1、34.1CHAPTER 34GEOTHERMAL ENERGYRESOURCES . 34.1Fluids . 34.2Present Use 34.3Renewability. 34.3DIRECT-USE SYSTEMS DESIGN. 34.3Cost Factors . 34.3Materials and Equipment. 34.5Residential and Commercial Building Applications 34.8Industrial Applications. 34.10GROUND-SOURCE HEAT PUMPS 34.10Terminology 34
2、.10Ground-Coupled Heat Pump System Design. 34.13Groundwater Heat Pumps 34.32Water Wells . 34.33Surface Water Heat Pumps. 34.38HE use of geothermal resources can be subdivided into threeTgeneral categories: high-temperature (300F) electric powerproduction, intermediate- and low-temperature ( 300FInte
3、rmediate temperature 195F 50 ppm at 200F, titanium would be used. At lower tempera-tures, much higher chloride exposure can be tolerated (see Figure 4).Downhole Heat Exchangers. The downhole heat exchanger(DHE) is an arrangement of pipes or tubes suspended in a wellbore(Culver and Reistad 1978). A s
4、econdary fluid circulates from theload through the exchanger and back to the load in a closed loop.The primary advantage of a DHE is that only heat is extracted fromthe earth, which eliminates the need to dispose of spent fluids. Otheradvantages are the elimination of (1) well pumps with their initi
5、aloperating and maintenance costs, (2) the potential for depletion ofgroundwater, and (3) environmental and institutional restrictions onsurface disposal. One disadvantage of a DHE is the limited amountof heat that can be extracted from or rejected to the well. Theamount of heat extracted depends on
6、 the hydraulic conductivity ofthe aquifer and well design. Because of the limitations of naturalconvection, only about 10% of the heat output of the well is avail-able from a DHE in comparison to pumping and using surface heatexchange (Reistad et al. 1979). With wells of approximately 200Fand depths
7、 of 500 ft, output under favorable conditions is sufficientto serve the needs of up to five homes.The DHE in low- to moderate-temperature geothermal wells isinstalled in a casing, as shown in Figure 5.Downhole heat exchangers with higher outputs rely on water cir-culation within the well, whereas lo
8、wer-output DHEs rely on earthconduction. Circulation in the well can be accomplished by twomethods: (1) undersized casing and (2) convection tube. Both meth-ods rely on the difference in density between the water surroundingthe DHE and that in the aquifer.Circulation provides the following advantage
9、s:Water circulates around the DHE at velocities that, in optimumconditions, can approach those in the shell of a shell-and-tubeexchanger.Hot water moving up the annulus heats the upper rocks and thewell becomes nearly isothermal.Some of the cool water, being denser than the water in the aquifer,sink
10、s into the aquifer and is replaced by hotter water, which flowsup the annulus.Figure 5 shows well construction in competent formation (i.e.,where the wellbore will stand open without a casing). An under-sized casing with perforations at the lowest producing zone (usuallynear the bottom) and just bel
11、ow the static water level is installed. Apacker near the top of the competent formation allows installation ofan annular seal between it and the surface. When the DHE is in-stalled and heat extracted, thermosiphoning causes cooler water in-side the casing to move to the bottom, and hotter water move
12、s up theannulus outside the casing.Because most DHEs are used for space heating (an intermittentoperation), heated rocks in the upper portion of the well store heatfor the next cycle.Where the well will not stand open without casing, a convectiontube can be used. This is a pipe one-half the diameter
13、 of the casingeither hung with its lower end above the well bottom and its upperend below the surface or set on the bottom with perforations at thebottom and below the static water level. If a U-bend DHE is used, itcan be either inside or outside the convection tube. DHEs operatebest in aquifers wit
14、h a high hydraulic conductivity and that providewater movement for heat and mass transfer.ValvesIn large (2.5 in.) pipe sizes, resilient-lined butterfly valves arepreferred for geothermal applications. The lining material protectsthe valve body from exposure to the geothermal fluid. The rotaryrather
15、 than reciprocating motion of the stem makes the valve lesssusceptible to leakage and build-up of scale deposits. For manydirect-use applications, these valves are composed of Buna-N orEPDM seats, stainless steel shafts, and bronze or stainless steeldisks. Where oil-lubricated well pumps are used, a
16、 seat material ofFig. 5 Typical Connection of Downhole Heat Exchanger for Space and Domestic Hot-Water Heating(Reistad et al. 1979)34.8 2015 ASHRAE HandbookHVAC Applicationsoil-resistant material is recommended. Gate valves have been usedin some larger geothermal systems but have been subject to ste
17、mleakage and seizure. After several years of use, they are no longercapable of 100% shutoff.PipingPiping in geothermal systems can be divided into two broadgroups: pipes used inside buildings and those used outside, typi-cally buried. Indoor piping carrying geothermal water is usuallylimited to the
18、mechanical room. Carbon steel with grooved endjoining is the most common material.For buried piping, many existing systems use some form of non-metallic piping, particularly asbestos cement (which is no longeravailable) and glass fiber. With the cost of glass fiber for larger sizes(6 in.) sometimes
19、prohibitive, ductile iron is frequently used.Available in sizes 2 in., ductile iron offers several positive charac-teristics: low cost, familiarity to installation crews, and wide avail-ability. It requires no allowances for thermal expansion if push-onfittings are used.Most larger-diameter buried p
20、iping is preinsulated. The basicductile iron pipe is surrounded by a layer of insulation (typicallypolyurethane), which is protected by an outer jacket of PVC or high-density polyethylene (HDPE).Standard ductile iron used for municipal water systems is some-times modified for geothermal use. The sea
21、l coat used to protect thecement lining of the pipe is not suitable for the temperature of mostgeothermal applications; in applications where the geothermalwater is especially soft or low in pH, the cement lining should beomitted, as well. Special high-temperature gaskets (usually EPDM)are used in g
22、eothermal applications. Few problems have beenencountered in using ferrous piping with low-temperature geother-mal fluids unless high chloride concentration, low (10 tons) and inareas with high electric rates and high cooling requirements(2000 full-load hours) would this type of equipment offer anat
23、tractive investment to the owner (Rafferty 1989a).2.4 INDUSTRIAL APPLICATIONSDesign philosophy for the use of geothermal energy in industrialapplications, including agricultural facilities, is similar to that forspace conditioning. However, these applications have the potentialfor much more economic
24、al use of the geothermal resource, primarilybecause they (1) operate year-round, which gives them greater loadfactors than possible with space-conditioning applications; (2) donot require extensive (and expensive) distribution to dispersed en-ergy consumers, as is common in district heating; and (3)
25、 often re-quire various temperatures and, consequently, may be able to makegreater use of a particular resource than space conditioning, which isrestricted to a specific temperature. In the United States, the primarynon-space-heating applications of direct-use geothermal resourcesare dehydration (pr
26、imarily vegetables), gold mining, and aquaculture.3. GROUND-SOURCE HEAT PUMPSGround-source heat pumps were originally developed in the resi-dential arena and are now widely applied in the commercial sector.Many of the installation recommendations and design guides appro-priate to residential design
27、must be amended for large buildings. Kava-naugh and Rafferty (1997) provide a more complete overview ofdesign of ground-source heat pump systems. Kavanaugh (1991) andOSU (1988a, 1988b) provide a more detailed treatment of the designand installation of ground-source heat pumps, but their focus is pri
28、-marily residential and light commercial applications. Comprehensivecoverage of commercial and institutional design and construction ofground-source heat pump systems is provided in CSA Standard C448.3.1 TERMINOLOGYThe term ground-source heat pump (GSHP) is applied to avariety of systems that use th
29、e ground, groundwater, or surfacewater as a heat source and sink. The general terms include ground-coupled (GCHP), groundwater (GWHP), and surface-water(SWHP) heat pumps. Many parallel terms exist e.g., geothermalheat pumps (GHP), geo-exchange, and ground-source (GS) sys-tems and are used to meet a
30、variety of marketing or institutionalneeds (Kavanaugh 1992). See Chapter 9 of the 2012 ASHRAE Hand-bookHVAC Systems and Equipment for a discussion of the meritsof various other nongeothermal heat sources/sinks.This chapter focuses primarily on the ground heat exchanger por-tion of GSHP systems, alth
31、ough the heat pump units used in thesesystems are unique to GSHP technology as well. GSHP systemstypically use extended-range water-source heat pump units, in mostcases of water-to-air configuration. Extended-range units are specif-ically designed for operation at entering water temperaturesbetween
32、23F in heating mode and 104F in cooling mode. Unitsnot meeting the extended-range criteria are not suitable for use inGSHP systems (except for some groundwater heat pump systems).Some applications can include a free-cooling mode when water-loop temperatures fall near or below 55F. This includes grou
33、ndwa-ter loops, deep-surface-water loops, and interior core zones ofground-coupled loops when perimeter zones require heating. This istypically accomplished by inserting a water coil in the return airstream before the refrigerant coil.Ground-Coupled Heat Pump SystemsThe GCHP is a subset of the GSHP
34、and is often called a closed-loopheat pump. A GCHP system consists of a reversible vapor compres-sion cycle that is linked to a closed ground heat exchanger (also calleda ground loop) buried in soil (Figure 9). The most widely used unit isa water-to-air heat pump, which circulates a water or a water
35、/antifreezesolution through a liquid-to-refrigerant heat exchanger and a buriedthermoplastic piping network. Heat pump units often include desuper-heater heat exchangers (shown on the left in Figure 9). These devicesuse hot refrigerant at the compressor outlet to heat water. A secondtype of GCHP is
36、the direct-expansion (DX) GCHP, which uses a bur-ied copper piping network through which refrigerant is circulated.The GCHP is further subdivided according to ground heatexchanger design: vertical and horizontal. Vertical GCHPs (Figure10) generally consist of two small-diameter, high-density polyeth
37、yl-ene (HDPE) tubes placed in a vertical borehole that is subsequentlyfilled with a solid medium. The tubes are thermally fused at the bot-tom of the bore to a close return U-bend. Vertical tubes range from0.75 to 1.5 in. nominal diameter. Bore depths normally range from 50to 400 ft depending on loc
38、al drilling conditions and available equip-ment, but can go to 600 ft or more if procedures for deep boreholesare followed (see the section on Pump and Piping System Options).Boreholes are typically 4 to 6 in. in diameter.To reduce thermal interference between individual bores, a min-imum borehole s
39、eparation distance of 20 ft is recommended whenloops are placed in a grid pattern. This distance may be reducedwhen bores are placed in a single row, the annual ground load isbalanced (i.e., energy released in the ground is approximately equalto the energy extracted on an annual basis), or water mov
40、ement orevaporation and subsequent recharge mitigates the effect of heatbuild-up in the loop field.Advantages of the vertical GCHP are that it (1) requires rela-tively small plots of ground, (2) is in contact with soil that variesvery little in temperature and thermal properties, (3) requires thesma
41、llest amount of pipe and pumping energy, and (4) can yieldthe most efficient GCHP system performance. Disadvantages are(1) typically higher cost because of expensive equipment needed toFig. 9 Vertical Closed-Loop Ground-Coupled Heat Pump System(Kavanaugh 1985)Geothermal Energy 34.11drill the borehol
42、e and (2) the limited availability of contractors toperform such work.Hybrid systems are a variation of ground-coupled systems inwhich a smaller ground heat exchanger is used, augmented in cool-ing mode by a fluid cooler or a cooling tower. This approach canhave merit in large cooling-dominated appl
43、ications. The groundheat exchanger is sized to meet the heating requirements. The down-sized loop is used in conjunction with the fluid cooler or coolingtower with an isolation heat exchanger to meet the heat rejectionload. Using the cooler reduces the capital cost of the ground heatexchanger in suc
44、h applications, but somewhat increases mainte-nance requirements. For heavily heating-dominant applications, adownsized loop also can be augmented with an auxiliary heat sourcesuch as electric resistance, solar collectors, or fossil fuel.Horizontal GCHPs (Figure 11) can be divided into several sub-g
45、roups, including single-pipe, multiple-pipe, spiral, and horizon-tally bored. Single-pipe horizontal GCHPs were initially placed innarrow trenches at least 4 ft deep. These designs require the great-est amount of ground area. Multiple pipes (usually two, four, orsix), placed in a single trench, can
46、reduce the amount of requiredground area. Trench length is reduced with multiple-pipe GCHPs,but total pipe length must be increased to overcome thermal inter-ference from adjacent pipes. The spiral coil is reported to furtherreduce required ground area. These horizontal ground heat ex-changers are m
47、ade by stretching small-diameter polyethylene tub-ing from the tight coil in which it is shipped into an extended coilthat can be placed vertically in a narrow trench or laid flat at thebottom of a wide trench. Recommended trench lengths are muchshorter than those of single-pipe horizontal GCHPs, bu
48、t pipe lengthsmust be much longer to achieve equivalent thermal performance.When horizontally bored loops are grouted and placed in the deepearth, as shown in the bottom of Figure 11, design lengths are nearthose for vertical systems, because annual temperature and moisturecontent variations approac
49、h deep-earth values.Advantages of horizontal GCHPs are that (1) they are typicallyless expensive than vertical GCHPs because relatively low-costinstallation equipment is widely available, (2) many residentialapplications have adequate ground area, and (3) trained equipmentoperators are more widely available. Disadvantages include, in addi-tion to a larger ground area requirement, (1) greater adverse varia-tions in performance because ground temperatures and thermalproperties fluctuate with season, rainfall, and burial depth; (2) slightlyhigher pumping-energy requirements; and (3)
