1、192 2009 ASHRAEABSTRACTFree cooling was not common in data centers in the past for a variety of reasons including the philosophy that data center cooling should be designed for maximum reliability and not for energy efficiency. Recently times have changed. Energy and sustainability are more importan
2、t to many data center owners now and sophisticated owners and designers know that free cooling can provide a good return on investment while still maintaining adequate reliability. Many data centers are being designed or retrofitted with airside economizers, waterside economizers, and even “wet bulb
3、” economizers (direct evap-orative coolers). This paper briefly compares airside and waterside economizers, then briefly compares the two types of waterside economizers (CRAC and chiller plant) and then focuses on design and control considerations for chiller plant waterside economizers serving data
4、 centers.AIR VERSUS WATERAn airside economizer will generally be more energy effi-cient than a waterside economizer, if space humidity is not required to be tightly controlled. Airside free cooling is imple-mented at each air handler and thus the amount of free cooling can be maximized at each air h
5、andler. The energy savings of a chiller plant economizer, on the other hand, can be reduced or eliminated for the entire installation by a single “rogue” zone. Furthermore, waterside economizers (WSE) require pump and tower energy and typically three steps of heat trans-fer (e.g. ambient air to cond
6、enser water, to chilled water, to supply air). Airside economizers do not require pump or tower energy or any steps of heat transfer.If humidity is required to be tightly controlled and humid-ification is provided by steam, or infrared humidifiers then the energy savings from airside economizing are
7、 significantly reduced but can still be substantial1. However, if adiabatic humidification (direct evaporation) is used then the savings can be even greater than airside economizing without humidification2. Furthermore, more and more data center managers are realizing that humidity control has littl
8、e if any impact on data center operations and therefore more and more data centers are relaxing or eliminating humidity control.On the other hand, air economizers have some concerns that waterside economizers do not share including introducing unwanted particulates, and gaseous contaminants. Lawrenc
9、e Berkeley National Labs recently measured particulate concen-trations in several data centers with and without airside econ-omizers. They found that with proper filtration particulate concentrations in data centers with airside economizers are not necessarily higher than in data centers without air
10、side econo-mizers and can easily be maintained below the most conser-vative standards3. Recent research projects by Microsoft and Intel have also showed that airside economizers do not affect data center reliability4,5. More research is needed but so far there is no research showing that airside eco
11、nomizers compro-mise data center reliability.Air side economizers will generally be more expensive than waterside economizers. Furthermore, the deal killer for airside economizers in many installations is the extra space required for outside air intake and exhaust louvers, dampers and ducts, particu
12、larly when the data center is buried at the bottom of an office building. In many cases the only option for economizing is a waterside economizer.TYPES OF WATERSIDE ECONOMIZERSThere are two basic types of waterside economizers: CRAC (computer room air conditioner) unit economizers and Waterside Econ
13、omizing in Data Centers: Design and Control ConsiderationsJeff Stein, PEMember ASHRAEJeff Stein is a principal at Taylor Engineering in Alameda, CA.LO-09-015 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009,
14、 vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 193chiller plant economizers. A typical water-side economizer in a CRAC unit is a water-coole
15、d direct expansion CRAC unit with a water coil upstream of the DX coil. The water coil can be served by chilled water or by condenser water (making it a waterside economizer coil). The free cooling provided by the waterside economizer coil is largely offset by the added pres-sure drop of the extra c
16、oil (e.g. selections from one major CRAC manufacturer indicate that the economizer coil increases fan brake horsepower by 40%).A chiller plant economizer consists of a heat exchanger that allows condenser water to cool the chilled water directly. Such an economizer would be used in a data center whe
17、re the data floor is served by chilled water computer room air handlers (CRAHs) rather than DX CRAC units. The CRAH units would only have the pressure drop of one coil, not two. Perhaps the biggest difference, however, between a CRAC unit economizer and a chiller plant economizer is that pack-aged D
18、X CRAC units usually have constant speed fans while chilled water CRAH units can have variable speed drives (VSDs) on the supply fans. Fan energy accounts for about half of data center cooling energy and properly controlled VSD CRAH fans can dramatically reduce fan energy. The rest of this paper is
19、devoted to plant economizers.INTEGRATED VERSUS NON-INTEGRATEDA chiller plant waterside economizer can be integrated, meaning the economizer can meet all or some of the load while the chiller meets the rest of the load, or non-integrated, mean-ing the economizer can only operate when it can meet the
20、entire load. Figure 1 shows an integrated waterside econo-mizer in a primary/secondary chiller plant with two chillers. The heat exchanger is in parallel with the chillers on the condenser water side and in series with the chillers on the chilled water return side.When the outdoor air wet-bulb tempe
21、rature is low, the cooling tower fans are run at high speed to produce cold condenser water (e.g. in the 40s or 50s). This water is pumped through the heat exchanger where it cools chilled water to within a couple degrees of the tower water tempera-ture. If the economizer cannot bring the chilled wa
22、ter temper-ature down to the supply temperature setpoint then the chiller(s) pick up the remaining load and bring the water leav-ing the plant down to setpoint.The HX is located in the secondary loop on the return side just before the common leg. Locating the HX in the secondary loop, rather than th
23、e primary loop is important because it allows the heat exchanger to see the warmest possible water which maximizes the hours when the economizer can operate. The secondary loop is also better than the primary loop because it allows the primary pumps to be shut off when the economizer can handle the
24、entire load.Figure 2 shows an integrated economizer in a primary-only chiller plant. The HX is located on the load side of the common leg so that it sees the warmest return water. If it were on the plant side of the common leg then it could see a blend of return water from the loads and cold supply
25、water from the common leg the valve in the common leg is modulated to maintain the required minimum flow rate through the chiller(s).Figure 3 shows a non-integrated waterside economizer piped in parallel with the chillers on both the condenser and chilled water sides. If the economizer cannot meet t
26、he entire load then it must be shut off. Otherwise, the relatively warm economizer leaving water will be mixed with the cold chiller leaving water and the plant leaving water temperature will be above setpoint. The chillers might be able to compensate by over cooling their leaving water but with chi
27、ller(s) operating the primary loop flow will likely exceed the secondary flow which will result in colder primary return water which will reduce or eliminate the economizer capacity. Non-integrated economizers do not save as much energy as integrated econ-omizers and there is no real advantage of no
28、n-integrated over integrated. Non-integrated economizers do not meet the econ-omizer requirement in California Title 24 or in ASHRAE 90.1-2007 for some climate zones.HEAD PRESSURE CONTROLA critical feature of a chilled water plant with a waterside economizer is head pressure control on all chillers,
29、 air condi-tioners and heat pumps served by the condenser water. To gain benefit from the economizer the cooling towers must produce very cold condenser water but if the condenser water leaving a chiller is too low then the chiller could trip on low head pres-sure. Most chillers require a minimum he
30、ad pressure (the pres-sure differential between the condenser and the evaporator) in order to insure adequate refrigerant flow and adequate oil flow to lubricate the compressor and provide the seal between the rotor or impeller and its housing. Without head pressure control most chillers cannot oper
31、ate with entering condenser water below about 70F during normal operation or below about 55F at start up.Adequate head pressure can be maintained by modulating the condenser water flow to maintain the head pressure at or above minimum head pressure setpoint. Most chiller control-lers have an analog
32、output signal that allow the chiller to directly control a modulating condenser water valve or condenser water pump in order to maintain adequate head pressure for that chiller. If the chiller does not have a head pres-sure control output, a stand alone controller can easily be added to measure head
33、 pressure (or leaving condenser water temperature) and modulate the condenser water valve. As with all control loops, the PID loop modulating the valve/pump must be tuned to avoid hunting.Some chillers are specifically designed to handle cold entering condenser water (e.g. 55F during normal operatio
34、n and 40F during start up) but even these chillers should have head pressure control to allow the economizer to maximize free cooling. If the chiller has a condenser water flow switch it is a good idea to jumper out the flow switch since modulating 194 ASHRAE Transactionsthe condenser water flow is
35、likely to cause the flow switch to trip. Condenser flow switches are generally unnecessary because chillers have high head safeties that will protect them from damage if there is insufficient condenser water flow.Head pressure control on water-cooled air conditions and heat pumps can be achieved in
36、two ways. One option is to use modulating condenser water control valves, like a chiller. A better option is to add a heat exchanger to the condenser water loop serving the heat pumps and control the temperature on the closed loop side of the heat exchanger by modulating the flow on the open loop si
37、de. This option has the advantage of protecting the heat pump condenser coils from the particulate matter that accumulates in an open loop condenser water system. Of course another option is to use chilled water fan coils instead of heat pumps. This is likely to be less expensive and more efficient
38、in a building with a data center and a water-side economizer.LIFECYCLE COST ANALYSISWaterside economizers are most cost effective in large data centers in cold or very dry climates where the wet-bulb temperature is often below 50F but even in moderate climates or relatively small data centers waters
39、ide economizers can still “pencil out” in large part due to the 24/7 nature of data center loads. Figure 4 shows annual simulation results for a chilled water plant serving a data center in four diverse climates. Even in a relatively warm climate like El Paso a waterside econo-mizer can reduce total
40、 HVAC energy by 30%.A waterside economizer was bid as an add alternate on two recent office/data center projects for which the authors firm designed the mechanical system. Project A consisted of a chiller plant serving a 500,000 ft2office building and a 2,000 ton data center. Project B consisted of
41、a plant serving a 150,000 ft2office building and a 110 ton data center. In both cases the office air handlers had airside economizers so the Figure 1 Integrated economizerprimary/secondary.ASHRAE Transactions 195waterside economizers were really only serving the data center CRAH units. Detailed life
42、cycle cost analyses were performed in both cases to determine the cost effectiveness of the waterside economizer. The simple payback for Project A was less than one year and the simple payback for Project B was about 5 to 10 years. In both cases, the owner elected to proceed with the waterside econo
43、mizer.In order to accurately calculate the energy savings from the economizer the plant must be simulated with an annual simulation program which allows accurate modeling of water-side economizers and water coil response to chilled water temperature setpoint reset. The DOE-2.2 DesignDay simula-tion
44、engine has these features. It is available for free from www.DOE. One of the most sensitive assumptions in the model is the expected data center load as a percentage of design load. A data center might be designed for 100 Watts/ft2but the actual load is not likely to match the design load, at least
45、not initially. If the data center is too lightly loaded then the savings potential is too small to justify the economizer but if the data center is fully loaded then the chilled water temper-ature reset is limited and the economizer will not be able to run as many hours. It is a good idea therefore
46、to run several para-metrics at different data center load levels (e.g. 25%, 50%, 75% and 100%) to see how load affects the payback.HX SELECTIONThere isnt necessarily a right way to select a waterside economizer heat exchanger (HX). The larger the heat exchanger the greater the savings but there is o
47、bviously a diminishing return on investment. The main defining vari-ables with a heat exchanger are the approach and the capac-ity. The approach is the difference between the entering condenser water and the leaving chilled water. The smaller the approach the bigger the HX. Ideally one would iterate
48、 on the HX size and re-run the HX lifecycle cost analysis for each selection to determine the lifecycle cost optimum. Unfortu-nately most heat exchanger manufacturers only allow their Figure 2 Integrated economizerprimary only.196 ASHRAE Transactionssales representatives (and not consulting engineer
49、s) to run their selection software, so iterating on heat exchanger selec-tion parameters can be cumbersome.The following procedure is one way to select a heat exchanger:1. CW FlowPick a condenser water flow and dP to match the number of chiller(s) needed for the expected data center load. The HX will typically be served by the same condenser water pumps as the chillers so it makes sense to select a HX with a condenser water flow and pressure drop similar to one chiller, or to the sum of multiple chill-ers. At one extreme you could size the heat exchanger with the same con
