Patent Publication Number: US-2012024502-A1

Title: Enclosed-aisle data center cooling system

Description:
BACKGROUND OF THE INVENTION 
     The disclosure relates generally to data centers. More particularly, the disclosure relates to an enclosed-aisle data center cooling system that reduces the power and flow capacity requirements of an air conditioning or air handling unit by diverting a portion of the flow of exhaust air from the air conditioning or air handling unit. 
     Traditional air-cooled data centers may include racks housing numerous computer servers. These racks are arranged in an alternating hot-aisle/cold-aisle configuration, where cooling of the computer servers is performed through front-to-back cooling in each rack, using a mixture of cold air (supplied through perforated tiles from an under-floor plenum) and hot air (re-circulated from the exhaust of the racks back to the inlet of the racks without being cooled). The portion of the exhaust air that is cooled leaves the exhaust of the racks (possibly mixed with cold air that short-circuits the racks) and is passed through a computer room air conditioning unit (CRAC) (or a computer room air handling unit (CRAH), interchangeably referred to herein as CRAC). Each of these units uses an air-liquid heat exchanger to cool the hot exhaust air down to a specified cold supply air temperature (a desired temperature of supply air provided by the plenum). After passing over the heat exchanger, the cold air from the CRAC units is discharged into the under-floor plenum, where it is finally reintroduced to the data center through the perforated tiles. 
     Because some servers (i.e., a server located in the bottom of a rack and in the middle of an aisle) are closer to the source of cold air, it is expected that these servers will receive cold inlet air almost completely from the perforated tiles. However, servers that are far from the tiles (i.e., a server located at the top of a rack and at the end of an aisle) will receive a mixture of the cold air from the tiles and hot air that has been re-circulated from the exhaust of the racks. This phenomenon leads to a highly non-uniform temperature distribution over the inlet face of racks in an aisle. The allowable inlet temperature to the servers is not arbitrary and a maximum allowable limit (or, redline temperature) is specified by the server manufacturer. Therefore, the cold air supply temperature is selected to ensure that no server in the rack receives an inlet air with a temperature greater than the redline temperature. This takes into account the fact that some servers will receive a mix of cold supply air (from the plenum) and hot re-circulated air. 
     As the non-uniformity of the server inlet temperatures increases, the power required to maintain all the servers under the redline temperature also increases. In order to overcome the non-uniformity in the inlet temperature distribution over the face of the racks in an aisle, attempts have been made at enclosing the cold aisle. The enclosure may include a roof and side barriers, which prevent warm air from being re-circulated from the exhaust of the racks. The enclosure also ensures that the temperature distribution over the face of the servers is approximately uniform (at the cold air supply temperature). This approach, however, requires the CRAC unit to supply all the air flow that is required by the servers through the under-floor plenum and the perforated tiles within the enclosed aisles. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects of the disclosure provide systems and methods for achieving desired operating conditions while reducing the power consumption of one or more fans and/or cooling units in a computer room air conditioner (CRAC) system. 
     A first aspect of the disclosure provides a system for cooling racks of computer servers in an enclosed aisle arrangement, the system comprising: an underfloor plenum for providing a cooling gas mixture to the racks of computer servers; a computer room air conditioner (CRAC) fluidly connected to the underfloor plenum, the CRAC for providing a portion of the cooling gas mixture to the underfloor plenum and for receiving a first portion of exhaust air from the racks of computer servers; a passage fluidly connected to the underfloor plenum, the passage for diverting a second portion of the exhaust air from the rack around the CRAC and to the underfloor plenum, the second portion mixing with the portion of the cooling air provided by the CRAC to form the cooling gas mixture in the underfloor plenum. 
     A second aspect of the disclosure provides a computer server cooling system comprising: racks of computer servers in an enclosed aisle arrangement, the racks of computer servers configured to intake a cooling gas mixture at a first temperature and output an exhaust gas at a second temperature higher than the first temperature; a computer room air conditioner (CRAC) configured to: receive a first portion of the exhaust gas; cool the first portion of the exhaust gas to a third temperature lower than the first temperature, forming a cooled gas; and provide the cooled gas to an underfloor plenum; and a passage for receiving a second portion of the exhaust gas and providing the second portion of exhaust gas to the underfloor plenum; wherein the underfloor plenum is configured to mix the second portion of exhaust gas with the cooled gas to form the cooling gas mixture. 
     A third aspect of the disclosure provides a method comprising: receiving an exhaust gas from racks of computer servers in enclosed aisle arrangement; directing a first portion of the exhaust gas to a cooling unit; cooling the first portion of the exhaust gas to form a cooled gas; diverting a second portion of the exhaust gas to an underfloor plenum; providing the cooled gas to the underfloor plenum for mixing with the second portion of the exhaust gas to form a cooling gas mixture; and providing the cooling gas mixture to an enclosed-aisle inlet of the racks of computer servers. 
     A fourth aspect of the disclosure provides a method comprising: providing a first portion of an exhaust gas from racks of computer devices in an enclosed aisle arrangement for cooling in a computer room air conditioner (CRAC); and diverting a second portion of the exhaust gas from the racks of computer devices to an underfloor plenum for mixing with a cooled gas provided by the CRAC, forming a mixed cooling gas; and providing the mixed cooling gas to the racks of computer devices. 
     A fifth aspect of the disclosure provides a method of reducing power consumption in a computer room air conditioner (CRAC), the method comprising: diverting a first portion of exhaust from racks of computer servers in an enclosed aisle arrangement away from the CRAC and to an underfloor plenum; and combining the first portion with a second portion of the exhaust in the underfloor plenum after cooling of the second portion of the exhaust in the CRAC. 
     A sixth aspect of the disclosure provides a method comprising: collecting a first portion of an exhaust air from racks of computer devices in an enclosed aisle arrangement for cooling in an air cooling device; diverting a second portion of the exhaust air from the racks of computer devices to an underfloor plenum for mixing with a cooled air provided by the air cooling device, forming a mixed cooling air; and providing the mixed cooling air to the racks of computer devices. 
     A seventh aspect of the disclosure provides a system for cooling racks of computer servers in an enclosed cold arrangement, the system comprising: an underfloor plenum for providing a cooling air mixture to the racks of computer servers; an air cooling device fluidly connected to the plenum, the air cooling device for providing a portion of the cooling air mixture to the underfloor plenum and for receiving a first portion of exhaust air from the racks of computer servers; and a passage fluidly connected to the underfloor plenum and the racks of computer servers, the passage for diverting a second portion of the exhaust air from the racks of computer servers to the underfloor plenum, the second portion bypassing the air cooling device and mixing with the portion of the cooling air provided by the air cooling device to form the cooling air mixture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
         FIG. 1  shows a schematic depiction of a system according to embodiments of the invention 
         FIG. 2  shows a graph illustrating data generated according to exemplary features of embodiments of the invention. 
         FIG. 3  shows a schematic depiction of a system according to embodiments of the invention. 
     
    
    
     It is noted that the drawings of the disclosure are not necessarily to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, the disclosure provides a data center cooling system that reduces the power requirement of an air conditioning or air handling unit by diverting a portion of the flow of exhaust air from the air conditioning or air handling unit to a plenum. 
     As indicated herein, enclosed-aisle data centers typically require greater fan power from the CRAC than open-aisle data centers, due to the greater quantity of airflow exhausting from the CRAC units in the enclosed-aisle configuration. Aspects of the disclosure provide methods and systems for reducing the power consumption of components within an enclosed-aisle data center cooling system. Particularly, aspects of the disclosure provide a method and system for reducing the power requirements of one or more fans within a CRAC in an enclosed-aisle data center cooling system. 
     Due to the relatively high pressure drop in a CRAC, passing all the airflow required by the racks over the heat exchangers and filters of the CRAC may be both wasteful and unnecessary. Typically, all the servers in an enclosed-aisle data center receive air at approximately the same temperature, and over-cooling of that air to prevent the warmest server from exceeding its redline temperature may be inefficient. It has been found by Applicants that raising the temperature of the cooling air as high as possible while keeping the warmest server from exceeding its redline temperature is desirable. In one embodiment, the temperature of the warmest server could be raised to approach its redline temperature (typically about 27° C.). Maintaining the warmest server at a temperature approaching its redline temperature may not require passing all of the air received by the CRAC over the heat exchangers and filters of the CRAC. 
     Accordingly, aspects of the invention provide for a bypass mechanism (e.g., passage external to the CRAC or an internal CRAC bypass passage) to send a fraction of the server&#39;s exhaust air around the CRAC in the cooling circuit. 
     Turning to  FIG. 1 , a schematic diagram of a system  10  according to an embodiment is shown. System  10  may include a rack  12 , a cooling unit  14  (e.g., a computer room air conditioning unit (CRAC), or computer room air handling unit (CRAH)) which may include a fan  16 , a plenum  18 , and a bypass passage  20  (which may be bifurcated to form multiple passages  20   a ,  20   b , etc. by one or more bypass fans  22 ). As shown, in one embodiment, plenum  18  may be located below the floor  23  of a data center in system  10 . In one embodiment, a fraction (φ) of the exhaust air from rack  12  bypasses the cooling unit  14  (e.g., CRAC unit) and flows via passage  20  to plenum  18 . This bypassed exhaust air may be recombined with cooled exhaust air entering plenum  18  via passage  24  to form a cooling gas mixture. Plenum  18  may then provide this cooling gas mixture to rack  12  via one or more apertures  26  (e.g., perforated tiles) in floor  23  of the data center. 
     Because of the enclosed-aisle configuration, the flow rate to rack  12  must still equal the flow rate through the perforated tiles. To assure flow uniformity, the exhaust air that is bypassed around the CRAC  14  (via passage  20 ) may be thoroughly mixed with the cooled exhaust air supplied by the CRAC  14  (via passage  24 ). This can be performed, for example, by introducing the fraction (φ) of exhaust air into the plenum  18  (via passage  20 ), where it will have ample opportunity to thoroughly mix with cold air from the CRAC  14 . In one embodiment, a fan  22  (e.g., a low-lift fan) is used to overcome the slight pressurization of plenum  18  (approximately 0.1-0.2 inches of water, contrasted with approximate pressure drops in the CRAC  14  that can be as high as 1 inch of water.) 
     The following may be realized from practicing the aspects of embodiments described herein. 
     Because a fraction (φ) of hot exhaust air is being mixed with the cooler air from CRAC  14  in plenum  18 , the temperature of the cooled gas from the CRAC  14  must be lower than the redline temperature of the rack  12  to assure that the cooling gas mixture supplied to the rack  12  (via apertures  26 , such as perforated tiles) is provided at no higher than the redline temperature after mixing. This means that the cooling unit  14  (e.g., CRAC) will have to work at a lower evaporator temperature and consume a relatively greater amount of input energy per unit of refrigeration (i.e., decreasing its coefficient of performance, COP). 
     The CRAC  14  has a relatively large pressure drop due to the heat exchanger and filters present in the CRAC  14 . Accordingly, bypassing some of the exhaust air (fraction, (φ)) around the CRAC  14  (via passage  20 ) may reduce the pumping power required by the fan  16  of CRAC  14 , which will cause system  10  to consume less total energy. The bypassed air (fraction (φ)) may still need to be pumped into plenum  18 ; however, the pressure differential between the data center space (above floor  23 ) and the plenum  18  is significantly smaller than the pressure drop in the CRAC  14  (typically ˜10% of the CRAC  14  pressure drop for the same flow rate), and may be overcome with, e.g., a low-lift fan  22 . 
     Therefore, although additional refrigeration power may be required by the CRAC  14 , the reduction in fan power requirement (e.g., in fan  16 ) results in an overall power savings, as system  10  allows for more bypass air than a traditional enclosed-aisle cooling system. 
       FIG. 2  shows the results of an example study performed for an enclosed cold aisle data center configuration that is based on the system  10  introduced herein. The left-hand vertical axis shows the power consumption of the cooling infrastructure, which includes the refrigeration power of the CRAC  14  and the pumping power of the CRAC  14  (e.g., including fan  16 ) and bypass (e.g., low-lift) fan  22  as a percentage of the information technology (IT) power required to run the racks of servers cooled by the system  10 . The horizontal axis shows the fraction (φ) of exhaust air from rack  12  that is bypassed around the CRAC  14  and injected directly in the plenum  18  to mix with the cooled gas from CRAC  14 . A recirculation factor of 0.0 represents a typical enclosed aisle, where all the flow goes through the CRAC  14 . As shown, the power consumption at φ=0 is higher than the minimum point on the curve, which corresponds to a recirculation factor (φ)˜0.5-0.55, in this example. As mentioned herein, this difference stems for the fact that if all the flow issuing from the enclosed perforated tiles emanated from the CRAC  14 , considerable fan power (e.g., from fan  16 ) may be consumed to overcome the high pressure drop across the heat exchanger and filters of CRAC  14 . This fan power may be unnecessary in view of the relatively high temperature at which air enters the server. The secondary vertical axis shows the required CRAC  14  exit temperature needed in order to provide the air to the rack  12  (via apertures  26 ) at no greater than the redline temperature (e.g., 27° C. in this example). It can be seen that for a φ=0 the air is supplied at 27° C., whereas at the minimum φ˜0.5-0.55, the CRAC is supplying air at ˜16.5° C. 
     Several possible techniques exist for reintroducing the bypassed air (traveling via passage  20 ) to the enclosed aisle (e.g., via apertures  26 ). These include, but are not limited to, a bypass duct with a low-lift fan (e.g., fan  22 ) to assist the flow into the plenum ( 18 ), as shown in  FIG. 1 ; louvers and small fans in the aisle enclosure; induction nozzles; or integrating the bypass branch (e.g., a passage) into the CRAC  14  structure. Another configuration could be to enclose the hot aisle (having hot racks, not shown), which may also include several practical advantages including, for example: recirculation bypass could be provided directly via a number of actively dampered tiles in the floor of the hot aisle. The tiles may be equipped with low-lift axial fans to overcome the additional few mm-of-water pressure difference in the under-floor plenum (e.g., plenum  18 ). In this embodiment, the air bypassing CRAC  14  (rack exhaust air) will thoroughly mix with the cold air exhausted from the CRAC  14  inside the plenum (e.g., plenum  18 ) before issuing from the cold-aisle apertures (e.g., apertures  26 , such as perforated tiles) to enter the racks. The remaining flow, which passes through the CRAC  14 , could be ducted from the enclosed hot aisle to the CRAC  14  (where the pressure loss associated with this duct may be overcome by the CRAC fans, e.g., fan  16 ). 
       FIG. 3  shows an additional schematic depiction of a system  10  according to embodiments of the invention. It is noted that similarly labeled elements between the figures may depict similar components. As shown, the system includes an enclosed cold aisle  100 , having a roof  102  thereover, and adjacent rows of racks  12  (e.g., similar to racks  12  shown and described with reference to  FIG. 1 ). These rows of racks  12  may discharge exhaust air  110  into hot aisles  112 , which may in some cases be open (e.g., not enclosed by a roof such as roof  102 ). A first portion  110   a  of the exhaust air  110  can be provided to the CRAC (or CRAH)  14 , where it is cooled and then provided to the underfloor plenum  18  (e.g., via fan  16 ). There, the first portion  110   a  can mix with a second portion  110 B of the exhaust air  110 , which is drawn through one or more passages  120  in the underfloor plenum  18 . In some embodiments, the passages  120  can include bypass fan or blower  22  (as shown and described with reference to  FIG. 1 . In any case, the system  10  can be configured to mix bypassed hot-aisle air  110   b  with cooled exhaust from the CRAC  14  in an underfloor plenum  18 , to provide a cooled mixture  122  to the enclosed-aisle inlet of the row of racks. It is understood that while an enclosed cold-aisle configuration is shown in  FIG. 3 , it is also possible to implement aspects of the invention using an enclosed hot-aisle configuration. 
     It is understood that while described herein as a system  10  for cooling a rack of computer servers (e.g., rack  12 ), the teachings described herein may be used in the cooling of other computer devices arranged in an enclosed-aisle configuration. The teachings need not be limited strictly to cooling racks of computer servers. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.