1、2009 ASHRAE 187ABSTRACTThere is a growing trend to utilize economical ways to cool data centers, which includes maximizing the use of “free cool-ing” techniques such as airside and waterside economizing. A new economizer cooling system is being developed and tested at this time and will be implement
2、ed in a number of data centers within the next year. This technique uses a heat wheel to trans-fer the heat from the computer room to the ambient environ-ment with a minimal transfer of air and moisture. It has been shown to have the capability of cooling a computer room with a heat density of 600 W
3、/ft and a cabinet heat load of 70 kW. It can provide cooling throughout the computer room within the existing ASHRAE guidelines up to an ambient condition of 22C (72F). It has been demonstrated to handle a temperature rise through cabinets of over 28C (50F).It can accomplish all this with a mechanic
4、al efficiency as low as 0.03, which for 550 kW load in the computer room only 17 kW of mechanical energy was being utilized. HISTORY OF THE HEAT WHEELThe use of heat wheels in energy recovery systems has existed for decades. Heat recovery wheels attain high levels of efficiency by transferring heat
5、or enthalpy between two air-streams. The heat wheel rotates through the two air streams and transfers the energy from the air being exhausted from the building to the incoming airstream.(1)In summer the exhaust air is cooler than the ambient conditions and precools the hot incoming airstream, thereb
6、y minimizing the energy required of the A/C refrigeration system. In winter the warm exhaust air preheats the incoming air, thereby minimizing the heating energy required to warm the building. (See Figure 1).With a desiccant coating on the wheel the same process can be used to either maintain or rem
7、ove moisture from the air stream as it enters the building.There are a number of materials used in heat wheels, the most prevalent is a honeycomb aluminum structure.Plumbing it Wrong Heat Wheels Introduction into the Data CenterThe way the normal energy recovery system is plumbed air circulates into
8、 the building, passes through the HVAC system, is circulated throughout the facility, and is exhausted out of the building.With the new configuration, used to dissipate the heat produced in the computer room, there are two separate recir-culation cycles. The first recirculates air from the computer
9、room, carrying the heat from the computer equipment through the heat wheel and back to the supply side of the computer room. The second circulates ambient air through the wheel to dissipate the computer room heat. This air is then exhausted outside the facility. (See Figure 2)The installation, calle
10、d a cell, is divided into four cham-bers, return and supply on the computer room side and supply and exhaust on the ambient side. (See Figures 3)The hot exhaust air from the computer room is circulated back to the heat wheel by a set of three fans. The airflow volume of these fans is controlled by t
11、he specified temperature drop across the heat wheel and the power being dissipated in the computer room, which is monitored by the controls system.Outside cold air is drawn into the lower chamber of the ambient side, cools the wheel, and is then exhausted back to the outside by one set of fans. The
12、rotational speed of the wheel Introducing Using the Heat Wheel to Cool the Computer RoomRobert Sullivan, PhD Marcel Van Dijk Mees LodderRobert Sullivan is a senior consultant at Uptime Institute, Morgan Hill, CA. Marcel Van Dijk and Mees Lodder are principal consultantsat Uptime Technology, NL, Nete
13、rlands.LO-09-014 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permi
14、tted without ASHRAEs prior written permission.188 ASHRAE Transactionsand the volume of outside air being circulated are varied to control the supply air temperature being fed into the computer room.For this new cooling system to function, especially at the high loads it is capable of handling, the h
15、ot air exhausted from the computer equipment is isolated from the cold air in the computer room. This is accomplished by sealing the Hot Aisle and having the hot exhaust air rise to the dropped ceiling area to be carried back to the computer room chamber of the heat wheel cell. It also requires that
16、 blanking plates be utilized in all unused areas of the equipment cabinets to avoid recircula-tion of hot air within the computer cabinets.The cold air coming off the heat wheel is allowed to flood the rest of the room, filling the Cold Aisles. As long as there is sufficient air introduced into the
17、computer room to satisfy the airflow demands of the server fans all the heat loads installed in the room can be cooled. The proper amount of cold air is ensured in two ways. The first is to establish the proper temperature differential across the wheel to match the average temperature rise through t
18、he servers installed in the room. The second is to match the required airflow of the installed load in the computer room. This second requirement is controlled by maintaining a positive static pressure in the room.If the ambient air temperature is below 10C (50F) there are sets of louvers connecting
19、 the two chambers on the ambi-ent side of the cell. Warm exhaust air is drawn from the exhaust side of the ambient chamber into the supply side to maintain a face temperature on the wheel that will neither condense moisture or cause it to freeze.Leakage of ambient air into the computer room is limit
20、ed to less than 1% of the total airflow through the system. (N) This leakage comes in two forms. First is leakage between the two chambers at the wheel interfaces. This leakage is mini-mized by seals on the top and bottom surfaces of the wheel. The most significant leakage comes from air trapped in
21、the wheel as it rotates through a chamber. Once the wheel rotates into the adjoining chamber the trapped air is released into the opposite chamber. So ambient air is transferred to the computer room chamber as the wheel rotates that chamber and visa versa. Both forms of leakage still account for les
22、s than 1% of the total airflow in the computer room chamber.(2)The heat wheel is driven by a 1.5 hp motor with a V-belt wrapped around the perimeter of the wheel. Wheel rotation can be varied from 0.4 RPM to 10 RPM.Modular Direct Expansion (DX) units are utilized for supplemental cooling when the am
23、bient temperature exceeds 22C. DX cooling capacity can be brought on line in 50 100 kW segments as supplemental cooling is required. Above approximately 32C the system would be on total supplemen-tal cooling, the wheel would be stopped, and louvers in both Figure 1 Typical Heat Wheel Application.Fig
24、ure 2 Recirculation cooling by Heat WheelPlumbed Wrong.ASHRAE Transactions 189chambers would be opened to eliminate the pressure drop through the heat wheel. (See Figure 5).Static pressure and moisture content of the air in the computer room are controlled by a separate building air handling unit (A
25、HU). This AHU recirculates air from the compute room, introducing only enough outside air to main-tain a positive static pressure in the room of 5 6 Pascals (0.01 0.02 in of water gage). In this AHU humidification is provided through a steam generator and dehumidification through a cold coil.Control
26、 of the SystemControl of the system is automatically handled by propri-etary software with only a minimum number of inputs. The inputs include the temperature drop through the wheel in the computer room chamber, the supply air temperature to the computer room, and the static pressure to be maintaine
27、d in the room. The control system continuously monitors the heat load being dissipated in the room and controls the computer room airflow volume. The speed of the wheel and the ambient airflow volume are them modulated to control the computer room supply air temperature.As computer room load and amb
28、ient temperatures vary so do the operating conditions. At temperatures below 10C (50F) the recirculation louvers between the two chambers on the ambient side of the cell maintain a 10C (50F) face temperature on the wheel. From 10 and 22C (50 and 72F) ambient temperature the speed of the wheel and th
29、e ambient airflow volume increase with increasing outside temperature, maintaining the preset temperature of the supply air to the computer room. Above 22C (72F) supplemental DX cooling is brought on in 50 kW 100 kW increments. As each incre-ment of DX cooling is added the wheel slows down so the DX
30、 compressors operate at 100% capacity. As the ambient temperature continues to rise the wheel again speeds up until the next increment of supplemental cooling is required. This process repeats itself until the cooling system is on 100% sensible cooling, at which time the wheel is stopped and the byp
31、ass louvers are opened. (See Figure 4)The ambient temperature at which supplemental cooling is required can be raised by employing adiabatic precooling of the ambient air. In one proposed installation in a very dry (low Dew Point) climate adiabatic cooling will extend the exclusive use of the heat w
32、heel cooling system to 36C (96F). If the ASHRAE recommended guidelines are extended to 27C (81F) the adiabatic cooling system would take this installa-tion of over 38C (100F) ambient temperature.Performance and Efficiency CharacteristicsThe measurement of cooling performance for this system will be
33、the input air temperatures to the four rows of 8 cabinets in the computer room of the demonstration site. Liquid crystal temperature strips were installed at the top and bottom of each of the 32 cabinets. Temperature variations were monitored Figure 3 Control mechanismHeat Wheel Cooling.Figure 4 Pri
34、nciple of Heat Wheel Cooling.190 ASHRAE Transactionsfrom top to bottom of each cabinet, between the first cabinet and the last cabinet in the row, and from row to row.For ambient conditions less than 22C (72F) the temper-ature variations are dependent on the amount of heat load in the room. With a h
35、eat load of just 150 kW (1500 W/m2or 150 W/ft2) the airflow through the wheel in the computer room chambers is quite low, 38 K m3/hr (23 K CFM). With these low airflows the variation from top to bottom of cabinets and from the first cabinet to the last cabinet was approximately 1C (2F). However, fro
36、m Row 1 to Row 4 the variation was 5C (8F).The row to row variation is the result of a temperature difference of the heat wheel as it rotates through the computer room portion of the cell. It is very cold when it enters and warms as it absorbs heat from the computer room. This vari-ation across the
37、computer room can be minimized by mixing the air in the supply chamber of the computer room side of the cell with a small auxiliary fan or by reducing the T across the wheel, which will increase the airflow through the computer room side of the cell.When the heat load in the computer room increases
38、to 550 kW the airflow volume through the computer room increases to 140 K m3/hr (85 K CFM) and the temperature differences in the computer room are minimized. Top to bottom and front to back are less that 1C (1F) and from Row 1 to Row 4 the variation is 1C (2F). With added circulation in the lower c
39、hamber of the computer room cell these varia-tions would all be less than 1C (1F) throughout the room.Other performance highlights include the verification that cabinet input air temperature less than 25C (77F) can be maintained up to an ambient temperature of 22C (72F). Another test demonstrated th
40、at the system can deliver 21C (70F) air to the computer room with a heat load of 550 kW and a T across the wheel of 28C (50F) at an ambient temperature of 20C (68F). With these last results it shows the heat wheel can easily handle blade server and other high density loads that produce high T values
41、 through the servers.The measurement of efficiency used to characterize this installation is a modified version of the Green Grids “Power Utilizations Efficiency (PUE)”.(3) Since the measurements being made only reference the mechanical energy utilized to cool the computer room heat load, a Mechanic
42、al PUE (PUEm) will be used. PUEm compares the computer equipment load (referred to as the “Critical Load”) to the mechanical load required to cool the critical load.PUEm = Critical Load + Mechanical Load/Critical LoadA summary of the PUEm values is listed below. (See Table 1) Highlights of these eff
43、iciencies include variations from 1.03 to 1.12 instantaneous measurements. The PUEm for this system varies from 1.04 for a heat load of 150 kW and a T across the wheel of 12C (22F) to 1.03 for a heat load of 550 kW and a T across the wheel of 28C (50F). Under the later circumstances only 17 kW of me
44、chanical energy is being utilized to dissipate 550 kW of computer load. For a load of 550 kW and a T through the wheel of 12C (22F) the PUEm is 1.08 requiring only 43 kW to dissipate the heat from the computer equipment.The primary difference in the PUEm is the amount of energy required by the fans
45、to provide the airflow through both the computer room and ambient chambers of the cell.An annual power utilization average was calculated using historic temperature and humidity data for the Netherlands. The annual PUEm for the heat wheel cooling system with a heat load of 550 kW and a T across the
46、wheel of 12C (22F) was 1.08.(2) This annual number includes a few hours per year of supplemental cooling to aid the heat wheel system as the temperature rises above 22C (72F). With the adoption of the new computer equipment environmental recommendations by ASHRAEs TC 9.9, with a maximum input air te
47、mperature of 27C (81F), the average efficiency will be reduced due the need for even less supplemental cooling.Going to ExtremesThe use of the Heat Wheel, with the isolated Hot Aisle, has been shown to cool 28 kW per cabinet, with modeling showing the ability to cool 70+ kW per cabinet.There is a sp
48、ecial cabinet in place that uses a chimney to isolate the hot air to the dropped ceiling area that has the ability Table 1. Mechanical Efficiency (PUEm) of Heat Wheel Cooling SystemAmersfoort, NL, March 24 - 27, 2008Critical Load kWWheel T C/FAmbient Temp. C/FTotal Mechanical Power kWPUEm150 12/22 1
49、5/60 6.5 1.04550 12/22 10/ 22 43.4 1.08550 12/22 22/72 67.3 1.12550 28/50 10/50 17.4 1.03550 25/45 20 /68 23.9 1.04Table 2. Comparison of Cooling TechniquesCoefficient of EfficienciesCooling TypeHot & DryCold & DryMarineHot & DampRefrigeration Pro-cess (Baseline)1.28 1.28 1.28 1.28Baseline with Air-side Economizing1.33 1.30 1.25 1.28Baseline with Water Free Cooling1.25 1.21 1.26 1.21Heat Wheel* 1.26 1.13 1.13 1.15*Improved using adiabatic cooling1.18 1.08 1.11 1.15ASHRAE Transactions 191to dissipate from 20 k
