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Drilling methods for shallowgeothermal installations

Burkhard Sanner1 and Olof Andersson2

1 Institute of Applied Geosciences, Justus-Liebig-UniversityDiezstrasse 15, D-35390 Giessen, Germany

2 Department of Engineering Geology, Lund University of TechnologyP.O.Box 118, S-22100 Lund, Sweden

Introduction

Drilling is a very old technique, for water supply and for exploitation of mineral resources.Water wells have been drilled in Egypt more than three millenia ago. Fig. 1 shows an exampleof a quite sophisticated, hand-operated drill rig used in China to drill several hundred meterdeep in a few month.

Fig. 1: Chinese percussion boring (after Chugh, 1985)

Today, a variety of mechanised drilling equipment is on the market and in use. The size range isfrom small augers for post-holes few meters deep to the deepest borehole on Kola peninsula inRussia with a depth of more than 12 km. Economic considerations as well as technicalproblems restrict the drilling depth for shallow geothermal applications generally to around100 m. In recent times, also deeper holes have been drilled for borehole heat exchangers (BHE)with ca. 250 m depth, and holes in the 400-m-range are in preparation in Switzerland.

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Although a deeper hole provides access to slightly higher temperature, the increasing problemswith insertion of heat exchanger, grouting and static pressure have to be solved.

For vertical ground heat exchangers, a distinction has to be made between two basic ways ofinstallation:

• Direct pressing or ramming of the heat exchanger into the ground (in soft ground only)

• Insertion of the heat exchanger in a borehole drilled beforehand

Boreholes can be drilled under almost any subsurface condition From site to site, an optimumchoice for type and installation method is necessary according to the geological situation.

Basic drilling methods

For shallow holes down to app. 100 m, not every drilling method is suitable. Hytti (1987)presents a diagram showing the optimal drilling methods in respect to hole diameter and rockstrength (Fig. 2).

Hard rock

Soft rock

Top hammer

DTH

Rotary crushing

Rotary cutting

20 60 100 140 180 220 260 300 340Borehole diameter [mm]

1 4 5 8 10 12 13Borehole diameter [inches]

2 3 6 7 9 11 14

Fig. 2: Applicable drilling methods (revised after Hytti, 1987)

Table 1 lists recommended drilling methods for the various ground conditions. Drilling rates ofapp. 10 m per hour are realistic when using rotary drilling with drag bits in soft/medium andDown-the-hole- (DTH) or Top-Hammer in hard and very hard rock. The advantage of DTH inhard rock can be seen in data from drilling for Schwalbach GCHP Reserach Station: In thesame quarzitic rock, app. 5 m apart, and with the same very light drill rig (~2 metric tons), a50-m-borehole could be completed using rotary drilling with rock- and button-bit in about 5days; with DTH, the 50 m were completed after 4½ hours.

A further restriction applies to the rotary method. Rotary drilling is widely used and can beadopted to almost every drilling problem, but drilling velocity normaly is not very high and canbecome extremly low in unfavourable conditions. Rotary cutting with drag bits in soft andmedium rock can be effective even with light rigs, but rotary crushing with rock- or roller bitsand even more with button bits requires heavy load on the bit to crush the rock when rollingover the teeth (fig. 3). In deep holes as in oil well drilling, the drill string alone brings enoughload onto the bit; for shallow holes, the required load often exceeds the weight of a lightdrilling rig, in spite of using heavy tubing. The optimum load increases with borehole diameter;

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Cambefort (1964) recommends 700-900 kg/in in soft and 1400-1800 kg/in in hard rock. For astandard 115 mm (4½") borehole, suitable for most of the European BHE, in hard rock a load of6.3 to 8.1 metric tons would result. Even in this optimum condition, the drilling rate in hardrock is only 1-2 m per hour, and can decrease to some 10 cm/h when the load is too small.Thus, for shallow holes in hard rock conventional rotary drilling is not a good choice.

Table 1: Drilling methods

Soil/Rock-type Method Remarks

soil, sand/gravel auger sometimes temporary casing required

rotary temporary casing or mud additivesrequired

soil, silty/clayey auger mostly best choice

rotary temporary casing or mud additivesrequired

rock, medium hard rotary roller bit, sometimes mud additivesrequired

DTH 1 large compressor required

rock, hard to very hard rotary with rock bit or hard-metal insert buttonbit, very slow

DTH 2 large compressor required

top hammer special equipment, depth range ca.70 m

rock under overburden ODEX 2 or similar in combination with DTH

Fig. 3: rock bit (left) and tungsten-carbid button bit (right), from Chugh (1985)

1 Down-the-Hole-Hammer2 Overburden Drilling Equipment; ODEX is a trademark of Atlas Copco

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With the hammer techniques, drilling in hard rock is very fast and cost-effective. Problemsarise, if intercalations of soft and instable layers are present. For unconsolidated overburden,the ODEX 3-method offers a way to install a casing down to the bedrock while drilling and toproceed drilling in the hard formation with normal DTH-equipment. If the overburden is toothick, or if instable rock is found at considerable depth, the DTH with pneumatic flushing of thecuttings can not be used. In rotary drilling, were a fluid (normally water) is used for flushing thehole, special drilling muds can stabilize the borehole wall. In table 2, some common mudadditives are listed.

Tab. 2: Additives for drilling mud in rotary drilling

Name Properties Remarks

bentonite thixotropic, stabilises the hole possible clogging of aquifers

CMC (cellulose) stabilises the borehole wall,reduces water losses

growth of bacteria

polyacrylamide minimises water losses to theformation

baryte, ilmenite etc. weighting materials, stabilise thehole, keep pressured water down

possible problems with BHEinstallation

foam generators facilitates flushing out of cuttings

Two phenomena are used to stabilize the hole with the listed additives. Bentonite and cellu-lose-products have thixotropic properties, i.e. they build up stable aggregates when stagnant butare fluid in motion. Heavy minerals as baryte (BaSO4), ilmenite (FeTiO3) or hematite (Fe2O3)increase the density of the drilling mud, can counteract the formation pressure and thus stabilizethe borehole. They are also used if groundwater under artesian pressure is present. However,thick or heavy muds make insertion of the heat exchanger pipes difficult.

The economy of drilling shallow holes is completely different than that of deep oil- or gaswells. Very easy methods, as the tractor-mounted auger (fig. 4) described by Reuss et al.(1990), can be used by a farmer for installing BHE with no external cost; and even drilling withhammer equipment is far from cost and time required for deep holes. Light, mobile rigs,suitable for both rotary and DTH, ensure cost-effective drilling (fig. 5).

Equations for calculating drilling cost have been discussed in the literature. Armstead (1983)cites two equations, and Schulz & Jobmann (1989) establish an equation for Germany (hereconverted into US-$):

C = 95,000 . 1.153D US geothermal wells

C = 52,000 . 1.0998D US oil and gas wells

C = 112,000 . D . e(0,01 . D) German geothermal wells

with C: Total cost for drilling in US-$D: Depth of hole in hundreds of meters

All three equations fit well the data for holes deeper than 500 m, but overestimate cost forshallow holes. A 100-m-hole would cost US-$ 109535, 57189 and 113125 resp., following

3 Overburden Drilling Equipment; ODEX is a trademark of Atlas Copco

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these equations. The realistic drilling cost including BHE and grouting in Europe are from 35-50 US-$/m (40-55 €/m) for holes from 50-100 depth, resulting in 3500 to 5000 US-$ (4000-5500 €) for the 100-m-hole. Only in very unfavourable geologic conditions, where temporarycasing etc. may be required, the cost will be higher.

Fig. 4: Auger principle (left, Atlas Copco) and light auger mounted on agriculturaltractor (photo from Beck et al., 1993)

Fig. 5: Drilling rig optimised for BHE installation, mounted on all-terrain vehicle(Unimog)

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BHE fields with a number of boreholes should yield netterr economy. So it is not unexpectedthat Garlick (1986) reports US-$ 311,134 for 324 holes each 52 m deep, which means only US-$ 18.5 per meter. Rammed or pressed steel earth probes in soft ground cost 22 - 40 US-$ permeter, and for the SIG-system Engvall (1986) calculated 1,640,000 SKr for a 100,000 m3

ground heat store with 54,750 m earth probe length, which would be less than 6 US-$ permeter. With the simple auger shown in fig. 4, Beck et al. (1993) achieved total cost for drilling,installation, ground connections, backfilling in a BHE field with 103 boreholes of 8-12 m depthof 27400 US-$ (30900 €), that is 26,4 US-$/m (30 €/m).

Drilling for exploration purposes

The methods for drilling and data collection vary between countries, but are basically the sameas those used for other exploration purposes, like in water well industry, foundation drillingand in the mining industry. The importance of geological exploration for shallow geothermalapplications is discussed by Andersson et al. (1997).

Exploration boreholes for shallow geothermal purposes would in most cases be performed byconventional rotary drilling. During drilling, samples of the drilled formations (cuttings) can beseparated from the drilling fluid. These samples give a basic information for the geologicaldescription. By measuring the rate of penetration as a function of rotation speed and thrust orweigth on the drillbit, the relative hardness of the penetrated formations can be described. Thisinformation is of value for property descriptions. Furthermore, losses (or gain) of drilling fluidindicates permeable layers or fractures. By measurement of level and amount of losses,important hydraulic properties can be detected (Andersson, 1981).

In boreholes for groundwater wells, after drilling, and based on the informations gained duringdrilling, a screen is properly set in the hole and a testwell is completed. By pumping this welland observation of groundwater level in the well (and in surrounding wells, if available), thehydraulic properties of the aquifer can be determined. Basic methods are described e.g. inKruseman & De Ridder (1970).

Exploration boreholes for BHE in hard rock (consolidated formations, igneous or metamorphicrock) would preferably be carried out by the hammer-drilling method (DTH). In this casecompressed air is used to drive the hammer and to flush the hole. Samples of cuttings, separatedfrom the outlet air, are used to describe lithology (type of rock, mineral composition). Similarto rotary drilling, rate of penetration can give information about lithological boundaries ,relative hardness, location of fractures etc. (Andersson, 1981). Because no water is added fromthe surface while drilling, this method furthermore allows a continuous recording of waterflowrate out of the borehole, and also water samples for chemical analysis can be taken.

If very exact geological data as well as larger samples for determination of thermal and/orhydraulic parameters are required, core drilling is the method to choose. For this method, ahollow pipe with an annular drilling bit ("core barrel") is attached to the drill string, andcylindrical rock or soil samples can be retrieved. Core drilling for the total length of the bore-hole is expensive, because the drill string has to be completely removed and inserted againafter each length of core barrel (1-1.5 m), or very specialized tools (wire-line core barrel)have to be used.

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Groundwater well drilling and completion

Groundwater wells are drilled by the drilling methods mentioned before. For large diameterwells, the reverse rotary method is well suited. While in normal rotary the fluid is pumpeddownwards inside the drill pipe and rises through the annulus, the sense of motion here isreversed. This allows for reliable removal of drill cuttings through the pipe and helps to keepthe borehole wall stable.

There are two basic ways for construction of a well:• Natural completion (lost filter completion)• Gravel packFor natural completion, the formation has to be sand and gravel with suitable grain-size distri-bution. The well screen is inserted, and then through strong pumping fine material is removed,thus forming a kind of natural gravel filter around the screen. If the formation does not exhibitthe composition required for natural completion, a gravel pack (or sand pack) has to beinstalled in the annulus around the screen, to avoid continuing production of fine material.

For a ground source heat pump, the type of well (production or injection) is always the same. Inaquifer storage, wells may have to be operated as production well half of the year and asinjection well for the other half (fig. 6).

Motor-valve

Valve

Cover

Concrete

Standpipe

Pipe

grouting(e.g. clay)

Overburden

Aquifer

grouting

control-electrodes

well screen

gravel filter

submersible pump

control electrode

riser pipeinjection pipe

GW-level (ca.)

Bedrock

Fig. 6: Well for ATES-applications (production and injection)

Methods for direct installation of ground heat exchangers in soft ground

In unconsolidated sediments, ground heat exchangers can be pressed or driven into the ground.Table 3 shows the most important techniques.

The major advantages of direct pressing or ramming the heat exchanger in place are:

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• No problems with stability of borehole walls• Good contact to the ground, no grouting required• Complete installation in one single stepFor an economic configuration of ground heat exchangers, steel pipes can be driven radiallyfrom one point as shown in Fig. 7. In this fan-shaped assemblage, the heads of the pipes areclose together and connection to a manifold is easy.

Tab. 3: Installation methods for soft ground

Soil type Method heat exchanger Remarks

sand/gravel ramming/driving coaxial only possible with steelpipes, corrosion problems

water jetting coaxial steel pipes (corrosion)

silt/clay ramming/pressing coaxial steel pipes (corrosion)

driving with SGI 4 tool single-U plastic tubes, with lost tool

The driven pipe remains in the ground and acts as outer wall of the heat exchanger. Because thepipe has to withstand the pressing-/ramming forces, the choice of materials is limited. Steel asheat exchanger material has good thermal properties, is cheap and common in constructionwork. A major drawback is the lacking resistivity to corrosion by groundwater and soilhumidity, which makes active electric corrosion protection necessary. In tests in a groundwaterwell over two years (Sanner & Knoblich, 1991b), steel proved to be badly eroded after somedecades, and the risk for leakage is high within few years (Tab. 4). The use of stainless steel,which shows very good anticorrosive properties, is limited due to the very high price. Thevalues in Tab. 4 have been measured in ground water with low chloride content; chloride ionscan initiate pit-hole corrosion even in stainless steel.

Tab. 4: Some results of In-Situ Corrosion Test in Schwalbach

Material Mass loss peryear (average)

extrapolated time for lossof half of the sample mass

Steel St37, tube 2.15 % 32 years

Copper, tube 1.74 % 40 years

Inox-steel, corrugated tube 0.0 % ---

To avoid problems with corrosion, some methods for using plastic pipes have been investi-gated. A summary of installation techniques is given in Bouma & Koppenol (1983). In Sweden,a method had been developed around 1990 (Engvall, 1986; Lehtmets, 1991), which also isshown in Fig. 7. A 32 mm (1¼") polyethylene pipe is pressed down into soft ground by using aguidance tool. Engvall (1986) reports tests with the first design of the tool. In clayey subsoil, ingeneral a depth similar to that attainable with a standard penetrometer can be achieved. Thedata in Tab. 5 summarise the logs of these tests.

Although the depth for direct installation of ground heat exchangers is limited to some 10 m, theeasy, fast and cheap methods are well suited for cost-effective installation under certain

4 Swedish Geotechnical Institute, Linköping, Sweden

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conditions. The thickness of unconsolidated sediments must exceed at least 10 m, and no largeboulders or coarse gravel should be in the sediment. In other places, drilling is unavoidable forheat exchanger installation.

force

reel with PE-pipe

groundground

ditch

installation tool of SGI, Sweden

ramming of steel tubes

Fig. 7: Pressing and ramming in soft ground

Tab. 5: Field tests for plastic pipe installation rig (after data from Engvall, 1986)

Location(Sweden)

Depth [m] Soil material Consist.index Ic

Pressingforce [kN]

Remarks

Kista 0-11-10

10-1414-20

dry crustclaysilt, clayeysilt

1.00.25-0.75

1.0>1.0

189-18

18-2222-26

pre-perforation

rig anchored

UpplandsVäsby

0-1.51.5-5.55.5-11

11-14.5

dry crustclayclayclay

>1.00.5-0.75

1.0>1.0

1810-1313-1919-20

pre-perforation

Uppsala 0-11-2.5

2.5-8.5

dry crustorg. clayorg. clay

>1.00.75-1.0

>1.0

2211

13-26

pre-perforationpre-perforationrig anchored

Drilling for borehole heat exchangers

In table 1 possible drilling and installation methods for vertical ground heat exchangers arelisted. As can be seen from that table, for the same rock or soil types often more than onesuitable method exists. Beside the auger method, where the material from inside the borehole isbrought to the surface mechanically, all methods listed use flushing of the borehole. Water orcompressed air (in DTH) is used to remove the cuttings in a constant flow.

Problems can arise when inserting the heat exchanger pipes into the completed borehole. Instable rocks, with clear water as drilling fluid, the heat exchanger (filled with water) slides intothe borehole under its own weight. Contrary to common perception, hard, stable rocks are very

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well suited for BHEs, with fast DTH-drilling, now stability problems and easy installation.These rocks, in particular if they contain a good amount of quartz, also exhibit very goodthermal conductivity.

If the borehole is filled with thick mud, and if the mud cannot be flushed out in order to keep thehole stable, additional techniques have to be used to get the pipes into the hole. It is of little useto push hard on the part of the pipes still out of the hole, as this would result in bending thepipes, pushing them towards the borehole wall, and increase friction (not to mention the dangerof damaging a plastic pipe). A force has to be applied to the bottom of the pipes:

• hanging a weight to the bottom part of the heat exchanger or tying one just above the bottompart

• applying pressure to the bottom part by a steel rod running between the pipes and fitting intoa receptable on the bottom part of the heat exchanger

• applying pressure to the bottom part by using the drill pipes fitting to a rim around the bottompart of the heat exchanger

Helpful is a reel-like device to hold the pipes while inserting (fig. 8 and 9). This method todayis standard in particular in Switzerland, where reels for 400 m deep double-U-tube-BHE andtremie pipe have been built in 2001.

Reel with BHE pipes

Device to straighten the BHE pipes

Ground

Fig. 8: Device for BHE insertion (as suggested by Hess, 1987)

Special problems for drilling in large, dense BHE-fields

In large BHE fields, several problems have to be dealt with. To allow completing of more than100 BHE in a relatively short time, several drilling rigs have to be on site simultanuosly. Theavailable space, the supply of water, BHE pipes and grout, and the disposal of drilling mud allhave to be planned diligently in advance. Fig. 10 shows the drilling for the largest central BHEfield in the world at Richard Stockton College in New Jersey, USA. Up to four drilling rigswere in use, and the connections in the field have impressive dimensions. Table 6 lists some ofthe most interesting examples of large BHE fields.

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Fig. 9: Reel with single-U-pipe in use for the Richard Stockton College wellfield(photo RSC)

Fig. 10: Drilling on the Richard Stockton college wellfield (left) and main collectingpipes (right); photos: RSC

The problem of borehole deviation becomes serious in large plants. No borehole is exactlystraight. With single boreholes, no problem arises, but with a number of boreholes closetogether like in an Underground Thermal Energy Storage system (e.g. Neckarsulm in table 6)Percussive drilling is particularily perceptible to borehole deviation. Sinkala (1987) reported afield study of borehole deviation in top-hammer blasthole drilling in Northern Scandinavia, andcould prove deviations of up to two meters in less than 20 m depth in rocks with dipping layers(fig. 11). This ratio of 10 % would mean a deviaton of 10 m for boreholes of 100 m depth andmore!

In order to avoid intersection of boreholes or damage of previously installed BHE, boreholedeviation must be minimised. The only way is to limit the pushing force from the top of the hole,and to increase the weight directly above the drill bit. That way the drill string is kept straight.

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Tab. 6: Examples of large borehole fields

Name Location No. of BHE Depth of BHE Total length

Central Valley School Buxton ND, USA 120 64 m 7680 m

Lulevärme BTES Luleå, S 120 65 m 7800 m

DFS Langen, D 154 70 m 10780 m

Max-Planck-Institute Golm, D 160 100 m 16000 m

Corporate Sqare Bldg. Terre Haute IN, USA 324 52 m 16848 m

Amorbach Neckarsulm, D > 600 30 m 18000 m

Sagamore Hotel Lake George NY, USA ca. 400 ca. 65 m 20800 m

Beaumont Bryn Mawr PA, USA 170 ca. 160 m 27200 m

Whitehorse Village Edgemont PA, USA 236 ca. 160 m 37760 m

Richard Stockton Coll. Pomona NJ, USA 400 130 m 52000 m

Fig. 11: Typical unilateral deviation of 19 m deep boreholes in rock with dipping ‘layers; wall of quarry in Bulken, Norway (from Sinkala, 1987)

Grouting of borehole heat exchangers

A good thermal contact between the pipes and the ground is paramount for the performance ofthe BHE. In Scandinavia, in hard, crystalline rock the borehole often is just filled with water.With increasing temperature (e.g. for borehole heat storage), the water shows convection, andheat transport is increased, as measured during thermal response tests (Gehlin & Hellström,2000).

In most other cases, and in particular if water authorities demand plugging the hole forgroundwater protection, the borehole annulus has to be filled („grouted“) with a suitablematerial. This grouting material has to be pumpable and is pressed through a tremie pipe to thebottom of the hole, from where it rises to the borehole mouth. Grouting from bottom to the topthat way is the only method to guarantee perfect filling of the hole. Grouting procedures aredealt with in the German guideline VDI 4640, part 2 (VDI, 2001). Some data of grouts aregiven in table 7. In the end, the BHE looks like shown in fig. 12, and is ready to be laid in aconnecting trench or hooked to a manifold.

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Since some time, thermally enhanced grouts have been used in USA to improve heat transfer. In2000 also in Germany two brands of thermally enhanced grouts did hit the market. They usevery fine-grained quartz or graphite to improve thermal conductivity. Reasonable ranges ofthermal conductivity of the finished grout are λ = 1.6 - 2.0 W/m/K.

Tab. 7: Properties of BHE grouting materials (after Sanner, 1992, revised)

Material thermal conduc-tivity [W/m/K]

hydraulicconductivity

pumpability impact dueto freezing

sand, water saturated 1.7-2.5 good - -

sand, dry 0.3-0.6 good - -

clay 0.9-1.4 low poor exists

bentonite 1.3 g/cm3 0.7 very low good high

bentonite with sand 1.4-1.8 very low poor medium

bentonite/cement 0.6-1.0 very low good low

therm. enhanced grout 1.6-2.0 very low good low

for comparison:

air 0.03

water 0.6

Fig. 12: left: BHE field for a school in Northern New Jersey, with single-U-tuber;right: BHEs for a residential house in Germany, with excess BHE-length to becoupled without further connection to the manifold at the house

References

Andersson, O. (1981): Borrning och Dokumentation, Borrningsteknik jämte Metodik förGeologisk Datainsamling under Borrnings Gång. - 281 pp., Diss. Thesis 8, Dept. Quartern.Geol. Lund Univ., Lund

Andersson, O., Mirza, C. & Sanner, B. (1997): Relevance of geology, hydrogeology andgeotechnique for UTES. - Proc. MEGASTOCK 97, p. 241-246, Sapporo

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Bouma, J.W.J. & Koppenol, A.D. (1983): Investigation into a complete earth-to-water heatpump system in a single family dwelling focussing on the application of a vertical subsoil heatexchanger. - Report EUR 8077 BF, Brussels

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Gehlin, S. & Hellström, G. (2000): Recent Status of In-Situ Thermal Response Tests for BTESApplications in Sweden. - Proc. TERRASTOCK 2000, pp. 159-164, Stuttgart

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Kruseman, G.P. & De Ridder, N.A. (1970): Analysis and evaluation of pumping test data. -Bull. Int. Inst. Land Reclamation and Improvement, 11, 200 pp., Wageningen

Lehtmets, M. (1991): Technical and economic basis for Swedish R&D in high temperaturestorage in ground and water. - Proc. 5th int. Conf. Energy Storage THERMASTOCK 91, pp.4.6.1-4.6.6, NOVEM, Utrecht

Reuss, M., Schulz, H. & Wagner, B. (1990): Solar assisted heat pump with duct storage inDonauwoerth. - Z. Angew. Geowiss. 9, pp. 79-91, Giessen

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Sanner, B. & Knoblich, K. (1991b): In-Situ Corrosion Tests for Ground Heat ExchangerMaterials in Schwalbach GCHP Research Station. - Newsletter IEA Heat Pump Center 9/3, p.27-29, Sittard

Sanner, B. (1992): Erdgekoppelte Wärmepumpen, Geschichte, Systeme, Auslegung, Installa-tion. - 328 S., Ber. IZW 2/92, Karlsruhe

Schulz, R. & Jobmann, M. (1989): Hydrogeothermische Energiebilanz und Grundwasser-haushalt des Malmkarstes im süddeutschen Molassebecken. - 56 p., Ber. 105 040, NLfB,Hannover

Schunnesson, H. (1985): Borrning av Värmelager. - Högskolan i Luleå, Teknisk Rapport1985:24T, 61 p., Luleå

Schunnesson, H. (1987): Longhole drilling with the top-hammer technique, its potential appli-cation in thermal heat storage. - Proc. DRILLEX 87, p. 151-160, IMM, London

Sinkala, T. (1987): Rock and hole pattern influences on percussion hole deviations: a fieldstudy. - Proc. DRILLEX 87, p. 161-174, IMM, London

VDI (2001): Thermal Use of the Underground - Ground Source Heat Pumps. - Guideline VDI4640, part 2, Beuth Verlag, Berlin