current generation mix on the UK grid [15]. 5. Conclusions Geothermal heat pumps are highly efficient heating technologies that allow for reductions in CO2 emissions, the potential avoidance of fossil fuel usage and economic advantages. Heat pumps utilize significantly less energy to heat a building than alternative heating systems. Many variations of geothermal systems for heating exist, with different configurations suitable in different situations and most locations around the world. In deciding among heating options, it is important to determine the benefits for different ground heat pump options, typically in terms of efficiency, emissions and economics.
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