1、 Signhild Gehlin is a technical expert at the Swedish Centre for Shallow Geothermal Energy, Lund, Sweden. Jeffrey D Spitler is a professor in the School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma. Gran Hellstrm is an Adjunct Associate Professor in the De
2、partment of Mathematical Physics at Lund University of Technology, Sweden. Deep Boreholes for Ground Source Heat Pump Systems Scandinavian Experience and Future Prospects Signhild E A Gehlin, PhD Jeffrey Spitler, PhD, PE Gran Hellstrm, PhD Member ASHRAE Fellow ASHRAE ABSTRACT Ground source heat pump
3、 (GSHP) systems are commonly used in Sweden for both residential and commercial buildings. However, there are several key differences compared with GSHP systems utilized in the USA. Scandinavian systems are often heating-only, and instead of using grouted boreholes, groundwater-filled boreholes are
4、often used. These boreholes are cased from the ground surface to the usually shallow bedrock. A single or double U-tube is commonly suspended in the borehole. These boreholes are often deeper than those commonly used in the USA. The average borehole depth has increased over time, and the average bor
5、ehole depth for ground heat exchangers installed in 2013 in Sweden was 171 m (561 ft.) Boreholes as deep as 250-300 m (820-984 ft) are not uncommon and there is interest among installers of using even deeper boreholes. Incentives for deeper boreholes include limited area for drilling, pre-existing b
6、oreholes on neighboring properties, and deeper-than-usual layers of soil and unconsolidated rock. This paper reviews current Scandinavian practice for borehole design and discusses installations with boreholes 300 m (984 ft) deep or deeper. Aspects of the design include using larger pipe sizes or do
7、uble U-tubes to keep pressure losses acceptable, larger borehole diameters to accommodate the larger pipe sizes, increased short-circuiting due to the long lengths, and design temperatures for heating-dominant systems due to the geothermal gradient. INTRODUCTION Approximately one fifth of the two mi
8、llion single-family houses in Sweden are today heated with a ground source heat pump (GSHP) (Gehlin et al. 2015). The typical domestic GSHP is a 5-10 kW (17,060-34,120 Btu/hr) capacity heat pump connected to a 100-200 m (328-656 ft) deep vertical groundwater-filled borehole in hard rock. Ground heat
9、 exchangers used in Scandinavia are commonly closed-loop systems, fitted with a single U-tube. The uppermost 6 m (20 ft.) or more of the borehole is cased with a steel casing and sealed to the bedrock to protect the groundwater from surface pollution (SGU 2008). According to the Swedish Geological S
10、urvey Well Database, the average borehole depth for GSHP systems has increased from 100 m (328 ft.) in 1995 to 171 m (561 ft.) in 2013 (Gehlin et al. 2015) (see Fig. 1). For single-family houses the increasing borehole depth is a result of more efficient heat pumps, as well as the trend to utilize G
11、SHP with higher installed capacities to avoid the use of auxiliary heating by electric resistance heaters. The trend towards deeper boreholes is however more pronounced for commercial GSHP systems, where there OR-16-C043is space, time and money to save by drilling fewer but deeper boreholes. Large G
12、SHP systems in Scandinavia are now commonly drilled to a depth of 200-300 m (656-984 ft.). The three largest GSHP systems in Scandinavia today are Akershus Hospital, Norway, with 228 boreholes to a depth of 200 m (656 ft.), Karlstad University Campus, Sweden, with 204 boreholes of 240-250 m (787-820
13、 ft.) depth, and SOK Logistic Centre in Sibbo, Finland, with 150 boreholes to 300 m (984 ft.). Incentives for deeper boreholes include limited area for drilling, pre-existing boreholes on neighboring properties, and deeper-than-usual layers of soil and unconsolidated rock. For pure heat extraction s
14、ystems there is also an interest in taking advantage in the increased temperature with depth, due to the geothermal gradient, however, as is shown in this paper, this issue is not as simple as some tend to believe. Figure 1 Average borehole depth and deepest borehole, from SGU Well Database (Gierup
15、2015). ! This paper reviews current Scandinavian practice for borehole design and discusses technical, thermal and economic considerations with boreholes drilled to 300 m (820 ft.) depth or more. Aspects discussed include using larger pipe sizes or double U-tubes to keep pressure losses acceptable,
16、larger borehole diameters to accommodate the larger pipe sizes, increased short-circuiting due to the long lengths, and design temperatures for heating-dominant systems due to the geothermal gradient. SCANDINAVIAN DESIGN OF GSHP BOREHOLES The typical Scandinavian GSHP design and design conditions di
17、ffer from GSHP systems in many other countries. This has to do with climatic and geological conditions, as well as issues related to building codes and other regulations, energy source availability and pricing. Geological and Climatic Conditions The Swedish geology is predominantly crystalline rock
18、with shallow overburden, generally high quartz content, and high thermal conductivity. Groundwater level is generally high (a few meters below the ground surface). There are areas with rich overburden and sedimentary deposits, mainly in the south and middle parts of Sweden and on the islands land an
19、d Gotland. Similar conditions with predominantly crystalline rock and high groundwater levels are found in Norway and Finland. The Swedish Geological Survey (SGU) has provided guidelines for construction of wells for GSHP systems since 1997(SGU 2008), and collects data on all groundwater wells and G
20、SHP boreholes in Sweden through the Well Database. The Swedish well construction guideline requires steel casing of the uppermost part to a minimum depth of 6 m (20 ft.) and with at least 2 m (7 ft.) drilled into hard bedrock and sealed with concrete. The guidelines allow for ungrouted, groundwater-
21、filled boreholes, which is how the vast majority of Scandinavian borehole heat exchangers OR-16-C043are constructed. The Scandinavian climate is heating dominated, but for commercial GSHP systems both heating and cooling are used. Average ground temperature varies between 11C (51.8F) in the south an
22、d 2C (35.6F) in the north. Design Conditions Scandinavian GSHP systems are generally designed for a minimum design EFT of 0C (32F), though in the northern part of Sweden the minimum temperature may be allowed to fall below 0C (32F). Boreholes are allowed to freeze at maximum heating load conditions.
23、 Closed-loop single U-tubes are the most commonly used collectors, though for commercial GSHP systems double U-tubes are often used. Tube size is typically 40x2.4 mm (1 ”) PN10 SDR17 PE100 for single U-tubes and 32x2.0 mm (1 ”) PN10 SDR17 PE100 for double U-tubes. Coaxial ground heat exchangers have
24、 been used occasionally. A 20-28% ethanol/water solution is predominantly used as heat carrier fluid in the collector pipes. DEEP BOREHOLES Boreholes for deep geothermal use (direct use and power production) have limited potential in Scandinavia. The market is completely dominated by shallow geother
25、mal energy systems such as typical GSHP systems with vertical boreholes in rock. These boreholes are rarely drilled deeper than 300 m (984 ft.) in Scandinavia, and less than that in central Europe and North America. Hence experience from ground heat exchangers in boreholes deeper than 300 m (984 ft.
26、) is scarce. Rybach and Hopkirk (1995) discuss two converted “dry” geothermal boreholes in Switzerland. The 1700 m (5577 ft.) borehole at Reinach, near Basel, and the 2300 m (7546 ft.) deep borehole at Weggis are both used for heating of buildings. They conclude that the economics depend not only on
27、 the size and proximity of the energy user, but also on the temperature at which heat is to be delivered. They also conclude that failed geothermal or hydrocarbon exploration boreholes are feasible to use for GSHPs provided that the full drilling cost is not included. Kohl et al. (2002) also cover t
28、he Weggis borehole and Kohl et al (2000) describe the performance over two consecutive years of operation of a 1200 m (3937 ft.) deep borehole that was drilled in the early 1990s in Weissbad, Switzerland. The borehole was initially meant to reach a waterfilled fracture for deep geothermal heat extra
29、ction, but the formations proved to be dry, and the project was abandoned. Instead a borehole heat exchanger was installed to use the borehole with a GSHP. The temperature at the bottom of the borehole was measured at 45C (113F). Huchtemann and Mller (2014) and Dijkshoorn et al. (2013) treat a 2500
30、m (8202 ft.) deep borehole with a coaxial borehole heat exchanger for space heating and cooling of a university building in the city Aachen, Germany. More recent examples of deep boreholes drilled purposely as borehole heat exchangers are reported from Switzerland and Norway. In mid-March 2015, a Eu
31、ropean geothermal drilling company reported (De Varreux 2015) that they had installed their first 750 m (2460 ft) deep borehole heat exchanger (“geothermal probe”) in Lausanne, Switzerland. The borehole is the first in a series of 150 deep borehole heat exchangers to be installed during the period 2
32、015-2018 to provide district heating to a new housing and commercial complex area. The heat exchanger was a HPR (high pressure) PN80 double U-tube with outer diameter 50 mm (2). In 2014 the same company installed eight borehole heat exchangers to a depth of 500 m (1640 ft.), heating 64 apartments in
33、 four buildings (De Varreux 2015). Schwenke (2013) describes the Skoger School project in Drammen, Norway, which is heated by five boreholes of 500 m (1640 ft) depth. The boreholes are fitted with single U-tube collectors with OD 50 mm (2”) and ID of 44 mm (1.7”). The borehole has a diameter of 140
34、mm (5.5”). Holmberg et al. (2015) show results from numerical model simulations of borehole heat exchangers of 600 m (1969 ft), 800 m (2625 ft) and 1000 m (3281 ft) depth. The borehole is fitted with a coaxial heat exchanger consisting of a central polypropylene pipe and an outer thin polyethylene l
35、iner, as described in more detail by Acua (2013). The simulations are done to describe performance over time, and to determine average specific thermal load and energy extracted. A geothermal gradient of 0.02 K/m (0.006 K/ft.) was used. The authors conclude that deep borehole heat exchangers can sus
36、tain a higher average specific heat load than conventional borehole heat exchangers due to the OR-16-C043higher temperature level in the borehole, and that deep boreholes may be suitable for GSHP installations in areas with limited space and negatively balanced load profiles. Pipe and Pressure Consi
37、derations The difference in density between the borehole filling and the heat transfer fluid in the piping may be an issue in deep boreholes. If the heat transfer fluid has a lower density the resulting external pressure on the pipe may cause buckling. Common SDR-17 polyethylene pipes can withstand
38、an external pressure of 1.4 bar (20.3 psi) whereas SDR- 11 pipes can handle 5.7 bar (82.7 psi) before buckling (Kalantar 2015). Figure 2 shows the differential pressure versus depth for three combinations of heat carrier fluid and borehole filling. The worst-case for a groundwater-filled borehole oc
39、curs when a less dense antifreeze mixture is used as shown in Figure 2, its possible that there is a risk of buckling when SDR-17 is used and borehole depth exceeds 300 m (984 ft). However, although the vast majority of boreholes in Scandiavia are groundwater filled, there are situations where grout
40、ing is required. During the grouting process the grouting mixtures are heavier than the heat transfer fluids and will cause an external pressure on the pipe. Available grouting mixtures vary in density from 1100 to 2000 kg/m 3(69- 125 lb/ft 3 ) during injection, where the heavier materials usually h
41、ave higher thermal conductivities. As shown in Figure 2, differential pressures significantly exceed buckling limits for thermally-enhanced grout, regardless of the heat carrier fluid. It is common practice to use pressurized SDR-11 pipes for grouted boreholes, but even then, it may be impossible to
42、 safely grout the borehole in one step without allowing part of the grout to set up. A multi-step grouting process adds complexity and costs. Figure 2 Differential pressure in borehole with un-pressurized pipes. Pipes are installed in the borehole when the ground temperature is at undisturbed condit
43、ions. In groundwater filled boreholes the pipe will expand or contract due to heating/cooling during operation. If the pipe is heated to temperatures above those experienced during installation, the expansion may cause the U-bend to repeatedly push against the bottom of the borehole. This has occasi
44、onally led to failure of the pipe. The linear coefficient of thermal expansion for PE100 is 1.3 10 -4m/m K (ft/ft K), which means that a 300 m (984 ft) pipe will change 0.6 m (1.9 ft.) in length when changing temperature from 0C (32F) to 15C (59F), while a 500 m (1640 ft) pipe will change approximat
45、ely 1 m (3.28 ft.). Therefore, care must be taken to suspend the U-tube higher in deeper boreholes. Considerations Related to Drilling The governing drilling techniques for GSHP borehole drilling in Scandinavia today are air-driven or water-driven down-the-hole-hammer (DTH) drilling. With common com
46、pressors suitable for urban use, of up to 35 bar (507.6 psi), and lift capacity of 50-70 kN (5.62-7.87 tons) for air-driven DTH drilling, the theoretical maximum borehole depth is 300 m (984 ft.), where the counter pressure from groundwater will be too high for the hammer to work. For water-driven D
47、TH drilling, there is no counter-pressure from groundwater, but a limiting factor is flow losses to OR-16-C043fractures in the rock. Even larger DTH drill rigs with capacity of DTH drilling to 800 m (2625 ft.) depth exist in limited numbers, but are infeasibly large for urban drilling. GSHP borehole
48、s are usually produced at low-cost with small demands on drilling precision. The deviation from targets depends partly on drilling speed and to some extent on the geological structure. The deviation increases with depth, and is not linear. This means the risk for intersecting another borehole increa
49、se with borehole depth. Higher demands on precision involve smaller tolerances, increased material cost and fuel consumption, and decreased overall rate of penetration, leading to higher costs for deep boreholes. Thermal Considerations The heat energy budget for the subsurface is largely a balance between the surface boundary conditions and geothermal heat flux from the centre of the earth. Insolation varies with time and location, but for Scandinavia the average annual net insolation is on the order of a 100 W/m 2(31.7 BTU/hr/ft 2 ), while the geot
