Patent Publication Number: US-8966922-B2

Title: Air-side economizer facilitating liquid-based cooling of an electronics rack

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. Ser. No. 13/187,561, entitled “Air-Side Economizer Facilitating Liquid-Based Cooling of an Electronics Rack”, filed Jul. 21, 2011, and which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The power dissipation of integrated circuit chips, and the modules containing the chips, continues to increase in order to achieve increases in processor performance. This trend poses a cooling challenge at both the module and system level. Increased airflow rates are needed to effectively cool high power modules and to limit the temperature of the air that is exhausted into the computer center. 
     In many large server applications, processors along with their associated electronics (e.g., memory, disk drives, power supplies, etc.) are packaged in removable node configurations stacked within an electronics (or IT) rack or frame. In other cases, the electronics may be in fixed locations within the rack or frame. Typically, the components are cooled by air moving in parallel airflow paths, usually front-to-back, impelled by one or more air moving devices (e.g., fans or blowers). In some cases it may be possible to handle increased power dissipation within a single node by providing greater airflow, through the use of a more powerful air moving device or by increasing the rotational speed (i.e., RPMs) of an existing air moving device. However, this approach is becoming problematic at the rack level in the context of a computer installation (i.e., data center). 
     The sensible heat load carried by the air exiting the rack is stressing the ability of the room air-conditioning to effectively handle the load. This is especially true for large installations with “server farms” or large banks of computer racks close together. In such installations, liquid cooling (e.g., water cooling) is an attractive technology to manage the higher heat fluxes. The liquid absorbs the heat dissipated by the components/modules in an efficient manner. Typically, the heat is ultimately transferred from the liquid to an outside environment, whether air or other liquid coolant. 
     BRIEF SUMMARY 
     In one aspect, a method of cooling at least one electronic subsystem of an electronics rack is provided. The method includes: obtaining a cooling apparatus comprising: a local cooling station, the local cooling station including a liquid-to-air heat exchanger, and ducting for directing cooling airflow across the liquid-to-air heat exchanger; at least one cooling subsystem for association with the at least one electronic subsystem, one cooling subsystem of the at least one cooling subsystem to provide cooling to a respective electronic subsystem of the at least one electronic subsystem, the one cooling system comprising at least one of a housing facilitating immersion cooling of one or more electronic components of the respective electronic subsystems, or a liquid-cooled structure providing conductive cooling of one or more electronic components of the respective electronic subsystem; and at least one coolant loop for coupling the one cooling subsystem to the liquid-to-air heat exchanger of a respective local cooling substation; disposing the electronics rack and the respective local cooling station adjacent to each other, and employing the one coolant loop to couple in fluid communication the one cooling subsystem associated with the respective electronic subsystem and the liquid-to-air heat exchanger of the respective local cooling station; and establishing cooling airflow through the ducting and across the liquid-to-air heat exchanger, and circulation of coolant through the liquid-to-air heat exchanger, the coolant loop and the one cooling subsystem, wherein heat is transferred via the circulating coolant from the respective electronic subsystem and rejected in the liquid-to-air heat exchanger of the respective local cooling station to the cooling airflow passing across the liquid-to-air heat exchanger. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       One or more aspects of the present invention are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts one embodiment of a conventional raised floor layout of an air-cooled data center; 
         FIG. 2  is a schematic of one embodiment of a data center comprising one or more electronics racks and a cooling apparatus, in accordance with one or more aspects of the present invention; 
         FIG. 3  is an enlarged, partial schematic of the cooling apparatus and electronics rack depicted in  FIG. 2 , in accordance with one or more aspects of the present invention; 
         FIG. 4  is a partial top plan view of the data center of  FIG. 2 , in accordance with one or more aspects of the present invention; 
         FIG. 5A  is a schematic of an alternate embodiment of a data center comprising a cooling apparatus, including an air-side economizer facilitating liquid-based cooling of one or more associated electronics racks, in accordance with one or more aspects of the present invention; 
         FIG. 5B  is an enlarged, partial schematic of the cooling apparatus and associated electronics rack of  FIG. 5A , in accordance with one or more aspects of the present invention; 
         FIG. 6A  is a schematic of an alternate embodiment of a data center comprising a cooling apparatus, including an air-side economizer facilitating liquid-based cooling of one or more electronics racks, in accordance with one or more aspects of the present invention; 
         FIG. 6B  is an enlarged, partial schematic of the cooling apparatus and associated electronics rack of  FIG. 6A , in accordance with one or more aspects of the present invention; 
         FIG. 6C  is another enlarged, partial schematic of the cooling apparatus and associated electronics rack of  FIG. 6A , in accordance with one or more aspects of the present invention; 
         FIG. 7  depicts one embodiment of processing for controlling operation of a cooling apparatus, such as depicted in  FIGS. 2-6C , in accordance with one or more aspects of the present invention; 
         FIG. 8A  depicts one embodiment of processing for controlling operation of the evaporative cooling system of a cooling apparatus, such as depicted in  FIGS. 2-6C , in accordance with one or more aspects of the present invention; 
         FIG. 8B  is a schematic of one embodiment of a controllable evaporative cooling system employed in a cooling apparatus, such as depicted in  FIGS. 2-6C , in accordance with one or more aspects of the present invention; and 
         FIG. 9  depicts one embodiment of a process for controlling a controllable recirculation fan coupling the airflow exhaust plenum to the cooling airflow supply plenum of a cooling apparatus, such as depicted in  FIGS. 2-6C , in accordance with one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the terms “electronics rack”, “rack-mounted electronic equipment”, and “rack unit” are used interchangeably, and unless otherwise specified include any housing, frame, rack, compartment, blade server system, etc., having one or more heat generating components of a computer system or electronic system, and may be, for example, a stand-alone computer processor having high, mid or low end processing capability. In one embodiment, an electronics rack may comprise a portion of an electronic system, a single electronic system, or multiple electronic systems, for example, in one or more sub-housings, blades, books, drawers, nodes, compartments, etc., having one or more heat-generating electronic components disposed therein. An electronic system(s) within an electronics rack may be movable or fixed relative to the electronics rack, with the rack-mounted electronic drawers of a multi-drawer rack unit and blades of a blade center system being two examples of systems (or subsystems) of an electronics rack to be cooled. 
     “Electronic component” refers to any heat generating electronic component of, for example, a computer system or other electronic system requiring cooling. By way of example, an electronic component may comprise one or more integrated circuit dies and/or other electronic devices to be cooled, including one or more processor dies, memory dies and memory support dies. As a further example, the electronic component may comprise one or more bare dies or one or more packaged dies disposed on a common carrier. 
     Unless otherwise specified herein, the terms “liquid-cooled structure” and “liquid-cooled cold plate” refer to thermally conductive structures having one or more channels (or passageways) or chambers formed therein or passing therethrough, which facilitate flow of coolant therethrough. In one example, tubing may be provided extending into or through the liquid-cooled structure (or liquid-cooled cold plate). 
     As used herein, “liquid-to-air heat exchanger” means any heat exchange mechanism characterized as described herein through which liquid coolant can circulate; and includes, one or more discrete liquid-to-air heat exchangers coupled either in series or in parallel. A liquid-to-air heat exchanger may comprise, for example, one or more coolant flow paths, formed of thermally conductive tubings (such as copper or other tubing) in thermal or mechanical contact with a plurality of air-cooled cooling fins. Size, configuration and construction of the air-to-liquid heat exchanger can vary without departing from the scope of the invention disclosed herein. Further, as used herein “data center” refers to a computer installation containing, for example, one or more electronics racks to be cooled. As a specific example, a data center may include one or more electronic racks, such as server racks. 
     One example of the coolant employed herein is water. However, the concepts disclosed herein are readily adapted to use with other types of coolant. For example, one or more of the coolants may comprise a brine, a dielectric liquid, a fluorocarbon liquid, a liquid metal, or other similar coolant, or refrigerant, while still maintaining the advantages and unique features of the present invention. 
     Reference is made below to the drawings, which are not drawn to scale to facilitate understanding thereof, wherein the same reference numbers used throughout different figures designate the same or similar components. 
       FIG. 1  depicts a raised floor layout of an air cooled data center  100  typical in the prior art, wherein multiple electronics racks  110  are disposed in one or more rows. A data center such as depicted in  FIG. 1  may house several hundred, or even several thousand microprocessors. In the arrangement illustrated, chilled air enters the computer room via perforated floor tiles  160  from a supply air plenum  145  defined between the raised floor  140  and a base or sub-floor  165  of the room. Cooled air is taken in through louvered covers at air inlet sides  120  of the electronics racks and expelled through the back (i.e., air outlet sides  130 ) of the electronics racks. Each electronics rack  110  may have one or more air moving devices (e.g., fans or blowers) to provide forced inlet-to-outlet airflow to cool the electronic devices within the subsystem(s) of the rack. The supply air plenum  145  provides conditioned and cooled air to the air-inlet sides of the electronics racks via perforated floor tiles  160  disposed in a “cold” aisle of the computer installation. The conditioned and cooled air is supplied to plenum  145  by one or more computer room air-conditioning (CRAC) units  150 , also disposed within the data center  100 . Room air is taken into each air conditioning unit  150  near an upper portion thereof. This room air may comprise in part exhausted air from the “hot” aisles of the computer installation defined, for example, by opposing air outlet sides  130  of the electronics racks  110 . 
     Due to the ever-increasing airflow requirements through electronics racks, and the limits of air distribution within the typical data center installation, novel cooling apparatuses and methods are needed. Disclosed herein, therefore, are cooling apparatuses and methods combining a liquid-based cooling approach with, for example, an air-side economizer, for extracting heat from the liquid-based cooling approach.  FIGS. 2-9  illustrate various embodiments of a data center implementing such cooling apparatuses and methods for cooling electronic subsystems of one or more electronics racks, in accordance with one or more aspects of the present invention. 
     As noted initially, data center equipment may house several hundred, or even several thousand heat-generating electronic components, such as microprocessors. Cooling computer and telecommunications equipment rooms can be a major challenge. In fact, cooling has been found to contribute about one-third of the energy use of a typical IT data center. 
     In a conventional data center, sub-ambient temperature, refrigerated water leaves a chiller plant evaporator and is circulated through one or more CRAC units (see  FIG. 1 ) using building chilled water pumps. This water carries heat away from the air-conditioned, raised floor room that houses the IT equipment, and rejects the heat into the refrigeration chiller evaporator via a heat exchanger. The refrigeration chiller operates on a vapor-compression cycle that consumes compression work (compressor). The refrigerant loop rejects the heat into a condenser water loop using another chiller heat exchanger (condenser). A condenser pump circulates water between the chiller condenser and the air-cooled, evaporative cooling tower. The air-cooled cooling tower uses forced air movement and water evaporation to extract heat from the condenser water loop, and transfer it into the ambient environment. Thus, in such a “standard” facility cooling design, the primary cooling energy consumption components are: the server fans; the computer room air-conditioning (CRAC) unit blowers; the building chilled water (BCW) pumps; the refrigeration chiller compressors; the condenser water pumps; and the cooling tower fans. 
     As a departure from this typical cooling approach, a cooling apparatus and method are disclosed herein which provide energy efficient cooling of electronic subsystems, such as servers, and other information technology equipment, of a data center. As described below, in one embodiment, outdoor air is drawn in and conditioned as a cooling airflow to which heat is rejected from one or more liquid-cooled electronic subsystems of one or more electronics racks within the data center. The outdoor air may be used “as is”, or may be conditioned using, for example, a filter and an evaporative cooling system in which water is sprayed onto a porous media, while the outdoor air is forced through the media, thus evaporating the water resident on the surfaces of the porous media directly into the air. Such evaporative cooling system, which reduces the dry bulb temperature of the air, may comprise a commercially available system, such as the evaporative cooling systems available from Munters Corporation, of Amesbury, Mass., U.S.A. Using such an evaporative cooling system can reduce the temperature of the outdoor air drawn into the cooling apparatus to be close to the air&#39;s wet bulb temperature. Thus, in hot summer months, the use of evaporative cooling (for example, at the inlet of a cooling airflow supply plenum of the cooling apparatus) can provide significant reduction in the intake air temperature, that is, significant reduction of the temperature of the outdoor air used for indoor cooling, as described hereinbelow. 
     Unfortunately, it can be problematic to use outdoor air directly inside a data center room, even with further cooling using evaporative methods. Outdoor air can often possess several undesirable attributes, such as containing particulate pollution or chemical or gaseous pollution, which both can be extremely harmful to electronic hardware. Thus, disclosed herein is a data center cooling system that possess the beneficial energy-saving attributes of an air-side, economizer-based cooling approach, but which also provides protection from the harmful properties of the outdoor air. 
       FIGS. 2 &amp; 3  depict a data center, generally denoted  200 , comprising one embodiment of such a cooling apparatus. As shown, cooling apparatus  200  comprises an air-side economizer  201  and liquid-based cooling of one or more electronic subsystems  220  of one or more electronics racks  210 . In the depicted embodiment, data center  200  includes multiple local cooling stations  240 , each of which is (in one embodiment) associated with, but free-standing from, a respective electronics rack  210  comprising one or more electronic subsystems to be cooled. Local cooling station  240  includes (in one embodiment) a vertically-extending, liquid-to-air heat exchanger  243  and supply and return ducting  241 ,  242  for directing a cooling airflow  244  across liquid-to-air heat exchanger  243 . 
     The liquid-based cooling aspect of the cooling apparatus includes, in one embodiment, multiple cooling subsystems  219  associated with the multiple electronic subsystems  220 , and together forming multiple liquid-cooled electronic subsystems. As shown in  FIG. 3 , each cooling subsystem  219  comprises (in this embodiment) a housing  221  which encloses a respective electronic subsystem  220  comprising a plurality of electronic components  223 . In this implementation, the electronic components are (by way of example) immersion-cooled in a coolant  224 , such as a dielectric coolant. The cooling system is designed for the dielectric coolant to boil in typical operation, generating dielectric coolant vapor  225 . As illustrated, electronic subsystems  220  are angled by providing upward-sloped support rails  222  within electronics rack  210  to accommodate the electronic subsystems  220  at an angle. Angling of the electronic subsystems as illustrated facilitates buoyancy-driven circulation of coolant vapor  225  between the cooling subsystem  219  and the liquid-to-air heat exchanger  243  of the associated local cooling station  240 . 
     In the embodiment depicted in  FIGS. 2 &amp; 3 , the cooling apparatus further includes multiple coolant loops  226  coupling in fluid communication the liquid-cooled electronic subsystems and a respective portion of liquid-to-air heat exchanger  243 . In particular, multiple sloped tubing sections  300  are provided, as illustrated in  FIGS. 2 &amp; 3 , passing through liquid-to-air heat exchanger  243 . Liquid-to-air heat exchanger  243  further includes, in this example, a plurality of air-cooling fins  310  which may be, in one example, oriented vertically within the liquid-to-air heat exchanger  243 . The configuration of the plurality of air-cooled fins  310  and the multiple sloped tubing sections  300  may be chosen to facilitate the passage of cooling airflow  244  across the liquid-to-air heat exchanger, which in this two-phase example, functions as a condenser for the coolant vapor circulating therethrough. 
     In the example of  FIGS. 2 &amp; 3 , the liquid-cooled electronic subsystems remain accessible through a front  212  of the electronics rack  210 , and multiple quick connect couplings  246  are provided in association with the multiple coolant loops  226  to facilitate connection or disconnection of the respective liquid-cooled electronic subsystem(s) from the local cooling station  240 . The multiple coolant loops may include flexible tubing, and quick connect couplings  246  may be any one of various types of commercially available couplings, such as those available from Colder Products Co., of St. Paul, Minn., USA, or Parker Hannifin, of Cleveland, Ohio, USA. 
     As noted, dielectric coolant vapor  225  is buoyancy-driven from housing  221  to the corresponding sloped tubing section  300  of liquid-to-air heat exchanger  243 , where the vapor condenses and is then returned as liquid to the associated liquid-cooled electronics subsystem. In one embodiment, the local cooling station  240  is free-standing and separate from electronics rack  210 , with the liquid coolant loops  226  being completed by positioning electronics rack  210  adjacent to the respective local cooling station  240 , and attaching the quick connect couplings. 
     As illustrated in  FIG. 2 , an airflow damper  245  is provided to control the amount of cooling airflow  244  flowing through supply ducting  241  to liquid-to-air heat exchanger  240 . When associated with no electronics rack or an empty electronics rack  211  that is awaiting the electronic subsystems (e.g., server units), the respective airflow damper  245  may be moved to a closed position, as illustrated on the right side of  FIG. 2 , to prevent cooling airflow from passing through the liquid-to-air heat exchanger  243  of the associated local cooling station  240 . In the embodiment of  FIG. 2 , air-side economizer  201  further includes a cooling airflow supply plenum  231  and an airflow exhaust plenum  232 . Cooling airflow supply plenum  231  receives outdoor air  230  after being drawn across a filter  233  via an outdoor air intake fan  234 . In the embodiment depicted, an evaporative cooling system  235  and associated controller  236  are provided to selectively cool the outdoor air, depending upon its temperature, as explained further below. 
     In one embodiment, cooling airflow  244  is provided in parallel to the supply ducting  241  of multiple local cooling stations  240  of data center  200 , and the heated airflow is exhausted via return ducting  242  in parallel from the multiple cooling stations to the airflow exhaust plenum  232 . In this embodiment, the cooling airflow supply plenum and airflow exhaust plenum comprise overhead plenums within the data center. 
       FIG. 4  illustrates one embodiment of a plurality of local cooling stations  240  and an associated row of electronics racks  210 , as well as an associated row of empty electronics racks  211 . In this embodiment, the liquid-to-air heat exchangers  243  are either door-mounted or pivotally-mounted, liquid-to-air heat exchangers, which facilitates access to, for example, the quick connect couplings (see  FIGS. 2 &amp; 3 ) disposed within or adjacent to return ducting  242 . As noted, cooling airflow  244  flows down the respective supply ducting  241 , is directed across the liquid-to-air heat exchanger  243  before being exhausted via return ducting  242  to, for example, the airflow exhaust plenum  232  (see  FIG. 2 ). In the example of  FIG. 4 , the airflow dampers  245  on the right side are shown in closed position for the local cooling stations  240  associated with the empty electronics racks  211 . 
       FIG. 2  also illustrates a controllable recirculation fan  250 , which comprises a fan that is selectively controlled (as explained further below), for example, during winter months, in order to recirculate a portion of the heated airflow exhaust in the airflow exhaust plenum  232  directly into the cooling airflow supply plenum  231  for mixing with the cold outdoor air  230 , drawn into the cooling apparatus. In winter operation, the evaporative cooling system  235  would be shut OFF by controller  236 . 
     To summarize, in operation, outdoor air  230  is drawn in through, for example, particulate filter  233 , and may be forced through an evaporative cooling system  235 , after which it is distributed via the cooling airflow supply plenum  231  to various parts of data center  200 . The cooling airflow supply plenum  231  feeds several vertical supply ducts  241  with cooling airflow  244 , and this cooling airflow passes through the respective liquid-to-air heat exchangers  243 , and returns via vertical return ducting  242 , to airflow exhaust plenum  232 , where it is exhausted through an exhaust vent  238  by an exhaust fan  237  to the outside of the data center. While the intake and exhaust openings to the cooling airflow supply plenum and airflow exhaust plenum, respectively, are shown in  FIG. 2  adjacent to each other, in reality, the intake and exhaust openings may be disposed remote from each other. By remotely disposing the intake and exhaust openings, any mixing of the warm exhaust air with the cooler intake air can be avoided. As explained further below, in winter months, when the outdoor air temperature may be quite cold, the outdoor air temperature may be heated by recirculating the warmer exhaust air (as shown in  FIG. 2 ) wherein the controllable recirculation fan  250  is provided, along with an appropriate opening, to facilitate controlled heating of the intake air using the warmer exhaust air stream. 
     As described above, in the embodiment depicted in  FIGS. 2-4 , the electronic subsystems (e.g., server nodes) are immersion-cooled, and are docked using sloped rack rails  222  which angle upwards from front side  212  of the rack. The immersion-cooled electronic subsystems contain electronic components  223 , such as a printed circuit board, microprocessor modules, and memory devices, that are packaged within housing (or container)  221  filled, in this example, with dielectric coolant  224 . The coolant boils where in contact with the electronic components, and the vapor exits the liquid-cooled electronic subsystem via exhaust tubing of a coolant loop  226  to a respective tubing section  300  in the liquid-to-air heat exchanger  243 . In this example, the liquid-to-air heat exchanger contains several parallel tubing sections  300  through which vapor from different subsystems is condensed, and subsequently returned back to the respective liquid-cooled electronic subsystem to repeat the cooling cycle. The sloped nature of the electronic subsystems facilitates the upwards and natural travel of the vapor to the heat exchanger tube sections, and then the natural downward return of the liquid condensate back to the electronic subsystems. Thus, dielectric coolant circulation (in one example) is via buoyancy-driven flow. 
     As noted, an airflow damper may be placed in open position to allow unimpeded flow of cooling airflow through the vertical supply duct and vertical return duct, or may be placed in closed position when an empty electronics rack (or no electronics rack) is disposed adjacent to the respective local cooling station. In the closed position, the vertical supply and return ducts are blocked by the damper, which cuts off airflow through the local cooling station, thus preventing wasting of pumped airflow within the data center. 
     Quick connect couplings provided at (for example) inlet and outlet ports of each of the parallel-coupled coolant loops facilitate connection and disconnection of the respective liquid-cooled electronic subsystems to the respective local cooling station. In operation, the cooling airflow, which cools the liquid-to-air heat exchanger fins, and thus the inside of the sloped tubing sections through which the dielectric vapor flows, should be at a temperature that is well below the condensation temperature of the vapor (i.e., the boiling point of the dielectric). This would allow for a temperature difference between the surface contacting the vapor and the cooling airflow. In one embodiment, at any time during operation of the cooling apparatus and electronics rack, there should be a prevailing liquid coolant level within the liquid-immersed electronic subsystem that submerges most of the heat-generating electronic components of the electronic subsystem. 
     As illustrated in  FIG. 4 , the shaped profile of the vertical duct sections  241 ,  242  facilitates the opening of one or more doors of the local cooling stations (which are substantially coplanar with the ducts) so as to allow servicing of the electronics rack, from the side of the electronics rack disposed adjacent to (or in contact with) the local cooling station. In one embodiment, the doors are coplanar and part of the duct structure, and can be opened as illustrated in  FIG. 4 . Once the door is opened, the respective liquid-to-air heat exchanger can then be rotated about a separate vertical hinge to allow for access to (for example) the back side of the associated electronics rack, either directly through an opening in return ducting  242 , or by selective removal of return ducting  242 . 
     Note that advantageously, the only cooling energy consumed in the cooling apparatus of  FIGS. 2-4  is at the intake and exhaust fans, the selectively operated recirculation fan, and the selectively operated evaporative cooling system (e.g., the water pump for distributing water to the evaporative cooling media of the system). 
       FIGS. 5A &amp; 5B  depict an alternate embodiment of a cooling apparatus such as described above in connection with  FIGS. 2-4 . In this alternate embodiment, the liquid-cooling approach is modified to incorporate a liquid-cooled structure  530  within (in one embodiment) each of the respective electronic subsystems  220  of the electronics rack  210 ′. As illustrated, the liquid-cooled structures  530  are configured, in one embodiment, to overlie and to provide conduction cooling to one or more electronic components  223  of electronic subsystem  220 . In one embodiment, the liquid-cooled structures  530  are water-cooled, with water being pumped through the respective liquid-cooled structures via one or more respective node-level pumps  531  disposed, in this example, within or at the electronic subsystem  220 . 
     As illustrated in  FIGS. 5A &amp; 5B , the local cooling station  500  includes ducting  241 ,  242 , as described above in connection with  FIGS. 2-4 , which receive cooling airflow  244  from a cooling airflow supply plenum and exhaust heated airflow to an airflow exhaust plenum, as described. The cooling airflow  244  passes through a liquid-to-air heat exchanger  510 , which is configured with a plurality of tube sections  512 , each of which is approximately aligned to a respective electronics subsystem  220  of the associated electronics rack  210 ′ disposed adjacent to the local cooling station  500 . Liquid-to-air heat exchanger  510  further includes a plurality of air-cooled fins  511  coupled in thermal communication with the coolant-carrying tube sections  512  of the heat exchanger. Coolant loops  532  are provided to couple the cooling subsystems, in this case, comprising liquid-cooled structures  530  to the respective coolant-carrying tube sections  512  of liquid-to-air heat exchanger  510 . Quick connect couplings  513 , such as the quick connect couplings described above with reference to  FIGS. 2-4 , may be employed in association with the coolant loops  532  to provide quick connection or disconnection of a respective liquid-cooled electronic subsystem to the local cooling station  500 . 
     In operation, coolant, such as water, or other single-phase liquid coolant, may be circulated via the node-level pumps through the liquid-cooled structures  530 , coolant loops  532 , and respective coolant-carrying tube sections  512  of the liquid-to-air heat exchanger  510 . The liquid-cooled structure  530  within a particular electronic subsystem may comprise a single liquid-cooled structure, or multiple liquid-cooled structures, such as a plurality of liquid-cooled cold plates, or other such conduction-based structures, coupled in fluid communication, either in series or in parallel within the liquid-cooled electronic subsystem. Since there is no buoyancy-driven flow in this embodiment, the electronic subsystems do not need to be sloped, as in the embodiment of  FIGS. 2-4 . 
       FIGS. 6A-6C  depict a further embodiment of a cooling apparatus, in accordance with one or more aspects of the present invention. This cooling apparatus includes an air-side economizer similar to that described above in connection with  FIGS. 2-4 , and liquid-cooled electronic subsystems, such as described above in connection with the embodiment of  FIGS. 5A &amp; 5B . In this embodiment, however, the node-level pumps are removed from the individual electronic subsystems  220  and one or more pumps are provided within the local cooling station  600  of the cooling apparatus. In addition, the dedicated coolant-carrying tube sections of the embodiments of  FIGS. 2-5B  are replaced with common coolant flow tubing through the local cooling station. As illustrated, the local cooling station  600  includes ducting  241 ,  242 , which facilitates passage of cooling airflow  244  across a liquid-to-air heat exchanger  610  of the local cooling station. In this embodiment, the local cooling station  600  includes a coolant distribution unit  640  which comprises, in one embodiment, a coolant reservoir  641  and one or more coolant pumps  642  for pumping cooled liquid coolant via a coolant supply manifold  620  in parallel to the individual liquid-cooled structures  530  within the electronic subsystems  220  of electronics rack  210 ′. Heated coolant is exhausted via a common coolant return manifold  630  for passage through liquid-to-air heat exchanger  610 . In the embodiment illustrated, the common coolant flow tubing  611  within liquid-to-air heat exchanger  610  is oriented vertically within the heat exchanger, and the air-cooled fins  612  are oriented substantially horizontally (by way of example only). Quick connect couplings  513  may also be provided to facilitate connection or disconnection of the respective liquid-cooled electronic subsystems from the local cooling station  600 . 
     Note that in the embodiments described herein, in operation, coolant (whether vapor or liquid) is circulated between the respective cooling subsystems within the associated electronics rack and the liquid-to-air heat exchanger of the adjacent, local cooling station. Heat is transferred via the circulating coolant from one or more heat-generating electronic components within the electronic subsystem, and rejected in the liquid-to-air heat exchanger of the respective cooling station to the cooling airflow passing across the liquid-to-air heat exchanger. The heated airflow is then exhausted via, for example, a common airflow exhaust plenum. Note also that, although described herein as having a one-to-one correspondence between the local cooling station and an electronics rack, a local cooling station could be configured to accommodate, for example, two or more electronics racks, if desired. 
       FIGS. 7-9  depicts various control processes of a cooling apparatus such as a described above in connection with  FIGS. 2-6C . In one embodiment, the control processes may be implemented by a controller associated with the cooling apparatus, such as controller  236  of the cooling apparatus of  FIG. 2 . 
     Referring to the process of  FIG. 7 , the controller collects data on the outdoor temperature at the air intake to the cooling apparatus  700 , and determines whether the outdoor temperature (T o ) is less than a first temperature threshold (T spec1 )  705 . In one embodiment, the first temperature threshold (T spec1 ) is the coldest air temperature allowable to the liquid-to-air heat exchangers of the local cooling stations without entering a winter mode. Assuming that the outdoor temperature (T o ) is greater than or equal to the first specified temperature (T spec1 ), then processing determines whether the outdoor temperature (T o ) is greater than a second specified temperature (T spec2 )  710 , which is a threshold of the warmest air temperature allowable to the liquid-to-air heat exchangers without entering a summer mode. Assuming that the outdoor temperature (T o ) is less than or equal to the second specified temperature (T spec2 ), then the data center is in regular operating mode  715 , and the winter or summer operating modes may be disengaged if previously engaged. Processing then waits a first time interval (t 1 )  720  before again collecting data on the outdoor air temperature at the intake of the cooling apparatus  700 , and repeats the process. 
     Assuming that the outdoor temperature (T o ) is less than the first specified temperature threshold (T spec1 ), meaning that the outdoor temperature has dropped below the coolest allowable air temperature threshold to the liquid-to-air heat exchangers, then the controller places the cooling apparatus in winter mode, meaning that the air inlet temperature requires heating  730 . Responsive to this, processing initiates recirculation mode to redirect a portion of the warm airflow exhausting via the airflow exhaust plenum into the cooling airflow supply plenum  735 . Processing then waits a second time interval (t 2 )  740 , before again collecting outdoor temperature readings  700 , and repeating the process. Note that in one embodiment, time interval t 1  and time interval t 2  may be the same time intervals, or may be different intervals. 
     Assuming that the outdoor temperature (T o ) is greater than the second temperature threshold (T spec2 ), then processing places the cooling apparatus in summer operating mode, and initiates a dry bulb temperature decrease of the outdoor air being drawn into the cooling airflow supply plenum across the evaporative cooling system  750 . Processing enters the evaporative cooling mode  755  to initiate evaporative cooling of the outdoor air drawn across the evaporative cooling media of the evaporative cooling system, and then waits second time interval (t 2 )  740  before again collecting outdoor temperature data, and repeating the process. 
       FIGS. 8A &amp; 8B  depict one embodiment of an evaporative cooling process and evaporative cooling system, respectively, in accordance with an aspect of the present invention. Referring to the process of  FIG. 8A , evaporative cooling mode  800  is entered with the controller collecting data for controlling evaporative cooler pump ON/OFF, including inlet duct air temperature (T in )  810 . Processing determines whether the inlet duct air temperature (T in ) is greater than the second specified temperature (T spec2 )  820 , and if “yes”, switches the evaporative cooling pump ON  830 . As illustrated in the embodiment of  FIG. 8B , the evaporative cooling system may comprise a porous media  801  through which the inlet air passes, a container  802 , a pump  803 , and one or more spray nozzles  804 . Water  805  is pumped via pump  803  to water spray nozzles  804  where it drips down porous media  801  and cools by evaporation the air passing through the porous media. Once the water level falls below a certain threshold (monitored, e.g., using a float valve (not shown)), additional water can be provided to container  802  via a water supply line  806 . 
     Continuing with the processing  840  of  FIG. 8A , if the inlet duct air temperature (T in ) is less than a third specified temperature threshold (T spec3 ) processing switches the pump OFF  850 . Otherwise, processing waits a time interval (t)  835  before again collecting the relevant data  810 , and repeating the process. Note that in this example, the third specified temperature threshold (T spec3 ) is a defined, acceptable air temperature for the liquid-to-air heat exchangers of the local cooling stations. 
       FIG. 9  illustrates one embodiment of processing for control of the airflow recirculation mode. As noted, airflow recirculation mode is entered  900  when the outdoor temperature (T o ) is below a first specified temperature threshold. Processing initially collects data for control of the recirculation fan&#39;s RPMs, including the inlet duct air temperature and the exhaust duct air temperature  910 . Processing determines whether the inlet duct air temperature (T in ) is less than the first temperature threshold (T spec1 )  920 , and if “yes”, increases the recirculation fan&#39;s speed (RPMs) by a set ΔRPM  930 . If the inlet duct air temperature (T in ) is greater than or equal to the first temperature threshold (T spec1 ), then processing determines whether the inlet duct air temperature (T in ) is greater than a fourth specified temperature threshold (T spec4 ). In this processing example, the inlet duct air temperature (T in ) is the air temperature downstream of the recirculation fan, and the fourth temperature threshold (T spec4 ) is a defined, acceptably cool air temperature that is allowable to the liquid-to-air heat exchanger coil of the local cooling stations. If the inlet duct air temperature (T in ) is greater than the fourth temperature threshold (T spec4 )  940 , then the recirculation fan&#39;s speed may be reduced by the set amount (ΔRPM)  950 . Thereafter, processing waits time interval (t)  935  before again collecting temperature data for control of the recirculation fan speed, as described above. 
     As will be appreciated by one skilled in the art, control aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, control aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system”. Furthermore, control aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible, non-transitory medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     In one example, a computer program product includes, for instance, one or more computer readable storage media to store computer readable program code means or logic thereon to provide and facilitate one or more aspects of the present invention. 
     Program code embodied on a computer readable medium may be transmitted using an appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language, such as Java, Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language, assembler or similar programming languages. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. 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 “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”), and “contain” (and any form contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises”, “has”, “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises”, “has”, “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, 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 invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention 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 invention.