Ground-source Heat Pumps State of the Art

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1 Ground-source Heat Pumps State of the Art Constantine Karytsas Abstract Geothermal or Ground Source Heat Pump (GSHP) systems provide heating, cooling as well as domestic hot water, by the use of the underground or bodies of surface water as heat source or sink. More specifically, a GSHP system comprises of a Heat Pump (usually waterto-water) and a ground source system (ground heat exchanger or groundwater well) in order to provide heating and cooling to the building through a low-temperature heating system and domestic hot water as well. The main concept of a GSHP system is the maximization of the heat pump efficiency i.e. minimization of electricity consumption mostly due to underground temperature which is almost invariant throughout the year. So, it is clear that GSHP systems have contribution to the environmental protection and reduction of greenhouse gas emissions. GSHPs are a mature industry, with a continuous trend of development and due to their increasing energy efficiency due to recent advances of the technology (described in the following chapters) are attractive and are more and more considered as an excellent substitute of conventional heating/cooling sources including air source heat pumps and VRVs. Due to these advances in several cases the high efficiency of GSHP compared to the initial capital cost minimizes the need for subsidies. 1. Introduction Geothermal Heat Pumps, or Ground Source Heat Pumps (GCHP), are systems combining a heat pump with a ground heat exchanger (closed loop systems), or are fed by ground water from a well or an open surface water body (open loop systems). They use the earth as a heat source when operating in heating mode, with a fluid (usually water or a water-antifreezemixture) as the media transferring the heat from the earth to the evaporator of the heat pump, utilising that way geothermal energy. In cooling mode, they use the earth as a heat sink. Geothermal heat pumps can offer both heating and cooling at virtually any location, with great flexibility to meet any demands. More than 20 years of R&D focusing on GSHPs in Europe resulted in a well-established concept of sustainability for this technology, as well as sound design and installation criteria (Lund et al, 2004). Recent developments include the Thermal Response Test, which allows in-situ-determination of ground thermal properties for design purposes, and thermally enhanced grouting materials to reduce borehole thermal resistance. For cooling purposes, but also for the storage of solar or waste heat, the concept of Underground Thermal Energy Storage (UTES) has been proven successful. Systems can be either open (aquifer storage) or can use BHE (borehole storage). While cold storage meanwhile is established on the market, heat storage, and in particular high temperature heat storage (>50 C) is still in the demonstration phase. A rapidly growing field of applications is emerging and developing in various European countries, utilizing shallow geothermal resources. Hence, a rapid market penetration of such systems is resulting; the number of commercial companies actively working in this field is

2 ever increasing and their products have reached the yellow pages stage (Geothermal Energy Barometer, 2003). 2. GSHP technology status A ground coupled (or ground source, or geothermal) heat pump system consists of three components: ground heat exchanger or groundwater well, water source heat pump and heating/cooling system in the building. 2.1 Ground heat exchanger The ground heat exchanger comprises pipes buried in the ground, either in a horizontal layout at 1,2-2,0 m depth within trenches (horizontal ground heat exchanger see Figures 1a and 3) or in a vertical layout within boreholes (borehole heat exchanger see Figure 1b). Typical piping material is HDPE, which gives a life span of at least 50 years to the pipe, and typical external pipe diameters are 32 or 40 mm. Depending on the design operation temperature range, the pipe may be filled with water or a mixture of water and antifreeze. In horizontal systems, also the refrigerant from the heat pump cycle may flow through the ground pipes, and in vertical systems, borehole heat exchangers following the heat pipe principle have been introduced in recent years. A further option is to use groundwater wells, where the water pumped from the ground is used as heat source or sink (open system see Figure 1c). Although horizontal ground heat exchangers are of lower cost, the majority of installed systems use Borehole Heat Exchangers (BHEs), which give better energy performance to the system, but most important, they have much less space requirements. Typical BHE technology comprises a single or double U-tube (Figure 2) placed within one or more vertical boreholes m deep each. The space between the U-tube and the walls of the borehole may be filled with groundwater (Scandinavian practice), if the local groundwater table is high enough, and interference between different groundwater horizons is not problematic, or, more often, it is filled by the grouting material (Figure 3). The grout isolates individual water bearing formations from one another, eliminating that way any vertical flow between them through the borehole. A good grout should adhere well to both the U-tube and the borehole walls, leaving no cavities which hinder heat transfer. Special thermally enhanced grouts are available in the market with high thermal conductivity and excellent heat transfer properties. Rohre in Graben Verteiler im Haus Grundwasserspiegel Pumpe Serienschaltung Figure 1: Ground heat exchanger types: horizontal (a), BHE (b), groundwater production and reinjection wells (c) (Mendrinos et al, 2007) 2

3 Meanwhile BHE are offered by several manufacturers as standard products, tested and certified (see Figure 2). Also a range of other products (grouting material, connection pipes, manifolds, circulation pumps) have been designed especially for GSHP installations and can be bought off the shelf. Figure 2: BHE lower part or footpart ; factory made BHE from different producers (Mendrinos et al, 2007) Figure 3: View of a grouted double-u BHE top (Mendrinos et al, 2007) Typical operating temperature range of ground heat exchangers are -3 C 0 C in Sweden and -3 C +2 C in Germany, where ground temperatures of 8,5-9,0 C respectively are observed in natural state. In these conditions a horizontal ground heat exchanger yields a typical geothermal energy supply to the heat pump of 8-32 W/m², and a borehole heat exchanger of W/m, depending on subsurface geology and water saturation. In cases of strong horizontal groundwater flow heat extraction rates up to 80 W/m are possible for BHE, in single plants. In BHE fields (Figure 4), the mutual interdependence of the BHE must also be considered; such a field develops a clear storage effect. 3

4 Figure 4: Field of BHEs just after completion (at the German/Belgian border, photo EWS) (Mendrinos et al, 2007) In South Europe, where ground temperatures at natural state of C are common, the above heat extraction values can be increased by 50% for the same operating temperatures of the ground heat exchanger. In case no antifreeze is used, the same heat extraction rates as in central Europe can be achieved, but with higher operating temperature of +3 C +8 C within the ground heat exchanger. This fact also results in superior energy performance of the whole ground coupled heat pump system. In large systems, where oversizing of the ground heat exchanger would result in severe cost penalties, the thermal properties of the ground and the thermal performance of the BHE can be measured by the thermal response test. During a thermal response test, heat is transferred to the fluid of a BHE and the output temperature is measured. The effective ground thermal conductivity and the thermal resistance of the BHE are two parameters that are calculated by using an approximation of the line-source-theory, or by fitting the resulting temperature transients from computer simulation with the measured ones. Usually the thermal response test is done immediately after the first BHE is constructed, and its role is to define with great accuracy the exact number of BHEs needed for a specific system. 2.2 Water source heat pumps In GSHP applications water source heat pumps (Figure 5), mainly of water-to-water type are installed, but some manufacturers offer water-to-air types as well. This choice is dictated by the building practice in the different areas, with the hydronic system being the majority choice in central Europe. The heat pumps are used for heating and cooling of buildings, as well as for supply of domestic hot water. Because they use water, which has much better heat transfer properties than air, and because of the stable temperature supplied by the ground heat exchanger, which is higher than extreme ambient conditions in cases of peak heating load and lower than ambient extremes in peak cooling load, a well designed and constructed ground coupled heat pump system operates with at least 30% higher energy efficiency than the best air source heat pumps. Figure 5: View of water source heat pumps at the Town Hall of Pylaia, Thessaloniki (Mendrinos et al, 2007) 4

5 Despite what was the case 10 years ago, nowadays water source heat pumps of high efficiency are available in the market. They usually use scroll compressors of on-off regulation and R407C or R134a as working fluids with the trend being to shift to R410A, which has better heat transfer properties and better performance for reversible systems for heating and cooling operation. A future trend is also to introduce variable capacity compressors. The coefficient of performance (COP) of ground source heat pumps is defined as the ratio of useful energy delivered over the electricity consumption. SPF is the integration of COP over the heating and/or cooling season. Unlike air source heat pumps, the values of COP and SPF of a ground source heat pump are closer to each other due to the stable operating parameters of a ground-coupled system. In general, the lower the temperature difference between the ground heat exchanger and the water of the building s heating/cooling system, the higher the COP. Values of both COP and SPF in the range of 4,2-5,0 are typical for operation with a ground heat exchanger and a floor heating system. In case the heat pump is coupled to a groundwater well instead, COP and SPF values in the range of 5,0-6,5 are typical. 2.3 Building heating/cooling system As mentioned above, the energy performance of a ground coupled heat pump system is enhanced when the operating temperature of the building heating system is lower. In case of cooling operation, higher temperature of the cooling system results in better energy performance. Heating systems that require low operating temperature are floor heating and wall heating, followed by fan-coils and air handling units coupled with air ducts. In case of cooling, the best systems are ceiling or wall cooling. COP measurements in Europe, mainly in the Swiss heat pump test centre in Töss, already show that for a source temperature of 0 C, values close to COP = 5 can be achieved for 35 C heating supply temperature, and still values around COP = 3.5 for 55 C supply temperature (Figure 6). Figure 6: Values of COP for brine/water heat pumps (as used typically in geothermal heat pump systems), measured in the Heat Pump Test Centre Toess (Sanner et al, 2003) Although the maximum COP of existing ground source heat pumps is around 4,5 their mean COP during operation is lower. This mean COP, usually called Seasonal Performance Factor (SPF), is defined as the mean COP during operation and varies at around SPF=3,0-3,8. In cases where high quality standards for all components of a geothermal heat pump system are applied and also an optimum building heating system exists, values of SPF=4,0 5

6 can be achieved; in these cases usually no domestic hot water can be provided by heat pump. 3. Ground source heat pumps market in Europe Although the technological know-how of ground coupled heat pumps is well developed in Germany, Sweden, Switzerland, France and Austria (Figures 7 and 8), only in Sweden and Austria the corresponding market position of GCHPs is leading, where they are one of the standard systems for heating of buildings. Of course, in Germany, Switzerland, France and Finland there is a developed market for GCHPs, the growth of which has accelerated during the last 12 months. Elsewhere in EU, however, we have a new market. In Sweden, a rule of thumb is to size the ground coupled heat pump at approximately 67% of peak load, which corresponds to 95% of heating needs. That way, the economics of the system are improved considerably and ground coupled heat pumps can effectively compete with fossil fuels and air source heat pumps with typical payback times between 5-10 years. Figure 7: Installed capacity of ground coupled heat pumps in Europe (Mendrinos et al, 2007) Figure 8: Number of ground coupled heat pump systems in Europe (Mendrinos et al, 2007) 6

7 Figure 9: Average size of ground coupled heat pump systems in Europe (Mendrinos et al, 2007) Heating power [kw] Year of commissioning No. of units Heating power Figure 10: Ground coupled heat pumps market evolution in Germany and Romania showing the different levels of market penetration of the technology between Central European and Mediterranean countries Unlike the other renewable energy sectors, geothermal sector growth appears to be on the right track for reaching the White Paper objectives outlined for Starting with 1995 up to 2008 the annual new installed heat pumps had been increased by 5 times. This means that more than systems had been installed in the year 2008 (Figure 11) (ETP-RHC, 2009). The quantitative development of the European geothermal market in the next ten years that is until 2020 is expected to be fuelled mainly through the introduction and consolidation of shallow geothermal systems, with a quite mature market in Sweden and Switzerland and developing markets in Austria, Germany and France.. In other emerging European markets in which a high growth is possible, it is expected over the next years (Italy, Greece, France, Spain, UK, Hungary, etc.). Mature countries (namely Sweden and Germany) will see a steady increase, mainly fuelled by sales in the building renovation sector, but all other countries will see a significant growth. Fast development for geothermal heat pumps illustrates how shallow geothermal energy resources, previously regularly neglected, have become very significant, and should be obviously taken into account in any energy development scenario. 7

8 Figure 11: Annual amount of geothermal heat pump installations in the European Union (ETP-RHC, 2009) Ktoe ,5 NREAP EGEC Years Figure 12: Development of GSHP in the next 10 years (2020) according to NREAP (National Renewable Action Plans) and EGEC (European Geothermal Energy Council). 4. Conclusions As described analytically in the previous chapters Geothermal or ground-source heat pumps (GHP/GSHP) are systems that transform the heat of the ground, the shallow earth and the earth s surface water bodies into useful space or water heating with the support of electricity. GSHP can be open- or closed-loop, and can be used also for cooling/domestic hot water in any type of built environment such as in single or multi-family houses/apartements, industrial, hotels, education, and office buildings. Open-loop systems may draw water from the ground 8

9 and from the water bodies of the earth s surface such as the sea, lakes, ponds and rivers. These sources are the means of the heat source/sink. Closed-loop systems, also called earth-coupled, use water or a water and antifreeze solution, circulate in ground loop of pipes, to extract heat from the earth. Direct evaporation of e.g. CO 2 is in usage as well. Ground loops can be vertical or horizontal. Vertical loops usually are consisted of borehole heat exchangers (BHEs), and are usually applied in cases where space is limited. The depth of the loop pipe will vary with soil type, loop configuration and system capacity, from 2 metres for a horizontal loop and 250 metres or more for borehole heat exchangers (Fernández, 2009). The GSHPs industry is mature, with a continuous trend of development and due to the increasing energy efficiency of GSHP mainly due to recent advances of the technology (described in the previous chapters) thus being attractive and more and more considered as an excellent substitute of conventional heating/cooling sources including air source heat pumps and VRVs. The GSHP efficiency is measured by its Seasonal Performance Factor (SPF, the ratio of total output heat/cool to input electricity), it is normally around 3 but it can reach 5. A lower SPF may be acceptable if a larger heat demand is covered, as in cold climates or when systems combine heating and cooling and/or domestic hot water production. The measured primary energy savings (Table 1) of an average GSHP compared to conventional systems is as high as 50% and for advanced systems, described in the previous chapters, it may be as high as 80%. Due to these advances in several cases the high efficiency of GSHP compared to the initial capital cost minimizes the need for subsidies. System Efficiency SPF Description GSHP - average SPF 3 1 kwhe consumed for 3 kwhth produced GSHP - advanced SPF 5 1 kwhe consumed for 5 kwhth produced Energy saving including cooling 30-50% 60-80% Table 1: Summary of efficiencies and respective energy saving factors of average and advanced GSHPs References European Technology Platform Renewable Heating and Cooling, GEOTHERMAL PANEL (2009) Vision , Fernández, A. (2009) Geothermal energy in Europe, today and in future. Presentation at the European Future Energy Forum 2009, Bilbao (ES), 9-11 June GEOTHERMAL ENERGY BAROMETER (2003) The Right Pace for 2010, Eurobserver 35, August Lund, J., Sanner B., Rybach L., Curtis R., and Hellström, G. (2004) Geothermal (Ground Source) Heat Pumps a World Overview, GHC Bulletin, September Mendrinos D., Karytsas C. and Sanner B. (2007) Project GROUND-REACH -Reaching the Kyoto targets by means of a wide introduction of ground coupled heat pumps (GCHP) in the built environment, Proceedings European Geothermal Congress 2007, Unterhaching, Germany, 30 May-1 June Sanner, B., Karytsas, C., Mendrinos, D., and Rybach, L. (2003) Current Status of Ground Source Heat Pumps and Underground Thermal Energy Storage in Europe, Geothermics, Vol. 32, pp