Patent Publication Number: US-2020284464-A1

Title: Hybrid direct and indirect air cooling system

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/556,250, filed on Sep. 8, 2017, the benefit of priority of which is claimed hereby, and which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     There are many applications where cooling is critical, such as, for example, data centers. A data center usually consists of computers and associated components working continuously (24 hours per day, 7 days per week). The electrical components in a data center can produce a lot of heat, which then needs to be removed from the space. Air-conditioning systems in data centers can often consume more than 40% of the total energy. 
     With the current data centers&#39; air-conditioning systems and techniques and significant improvements in IT components operating conditions and processing capacity, servers can roughly operate at 50% of their capacity. This capacity limitation is due, in part, to the cooling systems not being able to cool the servers efficiently and the servers reach their high temperature limit before reaching their maximum capacity. High density data center cooling seeks to cool servers more effectively and increase the density of the data centers. Consequently, this can result in savings in data center operating costs and increase the overall capacity of the data center. 
     Existing cooling systems for data centers and other enclosed spaces can include direct-air and indirect-air cooling technologies. Each of these technologies (direct-air and indirect-air) can have drawbacks or limitations, depending on the operating conditions. Such drawbacks or limitations can include, for example, increased water or energy consumption or decreased indoor air quality. 
     OVERVIEW 
     The present inventors recognize, among other things, an opportunity for improved performance in cooling an enclosed space using a hybrid system capable of direct and indirect cooling in combination. Thus the hybrid system can integrate the strengths of direct and indirect cooling technologies. The system can operate under a plurality of operating modes and a particular operating mode can be selected based on the outdoor air conditions. As shown below, the systems and methods disclosed herein can result in a reduced Water Usage Effectiveness (WUE) and a reduced partial Power Usage Effectiveness (pPUE) as compared to other existing designs. 
     Indirect cooling can be achieved by using liquid cooling technologies to reject the heat at the server. Data center liquid cooling affects the data center energy consumption in two ways: (1) utilizing maximum server processing capacity and data center processing density which will result in lower cooling power consumption per kW of processing power in the data center, and (2) generally liquid-cooling systems are more energy efficient than data centers air-cooling systems. The liquid cooling technology can capture up to 100% of the heat at the server which can eliminate the need for data centers air-cooling systems. The data center liquid cooling can save up to 90% in data centers cooling costs and up to 50% in data centers operating costs. Also, data center liquid cooling can increase the servers processing density by up to 100%, which can result in significant savings. 
     An external cooling unit, which can be physically separate from the enclosed space and the accompanying air handling unit, can be used to produce a reduced temperature cooling fluid. The reduced temperature cooling fluid can provide liquid cooling to a heat load from the enclosed space. In an example, the reduced temperature cooling fluid can be delivered to a plenum or air handling unit to cool return air from the enclosed space. In an example, the reduced temperature cooling fluid can be water. In an example, the reduced temperature cooling fluid can reduce a temperature of a second cooling fluid and the second cooling fluid can be delivered to the plenum or air handling unit to cool the return air. 
     The external cooling unit can include an evaporative cooler and one or more additional components that enable the external cooling unit to operate in a dry mode and a wet mode. In an example, the evaporative cooler of the cooling unit can include a Liquid-to-Air Membrane Energy Exchanger (LAMEE) operating as an evaporative cooler. 
     The direct mode can be achieved under certain ambient conditions in which the external cooling unit can be on or off and the outdoor air can be delivered to the enclosed space as supply air. In a mixed mode, the cooling unit can be off and a combination of outdoor air and return air can be delivered as supply air to the enclosed space. 
     Examples according to the present application can include a control system for operating the hybrid system in multiple modes depending at least in part on ambient temperature and humidity. The operating modes can include 100% indirect in which the return air from the enclosed space is cooled using the reduced temperature cooling fluid from the external cooling unit. The operating modes can include 100% direct in which the outdoor air conditions are sufficient such that the external cooling unit is off and the outdoor air can be delivered to the enclosed space as supply air. Hot return air from the enclosed space can be exhausted to outside. The operating modes can include 100% direct with the cooling unit on. The outdoor air can be within an acceptable humidity range and the cooling unit can be used to reduce a temperature of the outdoor air before the outdoor air is delivered to the enclosed space. In a mixed operating mode, the external cooling unit can be off, a portion of the return air from the enclosed space can be supplied back to the enclosed space and a portion of the return air can be exhausted to outside. In the mixed operating mode, the supply air to the enclosed space can also include outdoor air. 
     Examples according to the present application can include multiple sub-modes for 100% indirect operation, including operating the external cooling unit in a dry mode and a wet mode. In the dry mode, the evaporative cooler of the external cooling unit can be bypassed and water can be conserved. The wet mode can include multiple sub-modes, such as, adiabatic, evaporative and super-evaporative. 
     This overview is intended to provide an overview of subject matter in the present application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  is a schematic of an example hybrid cooling system, in accordance with the present application, for providing cooling to an enclosed space. 
         FIG. 1A  is a schematic of the hybrid cooling system of  FIG. 1  operating in an indirect mode. 
         FIG. 1B  is a schematic of the hybrid cooling system of  FIG. 1  operating in a direct mode with cooling from an external cooling unit. 
         FIG. 1C  is a schematic of the hybrid cooling system of  FIG. 1  operating in a direct mode without any cooling from the external cooling unit. 
         FIG. 1D  is a schematic of the hybrid cooling system of  FIG. 1  operating in a mixed mode. 
         FIG. 2  is a chart comparing the Water Usage Effectiveness (WUE) of the hybrid system disclosed herein to existing technologies. 
         FIG. 3  is a chart comparing the partial Power Usage Effectiveness (pPUE) of the hybrid system disclosed herein to existing technologies. 
         FIG. 4A  is a schematic of an example cooling unit, in accordance with the present application, for use within the hybrid system of  FIG. 1 . 
         FIG. 4B  is a schematic of an example cooling unit, in accordance with the present application, for use within the hybrid system of  FIG. 1 . 
         FIG. 5  is a psychometric chart illustrating various operating modes of the hybrid system, in accordance with the present application. 
         FIG. 6  is a flow chart of an example process for determining an operating mode of the hybrid system, in accordance with the present application. 
     
    
    
     DETAILED DESCRIPTION 
     The present application relates to systems and methods to condition air for an enclosed space using a direct and indirect hybrid cooling system. The hybrid system is able to leverage the advantages of each type of cooling system—direct cooling and indirect cooling, while minimizing or eliminating the disadvantages of each. In an example, the enclosed space can be a data center. 
     The hybrid system can use indirect cooling when 100% of the hot return air can be conditioned using an external cooling unit and the conditioned air can be returned to the enclosed space. The external cooling unit can include multiple components and can operate in multiple modes (or sub modes) including a dry mode and a wet mode. The hybrid system can operate in a direct mode when ambient temperature and humidity levels are such that 100% outdoor air can be delivered to the data center. For the direct mode, the outdoor air quality should be acceptable for the enclosed space. The direct mode can include a first direct mode with 100% outdoor air and the external cooling unit off (direct without cooling), such that the temperature and humidity of the outdoor air is acceptable. The direct mode can include a second direct mode with 100% outdoor air and the external cooling unit on (direct with cooling). In such mode, the humidity level of the outdoor air can be within an acceptable range for delivery of the outdoor air to the enclosed space, but a temperature of the outdoor air can be higher than an acceptable range. As such, the external cooling unit can be used to reduce a temperature of the outdoor air. The hybrid system can operate in a mixed mode in which the external cooling unit is off and a mix of hot return air and outdoor air can be delivered to the enclosed space. 
       FIG. 1  is a schematic of a hybrid system  10  to condition air for an enclosed space  12 . The system  10  can include a plenum  14  having an outdoor air inlet  16  and a supply air outlet  18 . The plenum  14  can be configured to deliver an air stream through at least a portion of the plenum  14  and deliver supply air to the enclosed space  12  through the supply air outlet  18 . The plenum  14  can also be referred to as a housing, cabinet, structure or air handling unit and can be configured to house one or more components used to condition air or water. 
     The hybrid system  10  can include a filter  20 , a coil  22 , and a fan  24 , all of which can be disposed inside the plenum  14 . The filter  20  can be configured to remove contaminants from the outdoor air entering the plenum  14  through the outdoor air inlet  16 . The fan  24  can be configured to deliver the supply air from the plenum  14  to the enclosed space  12  through the supply air outlet  18 . The fan  24  can be a single fan or multiple fans, including a fan array, such as, for example, FANWALL® Systems provided by Nortek Air Solutions. 
     The hybrid system  10  can include a return air duct  26  that can be configured to selectively deliver return air from the enclosed space (via an outlet  28 ) to at least one of the plenum  14  (to be used as supply air) through a return air inlet  30  and to outside (as exhaust air) through an exhaust air outlet  32 . If the hybrid system  10  is operating, regardless of the mode, the outlet  28  can be open; the operating mode can determine whether one or both of the return air inlet  30  and the exhaust air outlet  32  are open. 
     In an example, the hybrid system  10  can include a bypass duct  34  having a bypass inlet  36  and a bypass outlet  38 . As described below, the bypass duct  34  can be used in a direct mode (without cooling) or a mixed mode to divert air from the plenum  14  and bypass the coil  22 . Bypassing the coil  22  can eliminate a pressure drop that results from the air stream passing through the coil  22 , even if the coil  22  is not operational. In other examples, the hybrid system  10  can exclude the bypass duct  34 . In an example, even if the bypass duct  34  is included in the system  10 , in a direct mode with cooling (see  FIG. 1B ) or mixed mode ( FIG. 1D ) the outdoor air can pass through the coil  22  to cool the outdoor air before the outdoor air is delivered to the enclosed space as supply air. 
     The hybrid system  10  can include a cooling unit  40  which can be located external to the plenum  14  and the enclosed space  12 . The cooling unit  40  can include one or more components that can be selectively used in combination to produce a reduced temperature cooling fluid. The reduced temperature cooling fluid can selectively provide liquid cooling to the air flowing through the plenum  14 . As described below, depending on an operating mode of the system  10 , the cooling unit  40  can be off during some ambient conditions even though the system  10  is operating and air is flowing through the plenum  14  for delivery to the enclosed space  12 . 
     The reduced temperature cooling fluid from the cooling unit  40  can be delivered to the coil  22  via a supply line  42 . The reduced temperature cooling fluid can flow through the coil  22  and thus cool the air flowing through the coil  22 . As such, a temperature of the cooling fluid at an outlet  44  of the coil  22  can be higher than a temperature of the cooling fluid at an inlet  46  of the coil  22 . The increased temperature cooling fluid can be delivered back to the cooling unit  40  via a return line  48  and recirculated back through the cooling unit  40  to again reduce the temperature of the cooling fluid. In an example, the cooling fluid can be water or predominantly water. It is recognized that other types of evaporative cooling fluids can be used in combination with water or as an alternative to water for use as the cooling fluid circulating through the cooling unit  40  and the coil  22 . 
     In an example, the reduced temperature cooling fluid exiting the cooling unit  40  can circulate through the coil  22 . In another example, the reduced temperature cooling fluid from the unit  40  can cool a second fluid and the second fluid can pass through the coil  22 . This is described further below in reference to  FIG. 4A . Although not included in  FIG. 1 , the system  10  can include a liquid to liquid heat exchanger (LLHX) that can circulate the reduced temperature cooling fluid from the cooling unit  40  and the second fluid. 
     In an example, the cooling unit  40  can use scavenger air (outdoor air) that can selectively pass through the one or more components of the cooling unit  40  and reduce a temperature of the cooling fluid flowing there through. During operation in an indirect mode, the reduced temperature cooling fluid flows through the coil  22  in the plenum  14 . Thus, the indirect mode of the hybrid system  10  can be described as an air-to-liquid-to air cooling system. The cooling unit  40  can include an evaporative cooler and can operate in multiple modes, including a dry mode and a wet mode. Because the cooling unit  40  can use outdoor scavenger air to reduce a temperature of the cooling fluid, an operating mode of the cooling unit  40  can depend on ambient temperature and humidity, as described further below. In the dry mode, the evaporative cooler of the cooling unit  40  can be bypassed and a temperature of the cooling fluid can be reduced using the scavenger air stream passing through the cooling unit  40 . The wet mode can include multiple sub-modes, such as for example, an adiabatic mode, an evaporative mode, and a super-evaporative mode. Examples of a design and configuration of the cooling unit  40  is shown in  FIGS. 4A and 4B  and described below. 
     The hybrid system  10  can include a system controller  50  to control operation of the system  10 . The controller  50  can be used to determine an operating mode of the system  10  and vary the operating mode as needed and desired. The controller  50  can be manual or automated, or a combination of both. The controller  50  is described further below in reference to the controller  148 A of  FIG. 4A . 
       FIGS. 1A-1D  illustrate air flow through the system  10  in the indirect, direct and mixed modes of operation and aid in the description below of how such air flow varies as a function of the operating mode of the system  10 .  FIGS. 5 and 6  and the accompanying description below illustrate how the operating mode can be determined. 
       FIG. 1A  illustrates air flow (designated as arrows) in an indirect mode in which essentially 100% of the supply air delivered to the enclosed space  12  (via the supply air outlet  18 ) can be return air from the enclosed space that is indirectly cooled inside the plenum  14 . In the indirect mode, the outlet  28  and return air inlet  30  can be open, and supply air from the enclosed space  12  can flow through the return air duct  26  and into the plenum  14  at a location upstream of the filter  20 . In the indirect mode, the exhaust air outlet  32  can be closed such that essentially all of the return air in the return air duct can flow into the plenum  14 . The outdoor air inlet  16  can also be closed. 
     During the indirect mode, the cooling unit  40  can be on and reduced temperature cooling fluid can be supplied to the coil  22 . As the air flows through the coil  22 , the reduced temperature cooling fluid flowing through the coil  22  can reduce a temperature of the air in the plenum  14 . The fan  24  can then direct the reduced temperature air back to the enclosed space  12  as supply air. 
     In an example, the return air inlet  30  can be located downstream of the filter  20 . As such, in the indirect or mixed modes, the return air passing through the plenum  14  can avoid any pressure drop associated with passing through the filter  20 . The filter  20  can be arranged inside the plenum  14  such that the outdoor air passes through the filter  20  but any return air does not. Although not shown in  FIG. 1  or  FIG. 1A , it is recognized that the system  10  can include a make-up air unit to introduce fresh air into the enclosed space  12  when the system  10  is operating in the 100% indirect mode. 
       FIG. 1B  illustrates air flow (designated as arrows) in a direct mode with cooling. In this mode (direct with cooling), essentially 100% of the supply air delivered to the enclosed space  12  can be outdoor air. The outdoor air inlet  16  can be open and the return air inlet  30  can be closed. As a result of the inlet  30  being closed, all of the return air exiting the enclosed space  12  and flowing through the return air duct  26  can exit the duct  26  as exhaust air through the exhaust air outlet  32 . In this mode (direct with cooling), the outdoor air conditions are such that the outdoor air is within an acceptable humidity range for delivery to the enclosed space but the outdoor air requires some cooling to reduce a temperature of the outdoor air before delivery to the enclosed space. As shown in  FIG. 1B , the outdoor air can thus be passed through the coil  22  and the reduced temperature cooling fluid circulating through the coil  22  can reduce the temperature of the outdoor air. 
       FIG. 1C  illustrates air flow (designed as arrows) in a direct mode without cooling in which essentially 100% of the supply air delivered to the enclosed space  12  can be outdoor air. In this mode (direct without cooling), the cooling unit  40  and the coil  22  can be off (or non-operational) and the outdoor air can be delivered to the enclosed space  12  without having to cool or adjust a humidity of the outdoor air. The mode represented in  FIG. 1C  is referred to herein as “direct without cooling” because the external cooling unit  40  is off/not operating and the coil  22  is off/not operating (i.e. not circulating the reduced temperature cooling fluid from the cooling unit  40 ); however, in this mode, it is recognized that the enclosed space is being directly cooled with the outdoor air. 
     In an example, the system  10  can include the bypass duct  34 , which can divert the outdoor air, after it passes through the filter  20 , through the bypass inlet  36  such that the outdoor air bypasses the coil  22 . The bypass outlet  38  can be located downstream of the coil  22  and upstream of the fan  24 . In an example, the system  10  can exclude the bypass duct and the outdoor air can flow through the coil  22 , even though the coil  22  can be off (non-operational) in the direct without cooling mode. After bypassing the coil  22  or passing through the non-operational coil  22 , the outdoor air can be directed into the enclosed space  12  via the fan  24  and the supply air outlet  18 . 
     In either the direct with cooling mode ( FIG. 1B ) and the direct without cooling mode ( FIG. 1C ), as well as the mixed mode (see  FIG. 1D ), the outdoor air conditions are such that the outdoor air can be delivered to the enclosed space  12 . In an example, the outdoor air can pass through the filter  20  to remove contaminants from the outdoor air. Operation in the direct or mixed modes can be contingent on acceptable air quality of the outdoor air. If the outdoor air quality is poor, the system  10  can switch to the indirect mode shown in  FIG. 1A . 
       FIG. 1D  illustrates air flow (designated as arrows) in a mixed mode in which the cooling unit  40  and coil  22  are off, and a combination of outdoor air and return air can be delivered to the enclosed space  12 . In the mixed mode, both the return air inlet  30  and the exhaust air outlet  32  can be open such that the return air in the return air duct  26  can be split into two portions—a first portion can enter the plenum through the return air  30  and a remaining portion (or a second portion) can be exhausted through the exhaust air outlet  32 . The first portion of return air entering the plenum can mix with outdoor air entering the plenum through the outdoor air inlet  16 . In the mixed mode, the coil  22  can be off and consequently there is no cooling fluid running through the coil  22 . The mixed air stream of outdoor air and return air can either pass through the non-operational coil  22  or the mixed air stream can flow through the bypass duct  26 . The controller  50  of the system  10  can determine and vary the amount of return air in the mixed air stream relative to the amount of outdoor air in the mixed air stream. 
     The modes of operation for the system  10  are shown in  FIGS. 1A-1D . In an example, the cooling unit  40  can be on in the 100% indirect mode and the direct with cooling mode. There can be multiple sub-modes of indirect cooling, which are described below in reference to  FIGS. 4A and 4B . (These sub-modes can also be used in the direct with cooling mode.) The specific type and number of sub-modes under the indirect mode can depend, at least in part, on the type and arrangement of the components in the cooling unit  40 . 
     The cooling unit  40  can be described as being off when the system  10  is operating in the direct without cooling mode or the mixed mode, mainly because the reduced temperature cooling fluid is not circulating through the coil  22  when the system  10  is operating in these two modes. However, it is recognized that even if the system  10  is operating in one of these two modes, in an example, the cooling unit  40  can be on or operational and the reduced temperature cooling fluid produced by the cooling unit  40  can be stored in a reserve for future cooling of the air stream passing through the coil  22 . Thus, the “on” and “off” (or operational/non-operational) designation herein for the cooling unit  40  can refer to whether the coil  22  is receiving reduced temperature cooling fluid (from the cooling unit  40  or from a reserve or supply of cooling fluid) for circulating through the coil  22  to cool the air stream. 
     In an example, the configuration or layout of the hybrid system  10  can be based on or originate from an indirect cooling system that uses air-to-liquid-to-air cooling of return air from an enclosed space. As provided above, liquid cooling can provide significant advantages. Such indirect cooling system can be modified structurally such that it has the functionality to also provide direct cooling and operate as a hybrid system. The strengths of a direct cooling system can be leveraged with an existing indirect system. An intake damper for outdoor air (i.e. the outdoor air inlet  16 ) can be added to the plenum or air handling unit  14  for the return air so that the plenum  14  can direct outdoor through the plenum  14  and into the enclosed space  12 , either in combination with or as an alternative to the return air. An exhaust damper for return air (i.e. the exhaust air outlet  32 ) can be added to the return air duct  26  to discharge some or all of the return air when operating in a direct or mixed mode. Existing indirect systems can be retrofit with these additional components in order to operate as a hybrid system. 
     It is recognized that additional components, such as, for example, a filter, can be included inside or external to the plenum  14 . In an example, a side-stream filtration unit can be included in the system  10  so that a portion of the air in the enclosed space  12  can be continuously or selectively filtered. 
     As described above, the hybrid system  10  can leverage the strengths of direct and indirect cooling. By being able to switch back and forth between the various operating modes, the hybrid system  10  can minimize or eliminate the disadvantages or limitations of each of the direct and indirect cooling systems. A comparison was conducted to demonstrate potential energy and water savings of the hybrid system  10  as compared to existing cooling technologies. The various cooling systems that were evaluated are shown below in Table 1 and the evaluation was conducted in the state of Iowa (United States) which can have a challenging climate (a dry winter and a humid summer). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of Hybrid System to existing technologies 
               
            
           
           
               
               
               
            
               
                   
                   
                 Exemplary commercial 
               
               
                 Technology 
                 Description 
                 design 
               
               
                   
               
               
                 1 
                 Hybrid system disclosed herein 
                   
               
               
                 2 
                 Indirect-air cooling system with 
               
               
                   
                 liquid cooling - Nortek Air 
               
               
                   
                 Solutions 
               
               
                 3 
                 Indirect-air cooling system - 
                 Nortek Air Solutions 
               
               
                   
                 Nortek Air Solutions 
                 Cool3 IDEC 
               
               
                   
               
            
           
         
       
     
       FIG. 2  compares a Water Usage Effectiveness (WUE) of the hybrid system (#1) to technologies #2 and #3. As shown in  FIG. 2 , the hybrid system demonstrated the lowest water usage. 
       FIG. 3  compares a partial Power Usage Effectiveness (pPUE) of the hybrid system (#1) to technologies #2 and #3. The hybrid system demonstrated the lowest power usage relative to technologies #2 and #3. 
     Referring back to  FIG. 1 , the external cooling unit  40  of the hybrid system  10  can operate in multiple modes to provide multiple sub-modes of indirect cooling. The cooling unit  40  can include an evaporate cooler in combination with one or more other additional components that enable the cooling unit to operate in a dry mode and a wet mode. A particular operating mode of the external cooling unit  40  can be selected based on the outdoor air conditions (temperature and humidity). 
       FIGS. 4A and 4B  illustrate examples of a cooling unit  40 A and  40 B, respectively, suitable for use as the external cooling unit  40  in the hybrid system  10  of  FIG. 1 . The cooling unit  40 A of  FIG. 4A  can enable three sub-modes of indirect cooling for the hybrid system  10 . The cooling unit  40 B of  FIG. 4B  can enable four sub-modes of indirect cooling for the hybrid system  10 . 
       FIG. 4A  illustrates an example cooling unit  40 A for providing cooling to the enclosed space  12  via the coil  22  (see  FIG. 1 ). The cooling unit  40 A can include a scavenger air plenum  104 A which can include an air inlet  106 A and an air outlet  108 A through which a scavenger air stream can flow. The plenum  104 A can also be referred to as a housing, cabinet or structure, and can be configured to house one or more components used to condition air or water. The plenum  104 A can be disposed outside of the enclosed space  12  and the plenum  14 . 
     The cooling unit  40 A can include a pre-cooler  160 A, an evaporative cooler  110 A, a dry coil (or cooling coil)  112 A, and a fan (or fan array)  114 A, all of which can be arranged inside the plenum  104 A. The dry coil or cooling coil  112 A can also be referred to herein as a recovery coil. The pre-cooler  160 A can also be referred to herein as a pre-cooling coil, a pre-cooler coil, a pre-conditioner or a dry coil. The pre-cooler  160 A can be referred to herein as a first cooling component (upstream of the evaporative cooler  110 A) and the dry coil  112 A can be referred to herein as a second cooling component (downstream of the evaporative cooler  110 A). In some examples, a filter (not shown) can be arranged inside the scavenger plenum  104 A near the air inlet  106 A. 
     The scavenger air entering the plenum  104 A can pass through a pre-cooler  160 A to precondition the scavenger air. The pre-cooler  160 A is discussed further below. The scavenger air exiting the pre-cooler  160 A can then pass through the evaporative cooler  110 A. The evaporative cooler  110 A can be configured to condition the scavenger air passing there through using an evaporative fluid, such as water. The evaporative cooler  110 A can use the cooling potential in both the air and the evaporative fluid to reject heat. In an example, as scavenger air flows through the evaporative cooler  110 A, the evaporative fluid, or both the scavenger air and the evaporative fluid, can be cooled to a temperature approaching the wet bulb (WB) temperature of the air leaving the pre-cooler  160 A. Due to the evaporative cooling process in the evaporative cooler  110 A, a temperature of the evaporative fluid at an outlet  118 A of the evaporative cooler  110 A can be less than a temperature of the evaporative fluid at an inlet  116 A of the evaporative cooler  110 A; and a temperature of the scavenger air at an outlet of the evaporative cooler  110 A can be less than a temperature of the scavenger air at an inlet of the evaporative cooler  110 A. In some cases, a temperature reduction of the evaporative fluid can be significant, whereas in other cases, the temperature reduction can be minimal. Similarly, a temperature reduction of the scavenger air can range between minimal and significant. In some cases, the scavenger air temperature can increase across the evaporative cooler  110 A. Such temperature reduction of one or both of the evaporative fluid and the scavenger air can depend in part on the outdoor air conditions (temperature, humidity), operation of the pre-cooler  160 A, and operation of the evaporative cooler  110 A. For example, as described below and shown in  FIG. 4B , in an example, the evaporative cooler  110 B can selectively operate adiabatically, in which case a temperature of the evaporative fluid circulating through the evaporative cooler  110 B can remain relatively constant or undergo minimal changes. 
     The evaporative cooler  110 A can be any type of evaporative cooler configured to exchange energy between an air stream and a cooling fluid through evaporation of a portion of the fluid into the air. Evaporative coolers can include direct-contact evaporation devices in which the working air stream and the liquid water (or other fluid) stream that is evaporated into the air to drive heat transfer are in direct contact with one another. In what is sometimes referred to as “open” direct-contact evaporation devices, the liquid water may be sprayed or misted directly into the air stream, or, alternatively the water is sprayed onto a filler material or wetted media across which the air stream flows. As the unsaturated air is directly exposed to the liquid water, the water evaporates into the air, and, in some cases, the water is cooled. 
     Such direct-contact evaporation devices can also include what is sometimes referred to as a closed circuit device. Unlike the open direct-contact evaporative device, the closed system has two separate fluid circuits. One is an external circuit in which water is recirculated on the outside of the second circuit, which is tube bundles (closed coils) connected to the process for the hot fluid being cooled and returned in a closed circuit. Air is drawn through the recirculating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open circuit. In operation the heat flows from the internal fluid circuit, through the tube walls of the coils, to the external circuit and then by heating of the air and evaporation of some of the water, to the atmosphere. 
     These different types of evaporative coolers can also be packaged and implemented in specific types of systems. For example, a cooling tower can include an evaporative cooling device such as those described above. A cooling tower is a device that processes working air and water streams in generally a vertical direction and that is designed to reject waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers can transport the air stream through the device either through a natural draft or using fans to induce the draft or exhaust of air into the atmosphere. Cooling towers include or incorporate a direct-contact evaporation device/components, as described above. 
     Examples of evaporative coolers usable in the cooling unit  40  of  FIG. 1  can also include other types of evaporative cooling devices, including liquid-to-air membrane energy exchangers. Unlike direct-contact evaporation devices, a liquid-to-air membrane energy exchanger (LAMEE) separates the air stream and the liquid water stream by a permeable membrane, which allows water to evaporate on the liquid water stream side of the membrane and water vapor molecules to permeate through the membrane into the air stream. The water vapor molecules permeated through the membrane saturate the air stream and the associated energy caused by the evaporation is transferred between the liquid water stream and the air stream by the membrane. 
     Membrane exchangers may have some advantages over other types of evaporative coolers. For example, the LAMEE may eliminate or mitigate maintenance requirements and concerns of conventional cooling towers or other systems including direct-contact evaporation devices, where the water is in direct contact with the air stream that is saturated by the evaporated water. For example, the membrane barriers of the LAMEE inhibit or prohibit the transfer of contaminants and micro-organisms between the air and the liquid stream, as well as inhibiting or prohibiting the transfer of solids between the water and air. The use of a LAMEE as the evaporative cooler in the cooling unit  40 A is exemplary. As noted above, depending upon the application and a number of factors, examples according to this disclosure can include any type of evaporative cooler configured to exchange energy between an air stream and a cooling fluid through evaporation of a portion of the fluid into the air. 
     In an example, as shown in  FIG. 4A , the evaporative fluid from the evaporative cooler  110 A can be collected and delivered to a tank  122 A. In other examples, the evaporative fluid from the evaporative cooler  110 A is not collected for cooling the enclosed space. In yet other examples, the cooling unit  40 A can be configured to switch between the configuration shown in  FIG. 4A  (in which the evaporative fluid exiting the evaporative cooler  110 A is collected and transported to the tank  122 A) and operating the evaporative cooler  110 A adiabatically to circulate the evaporative fluid through the evaporative cooler  110 A only. This is shown in  FIG. 4B  and described below. 
     In an example, the evaporative fluid in the evaporative cooler  110 A can be water or predominantly water. In the cooling unit  40 A of  FIG. 4A , the cooling fluid is described as being water but the inlet  116 A and outlet  118 A can be described as a cooling fluid inlet and a cooling fluid outlet since a fluid in addition to, or as an alternative to, water can circulate through the evaporative cooler  110 A. It is recognized that other types of evaporative cooling fluids can be used in combination with water or as an alternative to water in the cooling unit  40 A (or  40 B in  FIG. 4B ). 
     The dry coil or recovery coil  112 A can be arranged inside the plenum  104 A downstream of the evaporative cooler  110 A. The recovery coil  112 A can cool a cooling fluid circulating through the recovery coil  112 A using the cooling potential of the scavenger air. The scavenger air exiting the evaporative cooler  110 A can be relatively cool and additional sensible heat from the cooling fluid passing through the recovery coil  112 A can be rejected into the scavenger air. The recovery coil  112 A can produce a reduced-temperature cooling fluid that can provide cooling to the coil  22  (see  FIG. 1 ). The reduced-temperature cooling fluid exiting the recovery coil  112 A can flow to the evaporative cooler  110 A or to a water tank  122 A. The flow path of the cooling fluid to and from the recovery coil  112 A is described below. The scavenger air exiting the recovery coil  112 A can be directed out of the plenum  104 A using the fan  114 A and can exit the plenum  104 A at the outlet  108 A as exhaust. 
     In an example, the cooling fluid circulating through the recovery coil  112 A can be water. In an example, the cooling fluid circulating through the recovery coil  112 A can be the same fluid as the evaporative fluid in the evaporative cooler  110 A. 
     As provided above, in an example, the evaporative fluid in the evaporative cooler  110 A can be water. In an example, as shown in  FIG. 4A , the reduced-temperature water from the outlet  118 A of the evaporative cooler  110 A can be used to provide cooling to the air passing through the coil  22 . The reduced-temperature water can flow from the outlet  118 A to the water tank  122 A via a water line  120 A. Although not shown in  FIG. 4A , the water tank  122 A can include a make-up valve and a drain valve to maintain the water level and hardness level inside the tank  122 A. The water tank  122 A can include one or more temperature sensors in or around the water tank  122 A to monitor a temperature of the water in the tank  122 A. In an example, a control of the cooling unit  40 A can be based, in part, on a measured temperature of the water in the tank  122 A compared to a set point water temperature. In an example, the set point water temperature can be pre-determined based on cooling needed for the enclosed space  12 . In an example, the set point water temperature can vary during operation of the system  10 , based in part on operation of the data center or other devices that produce the heat in the enclosed space  12 . 
     The water from the water tank  122 A can be pumped with a pump  124 A to the coil  22  via a water line  126 A. Alternatively, the water from the tank  122 A can be pumped to a cold water supply main configured to feed the cold water to the coil  22 . The reduced-temperature water can provide cooling to the coil  22  by transporting the water through the coil  22 . This design can eliminate the steps of moving hot supply air from the enclosed space  12  through the cooling unit  40 A and then back to the enclosed space  12 . Rather, the reduced temperature water produced by the unit  40 A can be delivered to the coil  22 . 
     After the water provides cooling to the coil  22 , the water can be recirculated back through the cooling unit  40 A. The water can be at an increased-temperature after providing cooling to the air in the plenum  14  because the rejected heat from the air has been picked up by the water. The increased-temperature water can be transported to the dry coil  112 A through a water line  128 A. Alternatively, the water can be transported to a hot water return configured to transport the increased-temperature water back to the dry coil  112 A. As provided above, the dry coil  112 A can cool the water using the scavenger air exiting the evaporative cooler  110 A. 
     The water can exit the dry coil  112 A at a reduced temperature through a water line  130 A, which can be split, using a bypass valve  132 A, into a water line  180 A to the evaporative cooler  110 A and a water line  129 A to the tank  122 A. The bypass valve  132 A can control how much of the water exiting the dry coil  112 A is sent to the evaporative cooler  110 A and how much is sent to the tank  122 A, depending on an operating mode of the cooling unit  40 A. 
     In an economizer mode, the bypass valve  132 A can be open such that all of the water from the dry coil  112 A can bypass the evaporative cooler  110 A and go directly to the tank  122 A. The economizer mode or winter mode can enable the cooling unit  40 A to cool the water using the scavenger air and dry coil  112 A, without having to run the evaporative cooler  110 A. In that situation, there may be no need for evaporation inside the evaporative cooler  110 A since the cold outdoor air (scavenger air) can pass through the dry coil  112 A and sufficiently cool the water. The dry coil  112 A can also be referred to herein as an economizer coil since it can be a primary cooling source for the water in the economizer mode. Three modes of operation are described further below for operating the cooling unit  40 A. 
     In another example, instead of the bypass valve  132 A controlling a flow between the evaporative cooler  110 A and the tank  122 A, the cooling unit  40 A can include two separate tanks or two separate tank sections. This is described below in reference to  FIG. 4B . 
     The pre-cooler  160 A, located upstream of the evaporative cooler  110 A, can be used to pre-condition the scavenger air entering the plenum  104 A, prior to passing the scavenger air through the evaporative cooler  110 A. The pre-cooler  160 A can be effective when the temperature of the water entering the pre-cooler  160 A is lower than the outdoor air dry bulb temperature. The pre-cooler  160 A can be used in typical summer conditions as well as in extreme summer conditions when the outdoor air is hot and humid. The pre-cooler  160 A can depress the outdoor air wet bulb temperature, thus pre-cooling the scavenger air and heating the water. The pre-cooler  160 A can provide more cooling potential in the evaporative cooler  110 A. 
     In an example as shown in  FIG. 4A , the pre-cooler  160 A can use water from the tank  122 A to condition the scavenger air. A pump  172 A can pump water from the tank  122 A to the pre-cooler  160 A through a water line  174 A. (Thus the reduced temperature water in the tank  122 A can leave the tank  122 A through two different water lines—line  126 A to the coil  22  and line  174 A to the pre-cooler  160 A.) In other examples, one water line and one pump can be used to deliver water out of the tank  122 A and a split valve can be used to control the delivery of water to the coil  22  and to the pre-cooler  160 A. 
     In an example, reduced temperature water is described above as being delivered to the coil  22  for providing liquid cooling to the air for the enclosed space  12 .  FIG. 4A  shows the line  126 A being directed to the coil  22  for delivery of the water to the coil  22  and the line  128 A being directed from the coil  22  for return of the water from the coil  22  to the dry coil  112 A. In other examples, instead of delivering water from the tank  122 A to the coil  22 , the reduced temperature water can be delivered to a liquid to liquid heat exchanger (LLHX) to use the water to reduce a temperature of a secondary coolant circulating through the LLHX. The secondary coolant can then be directed through a supply line to the coil  22  to provide cooling to the coil  22 , and the coolant can receive the heat rejected from the air in the plenum  14 , resulting in a temperature increase of the secondary coolant. The reduced temperature water can provide cooling to the increased temperature secondary coolant such that the secondary coolant can be delivered back to the coil  22  (via a return line from the coil  22 ) for continued cooling. Reference is made to International Application No. PCT/CA2016/050252, filed on Mar. 8, 2016, which is incorporated by reference herein and discloses an example of a design with a secondary coolant and LLHX. 
     Because the pre-cooler  160 A uses water from the tank  122 A as the cooling fluid in the pre-cooler  160 A, the design of the pre-cooler  160 A as shown in  FIG. 4A  can be referred to herein as a coupled pre-cooler. In other words, the pre-cooler  160 A is designed and configured to use a portion of the reduced-temperature water produced by the recovery coil  112 A or the evaporative cooler  110 A (and intended for cooling the air from the enclosed space  12 ) as the cooling fluid for the pre-cooler  160 A. In other examples illustrated and described herein, a cooling fluid circuit for the pre-cooler  160 A can be partially or wholly decoupled from the process circuit for the evaporative cooler  110 A and recovery coil  112 A. In that case, the pre-cooler  160 A can have an external cooling circuit partially or wholly separate from the reduced-temperature water produced by the evaporative cooler  110 A or recovery coil  112 A for process cooling. 
     In an example, and as shown in  FIG. 4A , the plenum  104 A can include two sets of bypass dampers—first dampers  176 A between the pre-cooler  160 A and the evaporative cooler  110 A, and second dampers  134 A between the evaporative cooler  110 A and the dry coil  112 A. The use of the bypass dampers  176 A and  134 A to direct the flow of scavenger air into the plenum  104 A can depend on the outdoor air conditions. Although the first and second bypass dampers  176 A and  134 A are each shown as having a pair of dampers on opposing sides of the plenum  104 A, it is recognized that one or both of the first  176 A and second  134 A bypass dampers can be a single damper on one side of the plenum  104 A. 
     The cooling unit  40 A can operate in at least three modes and selection of the mode can depend, in part, on the outdoor air conditions and the heat load of the enclosed space  12 . When the outdoor air is cold, the cooling unit  40 A can operate in a first mode, also referred to as an economizer mode, and the pre-cooler  160 A and the evaporative cooler  110 A can be bypassed. The scavenger air can enter the plenum  104 A through the dampers A 134  and pass through the dry coil  112 A. This can protect the evaporative cooler  110 A and avoid running the evaporative cooler  110 A when it is not needed. In the first mode or economizer mode, the scavenger air can be cool enough such that the dry coil  112 A can provide all cooling to the cooling fluid (water) delivered to the tank  122 A to provide cooling to the enclosed space  12 , without needing to operate the evaporative cooler  110 A. 
     In a second operating mode, which can also be referred to as a normal mode or an evaporation mode, the pre-cooler  160 A can be bypassed but the evaporative cooler  110 A can be used. The evaporation mode can operate during mild conditions, such as spring or fall, when the temperature or humidity is moderate, as well as during some summer conditions. The scavenger air may be able to bypass the pre-cooler  160 A, while still meeting the cooling load. The scavenger air can enter the plenum  104 A through dampers  176 A, and then can pass through the evaporative cooler  110 A and the dry coil  112 A. The cooling unit  40 A can modulate between a normal mode and an economizer mode to limit power consumption and based on outdoor air conditions. In another example, the dampers  176 A can be excluded from unit  40 A or the dampers  176 A may not be used in some cases. In such example, during the second operating mode, the scavenger air can enter through the inlet  106 A and pass through the pre-cooler  160 A but the pre-cooler  160 A can be turned off such that the water or cooling fluid is not circulating through the pre-cooler  160 A. 
     In a third operating mode, which can also be referred to as an enhanced mode or a super-evaporation mode, the cooling unit  40 A can run using both the pre-cooler  160 A and the dry coil  112 A. Under extreme conditions, or when the outdoor air is hot or humid, the cooling unit  40 A can provide pre-cooling to the scavenger air, using the pre-cooler  160 A, before the scavenger air enters the evaporative cooler  110 A. The pre-cooler  160 A can be used to improve the cooling power of the unit  40 A, allowing the evaporative cooler  110 A to achieve lower discharge temperatures at the outlet  118 A of the evaporative cooler  110 A. The pre-cooler  160 A can reduce or eliminate a need for supplemental mechanical cooling. 
     In summary, the three operating modes of the unit  40 A can include economizer mode, evaporative mode and super-evaporative mode. The economizer mode can also be referred to as a dry mode since the evaporative cooler  110 A is not operated in the economizer mode and thus the cooling unit  40 A has minimal to zero water consumption in the dry mode. In the economizer mode, the increased temperature water returning from the coil  22  can be cooled using scavenger air. The other two operating modes of the unit  40 A (evaporative and super-evaporative) can each also be referred to as a wet mode. 
     The water exiting the pre-cooler  160 A can be directed to the inlet  116 A of the evaporative cooler  110 A through a water line  178 A. A junction  181 A of the water lines  178 A and  180 A is shown in  FIG. 4A . It is recognized that the water lines  178 A and  180 A do not have to merge or join together prior to the inlet  116 A and two separate water lines can be in fluid connection with the inlet  116 A. 
     As provided above, the cooling fluid circuit of the pre-cooler  160 A of  FIG. 4A  can be coupled with the evaporative cooler  110 A since the cooling fluid for the pre-cooler  160 A comes from the water in the tank  122 A, which is produced by the evaporative cooler  110 A. The pre-cooler  160 A is further coupled in the design of  FIG. 4A  given that the cooling fluid, after exiting the pre-cooler  160 A, flows through the evaporative cooler  110 A. 
     The cooling unit  40 A can include a system controller  148 A to control operation of the cooling unit  40 A and control an amount of cooling provided from the cooling unit  40 A to the coil  22  (and thus to the enclosed space  12 ). The system controller  148 A can be manual or automated, or a combination of both. The system controller  148 A can be part of the system controller  50  of the system  10  (see  FIG. 1 ) or the system controller  148 A can be separate from the system  50 . 
     The cooling unit  40 A can be operated so that a temperature of the water in the tank  122 A can be equal to a set point temperature that can be constant or variable. In a cooling unit  40 A including a LLHX and a secondary coolant loop, the cooling unit  40 A can be operated so that a temperature of the coolant leaving the LLHX can be equal to a set point temperature that can be constant or variable. Controlling to the temperature of the coolant can be in addition to or as an alternative to controlling to the temperature of the water in the tank  122 A or the water leaving the tank  122 A. The set point temperature can be determined based in part on the cooling requirements of the enclosed space  12 . Water or coolant delivered to the coil  22  from the cooling unit  40 A can cool the air in an enclosed space or cool one or more electrical components that can be enclosed or open to the atmosphere. The cooling unit  40 A can be controlled to reduce overall water usage and power consumption, and increase heat rejection from the air in the enclosed space  12 . The system controller  148 A is described in further detail below. 
     Operation of the cooling unit  40 A can be aimed at increasing the portion of sensible heating between the water and the scavenger air and decreasing the portion of latent heating between the water and the scavenger air. Water evaporation inside the evaporative cooler  110 A can be optimized to minimize water consumption in the cooling unit  40 A by at least one of using cooling coils before or after the evaporative cooler  110 A and modulating a scavenger air flow rate through the cooling unit  40 A. A greater portion of the heat load can be rejected in the dry coil  112 A downstream of the evaporative cooler  110 A, if the water returning to the cooling unit  40 A is at a higher temperature. As a result, the scavenger air temperature at an outlet of the dry coil  112 A can be higher. The evaporative cooler  110 A can consume less water when the latent portion of the work performed in the evaporative cooler  110 A is reduced. 
     In an example, the cooling unit  40 A can be operated in an economizer mode in which the evaporative cooler  110 A is turned off and bypassed so long as the set point temperature of the water delivered to the tank  122 A can be met using the dry coil  112 A. However, if the water in the tank is at a temperature above the set point, the cooling unit  40 A can be operated in a normal mode which includes using the evaporative cooler  110 A to cool the water. Similarly, if the set point temperature cannot be achieved in the normal mode, an enhanced mode can include using the pre-cooler  160 A to condition the scavenger air before the scavenger air enters the evaporative cooler  110 A. 
     The reduced-temperature water from the recovery coil  112 A or evaporative cooler  110 A can be part of a cooling fluid circuit that can extend from the plenum  104 A and be delivered to the coil  22 . After the water provides cooling to the air passing through the coil  22 , the water can be recirculated through the cooling unit  40 A. One or both of the tank  122 A and pump  124 A can be located physically in the plenum  104 A, or one or both of the tank  122 A and pump  124 A can be physically located in the plenum  14  (see  FIG. 1 ). Alternatively, one or both of the tank  122 A and pump  124 A can be located in a structure separate from the plenum  104 A or plenum  14  and the enclosed space  12 . Each of the water lines  129 A,  130 A,  178 A and  180 A can be inside or outside the plenum  104 A, or partially inside and partially outside the plenum  104 A. A location of the other water lines relative to the plenum  104 A can depend in part on whether the tank  122 A is inside or outside of the plenum  104 A. 
     As provided above, the water line  126 A can transport the water from the tank  122 A to a cold water supply main, which can deliver the water to the coil  22 . In an example, the enclosed space  12  can utilize multiple cooling units  40 A for cooling and the cold water supply can be fluidly connected to each cooling unit  40 A. 
     The system controller  148 A can include hardware, software, and combinations thereof to implement the functions attributed to the controller herein. As provided above, the system controller  148 A can be part of the controller  50  (see  FIG. 1 ) or separate from the controller  50 . The description of the system controller  148 A below can also apply to the main controller  50  of the system  10 . The system controller  148 A can be an analog, digital, or combination analog and digital controller including a number of components. As examples, the controller  148 A can include ICB(s), PCB(s), processor(s), data storage devices, switches, relays, etcetera. Examples of processors can include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. Storage devices, in some examples, are described as a computer-readable storage medium. In some examples, storage devices include a temporary memory, meaning that a primary purpose of one or more storage devices is not long-term storage. Storage devices are, in some examples, described as a volatile memory, meaning that storage devices do not maintain stored contents when the computer is turned off. Examples of volatile memories include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories known in the art. The data storage devices can be used to store program instructions for execution by processor(s) of the controller  148 A. The storage devices, for example, are used by software, applications, algorithms, as examples, running on and/or executed by the controller  148 A. The storage devices can include short-term and/or long-term memory, and can be volatile and/or non-volatile. Examples of non-volatile storage elements include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. 
     The system controller  148 A can be configured to communicate with the cooling unit  40 A and components thereof via various wired or wireless communications technologies and components using various public and/or proprietary standards and/or protocols. For example, a power and/or communications network of some kind may be employed to facilitate communication and control between the controller  148 A and the cooling unit  40 A. In one example, the system controller  148 A can communicate with the cooling unit  40 A via a private or public local area network (LAN), which can include wired and/or wireless elements functioning in accordance with one or more standards and/or via one or more transport mediums. In one example, the unit  40 A can be configured to use wireless communications according to one of the 802.11 or Bluetooth specification sets, or another standard or proprietary wireless communication protocol. Data transmitted to and from components of the unit  40 A, including the controller  148 A, can be formatted in accordance with a variety of different communications protocols. For example, all or a portion of the communications can be via a packet-based, Internet Protocol (IP) network that communicates data in Transmission Control Protocol/Internet Protocol (TCP/IP) packets, over, for example, Category 5, Ethernet cables. 
     The system controller  148 A can include one or more programs, circuits, algorithms or other mechanisms for controlling the operation of the cooling unit  40 A. For example, the system controller  148 A can be configured to modulate the speed of the fan  114 A and/or control actuation of the valve  132 A to direct cooling fluid from the outlet of the dry coil  112 A to either the inlet  116 A of evaporative cooler  110 A or the tank  122 A. The system controller  148 A can also be configured to operate the unit  40 A in the three modes described above. 
     The cooling unit  40 A can maximize the cooling potential in the evaporative cooler  110 A and modulate the scavenger air through the plenum  104 A based on the outdoor air conditions. The economizer mode, for example, in winter, can provide a reduction in water usage and power consumption compared to conventional cooling systems. 
     The unit  40 A can utilize reduced-temperature water (from the dry coil  112 A or the evaporative cooler  110 A) to provide cooling to the enclosed space  12 . In an example, the enclosed space  12  can be a data center. In the systems described herein, less energy can be used to deliver the reduced-temperature water from the cooling unit  40 A to the data center, as compared to existing air designs. Such existing designs can include hot process air from the data center being delivered to the cooling system which can be configured as a larger unit for two air flow paths—the process air and the scavenger air. Thus more energy is used in such designs to move the hot process air from the data center to the cooling system and then condition the process air. Moreover, water has a higher thermal capacity than air; thus a lower flow rate of water can be used, compared to air, to reject a certain amount of heat directly from one or more electrical components in the data center (or other components needing cooling) or from the air in the data center. 
       FIG. 4B  illustrates another example cooling unit  40 B for providing liquid cooling to the enclosed space  12  via the coil  22 . The cooling unit  40 B can be similar in many aspects to the cooling unit  40 A of  FIG. 4A  and can include a pre-cooler  160 B, an evaporative cooler  110 B, a dry coil  112 B and a fan  114 B, all of which can be arranged within a scavenger plenum  104 B as described above for the unit  40 A. However, in contrast to the unit  40 A of  FIG. 4A , the cooling unit  40 B can have two separate water tanks, as well as an additional pump and flow path to the evaporative cooler  110 B. As described below, the design in  FIG. 4B  can allow for additional operating modes of the unit  40 B, as compared to the unit  40 A. The unit  40 B can include a system controller  148 B that can be similar to the system controller  148 A described above for the unit  40 A. 
     The cooling unit  40 B can include a first tank  122 B and a second tank  123 B. The first tank  122 B can be generally configured to provide a reduced temperature cooling fluid to the coil  22  and the second tank  123 B can be generally configured as the water supply for the evaporative cooler  110 B. However, each of tanks  122 B and  123 B can receive water from the evaporative cooler  110 B and the recovery coil  112 B, depending on an operating mode of the unit  40 B. 
     In an example, the first tank  122 B can be fluidly connected to the coil  22  such that the reduced-temperature water can flow from the tank  122 B to the coil  22  through a water line  126 B using a pump  124 B, as configured with the unit  40 A. In another example, water can drain out of the tank  122 B to another external collection reservoir, where it can then be pumped to the coil  22 . This can eliminate a supply pump ( 124 B) inside the plenum  104 B. 
     The increased-temperature water leaving the coil  22  can be returned to the recovery coil  112 B (via a water line  128 B) in order to cool the increased temperature water, which can then exit the recovery coil  112 B through a water line  130 B. The flow path into and out of the recovery coil  112 B can be the same as in the unit  40 A. However, a bypass valve  132 B can control distribution of the reduced-temperature water either to the first tank  122 B through a water line  129 B or to the second tank  123 B through a water line  180 B. This is different from the unit  40 A in which the bypass valve  132 A can direct water in the water line  130 A to the evaporative cooler  110 A directly, rather than to the second tank  123 B as shown in  FIG. 4B . As provided below, a position of the valve  132 B can depend on the operating mode of the unit  40 B. 
     The second tank  123 B can provide water to an inlet  116 B of the evaporative cooler  110 B using a pump  154 B and a water line  121 B. The separation of the two tanks  122 B and  123 B in the design of  FIG. 4B  can replace the junction  181 A of the design of  FIG. 4A . Moreover, the design of  FIG. 4B  having the two tanks  122 B and  123 B can facilitate operation of the evaporative cooler  110 B in an evaporation mode and an adiabatic mode, as described further below. 
     After flowing through the evaporative cooler  110 B, the water can exit the evaporative cooler  110 B through a water line  120 B. A bypass valve  182 B can control the distribution of water from the evaporative cooler  110 B to the first tank  122 B (via a water line  135 B) and the second tank  123 B (via a water line  131 B). The valve  182 B is not included in the design of  FIG. 4A  and is described further below in reference to the operating modes of the unit  40 B. 
     As provided above in reference to the unit  40 A, the pre-cooler  160 B can selectively be used depending on the outdoor air conditions and an operating mode of the unit  40 A. Similar to the design of the unit  40 A, the pre-cooler  160 B can receive reduced-temperature water from the first tank  122 B using a pump  172 B and water line  174 B. The water can exit the pre-cooler  160 B at an increased temperature. In contrast to the design of the unit  40 A, the increased-temperature water from the pre-cooler  160 B can be directed to the second tank  123 B through a water line  178 B, rather than through the evaporative cooler  110 B. Similar to the design of the unit  40 A, the pre-cooler  160 B of the unit  40 B, as shown in  FIG. 4B  can have a coupled design and the cooling fluid for the pre-cooler  160 B can come from the first tank  122 B. In other examples, the pre-cooler  160 B can be partially or fully decoupled. 
     In an example, the unit  40 B can operate in the three modes described above for the unit  40 A, but the unit  40 B can also operate in at least two additional modes as compared to the unit  40 A. 
     In an economizer mode (first mode of the unit  40 A), only the recovery coil  112 B is used to cool the water or other cooling fluid that provides liquid cooling to the coil  22  for the enclosed space  12 . The cold water exiting the recovery coil  112 B can pass through the three-way valve  132 B which can divert essentially all of the water in the water line  130 B to the first tank  122 B. The first tank  122 B can supply the cold water to the coil  22  using the pump  124 B. In the economizer mode, the pumps  154 B and  172 B can be turned off since the evaporative cooler  110 B and pre-cooler  160 B are not being used. The scavenger air can enter the plenum  104 B through the bypass dampers  134 B. 
     The unit  40 B can operate in an adiabatic mode that can considered to be between the economizer mode and the evaporation mode (second mode of the unit  40 A) in terms of the energy usage and the cooling requirements needed by the enclosed space  12 . The bypass valve  132 B can be in the same position and the delivery of cold water to the coil  22  can be the same as described above in the economizer mode. In the adiabatic mode, the evaporative cooler  110 B can be configured to circulate water from the second tank  123 B through the evaporative cooler  110 B in a closed fluid circuit. The pump  154 B can be on and water can be provided through the water line  121 B to the inlet  116 B of the evaporative cooler. The bypass valve  182 B can be positioned such that essentially all of the water exiting the evaporative cooler  110 B at the outlet  118 B can be directed to the second tank  123 B. Thus the flow of water through each of the evaporative cooler  110 B and the recovery coil  112 B can be separate from one another via the two tanks  122 B and  123 B. In this adiabatic mode, the tank  123 B can be essentially dedicated to the recovery coil  112 B and the tank  122 B can be essentially dedicated to the evaporative cooler  110 B. 
     During operation of the evaporative cooler  110 B in the adiabatic mode, a temperature of the water (or other cooling fluid) can remain generally constant or have minimal temperature fluctuations. The outdoor air conditions can be such that sufficient conditioning of the scavenger air stream can be provided by the water in the tank  123 B through recirculation of the water in the closed fluid circuit. As the scavenger air passes through the evaporative cooler  110 B, it can be cooled adiabatically such that its temperature can be reduced, but its humidity level can increase, while its overall enthalpy can remain constant. The reduced-temperature air can be supplied to the recovery coil  112 B and the recovery coil  112 B can supply water at the required temperature set point. This adiabatic process or mode can significantly reduce or minimize water consumption by the cooling unit  40 B and can be used when operation of the unit  40 B in the economizer mode is not able to reach the set point temperature for the cold water supply to the coil  22 . 
     In an evaporation mode (second mode of the unit  40 A), the evaporative cooler  110 B can be switched over from operating adiabatically. A position of the bypass valve  132 B can be changed to direct water from the recovery coil  112 B to the second tank  123 B. Similarly, a position of the bypass valve  182 B can be changed to direct water from the evaporative cooler  110 B to the first tank  122 B. An equalization valve  137 B can be located between the two tanks  122 B and  123 B. The valve  137 B can be closed during the economizer and adiabatic modes, and can be opened in the evaporation mode to stabilize the tank levels. The evaporation mode in the unit  40 B can be similar to that described above for the unit  40 A in that the fluid circuit through the evaporative cooler  110 B can be in fluid connection with the fluid circuit through the recovery coil  112 B. 
     In an example, in the evaporation mode, essentially all or a majority of the water from the recovery coil  112 B can be redirected to the second tank  123 B and essentially all or a majority of the water from the evaporative cooler  110 B can be redirected to the first tank  122 B. In another example, in the evaporation mode, the distribution to each tank  122 B and  123 B can be split for one or both of the water from the evaporative cooler  110 B and the recovery coil  112 B. In an example, instead of the equalization valve  137 B, the tanks  122 B and  123 B can be separated by a dividing wall and a height of the wall can be lowered such that the wall can function as a weir. If one tank level rises too high, the water can spill over the weir into the other tank. 
     During operation in the adiabatic and evaporation modes, the scavenger air can enter the plenum  104 B at an inlet  106 B and the pre-cooler  160 B can be off. In another example, the plenum  104 B can include bypass dampers downstream of the pre-cooler  160 B and upstream of the evaporative cooler  110 B to bypass the pre-cooler  160 B and direct the scavenger air into the evaporative cooler  110 B. 
     In an enhanced mode or a super-evaporation mode (third mode of the unit  40 A), the pump  172 B can be turned on to direct water through the pre-cooler  160 B. The cold water for the pre-cooler  160 B can come from the first tank  122 B. After exiting the pre-cooler  160 B at an increased-temperature, the water can be delivered to the second tank  123 B. Similar to the unit  40 A, as shown in  FIG. 4B , the pre-cooler  160 B can have a coupled design within the cooling unit  40 B. In other examples, the pre-cooler  160 B can have a partially decoupled or fully decoupled design. 
     The unit  40 B can be controlled to run at the lowest operating mode (in terms of energy and water usage) that is sufficient for meeting the liquid cooling requirements for the enclosed space  12  in the indirect mode. The design of the unit  40 B can allow for an additional mode that can include operating the evaporative cooler  110 B adiabatically and running the pre-cooler  160 B. This mode can be considered somewhat of a hybrid mode that is generally between the adiabatic mode and the enhanced mode. The four operating modes of the unit  40 B can include an economizer mode, adiabatic mode, evaporative mode and super-evaporative mode. As described in reference to the unit  40 A, the economizer mode for the unit  40 B can be referred to as a dry mode. The other three modes can include operating the evaporative cooler  110 B and thus each of the three modes can be referred to as a wet mode. 
     It is recognized that the cooling units  40 A and  40 B of  FIGS. 4A and 4B , respectively, are two examples of a cooling system for produced a reduced temperature cooling fluid (for example, cold water) that can be used to provide liquid cooling to the air from the enclosed space  12  when the system  10  of  FIG. 1  is operating in an indirect mode. Other designs of a cooling system that includes an evaporative cooler (and can operate in a dry mode and a wet mode) can be used in addition to or as an alternative to the cooling units  40 A and  40 B of  FIGS. 4A and 4B . 
     In an example, the hybrid system  10  of  FIG. 1  can operate in seven modes which are listed in Table 2 below. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Operating Modes for Hybrid System of FIG. 1 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Cooling Unit: 
                   
               
               
                 Number 
                 System Mode 
                 On or Off? 
                 Cooling Unit Mode 
               
               
                   
               
               
                 1 
                 Indirect 
                 ON 
                 Economizer 
               
               
                 2 
                 Indirect 
                 ON 
                 Adiabatic 
               
               
                 3 
                 Indirect 
                 ON 
                 Evaporative 
               
               
                 4 
                 Indirect 
                 ON 
                 Super-Evaporative 
               
               
                 5 
                 Direct 
                 ON 
                 Any wet or dry mode 
               
               
                 6 
                 Direct 
                 OFF 
                 N/A 
               
               
                 7 
                 Mixed 
                 OFF 
                 N/A 
               
               
                   
               
            
           
         
       
     
       FIG. 5  is a psychometric chart illustrating the conditions for each of the seven modes in Table 2 for the hybrid system  10 . The numbers in Table 2 for each mode correspond with the numbers in  FIG. 5 . As demonstrated by Table 2, the operating modes of the system  10  can include 100% indirect ( FIG. 1A ), 100% direct with cooling ( FIG. 1B ), 100% direct without cooling ( FIG. 1C ), and mixed ( FIG. 1D ); for the 100% indirect mode, there are four sub-modes. Each of the four sub-modes for indirect cooling correspond to the four operating modes of the cooling unit  40 B of  FIG. 4B —economizer, adiabatic, evaporative and super-evaporative. 
     The number of sub-modes of the indirect cooling mode can be a function of the design of the cooling unit  40  of the system  10 . If the cooling unit  40 B is replaced in the hybrid system  10  with the cooling unit  40 A of  FIG. 1A , the hybrid system  10  can have three-sub modes of indirect cooling, and thus a total of six modes, instead of the seven modes listed above in Table 2. 
     As provided below, in mode #5 (direct with cooling), any of the sub-modes for indirect cooling mode can be used. In other words, in mode #5, 100% outdoor air can be delivered to the enclosed space  12  but the outdoor air can be cooled using the coil  22  (see  FIG. 1B ); the reduced temperature cooling fluid used in the coil  22  can be produced under a dry mode or a wet mode and thus under any of the sub-modes described herein for indirect cooling—economizer, adiabatic, evaporative, and super-evaporative. 
     Referring back to  FIG. 5 , the psychometric chart shows what mode can be selected for operation of the hybrid system  10 . The selection can depend on the outdoor air conditions—including a dry bulb temperature, a wet bulb temperature, a humidity ratio, and relative humidity. In an example, selection of the operating mode can also depend on additional factors, such as, for example, outdoor air quality and restrictions on water usage. 
     When the ambient humidity levels permit, the hybrid system  10  can operate in modes #5, #6 or #7. In modes #6 or #7, the external cooling unit  40  can be off. In mode #6, 100% outdoor air can be delivered to the enclosed space  12 , as is, and all of the hot return air exiting the enclosed space  12  through the return air duct  26  can be outlet as exhaust air. (See  FIG. 1C .) The outdoor air conditions in mode #6 can be such that the outdoor air can replace the hot return air in the enclosed space  12 , without reducing a temperature of the air or adjusting the humidity of the air. In mode #7, return air from the enclosed space  12  can mix with outdoor air. (See  FIG. 1D .) The mixed mode can be used when the dry bulb temperature of the outdoor air is low or the relative humidity is high, and the mixed air stream of outdoor air and return air can maintain the humidity levels inside the enclosed space  12  without having to humidify the outdoor air before the outdoor air is delivered to the enclosed space  12 . 
     In modes #6 and #7, there is essentially no conditioning of the air being delivered to the enclosed space  12 . Thus modes #6 and #7 can contribute to the efficiency in operating the system  10 , in terms of at least energy and water. 
     In mode #5, the cooling unit  40  can be on. The temperature of the outdoor air can be higher than an acceptable range for the enclosed space  12  but the humidity of the outdoor air can be within an acceptable range. Thus the cooling unit  40  can be used to provide reduced temperature cooling fluid to the coil  22  and reduce the temperature of the outdoor air passing through the coil  22  (see  FIG. 1B ). This can reduce a cooling load on the coil  22 , as compared to if the system  10  were operating in modes #3 or #4 in which evaporative cooling in the unit  40  is used in the indirect mode to condition the return air from the enclosed space. 
     If there is any risk of degrading the air quality inside the enclosed space  12  by using outdoor air, the hybrid system  10  can switch from mode #6 or #7 to one of the indirect cooling modes (modes #1-4), regardless of the temperature and humidity of the outdoor air. As such, the air quality inside the enclosed space  12  can be maintained within desired levels, regardless of the air quality of the outdoor air. 
     If the system  10  switches from either of modes #6 or #7 to an indirect cooling mode (due to air quality rather than a change in humidity or temperature), the system  10  can likely switch to mode #1 in which the cooling unit  40  operates in a dry mode or economizer mode. This is exemplified in  FIG. 5 , given the proximity of modes #6 and #7 to mode #1. In other words, the outdoor air conditions that equate to operating in modes #6 and #7 can be similar or overlap with the outdoor air conditions that equate to operating in mode #1. In mode #1, the outdoor air conditions can be sufficient to provide liquid cooling to the return air without having to use the evaporative cooler in the cooling unit  40  and without having to humidify the return air. Similar to modes #6 and #7, mode #1 can minimize energy or water consumption in operating the system  10 . 
     If the system  10  is operating in mode #6 or #7 and the humidity levels of the outdoor air decrease below a predetermined limit, the system  10  can switch to mode #1 in order to maintain the humidity level of the air in the enclosed space  12  within an acceptable range. Operation of the system  10  in mode #1 can address the excessive humidification requirements associated with direct-air optimization (DAO) systems when such systems are operating in cold and dry air conditions. These types of DAO systems can commonly consume excessive amounts of water in the cold/dry operating months in order to maintain the air in the enclosed space  12  within an acceptable range of conditions. Rather than use outdoor air and humidify the outdoor air before delivering the outdoor air to the enclosed space, mode #1 can enable the system  10  to efficiently condition the return air from the enclosed space by operating in a dry mode or an economizer mode of the external cooling unit  40 . 
     On the other hand, if the outdoor air conditions are such that the humidity levels exceed the acceptable range, the system  10  can operate in an indirect mode to condition the return air, rather than dehumidify the outdoor air. This can eliminate the need for dehumidification of the outdoor air, as is commonly required in DAO systems. Such dehumidification capabilities in DAO systems can result in oversizing of the system to account for sensible and latent loads of the outdoor air. 
     The indirect cooling modes #2-4 can each be defined as a wet mode since each of modes #2-4 can include operation of the evaporative cooler in the cooling unit  40 . Mode #2 can involve adiabatic operation of the evaporative cooler. (See description of the cooling unit  40 B of  FIG. 4B .) Mode #3 can involve evaporative cooling of the evaporative cooler in the cooling unit  40 . Mode #4 can involve super-evaporative cooling in which a pre-cooler located upstream of the evaporative cooler can be used to pre-condition the scavenger air prior to the scavenger air passing through the evaporative cooler. In an example, all of the cooling requirements for the enclosed space  12  can be met by one of the wet modes #2-4 without requiring the addition of mechanical chillers. The particular mode selected from wet modes #2-4 can be based on providing enough cooling to the reduced temperature cooling fluid such that the cooling fluid can sufficiently condition the air passing through the coil  22 , while minimizing energy and water usage. 
     In summary, a direct mode (with or without cooling) can be used when ambient conditions permit and 100% indirect cooling can be used when ambient conditions are not sufficient or outdoor air quality is not acceptable. A mixed mode can be used when the outdoor air is too cold or the relative humidity is too high, but the humidity range is acceptable. The indirect cooling modes of the system  10  can eliminate the need to humidify or dehumidify the outdoor air stream before supplying outdoor air to the enclosed space  12 . The particular sub-mode of indirect cooling can be determined to meet the cooling load for the enclosed space, but minimize energy and water consumption. 
     The hybrid system  10  of the present application focuses on the capability to operate in a direct mode (with or without cooling), an indirect cooling mode, and a mixed mode. It is recognized that the external cooling unit  40  utilized in the indirect mode can have additional or alternative components, or a different configuration, to what is shown in  FIGS. 4A and 4B  for use in combination with the evaporative cooler of the external cooling unit  40 . Although four sub-modes for indirect cooling are described herein (one dry mode; three wet modes), it is recognized that the external cooling unit  40  can exclude one or more of these wet modes. As provided above, the benefits of the system  10  can include the ability to switch between modes as needed or desired, for example, to save water or to preserve air quality inside the enclosed space  12 . 
       FIG. 6  is a flow chart illustrating a process  200  for determining an operating mode of the hybrid system  10 . An initial inquiry at  202  can include whether ambient conditions are within allowed indoor air conditions. If yes at  202 , at  204  an inquiry can be whether there is a risk for indoor air quality (IAQ) degradation. If no at  204 , an operating mode at  206  can be 100% direct without cooling (mode #6 above). If yes at  204 , at  208  the inquiry can be whether ambient conditions are within indirect economizer mode limits. If yes at  208 , an operating mode at  210  can be 100% indirect—economizer (mode #1). If no at  208 , at  212  an inquiry can be whether ambient conditions are within adiabatic mode limits. If yes at  212 , an operating mode at  214  can be 100% indirect—adiabatic (mode #2). If no at  212 , at  216  an inquiry can be whether ambient conditions are within evaporative mode limits. If yes at  216 , an operating mode at  218  can be 100% indirect—evaporative (mode #3). If no at  216 , an operating mode at  220  can be 100% indirect—super-evaporative (mode #4). 
     Referring back to  202 , if the answer is no (i.e. ambient conditions are not within allowed indoor air conditions), at  222  an inquiry can be whether ambient temperature is less than or equal to an allowed indoor air temperature. If yes at  222 , an inquiry at  224  can be whether ambient humidity is within allowed indoor air conditions. If yes at  224 , an inquiry at  226  can be whether there is a risk for indoor air quality (IAQ) degradation. If no at  226 , an operating mode at  228  can be mixed (mode #7) and the cooling unit  40  can be off. Referring back to  224 , if the answer is no (i.e. ambient humidity is not within allowed indoor air conditions), at  230 , an inquiry can be whether ambient conditions are within economizer mode limits. Note this is the same inquiry as at  208 . If yes at  230 , an operating mode at  232  can be 100% indirect—economizer (mode #1). If the answer is no at  230 , an inquiry at  212  can be whether the ambient conditions are within adiabatic mode limits. The answer at  212  determines whether the particular indirect operating mode is adiabatic, evaporative or super-evaporative, as described above in reference to  214 - 220 . 
     Referring back to  222 , if the answer is no (i.e. ambient temperature is not less than or equal to the allowed indoor air temperature), an inquiry at  234  can be whether the return air temperature is greater than the ambient air temperature. If no at  234 , the next inquiry can be at  212  which is described above and determines whether the particular indirect operating mode is adiabatic, evaporative or super-evaporative. If yes at  234 , an inquiry at  236  can be whether the ambient humidity is within allowed indoor air conditions. If no at  236 , the next inquiry can be at  212 . If yes at  236 , an inquiry at  238  can be whether there is a risk for indoor air quality (IAQ) degradation. If yes at  238 , the next inquiry can be at  212 . If no at  238 , an operating mode at  240  can be 100% direct with cooling (mode #5). The sub-mode of the cooling unit  40  under mode #5 can be the dry mode or any of the wet modes (adiabatic, evaporative or super-evaporative). The sub-mode can be determined based on ambient conditions. 
     It is recognized that a control system, including but not limited to the controllers  50 ,  148  and  148 B described above, can be used to determine the operating modes. It is recognized that the process  200  can vary from what is shown in  FIG. 6 . For example, the process  200  does not have to follow in the specific sequence presented in  FIG. 6 . Moreover, it is recognized that additional or alternative inquires or decision points can be used to determine an operating mode for the hybrid system  10 . The flow chart for the process  200  can also depend on the operating modes of the cooling unit  40  that is used to provide indirect cooling in the hybrid system  10 . 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, the code can be tangibly stored on one or more volatile or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     Examples, as described herein, may include, or may operate on, logic or a number of components, modules, or mechanisms. Modules may be hardware, software, or firmware communicatively coupled to one or more processors in order to carry out the operations described herein. Modules may hardware modules, and as such modules may be considered tangible entities capable of performing specified operations and may be configured or arranged in a certain manner. In an example, circuits may be arranged (e.g., internally or with respect to external entities such as other circuits) in a specified manner as a module. In an example, the whole or part of one or more computer systems (e.g., a standalone, client or server computer system) or one or more hardware processors may be configured by firmware or software (e.g., instructions, an application portion, or an application) as a module that operates to perform specified operations. In an example, the software may reside on a machine-readable medium. In an example, the software, when executed by the underlying hardware of the module, causes the hardware to perform the specified operations. Accordingly, the term hardware module is understood to encompass a tangible entity, be that an entity that is physically constructed, specifically configured (e.g., hardwired), or temporarily (e.g., transitorily) configured (e.g., programmed) to operate in a specified manner or to perform part or all of any operation described herein. Considering examples in which modules are temporarily configured, each of the modules need not be instantiated at any one moment in time. For example, where the modules comprise a general-purpose hardware processor configured using software; the general-purpose hardware processor may be configured as respective different modules at different times. Software may accordingly configure a hardware processor, for example, to constitute a particular module at one instance of time and to constitute a different module at a different instance of time. Modules may also be software or firmware modules, which operate to perform the methodologies described herein. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     The present application provides for the following exemplary embodiments or examples, the numbering of which is not to be construed as designating levels of importance: 
     Example 1 provides a system to condition air for an enclosed space, the system comprising: a plenum for supplying air to the enclosed space, the plenum having an outside air inlet, a return air inlet, and a supply air outlet in fluid communication with the enclosed space, wherein the outdoor air inlet and the return air inlet selectively open and close such that an amount of outdoor air and an amount of return air supplied to the enclosed space varies and depends on an operating mode of the system; a cooling unit comprising an evaporative cooler and located external to the plenum and the enclosed space, the cooling unit having a first cooling fluid circuit configured to selectively circulate a first cooling fluid to reduce a temperature of the first cooling fluid, the cooling unit configured to operate in a dry mode and a wet mode; a coil disposed inside the plenum and having a second cooling fluid circuit configured to selectively circulate a second cooling fluid through the coil to condition air passing through the coil, depending on the operating mode of the system, wherein the first cooling fluid exiting the cooling unit reduces a temperature of the second cooling fluid prior to circulating the second cooling fluid through the coil; and a return air duct in fluid communication with the enclosed space, the return air duct configured to deliver return air from the enclosed space to at least one of the return air inlet and an exhaust air outlet, depending on the operating mode of the system. 
     Example 2 provides the system of Example 1 optionally configured such that the operating mode of the system comprises an indirect mode in which the outdoor air inlet and the exhaust air outlet are closed, the return air inlet is open, and the cooling unit is on, and wherein return air from the enclosed space is directed through the plenum and conditioned by the coil before being delivered back to the enclosed space as supply air. 
     Example 3 provides the system of Example 1 and/or 2 optionally configured such that the cooling unit comprises a scavenger air plenum configured to direct outdoor air from an inlet to an outlet of the scavenger air plenum, and wherein the evaporative cooler is disposed inside the scavenger air plenum. 
     Example 4 provides the system of Example 3 optionally configured such that the cooling unit comprises a dry coil disposed inside the scavenger air plenum. 
     Example 5 provides the system of Example 4 optionally configured such that the dry coil is disposed downstream of the evaporative cooler. 
     Example 6 provides the system of Example 4 and/or 5 optionally configured such that the outdoor air bypasses the evaporative cooler in the dry mode and passes through the dry coil such that, in the dry mode, the outdoor air is sufficient to reduce a temperature of the first cooling fluid flowing through the dry coil. 
     Example 7 provides the system of any of Examples 2-6 optionally configured such that the wet mode comprises an adiabatic mode in which an evaporative cooling fluid flowing through the evaporative cooler is contained within a closed fluid circuit of the evaporative cooler, and an evaporative mode in which the evaporative cooling fluid flowing through the evaporative cooler is in fluid connection with the first cooling fluid of the cooling unit. 
     Example 8 provides the system of any of Examples 2-7 optionally configured such that the cooling unit comprises a pre-cooler disposed inside the scavenger air plenum upstream of the evaporative cooler. 
     Example 9 provides the system of Example 8 optionally configured such that the wet mode comprises a super-evaporative mode in which the pre-cooler circulates a pre-cooling fluid to selectively condition the outdoor air, prior to passing the outdoor air through the evaporative cooler. 
     Example 10 provides the system of any of Examples 1-9 wherein the operating mode of the system comprises a direct mode in which the outdoor air inlet and the exhaust air inlet are open, the cooling unit is off, and wherein outdoor air enters the plenum and is delivered to the enclosed space as supply air. 
     Example 11 provides the system of Example 10 optionally configured such that the direct mode includes a first direct mode in which the cooling unit is off and the outdoor air is delivered to the enclosed space without reducing a temperature of the outdoor air. 
     Example 12 provides the system of Example 11 optionally further comprising a bypass duct having a bypass inlet located upstream of the coil and a bypass outlet located downstream of the coil, wherein the bypass duct is configured to selectively direct air flowing through the plenum into the bypass duct and return the air to the plenum downstream of the coil such that the air bypasses the coil when the system is operating in the first direct mode. 
     Example 13 provides the system of Example 11 optionally configured such that when the system is operating in the first direct mode, the coil is not operational and the outdoor air passes through the non-operational coil prior to be delivered to the enclosed space. 
     Examples 14 provides the system of any of Examples 10-13 optionally configured such that the direct mode includes a second direct mode in which the cooling unit is on and the outdoor air passes through the coil to reduce a temperature of the outdoor air prior to delivering the outdoor air to the enclosed space. 
     Example 15 provides the system of Example 14 optionally configured such that the cooling unit operates in a plurality of modes, and a selected mode from the plurality of modes depends on outdoor air conditions. 
     Example 16 provides the system of any of Examples 10-15 optionally configured such that the direct mode includes a mixed mode in which the cooling unit is off, the return air inlet is open, and wherein a first portion of the return air flowing through the return air duct is exhausted to outside through the exhaust air outlet and a second portion of the return air flowing through the return duct enters the plenum at the return air inlet and mixes with the outdoor air for delivery of a mixed air stream to the enclosed space. 
     Example 17 provides the system of Example 16 optionally configured such that the return air mixes with the outdoor air upstream of a bypass inlet of a bypass duct, the bypass duct configured to selectively direct the mixed air stream into the bypass duct to bypass the coil, and wherein the mixed air stream exists the bypass duct through a bypass outlet in fluid connection with the plenum. 
     Example 18 provides the system of Example 16 optionally configured such that the coil is not operational in the mixed mode, and wherein the outdoor air and the return air mixes upstream of the coil, and the mixed air stream passes through the non-operational coil prior to be delivered to the enclosed space. 
     Example 19 provides the system of any of Examples 10-15 optionally configured such that the return air inlet is closed, and wherein essentially all of the return air flowing through the return duct is exhausted to outside. 
     Example 20 provides the system of any of Examples 1-19 optionally configured such that the first cooling fluid and the second cooling fluid are the same and the first cooling fluid circuit is fluidly connected to the second cooling fluid circuit. 
     Example 21 provides the system of any of Examples 1-19 optionally configured such that the first cooling fluid circuit and the second cooling fluid circuit are separate from each other, and the system comprises: a liquid to liquid heat exchanger configured to circulate the first and second cooling fluids such that the first cooling fluid reduces a temperature of the second cooling fluid, prior to circulating the second cooling fluid through the coil. 
     Example 22 provides the system of any of Examples 1-21 optionally configured such that the reduced temperature cooling fluid is water. 
     Example 23 provides the system of any of Examples 1-22 optionally configured such that the evaporative cooler is a liquid-to-air membrane energy exchanger (LAMEE). 
     Example 24 provides the system of any of Examples 1-23 optionally configured such that the enclosed space is a data center. 
     Example 25 provides the system of any of Examples 1-24 optionally further comprising at least one filter disposed inside the plenum at a location upstream of the coil. 
     Example 26 provides the system of any of Examples 1-25 optionally further comprising a fan downstream of the coil and upstream of the supply air outlet. 
     Example 27 provides the system of Example 26 optionally configured such that the fan comprises a fan array of multiple fans. 
     Example 28 provides a method of conditioning air for an enclosed space, the method comprising directing air through a plenum, the air including outdoor air, return air from the enclosed space, or a combination thereof; delivering the air from the plenum to the enclosed space as supply air; selectively operating an external cooling unit having a first cooling fluid circuit configured to circulate a first cooling fluid, the external cooling unit located external to the plenum and the enclosed space, the external cooling unit comprising an evaporative cooler and configured to operate in a dry mode and a wet mode to reduce a temperature of the first cooling fluid; and selectively directing a second cooling fluid through a coil disposed inside the plenum to provide liquid cooling to air directed through the coil, wherein the second cooling fluid is fluidly connected to the first cooling fluid or the second cooling fluid is cooled by the first cooling fluid prior to being directed through the coil. 
     Example 29 provides the method of Example 28 optionally further comprising determining an operating mode of the system based on an ambient temperature and humidity, the operating mode comprising: a direct mode in which an outdoor air inlet of the plenum is open and outdoor air enters the plenum, an exhaust air outlet is open and a portion of the return air from the enclosed space is exhausted to outside; an indirect mode in which the outdoor air inlet and the exhaust air outlet are closed, the external cooling unit is on, and return air from the enclosed space is conditioned by the second cooling fluid flowing through the coil; and a mixed mode in which the outdoor air inlet and the exhaust air outlet are each at least partially open, the cooling unit is off and return air from the enclosed space mixes with outdoor air to create a mixed air stream that is delivered to the enclosed space as supply air. 
     Example 30 provides the method of Example 29 optionally further comprising: diverting the air in the plenum through a bypass duct to bypass the coil, when the system is operating in the direct mode or mixed mode. 
     Example 31 provides the method of Example 30 optionally configured such that diverting the air through the bypass duct includes directing the air through a bypass inlet upstream of the coil and directing the air through a bypass outlet downstream of the coil. 
     Example 32 provides the method of any of Examples 29-31 optionally configured such that the direct mode comprises a first direct mode in which the external cooling unit is off and the outdoor air is delivered to the enclosed space without reducing a temperature of the outdoor air. 
     Example 33 provides the method of any of Examples 29-32 optionally configured such that the direct mode comprises a second direct mode in which the external cooling unit is on and the outdoor air passes through the coil to reduce a temperature of the outdoor air prior to delivering the outdoor air to the enclosed space. 
     Example 34 provides the method of Example 33 optionally configured such that selectively operating the external cooling unit in the second direct mode comprises operating the external cooling unit in a plurality of modes, and a selected mode from the plurality of modes depends on outdoor air conditions. 
     Example 35 provides the method of any of Examples 28-34 optionally configured such that the external cooling unit comprises a scavenger air plenum configured to receive an outdoor air stream, and wherein the evaporative cooler is disposed inside the scavenger air plenum. 
     Example 36 provides the method of Example 35 optionally configured such that the evaporative cooler is not operational in the dry mode of the external cooling unit, and wherein the external cooling unit comprises a dry coil arranged in the scavenger air plenum downstream of the evaporative cooler. 
     Example 37 provides the method of Example 36 optionally configured such that selectively operating the external cooling unit comprises: directing the first cooling fluid through the dry coil to cool the first cooling fluid with the outdoor air stream. 
     Example 38 provides the method of Example 36 and/or 37 optionally configured such that selectively operating the external cooling unit in the wet mode comprises: operating the external cooling unit in an adiabatic mode, wherein an evaporative cooling fluid flowing through the evaporative cooler is separate from the first cooling fluid. 
     Example 39 provides the method of any of Examples 36-38 optionally configured such that operating the external cooling unit in the wet mode comprises: operating the external cooling unit in an evaporative mode, wherein the first cooling fluid circulates through the evaporative cooler and the dry coil. 
     Example 40 provides the method of any of Examples 36-39 optionally configured such that the external cooling unit comprises a pre-cooler arranged in the scavenger air plenum upstream of the evaporative cooler, and wherein operating the external cooling unit in the wet mode comprises: directing the outdoor air stream through the pre-cooler to condition the outdoor air; and reducing a temperature of the first cooling fluid using the evaporative cooler and the dry coil. 
     Example 41 provides the method of any of Examples 28-40 optionally configured such that the first cooling fluid and the second cooling fluid are different, and the method further comprises reducing a temperature of the second cooling fluid in a liquid to liquid heat exchanger (LLHX) using the reduced temperature first cooling fluid exiting the external cooling unit. 
     Example 42 provides a system or method of any one or any combination of Examples 1-41, which can be optionally configured such that all steps or elements recited are available to use or select from. 
     Various aspects of the disclosure have been described. These and other aspects are within the scope of the following claims.