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Geothermal space heating

R. Gordon Bloomquist *Washington State University Energy Program, 925 Plum St SE, Town Square Building 4,

Olympia, WA 98504, USA

Received 5 June 2003; accepted 24 June 2003

Abstract

The use of geothermal resources for space heating dominates the direct use industry, withapproximately 37% of all direct use development. Of this, 75% is provided by district heatingsystems. In fact, the earliest known commercial use of geothermal energy was in Chaudes-Aigues Cantal, France, where a district heating system was built in the 14th century. Today,

geothermal district space heating projects can be found in 12 countries and provide some44,772 TJ of energy yearly. Although temperatures in excess of 50 C are generally required,resources as low as 40 C can be used in certain circumstances, and, if geothermal heat pumpsare included, space heating can be a viable alternative to other forms of heating at temperatureswell below 10 C.# 2003 CNR. Published by Elsevier Ltd. All rights reserved.

Keywords: Direct uses; Space heating; District heating; Absorption technology

1. Introduction

Space heating with geothermal energy is one of the most common and widespreaddirect uses of geothermal resources. Space heating comprises over 37% of the totaldirect use worldwide ( Fig. 1 ). If geothermal heat pumps are included, space heatingaccounts for more than 50% of all geothermal non-electric uses. Space heating isalso one of the oldest direct uses of geothermal energy; as early as the 14th centurythe inhabitants of the French village of Chaudes-Aigues Cantal were enjoying thebenets of geothermal space heating delivered to them via a district heating network

Geothermics 32 (2003) 513–526

www.elsevier.com/locate/geothermics

0375-6505/$30.00 # 2003 CNR. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.geothermics.2003.06.001

* Corresponding author. Tel.: +1-360-956-2016; fax: +1-360-956-2030.E-mail address: [email protected] (R.G. Bloomquist).

http://www.elsevier.com/locate/geothermics/a4.3dmailto:[email protected]:[email protected]://www.elsevier.com/locate/geothermics/a4.3dhttp://www.sciencedirect.com/http://www.sciencedirect.com/http://www.sciencedirect.com/

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from the reservoir as well as the need for a disposal system. Heat is provided eitherthrough a forced-air system (common in the USA), through a hydraulic heatingsystem (radiators), or through a radiant oor or ceiling system. Most systems arealso designed to provide domestic hot water.

Critical system components include the well, pump(s) if the well does not owartesian, a downhole or surface heat exchanger and the heat delivery system. Somesystems retain the ability to provide backup and/or peaking through the use of a

conventional fossil fuel or electric boiler or furnace and some may contain a storagefacility to help reduce peak pumping requirements.

Small individual geothermal space heating systems are found throughout much of the world. The USA, many countries of Western and Central Europe, Russia,China, New Zealand and Iceland all contain excellent examples of such systems. Theuse of downhole heat exchangers has been especially popular in the United States(primarily Klamath Falls, Oregon), New Zealand (primarily Rotorua) and Turkey.

3. District heating

District heating, and in some cases district cooling, networks are designed to pro-vide space heating and/or cooling to multiple consumers from a single well or from

Fig. 2. Downhole heat exchanger geothermal direct-use energy guidebook ( Lund et al., 1991 ).

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multiple wells or elds. The development of geothermal district heating, led by theIcelanders, has been one of the fastest growing segments of the geothermal spaceheating industry and now accounts for over 75% of all space heating provided fromgeothermal resources worldwide. District heating is also one of the oldest uses of geothermal energy, and the rst documented geothermal district heating system wasbuilt in Chaudes-Aigues Cantal, France, in the 14th century. In the United Statesthe Boise, Idaho system known originally as the Artesian Hot and Cold WaterCompany, and later as the Boise Warm Springs Water District, went online in 1893and is now one of the four independent systems that serve the Boise metropolitanarea ( Bloomquist, 2000 ). The early-to-mid 1980s saw the development of severalgeothermal district heating systems in Idaho, Oregon, New Mexico and California,and although growth of these systems has continued, no new system has comeonline since the late 1980s due to extremely low natural gas prices. Iceland has,however, been the leader in the development of geothermal district heating systems,and as of today over 97% of the inhabitants of the capital city of Reykjavik enjoythe benets of geothermal district heating, and more than 90% of the total Icelandicpopulation can now count on geothermal district heating to supply their spaceheating as well as domestic hot water heating requirements. Iceland may soon loseits leadership role as Turkey is quickly emerging as a leader in the development of new geothermal district heating systems. By 2000, Turkey had over 51,600 resi-dences connected to district heating networks and projects to supply geothermaldistrict energy services to approximately 500,000 residences, or 30% of the resi-

dences in the country, by 2010 ( Lund, 2002 ). Several other countries have alsodeveloped or are developing geothermal district heating systems, including Hun-gary, Romania, France, Poland, China and even Sweden and Denmark, to name just a few.

Although the demand for district cooling is growing at a tremendous rate world-wide, the author is aware of only one district cooling system that derived its energyfrom geothermal. That system was established at the Oregon Institute of Technol-ogy in Klamath Falls, Oregon in the early 1980s and was based on the use of the88 C geothermal water that also supplies the Institute’s district heating system. Thesystem consisted of a 1055 kW lithium bromide absorption heat pump and a con-

ventional cooling tower. The system operated until 2000 but was replaced at thattime by a conventional electrical-driven chiller. The primary reason for the decom-missioning of the geothermally-driven absorption heat pump was the low efficiencyof the system due to the moderate temperature of the geothermal uid available. Infact, the 1055 kW unit could only produce approximately 528 kW of cooling at thetemperature of the geothermal uid available. Most lithium bromide absorptionunits are designed to operate optimally at temperatures in excess of 120 C.Advances in absorption technology and/or the use of binary turbine-driven refrig-eration equipment could, however, open the door to a much greater use of geother-mal in district cooling applications.

A geothermal district energy system consists of one or multiple wells or in somecases even well elds, well and circulating pumps, transmission and distributionpiping, central or individual building heat exchangers, peaking/backup boilers

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and/or thermal storage units and a system for metering. For the individual con-sumer the equipment is identical or similar to that used in individual systems, i.e.either a forced air or radiant system.

Because of the scale of most geothermal district energy systems, wells will gen-erally be drilled with a larger diameter and often much deeper than would be cost– effective for an individual system. Depths in excess of several hundred meters are notuncommon in many systems, and some may be as deep as 2000–3000 m or more.Most wells will require pumping to bring the water to the surface and to maintainthe pressure so as to prevent release of gases that could result in scale formation.Both line-shaft and downhole pumps have been used successfully, with line-shaftpumps being preferred in the USA and downhole pumps more common in Europeand much of the rest of the world. New advances in downhole pump technologymay pave the way for their use in higher temperature wells, and their ability to beused in deviated wells makes them an obvious choice in many applications. Mostpumps are equipped with variable speed drives in order to conserve energy as well asbetter meet system requirements.

Transmission and distribution piping generally consists of pre-insulated and jacketed welded steel. However, asbestos cement and some other types of non-metallic pipe have been used successfully in some applications. The use of non-metallic pipe is more common in cooling systems than in heating systems and thepipe may or may not be insulated. Both above and below ground transmission linesare common, but distribution piping is almost always underground except in unique

applications such as military bases where above-ground piping appears to be muchmore acceptable and common. When placed above ground, expansion loops andanchors are a necessity and supports must be constructed so as to allow for somepipe movement. When installed below ground, direct burial is preferred and mostcommon. Expansion is accounted for through the use of compensators, and pre-stressing the pipe before burial is all the more common. During construction, it isvital that welds be thoroughly checked and that water-tight mufflers are used wher-ever jointing occurs. Once the muffler is installed, the area between the pipe and themuffler jacket is insulated. The muffler ensures that no water will reach the carrierpipe and result in severe external corrosion. In direct buried systems the use of a leak

detection system is often employed and although not necessary, the minimal extracost is often considered to be very inexpensive insurance. A three-wire leak detectionsystem may also double as a signal mechanism for the control of valves. In somebelow-ground applications, covered trenches or utilidors are used, but their extracost is very difficult to justify in most applications. They do, however, provide easyaccess to the piping system for maintenance and repair. If the pipes are installed incovered trenches or utilidors, proper anchors and expansion loops must be used tominimize stress. In larger systems, valves and bypasses are used to reduce the risk of water hammer when ow velocity must be rapidly changed or stopped. Metersinstalled at critical locations provide data used by operations personnel, and

increasingly common real-time computer models provide operators with not onlyenhanced system control but also reduced operation costs by controlling and opti-mizing the system to best meet load. Savings may occur in electricity to drive pump


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motors, in the fossil fuels used in peaking and in a reduction in required personnel(Fig. 3 ) (Bloomquist et al., 2002 ).

Although geothermal uids may be circulated through the transmission and dis-tribution system directly to end users and in some cases even through the end users’heating system, the practice is discouraged due to the serious risk of corrosion and/or scaling. The heat in the geothermal uid is instead most often transferred to asecondary uid for transmission and/or distribution. Such systems are often referredto as closed loop systems. The geothermal uid is then returned to the reservoirthrough an injection well. The distribution uid used is generally water containingfreeze and corrosion protection additives, although in some countries the use of de-ionized water is common. The heat transfer is accomplished through a heatexchanger. Although both tube-and-shell and plate-and-frame exchangers have beensuccessfully utilized, plate-and-frame exchangers command the largest part of themarket due to their better heat transfer characteristics (lower approach tempera-tures), relatively compact size and the ease with which they can be expanded andcleaned ( Fig. 4 ).

Both closed and open loop systems may incorporate thermal storage and/orpeaking and/or backup equipment into the ‘‘production plant’’. Thermal storageequipment can minimize the ow required by the geothermal wells needed to meetpeak demand. However, despite the availability of thermal storage facilities, meetingpeak demand solely with geothermal requires substantially increased ows—some-times as much as a 50% increase in ow with increased demand. This is because the

temperature that is available from the geothermal wells cannot be varied, forcingsystem operators to meet peak only by increasing ow. Peaking can be provided byfossil fuel boilers, electric boilers or heat pumps. Meeting peak solely by increasinggeothermal ow also requires a substantial over-sizing, by 40–60%, of the trans-mission and/or distribution piping network Alternatively, peak can be met by vary-ing temperatures while holding ow constant. Since, in an idealized load durationcurve, peak demand occurs only 3–5% of the time ( Fig. 5 ), the penalty suffered fromusing fossil fuels to meet peak demand is easily offset by the savings that can beachieved by limiting the number of production and injection wells and the cost sav-ings that result from optimizing (i.e. minimizing) the diameter of the transmission

and distribution piping networks. Research conducted by the author revealed thatincorporation of peaking into the system design can reduce the number of produc-tion and injection wells by as much as 50% and reduce the diameter of transmissionand/or distribution piping by 40–60%. Reducing the pipe diameter not only reducescapital cost but also results in savings from reduced thermal losses during a baseloadsystem operation.

District cooling is becoming an increasingly important utility service in manyparts of the world. In the United States, the development of district cooling systemshas far outpaced the development of new district heating systems and even in rela-tively mild climatic areas, such as Sweden, district cooling is becoming a major area

for new development. Unfortunately, none of these new systems are based on geo-thermal energy, and, as was noted above, the only such system with which theauthor is familiar is the one that served the Oregon Institute of Technology campus.


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Fig. 3. TERMIS plot of heat loss from pipes and a dynamic simulation showing the temperature propagation (

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Recent advances in absorption technology (primarily lithium bromide systems)could, however, allow for much greater utilization of low-to-moderate temperaturegeothermal resources (110–150 C) in district cooling applications. Absorptiontechnology ( Fig. 6 ) may be applied through the production of cooling at a centrallocation with subsequent distribution to multiple consumers or, as is occurring inmany European countries that have existing district heating systems, on a building-by-building basis, with the heat input provided through the circulating districtheating loop. The one downside of using lithium bromide absorption technology isthat the minimum temperature achievable is approximately 5 C, allowing for

Fig. 4. Plate and frame heat exchanger ( Lund et al., 1991 ).

Fig. 5. Typical load duration curve.


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storage only in the liquid phase. Ammonia absorption can achieve sub-freezingtemperatures, thus allowing for ice storage, but the COP is extremely low, e.g. 0.2– 0.3. Recent studies on using an organic Rankine Cycle-driven refrigeration machinecould provide the breakthrough that is needed to make low temperature geothermaldistrict cooling a reality.

End users of a district energy system may be connected directly to the distributionnetwork with system distribution uids circulated directly through the user’s HVACsystem or each end user may be connected to the central distribution system via aheat exchanger or exchangers—one for heating and a second for the provision of

Fig. 7. Brazed plate conguration ( Lund et al., 1991 ).

Fig. 6. Absorption technology ( IEA, 1992 ).


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domestic hot water. Generally users are connected through a plate-and-frame heatexchanger (sometimes of brazed plate conguration; Fig. 7 ) or a spiral exchanger(Fig. 8 ) (Bloomquist and Wegman, 1998 ). In some systems, where de-ionized wateris circulated through the distribution networks, the heat exchanger may be elimi-nated and the system circulating uids carried directly to the in-building equipment.In-building equipment is most common in room radiators, but in the United Statesforced air or ceiling or oor radiant heat is in common use. Heat exchangers areoften a requirement in multiple-storey buildings where additional pumping isrequired and/or where liability issues preclude direct connections.

Most district energy systems utilize some sort of metering of energy consumptionfor billing purposes, although some systems use a at-rate approach based on billingon a per square meter of conditioned space or ow basis. In the case of at-ratebilling on the basis of ow, ow limiters are installed that restrain ow to a certainpre-determined limit. Where energy consumption is used as the basis for billing,energy meters that integrate in-ow and out-ow temperatures and ow are thestandard (in the USA they are referred to as BTU meters). Energy or ow meters

Fig. 8. Spiral exchanger ( Bloomquist and Wegman, 1998 ).


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Fig. 9. Thin client on GIS GUI including SQL analysis on results ( Bloomquist et al., 2

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remotely monitorable may also provide real-time data for system operation as wellas eliminating the personnel needed for meter reading and billing, and updatingdatabases and system maps ( Fig. 9 ) (Bloomquist et al., 2002 ).

4. Geothermal heat pumps

Geothermal heat pumps are an ever increasing means of providing space con-ditioning through the use of low temperature ( < 10–ca. 40 C) geothermal resources(Fig. 10 ). Geothermal heat pumps are mechanical devices that can provide heatingand/or cooling ( Fig. 10 ). In the heating mode, heat extracted from the earth or fromground water is boosted by the heat pumps to meet space heating requirements. Inthe cooling mode, heat is extracted from the interior of the building and the earth orground-water serves as a heat sink.

Heat can be extracted or returned through a horizontal heat exchanger array,vertical bores or pumped wells ( Fig. 11 ). While a single family house will require asingle heat pump and a horizontal loop, single vertical bore or a pumped well,commercial or institutional building(s) or complexes may require multiple verticalbores or pumped wells and either a single large central heat pump or multiple dis-tributed heat pumps.

At the present time approximately 50,000 geothermal heat pumps are installed inthe United States annually with a total capacity of approximately 600,000 kW t .

Geothermal heat pumps are also popular in many European countries, with Sweden,Switzerland, Germany and France among the leaders in utilizing this technology.Many of the heat pumps in Europe are, however, used only in heating mode whilemost of those installed in the United States provide both heating and cooling andare most often sized to meet the cooling load since cooling demand often exceedspeak heating demand.

Fig. 10. Geothermal heat pump ( Electric Ideas Clearinghouse, 1993 ).


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Geothermal heat pumps have been found to be an extremely attractive alternativeto conventional electric or fossil fuel space-conditioning equipment, often providinga cost advantage on rst cost basis as well as in operation and maintenance(Bloomquist, 2001 ).

Geothermal heat pumps are also an important component of some district heatingsystems. One of the most signicant of these systems is located in Lund, Sweden,where over 50 MW t are provided by two large industrial heat pumps (20 and 30MW t respectively) ( Lunds Energiverk, 1985 ). Four production wells provide ca.23 C geothermal water. The heat pump boosts the temperature to approximately80 C for supply to the municipal district heating system. The spent geothermal uidis returned to the aquifer through four injection wells.

Fig. 11. Heat pump congurations ( Lund, 2002 ).


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5. Summary

Space heating and, to a lesser extent, space cooling can be provided directly,through district energy networks or through the application of heat pump technol-ogy. The earliest recorded district space-conditioning system dates back to 14thcentury France; space heating now accounts for over 75% of all direct use geothermalspace heating applications.

Increased demand for space cooling and recent technological advances, as well asgrowing research into low temperature absorption cooling, can be expected to resultin a rapid growth in the use of geothermal energy for space cooling.

References

Bloomquist, R.G., 2000. Geothermal District Energy System Analysis, Design and Development. WorldGeothermal Congress 2000, Japan.

Bloomquist, R. G., 2001. The economics of geothermal heat pump systems for commercial and institu-tional buildings. In: Proceedings Conference on Geothermal Energy in Underground Mines, Ustron,Poland, 2001.

Bloomquist, R. G., Strunge, S., Van Blaricum, V., 2002. Computer modeling for district energy— HEATMAP, TERMIS, & HEATER. IDEA 93rd Annual Conference & Trade Show, Baltimore, 2002.

Bloomquist, R. G., Wegman, S., 1998. Municipal Water-Based Heat Pump Heating and/or Cooling Sys-tems. Washington State University Energy Program Contract No. DE-FG07-97ID13480.

Electric Ideas Clearinghouse, 1993. Geothermal Heat Pumps.

IEA, 1992. Heat Pumps, an Opportunity for Reducing the Greenhouse Effect. IEA Heat Pump Centre,The Netherlands.Lund, J.W., 2002. Direct Heat Utilization of Geothermal Resources In: (Chandrasekharam & Bund-

schuh, eds.), Geothermal Energy Resources for Developing Countries. pp. 129–147.Lund, J.W., Freeston, D.H., 2001. World-wide direct uses of geothermal energy 2000. Geothermics 30,

29–68.Lunds Energiverk, 1985. Geothermal Energy, Trollebergsvagen 5, Box 1726, S-221 01 Lund, Sweden.Lund, J. W., Lienau, P., Lunis, B., 1991. Geothermal Direct-Use Energy Guidebook.


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