Patent Publication Number: US-2011075373-A1

Title: System and method for standby mode cooling of a liquid-cooled electronics rack

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under DARPA Contract No. HR0011-07-9-0002, awarded by the Department of Defense. Accordingly, the United States government may have certain rights in the invention 
    
    
     BACKGROUND 
     The present invention relates in general to apparatuses and methods for facilitating operation of liquid-cooled, rack-mounted assemblages of individual electronics units, such as rack-mounted computer server units. 
     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 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 drawer configurations stacked within a 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 drawer 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 availability 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 
     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a system for facilitating cooling of components of an electronics rack comprising at least one heat-generating electronics subsystem and at least one bulk power assembly providing power to the at least one electronics subsystem. The system includes: at least one modular cooling unit (MCU) associated with the electronics rack and configured to provide, via a system coolant loop, system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly for facilitating cooling thereof, wherein each MCU of the at least one MCU comprises a liquid-to-liquid heat exchanger, a facility coolant loop portion and a system coolant loop portion, and when in normal operating mode, a facility coolant loop receives chilled coolant from a source and passes at least a portion thereof through the liquid-to-liquid heat exchanger of each MCU via the facility coolant loop portion thereof, and the system coolant loop provides cooled system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and expels heat in the liquid-to-liquid heat exchanger from the at least one heat-generating electronics subsystem and the at least one bulk power assembly to the chilled coolant in the facility coolant loop; an air-to-liquid heat exchanger associated with the electronics rack and in fluid communication with the system coolant loop; at least one pump in fluid communication with the system coolant loop; and a controller coupled to the at least one pump for adjusting operation of the at least one pump to control flow rate of system coolant through the system coolant loop dependent upon a mode of operation, wherein in the normal operating mode, a first system coolant flow rate is provided through the system coolant loop to cool the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and in standby mode, a second system coolant flow rate is provided through the system coolant loop to cool the at least one bulk power assembly, wherein the first system coolant flow rate is greater than the second system coolant flow rate, and wherein in standby mode, at least a portion of the system coolant flowing through the system coolant loop passes through the air-to-liquid heat exchanger and expels heat in the air-to-liquid heat exchanger from the at least one bulk power assembly to ambient air. 
     In another aspect, a cooled electronics system is provided. The cooled electronics system includes: an electronics rack comprising at least one heat-generating electronics subsystem and at least one bulk power assembly providing power to the at least one heat-generating electronics subsystem; at least one modular cooling unit (MCU) associated with the electronics rack and configured to provide, via a system coolant loop, system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly for facilitating cooling thereof, wherein each MCU of the at least one MCU comprises a liquid-to-liquid heat exchanger, a facility coolant loop portion and a system coolant loop portion, and wherein in normal operating mode, a facility coolant loop receives chilled coolant from a source and passes at least a portion thereof through the liquid-to-liquid heat exchanger of each MCU via the facility coolant loop portion thereof, and the system coolant loop provides cooled system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and expels heat in the liquid-to-liquid heat exchanger from the at least one heat-generating electronics subsystem and the at least one bulk power assembly to the chilled coolant in the facility coolant loop; an air-to-liquid heat exchanger associated with the electronics rack and in fluid communication with the system coolant loop; at least one pump in fluid communication with the system coolant loop; and a controller coupled to the at least one pump for adjusting operation of the at least one pump to control flow rate of system coolant through the system coolant loop dependent upon a mode of operation, wherein in the normal operating mode, a first system coolant flow rate is provided through the system coolant loop to cool the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and in a standby mode, a second system coolant flow rate is provided through the system coolant loop to cool the at least one bulk power assembly, wherein the first system coolant flow rate is greater than the second system coolant flow rate, and wherein in standby mode, at least a portion of the system coolant flowing through the system coolant loop passes through the air-to-liquid heat exchanger and expels heat in the air-to-liquid heat exchanger from the at least one bulk power assembly to ambient air. 
     In a further aspect, a method of cooling components of an electronics rack comprising at least one heat-generating electronics subsystem and at least one bulk power assembly providing power to the at least one heat-generating electronics subsystem is provided. The method includes: employing at least one modular cooling unit (MCU) associated with the electronics rack to provide, via a system coolant loop, system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly for facilitating cooling thereof, wherein each MCU of the at least one MCU includes a liquid-to-liquid heat exchanger, a facility coolant loop portion and a system coolant loop portion, and when in normal operating mode, a facility coolant loop receives chilled coolant from a source and passes at least a portion thereof through the liquid-to-liquid heat exchanger of each MCU via the facility coolant loop portion thereof, and the system coolant loop provides cooled system coolant to the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and expels heat in the liquid-to-liquid heat exchanger from the at least one heat-generating electronics subsystem and the at least one bulk power assembly to the chilled coolant in the facility coolant loop; pumping at least a portion of the system coolant in the system coolant loop through an air-to-liquid heat exchanger associated with the electronics rack and in fluid communication with the system coolant loop; and controlling at least one pump in fluid communication with the system coolant loop to control flow rate of system coolant through the system coolant loop dependent upon a mode of operation, wherein in normal operating mode, a first system coolant flow rate is provided through the system coolant loop to cool the at least one heat-generating electronics subsystem and the at least one bulk power assembly, and in a standby mode, a second system coolant flow rate is provided through the system coolant loop to cool the at least one bulk power assembly, wherein the first system coolant flow rate is greater than the second system coolant flow rate, and wherein in standby mode, at least a portion of the system coolant flowing through the system coolant loop passes through the air-to-liquid heat exchanger and expels heat in the air-to-liquid heat exchanger from the at least one bulk power assembly to ambient air. 
     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 part of a 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 computer installation; 
         FIG. 2  depicts one problem addressed by the present invention, showing recirculation airflow patterns in one implementation of a raised floor layout of an air-cooled computer installation, in accordance with an aspect of the present invention; 
         FIG. 3  is a cross-sectional plan view of one embodiment of an electronics rack utilizing at least one air-to-liquid heat exchanger disposed at the air outlet side of the electronics rack, in accordance with an aspect of the present invention; 
         FIG. 4  is a front elevational view of one embodiment of a liquid-cooled electronics rack comprising multiple electronics subsystems cooled by an apparatus, in accordance with an aspect of the present invention; 
         FIG. 5  is a schematic of one embodiment of an electronics subsystem of an electronics rack, wherein an electronics module is liquid-cooled by system coolant provided by one or more modular cooling units disposed within the electronics rack, in accordance with an aspect of the present invention; 
         FIG. 6  is a schematic of one embodiment of a modular cooling unit disposed within a liquid-cooled electronics rack, in accordance with an aspect of the present invention; 
         FIG. 7  is a plan view of one embodiment of an electronics subsystem layout illustrating an air and liquid cooling subsystem for cooling components of the electronics subsystem, in accordance with an aspect of the present invention; 
         FIG. 8  depicts one detailed embodiment of a partially-assembled electronics subsystem layout, wherein the electronics subsystem includes eight heat-generating electronics components to be actively cooled, each having a respective liquid-cooled cold plate of a liquid-based cooling system coupled thereto, in accordance with an aspect of the present invention; 
         FIG. 9  is a schematic of one embodiment of a system comprising a liquid-cooled electronics rack and a cooling system associated therewith, wherein the cooling system includes two modular cooling units (MCUs) for providing in parallel liquid coolant to the electronics subsystems of the rack, and to an air-to-liquid heat exchanger disposed, for example, at an air outlet side of the electronics rack for cooling air egressing therefrom, in accordance with an aspect of the present invention; 
         FIG. 10  is a schematic of one embodiment of a multi-mode cooling system for cooling electronics of the liquid-cooled electronics rack of  FIG. 9 , shown with system coolant flow in normal operating mode, in accordance with an aspect of the present invention; 
         FIG. 11  depicts the cooling system of  FIG. 10 , shown with system coolant flow in standby mode, in accordance with an aspect of the present invention; 
         FIG. 12  depicts one embodiment of a bulk power drawer of a bulk power assembly of a system such as depicted in  FIGS. 10 &amp; 11 , in accordance with an aspect of the present invention; and 
         FIG. 13  is a flowchart of one embodiment of a process for transitioning the cooling system of  FIGS. 10 &amp; 11  into standby mode, and from standby mode into normal operating mode, in accordance with an aspect 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 electronics 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 multiple electronics subsystems, each having one or more heat generating components disposed therein requiring cooling. “Electronics subsystem” refers to any sub-housing, blade, book, drawer, node, compartment, etc., having one or more heat generating electronic components disposed therein. Each electronics subsystem of an electronics rack may be movable or fixed relative to the electronics rack, with the rack-mounted electronics drawers of a multi-drawer rack unit and blades of a blade center system being two examples of 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 electronics unit 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. Further, unless otherwise specified herein, the term “liquid-cooled cold plate” refers to any conventional thermally conductive structure having a plurality of channels or passageways formed therein for flowing of liquid coolant therethrough. In addition, “metallurgically bonded” refers generally herein to two components being welded, brazed or soldered together by any means. 
     As used herein, “air-to-liquid heat exchanger” means any heat exchange mechanism characterized as described herein through which liquid coolant can circulate; and includes, one or more discrete air-to-liquid heat exchangers coupled either in series or in parallel. An air-to-liquid 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 exchange assembly and/or air-to-liquid heat exchanger thereof can vary without departing from the scope of the invention disclosed herein. A “liquid-to-liquid heat exchanger” may comprise, for example, two or more coolant flow paths, formed of thermally conductive tubing (such as copper or other tubing) in thermal or mechanical contact with each other. Size, configuration and construction of the liquid-to-liquid heat exchanger can vary without departing from the scope of the invention disclosed herein. Further, “data center” refers to a computer installation containing one or more electronics racks to be cooled. As a specific example, a data center may include one or more rows of rack-mounted computing units, such as server units. 
     One example of facility coolant and system coolant is water. However, the concepts disclosed herein are readily adapted to use with other types of coolant on the facility side and/or on the system side. 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 for ease of understanding), wherein the same reference numbers used throughout different figures designate the same or similar components. 
     As shown in  FIG. 1 , in a raised floor layout of an air cooled computer installation  100  typical in the prior art, multiple electronics racks  110  are disposed in one or more rows. A computer installation such as depicted in  FIG. 1  may house several hundred, or even several thousand microprocessors. In the arrangement of  FIG. 1 , chilled air enters the computer room via floor vents 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 an air moving device (e.g., fan or blower) to provide forced inlet-to-outlet air flow to cool the electronic components within the drawer(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 conditioned air units  150 , also disposed within the computer installation  100 . Room air is taken into each conditioned air unit  150  near an upper portion thereof. This room air comprises in part exhausted air from the “hot” aisles of the computer installation defined by opposing air outlet sides  130  of the electronics racks  110 . 
     Due to the ever increasing air flow requirements through electronics racks, and limits of air distribution within the typical computer room installation, recirculation problems within the room may occur. This is shown in  FIG. 2  for a raised floor layout, wherein hot air recirculation  200  occurs from the air outlet sides  130  of the electronics racks back to the cold air aisle defined by the opposing air inlet sides  120  of the electronics rack. This recirculation can occur because the conditioned air supplied through tiles  160  is typically only a fraction of the air flow rate forced through the electronics racks by the air moving devices disposed therein. This can be due, for example, to limitations on the tile sizes (or diffuser flow rates). The remaining fraction of the supply of inlet side air is often made up by ambient room air through recirculation  200 . This re-circulating flow is often very complex in nature, and can lead to significantly higher rack unit inlet temperatures than might be expected. 
     The recirculation of hot exhaust air from the hot aisle of the computer room installation to the cold aisle can be detrimental to the performance and reliability of the computer system(s) or electronic system(s) within the racks. Data center equipment is typically designed to operate with rack air inlet temperatures in the 18-35° C. range. For a raised floor layout such as depicted in  FIG. 1 , however, temperatures can range from 15-20° C. at the lower portion of the rack, close to the cooled air input floor vents, to as much as 45-50° C. at the upper portion of the electronics rack, where the hot air can form a self-sustaining recirculation loop. Since the allowable rack heat load is limited by the rack inlet air temperature at the “hot” part, this temperature distribution correlates to a lower processing capacity. Also, computer installation equipment almost always represents a high capital investment to the customer. Thus, it is of significant importance, from a product reliability and performance view point, and from a customer satisfaction and business perspective, to maintain the temperature of the inlet air uniform. The efficient cooling of such computer and electronic systems, and the amelioration of localized hot air inlet temperatures to one or more rack units due to recirculation of air currents, are addressed by the apparatuses and methods disclosed herein. 
       FIG. 3  depicts one embodiment of a cooled electronics system, generally denoted  300 , in accordance with one aspect of the present invention. In this embodiment, electronics system  300  includes one electronics rack  310  having an inlet door cover  320  and an outlet door cover  330  which have openings to allow for the ingress and egress of external air from the inlet side to the outlet side of the electronics rack  310 . The system further includes at least one air moving device  312  for moving external air across at least one electronics drawer unit  314  positioned within the electronics rack. Disposed within outlet door cover  330  is a heat exchange assembly  340 . Heat exchange assembly  340  includes an air-to-liquid heat exchanger through which the inlet-to-outlet air flow through the electronics rack passes. In this embodiment, a computer room water conditioner (CRWC)  350  is used to buffer heat exchange assembly  340  from the building utility or local chiller coolant  360 , which is provided as input to CRWC  350 . The CRWC  350  provides system water or system coolant to heat exchange assembly  340 . Heat exchange assembly  340  removes heat from the exhausted inlet-to-outlet air flow through the electronics rack for transfer via the system water or coolant to CRWC  350 . Advantageously, providing a heat exchange assembly with an air-to-liquid heat exchanger such as disclosed herein at the outlet door cover of one or more electronics racks in a computer installation can significantly reduce heat loads on existing air conditioning units within the computer installation, and facilitate the cooling of the rack-mounted electronics units. 
       FIG. 4  depicts one embodiment of a liquid-cooled electronics rack  400  which employs a cooling system to be operated utilizing the systems and methods described herein. In one embodiment, liquid-cooled electronics rack  400  comprises a plurality of electronics subsystems  410 , which are processor or server nodes. A bulk power assembly  420  is shown disposed at an upper portion of liquid-cooled electronics rack  400 , and two modular cooling units (MCUs)  430  are disposed in a lower portion of the liquid-cooled electronics rack. In the embodiments described herein, the coolant is assumed to be water or an aqueous-based solution, again, by way of example only. 
     In addition to MCUs  430 , the cooling system includes a system water supply manifold  431 , a system water return manifold  432 , and manifold-to-node fluid connect hoses  433  coupling system water supply manifold  431  to electronics subsystems  410 , and node-to-manifold fluid connect hoses  434  coupling the individual electronics subsystems  410  to system water return manifold  432 . Each MCU  430  is in fluid communication with system water supply manifold  431  via a respective system water supply hose  435 , and each MWCU  430  is in fluid communication with system water return manifold  432  via a respective system water return hose  436 . 
     As illustrated, heat load of the electronics subsystems is transferred from the system water to cooler facility water supplied by facility water supply line  440  and facility water return line  441  disposed, in the illustrated embodiment, in the space between a raised floor  145  and a base floor  165 . 
       FIG. 5  schematically illustrates operation of the cooling system of  FIG. 4 , wherein a liquid-cooled cold plate  500  is shown coupled to an electronics module  501  of an electronics subsystem  410  within the liquid-cooled electronics rack  400 . Heat is removed from electronics module  501  via the system coolant circulated via pump  520  through cold plate  500  within the system coolant loop defined by liquid-to-liquid heat exchanger  521  of modular cooling unit  430 , lines  522 ,  523  and cold plate  500 . The system coolant loop and modular cooling unit are designed to provide coolant of a controlled temperature and pressure, as well as controlled chemistry and cleanliness to the electronics module(s). Furthermore, the system coolant is physically separate from the less controlled facility coolant in lines  440 ,  441 , to which heat is ultimately transferred. 
       FIG. 6  depicts a more detailed embodiment of a modular cooling unit  430 , in accordance with an aspect of the present invention. As shown in  FIG. 6 , modular cooling unit  430  includes a facility coolant loop wherein building chilled, facility coolant is supplied  610  and passes through a control valve  620  driven by a motor  625 . Valve  620  determines an amount of facility coolant to be passed through heat exchanger  521 , with a portion of the facility coolant possibly being returned directly via a bypass orifice  635 . The modular cooling unit further includes a system coolant loop with a reservoir tank  640  from which system coolant is pumped, either by pump  650  or pump  651 , into the heat exchanger  521  for conditioning and output thereof, as cooled system coolant to the electronics rack to be cooled. The cooled system coolant is supplied to the system supply manifold and system return manifold of the liquid-cooled electronics rack via the system water supply hose  435  and system water return hose  436 . 
       FIG. 7  depicts one embodiment of an electronics subsystem  410  component layout wherein one or more air moving devices  711  provide forced air flow  715  in normal operating mode to cool multiple components  712  within electronics subsystem  713 . Cool air is taken in through a front  731  and exhausted out a back  733  of the drawer. The multiple components to be cooled include multiple processor modules to which liquid-cooled cold plates  720  (of a liquid-based cooling system) are coupled, as well as multiple arrays of memory modules  730  (e.g., dual in-line memory modules (DIMMs)) and multiple rows of memory support modules  732  (e.g., DIMM control modules) to which air-cooled heat sinks are coupled. In the embodiment illustrated, memory modules  730  and the memory support modules  732  are partially arrayed near front  731  of electronics subsystem  410 , and partially arrayed near back  733  of electronics subsystem  410 . Also, in the embodiment of  FIG. 7 , memory modules  730  and the memory support modules  732  are cooled by air flow  715  across the electronics subsystem. 
     The illustrated liquid-based cooling system further includes multiple coolant-carrying tubes connected to and in fluid communication with liquid-cooled cold plates  720 . The coolant-carrying tubes comprise sets of coolant-carrying tubes, with each set including (for example) a coolant supply tube  740 , a bridge tube  741  and a coolant return tube  742 . In this example, each set of tubes provides liquid coolant to a series-connected pair of cold plates  720  (coupled to a pair of processor modules). Coolant flows into a first cold plate of each pair via the coolant supply tube  740  and from the first cold plate to a second cold plate of the pair via bridge tube or line  741 , which may or may not be thermally conductive. From the second cold plate of the pair, coolant is returned through the respective coolant return tube  742 . 
       FIG. 8  depicts in greater detail an alternate electronics drawer layout comprising eight processor modules, each having a respective liquid-cooled cold plate of a liquid-based cooling system coupled thereto. The liquid-based cooling system is shown to further include associated coolant-carrying tubes for facilitating passage of liquid coolant through the liquid-cooled cold plates and a header subassembly to facilitate distribution of liquid coolant to and return of liquid coolant from the liquid-cooled cold plates. By way of specific example, the liquid coolant passing through the liquid-based cooling subsystem is chilled water. 
     The planar server assembly depicted in  FIG. 8  includes a multi-layer printed circuit board to which memory DIMM sockets and various electronic components to be cooled are attached both physically and electrically. In the cooling system depicted, a supply header is provided to distribute liquid coolant from a single inlet to multiple parallel coolant flow paths and a return header collects exhausted coolant from the multiple parallel coolant flow paths into a single outlet. Each parallel coolant flow path includes one or more cold plates in series flow arrangement to cool one or more electronic components to which the cold plates are mechanically and thermally coupled. The number of parallel paths and the number of series-connected liquid-cooled cold plates depends, for example on the desired device temperature, available coolant temperature and coolant flow rate, and the total heat load being dissipated from each electronic component. 
     More particularly,  FIG. 8  depicts a partially assembled electronics system  813  and an assembled liquid-based cooling system  815  coupled to selected heat generating components (e.g., including processor dies) to be cooled. In this embodiment, the electronics system is configured for (or as) an electronics drawer of an electronics rack, and includes, by way of example, a support substrate or planar board  805 , a plurality of memory module sockets  810  (with the memory modules (e.g., dual in-line memory modules) not shown), multiple rows of memory support modules  832  (each having coupled thereto an air-cooled heat sink  834 ), and multiple processor modules (not shown) disposed below the liquid-cooled cold plates  820  of the liquid-based cooling system  815 . 
     In addition to liquid-cooled cold plates  820 , liquid-based cooling system  815  includes multiple coolant-carrying tubes, including coolant supply tubes  840  and coolant return tubes  842  in fluid communication with respective liquid-cooled cold plates  820 . The coolant-carrying tubes  840 ,  842  are also connected to a header (or manifold) subassembly  850  which facilitates distribution of liquid coolant to the coolant supply tubes and return of liquid coolant from the coolant return tubes  842 . In this embodiment, the air-cooled heat sinks  834  coupled to memory support modules  832  closer to front  831  of electronics drawer  813  are shorter in height than the air-cooled heat sinks  834 ′ coupled to memory support modules  832  near back  833  of electronics drawer  813 . This size difference is to accommodate the coolant-carrying tubes  840 ,  842  since, in this embodiment, the header subassembly  850  is at the front  831  of the electronics drawer and the multiple liquid-cooled cold plates  820  are in the middle of the drawer. 
     Liquid-based cooling system  815  comprises (in one embodiment) a preconfigured monolithic structure which includes multiple (pre-assembled) liquid-cooled cold plates  820  configured and disposed in spaced relation to engage respective heat generating electronic components. Each liquid-cooled cold plate  820  includes, in this embodiment, a liquid coolant inlet and a liquid coolant outlet, as well as an attachment subassembly (i.e., a cold plate/load arm assembly). Each attachment subassembly is employed to couple its respective liquid-cooled cold plate  820  to the associated electronic component to form the cold plate and electronic component assemblies. Alignment openings (i.e., thru-holes) are provided on the sides of the cold plate to receive alignment pins or positioning dowels during the assembly process. Additionally, connectors (or guide pins) are included within attachment subassembly (not shown) which facilitate use of the attachment assembly. 
     As shown in  FIG. 8 , header subassembly  850  includes two liquid manifolds, i.e., a coolant supply header  852  and a coolant return header  854 , which in one embodiment, are coupled together via supporting brackets. In the monolithic cooling structure of  FIG. 8 , the coolant supply header  852  is metallurgically bonded and in fluid communication to each coolant supply tube  840 , while the coolant return header  854  is metallurgically bonded and in fluid communication to each coolant return tube  852 . A single coolant inlet  851  and a single coolant outlet  853  extend from the header subassembly for coupling to the electronics rack&#39;s coolant supply and return manifolds (not shown). 
       FIG. 8  also depicts one embodiment of the preconfigured, coolant-carrying tubes. In addition to coolant supply tubes  840  and coolant return tubes  842 , bridge tubes or lines  841  are provided for coupling, for example, a liquid coolant outlet of one liquid-cooled cold plate to the liquid coolant inlet of another liquid-cooled cold plate to connect in series fluid flow the cold plates, with the pair of cold plates receiving and returning liquid coolant via a respective set of coolant supply and return tubes. In one embodiment, the coolant supply tubes  840 , bridge tubes  841  and coolant return tubes  842  are each preconfigured, semi-rigid tubes formed of a thermally conductive material, such as copper or aluminum, and the tubes are respectively brazed, soldered or welded in a fluid-tight manner to the header subassembly and/or the liquid-cooled cold plates. The tubes are preconfigured for a particular electronics system to facilitate installation of the monolithic structure in engaging relation with the electronics system. 
     Liquid cooling of heat-generating electronics components within an electronics rack can greatly facilitate removal of heat generated by those components. However, in certain high performance systems, the heat dissipated by certain components being liquid-cooled, such as processors, may exceed the ability of the liquid cooling system to extract heat. For example, a fully configured liquid-cooled electronics rack, such as described hereinabove may dissipate approximately 72 kW of heat. Half of this heat may be removed by liquid coolant using liquid-cooled cold plates such as described above. The other half of the heat may be dissipated by memory, power supplies, etc., which are air-cooled. Given the density at which electronics racks are placed on a data center floor, existing air-conditioning facilities are stressed with such a high air heat load from the electronics rack. Thus, a solution presented herein is to incorporate an air-to-liquid heat exchanger, for example, at the air outlet side of the electronics rack, to extract heat from air egressing from the electronics rack in normal operating mode. This solution is presented herein in combination with liquid-cooled cold plate cooling of certain components within the electronics rack. To provide the necessary amount of coolant, two MCUs are associated with the electronics rack (in one embodiment), and system coolant is fed from each MCU to the air-to-liquid heat exchanger in parallel to the flow of system coolant to the liquid-cooled cold plates disposed within the one or more electronics subsystems of the electronics rack. Note that if desired, flow of system coolant to the individual liquid-cooled cold plates may be in any one of a multitude of series/parallel arrangements. 
     Also, for a high availability system, techniques can be provided for maintaining operation of one modular cooling unit, notwithstanding failure of another modular cooling unit of an electronics rack. This allows continued provision of system coolant to the one or more electronics subsystems of the rack being liquid-cooled. To facilitate liquid cooling of the primary heat-generating electronics components within the electronics rack, one or more isolation valves are employed (upon detection of failure at one MCU of the two MCUs) to shut off coolant flow to the air-to-liquid heat exchanger, and thereby, conserve coolant for the direct cooling of the electronics subsystems. 
       FIG. 9  illustrates one embodiment of a system wherein an electronics rack  900  includes a plurality of heat-generating electronic subsystems  910 , which are liquid-cooled employing a cooling system comprising at least two modular cooling units (MCUs)  920 ,  930  labeled MCU  1  &amp; MCU  2 , respectively. The MCUs are configured and coupled to provide system coolant in parallel to the plurality of heat-generating electronic subsystems for facilitating liquid cooling thereof. Each MCU  920 ,  930  includes a liquid-to-liquid heat exchanger  921 ,  931 , a first (facility) coolant loop  922 ,  932 , and a second (system) coolant loop,  923 ,  933 , respectively. The first coolant loops  922 ,  932  are coupled to receive chilled coolant, such as facility coolant, via (for example) facility water supply line  440  and facility water return line  441 . Each first coolant loop  922 ,  932  passes at least a portion of the chilled coolant flowing therein through the respective liquid-to-liquid heat exchanger  921 ,  931 . Each second coolant loop  923 ,  933  provides cooled system coolant to the plurality of heat-generating electronic subsystems  910  of electronics rack  900 , and expels heat via the respective liquid-to-liquid heat exchanger  921 ,  931  from the plurality of heat-generating electronic subsystems  910  to the chilled coolant in the first coolant loop  922 ,  932 . 
     The second coolant loops  923 ,  933  include respective coolant supply lines  924 ,  934 , which supply cooled system coolant from the liquid-to-liquid heat exchangers  921 ,  931  to a system coolant supply manifold  940 . System coolant supply manifold  940  is coupled via flexible supply hoses  941  to the plurality of heat-generating electronics subsystems  910  of electronics rack  900  (e.g., using quick connect couplings connected to respective ports of the system coolant supply manifold). Similarly, second coolant loops  923 ,  933  include system coolant return lines  925 ,  935  coupling a system coolant return manifold  950  to the respective liquid-to-liquid heat exchangers  921 ,  931 . System coolant is exhausted from the plurality of heat-generating electronics components  910  via flexible return hoses  951  coupling the heat-generating electronics subsystems to system coolant return manifold  950 . In one embodiment, the return hoses may couple to respective ports of the system coolant return manifold via quick connect couplings. Further, in one embodiment, the plurality of heat-generating electronics subsystems each include a respective liquid-based cooling subsystem, such as described above in connection with  FIGS. 7 &amp; 8 , coupled to flexible supply hoses  941  and flexible return hoses  951  to facilitate liquid cooling of one or more heat-generating electronics components disposed within the electronics subsystem. 
     In addition to supplying and exhausting system coolant in parallel to the plurality of heat-generating electronics subsystems of the electronics rack, the MCUs  920 ,  930  also provide in parallel system coolant to an air-to-liquid heat exchanger  960  disposed, for example, for cooling air passing through the electronics rack from an air inlet side to an air outlet side thereof in normal operating mode. By way of example, air-to-liquid heat exchanger  960  is a rear door heat exchanger disposed at the air outlet side of electronics rack  900 . Further, in one example, air-to-liquid heat exchanger  960  is sized to cool substantially all air egressing from electronics rack  900 , and thereby reduce air-conditioning requirements for a data center containing the electronics rack. In one example, a plurality of electronics racks in the data center are each provided with a cooling system such as described herein and depicted in  FIG. 9 . 
     In the embodiment of  FIG. 9 , system coolant flows to and from air-to-liquid heat exchanger  960  via a coolant supply line  961  coupling system coolant supply manifold  940  to air-to-liquid heat exchanger  960 , and a coolant return line  962  coupling the air-to-liquid heat exchanger to system coolant return manifold  950 . Quick connect couplings may be employed at the inlet and outlet of air-to-liquid heat exchanger  960  and/or at corresponding ports at the system coolant supply and return manifolds to facilitate connection of coolant supply and return lines  961 ,  962 . In one embodiment, it is assumed that one MCU of the two MCUs illustrated is incapable of being sized to function within required design parameters as a primary MCU (with the other MCU being a backup MCU) to extract the full heat load from both the plurality of heat-generating electronics subsystems and the air-to-liquid heat exchanger. Therefore, the two MCUs  920 ,  930  are assumed in normal operation to be functioning in parallel. This also ensures a measure of redundancy to the cooling system. 
     As shown, the cooling system further includes a system controller  970 , and an MCU control  1   980  and an MCU control  2   990 , which cooperate together to monitor system coolant temperature of each MCU, and automatically isolate air-to-liquid heat exchanger  960  upon detection of failure of one MCU (as well as to ensure shut down of a failing MCU) so as not to degrade cooling capability of the system coolant provided by the remaining operational MCU to the electronics subsystems of the rack. In one embodiment, the MCU control  1  and the MCU control  2  are control cards, each associated with a respective MCU. 
     As shown, system controller  970  is coupled to both MCU control  1  and the MCU control  2 . MCU control  1   980  is coupled in this embodiment to a temperature sensor T 1    981 , which is disposed to sense system coolant temperature within system coolant supply line  924 , for example, near a coolant outlet of liquid-to-liquid heat exchanger  921  within MCU  1   920 . Additionally, MCU control  1   980  is coupled to a solenoid-actuated isolation valve  982 , which in the embodiment depicted, is disposed within coolant supply line  961  coupling in fluid communication system coolant supply manifold  940  to air-to-liquid heat exchanger  960 . Similarly, MCU control  2   990  couples to MCU  2   930 , as well as to a second temperature sensor T 2    991 , disposed for sensing system coolant temperature within system coolant supply line  934 , and to a second isolation valve S 2    992 , which in the example depicted, is coupled to coolant return line  962  coupling air-to-liquid heat exchanger  960  to system coolant return manifold  950 . 
     Details on processings implemented by MCU control  1 , MCU control  2  and the system controller are provided in co-pending, commonly assigned U.S. patent application Ser. No. 11/942,207, filed Nov. 19, 2007, entitled “System and Method for Facilitating Cooling of a Liquid-Cooled Electronics Rack”, and published on May 21, 2009, as U.S. Patent Publication No. 2009/0126909 A1, which is hereby incorporated herein by reference in its entirety. 
       FIGS. 10-13  depict a further enhanced cooling system and method for facilitating (in part) cooling of a bulk power assembly within the liquid-cooled electronics racks described above. Bulk power supplies for current and future high-end and high-performance computing systems continue to increase in power delivery to meet the needs of these systems. Currently, space limitations necessitate an increase in component density in the power assemblies. The high resulting power dissipation as heat and the small space allocation drive high heat fluxes that may require liquid cooling to maintain appropriate component temperatures for function and reliability. Further, the need for energy efficiency suggests the implementation of a standby power state (or mode) where minimum energy is consumed maintaining the system ready for utilization within a minimum waiting period. Such a standby mode involves networking and power components of the power assembly that must be powered with the expectation that a signal will eventually be sent to activate the entire system. One solution to cooling the bulk power assembly for such high-end computing systems is to employ air-cooling. However, more and more air will necessarily need to be impelled though the power supply assembly to meet ever-increasing loads during normal operating mode (with the air-moving devices being slowed in standby mode). Thus, a liquid-based solution to cooling the bulk power supply assembly is believed advantageous. 
       FIG. 10  illustrates one example of a bulk power assembly (BPA) cooling system and method (in accordance with an aspect of the present invention), wherein the bulk power assembly  1000  is shown within electronics rack  900 , in an upper portion thereof. Bulk power assembly  1000  includes (in this embodiment) a plurality of bulk power drawers  1010 , within which components may be immersed in a dielectric coolant, with an integrated cold plate being provided to transfer heat from the immersion coolant to a system coolant (such as employed above in connection with the liquid-cooled electronics rack of  FIG. 9 ). 
     In the liquid-cooled electronics rack embodiment of  FIG. 10 , electronics rack  900  includes a plurality of heat-generating electronics subsystems  910 , which are liquid-cooled employing a cooling system comprising one or more modular cooling units (MCUs), with two MCUs  920 ,  930  being illustrated, disposed in a lower portion of electronics rack  900 . The MCUs are configured and coupled to provide, via the system coolant loop, system coolant in parallel to the plurality of heat-generating electronics subsystems  910 , and to at least one bulk power assembly  1000  for facilitating liquid cooling thereof. Each MCU  920 ,  930  includes a liquid-to-liquid heat exchanger, a facility coolant loop portion and a system coolant loop portion. The facility coolant loop portions are coupled to a facility coolant loop to receive chilled coolant, such as facility coolant, via (for example) facility water supply line  440  and facility water return line  441 , and to pass at least a portion of the chilled coolant flowing therein through the respective liquid-to-liquid heat exchanger. Each system coolant loop portion provides (in normal operating mode) cooled system coolant to the plurality of heat-generating electronics subsystems  910  of electronics rack  900  and to bulk power drawers  1010  of bulk power assembly  1000 , and expels heat via the respective liquid-to-liquid heat exchanger from the plurality of heat-generating electronics subsystems  910  and bulk power drawers  1010  to the chilled coolant in the facility coolant loop. 
     The system coolant loop includes the respective coolant supply lines which supply cooled system coolant from the liquid-to-liquid heat exchangers of the MCUs to a system coolant supply manifold  940 . System coolant supply manifold  940  is coupled via flexible supply hoses to the plurality of heat-generating electronics subsystems  910  of electronics rack  900 . Similarly, system coolant return lines couple a system coolant return manifold  950  to the respective liquid-to-liquid heat exchangers of the MCUs  920 ,  930 . System coolant is exhausted from the plurality of heat-generating electronics subsystems  910  via flexible return hoses coupling the heat-generating electronics subsystems to system coolant return manifold  950 . In one embodiment, and by way of example only, the plurality of heat-generating electronics subsystems each include a respective liquid-based cooling subsystem, such as described above in connection with  FIGS. 7 &amp; 8  to facilitate cooling of one or more electronic components disposed within the electronics subsystem. 
     In addition to supplying and exhausting system coolant in parallel to the plurality of heat-generating electronic subsystems of the electronics rack, the MCUs  920 ,  930  also provide in parallel system coolant to an air-to-liquid heat exchanger  960  disposed, for example, for cooling air passing through the electronics rack from an air inlet side to an air outlet side thereof. By way of example, air-to-liquid heat exchanger  960  is a rear door heat exchanger disposed at the air outlet side of electronics rack  900 . Further, in one example, air-to-liquid heat exchanger  960  is sized to cool in normal operating mode substantially all air egressing from electronics rack  900 , and thereby reduce air-conditioning requirements for a data center containing the electronics rack. In one example, a plurality of electronics racks in the data center are each provided with a cooling system such as described herein and depicted in  FIG. 10 . 
     In the embodiment of  FIG. 10 , system coolant flows to and from air-to-liquid heat exchanger  960  via a coolant supply line  961  coupling system coolant supply manifold  940  to air-to-liquid heat exchanger  960 , and a coolant return line  962  coupling the air-to-liquid heat exchanger to system coolant return manifold  950 . In the embodiment depicted, a first solenoid-actuated isolation valve  982  is disposed within coolant supply line  961  coupling in fluid communication system coolant supply manifold  940  and air-to-liquid heat exchanger  960 , and a second solenoid-actuated isolation valve S 2    992  is coupled to coolant return line  962  connecting air-to-liquid heat exchanger  960  to system coolant return manifold  950 . These isolation valves allow for selective isolation of the air-to-liquid heat exchanger from the system coolant loop, for example, for servicing of the air-to-liquid heat exchanger. 
     In normal operating mode, system coolant flows from system coolant supply manifold  940  to electronics subsystems  910  and from electronics subsystems  910  to system coolant return manifold  950 , as indicated by direction arrow  1060 . Additionally, in normal operating mode, cooled system coolant is supplied from system coolant supply manifold  940  via coolant supply line  961  to air-to-liquid heat exchanger  960  for cooling airflow  1050  passing through electronics rack  900 . The heated system coolant is then exhausted via system coolant return line  962  to system coolant return manifold  950 , as noted above, for return to the MCU(s). 
     In the enhanced cooling system embodiment depicted in  FIG. 10 , liquid-based cooling of one or more bulk power drawers  1010  of bulk power assembly  1000  is also provided. Numerous approaches for liquid-cooling the bulk power drawers of bulk power assembly  1000  are implementable.  FIG. 12  illustrates one such cooling approach, wherein a liquid-cooled cold plate  1200  is coupled to an immersion-cooled bulk power drawer  1010  of the bulk power assembly. In this embodiment, a coolant inlet manifold (or line)  1020  is coupled to a system coolant inlet  1201  of liquid-cooled cold plate  1200  and a coolant outlet manifold (or line)  1030  is coupled to a system coolant outlet  1202  of liquid-cooled cold plate  1200 . The coolant inlet and outlet manifolds  1020 ,  1030  are illustrated in  FIG. 10 , by way of example only, as being in fluid communication with each bulk power drawer  1010  of bulk power assembly  1000 . 
     Note that, depending upon the implementation, only one bulk power drawer or multiple bulk power drawers may be liquid-cooled via the system coolant. Further, the liquid-cooling approach depicted in  FIG. 12  is provided by way of example only. Alternative liquid-cooling embodiments may be utilized. For example, if the system coolant is a dielectric coolant, then the system coolant could be pumped directly through the immersion-cooled bulk power drawers  1010 . Alternatively, depending upon the bulk power drawer implementation, immersion-cooling of the drawer may be omitted, with a liquid-cooled cold plate being attached to one or more components thereof in a manner similar to that described above in connection with  FIGS. 7 &amp; 8 . 
     By way of further detail, a bulk power assembly may include a chassis housing several electronics drawers configured to convert available utility power to voltages and frequencies usable by the computing system (or more generally, the electronics rack). At least three different types of drawers are typically employed. For example, a bulk power regulator (BPR) drawer does the bulk power conversion from, for example, 400V three-phase AC to 350V DC, a bulk power communications hub (BPCH) provides control and communications functions for the bulk power assembly, and a bulk power distribution drawer (BPD) distributes the converted electrical energy to the computing system. One aspect of the invention disclosed herein is to sealably enclose each bulk power drawer, with the heat-generating components thereof immersed in a dielectric coolant, such as a refrigerant, polyalphaolefin (PAO) oil, a hydrofluoroether liquid, a fluorocarbon liquid, etc., and to provide at least one heat exchange surface, such as the surface of a cold plate, or a finned surface, or a fin and tube heat exchanger, etc., where the dielectric coolant transfers heat to the system coolant circulating through the liquid-cooled electronics rack. 
     In the embodiment illustrated in  FIGS. 10 &amp; 11 , system coolant supply manifold  940  and system coolant return manifold  950  are illustrated (by way of example only) as being separate from the respective coolant inlet manifold  1020  and coolant outlet manifold  1030  of the bulk power assembly. Alternatively, these manifold could be common respective supply and return manifolds. In either case, the supply manifold(s) is modified to include a check valve  1041  and a standby pump  1040  disposed in parallel fluid communication with the check valve, as illustrated in  FIGS. 10 &amp; 11 . Standby pump  1040  is controlled by system controller  970 , and is OFF in normal operating mode, and is ON in standby mode. The arrows in  FIG. 10  illustrate the direction of system coolant flow through the electronics rack in normal operating mode. Specifically, in normal operating mode, system coolant moves through system coolant inlet manifold  940  and coolant inlet manifold  1020  to electronics subsystems  910  and bulk power drawers  1010  of the BPA in parallel, and is exhausted via coolant outlet manifold  1030  and system coolant return manifold  950  to the operating MCUs  920 ,  930 . The operating MCUs cool the exhausted system coolant via facility coolant passing through the facility coolant loop, and provide cooled system coolant back to the supply manifolds  940 ,  1020 . Additionally, in normal operating mode, cooled system coolant is provided from system coolant supply manifold  940  to air-to-liquid heat exchanger  960  for cooling air  1050  passing through the electronics rack. The warmed system coolant is returned via system coolant return manifold  950  from air-to-liquid heat exchanger  960  to the operating MCUs. Depending upon the implementation, a majority of the heat load of the electronics rack can be removed using the liquid-cooling approach depicted in  FIG. 10 . 
     When the system is in normal operating mode, each of the bulk power drawers is active and the heat-generating components are in normal operation, that is, there is a large amount of heat dissipation (e.g., 2.8 kW) in the drawers that is to be dissipated via the immersion coolant to the system coolant and eventually away from the system coolant entirely (by, for example, the facility coolant). To conserve energy when the system is not in use, and to also maintain the system prepared to react quickly to a new workload, it is deemed desirable to implement a standby mode, wherein a minimum amount of heat-generating components are active in the bulk power assembly, but few (if any) of the electronics subsystems within the computing system are powered. In this state, a network signal to the BPCH can trigger the system to resume normal operation. In the standby mode, cooled system coolant is assumed to not be provided by the MCUs of the electronics rack (or computing system), that is, the MCUs are assumed to be off. Since some heat dissipation takes place within the bulk power assembly in standby mode, some coolant flow is desired to prevent the storage of heat within the bulk power assembly (that is, to prevent any heat storage and the associated temperature increase which may result in a failure). 
     In one embodiment, standby pump  1040  and check valve  1041  are provided within the plumbing line between the system coolant supply manifold and the bulk power coolant supply manifold. The standby pump is coupled in parallel with the plumbing line such that flow impelled by the pump flows in the same direction as the flow between the supply manifolds during normal operating mode, with the check valve being in series with the plumbing line to prevent the flow of coolant in the opposite direction to coolant flow in the line during normal operation. When the system is in normal operating mode, system coolant flows through the check valve and the pump is inactive (i.e., off). However, as illustrated in  FIG. 11 , in standby mode, standby pump  1040  begins to impel the flow of system coolant through the system coolant loop, for example, at a substantially lower flow rate than in normal operating mode. Ideally, the standby pump is disposed in close association to the bulk power assembly, and may even be part of the bulk power assembly, to ensure pumping of system coolant through the bulk power assembly in standby mode, while minimizing the pumping power required. At least a portion of the system coolant circulating through the bulk power assembly in standby mode passes from the system coolant return manifold  950  (via coolant return line  962 ) to air-to-liquid heat exchanger  960  and from air-to-liquid heat exchanger  960  (via coolant supply line  961 ) to the deactivated MCU(s) and thereafter to system coolant supply manifold  940 . As one example, if the flow rate of coolant in standby mode is to be 10% of the normal operating mode system flow rate, then the required pump impeller diameter might be about half the size in the standby pump compared with the MCU pump(s). 
     As illustrated, air-side natural convection  1100  may be employed (in one embodiment) in expelling heat from the bulk power assembly via system coolant passing through the air-to-liquid heat exchanger. Note that in the standby mode, the direction  1110  of system coolant flow through electronics subsystems  910  is reversed from direction  1060  (see  FIG. 10 ) of system coolant flow through the electronics subsystems in normal operating mode, and the direction of system coolant flow through the air-to-liquid heat exchanger is reversed from the direction of system coolant flow through the air-to-liquid heat exchanger in normal operating mode. Note also that, in standby mode, it is contemplated that MCUs  920 ,  930  will be off, and that electronics subsystems  910  will be powered down. Further, note that standby power loss in the bulk power assembly is approximately 1.5% of the normal operating dissipation (in one embodiment). 
     In an alternate embodiment, standby pump  1040  may be replaced with control of the system coolant pumps within MCUs  920 ,  930 . Specifically, the pumps within the MCUs could be modulated between a higher pump speed for normal operating mode and a lower pump speed for standby mode, if desired, with the system controller controlling the mode of operation via, for example, appropriate network command(s). 
       FIG. 13  illustrates one embodiment of processing implemented (for example, by system controller  970 ) in switching between normal operating mode and standby mode. Initially, the controller determines whether standby mode is enabled  1300 , and if “no”, waits a time interval t  1305  before again determining whether standby mode is enabled. Once standby mode is enabled, the controller powers off the electronics subsystems (i.e., the nodes), the MCUs, the high voltage supply (in the BPA), and any other unnecessary subsystems  1310 , and powers on the standby pump  1320 , which transitions the system to standby mode. From standby mode, processing inquires whether normal operating mode is to be resumed  1330 , and if “no”, then waits a time interval t  1335  before again inquiring whether normal operating mode is to be resumed. Once normal operating mode is to be resumed, the controller powers off the standby pump  1340 , and powers on the electronics subsystems (i.e., the nodes), the MCUs, the high voltage supply, and any subsystems powered off in standby mode  1350 . 
     As will be appreciated by one skilled in the art, aspects of the controller described above may be embodied as a system, method or computer program product. Accordingly, aspects of the controller 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, aspects of the controller 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, 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 medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer-readable signal medium may include a propagated data signal with computer-readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer-readable signal medium may be any computer-readable medium that is not a computer-readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus or device. 
     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 or similar programming languages. 
     Aspects of the present invention are described above 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 flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowcharts or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the blocks may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     Although embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims