Ground-source Heat Pump Systems the European Experience

Ground-source heat pumps play a key role in geothermal development in Central and Northern Europe. With borehole heat exchangers as heat source, they offer de-central geothermal heating at virtually any location, with great flexibility to meet given demands. In the vast majority of systems, no space cooling is included, leaving ground-source heat pumps with some economic constraints. Nevertheless, a promising market development first occurred in Switzerland and Sweden, and now also is obvious in Austria and Germany. Approximately 20 years of R&D focusing on borehole heat exchangers resulted in a well-established concept of sustainability for this technology, as well as in sound design and installation criteria. The market success brought Switzerland to the third rank worldwide in geothermal direct use. The future prospects are good, with an increasing range of applications including large systems with thermal energy storage for heating and cooling, ground-source heat pumps in densely populated development areas, borehole heat exchangers for cooling of telecommunication equipment, etc. INTRODUCTION Most European countries do not boast abundant hydrothermal resources that could be tapped for direct use (the notable exceptions are Iceland, Hungary and France). The utilization of low-enthalpy aquifers that enable the supply of a larger number of customers by district heating is limited so far to regions with specific geological settings. In this situation, the utilization of the ubiquitous shallow geothermal resources by de-central ground-coupled heat pump systems is an obvious option. Correspondingly, a rapidly growing field of applications is emerging and developing in various European countries. A rapid market penetration of such systems is resulting. The number of commercial companies actively working in this field is ever increasing and their products have reached the “yellow pages” stage. The climatic conditions in Central and Northern Europe, where most of the market development takes place, are such that by far the most demand is for space heating and air conditioning is rarely required. Therefore, unlike the “geothermal heat pumps” in the USA, the heat pumps usually operate in the heating mode only. The following sections describe the technology, the market situation, future trends and questions in Europe, with special emphasis on the experience in Switzerland where a veritable boom in installing such systems took place in the last couple of years. 16 DEFINITIONS, TECHNOLOGY Shallow geothermal resources (<400 m depth by governmental definition in several countries) are omnipresent. Below 15 20 m depth, everything is geothermal (Figure 1). The temperature field is governed by terrestrial heat flow and the local ground thermal conductivity structure (± groundwater flow). In some countries, all energy stored in form of heat beneath the earth surface is per definition perceived as geothermal energy (VDI 1998; BFE, 1998). The same approach is used in North America. The ubiquitous heat content of shallow resources can be made accessible either by extraction of groundwater or, more frequent, by artificial circulation like the borehole heat exchanger (BHE) system. This means, the heat extraction occurs–in most cases–by pure conduction, there are no formation fluids required. The most popular BHE heating system with one of more boreholes typically 50 200 m deep is a closed circuit, heat pump coupled system, ideally suited to supply heat to smaller, de-central objects like single family or multifamily dwellings (see Figure 2). The heat exchangers (mostly double U-tube plastic pipes in grouted boreholes) work efficiently in nearly all kinds of geologic media (except in material with lowthermal conductivity like dry sand or dry gravel). This means to tap the ground as a shallow heat source comprise: • Groundwater wells (“open” systems), • Borehole heat exchangers (BHE), • Horizontal heat exchanger pipes (including compact systems with trenches, spirals, etc.), and • “Geostructures” (foundation piles equipped with heat exchangers). A common feature of these ground-coupled systems is a heat pump, attached to a low-temperature heating system like floor panels/slab heating. They are all termed “ground-source heat pumps” (GSHP) systems. In general, these systems can be tailored in a highly flexible way to meet locally varying demands. Experimental and theoretical investigations (field measurement campaigns and numerical model simulations) have been conducted over several years to elaborate a solid base for the design and for performance evaluation of BHE systems (Knoblich, et al., 1993; Rybach and Hopkirk, 1995; Rybach and Eugster, 1997). While in the 80s, theoretical thermal analysis of BHE systems prevailed in Sweden (Claesson and Eskilson, 1988; Eskilsson and Claesson, GHC BULLETIN, MARCH 2000 Figure 1. Geothermal energy, comprising geothermal and mixed resources in the shallow subsurface. Figure 2. Typical application of a borehole heat exchanger (BHE) heat pump system in a central European home. Typical BHE length: 100 m. GHC BULLETIN, MARCH 2000 17 1988), monitoring and simulation was done in Switzerland (Gilby and Hopkirk, 1985; Hopkirk, et al., 1988), and measurements of heat transport in the ground were made on a test site in Germany (Sanner, 1986). In the German test system at SchöffengrundSchwalbach near Frankfurt/Main, a 50-m BHE was surrounded by a total of 9 monitoring boreholes at 2.5, 5 and 10 m distance, also 50 m deep. Temperatures in each hole and at the BHE itself were measured with 24 sensors at 2 m vertical distance, resulting in a total of 240 observation locations in the underground. This layout allowed to investigate the temperature distribution in the vicinity of the BHE, as shown in Figure 3. The influence from the surface is visible in the uppermost approximately 10 m (see Figure 1), as well as the temperature decrease around the BHE at the end of the heating season. Measurements from this system were used to validate a numerical model for convective and conductive heat transport in the ground (Sanner and Brehm, 1988; Sanner, et al., 1996). Starting in 1986, an extensive measurement campaign has been performed at a commercially delivered BHE installation in Elgg near Zurich. The object of the campaigns is a single, coaxial, 10m long BHE in use since its installation in a single family house. The BHE supplies a peak thermal power of about 70 W per m of length. The ground temperature results are highly informative with respect to the long-term performance (for details see Rybach and Euster, 1998). Atmospheric influences are clearly visible in the depth range 0 15 m. Below 15 m, the geothermal heat flux dominates. The results show that in the near field around the BHE, the ground coils down in the first 2 3 years of operation. However, the temperature deficit decreases from year to year until a new stable thermal equilibrium is established between BHE and ground, at temperatures that are some 1 2 K lower than originally. Thus, a “thermal collapse” (i.e., sudden drop of heat extraction efficiency) will not happen. After calibration of a numerical model with the data from the Elgg system, the extrapolation for an operation over a 30-year period as well as the thermal recovery for 25 years following the end of the operation period, has been simulated. Figure 4 shows the calculated difference of ground temperature to the initial temperature before start of operation, at various distances from the BHE. Temperature close to the BHE in winter drops quickly in the first years, only to stay more or less stable over the next years. In summertime, initial temperatures are not achieved again, but the temperature drop is decreasing from year to year. After termination of the operation, a rapid thermal recovery can be seen in the first spring, followed by a slowing down of the recovery process due to the decreasing temperature gradients. In the numerical simulation, a complete recovery will occur only after an indefinitely long time period; nevertheless, the remaining temperature deficit 25 years after the operation is stopped, is only in the order of 0.1 K. Figure 3. Measured temperature distribution in the ground at the beginning of the monitoring period (left, on October 10, 1986, after a total of ca. 2 hours of test operation) and at the end of the first heating season (right, on January 5, 1987), Schwallbach GSHP test system, Germany. 18 GHC BULLETIN, MARCH 2000 Figure 4. Changes in ground temperature at various distances from the BHE over many heating seasons, measurement and extrapolation (simulation) for the system in Elgg, Zurich, Switzerland. The long-term reliability of BHE-equipped heat pump systems, along with economic and ecological incentives (see below), led to rapid market penetration. This was accomplished by the development of design standards (e.g., VDI 1998) and easy-to-use design tools (Hellstrom, et al., 1997). MARKET PENETRATION Within the full swing of heat pump applications in Europe, ground-coupled heat pumps play a significant role. The development started around 1980 when the first BHEcoupled heat pump systems were built in Germany and Switzerland. Following a larger number of new units installed during the oil price crises and a subsequent low (except for Switzerland), the number of new installations is again increasing in the 90s. Table 1 shows the number of ground-source heat pumps (GSHP) installed in various European countries. The GSHP fraction is especially high in Sweden and Switzerland. In some other countries as Italy, Greece and Spain, there is so far only a negligible number of GSHPs installed. Table 1.General Heat Pump (Total) and Ground-Source Heat Pump Systems (GSHP) Installed 1993-1996 in Various European Countries (residential sector, in 1000 units, after data from Breembroek and Lazáro, 1999). Country All Heat Pumps Ground-Source Fraction % GSHP Systems Austria 22.2 11 2.42