Word Review Geothermal Heat Pumps

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<ul><li><p>HEAT EXCHANGERREFRIGERANT / AIR</p><p>(CONDENSER)</p><p>COOL RETURN AIRFROM CONDITIONED</p><p>SPACE</p><p>EXPANSION VALVE</p><p>REFRIGERANTREVERSING VALVE</p><p>WARM SUPPLY AIR TOCONDITIONED SPACE</p><p>HEAT EXCHANGERREFRIGERANT / WATER</p><p>(EVAPORATOR)</p><p>TO / FROM GROUNDHEAT EXCHANGER</p><p>(GEOTHERMAL)</p><p>REFRIGERANTCOMPRESSOR</p><p>DOMESTIC HOT WATEREXCHANGER</p><p>(DESUPERHEATER)</p><p>DOMESTIC WATER</p><p>INOUT</p><p>HEAT EXCHANGERREFRIGERANT / AIR</p><p>(EVAPORATOR)</p><p>WARM RETURN AIRFROM CONDITIONED</p><p>SPACE</p><p>EXPANSION VALVE</p><p>REFRIGERANTREVERSING VALVE</p><p>COOL SUPPLY AIR TOCONDITIONED SPACE</p><p>HEAT EXCHANGERREFRIGERANT / WATER</p><p>(CONDENSER)</p><p>TO / FROM GROUNDHEAT EXCHANGER</p><p>(GEOTHERMAL)</p><p>REFRIGERANTCOMPRESSOR</p><p>DOMESTIC HOT WATEREXCHANGER</p><p>(DESUPERHEATER)</p><p>DOMESTIC WATER</p><p>INOUT</p><p>GEOTHERMAL (GROUND-SOURCE) HEAT PUMPSA WORLD OVERVIEW</p><p>J. Lund1, B. Sanner2, L. Rybach3, R. Curtis4, G. Hellstrm5</p><p>1Geo-Heat Center, Oregon Institute of Technology, Klamath Falls, Oregon, USA2Institute of Applied Geosciences, Justus-Liebig University, Giessen, Germany</p><p>3Institute of Geophysics, ETH, Zurich and GEOWATT AG, Zurich, Switzerland4EarthEnergy Systems, GeoScience Ltd., Falmouth, Cornwall, UK</p><p>5Department of Mathematical Physics, Lund University of Technology, Lund, Sweden</p><p>INTRODUCTIONGeothermal (ground-source) heat pumps (GHP) are</p><p>one of the fastest growing applications of renewable energy(see discussion at the end of this article) in the world, withannual increases of 10% in about 30 countries over the past 10years. Its main advantage is that it uses normal ground orgroundwater temperatures (between about 5 and 30oC), whichare available in all countries of the world. Most of this growthhas occurred in the United States and Europe, though interestis developing in other countries such as Japan and Turkey.The present worldwide installed capacity is estimated at al-most 12,000 MWt (thermal) and the annual energy use isabout 72,000 TJ (20,000 GWh). The actual number of install-ed units is around 1,100,000, but the data are incomplete.Table 1 lists the countries with the highest use of GHPs.</p><p>Table 1. Leading Countries Using GHP________________________________________________</p><p> NumberCountry MWt GWh/yr InstalledAustria 275 370 23,000Canada 435 600 36,000Germany 640 930 46,400Sweden 2,300 9,200 230,000Switzerland 525 780 30,000USA 6,300 6,300 600,000________________________________________________</p><p>GHPs use the relatively constant temperature of theearth to provide heating, cooling and domestic hot water forhomes, schools, government and commercial buildings. Asmall amount of electricity input is required to run acompressor; however, the energy output is of the order of fourtimes this input. These machines cause heat to flowuphill from a lower to higher temperature location- reallynothing more than a refrigeration unit that can be reversed.Pump is used to described the work done, and thetemperature difference is called the lift-- the greater the lift,the greater the energy input. The technology isnt new, asLord Kelvin developed the concept in 1852, which was thenmodified as a GHP by Robert Webber in the 1940s. Theygained commercial popularity in the 1960s and 1970s. SeeFigure 1 for diagrams of typical GHP operation.</p><p>GHPs come in two basic configurations: ground-coupled (closed loop) and groundwater (open loop) systems,</p><p>GHC BULLETIN, SEPTEMBER 2004</p><p>Figure 1a. GHP in the heating cycle (source:Oklahoma State University).</p><p>Figure 1b. GHP in the cooling cycle (source:Oklahoma State University).</p><p>which are installed horizontally and vertically, or in wells andlakes. The type chosen depends upon the soil and rock type atthe installation, the land available and/or if a water well canbe drilled economically or is already on site. See Figure 2 fordiagrams of these systems. As shown in Figure 1, adesuperheater can be provided to use reject heat in the summerand some input heat in the winter for the domestic hot waterheating.</p><p> 1</p></li><li><p>Figure 2a. Closed loop heat pump systems (source:Geo-Heat Center).</p><p>Figure 2b. Open loop heat pumps systems (source:Geo-Heat Center).</p><p>In the ground-coupled system, a closed loop of pipe,placed either horizontally (1 to 2 m deep) or vertically (50 to100 m deep), is placed in the ground and a water-antifreezesolution is circulated through the plastic pipes to either collectheat from the ground in the winter or reject heat to the groundin the summer (Rafferty, 1997). The open loop system usesgroundwater or lake water directly in the heat exchanger andthen discharges it into another well, into a stream or lake, oron the ground (say for irrigation), depending upon local laws.</p><p>The efficiency of GHP units are described by theCoefficient of Performance (COP) in the heating mode and theEnergy Efficiency Ratio (EER) in the cooling mode (COPh andCOPc, respectively in Europe) which is the ratio of the outputenergy divided by the input energy (electricity for thecompressor) and varies from 3 to 6 with present equipment(the higher the number the better the efficiency). Thus, a COPof 4 would indicate that the unit produced four units of heatingenergy for every unit of electrical energy input. Incomparison, an air-source heat pump has a COP of around 2and is dependent upon backup electrical energy to meet peakheating and cooling requirements. In Europe, this ratio issometimes referred to as the Seasonal Performance Factor(Jahresarbeitszahl in German) and is the average COP overthe heating and cooling season, respectively, and takes intoaccount system properties. </p><p>UNITED STATES EXPERIENCEIn the Unites States, most units are sized for the peak</p><p>cooling load and are oversized for heating (except in thenorthern states), and thus, are estimated to average only 1,000full-load heating hours per year. In Europe, most units aresized for the heating load and are often designed to providejust the base load with peaking by fossil fuel. As a result, theEuropean units may operated from 2,000 to 6,000 full-load</p><p>2</p><p>hours per year, with an average of around 2,300 annual fullload hours. Even though the cooling mode rejects heat to theearth, and thus is not geothermal, it still saves energy andcontributes to a clean environment. In the United States,GHP installations have steadily increased over the past 10years with an annual growth rate of about 12%, mostly in themid-western and eastern states from North Dakota to Florida.Today, approximately 80,000 units are installed annually, ofwhich 46% are vertical closed loop systems, 38% horizontalclosed loops systems and 15% open loop systems. Over 600schools have installed these units for heating and cooling,especially in Texas. It should be noted at this point, that inthe United States, heat pumps are rated on tonnage (i.e. oneton of cooling power--produced by a ton of ice) and is equal to12,000 Btu/hr or 3.51 kW (Kavanaugh and Rafferty, 1997).A unit for a typical residential requirement would be aroundthree tons or 10.5 kW installed capacity.</p><p>One of the largest GHP installations in the UnitedStates is at the Galt House East Hotel in Louisville, Kentucky.Heat and air conditioning is provided by GHPs for 600 hotelrooms, 100 apartments, and 89,000 square meters of officespace for a total area of 161,650 square meters. The GHPs use177 L/s from four wells at 14EC, providing 15.8 MW ofcooling and 19.6 MW of heating capacity. The energyconsumed is approximately 53% of an adjacent similar non-GHP building, saving $25,000 per month.</p><p>One of the recent converts to this form of energysavings is President George W. Bush, who installed ageothermal heat pump on his Texas ranch during the electioncampaign (Lund, 2001). Even though he is not known as anenvironmentalist, he referred to his system asenvironmentally hip. This vertical closed loop installationcuts his heating and cooling cost by 40%.</p><p>EUROPEAN SITUATIONGround-source heat pumps (GSHP) can offer both</p><p>heating and cooling at virtually any location, with greatflexibility to meet any demands. In western and centralEuropean countries, the direct utilization of geothermal energyto supply heat through district heating to a larger number ofcustomers so far is limited to regions with specific geologicalsettings. In this situation, the utilization of the ubiquitousshallow geothermal resources by decentralized GSHP systemsis an obvious option. Correspondingly, a rapidly growing fieldof applications is emerging and developing in variousEuropean countries. A rapid market penetration of suchsystems is resulting; the number of commercial companiesactively working in this field is ever increasing and theirproducts have reached the yellow pages stage.</p><p>More than 20 years of R&amp;D focusing on GSHP inEurope resulted in a well-established concept of sustainabilityfor this technology, as well as sound design and installationcriteria. A typical GSHP with borehole heat exchanger (BHE;a vertical loop in U.S.-terms) is shown in Figure 3. Thesesystems require currently for each kWh of heating or coolingoutput 0.22 - 0.35 kWh of electricity, which is 30 - 50% lessthan the seasonal power consumption of air-to-air heat pumps,which use the atmosphere as a heat source/sink.</p><p>GHC BULLETIN, SEPTEMBER 2004</p></li><li><p>Low-temperatureunderfloor heating</p><p>Heat pump</p><p>Hot watertank</p><p>Boreholeheat exchanger</p><p>Dataacquisition</p><p>Elec. power</p><p>Heating</p><p>T1 T2</p><p>Mobile TestEquuipment</p><p>Figure 3. Typical application of a BHE / heat pumpsystem in a Central European home,typical BHE length $100 m.</p><p>The climatic conditions in many European countriesare such that by far the most demand is for space heating; airconditioning is rarely required. Therefore, the heat pumpsusually operate mainly in the heating mode. However, withthe increasing number of larger commercial applications,requiring cooling, and the ongoing proliferation of thetechnology into southern Europe, the double use for heatingand cooling will become of more importance in the future.</p><p>It is rather difficult to find reliable numbers ofinstalled heat pumps in Europe, and in particular for theindividual heat sources. Figure 4 gives some recent data forthe number of installed units in the main European heat pumpcountries. The extremely high number for Sweden in 2001 isthe result of a large number of exhaust-air and other air-to-airheat pumps; however, Sweden also has the highest number ofGSHP in Europe (see Table 1). In general it can beconcluded, that market penetration of GSHP still is modestthroughout Europe, with the exception of Sweden andSwitzerland.</p><p>On the field of technical optimization, somedevelopments of recent years should be mentioned:</p><p> Thermal Response Test to determine the thermalparameters of the underground in situ;</p><p> Grouting material with enhanced thermalconductivity; and</p><p> Heat Pumps with increased supply temperatures forretrofit purposes.</p><p>GHC BULLETIN, SEPTEMBER 2004</p><p>Figure 4. Number of installed heat pump units insome European countries (after data fromSanner, 1999; and Donnerbauer, 2003).</p><p>For a thermal response test, basically a defined heatload is put into the BHE and the resulting temperaturechanges of the circulating fluid are measured (Figure 5)(Eugster and Laloui, 2002). Since mid-1999, this technologyis in use in Central Europe for the design of larger plants withBHE, allowing sizing of the boreholes based upon reliableunderground data. Thermal response testing was firstdeveloped in Sweden and USA in 1995, and now is used inmany countries worldwide. Together with reliable designsoftware (Hellstrm and Sanner, 2001), BHE can be made asound and safe technology also for larger applications.</p><p>Figure 5. Schematic of a Thermal Response Test. </p><p> 3</p></li><li><p>Thermally enhanced grouting material has beenavailable in USA for more than 10 years. Meanwhile, inEurope such material is also on the market, optimized for therelevant drilling habits and ground situation. The advantageof its use is a significant reduction in the borehole thermalresistance, which governs the temperature losses between theundisturbed ground and the fluid inside the BHE pipes. Thetable in Figure 6 gives some values for typical BHE; the effectcould meanwhile also be demonstrated in situ, using theThermal Response Test on BHE with different groutingmaterials.</p><p>Type of BHE ? grout rbsingle-U, PE 0.8 W/m/K 0.196 K/(W/m)</p><p>1.6 W/m/K 0.112 K/(W/m)double-U, PE 0.8 W/m/K 0.134 K/(W/m)</p><p>1.6 W/m/K 0.075 K/(W/m)</p><p>Figure 6. Table with data for rb for differentgrouting materials.</p><p>Up to now, the upper temperature limits encounteredin commercially available heat pumps limit their applicationto low temperature heating systems. However, traditionalheating systems already installed in older buildings inGermany require higher supply temperatures. To allow forretrofit of such systems with a heat pump, development of heatpumps allowing for 65EC and more are under way. </p><p>GERMAN EXPERIENCESince 1996, the statistics for heat pump sales in</p><p>Germany allow the distinction of different heat sources(Figure 7). Within the last years, sales of GSHP have showna steady increase, after the all-time low in 1991 with less than2,000 units shipped. The share of GSHP (ground and water),which was less than 30% in the late-1980s, has risen to78% in 1996 and 82% in 2002. Also from 2001 to 2002,when the building market in Germany was shrinking due tothe poor economic situation, GSHP sales numbers still had aslight increase. There is still ample opportunity for furthermarket growth, and the technological prospects endorse thisexpectation. </p><p>The application of GSHP in Germany is larger innumbers in the residential sector, with many small systemsserving detached houses (Photo 1), but larger in installedcapacity in the commercial sector, where office buildingsrequiring heating and cooling dominate. In most regionsof Germany, the humidity in summertime allows for coolingwithout de-humidification (e.g. with cooling ceilings). Thesesystems are well suited to use the cold of the ground directly,without chillers, and they show extremely high efficiency withcooling COP of 20 or more. The first system with BHEs anddirect cooling was built already in 1987 (Sanner, 1990);meanwhile, the technology has become a standard designoption. Some recent examples of GSHP systems in Germanycan be found in Sanner and Kohlsch (2001).</p><p>4</p><p>Figure 7. Number of annual heat pump sales inGermany, according to heat sources (afterdata from IZW e.V., Hannover and BWPe.V., Munich; heat pumps used for hot tapwater production only are not included).</p><p>Photo 1. Installation of borehole heat exchanger ata small house in Bielefeld, Germany,using a small, powerful Rotomax drillingrig.</p><p>In Germany, the GSHP has left the R&amp;D &amp; D-statusway behind, and the emphasis nowadays is on optimisationand securing of quality. Measures like technical guidelines(VDI 4640), certification of contractors, quality awards, etc.,are beginning to be set into force to protect the industry andthe consumers against poor quality and insufficient longevityof geothermal heat...</p></li></ul>

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