Comparative Analysis of Life-Cycle Costs of Geothermal Heat Pumps and Three Conventional HVAC Systems

Transcription

1 AIVC #12,983 Comparative Analysis of Life-Cyle Costs of Geothermal Heat Pumps and Three Conventional HVAC Systems John A. Shonder Member ASHRAE Howard A. Mlain, Ph.D. Mihaela A. Martin, P.E. Member ASHRAE Patrik J. Hughes, P.E. Member ASHRAE Member ASHRAE ABSTRACT Beause of their low operating and mai11tenane osts, geothermal heat pumps (GHPs) are an inreasingly popular hoie for providing heating, ooli11g, and water heati11g lo sthool.1 and other i11slitutio11s. Reog11izi11g these be11eflts, the Linoln, Nebraska, shool distrit in I 99 5 om111issio11ed four new idential 69,000jl.2 (6400 m 2 ) elementary shools served by GHPs. T//e deision to use GHPs was based on an analysis thal ompared the estimated life-yle ost of a number of dijfere11t HVAC tehnologies. The existene of extensive operating data from tire shools energy management systems allowed the authors lo develop a alibrated simulation model thm (1urately predits the energy use of a/f major building subsystems, i11ludi11g the GHPs. for one of tlte four shools. This model, ombined with i11depe11de11t estimates of installed osts and atual ost information from the shool distrits maintenane database, allowed a more rigorous omparison of the life-yle osts of GHPs and three other spae-onditioning options. This paper presents the results of/'hat omparison and shows that when apital, operating, and 111ai11te11a11e osts are onsidered for the Li11oln appliatio11, geothermal heat pumps have the lowest life-yle ost, about 15% lower than the next most attrative option. The GHPs also have the lowest soure energy onsumption and the lowest total pollutant emissions of any of the teli11ologies onsidered. For this appliatio11, geothermal heat pumps also /rave a lower first ost than any of the other systems urremly used for spae onditioning in new shools. INTRODUCTION In the fall of 1995, the Linoln, Nebraska, shool distrit opened four new elementary shools (Campbell, Cavett, Max.ey, and Roper) served by geothermal heat pumps (GHPs). The shools have idential floorplans eah witl1 approximately 69,000 square feet (6400 m 2 ) of spae dediated to lassrooms, offies, meeting room, a afeteria and a gymnasium (the atual floor spae al eah shool varies slightly due to differenes in spae usage). Eah shool serves approximately 500 student. The design of the shools' mehanial systems was the result ofa ollaborative effort between the engineer, the shool distrit, and the loa.1 eletrial utility (Ba1ltam and Benson 1995). Life-yle osts for five alternative oneptual designs were analyzed using energy on umptiou inputs from simulations perfonned with a ommerially available software pakage. Considering estimated apital, operating and maintenane osts, the analysis showed the geothenna.i heal pumps to have the lowest life-yle ost. Based on our experiene, pre-onstrution estimates of energy onsumption by spae-onditioning equipmenl may vary by 20% or more from the atual energy onsumption of an oupied building. Maintenane osts are also diffiult to estimate espeially for geothermal beat pumps, whih do not have a long history of appliation in. shools ompared with other spae-onditioning tehnologies. For these reasons we deided to repeat the omparison of life-yle osts usi11g atual energy onsumption data from the shools and maintenane osts from the shool distrit s maintenane database. The energy onsumption data allowed us to develop a alibrated simulation model that aurately predits hourly heatillg and ooling loads for one of the shools ba ed on oupany and ambient weather onditions. These loads were u od to design three alternative o1wentional spae-onditioning systems for U1e hools, and independent onstrution ost John A. Shonder and Mihaela A. Martin are staff engineers, Howard A. MLain is a development staff member, and Patrik J. Hughes is a researh/development engineer at Oak Ridge National Laboratory, Oak Ridge, Tenn. THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY. FOR INCLUSION IN ASHRAE. TRANSACTIONS 2000, V , Pt 2_ Not to be reprinted In wholo 01 In part wlltiout written parmlsslon of tho Amerian Soiety of Heating, Refrigerating and Alr-Condilloning Engineers, In., 1791 Tullis Cirle, NE, Atlanta, GA Opinions. findings. onlwions. or reommandalions expressed n this paper are those of the aulhor(s) end do not neessarily reflet the views of ASHRAE. Wrillen questions and ommon ls regarding this paper should be reeived et ASHRAE no later than July 7, 2000.

2 estimates were prepared for the GHP system and for the three alternatives. The alibrated simulation was then used to predit the annual energy onsumption of the shool in a typial year with spae-onditioning loads served by eah of the four system types. Atual maintenane osts for the installed geothermal heat pumps were used, and maintenane osts of the onventional alternatives were determined from the ost of maintaining similar equipment at other shools in the distrit. This information was used to alulate the life-yle ost of eah system. DESCRIPTION OF THE SCHOOL AND BASELINE GHP SYSTEM Of the four GHP shools in the Linoln distrit, Maxey elementary shool was hosen as the basis for this study, primarily beause of the ompleteness of the energy onsumption data available. Figure 1 is a photograph of the front entrane of the shool, and a floorplan is presented in Figure 2. The total floor spae of the hool is 69,670 ft2 (6472 m2). Spae-onditioning loads in the shool are met by 54 geothermal heat pumps ranging in size from 1.4 to 15 tons (4.9 to 53 kw), with a total ooling apaity of 180 tons (630 kw). Four gas-fired boilers, eah with a apaity of 330,000 Btu/h (97 kw), provide hot water for pre-heating of outdoor air and to terminal heating units in vestibule areas. Humidifiation is provided by eletri spray humidifiers, and domesti hot water for the kithen and wash basins is provided by gas-fired hot water heaters. Introdution of outdoor air onforms to ASHRAE Standard The heat pumps absorb and rejet heat by way of a variable-speed pumping system to a borefield onsisting of 120 vertial heat exhangers arranged in a 12-by-10 pattern with 20 ft (6 m) spaing. The bores are 4\1.i in. (11 m) in diameter by 240 ft (73 m) deep with fine gravel bakfill to within 10 ft (3 m) of the surfae and the remainder filled with a bentonite plug. The vertial loops onsist of thermally fused SDR-11 high-density polyethylene pipe, and the heat exhange fluid is water with 22% (by volume) propylene glyol. Complete details of the system design and operation have been published in Shonder et. al. (1999). CONVENTIONAL SPACE-CONDITIONING ALTERNATIVES Three alternative onventional spae-onditioning systems were hosen for omparison with the baseline GHP system. An important onern was to maintain onsisteny Figure 1 Maxey elementary shool, Linoln, Nebraska. Figure 2 Floor plan of Maxey elementary shool. 2

3 with the systems installed in other shools in the Linoln distrit. At present, there is some unertainty in the engineering ommunity about maintenane osts for HVAC equipment in general and geothermal heat pumps in partiular. A number of publiations (e.g., Cane et al. 1997; Shonder and Hughes 1997; Martin et al. 1999) have appeared that indiate maintenane osts for GHPs may be onsiderably lower than the values given by ASHRAE (1999) for water-soure heat pumps. ASHRAE is known to be reexamining the issue. To mi nimize the unertainty in the life-yle ost analysis, we deided to use atual maintenane ost data from the shool distrit's maintenane database. This influened the hoie of the alternative systems, whih are desribed below. Option 1: Air-Cooled Chiller with VAV Air-Handling System (ACCNAV) This system onsists of a entral air-handling unit with filter, ooling and heating oils, and a supply air fan. A dut system distributes supply air to variable air volume terminal units loated in the zones, whih are idential to the zones supplied by GHPs in the baseline system. A reheat oil is inluded in eah zone. An air-ooled entrifugal hiller with apaity of 150 tons (530 kw) and an effiieny of 1.0 kw/ton (COP= 3.57) produes 44 F (7 C) water for the ooling oils. Heating oils are supplied by a 2800 MBH (820 kw) hot water boiler with 82.5% effiieny. Terminal units in the vestibules, idential to those installed in the baseline design, are also supplied by the boiler. As in the baseline GHP design, humidifiation is provided by eletri spray humidifiers, and gasfired water heaters are used for domesti hot water. Option 2: Water-Cooled Chiller with Constant Volume Air-Handling System (WCC/CV) In this system, onstant volume fored-flow heating and ooling is provided to individually ontrolled zones (again, idential to the zones supplied by GHPs in the baseline system installed in the shool) from an air handler onsisting of a filter, heating and ooling oils, and a draw through supply fan. A reheat oil is installed in the supply air distribution dut serving eah zone. Chilled water at 44 F (7 C) is provided to the ooling oils by a 150 ton (528 kw) entrifugal hiller with an effiieny of 0.6 kw/ton (COP= 5.86). The ooling tower inludes two-speed fan ontrol. A 2800 MBH (820 kw) hot water boiler with 82.5% effiieny provides hot water for heating oils, vestibule units, and outdoor air pre-heating by way of variable-speed irulation pumps. As in the baseline GHP design, humidifiation is provided by eletri spray humidifiers, and gas-fired water heaters are used for domesti hot water. Although systems like this are no longer ommonly installed in shools, it is atually the most ommon system in the Linoln shool distrit due to the age of the building stok. The extensive maintenane ost data for this type of equipment allowed us to alulate the annual rate of inrease in mai ntenane osts, and the system was therefore inluded in the omparison to mai ntai n onsisteny with the database. Option 3: Water-Cooled Chiller with VAV Air-Handling System (WCCNAV) This system is idential to Option 1, exept that the 150 ton (528 kw) hiller is water-ooled and has an effiieny of 0.6 kw/ton (COP= 5.86). Two-speed fan ontrol is provided for the ooling tower. DEVELOPMENT OF THE SIMULATION MODEL The simulation model of the shool was developed using the DOE-2 software, whih is an hourly simulation tool used widely in the building industry to evaluate energy use and ost alternatives in building onstrution and assoiated HVAC systems (Winkelman et al ). A representation of geothermal heat pump systems was introdued in 1995 in version DOE-2. IE (Gates and Hirsh 1996) and later inorporated without hange into version DOE-2.2, the engine that runs PowerDOE (Gates and Hirsh 1997). Version 2.IE was used for this study. Modeling of the shool began with as-built onstrution plans from whih onstrution details and material speifiatio ns were obtained for floors, walls, eilings, and wi ndows. Internal loads were estimated based on oupany shedules and an on-site sur vey of light fixtures, omputers, and other heat-generating equipment. In parallel with this effort, 15-minute interval data on water flow rate and inlet and outlet temperatures from the borefield, olleted from the shool's energy management system, were used to perform an independent alibration of the borefield model in DOE-2. The atual dimensions of the installed borefield (diameter, depth, spaing, and onfiguration) were entered into the model, and the operation of the ' borefield was simulated for one year using fluid flow rate and the inlet fluid temperature to the borefield as inputs. Soil formation thermal properties (far-field temperature, thermal ondutivity, and volumetri heat apaity) were adjusted until the predited outlet temperatures mathed the measured outlet temperatures in a least-squares sense for the entire year. Details of this alibration have been presented by MLain and Martin (1999). Shonder and Bek (2000) present results from an independent, 50-hour soil formation thermal property test performed at Maxey shool. In that test, the thermal ondutivity was measured at 1.36 Btu/h ft F (2.35 W/m K) with a deep earth temperature of 55.7 F (13.2 C). It is important to note that the soil thermal ondutivity that resulted in the best fit to the outlet water temperature data usi ng the DOE-2 borefield model was 1.60 Btu/h ft F (2.77 W/m K), some 18% higher than the measured value. Calibrations of the borefield model in another building simulation pakage using the same data did in fat onverge to soil properties very lose to the measured values (Shonder et al. 2000). One possible reason why the DOE-2 borefield model seems to require a higher 3

4 ondutivity is that the software does not allow the user to speify the borehole bakfill material; effetively, the loops are assumed to be surrounded by soil. Sine the thermal ondutivity of the fine gravel used to bakfill the bores at the site is somewhat higher than the ondutivity of the soil, the algorithm in the DOE-2 model may be ompensating by requiring a higher soil thermal ondutivity in order to repliate the field data. Although further alibration exerises at other sites would be required to determine whether this is a general problem with the software, engineers using DOE-2 to design geothermal heat pump systems should be aware of the situation. The building model, the alibrated borefield model, and representations of the heat pumps and assoiated equipment were ombined to form the overall simulation model. The shedule for the shool year was used to model oupany, and a year of operation was simulated using atual weather onditions for the period. Based on the initial run, minor adjustments were made to outdoor air infiltration rates and other parameters suh as window shading until the annual energy use mathed the monitored energy use at the site. The utility data and EMS data inluded the eletrial use of the spae-onditioning system separately from the total eletrial use. Atual eletrial use by the spae-onditioning systems from August 1995 to July 1996 was 322,111 kwh, whereas the a'librated DOE-2 model predited 316,412 kwh, whih is an error of less than 2%. The auray of the simulation is illustrated in Figu re 3, whih presents measured and predited daily eletrial energy use by the spae-onditioning systems vs. daily average temperature. Figure 4 presents a omparison of measured and simulated natural gas onsumption per month for the study period. Sine the utility data did not separate the use of gas speifially for spae onditioning, this figure inludes the use for domesti water heating as well. Measured gas onsumption for the monitored period was 14,258 therms (1504 GJ), while the predited onsumption was 14,274 therms (1506 GJ), an error of less than 1 %. ENERGY AND WATER USE One the simulation was alibrated with measured data, the model was run again using typial meteorologial year data for Linoln, Nebraska, and oupany based on the shool alendar for the shool year. The GHPs were then replaed with eah of the onventional spae-onditioning systems in tum, and the simulation was repeated for the typial meteorologial year. The annual gas and eletri use as alulated by the DOE-2 simulations for the baseline GHP system and the three onventional alternative systems is presented in Table I. The annual soure energy use of eah option (assuming 70% losses in. onve1 ion, transmissi.on, and distribution of eletriity) is presented graphially in Figure 4. The baseline GHP system installed at the shool is the lowest user of energy, onsuming 17% less soure energy than. the Figure 3 Average Dally Temperature ("F) August 1995-July 1996 daily energy use by geothermal heat pump system vs. daily average temperature. Simulation results and sitemonitored data r.:===== ,.I :!l 2000 OMoa urad XDOE-2 Simulation i! 1500 z x j! 1000 t-----m ---JJ- soo. 0 :Ii Figure 4 D x BOO Baae 65"F Heating Degree Days per Month August 1995-July 1996 total monthly gas use vs. base 65 F degree-days. Simulation results and site-monitored data. next highest system, a water-ooled hiller/gas boiler ombination. Sine the DOE-2 omponent for the ooling towers does not alulate water loss due to evaporation, makeup and blowdown losses for Options 2 and 3 were estimated to be 2% of annual water flow. The losses are approximately 260,000 gallons (984,000 liters) per year for eah system. In the life-yle ost analysis, the ost of water and water treatment is inluded as an annual reurring ost for these two systems. INSTALLED SYSTEM COSTS The installed osts of the baseline system and the three alternatives were estimated using designs produed to meet the loads seen in the alibrated DOE-2 simulation. In general, total osts were built up from the individual osts ofall system omponents inluding hillers boilers, piping, valves, dutwork ai r-handling units, ontrols, and eletrial equipment plus installation osts, management osts, and overhead. In x x 0 4

5 TABLE 1 Annual Energy Use for Baseline GHP System and Three Conventional Alternatives* Baseline Geothermal Option 1 Option 2 Option 3 ACCNAV WCC/CV WCCNAV Eletrial use Non-HVAC 255,807 HVAC systems 288,197 Total eletrial use, kwh 544, , , , , , , , , ,813 Natural gas use HVAC systems, therms (GJ) 7535 (795) Domesti hot water, therms (OJ) 5547 (585) Total gas use, therms (GJ) 13,082 (1380) Soure energy use, kbtu/ft 2 (kj/m 2 ) 99.1(9.7) alulated by DOE-2 simulations. most ases, quoted pries from suppliers and manufaturers in the Linoln area were used as the basis for the estimates. Based on urrent labor rates in Linoln, Nebraska, an average rate of $20 per hour was used. General ontrator total overhead was assumed to be 25% on labor and 10% on materials, and onstrution management fees were assumed to be 3% of the onstrution ost. Beause the shools are government owned, the estimates did not inlude sales taxes. For the geothermal systems, the prie for installation of the ground heat exhangers was assumed to be the atual installation ost of $5.93 per bore foot ($19.46 per bore meter) harged for the original installation. Costs for all other omponents of the geothermal systems were independently estimated, without referene to atual installed osts. The ost estimates were examined by the prinipal of a Knoxville, Tennessee, engineering firm with extensive experiene in design and ost estimating for geothermal heat pumps and other HVAC systems for shools. A number of 22,648 (2389) 49,021 (5172) 22,781 (2403) 5547 (585) 5547 (585) 5547 (585) 28,195 (2974) 54,568 (5757) 28,328 (2979) (12.1) (20.2) (11.7) suggestions were made to improve both the designs and the estimates (Regen 1999). Revised ost estimates were then produed based on these suggestions. The final ost estimates for the baseline geothermal system and the three onventional alternatives are presented in Table 2. Exept for the onstant volume system, whih is no longer ommonly installed in shool appliations, the baseline GHP system had the lowest installed ost, about 9.5% lower than the next most attrative option. The per unit area ost estimates for the three alternative systems ompares well with data presented by Mean (1997), whih lists a range of$9.55 to $30.55 per ft2 ($102 to $329 per m2) for HVA.C systems installed in shools, with a national average of per ft 2 ($233 per m2). However, Kavanaugh and Rafferty (1998) have published information indiating that the ost to install the geothermal heat pumps in the Linoln shools was approximately TABLE 2 Installed Cost Estimates for the Baseline GHP System and the Three Conventional Alternatives Baseline Option 1 Option 2 Option 3 Geothermal ACCNAV WCC/CV WCCNAV Engineering Title I & II $41,774 $44,664 $33,270 $45,922 Title III $20,887 $22,332 $16,635 $22,961 Total engineering $62,661 $66,996 $49,905 $68,883 Constrution Phase Loop system $170,910 $- $- $- HVAC system $756,356 $1,028,792 $761,059 $1,060,944 Constrution Management $31,330 $33,498 $24,952 $34,441 Total Estimated Cost $1,021,257 $1,129,286 $835,916 $1,164,268 Cost per ft 2 (ost per m 2 ) $14.66 (1.36) $16.21 (1.51) $12.00 (1.11) $16.71 (1.55) 5

6 $9.44 per ft2 ($102 per m2). This figure is based on osts in three broad ategories reported as well-field ontrator, HVAC ontrator, and other fees. We were unable to determine whether the information presented by Kavanaugh and Rafferty inluded suh omponents as design fees, power and ontrol eletrial wiring, and system ontrols as our estimate did. MAINTENANCE COSTS Using the Linoln shool distrit's database, Martin et. al. (1999, 2000) analyzed two to three years of maintenane reords for 20 shools to isolate the osts assoiated with maintaining HVAC systems of various ages and types. Data for 16 of these shools are presented in Table 3 (4 of the 20shools used gas-fired steam boilers, whih are not onsidered here). Of the four system types onsidered in the present study, there are four shools using geothermal heat pumps (the baseline system), two using air-ooled hillers and gas-fired hot water boilers with VAY air handlers (Option 1 ), nine using water-ooled hillers and gas-fired hot water boilers with onstant volume air handlers (Option 2), and three using water-ooled hillers and gas-fired hot water boilers with VAY air handlers (Option 3). The nine ases of systems similar to Option 2 provided suffiient data to develop a orrelation between maintenane ost and system age. The data are plotted in Figure 5 and are fit to a ur ve of the form = 0( 1 + r)11 1, where is the system maintenane ost in year n, 0 is the first-year maintenane ost, and r is the annual rate of inrease in maintenane ost. Although the data show a great deal of variation from this ur ve, both the first year maintenane ost of 16.6 ents per ooled square foot ($1.79 per ooled square meter) and the 1.5% annual rate of maintenane ost inrease appear reasonable. TABLE 3 Total Preventive and Corretive Maintenane Costs Per Unit Area for 16 Shools * Total HVAC Maintenane Cost, /yr-ooled ft 2 Shool Name Cooling System Age Air-Handler Type ($/yr-ooled m 2 ) Geothermal Heat Pumps Campbell 3 VAY 9.50 (1.02) Cavett 3 VAY 9.57 (1.03) Maxey 3 VAY (1.08) Roper 3 VAY 7.97 (0.85) Air-Cooled Chiller and Gas-Fired Hot Water Boiler Belmont 5 VAY (1.37) Humann 8 VAY 8.09 (0.87) Water-Cooled Chiller and Gas-Fired Hot Water Boiler Zeman 24 Constant Volume ( 1.49) Everett 6 VAY (l.16) Fredstrom 15 VAY (1.49) Goodrih 8 Constant Volume (2.31) Hill 22 Constant Volume (1.30) Kahoa 26 Constant Volume (1.95) MPhee 3 Constant Volume (2.04) Morley 23 Constant Volume (4.82) Park 8 VAY 8.28 (0.89) Pyrtle 32 Constant Volume (4.55) Rousseau 2 Constant Volume (1.78) Bryan 26 Constant Volume (2.95) From the Linoln ShooJ Distrit Maintenane Database 6

7 C] -- -;; 30-8 First-year maintenane ost: 16.6 /ft' ($1.791m2) 4 Annual rate of inrease: 1.5% ; < 5 Figure Year In whih maintenane ours D Maintenane osts per unit ooled area vs. ooling system age for shools with water-ooled hiller/gas-fired hot water boiler/onstant volume air distribution systems. Sine insuffiient data were available to perform similar orrelations for the other three system types, the 1.5% annual rate of inrease was assumed to apply for the geothermal heat pumps and for the other alternatives. For the two shools with systems like Option 1 and the three shools with systems like Option 3, the data were o rrelat ed to urves of the form = 0(1.015)'1-1 to determine first-year maintenane osts. For the GHP shools, the maintenane ost in year three was assumed to be the average of the maintenane osts presented for the four GHP shools; the first year maintenane ost was determined by bringing this average bak to year 1 assuming a 1.5% rate of annual inrease. Based on these alulations the first-year maintenane osts are as follows: Baseline system (GHP): Alternative 1 (ACC/VAV): Alternative 2 (WCC/CV): Alternative 3 (WCC/VAV): D /ft2 (0.97 $/m2) 9.5 /ft2 (1.03 $/m2) 16.6 /ft2 (1. 79 $/m2) 9.7 /ft.2 (l.04 $/m2) It is reognized that these are less than half the osts reported by ASHRAE (1999) for similar system types. Data from a 1994 survey of ommerial buildings performed by the Building Owners and Managers Assoiation (BOM A 1995) are also higher tha n the figures above, with an average annual HVAC maintenane ost of 29 /ft.2 (3.12 $/m2) for federal, state, and loal government buildings. However, in a report prepared for a utilities ompany (KUC 1994) several mehanial ontrators provide estimates of installation, maintenane, and operating osts of geothermal and other HVAC systems to support life-yle ost omparisons. Here the first year ma inten an e osts are estimated to be from 10.0 to I 0.8 /n2 (I.08-1.l 6 /m2) for geothermal heat pumps and from 13.0 to 45.0 /fl:2 ( $/m2) for other system 1 0 "e i 8 u u...it ; E ;; " " < types. Cane et al. (1997) report a range of0.94 to /ft.2 ( $/m2) and an average of ftl (0.44 $/m2) for annual maintenane osts for a sample of 15 shools using geothermal heat pumps. The wide variation in maintenane osts reported in the literature may be due to a number of fators, inluding whether sheduled maintenane is performed vs. a "fix it when it breaks" philosophy, the use of ontrat vs. in-house labor, differenes in labor and material osts aross regions, and differenes in aounting praties that sometime make it diffiult to separate HVAC maintenane osts from other maintenane osts. Beause the Linoln database was analyzed in detail, we are onfident that these figures represent the best estimate of the osts required to maintain the various system types in that partiular shool distrit. ENERGY AND WATER COSTS Current gas and eletri osts were obtained from the loal utility (LES 1998). For eletriity, the shool distrit is harged 2.5 ents per kwh and a demand harge of $8.50 per kw. There is also a ustomer harge of $17 per month. Gas rates are $0.493 per therm, with a $12 monthly ustomer ha rge. The life-yle ost analysis onsiders only the east of operating the spae-onditioning systems sine U1e ost of operating other equipment is the same for eah option. The ustomer harges are not inluded in the analysis. Water rates harged by the Linoln Water System are $1.20 per thousand gallons for the first 120,000 gallons and $1.48 per thousand gallons thereafter. In the life-yle ost analysis makeup water for ll1e ooling tower was assumed to be harged at the lower rate. Sine the shool distrit's maintenane database inludes the osts of boiler water treatment and ooling tower water treatment as a maintenane item, these osts are aounted for as neessary in the annual maintenane ost used for eah system type. LIFE-CYCLE COST ANALYSIS One all initial and reurring osts were determined for the baseline GHP system and the three onventional alternatives, the life-yle ost of eah one was alulated using the BLCC software (NIST 1995a) for a 20-year system life. In aordane with NIST guidelines for life-yle ost analysis (NIST l 995b ), BLCC alulates present value in onstant dollars, using the urrent (1999) U.S. Department of Energy (DOE) real disou11t rate of3. l % ba ed on the long-term Trea Sll!Y Bond rates (net the inflation rate, whih wtently stands at approximately 2.5%) averaged over the 12-month period from April 1998 to Marh The life-yle ost analysis onsidered only the gas and eletriity (both demand and energy) used by the spae-onditioning systems. Sine the energy use of all non-hvac loads is idential aross the four simulations, this does not affet the omparison. To determine future gas and eletri rates, BLCC uses energy prie esalation rate projetions from the Energy 7

8 Information Administration. The urrent projetions were published in April of 1999 (NIST 1999). Maintenane osts for all systems were assumed to inrease at an annual rate of 1.5% above the inflation rate, as determined from the database. The ost of water (for the two systems that inlude ooling towers) was assumed to rise only at the general rate of inflation. The systems were assumed to have no salvage value at the end of their 20-year life. All alulations begin with a base date of , when the systems are assumed to be installed and begin operation. End-of-year disounting was used throughout the analysis. Table 4 summarizes the inputs for the BLCC analysis of the baseline GHP system and the three onventional systems, as well as their life-yle osts. Of the four systems analyzed for this appliation, the geothermal heat pump system has the lowest life-yle ost, whih is $1,498,835 over the 20-year life of the system. The next most attrative option, the VAV system with a water-ooled hiller/gas hot water boiler ombination, has a life-yle ost about $230,000 higher-or about 15% higher-than the life-yle ost of the GHP system. There is only a small differene in life-yle ost (less than 1 %) between the two VAV systems that use water-ooled and air-ooled hillers. The system with the lowest first ost (the onstant volume system) has the highest life-yle ost of all the options examined, about 26% higher than the life-yle ost of the GHP system. POLLUTANT EMISSIONS Life-yle pollutant emissions for the four tehnologies, as alulated by BLCC using U.S. average emission fators, are presented in Table 5. In terms of arbon emissions, geothermal heat pumps are the lowest produer of pollutants. Over the 20-year life of the equipment, GHPs produe TABLE 4 Summary of Inputs and Outputs from BLCC for the Four Systems Option 1 Option 2 Option 3 Baseline Geothermal.ACCNAV WCC/CV WCCNAV Initial Cost $1,021,257 $1,129,286 $835,916 $1,164,268 First year maintenane ost $7,383 $7,824 $13,651 $7,928 First year eletri ost $22,138 $23,037 $34,152 $19,448 First year gas ost $3,533 $10,963 $23,944 $11,034 Water ost $385 $385 Total Annual O&M Costs $33,054 $41,824 $73,826 $38,795 Life-Cyle Cost $1,498,835 $1,734,327 $1,912,297 $1,728,736 TABLE 5 Life-Cyle Emissions in Pounds (Kilograms) for Baseline GHP and Three Conventional Alternatives, Using U.S. Average Emission Fato.rs Option 1 Option 2 Option 3 Baseline Geothermal ACCNAV WCC/CV WCCNAV Eletriity C02 12,320,000 (5,587,000) 13,118,000 (5,949,000) 26,222,000 (11,892,000) 11,970,000 (5,429,000) ,500 (18,000) 42,000 (19,070) 84,000 (38,000) 38,000 (17,000) NO X 37,000 (17,000) 40,000 (18,000) 79,000 (36,000) 36,000 ( 16,000) Natural Gas C02 1,673,000 (759,000) 5, 190,000 (2,354,000) 11,335,000 (5,140,000) 5,223,000 (2,369,000) (3) 20 (9) 44 (20) 20 (9) NO X 1300 (590) 4000 (1800) 8800 (4000) 4000 (1800) Total C02 13,993,000 (6,346,000) 18,308,000 (8,303,000) 37,557,000 (17,032,000) 17,193,000 (7,798,000) ,500 (18,000) 42,000 (18,000) 84,000 (38,000) 38,000 (17,000) NO X 38,000 (18,000) 44,000 (20,000) 88,000 (39,829) 40,000 (18,000) 8

9 437 equivalent tons of arbon less than the next highest produer, Option 3. Geothermal heat pumps also produe about 1700 pounds less NOx than Option 3 over the equipment life. However, the analysis indiates that geothermal heat pumps produe a small amount more of S02 than the equipment of Option 3. This is due to the fat that while the GHPs use 17% less soure energy than the water-ooled hiller/gas-fired boiler ombination, they do use slightly more eletriity (about l.5%). DISCUSSION AND CONCLUSIONS The objetive of this paper was to use site-monitored energy use data, atual maintenane ost data, and independent, onsistent installed ost estimates to determine the lifeyle ost of geothermal heat pumps and three alternative onventional spae-onditioning systems for an elementary shool in Linoln Nebraska. The results show that the geothermal heat pump system installed in the four Linoln shools has a lower life-yle ost than other spae-onditioning systems ommonly used in the shool distrit would have had in the same appliation, saving about $235,000 over the next most attrative option for a 20-year system life. Exept for one system no longer ommonly installed in shools, geothermal heat pumps have the lowest first ost as well. The geothermal heat pumps also have the lowest annual operating and maintenane osts of all the systems analyzed and the lowest onsumption of soure energy. Maintenane osts for all four system types were determined from the shool's maintenane database. From a sample of nine systems, the annual rate of inrease in maintenane osts was determined to be 1.5%. As Figure 5 indiates, there was a great deal of variation in this small sample. To determine the effet of a higher rate of ost inrease, the lifeyle ost analysis was repeated using a rate of 3%. Life-yle osts of the baseline GHP system and Options l, 2, and 3 inreased by 1.4%, 1.2%, 2.2%, and 1.5%, respetively, but the ranking of the options did not hange. Thus, the rate of maintenane ost inrease has only a small effet on life-yle ost for this analysis. A disount rate of3. l % was used in the study to maintain onsisteny with DOE guidelines. This rate is based on the long-term yield of U.S. Treasury bonds, but shool onstrution projets are ommonly finaned with tax free muniipal bonds. A survey of 20-year shool bonds in the state of Nebraska (see Bonds in referene list) shows their average rate ofreturn to be 5.65%, whih orresponds to a real disount rate of 3.15%. Thus, the use of the DOE disount rate appears warranted. Beause system renovation osts involve even greater unertainty than first ost, we did not attempt to arry the analysis past 20 years. No salvage value was assumed for any of the systems at the end of this period, but it is likely that the geothermal heat pump system would have some salvage value if the deision were made to install similar equipment. The loops that exhange heat with the borefield are warranted for 50 years and would not have to be replaed. On the other hand, entrifugal hillers generally have a useful life longer than 20 years, so the onventional systems would have some salvage value as well. Further analysis would be required to determine renovation osts, but we feel that they would not signifiantly affet the life-yle ost omparison. Table 4 learly shows that the installed ost of the spaeonditioning systems is the most important parameter in the life-yle ost analysis. Although every effort was made to produe aurate, omplete, and onsistent ost estimates for the four systems, we reognize that different estimators might produe estimates that differ from ours perhaps by as muh as 5%. Nevertheless, we are onfident that the figures presented here provide an aurate omparison of the four systems. The figures show that for shool distrits suh as the one in Linoln Nebraska, geothermal heat pumps offer lear advantages in first ost, operating and maintenane osts, and life-yle ost. Also, given their lower soure energy onsumption and redued pollutant emissions, the tehnology offers further advantages for the nation as a whole. ACKNOWLEDGMENTS The authors would like to thank Larry Hennings and Tim Pratt of the Linoln Shool Distrit for their valuable assistane in this work. NOMENCLATURE Abbreviations ACC = air-ooled hiller ASHRAE = Amerian Soiety of Heating, Refrigerating and Air-Conditioning Engineers BLCC = building life-yle ost omputer program BOMA = Building Owners and Managers Assoiation CV = onstant volume air-handling system DOE = U.S. Department of Energy GHP = geothermal heat pump HVAC = heating, ventilating, and air onditioning LCC = life-yle ost NIST = National Institute of Standards and Tehnology VAY = variable air volume air-handling system wee = water-ooled hiller Symbols = annual maintenane ost per unit area 0 = first year maintenane ost per unit area r = annual rate of inrease of maintenane ost per unit area n = year in whih maintenane ost ours 9

10 REFERENCES ASHRAE ASHRAE Handbook-HVAC Appliations, p Atlanta: Amerian Soiety of Heating, Refrigerating and Air-Conditioning Engi neers. Bantam, D.D., and S.J. Benson A publi utility strategy for implementing and monitoring a ground oupled heat pump system in a publi shool. Proeedings of the J 995 APPA Energy/Customer Servies and Communiations Workshop, Kansas City, Mo. BOM A BOMA experiene exhange report. Building Owners and Managers Assoiation International. Bonds Online, website aessed 11/06/99. Cane, D., A. Morrison, B. Clemes, and C. Ireland Survey and analysis of maintenane and servie osts in ommerial building geothermal systems-final report. Geothermal Heat Pump Consortium Report RP-024, Rev. 6/97. Gates, S.D., and J.J. Hirsh DOE-2. le Enhanements. Camarillo, Calif.: James J. Hirsh & Assoiates. Gates, S.D., and J. J. Hirsh DOE-2.2/PowerDOE-l.O HVAC Simulation Enhanements. Camarillo, Calif.: James J. Hirsh & Assoiates. Kavanaugh, S., and K. Rafferty Cost and performane of ground soure heat pump buildings. Outside the Loop: A Newsletter for Geothermal Heat Pump Designers and Installers l (2), April. KUC (Kentuky Utilities Company) Shool life yle ost studies and ase histories. Lexington: Kentuky Utilities Company. LES (Linoln Eletri System) Rates shedules and servie regulations for rates effetive Marh 1, Martin, M.A., M.G. Madgett, and P. J. Hughes Comparing maintenane osts of geothermal heat pump systems with other HVAC systems in Li noln Publi Shools: Preventive maintenane ations and total maintenane osts. ASHRAE Tr ansations 106 (1 ): Martin, M.A., D.J. Durfree, and P.J. Hughes Comparing maintenane osts of geothermal heat pump systems with other HVAC systems in Linoln Publi Shools: Repair, ser vie, and orretive ations. ASHRAE Transations 105 (2): MLain, H.A., and M. A. Martin A preliminary valuation of the DOE-2. le ground vertial well model using Maxey Shoo l measured data. ASHRAE Tr ansations 105 (2) : Means Mehanial Cost Data, 21st Annual Edition (HVAC and Controls). Kingston, Mass.: R.S. Means Company, In. NIST. 1995a. BLCC: The NIST building life yle ost program, Version 4.3 user 's guide and referene manual (NISTIR ). Washi ngton, D.C.: National Institute of Standards and Tehnology. NIST. 1995b. Life yle osting manual for the Federal Energy Management Program, NIST Ha ndbook 135. Washington, D.C. : National Institute of Standards and Tehnology. NIST Energy pries and disount fators for life-yle ost analysis-april 1999 (NISTIR r). Washington, D.C.: Natio nal Inst itute of Standards and Tehnology. Regen, J Personal ommuniation to J. A. Shonder, September. Shonder, J. A., and P.J. Hughes Estimated maintena ne ost savings from a geothermal heat pump energy savings performane ontrat at Fort Polk, La. ASHRAE Tr ansations 103 (2): Shonder, J. A., M.A. Martin, T.R. Sharp, D.J. Durfee, and P.J. Hughes Benhmark for performane: Geothermal appliations in Linoln Publi Shools. ASHRAE Tr ansations 105 (2): Shonder, J. A., and J. V. Bek Field test of a new method for determining soil formation thermal ondutivity and borehole resistane from in-situ measurements. ASHRAE Transations 106 (1): Shonder, J. A., V.D. Baxter, P. J. Hughes, and J. Thornton A omparison of vertial ground heat exhanger design software for ommerial appliations. ASHRAE Tr ansations 106 (1): Winkelman, F. C., B.E. Birdsall, W.F. Buhl, K.L. Ellington, and A.E. Erdem DOE-2 Suppleme nt, Version 2. IE.L BL Berkeley, Calif. : Lawrene Berkeley National Laboratory. 10 MN