Abstract:
A building air conditioning system control scheme to optimize water terminal capacity and create energy savings by utilizing a building management system signal to run in a mode that maximizes the conditions of the outside air to condition their local zones and potentially require no thermal pre-treatment of outside air by an air handling unit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to another patent application identified by Attorney docket number 210-1050PCT. Both are subject to assignment to Carrier Corporation and each is being filed on an even date herewith. 
       FIELD OF THE INVENTION 
       [0002]    The present invention is related to air-conditioning systems, and more particularly to an algorithm to optimize the free cooling mode related to controllers and systems driving hydronic products such as water terminals and, air handling units. 
       BACKGROUND OF THE INVENTION 
       [0003]    Usually, commercial building hydronic air-conditioning systems are made of one or more chillers or heat pumps that produce cooling and/or heating water, several water terminals, also referred to as fan coil units, and one or more air handling units that supply fresh air to the building through a duct network in order to maintain a minimum Indoor Air Quality (IAQ) for the comfort of building occupants. Some of these air handlers are energy recovery ventilation systems. 
         [0004]    In normal operation, coolant fluid, usually cooling and/or heating water, possibly having additives, is distributed through a pipe network from the air handling units to the local zone water terminals where it is circulated within a local zone temperature adjusting coil. Additionally, the air handling unit performs IAQ functions such as purification, filtration, or fresh air flow management. With the appropriate coolant fluid at the water terminal, its fan pushes air through its coil to condition it as needed to provide personalized local comfort such as cooling, accomplished by passing cool coolant fluid through the coil, or heating, accomplished by passing heated coolant fluid through the coil. 
         [0005]    Usually there is one water terminal per working zone, for example, an office space, meeting room, or washroom, with each having their own local water terminal and water terminal controller. Typically, a water terminal controller is connected to a local user interface that allows for temperature selection and fan speed control. Many local zone temperature controllers are capable of being connected to a building air-conditioning system communication network enabling multiple components of the entire building air-conditioning system to communicate with each other or be monitored or controlled by a building management system that is also connected to the building air-conditioning communication network. 
         [0006]    In usual operation, the outside air is aspirated by the air handling unit, then filtered and thermally treated (cooled or heated depending on the need) by its coil where the coolant fluid is circulated. The coil is equipped with a proportional coolant flow valve that opens or closes the coolant fluid pipe and therefore enables or disables the heat transfer. After treatment in the air handling unit, the “fresh” supply air is blown through a network of ducts to all the local zone water terminals through fresh air dampers that control the fresh airflow, usually depending on the demand. The water terminals control their own proportional coolant flow valve to provide the demanded comfort inside the controlled zone. 
         [0007]    The outside air is conditioned in the air handler units to a level where each water terminal will have the capacity to condition it locally as needed to provide the air-conditioning required by the zone user. If the local water terminal does not have the capacity to meet the requirements of the zone, their controls open their local fresh air damper to receive the air in the ductwork that was pre-conditioned by the air handling unit. However, this control scheme does not take advantage of information relating to the condition of the outside air to create energy savings. 
         [0008]    Other systems in use do not have an air handler but simply have a fan and a filter to provide fresh air flow into the building. Therefore, there is no thermal pre-treatment of the fresh air, and the water terminals control the temperature of their zones using the outside fresh air as needed without regard for the outside air temperature. 
         [0009]    This is problematic because achievement of the desired zone temperature may be beyond the capacity of the water terminal because the outside air temperature is largely different than the zone setpoint and opening the fresh air damper may result in a change in zone temperature that is even further away from the setpoint. 
         [0010]    In advanced technology water terminals such as demand control ventilation systems, the exact amount of fresh air needed to maintain a minimum IAQ is determined by a system that senses the level of carbon dioxide and its dilution in the zone. Typically, the carbon dioxide detection system includes a carbon dioxide sensor that communicates with a carbon dioxide controller which deduces presence or absence of humans in the zone. Based on this dilution level and the minimum IAQ, the aperture of the fresh air intake damper is adjusted to reduce the carbon dioxide level in the zone. In some cases, the resultant air flow information is returned to the local water terminal controller or to a building management system which might control a system through a physical communication bus or other remote communication technology. 
         [0011]    While most systems use the local water terminal controller to control the air temperature, and the carbon dioxide system to control the carbon dioxide dilution in the zone by actuating their fresh air dampers, they do not employ additional sensors or controls to optimize the system by making full use of the combined information relating to the carbon dioxide dilution and outside air conditions. 
       SUMMARY OF THE INVENTION 
       [0012]    An air-conditioning system control scheme is provided for implementation in local water terminals of a building air conditioning system wherein each water terminal can receive a command from a building management system to run in a variety of modes to achieve energy savings and increased water terminal cooling capacity. These modes are dependent on the temperature of the outside air measurement provided to the building management system, the local zone temperature setpoints, and the human occupancy of the air-conditioned zone. 
         [0013]    Where the outside air is of a temperature to entirely satisfy the air-conditioning demand of the zone with no thermal pre-treatment of the air by the air handling unit, the zone is said to be in free cooling mode. There are two modes within which to run free cooling. One is when the air-conditioned zone is occupied by humans, and the other is when the air-conditioned zone is not occupied by humans. 
         [0014]    Where the outside air is of a temperature to partially satisfy the air-conditioning demand of the zone with no thermal pre-treatment of the air by the air handling unit, the zone is said to be in pre-free cooling mode. Pre-free cooling mode is only available when the zone is unoccupied. 
         [0015]    In one embodiment, a new water terminal control scheme achieves energy savings and free cooling of a building zone by accepting a command from a building management system to run in the free cooling mode. 
         [0016]    In another embodiment, a new water terminal control scheme achieves energy savings and pre-free cooling of a building zone by accepting a command from a building management system to run in the pre-free cooling mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a further understanding of these and other objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where: 
           [0018]      FIG. 1  diagrammatically depicts an air handling unit and a connection to an exemplary water terminal wherein the controllers of both are connected to a building communication network that includes a Building Management System; and 
           [0019]      FIG. 2  depicts a block diagram of the new water terminal control algorithm to illustrate programmable and sensor signals to the water terminal controller and signals to various mechanical components of the water terminal. 
           [0020]      FIG. 3  is a depiction of the operation of the new water terminal control algorithm operating in free cooling mode in the occupied mode; and 
           [0021]      FIG. 4  is a depiction of the operation of the new algorithm operating in free cooling mode in the unoccupied mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Referring initially to  FIG. 1 , there is illustrated a diagrammatical depiction of a building air-conditioning system generally referenced at  5 , for conditioning the air of a building  64 , wherein the air handling unit generally referenced at  60 , the exemplary water terminal generally referenced at  10 , equipped with a fresh air damper  21 , the building air-conditioning system communication network  112 , and the building fresh air duct network  113 , comprise the major components of the system. 
         [0023]    The air handling unit  60 , is illustrated with a direction of outside air flow  72 , coming into it from outdoors, and a direction of air exiting the air handling unit  60 , as air flow  73 , to the building  64 . Outside air flow  72 , entering the air handling unit  60 , passes over an outside air temperature sensor  76 , then through an air handling unit filter  78 , at least one air handling unit fan  80 , an air handling unit temperature adjusting coil  92 , and finally over an air handling unit supply air temperature sensor  84 . 
         [0024]    An air handling unit supply side proportional coolant fluid flow valve  94 , is disposed in the piping that the supplies the air handling unit supply coolant fluid  95 , to the supply side of the air handling unit air temperature adjusting coil  92 . The temperature of the air handling unit supply coolant fluid  95 , is monitored by an air handling unit supply coolant fluid temperature sensor  96 . The temperature of the air handling unit return coolant fluid  98 , from the air handling unit air temperature adjusting coil  92 , is monitored by an air handling unit return coolant fluid temperature sensor  97 . 
         [0025]    Also shown is an air handling unit controller  111 , which runs an air handling unit control algorithm  110 , that communicates with a building management system  54 , through a building air-conditioning system communication network  112 . The building air-conditioning system communication network  112 , can be hard wired or wireless, and may or may not include a building management system. 
         [0026]    As shown in  FIG. 1 , a building management system  54 , uses the building air-conditioning system communication network  112 , to communicate with numerous air-conditioning system components.  FIG. 1  depicts exemplary component communications between an air handling unit controller  111 , a building management system  54 , and a water terminal controller  51 , which is used to control a local zone water terminal generally shown at  10 . 
         [0027]    The water terminal controller  51 , that executes the water terminal control algorithm  50 , contains a microprocessor having a clock speed of at least 16 MHz, internal RAM memory of at least 3.84 Kbytes, internal FLASH memory of at least 128 Kbytes, internal E 2  memory of at least 1 K Byte, a built in A/D converter of at least 10 bits with a 1 LSB error, and a watchdog that is on chip hardware. 
         [0028]    Local zone water terminal  10 , provide air-conditioning for a zone  14 , in the air conditioned building  64 . In this example, where the air handler unit controller  111 , the building management system  54 , and the water terminal controller  51 , are all communicating on the building air-conditioning system communication network  112 , any data input by a building management system user or collected by any sensor on any of these components can be communicated to any of the other components according to their need for the data. 
         [0029]    Turning now to the lower portion of  FIG. 1 , there is illustrated a diagrammatical depiction of local zone water terminal generally referenced at  10 , that illustrates a direction of air flow coming into the system  12 , from the air-conditioned zone  14 , or fresh air duct with damper  21 , and a direction of conditioned air flow exiting the system  13 . Entrance of the air flow from the zone  14 , or fresh air damper  21 , passes over a return air temperature sensor  16 , then through a supply side filter  18 , at least one supply side air fan  20 , a supply side air temperature adjusting coil  32 , and finally over a supply air temperature sensor  24 . The conditioned air is then supplied to the zone  14 . The air in the zone  14 , is monitored by carbon dioxide sensor  26 , which is connected to a carbon dioxide controller  25 , that is capable of providing signals to the water terminal controller  51 , to determine the occupancy status of the zone  14 . 
         [0030]    A supply side proportional coolant fluid flow valve  34  is disposed in the piping that the supplies the supply coolant fluid  35  to the supply side of the air temperature adjusting coil  32 . The temperature of the supply coolant fluid  35  is monitored by a supply coolant fluid temperature sensor  36 . The temperature of the return coolant fluid  38 , from the air temperature adjusting coil  32  is monitored by a return coolant fluid temperature sensor  37 . 
         [0031]    Free Cooling is a air-conditioning system control scheme wherein energy savings are achieved by reducing the speeds of the local water terminal cooling fans  20 , and disabling the thermal pre-treatment functions of the air handling units  60 , to allow outside air  72 , to pass directly through the air handlers  60 , the building fresh air duct network  113 , and fresh air damper  21 , of the local water terminal  10 , that can locally condition the air with only minor temperature adjustments as necessary to provide the desired air-conditioning to the zone  14 . 
         [0032]    Turning now to  FIG. 2 , a block diagram of the new water terminal control algorithm  50 , is provided which illustrates the various programmable and sensor signals to the water terminal controller  51 , and signals to various mechanical components of the water terminal  10 . 
         [0033]    Most clearly relevant is the Free Cooling Enable Signal  200 , to the water terminal controller  51 , from a building management system  54 , that continuously monitors the signal provided by the outside air temperature sensor  76 . If the building management system determines that free cooling will be effective, it will enable the Free Cooling Enable Signal  200 , in the water terminal controller  51 . The next signal depicted in the block diagram is the Occupancy Status Signal  202  of the zone  14 . This is sensed by a local carbon dioxide sensor  26 , and communicated to the water terminal controller  51 , by the carbon dioxide controller  25 . The Occupancy Status Signal  202 , is used to determine the mode, occupied or unoccupied, of free cooling to run when a Free Cooling Enable Signal  200 , is received by the water terminal controller  51 . 
         [0034]    The next signal to the water terminal controller  51 , is a user programmable Temperature Error Threshold Signal  204 . Finally, there is a Local Temperature Error Point Signal  210 , which is the resultant value of the combination in symbolic sigma block  207 , that combines the values of the zone temperature  206 , and the zone setpoint  208 . 
         [0035]    The Heating System Enable Variable  201 , is controlled outside the new water terminal control algorithm  50 , and is directly based on the Free Cooling Enable Signal  200 . If the Free Cooling Enable Signal  200 , is enabled by the building management system  54 , the Heating System Enable Mode  201 , is disabled. Additionally, if the Free Cooling Enable Signal  200 , is enabled by the building management system  54 , the Proportional Coolant Fluid Valve Percent Opening signal  214 , is simply generated by the Local Temperature Error Point Signal  210 , after it passes through the PI block  212 , for conditioning thereby placing the valve in a simple proportional-integral control loop depending on the zone local temperature error. 
         [0036]    Water terminal control algorithm  50 , takes the aforementioned signals and logically processes them to yield a Fresh Air Damper and Cooling Fan signal  217 , which is separately conditioned through PI block  218 , to generate an Air Damper Percent Opening Signal  220 , and through PI block  222 , to generate a Cooling Fan Percent Speed Signal  224 . When free cooling mode is enabled, the fresh air damper  21 , will be fully opened to intake as much air from the air handling unit  60 , as possible. 
         [0037]    If in the occupied mode, the speed of the water terminal cooling fans  20 , is minimized. In unoccupied mode, the speed of the water terminal cooling fans  20 , is also minimized unless the Zone Temperature  206 , is greater than the user programmable Temperature Error Threshold Signal  204 , at which point the speed of the water terminal cooling fans  20 , will be set to an automatic mode to until the local water terminal  10 , reduces the zone temperature  206 , to a point below the user programmable Temperature Error Threshold Signal  204 . 
         [0038]    Turning the  FIG. 3 , there is an exemplary graphical depiction of the operation of the water terminal control algorithm  50 , implementing free cooling in the occupied mode. An axis of the graph is depicted as the zone temperature  206 , increases moving from left to right along the axis  300 . 
         [0039]    While various Occupied Mode Zone Setpoints  302 , and Occupied Mode Deadbands  308 , may be selected based on a particular application, the calculation and determination of resultant control points by the control algorithm remains the same. 
         [0040]    In  FIG. 3 , an Occupied Mode Zone Setpoint  302 , is shown to be 20 degrees Celsius, an Occupied Mode Lower Deadband Temperature Limit  304 , is shown to be 19.5 degrees Celsius, and an Occupied Mode Upper Deadband Temperature Limit  306 , is shown to be 20.5 degrees Celsius, to yield an Occupied Mode Deadband  308 , which in this case, is 1.0 degree Celsius, about the Occupied Mode Zone Setpoint  302 . The overall function of the air handling unit  60 , is to provide fresh air to ensure that the local water terminal can maintain their zone  14 , temperature within Occupied Mode Deadband  308 . 
         [0041]    Within the Occupied Mode Deadband  308 , is a control point that implements a 0.2 degree Celsius Occupied Mode Deadband Hysteresis Control Point  310 . This value is calculated using the Occupied Mode Zone Setpoint  302 , plus one half of the Occupied Mode Deadband  306 , minus 0.2 degrees Celsius. 
         [0042]    When the zone temperature  206 , is being reduced to any temperature below the Occupied Mode Hysteresis Control Point  310 , or is being increased to the Occupied Mode Upper Satisfied Temperature Point  312 , which is calculated by adding the zone setpoint  302 , plus the deadband  308  plus 1 degree Celsius, to the Occupied Mode Upper Satisfied Temperature Limit  312 , in this case 22 degrees Celsius, the control algorithm  50 , determines that the occupied mode cooling demand is satisfied. In all other instances, the control algorithm  50 , determines that the system is in cooling demand mode. 
         [0043]    Turning the  FIG. 4 , there is an exemplary graphical depiction of the operation of the water terminal control algorithm  50 , implementing free cooling in the unoccupied mode. An axis of the graph is depicted as the zone temperature  206 , increases moving from left to right along the axis  400 . 
         [0044]    While various Unoccupied Mode Zone Setpoints  402 , and Unoccupied Mode Deadbands  408 , may be selected based on a particular application, the calculations and determination of resultant control points by the control algorithm remains the same. 
         [0045]    In  FIG. 4 , an Unoccupied Mode Zone Setpoint  402 , is shown to be 20 degrees Celsius, an Unoccupied Mode Lower Deadband Temperature Limit  404 , is shown to be 15 degrees Celsius, and an Unoccupied Mode Upper Deadband Temperature Limit  406 , is shown to be 25 degrees Celsius, to yield an Unoccupied Mode Deadband  408 , in this case, 10 degrees Celsius, about the Unoccupied Mode Zone Setpoint  402 . The overall function of the air handling unit  60 , is to provide fresh air to ensure that the local water terminal can maintain their zone temperature  206 , within Deadband  408 . 
         [0046]    Within the Unoccupied Mode Deadband  408 , is a control point that implements a 0.2 degree Celsius lower Unoccupied Mode Deadband Hysteresis Control Point  410 . This value is calculated by adding 0.2 degrees Celsius to the Unoccupied Mode Zone Setpoint  402 . 
         [0047]    Also within the Unoccupied Mode Deadband  408 , is an Unoccupied Mode Upper Deadband Hysteresis Control Point  411 , that is calculated by adding the zone setpoint  402 , plus one half of the Unoccupied Mode Upper Deadband Temperature Limit  406 , and subtracting 0.2 degrees Celsius for a result, in this case, of 24.8 degrees Celsius. 
         [0048]    When the zone temperature  206 , is being reduced to any temperature below the Unoccupied Mode Upper Hysteresis Control Point  411 , or is being increased to the Unoccupied Mode Upper Deadband Temperature Limit  406 , the control algorithm  50 , determines that the cooling demand is satisfied. In all other instances, the control algorithm  50 , determines that the system is in cooling demand mode. 
         [0049]    As noted above, in unoccupied mode, the speed of the water terminal cooling fans  20 , are minimized unless the zone temperature  206 , is above the programmable Temperature Error Threshold Signal  204 , at which point the speed of the water terminal cooling fans  20 , will be set to an automatic mode to allow them to reduce the zone temperature  206 , below the programmable Temperature Error Threshold Signal  204 . 
         [0050]    Pre-Free Cooling, used in the unoccupied mode, is an air-conditioning control scheme where the desired fresh air  73 , for cooling a zone  14 , is lower than the outside air  72 , temperature, zone temperature  206 , is higher than the outside air  72 , so pushing non-conditioned outside air  72 , into the system can still yield significant building temperature reduction without having to use the air-conditioning component of the air handling unit  60 . This will be effective until the outside air  72 , brings the building to as low a temperature as possible, equal to the outside temperature, at which point the air-conditioning components of the air handling unit  60 , and water terminals  10 , will need to be activated to finish the cooling to the desired zone temperature setpoint  208 . 
         [0051]    Much in the same way as Free Cooling and Pre-Free Cooling are implemented using outside air, Free-Heating is contemplated a variation of these aforementioned air-conditioning control schemes. 
         [0052]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.