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 . 34.34Surface Water Heat Pumps. 34.38HE use of geothermal resources can be subdivided into threeTgeneral categories: high-temperature (150C) electric powerproduction, intermediate- and low-temperature ( 150CInte
3、rmediate temperature 90C 50 mg/kg at 93C, 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 well-bore (Culver and Reistad 1978). A
4、 secondary fluid circulatesfrom the load through the exchanger and back to the load in aclosed loop. The primary advantage of a DHE is that only heat isextracted from the earth, which eliminates the need to dispose ofspent fluids. Other advantages are the elimination of (1) wellpumps with their init
5、ial operating and maintenance costs, (2) thepotential for depletion of groundwater, and (3) environmental andinstitutional restrictions on surface disposal. One disadvantage ofa DHE is the limited amount of heat that can be extracted from orrejected to the well. The amount of heat extracted depends
6、on thehydraulic conductivity of the aquifer and well design. Because ofthe limitations of natural convection, only about 10% of the heatoutput of the well is available from a DHE in comparison topumping and using surface heat exchange (Reistad et al. 1979).With wells of approximately 95C and depths
7、of 150 m, outputunder favorable conditions is sufficient to serve the needs of up tofive 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 lower
8、-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 advantages:W
9、ater 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,sinks i
10、nto 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 below
11、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 moves u
12、p 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 of
13、 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 with a
14、 high hydraulic conductivity and that providewater movement for heat and mass transfer.ValvesIn large (65 mm) pipe sizes, resilient-lined butterfly valves arepreferred for geothermal applications. The lining material protectsthe valve body from exposure to the geothermal fluid. The rotaryrather than
15、 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 seat
16、 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 Applications (SI)oil-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 nonme-tallic piping, particularly asbestos cement (which is no longer available)and glass fiber. With the cost of glass fiber for larger sizes (150 mm)sometimes
19、 prohibitive, ductile iron is frequently used. Available insizes 50 mm, ductile iron offers several positive characteristics: lowcost, familiarity to installation crews, and wide availability. It requiresno allowances for thermal expansion if push-on fittings are used.Most larger-diameter buried pip
20、ing 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 seal
21、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 geo
22、thermal applications. Few problems have beenencountered in using ferrous piping with low-temperature geother-mal fluids unless high chloride concentration, low (35 kW) and inareas with high electric rates and high cooling requirements(2000 full-load hours) would this type of equipment offer anattrac
23、tive 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 economical u
24、se 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) oft
25、en 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 (primar
26、ily vegetables), gold mining, and aquaculture.3. GROUND-SOURCE HEAT PUMPSGround-source heat pumps were originally developed in the res-idential arena and are now widely applied in the commercial sector.Many of the installation recommendations and design guides appro-priate to residential design must
27、 be amended for large buildings.Kavanaugh and Rafferty (1997) provide a more complete overviewof design of ground-source heat pump systems. Kavanaugh (1991)and OSU (1988a, 1988b) provide a more detailed treatment of thedesign and installation of ground-source heat pumps, but their focusis primarily
28、residential and light commercial applications. Compre-hensive coverage of commercial and institutional design and con-struction of ground-source heat pump systems is provided in CSAStandard C448.3.1 TERMINOLOGYThe term ground-source heat pump (GSHP) is applied to a vari-ety of systems that use the g
29、round, groundwater, or surface water asa 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., geothermal heatpumps (GHP), geo-exchange, and ground-source (GS) systemsand are used to meet a variet
30、y of marketing or institutional needs(Kavanaugh 1992). See Chapter 9 of the 2012 ASHRAE HandbookHVAC Systems and Equipment for a discussion of the merits of var-ious other nongeothermal heat sources/sinks.This chapter focuses primarily on the ground heat exchangerportion of GSHP systems, although th
31、e heat pump units used inthese systems are unique to GSHP technology as well. GSHP sys-tems typically use extended-range water-source heat pump units, inmost cases of water-to-air configuration. Extended-range units arespecifically designed for operation at entering water temperaturesbetween 5C in h
32、eating mode and 40C 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 13C. This includes ground-water
33、loops, deep-surface-water loops, and interior core zones ofground-coupled loops when perimeter zones require heating. Thisis typically 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 and is of
34、ten 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/antifree
35、zesolution 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 the direc
36、t-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-ylene (H
37、DPE) tubes placed in a vertical borehole that is subse-quently filled with a solid medium. The tubes are thermally fused atthe bottom of the bore to a close return U-bend. Vertical tubes rangefrom 20 to 40 mm nominal diameter. Bore depths normally rangefrom 15 to 120 m depending on local drilling co
38、nditions and avail-able equipment, but can go to 180 m or more if procedures for deepboreholes are followed (see the section on Pump and Piping SystemOptions). Boreholes are typically 100 to 150 mm in diameter.To reduce thermal interference between individual bores, a min-imum borehole separation di
39、stance of 6 m 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 movement orevapor
40、ation 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 thesmallest amount o
41、f 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 borehole and (2) the
42、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 applications. The
43、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 such applications
44、, 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-groups, includi
45、ng single-pipe, multiple-pipe, spiral, and horizon-tally bored. Single-pipe horizontal GCHPs were initially placed innarrow trenches at least 1.2 m deep. These designs require thegreatest amount of ground area. Multiple pipes (usually two, four,or six), placed in a single trench, can reduce the amou
46、nt 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 made by stretchi
47、ng 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, but pipe lengthsm
48、ust 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 approach deep-earth va
49、lues.Advantages of horizontal GCHPs are that (1) they are typicallyless expensive than vertical GCHPs because relatively low-cost in-stallation equipment is widely available, (2) many residentialapplications have adequate ground area, and (3) trained equipmentoperators are more widely available. Disadvantages include, in ad-dition to a larger ground area requirement, (1) greater adverse vari-ations in performance because ground temperatures and thermalproperties fluctuate with season, rainfall, and burial depth; (2) slightlyhigher pumping-energy requirements; and (3) lower syste
