Abstract:
A method and system of controlling a water heat pump system. The water heat pump system includes a fan, a water pump, and a boiler. The method includes determining a system time characteristic, determining a heat rejection rate based on the system time characteristic, and determining a loop flow rate based on the heat rejection rate. The method also includes sensing a loop flow rate of the water heat pump system, comparing the sensed loop flow rate with the determined loop flow rate, and modulating a speed of the water pump based on the comparing.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/701,597, filed on Jul. 22, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD  
       [0002]     Embodiments of the invention relate generally to control systems and methods, and particularly to systems and methods to improve efficiency of heat pump systems.  
       BACKGROUND  
       [0003]     Various types of facilities, such as buildings, industrial production facilities, medical buildings, manufacturing assemblies, and laboratories, often use heat pump systems to condition various spaces of the facilities. Such heat pump systems can generally provide both heating and cooling using heat pumps tied to one or more water sources.  
         [0004]     The effectiveness of water-source heat pump systems often depends on system processes that add heat to, or reject heat from, spaces to be heated or cooled. Such systems may use heat pumps to control a loop water temperature between 55° F. and 90° F. In some cases, such systems use a cooling tower to remove heat if the loop water temperature exceeds 90° F., and a boiler to add heat if the temperature falls below 55° F.  
         [0005]     The loop water temperature can fluctuate significantly due to loads present in a facility. If a heat loss (e.g., through ventilation) exceeds those loads, a significant energy surge occurs as additional pumps and/or a boiler are activated to replenish heat. Low compressor efficiency and high pump power consumption can result, particularly if inefficient heat pumps are present in a water-source heat pump system.  
       SUMMARY  
       [0006]     In one embodiment, the invention provides a method of controlling a water heat pump system. The water heat pump system includes a fan, a water pump, and a boiler. The method includes determining a system time characteristic, determining a heat rejection rate based on the system time characteristic, and determining a loop flow rate based on the heat rejection rate. The method also includes sensing a loop flow rate of the water heat pump system, comparing the sensed loop flow rate with the determined loop flow rate, and modulating a speed of the water pump based on the comparing.  
         [0007]     In another embodiment, the invention provides a controller for controlling a water heat pump system. The heat pump system includes a variable speed cooling tower fan, a water pump, a boiler operable to supply water at a plurality of temperatures, and a sensing device operable to sense a loop flow rate of the water heat pump system. The controller includes a timing module, a heat rejection module, a loop flow module, a comparator, and a modulator. The timing module determines a system time characteristic. The heat rejection module determines a heat rejection rate based on the system time characteristic. The loop flow module determines a loop flow rate based on the heat rejection rate. The comparator compares the sensed loop flow rate with the determined loop flow rate. The modulator modulates a speed of the water pump based on the comparing by the comparator.  
         [0008]     Embodiments of the invention can optimize a loop pump temperature and water flow rate to ensure optimal heat pump efficiency and minimal pump energy consumption. Some embodiments herein can reduce loop pump power by about 50 percent and compressor power by about 30 percent.  
         [0009]     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic diagram of a water-source heat pump system according to an embodiment of the invention.  
         [0011]      FIG. 2  is a block diagram of a controller according to an embodiment of the invention.  
         [0012]      FIG. 3  is a flow chart illustrating exemplary processes carried out in the controller of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0013]     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.  
         [0014]     As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of what actual systems might be like. Many of the modules and logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” may include or refer to both hardware and/or software. Furthermore, throughout the specification capitalized terms are used. Such terms are used to conform to common practices and to help correlate the description with the coding examples, equations, and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware.  
         [0015]     Embodiments of the invention provide control systems and methods that can be retrofitted in existing water-source heat pump systems, or can be incorporated in new systems.  
         [0016]      FIG. 1  shows a water-source heat pump system  100  that includes a boiler  104  coupled to a valve  108  that limits an amount of return water from a plurality of heat pumps  112 . The boiler  104  heats water collected by the heat pumps  112  and supplies the heated water downstream in the system  100 . Although the embodiment shown in  FIG. 1  includes only two heat pumps  112 , the system  100  can include more or fewer heat pumps. The system  100  also includes a plurality of loop pumps  116  coupled to a plurality of respective variable frequency drives (“VFDs”)  120  to drive the loop pumps  116 . Across each of the loop pumps  116  is a pressure differential sensor or a pump head  124  that measures a pressure differential between an input and an output of the loop pump  116 . In other embodiments, the system  100  includes more or fewer loop pumps  116 , VFDs  120 , and loop pump heads  124 .  
         [0017]     The system  100  includes a heat exchanger  128  to collect water from the boiler  104  and the loop pumps  116 . A plurality of tower pumps  132  located downstream from the heat exchanger  128  pump the water from the heat exchanger  128  to a cooling tower  136  located further downstream and typically on a rooftop. Like the loop pumps  116 , a plurality of VFDs  140  control the respective tower pumps  132 , and a plurality of pump heads  144  measure a plurality of pressure differentials across the respective tower pumps  132 . In other embodiments, the system  100  includes more or fewer tower pumps  132 , VFDs  140 , and tower pump heads  144 .  
         [0018]     The cooling tower  136  receives water from the tower pumps  132 , and cools the water with a fan  148  coupled to another VFD  152  that controls a speed of the fan  148 . The heat exchanger  128  collects the water from the cooling tower  136 , and supplies the water back to the heat pumps  112 , thus completing a water flow path.  
         [0019]     The system  100  also includes a controller  160  to collect and process information. In the embodiment shown, the system  100  includes a loop supply water temperature sensor  164  that senses temperature of the water being supplied to the heat pumps  112 . A loop return water temperature sensor  168  measures temperature of the water being collected from the heat pumps  112 . The controller  160  also receives signals from an outside air temperature sensor  176  and an outside air relative humidity sensor  180  configured to measure the temperature and the relative humidity of the outside air, respectively. The controller  160  also receives signals from a tower supply water temperature sensor  184  that measures the temperature of the water being supplied to the cooling tower  136 . Similarly, the controller  160  receives signals from a tower return water temperature sensor  188  that measures the temperature of the water being returned to the heat exchanger  128  from the cooling tower  136 .  
         [0020]      FIG. 2  shows a block diagram of the controller  160  of  FIG. 1 . The controller  160  includes an interface module  203  that is configured to receive a plurality of air-related conditions and system operating conditions from sensors of the system  100  of  FIG. 1 , such as the outside air temperature sensor  176 , the relative humidity sensor  180 , the loop pump head sensors  124 , and the tower pump head sensors  144 . Based on one or more of the sensed conditions, a loop flow module  206  determines a flow rate of the water at the loop pumps  116 ; a tower flow module  209  determines a flow rate of the water at the tower pumps  132 ; and an initialization module  212  initializes operating parameters, as described in greater detail below.  
         [0021]     The controller  160  also includes a timing module  215  to determine a time characteristic of the system  100 , and a heat rejection module  218  to determine a heat rejection rate based on the time characteristic. A comparator module  221  receives and compares inputs. For example, the comparator module  221  compares a loop pump flow rate with a loop pump set point that can be retrieved from a memory module  224 . Similarly, the comparator module  221  compares a tower pump flow rate with a tower pump set point. Additionally, the comparator module  221  compares sensed temperatures with a temperature set point retrieved from the memory module  224 . The comparator module  221  also compares a heat rejection rate with a plurality of heat rejection rate set points.  
         [0022]     The controller  160  enables the boiler  104  and the cooling tower  136  of  FIG. 1  via a boiler enable module  227  and a cooling tower enable module  230 , respectively. A heat pump module  233  activates or enables the heat pumps  112 . A fan speed module  236  adjusts a speed of the fan  148 , while a VFD module  239  sends a plurality of control signals to control a plurality of VFDs  120  and  140 . The controller  160  also includes a valve module  242  to control the valve  108 .  
         [0023]      FIG. 3  is a flow chart illustrating a water-source heat pump control process  300  carried out by the controller  200  of  FIG. 2 . At block  304 , the process  300  initializes system operating conditions, such as a loop pump speed (“N”) measured in revolutions-per-minute (“RPM”), a design loop pump speed (“N d ”) measured in RPM, and a time characteristic of the system  100  of  FIG. 1 . The system operating conditions can be determined, for example, by using sensed parameters directly, performing one or more computations using sensed parameters, etc.  
         [0024]     At block  308 , the process  300  determines a loop flow rate (“Q”) of the loop pumps  116  of  FIG. 1  as follows. A specific equation for determining the water flow rate is used depending on a type of pump curve associated with the loop pumps  116 . Pumps typically can be characterized by a pump curve, which may be steep or flat. Pumps with a steep pump curve include pumps whose differential pressure or pump head increases as a result of decreasing water flow rates (“Q”) at the same pump speed (“N”). Pumps with a flat pump curve include pumps whose differential pressure or pump head remains generally constant when the pump flow rate (“Q”) changes. For such pumps, the pump power varies significantly when the pump flow rate changes at the same pump speed.  
         [0025]     For example, the process  300  can use EQN. (1) to determine the flow rate (“Q”) of the loop pumps and the tower pumps, which is measured in gallons-per-minute (“GPM”), for pumps with a steep pump curve. EQN. (1) is based on a measured pump head (“H”), and a ratio (“ω”) between the pump speed (“N”) that is measured in revolutions-per-minute (“RPM”) and a design pump speed (“N d ”) that is also measured in RPM. In some embodiments, the design pump speed is about 1,450 RPM.  
             Q   =       (         -     a   1       -         a   1   2     -     4   ⁢       a   2     ⁡     (       a   0     -     H     ω   2         )                 2   ⁢     a   2         )     ⁢   ω             (   1   )             
 
 In EQN. (1), a 0 , a 1 , and a 2  are pump curve coefficients obtained from the pump curve, typically provided by manufacturers of the pumps  116 ,  132 . 
 
         [0026]     Further, the process  300  can use EQN. (2) to determine the pump airflow rate (“Q”) for pumps with a flat pump curve. EQN. (2) is based on the ratio (“ω”), and a pump power (“w f ”).  
             Q   =           -     b   1       ⁢     ω   2       -           b   1   2     ⁢     ω   4       -     4   ⁢     b   2     ⁢     ω   ⁡     (         b   0     ⁢     ω   3       -     w   f       )                 2   ⁢     b   2     ⁢   ω               (   2   )             
 
 In EQN. (2), b 0 , b 1 , and b 2  are pump power curve coefficients, also provided by manufacturers of the pumps  116 ,  132 . In this way, the process  300  can determine the pump water flow rate (“Q”) of the pumps  116 ,  132  using either of the above equations as appropriate. 
 
         [0027]     At block  312 , the process  300  determines a time characteristic of the system  100 . The time characteristic of the system  100  (also referred to as the system time characteristic) generally indicates an amount of time for water to completely flow through the system  100  (e.g., in the water flow path as described above). The process  300 , after the initialization process at block  304 , determines the system time characteristic (“T”) as follows.  
         [0028]     The process  300  stores in the memory module  224  a plurality of times at which maximum and minimum supply water temperatures are recorded, and their corresponding maximum and minimum supply water temperatures. Similarly, the process  300  stores in the memory module  224  a plurality of times at which maximum and minimum return water temperatures are recorded, and their corresponding maximum and minimum supply water temperatures. The maximum and minimum return water temperatures generally occur after the corresponding times at which the maximum and minimum supply water temperatures are recorded. The process  300  then determines a difference of the times if the difference between the minimum and the maximum supply water temperatures is higher than 4° F. The process  300  then compares the time difference with a predetermined time value. When the time difference is greater than the predetermined time value, the process  300  generates a system time characteristic.  
         [0029]     At block  316 , the process  300  determines a heat rejection rate (“E”) of the system  100  with EQN. (3) as follows.  
             E   =             ∑     i   =   1     n     ⁢       T     r   ,   i       ⁢     Q   i         -       ∑     i   =     m   +   1         m   +   1   +   n       ⁢       T     s   ,     i   -   m         ⁢     Q     i   -   m                   ∑     i   =   1     n     ⁢       Q   i     2       +       ∑     i   =     m   +   1         m   +   1   +   n       ⁢       Q     i   -   m       2           /       nQ   d     ⁡     (       T     r   ,   d       -     T     s   ,   d         )                 (   3   )             
 
         [0030]     In EQN. (3), the parameters include a loop return water temperature which is measured in ° F. at time i (“T r,i ”), a loop water flow rate at time i (“Q i ”), a loop supply water temperature at time (i−m) (“T s.i−m ”) a loop water flow rate at time (i−m) (“Q i−m ”), a design loop return water temperature (“T r,d ”), a design loop supply water temperature (“T s,d ”), a design loop water flow rate (“Q d ”), an average time period as a number of sampling intervals (“n”), and a number of sampling intervals in the system time characteristic, (“m=T/Δτ”) In general, the heat rejection ratio is between 0 and 1. For example, if E is greater than zero, the return water temperature is greater than the supply water temperature.  
         [0031]     At block  320 , the process  300  determines a loop pump speed (“Q loop.set ”) and a tower pump speed (“Q tower.set ”) with EQN. (4) and EQN. (5) as follows. 
 
 Q   loop.set =max(ε 0   ,√{square root over (|E|)})   Q   loop.d    (4) 
 
 Q   tower.set =max(ε 0   ,√{square root over (|E|)} ) Q   tower.d    (5) 
 
 In EQN. (4) and EQN. (5), Q loop.d  and Q tower.d  are design loop flow rate and tower flow rate, respectively. In some embodiments, the constant (“ε 0 ”) is about 0.3. 
 
         [0032]     The process  300  then proceeds to evaluate a plurality of system conditions, such as a loop pump condition, a tower pump condition, and a heat rejection rate condition. At block  324 , the process  300  compares the heat rejection rate determined at block  316  with a predetermined threshold (“ε 1 ”), such as 0.15. If the process  300  determines at block  324  that the heat rejection rate is less than the predetermined threshold (“ε 1 ”), the process  300  proceeds to compare the heat rejection rate with a second predetermined threshold (“ε 2 ”), such as −0.05, at block  328 . If the process  300  determines at block  328  that the heat rejection rate is less than the second predetermined threshold (“ε 2 ”), the process  300  proceeds to compare the heat rejection rate with a third predetermined threshold (“ε 3 ”), such as −0.1, at block  332 . If the heat rejection rate is less than the third predetermined threshold (“ε 3 ”), the process  300  proceeds to block  336 . Otherwise, if the heat rejection rate is greater than the third predetermined threshold (“ε 3 ”), the process  300  proceeds to block  340 .  
         [0033]     In block  336 , the process  300  compares the loop supply water temperature (“T s ”) with the loop supply water temperature set point (“T s.set ”) such as 80° F. If the process  300  determines that the loop supply water temperature is less than the loop supply water temperature set point, the process  300  opens the valve  108  by an amount (“Δv”) at block  344 , and repeats block  336 . Otherwise, if the process  300  determines that the loop supply water temperature is greater than the loop supply water temperature set point, the process  300  closes the valve  108  by Δv at block  348  and repeats block  336 . In some embodiments, the process  300  uses a proportional-integral controller (not shown) to adjust the valve  108 . At block  340 , the process  300  sets the loop supply water temperature set point as its maximum allowable value, (“T s.max ”) and repeats block  336 .  
         [0034]     At block  328 , when the heat rejection rate is greater than the second predetermined threshold (“ε 2 ”), the process  300  proceeds to compare the loop supply water temperature (“T s ”) with a predetermined loop temperature (“τ 2 ”), such as 55° F, at block  352 . If the process  300  determines that the loop supply water temperature (“T s ”) is less than the predetermined loop temperature (“τ 2 ”), the process  300  opens the valve  108  by the amount (“Δv”) at block  356 , and repeats block  352 . Otherwise, if the process  300  determines that the loop supply water temperature (“T s ”) is greater than the predetermined loop temperature (“τ 2 ”), the process  300  proceeds to compare the loop supply water temperature (“T s ”) with the loop supply water temperature set point (“T s.set ”) such as 80° F., at block  360 . If the process  300  determines that the loop supply water temperature (“T s ”) is greater than the loop supply water temperature set point (“T s.set ”), the process  300  proceeds to turn on the cooling tower at block  364 , and repeats block  360 . Otherwise, in block  360 , if the process  300  determines that the loop supply water temperature (“T s ”) is less than the loop supply water temperature set point (“T s.set ”), the process  300  keeps the cooling tower  136  deactivated at block  366 .  
         [0035]     At block  324 , if the process  300  determines that the heat rejection rate is greater than the predetermined threshold (“ε 1 ”), the process  300  proceeds to carry out operations defined in block  368 . At block  368 , the process  300  sets the loop supply water temperature (“T s.set ”) according EQN. (6) as follows. 
 
 T   s.set   =T   wet +ε 4   E    (6) 
 
 In EQN. (6), T wet  is an outside air wet bulb temperature, which can be determined based on the outside air temperature and the relative humidity ratio. The process  300  then compares the loop supply water temperature (“T s ”) with the loop supply water temperature set point (“T s.set ”), such as 80° F., at block  372 . If the process  300  determines that the loop supply water temperature (“T s ”) is greater than the loop supply water temperature set point (“T s.set ”), the process  300  proceeds to speed up the cooling fan  148  by an amount (“Δ cool ”) at block  376 , and repeats block  372 . Otherwise, if the process  300  determines that the loop supply water temperature (“T s ”) is less than the loop supply water temperature set point (“T s.set ”), the process  300  slows down the cooling fan  148  by the amount (“Δ cool ”) at block  376 , and repeats block  368 . 
 
         [0036]     Referring back to block  320 , the process  300  also checks to determine the loop pump conditions. At block  382 , the process  300  compares an actual loop pump flow rate with the loop pump flow rate set point determined at block  320 . If the process  300  determines that the actual loop flow rate (“Q loop ”) is greater than the set point (“Q loop ”), the process  300  slows down the pump to maintain the flow rate set point by an amount (“Δ loop ”) at block  384 , and repeats block  382 . Otherwise, if the process  300  determines that the actual loop flow rate (“Q loop ”) is less than the set point (“Q loop ”), the process  300  speeds up the pump to maintain the flow rate set point by the amount (“Δ loop ”) at block  386 , and repeats block  382 . In some embodiments, the process  300  uses a proportional-integral controller (not shown) to adjust the loop pumps  116 .  
         [0037]     Similarly, referring back to block  320 , the process  300  also checks to determine the tower pump conditions. At block  388 , the process  300  compares an actual tower pump flow rate with the tower pump flow rate set point determined at block  320 . If the process  300  determines that the actual tower pump flow rate (“Q tower ”) is greater than the set point (“Q tower ”) the process  300  slows down the tower pump to maintain the flow rate set point by an amount (“Δ tower ”) at block  390 , and repeats block  388 . Otherwise, if the process  300  determines that the actual tower flow rate (“Q tower ”) is less than the set point (“Q tower ”), the process  300  speeds up the tower pump to maintain the tower pump flow rate set point by the amount (“Δ tower ”) at block  392 , and repeats block  388 . In some embodiments, the process  300  uses a proportional-integral controller (not shown) to adjust the tower pumps  132 .  
         [0038]     Various features and advantages of the invention are set forth in the following claims.