STATUS AND PROSPECTS OF GEOTHERMAL HEAT PUMPS …sustainable production from shallow geo-thermal resources. GHPs in the world and in Europe The following compilation is mainly based

Ladislaus Rybach: STATUS AND PROSPECTS OF GEOTHERMAL HEAT PUMPS______________________________________________________________________________________

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Chapter 2.1

STATUS AND PROSPECTS OFGEOTHERMAL HEAT PUMPS (GHP) IN

EUROPE AND WORLDWIDE;SUSTAINABILITY ASPECTS OF GHPs

L. Rybach,

Institute of Geophysics ETH Zurich, Switzerland([email protected])

Abstract

The ubiquitous shallow geothermal re-sources can feasibly be utilized by geo-thermal heat pumps. They represent the fast-est growing segment on the geothermal ma-rket. The present status worldwide and inEurope is summarized, with special empha-sis on Switzerland, the lead country in thedissemination of GHP technology. Further,the sustainability of GHP systems is addres-sed, mainly the basic questions of long-termoperation. The sustainability of productionis demonstrated for a single borehole heatexchanger (BHE) by a combined experi-mental and theoretical (=numerical model-ling) approach. It turns out that BHE/HPsystems operate, if properly designed, fullyreliable on the long term, i.e. sustainableproduction can be guaranteed.

Introduction

Geothermal heat pumps (GHP) repre-sent nowadays the fastest growing segmentof geothermal energy utilization (Lund

2001). Central to this technology is the heatpump (HP) which can raise (or lower) thetemperature of a working fluid. Therefore,GHPs can be used for heating and/or cool-ing. In moderate climate they can be opera-ted as monovalent systems. In the great ma-jority of cases the HP is driven by electricpower.

The base for GHPs is provided by theubiquitous shallow geothermal resource, theground right below our feet, in the upper-most few hundred meters of the earth’scrust. Traditionally geothermal direct useaims to utilize the heat content of formationfluids (if present). The heat content of therock matrix is generally higher; this heat isthe target of shallow resources utilization.

Heat can be extracted from as well asstored in this shallow reservoir. There areseveral types of access to the ground toextract heat:• Groundwater wells• Horizontal coils• Borehole heat exchangers• Geostructures (energy piles, concrete

walls).

mailto:[email protected]

International Course on GEOTHERMAL HEAT PUMPS______________________________________________________________________________________

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The latter two can also serve to storeheat (or cold) in the ground. The heat sourceis coupled to the evaporator side of the heatpump whereas the compressor side suppliesthe heating/cooling circuit. In Europe thelatter is generally a hydronic system wherease.g. in the USA it mainly comprises forcedair systems. Sometimes systems which useopen surface waters (ponds, creeks) as heatsource are also considered as GHPs, e.g. inthe USA. Groundwater wells and surfacewaters belong to the category of opensystems.

There is a whole spectrum of GHP ap-plications for various buildings (single- ormulti-family dwellings, administrative buil-dings, schoolhouses, factories etc.) likespace heating only, heating plus domestichot water, combined heating and cooling/airconditioning, in addition to other applica-tions such as road/runway de-icing or lawnheating in stadions.

In most cases the HP is driven by elec-tric power. In the case of heating, the small-ler the temperature lift needed (outlet tempe-rature at the compressor to the heating cir-cuit – heat source/inlet temperature at theevaporator) the smaller is the power uptakeof the HP and thus its efficiency. For thisreason, underfloor heating tubes which re-quire relatively low delivery temperatures(35 – 40 oC) are used in many countries.

There is a real boom in the dissemi-nation of GHP technology, with spectaculargrowth rates in several countries. In thefollowing, the current status of GHPs aroundthe world is summarized and special featuresin various European countries are presented.In addition, the question of reliable longterm operation of GHPs is addressed, i.e sustainable production from shallow geo-thermal resources.

GHPs in the world and in Europe

The following compilation is mainlybased on three publications: SANNER et al.1998, RYBACH and SANNER 2000, andLUND 2001.

The situation worldwide is summarizedin Table 1. By far the largest GHP user is

the USA. Per capita, Switzerland has clearlythe lead. In fact, there is one GHP install-lation every 2 km2 in Switzerland (detailssee below).

There are significant differences fromcountry to country. Whereas there is a largenumber already in operation and rapid deve-lopment in several countries there is prac-tically no GHP boom in countries like Japan(although a major HP producer) or nume-rous European countries in the Mediter-ranean. And still enormous markets arepractically undeveloped (e.g. China). It canbe expected that this situation will changerapidly and significantly in the comingyears.

Over the past ten years there was a lar-ge increase in the installation and use ofGHPs worldwide, with almost a 10 %annual increase. Most of the growth is con-centrated in the USA and in some Europeancountries. The present worldwide capacity isnear 7 GWt and the production is around25'000 TJ/yr. The data about the actualnumber of GHP systems are incomplete; it iscertainly above 500'000. The size of indivi-dual units can be quite different, rangingfrom a few to over hundred kW.

For systems with electric HPs theseasonal performance coefficients (whichtake into account the power uptake for theHP as well as for the circulation pumps) arenow generally > 3.0. Load factors (full loadhours per year) can vary according to loca-tion and climate. In Switzerland, for exam-ple, the actual number of full load hours isabout 1600 h/yr. Low load factors are notnecessarily a disadvantage: short runningtimes mean less electricity consumption andthus lower running costs.

The worldwide abundance and distribu-tion of GHP systems is highly inhomo-genous and the data base about existing andnewly installed systems (number, type, size)is incomplete. Some information is givenbelow; it should be kept in mind that due tothe uneven but rapid developments thesituation is changing rapidly. Compilationsfor Europe are given in Tables 2 and 3.


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Table 1. Geothermal Heat Pumps Worldwide (status in 2000),from LUND (2001)

Country Installed capacity(MWt)

Energy produced(TJ/yr)

Australia 24.0 57.6Austria 228.0 1'094.0

Bulgaria 13.3 162.0Canada 360.0 891.0

Czech Rep. 8.0 38.2Denmark 3.0 20.8Finland 80.5 484.0France 48.0 255.0

Germany 344.0 1'149.0Greece 0.4 3.1

Hungary 3.8 20.2Iceland 4.0 20.0

Italy 1.2 6.4Japan 3.9 64.0

Lithuania 21.0 598.8Netherlands 10.8 57.4

Norway 6.0 31.9Russia 1.2 11.5Poland 26.2 108.3Serbia 6.0 40.0

Slovak Rep. 1.4 12.1Slovenia 2.6 46.8Sweden 377.0 4'128.0

Switzerland* 300.0 1'962.0Turkey 0.5 4.0

UK 0.6 2.7USA 4'800.0 12'000.0Total 6'675.4 23'268.9

*) from RYBACH et al. (2000)

For systems with electric HPs theseasonal performance coefficients (whichtake into account the power uptake for theHP as well as for the circulation pumps) arenow generally > 3.0. Load factors (full loadhours per year) can vary according to loca-tion and climate. In Switzerland, for exam-ple, the actual number of full load hours isabout 1600 h/yr. Low load factors are notnecessarily a disadvantage: short runningtimes mean less electricity consumption andthus lower running costs.

The worldwide abundance and distribu-tion of GHP systems is highly inhomo-

genous and the data base about existing andnewly installed systems (number, type, size)is incomplete. Some information is givenbelow; it should be kept in mind that due tothe uneven but rapid developments thesituation is changing rapidly. Compilationsfor Europe are given in Tables 2 and 3.

USAMost of the GHP growth is in the

midwestern and eastern states, from NorthDakota and Florida. In 2000, there wereabout 500'000 units, with 50'000 new onesinstalled annually. Of these are 46 % vertical


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closed loop, 38 % closed loop, 38 % horizontal closed loop and 15 % open loopTable 2. General heat pump (“total”) and ground source heat pump systems (GSHP) installed

1993–1996 in various European countries (residential sector, in 1000 units,from RYBACH and SANNER 1999)

Country All heatpumps

Groundsourcefraction (% )

GSHPsystems

Austria 22.2 11 2.42

Denmark 3.3 18 0.59

France 25.0 11 2.75Germany 5.7 4 0.23Netherlands 0.12 7 0.01

Norway 4.0 8 0.32

Sweden 42.3 28 11.8Switzerland 15.0 40 6.0

Total 24.12

Table 3. Share of ground coupled heat pumps in total residential heating demand(RYBACH and SANNER 1999)

Country %

Austria 0.38Denmark 0.27Germany 0.01

Norway 0.25Sweden 1.09

Switzerland 0.96

systems. Even President Bush has a bore-hole GHP system in his home in Texas.

AustriaIn 1996 the percentage of closed loop

systems was 83 % and that of the open-loopsystems 12 %. There are some spectacularsystems with geostructures like foundationpiles or supporting walls equipped with heatexchanger tubes like the architecturally fa-mous Museum of Fine Arts and the FestivalHall in Bregenz, or the Convention Hall inDornbirn.

BelgiumHere the combined use for heating/cool-

ing by means of underground storage ingaining increasing attention, mainly for

larger complexes. Various building typesare supplied like banks, factories, hospitalsetc.

FranceThe dominating electricity company

Electricité de France (EdF) has started withthe promotion of GHP systems. A number ofdifferent test facilities are in operation andthe results shall serve for a country-wideinitiative. The prestigeous Palais d’Europein Strasbourg uses groundwater-based GHP.

GermanyThe potential of GHPs using shallow

geothermal resources is estimated to bearound 960 PJ/yr (KALTSCHMITT et al.,1997). This means about 10 % of the end


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energy use. The installed GHP capacity in1995 was estimated to be around 250-450

MWt, with an annual increase of some 2'000new systems. In some regions, promoted bylocal utilities, there is remarkable progresslike in the Rheinland-Westfalen area sup-plied by the RWE utility (some 200 newinstallations in 1997 versus only 9 in 1993).

The NetherlandsThe development of GHPs –which aims

to replace traditional gas-fired heatingsystems by this envoronmentally friendlytechnology – is coordinated by the DutchAgency for Environment and EnergyNOVEM. The GHP sytsems are mainly ofthe ground-coupled type. Although in 1997only about 1'000 GHP systems were inoperation their number increased since 1994by more thank 300 %. The Netherlands arealso known as one of the pioneers of heatstorage in aquifers, with some 50 unitsalready in operation for important objectslike the Rijksmuseum in Amsterdam, theKLM Operations Center at Schiphol Airport,the IBM Netherlands Headquarters inZoetemeer, the Europe Center of Nike inHilversum, the IKEA Center in Deuven orthe EU Patent Office in Den Haag.

PolandOne of the first GHP in Poland was built

in 1993 for the hotel "Ornak" in Zakopane,with horizontal ground loops. On of themost recent horizontal loop installations isfor heating of a block of flats in Lomza.There are also groundwater heat pumps, asfor heating apartment- and administrationbuildings in the Slowinski National Park inSmlodzin with 150 kW. Some further, recentexamples of GHP in Poland include objectslike churches, banks and hospitals. Polishmanufacturers of heat pumps exist mean-while, offering a range of thermal capacityfrom 4-200 kW.

SwedenSweden is one of the classic countries of

heat pump use. Around 55,000 BHE-sys-tems are operational, with a total installedcapacity of ca. 330 MWth. GSHP are a gene-

rally accepted form of heating, and due tothe high share of hydropower in the electricpower supply, heat pumps always offer anopportunity for reduction of emissions.

Besides in GSHP use, Sweden is alsoleading in underground thermal energy sto-rage technology. Here often BHE are used,and in areas with glacio-fluviatile sedimentsor, in the Southern part of the country, withfractured limestone, groundwater is useddirectly.

Cooling of telecommunication stationsis done using BHE, like those in Ängby,Devikstrand, and Stockholm. Besides, 54television relay stations are supplied byBHE-coupled HPs (for cooling).

A particular success story can be told ofStrömstad, a town of 6000 about 200 km tothe North of Gothenburg. The rocky subsoilis not suited for district heating, and thus140 GSHP with a total of 400 BHE havebeen installed for heating of houses andapartments for 3000 people (SANNER andHELLSTRÖM 1998).

SwitzerlandWith a total of 60,000 presently installed

heat pumps for space heating/warm watersupply Switzerland is, per capita, worldleader in this environmentally friendly tech-nology. The general popularity of heatpumps in Switzerland lead also to a realboom of heat pump coupled BHE systems.Today, every third newly built single familyhouse is equipped with a heat pump system.Although air-source heat pumps aresignificantly lower in installation cost (thereare no drilling costs as for a BHE system)nearly 40 % of the heat pumps installedtoday have a geothermal (BHE) source. Thegenerally lower seasonal performancecoefficient of air-source heat pumps (due tothe low source temperature in winter) is themain reason of this high percentage. Themajority of GHPs have a BHE source, withan energy share of 71% (Fig. 1)

The share of heat delivered by GHPsystems in the Swiss geothermal mix isoverwhelming (75 % of a total of 439 GWhin 1997, RYBACH and WILHELM 1999).


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The boom resulted in the installation ofover 25,000 BHE systems to date, with atotal of about 4,000 km of BHE length. Atpresent, 1 m of BHE costs (drilling andinstallation included) about 40 US-$. Figure2 shows the spatial distribution of BHEinstallations, delivered by just one comer-cial company (GRUNDAG, Gossau/SG:7,900 BHE’s with 695 km total length;status in mid 1997). The pattern of BHEsystem locations corresponds roughly to thepopulation density. The widespread BHEinstallations secure Switzerland a leadingposition: Areal BHE density in Switzerlandis highest worldwide (1 BHE installationevery 2 km2). The number of installationsincreases yearly by >10 %, as well as theheat production. The annual growth is alsoimpressive (Fig. 3).

Borehole heat

exchangers 1393 TJ/yr

71%

Horizontal coils

15.2 TJ/yr 6%

Ground-water wells 454 TJ/yr

23%

Figure 1: Geothermal heat pump systems inSwitzerland in 1999.

New technologies like geostructures(“energy piles”), combined heating/coolingare rapidly progressing on the market. Aprominent example is Dock Midfield, thenew terminal at Zurich International Airport,This terminal building is a 30 meters widenew construction. It is funded on 440 fund-ation piles 0.9 – 1.5 m in diameter, of which315 are equipped with heat exchanger tubes(=”energy piles”). The piles stand at 30 mdepth on a tight ground moraine formation.Through this special BHE system 1.1 GWhheat are extracted annually from the ground

for heating in the winter. In the summer, thebuilding heat is deposited in the groundwhich thus acts as a storage medium. For airconditioning, some 500 kWh cooling energyis supplied by the energy piles.

Sustainability of GHP systems

The original definition of sustainabilitygoes back to the Bruntland Commission(1987; reinforced at the Rio 1991 and Kyoto1997 Summits):

“Meeting the needs of the presentgeneration without compromising the

needs of future generations”.

In relation to geothermal resources and,especially, to their exploitation for geother-mal energy utilization, sustainability meansthe ability of the production system appliedto sustain the production level over longtimes. Often the resources are taken intoproduction (of the reservoir fluid as the heatcarrier), mainly to meet economic goals likea quick pay-back of investments forexploration and equipment, in such a waythat reservoir depletion is the result. Thereare numerous examples for this worldwide,the most prominent is the vapor-dominatedfield of The Geysers/USA. Sustainableproduction of geothermal energy thereforesecures the longevity of the resource, at alower production level.

A definition of sustainable productionfrom an individual geothermal system hasbees suggested recently (ORKUSTOFNUNWORKING GROUP, 2001):

“For each geothermal system, and for eachmode of production, there exists a certain

level of maximum energy production, belowwhich it will be possible to maintain con-stant energy production from the system for a very long time (100 – 300 years)”.

The definition applies to the total extrac-table energy (=the heat in the fluid as well asin the rock) and depends on the nature of thesystem but not on load factors or utilizationefficiency. The definition does also not con-


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sider economic aspects, environmentalissu-es nor technological advances, all of

which may be expected to change with time.

Figure 2: Location of BHE systems in Switzerland, delivered by a single company (GRUNDAG AG, Gossau/SG; from RYBACH and EUGSTER 1998).

0

50

100

150

200

250

300

350

400GWh

1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999

Geothermal heat production from BHE systems in Switzerland


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Figure 3: Geothermal heat production from borehole heat exchangers. The fluctuations aroundthe development trend are caused by annual changes in heating demand (strong/mild winters).

From RYBACH et al. (2000).

In the case of GHPs the issue of sustai-nability concerns the various heat sources.In the horizontal systems the heat exchangerpipes are buried at shallow depth; the longe-vity of their smooth operation is guaranteedby the constant heat supply from the atmo-sphere by solar radiation. In the case ofcombined heating/cooling by GHPs the heatbalance (in/out) is given by the system de-sign itself. In the case of groundwater-coup-led GHPs the resupply of fluid is secured bythe hydrologic cycle (infiltration of precipi-tation) and the heat comes either “fromabove” (atmosphere) and/or “from below”(geothermal heat flow); the relative propor-tions depend on aquifer depth. This leads toa ± constant aquifer temperature all over theyear without any significant seasonalvariation.

The situation with BHE-coupled GHPsystems is different. During heat extractionoperation the BHE evolves more and moreto a heat sink. False design, especially withforced extraction rates (several tens of Wper meter BHE length, in low thermalconductivity materials like dry gravel) canlead to freezing of the surrounding groundand thus to system collaps. Therefore theconditions by which a reliable operation canbe secured also on the long term (i.e. sustai-nable operation) need to be established. Se-veral such attempts have been published inthe literature; one of the first –and rathercomplete, supported by theory and experi-ments– such studies will be summarizedbelow.

GHP sustainability with BHE

The question of sustainability of GHPsin general and of BHE coupled HPs boilsdown to the question: for how long can suchsystems operate without a significant draw-down in production, i.e. reaching a levelwhich is beyond economic viability. There-fore the long-term production behavior ofBHE based GHPs needs to be addressed.

The oldest BHE installations are not ol-der than about 15 - 20 years, thus experienceand especially detailed studies on long-termperformance (decades) are lacking. There-fore the question arises about the reliabilityof such systems on the long run. Along thesame line come the questions: can suchsystems operate in a sustainable manner? Isthe shallow geothermal resource renewable?I.e. does the ground recover thermally aftershut-down of the BHE heat extractionoperation which is customarily designed torun over a few decades?.

To answer these questions a combinedtheoretical/experimental approach has beenfollowed to establish a solid, verified basefor the confirmation of reliable long-termperformance on one hand, and to clarify theterms of renewability on the other.

Study of long-term performance

The verified base to confirm thereliability of BHE/HP systems on the longterm has been elaborated by combining fieldmeasurements with numerical model simu-lations. For this basic study, a single BHEwas treated. The approach used is describedin detail in EUGSTER and RYBACH 2000.

An extensive measurement campaignhas been performed at a commercially ins-talllled BHE system in Elgg near Zurich,Switzerland. Object of the study is a single,coaxial, 105 m long BHE, in use since itsinstallation (1986) in a single family house.The BHE stands isolated and supplies a peakthermal power of about 70 W per m length.By this, the BHE is rather heavily loaded.Thus the installation is by no means aparticularly favorable example.

The aim of the measurement campaignis the acquisition of ground temperature datain the surroundings of the BHE as well as ofoperational parameters of the entire system.For this purpose, 105 m long measuringprobes were installed in boreholes at 0.5 and1.0 m distance from the BHE, backfilled


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with a bentonite/cement mixture like theBHE itself. Both probes are equipped with

temperature sensors at 1, 2, 5, 10, 20, 35, 50,65, 85, and 105 m depth. The use of pre-aged Pt100 sensors, in combination with ahigh-resolution multimeter (DATRON 1061A), provides maximum long-term stability(± 0.1 K accuracy, ± 0.001 K precision) overthe entire measurement period. In addition

to the ground temperatures, the atmospherictemperature variations and all parametersrelevant to the operation for the entire sys-

tem (hydronic system flowrates, circuittemperatures, power consumption of the HPetc.) have also been recorded in 30 minuteintervals.

December 1986(undisturbed profile)

September 1988

September 1987

September 1989

September 1990

September 1991

September 1996

4 6 8 10 12 14 16

110

100

90

80

70

60

50

40

30

20

10

0

Dep

th [m

]

September 1997

4 6 8 10 12 14 16Temperature [°C]

110

100

90

80

70

60

50

40

30

20

10

0

September 1998

Figure 4: Measured ground temperature profiles at a distance of 1m from the BHE in Elgg.Curve "December 1986" marks the undisturbed profile at the start of the first heating season. The

subsequent curves show the conditions after winter heat extraction and summer recovery, justbefore the start of the next heating season

. The first campaign extended over theyears 1986-1991 and supplied a unique database (details in EUGSTER, 1991). Theground temperature results are displayed inFigure 4. Atmospheric influences are clearlyvisible in the depth range 0 - 15 m; below

15 m the geothermal heat flux dominates. Itis obvious that in the near field around theBHE the ground cools down in the first 2 - 3years of operation. However, the tempera-ture deficit decreases from year to year untila new stable thermal equilibrium is estab-


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lished between BHE and ground, at tempe-ratures which are some 1 - 2 K lower thanoriginally. (This temperature deficit is cha-racteristic of the measurement site withtypical Tertiary "Molasse" formations).

In the autumn of 1996 (i.e. after 10years of BHE operation) the measurementsystem was restarted. Due to the forcedaging of the Pt100 sensors the high qualityof temperature measurement has beenmaintained and the repeatability of themeasurements is still better than ± 0.01 K.The new temperature profiles ("September1996", "September 1997", "September1998", Figure 3) do not show any furthersignificant shift towards lower temperatures,thus demonstrating that a quasi-steady equi-librium has been reached after the first fewyears. The small differences between theprofiles of subsequent years after at leastthree years of operation are a result of thedifferent yearly heating demands which,given unchanged living habits of the owners,are uniquely a product of the outsidetemperature. In the following years theground temperatures fluctuate within alimited interval of about 0.5 K, dependingon the specific annual heating demand. For acorrectly designed BHE system in theabsence of groundwater and with boreholedepths of this order this corresponds to ourtheoretical expectations.

These measurements represent a uniquedata base which in turn was used to validatea numerical model. First, the temperaturecurve "September 1996" was predicted bysimulation and in turn compared with themeasured curve. The agreement was excel-lent; the deviations were within measure-ment error (± 0.1 K), see RYBACH andEUGSTER (1998). The excellent agreementbetween measured an calculated timehistories at a number of specific points in theunderground gives confidence to extrapolatefuture trends and situations by modelling.

For this, the results of the first measu-rement campaign (1986-1991) were used tocalibrate a 2D numerical code (COSOND, incylindrical coordinates). The code treats dif-fusive heat transfer in the ground, advectionin the BHE, heat transfer between the BHE

fluid and the wall materials, as well as heattransfer between atmosphere and ground.The program flow is controlled by a loadprofile which contains the atmospherictemperatures and the operational data of theheat pump. Details are given in GILBY andHOPKIRK (1985) and EUGSTER (1991).Ground temperatures over the first five yearsof measurement were fitted to within one ortwo tenths of a degree Celsius. Additionallythe formation temperature was predicted forseveral further years using assumed loadprofiles (EUGSTER, 1991).

These computer simulations have nowbeen recalculated using an adapted loadprofile based on the atmospheric tempera-tures of the years 1991-1997 actually measu-red in the meantime at a nearby meteo-rological station (Tänikon/TG) as well as onthe homeowner’s records about heat pumpoperation times. The model grid had 11'700grid cells in a model volume of 2x106 m3

(for details see EUGSTER and RYBACH,2000).

The operation of the Elgg BHE plant hasbeen extrapolated for additional 19 years toa final period of 30 years (1986 - 2015). Theload profiles for these extrapolation runs arebased on the new Swiss Standard ClimaticDatabase (METEONORM, 1997).

Thermal conditions around a BHE

The transient thermal conditions arounda BHE in operation are very complex.Several processes are superimposed:

• a heavy cooling-down and a subsequentrewarming of the immediate vicinity ofthe BHE up to some 10 cm during aoperational cycle (hourly cycle);

• the dissemination of this cooling andrewarming period up to several meter asa funnel-like temperature effect during aseasonal operation (yearly cycle);

• a large-scale, but only minimal cooling-down of the surrounding underground upto a distance of several 10's of metersduring the full life cycle of the BHE (30-years-cycle);


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• both the horizontal and vertical heatfluxes increase around the BHE. The

massive cooling down of the BHEvicinity enlarges the heat flows from theatmosphere and from the underground.

These pure conductive processes are rathercomplicated and visualized in Figure 5. But

in free nature, flowing groundwater and - insaturated formations - water vapor diffusion

processes add their effects to this complexsystem.

Figure 5: Funnel-like temperature distribution and long term cooling-down around a BHE. Theshort term and the long term influences are well documented.

The operating BHE creates a heat sinkin the ground which has cylindrical shape.The isotherms are, after a certain operationaltime, concentrated near the BHE. Fig. 6shows the measured temperature distributionaround a 50 m long BHE, at the German testplant at Schöffengrund-Schwalbach nearFrankfurt/Main. Here a 50-m-BHE was sur-rounded by a total of 9 monitoring boreholesat 2.5, 5 and 10 m distance, also 50 m deep.Tem-peratures in each hole and at the BHEitself were measured with 24 sensors at 2 mvertical distance, resulting in a total of 240observation locations in the underground.This layout allowed to investigate thetemperature distribution in the vicinity of theBHE, as shown in Fig. 6. The influencefrom the surface is visible in the uppermostca. 10 m (see also Fig. 4), as well as the

temperature decrease around the BHE at theend of the heating season. The latter createsstrong temperature gradients in the BHEvicinity which in turn leads to heat inflow,directed radially towards the BHE, to rep-lenish the deficit created by the heatextraction. This heat flow density attains,compared to the terrestrial heat flow (80 –100 mW/m2), rather high values (up toseveral W/m2). A similar situation is depic-ted in Fig. 7 from a site in Elgg/ZH, Switz-erland.

Thermal recovery

The long-term behavior of the singleBHE-HP system was further investigated bynumerical modelling. The results of thesimulation runs show on one hand the


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expected decrease of the yearly temperaturedeficit and on the other hand an increasingvolume around the BHE which is affectedby the cooling (see Figure 8). After shut-down of heat extraction, regeneration of theground begins. During the production periodof a BHE, the draw-down of the temperature

Figure 6: Measured temperature distribution in the ground at the beginning of the monitoringperiod (left, on 1.10.1986, after a total of ca. 2 hours of test operation) and at the end of the firstheating season (right, on 1.5.1987), Schwalbach GSHP test plant, Germany (after RYBACH and

SANNER, 1999).

11

9

9

10D

epth

[m

]

BH

E

Distance from BHE [m] Distance from BHE [m]

De

pth

[m

]

-5 5 100

36

32

28

24

20

16

12

8

4

40

44

48

36

32

28

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40

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48-5 5 100

34

6

66

77

8

9 °C

98

7

5

5

5

4B

HE

Heat flow

Ground temperature

Dep

th [

m]

Distance from BHE [m]

BHE

Heat flow

Ground temperature100

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40

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20

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0

110

-20 -10 10 200

9

9

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11 °C11

77

9

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88

5

5

44


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Fig. 7: Calculated temperature isolines around a 105 m deep BHE, during the coldest period ofthe heating season 1997 in Elgg/ZH, Switzerland. The radial heat flow in the BHE vicinity is

around 3 W/m2.around the BHE is strong during the firstfew years of operation. Later, the yearlydeficit decreases asymptotically to zero.During the recovery period after a virtualstop-of-operation , the ground temperatureshows a similar behavior: during the firstyears, the temperature increase is strong andtends with increasing recovery timeasymptotically to zero. The time to reach acomplete recovery depends on how long theBHE has been operational. Principally, therecovery period equals nearly the operationperiod. This is shown in Figure 9 for

different distances from the BHE and fordifferent final temperature deficits.

In summary, the measurements andmodel simulations prove that sustainableheat extraction can be achieved with suchsystems. The installation in Elgg supplies onthe average about 13 MWh per year. In fact,the BHE’s show stable and reliable perfor-mance which can be considered renewable.Reliable long-term performance provides asolid base for problem-free application;correct dimensioning of BHE gives greatscope of widespread use and optimisation.

Figure 8: Measured and modelled ground temperature changes of the BHE at Elgg relativeto the undisturbed situation in December 1986 over 30 years of operation and 30 years of

recovery.


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0 5 10 15 20 25 30Time of Operation [years]

0

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30

Tim

e to

reac

h ∆T

[yea

rs]

0

5

10

15

20

25

30

1m

10m

20m

1m

10m20m

1m

10m 20m

∆T=0.15K

∆T=0.25K

∆T=0.50K

Figure 9: Duration of recovery period to reach a minimal final temperature deficit (∆T) of 0.5,0.25 and 0.15K for different distances from the BHE as a function of the time of operation.The above investigations have been

performed for a single BHE. Similar studieshave now been initiated for BHE groups inregular as well as irregular patterns.

Conclusions

ß Geothermal heat pumps (GHP)represent the fastest growing segment ofgeothermal energy utilization. There is awhole spectrum of GHP applications forvarious buildings (single- or multi-family dwellings, administrative build-ings, schoolhouses, factories etc.) likespace heating only, heating plus domes-tic hot water, combined heating andcooling/air conditioning, in addition toother applications such as road/runwayde-icing or lawn heating in stadions.

ß There are significant differences in theGHP technology dissemination fromcountry to country. Whereas there aregreat advances in countries like theUSA, Sweden and, especially, Switz-erland (one GHP system every 2 km2)there is little to no development in othercountries. Nevertheless the generaldevelopment trend is clear; in somecountries the annual increase is > 10 %.

ß Sustainability aspects of GHP systemsalso been addressed, with emphasis onBorehole Heat Exchanger (BHE)/heatpump(HP) systems. They prove to be a

feasible way to tap shallow geothermalresources which, located directly belowour feet, represent a unique, ubiquitousand therefore enormous geothermal po-tential. They operate reliably also on thelong term. This has been proven byexperimental and theoretical investigati-ons: data of an extensive measurementcampaign over several years were usedto calibrate a numerical modelling code.The results of modelling with this codefor a single BHE show that the long-term performance of the BHE/HP sys-tem stabilizes, relatively to initial condi-tions, at a somewhat lower but constantlevel after the first few years. Thussustainable operation can be achieved.The ground around the BHE behaves inthe following way: the long-term heatextraction causes heat depletion/tempe-rature decrease. The temperature drop(which decreases with radial distancefrom the BHE) is significant after thefirst years of operation but then it tends,in the subsequent years, asymptoticallytowards zero.

ß The BHE operation creates a local heatsink and thus strong temperature gra-dients in the BHE vicinity which in turnleads to heat inflow, directed radiallytowards the BHE, to replenish the deficitcreated by the heat extraction. This heatflow density attains, compared to the


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15

terrestrial heat flow (80 – 100mW/m2), rather high values (up to

several W/m2).ß After shut-down of BHE operation ther-

mal recovery begins, strong in the be-ginning and decreasing asymptoticallyafterwards. Model simulations with dif-ferent operation recovery periods showthat recovery duration roughly equalsthat of operation: e.g. for 30 years ofBHE operation the thermal recovery ofthe ground needs 30 years.

ß The basic studies about long-term per-formance presented here apply to a sing-le BHE. Similar studies are underwayfor BHE groups/patterns.

Acknowledgement

The constant support of Dr. H. Gorhan,Project Leader for geothermal energy of theSwiss Federal Office of Energy, is gratefullyacknowledged.

References

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Lund, J. W. (2001): Geothermal heat pumps –An overview. GeoHeatCenter Bull. 22/1, 1-2Meteonorm (1997). Global meteorological data-base for solar energy and applied climatology.Manual, Swiss Federal Office of Energy, Berne,84 pp.ORKUSTOFNUN Working Group, Iceland(2001): Sustainable production of geothermalenergy – suggested definition. IGA News no. 43,January-March 2001, 1-2Rybach, L. and Eugster, W.J. (1998). Reliablelong term performance of the BHE systems andmarket penetration - the Swiss success story. In: Proc. 2nd Stockton International GeothermalConference, 87-101.

Rybach, L., and Wilhelm, J. (1999). Swiss Geo-thermal Society (SGS) Presentation. IGA Newsno. 35, January-March 1999, 9-10.Rybach, L. and Sanner, B. (2000): Ground-sour-ce heat pump systems – The European experi-ence. GeoHeatCenter Bull. 21/1, 16-26Rybach, L., Brunner, M., Gorhan, H. (2000):Swiss Geothermal Update 1995-2000. In: Proc.World Geothermal Congress 2000, 413-426.Rybach, L., Mégel, T., Eugster, W.J. (2000): Atwhat time sace are geotehrmal resources rene-wable? In: Proc. World Geothermal Congress2000, 867-872Sanner, B. and Hellström, G. (1998): Examplesfor Underground Thermal Energy Storage withBorehole Heat Exchangers in Central and North-ern Europe. Newsletter IEA Heat Pump Centre16/2, 25-27.Sanner, B., Boissavy, Ch., Eugster, W.J., Ritter,W., van Eck, H. (1998): Stand der Nutzung ober-flächenaher Geothermie in Mitteleuropa. In: Ta-gungsband 5. Geothermische Fachtagung Stra-ubing, GtV, Geeste, S. 461-478

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STATUS AND PROSPECTS OF GEOTHERMAL HEAT PUMPS …sustainable production from shallow geo-thermal resources. GHPs in the world and in Europe The following compilation is mainly based