1、2008 ASHRAE 37ABSTRACTThis paper investigates the potential of a geothermal heatreject system (GHRS) to improve the performance of a datacenter (DC) cooling system. The paper compares the perfor-mance of conventional systems rejecting heat to the atmo-sphere to a similar system rejecting heat to the
2、 ground. The firstsystem uses water cooled direct expansion (DX) computerroom air conditioners (CRAC); the second system uses watercooled chillers with chilled water computer room air handlers(CRAH) and the third system uses GHRS with CRAH and norefrigeration compressors. The paper also evaluates th
3、e poten-tial of replacing an emergency thermal storage system (TSS)using chilled water storage tank with a GHRS.Deep earth temperatures of the United States will be usedto depict the areas of the country that can be cooled withoutmechanical refrigeration, and those that can be cooled withfewer tons
4、of refrigeration on a percentage basis.GROUND SOURCE COOLING SYSTEMINTRODUCTIONIn most data centers, refrigeration systems are used toremove the heat dissipated by the electrical and computerequipment. The performance of the refrigeration system islimited by the Laws of Thermodynamics to a coefficie
5、nt ofperformance (COP) less than the COP of the Carnot cycle.Figure 1 Deep earth temperature in U.S. (ASHRAE 2007).Geothermal Heat Rejection Systemsfor Data CentersDennis R. Landsberg, PhD, PE Doug K. McLellan, PE Christopher W. Kurkjian, PEMember ASHRAE Member ASHRAE Member ASHRAEDennis R. Landsber
6、g is President of Landsberg Engineering, P.C. and President of L the two major concernsof the modern data center.This paper will investigate the feasibility of a GHRS fora data center. The paper will compare conventional data centersystems to a GHRS.To evaluate and compare the systems, a set of eval
7、uationcriteria were selected as shown below:System Criteria are: ReliabilityAll systems will have sufficient redundancy so that nosingle point of failure will cause system failure. To determinesystem reliability the number of active components andcomplexity of the system will be considered. Every ac
8、tivecomponent such as a motor or belt is a potential failure point.A drive belt is generally considered more likely to fail than apassive component such as a pipe or a duct. Equipment oper-ating at more severe conditions will also be more likely to fail.A compressor operating continuously at 90% of
9、the design liftis more likely to fail than same compressor operating at 80%of the design lift. The system must be able to function duringnatural disasters such as floods and storms. To the extentpossible the system should be protected from vandalism andman made disasters such as gun fire and bombs.M
10、aintainabilityEach system will have redundancy so that normal main-tenance will not interrupt services. Some active componentsare easily replaced. In-house personnel can easily replace abelt or fan motor, but not a compressor. Therefore, somecomponents may require a greater degree of redundancy toac
11、hieve the same maintainability.FlexibilityIn the modern data center, the air-conditioning systemshould be able to adapt to future requirements. The precisenature of the future requirements are usually not known, butfor this evaluation the system that can be most easily modifiedwill be scored highest
12、.Energy EfficiencyEnergy efficiency is the easiest of the criteria to quantify.The systems will be scored on the total power consumptionand peak power demand.All systems will be assumed to provide similar perfor-mance. Each system can provide satisfactory cooling to all thecritical loads.SYSTEM DESC
13、RIPTIONSThe data center (DC) for each system will consist of a10,000 ft2raised floor area with a critical load of 1MW. Theother building loads will be 250 KW. The total design coolingload is therefore 1.25 MW or about 360 tons. The DC locationwill be the Mid Atlantic region. The first two systems ar
14、e stan-dard refrigeration systems and the third system is system withground source heat rejection.System 1 is a direct expansion system with water cooledcondensers inside the CRAC. See Figure 2.The system will have 18 CRAC, 15 operating and 3standbys. Heat is rejected to the atmosphere through remot
15、eFigure 2ASHRAE Transactions 39Figure 3a Summer operation.Figure 3b Winter operation.Figure 4a Summer operation.40 ASHRAE Transactionsevaporative coolers (EC). Redundant pumps circulate thecondenser water between the CRAC and EC. A TSS system isnot included. In order to provide continuous cooling in
16、 theevent of loss of utility the CRAC system will be assumed tooperate from an uninterruptible power supply (UPS). Figure 2represents the heat balance for the System 1 summer opera-tion. During the winter, System 1 operation is the same assummer except that in the CRAC the economizer coil providesth
17、e cooling and the compressor energy is saved.The CRAC have economizer coils so that during thewinter, the CRACs compressors do not need to operate.Figure 2 also represents the heat balance for the System 1winter operation.System 2 is a chiller system with redundant water cooledchillers and 18 CRAH,
18、15 operating and 3 standbys. Figure 3aand 3b represents the heat balance for the System 2 summerand winter operation respectively.Heat is rejected to the atmosphere with redundant evap-orative coolers. The system has redundant primary, secondaryand condenser water pumps. The system has the secondary
19、pumps and CRAH on UPS and incorporates a TSS. The TSSencompasses the thermal storage tank (TST) and associatedvalves and controls. During the winter the chilled water can becooled by the EC without operating the chiller.System 3 is a GHRS. The system rejects heat from thedata center without refriger
20、ation compressors. The GHRShas 18 CRAH similar to the System 2, but they operate athigher temperatures than in System 2. The CRAH have anentering air temperature of 95F and a leaving air tempera-ture of 70F. The chilled water enters the CRAC at 65F andleaves at 75F. The corresponding temperatures in
21、 the chillersystem are 75, 55, 45, and 55F. Heat is rejected to the atmo-sphere through either an EC or to the ground through aground source heat exchanger (GSHE). Cooling water iscirculated by redundant pumps between the CRAH, EC, andGSHE. The pumps and CRAH are on UPS and system doesnot have a TSS
22、. Figure 4a represents the heat balance for theSystem 3 summer operation.Figure 4b Winter operation.Figure 4c Winter operation.ASHRAE Transactions 41As heat is continuously rejected to the ground, the groundtemperature may rise. After several years the ground sourcesystem may loose capacity due to t
23、he high ground tempera-ture. System 3 has an EC to extend the service life of theground source system indefinitely. During the winter there aretwo modes of operation, charge mode and EC only mode.When the outdoor wet bulb is less than 55F, the system canoperate with either the EC or GSHE. If the wat
24、er temperatureof the water leaving the GSHE is above 45F and watertemperature leaving the EC is less than the temperature leav-ing the GSHE, then System 3 operates in the charge mode.During the charge mode, the system cools the DC and “stores”cooling in the ground by removing heat from the ground. O
25、ncethe water temperature leaving the GSHE is 45F or less, thesystem operates in the EC only mode and energy consumptionis reduced. Figure 4b represents the heat balance for theSystem 3 winter operation during the charge mode and Figure4c represents the EC only mode.Table 1 summarizes the operating t
26、emperatures andpower demands of the three systems.Table 1. Load 1,250 kWChiller Conventional Chiller ConventionalSummer Operation Winter OperationEffi-ciency HeadFlow (gpm) hp kWEffi-ciency HeadFlow (gpm) hp kWSecondary Pump 0.9 100 938 26 22 Secondary Pump 0.9 100 938 26 22Primary Pump 0.9 50 1004
27、14 12 Primary Pump 0.9 50 0 0 0Condenser Pump 0.9 50 1254 18 15 Condenser Pump 0.9 50 1004 18 15EC Pump 30 25 EC Pump 30 25CRAH 150 124 CRAH 150 124EC fan 100 83 EC fan 100 83Chiller 251 Chiller 0COP = 2.4 532 COP = 4.6 269CRAC Conventional CRAC ConventionalSummer Operation Winter OperationEffi-cien
28、cy HeadFlow (gpm) hp kWEffi-ciency HeadFlow (gpm) hp kWSecondary Pump 0 0 Secondary Pump 0 0Primary Pump 0.9 50 1004 42 35 Primary Pump 0.9 50 1004 42 35Condenser Pump 0 0 Condenser Pump 0 0EC Pump 30 25 EC Pump 30 25CRAH 150 124 CRAH 150 124EC fan 100 83 EC fan 100 83Chiller 418 Chiller 0COP = 1.8
29、685 COP = 4.7 269GSCSSummer OperationWinter Operation Winter Operation (Charging)Effi-ciency HeadFlow (gpm) hp kWEffi-ciency HeadFlow (gpm) hp kWEffi-ciency HeadFlow (gpm) hp kWSecondary Pump0.9 100 938 26 22 0.9 100 938 26 22 0.9 100 938 26 22EC loop 0.9 50 0 0 0 0 0.9 50 1004 26 22 0.9 50 1004 26
30、22GS loop 0.9 75 1250 26 22 0.9 75 0 0 0 0.9 75 1254 26 22EC pump 0 0 30 25 30 25CRAH 150 124 150 124 150 124EC fan 0 0 100 83 100 83Chiller 0 0 0COP = 7.4 168 COP = 4.5 276 COP = 4.2 29842 ASHRAE TransactionsPERFORMANCESystems 1 and 2 have very good performance. Bothsystems meet the cooling needs o
31、f the data center and canmaintain all areas at or below design temperatures. System 1may have poorer humidity control because the refrigerantevaporator absorbs heat directly from the data center. If theevaporator temperature falls too far below room dew point, theSystem 1 evaporators will perform de
32、humidification. In bothsystems 1 and 2, dirty filters can reduce the air flow across theevaporator or cooling coil and cause a lower discharge airtemperatures. The lower discharge air temperatures may lowerthe data center humidity and required humidifiers to replacethe moisture removed by the coolin
33、g coils. Controls can beadded to System 1 to help prevent simultaneous dehumidifi-cation and humidification, but the controls must coordinate theoperation of all 18 CRAC units. In System 2 simultaneousdehumidification and humidification can be prevented bycirculating chilled water at or above the de
34、sired data centerdew point. An alternate approach would be to provide a sepa-rate unit that can humidify or dehumidify as needed. This unitcould also provide make-up air to the data center.GRHS systems for commercial air-conditioning normallyincorporate a water chiller to ensure that supply air from
35、 theair-conditioners can be maintained below 60F. However, thepotential for energy savings in data center cooling applicationsis much greater since the potential exists to eliminate therefrigeration equipment entirely. The system uses the naturallow temperature of the ground as the low temperature h
36、eat sinkto dissipate heat. The temperature of the ground is low enoughto absorb the heat from the data center without a conventionalrefrigeration system. In order for System 3 to provide satis-factory cooling to the data center without mechanical cooling,the center must operate with supply air tempe
37、ratures over65F. This is achievable in modern data centers.A typical CRAH is specified to produce 50 to 60F supplyair with an entering air or return air temperature of 72F with45F chilled water and a 10 degree chilled water temperaturerise. The wet bulb temperature of the entering air is a key facto
38、rin determining the supply temperature. If the air enter theCRAH has low humidity the CRAH can provide 100% sensi-ble cooling. As a design point for the geothermal system, wewill stay with 72F and 45% RH entering the servers. InSystem 3 the air discharged from the servers is not allowed tomix with c
39、ooling air supplied to the servers. The cold air fromthe CRAH circulates through the raised floor to the racks, isdrawn into the servers and discharged from the servers or otherheat dissipating equipment. The racks are assumed to operateon a 23 temperature difference. That is the air enter the racks
40、will be 72F and the air leave the racks will be 95F. No airleaving the racks will mix with cold aisle air at 72F so all thereturn air to the CRAH will be 95F. With 95F air entering theCRAH, the CRAH can cool the air to 72F with 65F chilledwater entering and 75F water leaving. In any area where thegr
41、ound is below 65F, the heat from the DC can be rejected tothe ground no matter the outdoor air temperature. Obviouslythe colder the ground the smaller the heat exchanger needed toreject the heat. The heat capacity, heat transfer coefficients,and underground water movement are also important factor i
42、nsizing the GHRS network.One problem that must be overcome with a GHRS systemis the potential for heat saturation of the ground. The groundwarms as it absorbs heat. This problem is more severe whenthe vertical wells are too close and the warmed areas of adja-cent loops over lap. To reduce the impact
43、 of this problem in acooling only application, the vertical wells should be spacedat 20 to 25 ft centers, rather than 15 ft centers. Additionally, theground temperature can be given time to recover by onlyrejecting heat during the warm periods. An evaporative coolercan produce 65F water whenever the
44、 outdoor wet bulb is lessthan 60F. When the wet bulb is below 50F, the evaporativecooler can cool water to below 65F. During periods of lowoutdoor temperature, the evaporative cooler can cool the datacenter and allow the ground temperature to recover. TheGHRS can also be coupled with hydronic heat p
45、umps so thatthe heat from the data center can be used to heat other areas inthe building or domestic water. This reduces the amount ofheat that must be rejected.Despite careful design, some heating of the ground islikely to occur. The system should be designed to operate at anincrease of about 6F ov
46、er the deep earth temperatures shownin Figure 1. Therefore, to cool to 65 with no mechanical cool-ing a deep earth temperature of no greater than 54F should beutilized. This will provide a temperature differential of at least5F for the geothermal heat exchanger. The deep earth temper-ature meets thi
47、s criteria in large areas of the U.S. (Figure 1),including all of New England, Pennsylvania, New York, halfof New Jersey, Ohio, Michigan, Wisconsin, Minnesota,Montana, Idaho, the Dakotas and the Pacific Northwest.Clearly a larger temperature differential will permit a smallerand less costly earth lo
48、op. Therefore sites with lower deepearth temperatures will have lower installation costs.Space is required to install the geothermal loop. Thesample system requires 360 tons of cooling. If 500 ft verticalwells are considered, the cooling obtained per well is a func-tion of the deep earth temperature
49、, soil conductivity andrequired chilled water temperatures. If one assumes 3 tons ofcooling per well, there will be 120 wells. With 2 tons of cool-ing per well, 180 wells will be required. If the wells are spacedon 25 ft centers, 120 wells require an area of 225 275 ft, while180 wells require an area of 225 425 ft. These areas can bereduced by reducing t
