Patent Publication Number: US-10767908-B2

Title: Cascading heat recovery using a cooling unit as a source

Description:
FIELD 
     Embodiments disclosed herein relate generally to recovery of rejected heat from a refrigeration circuit used to operate in a cooling cycle, where a heat exchanger from the refrigeration circuit is the source of rejected heat. In particular, methods, systems and apparatuses are disclosed that employ for instance a connection from a heating unit to any one of a heat recovery condenser or an auxiliary condenser or an evaporator, depending on system conditions to recover the rejected heat. 
     BACKGROUND 
     A refrigeration or heating, ventilation, and air conditioning system (HVAC) system would typically include a compressor, a condenser, an expansion device, and an evaporator that form a refrigerant circuit. Such a circuit can be embodied in what is known as a chiller. 
     Chillers for example can be used to cool a process fluid, such as water, where such process fluid can be directly used or may be used for various other cooling purposes, such as for example cooling a space. In a cooling cycle, refrigerant vapor is generally compressed by the compressor, and then condensed to liquid refrigerant in the condenser. The liquid refrigerant can then be directed through the expansion device to reduce a temperature and can become, at least in part, a liquid/vapor refrigerant mixture (two-phase refrigerant mixture). The refrigerant, e.g. including two-phase mixture, is directed into the evaporator to exchange heat with a fluid moving through the evaporator. The refrigerant mixture can be vaporized to refrigerant vapor in the evaporator, and the refrigerant vapor can then be returned to the compressor to repeat the refrigerant cycle. 
     SUMMARY 
     In some refrigeration circuits, such as chillers, a heat recovery cycle is available and designed to salvage the heat that may normally be rejected for example to the atmosphere through what is known as a cooling tower, and can put such recovered heat to beneficial use. For example, a high rise building may require simultaneous heating and cooling during certain months of the year, such as for example the winter months. With the addition of a heat recovery cycle, heat removed from the building cooling load can be transferred to areas, e.g. of the building, that may require heat. 
     To provide a heat recovery cycle, a heat recovery condenser or auxiliary condenser may be added to the unit, e.g. chiller. Though the heat recovery condenser may be structurally similar to the “main” condenser of the refrigerant cycle e.g. that may be used in support of the cooling load, the heat recovery condenser can often be piped into the heating water circuit rather than to the cooling tower. During a heat recovery cycle, the unit can operate just as it does in the “cooling only” mode except that the cooling load heat is rejected to the heating water circuit rather than to the cooling tower water circuit. When hot water is required, the heating water circuit pumps can energize, and water circulated through the heat recovery (or auxiliary) condenser tube bundle by the pumps can absorb the cooling load from the compressed refrigerant gas discharged by the compressor. The heated water can then be used to satisfy desired and/or required heating. Somewhat different from the heat recovery condenser, which has often been known for use in comfort heating applications, is the auxiliary condenser which can serve a preheat function. The auxiliary condenser can be used in such applications where hot water is used for example in kitchens, lavatories, or other utility. While the operation of the auxiliary condenser can also be structurally the same as the heat recovery condenser, in some cases it can be somewhat smaller in size/scale, and its heating capacity may not be controlled. 
     As such it can be desirable in some cases to use the waste heat from a cooling process to meet the heating load except that there can be challenges to this. First, standard cooling equipment often reject low grade heat which may not provide adequate heating. However, selecting cooling equipment that would provide an adequate temperature to provide heating can negatively impact the cooling efficiency of the cooling equipment. For example, when the cooling unit may be used for cold thermal storage production there can be issues with obtaining a high enough condenser water temperature to enable the heat recovery. Second, in some cases the cooling equipment may not be properly sized to match the heating load, so using such equipment for heat recovery can impact both the cooling and heating system efficiencies. However, oversizing the cooling equipment to meet higher heating loads can add cost to the system and also affect the system efficiency and reliability. Also, selecting or splitting the cooling equipment into multiple smaller units to meet the heating load can also add cost to the system. Third, the control of a single unit to meet both heating and cooling loads optimally may cause improper control which can compromise cooling and/or heating production or unit reliability. Fourth, the use of a dedicated properly sized heat recovery unit, while it may be effective, can add cost and can involve control complexity. For example, the efficiency of a dedicated heat recovery unit is limited by the fact that it may need to produce high refrigerant side lift to elevate the chilled water temperatures, e.g. about 40 F to about 56 F, to heating water temperatures, e.g. about 100 F to 140 F. 
     In the past cascade heat recovery has been done by connecting the evaporator of the heat recovery unit to the cooling unit or equipment, for example a chiller. The evaporator of the heat recovery unit would be connected to the condenser water/cooling tower circuit. However, such a configuration can subject the evaporator to dirty cooling water tower water, which can involve periodic cleaning, which involves maintenance cost. In addition, the heat recovery pump, net positive suction head, may be limited by open system pressure in the condenser water circuit potentially causing issues with pumping designs. Another approach has been to use an intermediate heat exchanger between the condenser/cooling tower water circuit to prevent fouling of the heat recovery unit evaporator. However, such a configuration can increase the complexity of the system, requiring the addition of a pump set, can introduce heat transfer loss reducing system efficiency, and can require added cost due to the additional pump set and due to periodic cleaning. 
     The systems, methods, apparatuses herein are directed to recovering heat from a cooling unit, where the cooling unit is, for example a water chiller equipped with dual condensers, one of which is the cooling condenser, the other being one of heat recovery condenser (e.g. full heat recovery) or an auxiliary condenser (e.g. partial heat recovery). In some embodiments, the cooling unit is a centrifugal water chiller. It is to be appreciated that the cooling unit from which there is heat recovery, can be other chiller types that run on different compressors, may be other cooling unit types that may be equipped with dual condensers. 
     In one embodiment, a cascade heat recovery system connects an evaporator from a heating unit to a recovery condenser of the cooling unit, where the recovery condenser is different from a cooling condenser of the cooling unit. The evaporator is connected to the recovery condenser such that the circuit of the heating unit is isolated or decoupled from the heat rejection circuit on which the cooling condenser runs. The use of the recovery condenser can use heat rejected by the evaporator of the cooling unit to be available to the heating unit so as to provide lift to the heating unit and improve its operating efficiency. 
     In another embodiment, a cascade heat recovery system can include connecting the evaporator of the heating unit to the cooling loop of the cooling unit, for example in fluid communication with the cooling unit evaporator, such as for example upstream of the evaporator water inlet and in some examples downstream from water pump of the cooling loop. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the heat recovery systems and methods will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein: 
         FIG. 1  is a diagrammatic plan view of one system that uses a cooling unit equipped with dual condensers, according to one embodiment. 
         FIG. 2  is a diagrammatic plan view of one system that uses a cooling unit equipped with dual condensers, according to another embodiment. 
     
    
    
     While the above-identified figures set forth particular embodiments of the heat recovery systems and methods, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the heat recovery systems and methods by way of representation but not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of the heat recovery systems and methods described herein. 
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic plan view of one system  100  that uses a cooling unit  110  equipped with dual condensers, one condenser  116  is sized for normal heat rejection (B) in the cooling cycle and the other condenser  120  is sized as a heat source (C) for a dedicated heating unit  150  (D), which may be a heat pump. 
     In the embodiment shown, the system  100  can have the appropriate piping to the components which may be included in the system such as those depicted in  FIG. 1 . 
     The cooling unit  110  has a cooling loop on which an evaporator  114  can undergo a heat exchange to vaporize refrigerant while cooling a process fluid, such as water. The cooled process fluid leaves the evaporator  114  to be used to cool a cooling load  112  and then returns to the evaporator, such as through a water pump  118  to repeat the cooling loop. 
     The cooling loop includes a heat rejection circuit  130 , on which the condenser  116  is in fluid communication with the evaporator  114  and undergoes a heat exchange with the evaporator  114  by condensing the refrigerant vaporized by the evaporator  114 . The condenser  116  can be in fluid communication with a cooling tower  132  which can use water to be pumped by, e.g. condenser water pump  138 , into the condenser  116  to condense the refrigerant vapor from the evaporator  114 . The leaving water from the condenser  116  can return to the cooling tower via, e.g. a three way valve  126 . The condensed refrigerant can return to the evaporator  114  from the condenser  116 , which can then deliver the vaporized refrigerant to a compressor (not shown) which can then deliver the high pressure, high temperature refrigerant back to the condenser  116  to repeat the cooling loop cycle. It will be appreciated that any suitable compressor and/or expansion device(s) can be employed in the system  100 , which may be a chiller unit, such as for example a centrifugal chiller unit (A). 
     Referring to the heating unit  150 , the heating unit  150  includes an evaporator  154  fluidly connected with a heating condenser  156 , which is in fluid communication with a heating load  152 . Optionally, an auxiliary heat source  140  may be employed as may be desired and/or suitable. A heat pump  158  may be employed to circulate fluid, e.g. water, through the heating unit  150 . 
     In one embodiment, the cascade heat recovery system  100  connects the evaporator  154  from the heating unit  150  to the recovery condenser  120  of the cooling unit  110 , where the recovery condenser  120  is different from the cooling condenser  116  of the cooling unit  110 . The evaporator  154  is connected to the recovery condenser  120  such that the circuit of the heating unit  150  is isolated or decoupled from the heat rejection circuit  130  on which the cooling condenser  116  runs. The use of the recovery condenser  120  can use heat rejected by the evaporator  114  of the cooling unit  110  to be available to the heating unit  150  so as to provide lift to the heating unit  150  and improve its operating efficiency. 
     In some embodiments, the evaporator  154  of the heating unit  150  may be connected to the recovery condenser  120  through, e.g. a three way valve  126 , and where in some examples a heat recovery pump  128  may be employed. 
       FIG. 2  is a diagrammatic plan view of another cascade heat recovery system  200  that uses a cooling unit  210  equipped with dual condensers, one condenser  216  is sized for normal heat rejection (B) in the cooling cycle and the other condenser  220  is sized as a heat source (C) for a dedicated heat pump unit (D). 
       FIG. 2  adds a connection between the heating unit evaporator  254  and another cooling loop  280  (F) in fluid communication with the cooling unit evaporator, e.g. the water side. In this configuration, the cascade heat recovery system  200  can include a dedicated heat recover option. In the embodiment shown, the evaporator  254  of the heating unit  250  is connected to the cooling loop of the cooling unit, for example in fluid communication with the cooling unit  210  evaporator  214 , such as for example upstream of the evaporator  214  water inlet and in some examples downstream from a water pump  218  of the cooling loop. Like reference numerals depict similar structures as illustrated in  FIG. 1 , but with the  200  reference numeral series and most are not further described. 
     In some embodiments, the evaporator  254  of the heating unit  250  may be connected to the cooling loop  280 , such as through, e.g. a three way valve  246 , so as to allow access to the cooling loop  280 , under certain conditions. 
     For example, this connection can allow the heating unit  250  (D) to operate as a dedicated heat recovery chiller A when beneficial under the system conditions, such as for example when the cooling load is not so high or relatively low. 
       FIGS. 1 and 2  show three way valves, e.g.  126 ,  136 ,  226 ,  236 ,  246 , by way of illustration. However, it will be appreciated that any one or more of the three way valves may be replaced for example by a variable drive if desired and/or suitable. 
     It will be appreciated that each of the systems  100 ,  200  may be controlled by a unit controller, e.g.  160 ,  260 , that includes a processor, a memory, and an input/output (I/O) interface as may be needed and/or suitable to control operation of the valves and equipment, such as shown in  FIGS. 1 and 2 , as well as any drives to communicate with available sensors or transducers, such as may be used. In particular, depending upon the operating system conditions, either of the heat recovery approaches may be employed, such as for example through the recovery condenser of  FIGS. 1 and/or 2 , or through the access to the cooling loop  280  in  FIG. 2 . It will be appreciated that the controller is suitably configured to communicate, receive, and/or command the necessary components shown in  FIGS. 1 and/or 2  to obtain the heat recovery approach desired. 
     The systems and methods described herein can solve several issues and improve system operation. First, the cooling unit(s) can be optimally sized and operated to meet the cooling load without regard to the heating load and/or heating unit(s) operation. This can simplify cooling unit selection, control, and system optimization. As another example, the cooling unit(s) can avoid being subjected to high condenser temperature operation, which will reduce cooling system inefficiency and/or improve cooling system efficiency, and which can reduce concern about surge in the cooling unit, e.g. in centrifugal compressor based cooling units. As yet another example, the control of the standard condenser water temperature from its heat sink, e.g. the cooling tower, may basically remain unchanged from the cooling unit&#39;s standard operating conditions. In yet another example, control of the standard condenser water temperature from its heat sink, e.g. cooling tower, could be optimized under some system operating conditions by slowing the cooling tower fans, reducing fan energy use, so as to raise the heat recovery source condenser temperature available to the heating unit to raise the heating unit&#39;s efficiency and/or capacity. (See e.g.  FIGS. 1 and 2 .) 
     Second, the heating unit(s) can be optimally sized and operated to meet a heating load without regard to the cooling load, for example as long as the cooling load exceeds the heating load. This can simplify heating unit selection, control, and system optimization. For example, this can be useful where many commercial buildings may operate the majority of hours in a cooling mode. As another example, this can be useful where a heating unit can limit its heating capacity to the lesser of the required heating load or heat recovery source condenser available heat. As yet another example, some high efficiency boilers can be used to provide the balance of the heating requirement/load, when for example the heating load exceeds the heat recovery source condenser available heat. (See e.g.  FIGS. 1 and 2 .) 
     Third, the efficiency, capacity, and reliability of the heat pump unit can be significantly increased as a result of the entering evaporator water temperature supplied from the cooling unit&#39;s heat recovery source condenser, e.g. at about 65 F to about 95 F, may be significantly higher than would be available to a dedicated heat recovery chiller from the system chilled water, e.g. at about 44 F to about 56 F. For example, the heating unit(s) efficiency and/or capacity improvement could be in the range of ten to forty percent. (See e.g.  FIGS. 1 and 2 .) 
     Fourth, the efficiency, capacity, and reliability of the cooling unit can be significantly increased as a result of the entering condenser water temperature supplied from the heat recovery unit&#39;s evaporator, e.g. at about 40 F to about 60 F, can be significantly lower than would be available to the standard condenser system cooling tower, e.g. at about 55 F to about 85 F. For example, the cooling unit(s) efficiency and capacity can improve as much as 20% as a result of the heating unit depressing the water temperature supplied from the heat recovery source condenser back to the cooling unit(s). (See e.g.  FIGS. 1 and 2 .) 
     Fifth, periodic cleaning can be avoided because the additional cooling unit heating source condenser waterside is a closed circuit isolated from the “dirty” condenser/cooling tower water circuit avoiding contamination. Additionally, the closed circuit can be designed such that the pump net positive suction head is suitable at various and/or all operating points. (See e.g.  FIG. 1  and  FIG. 2 .) 
     Sixth, when the cooling unit functions to provide thermal storage cooling, the cooling unit can operate with normal condenser temperatures and the heat pump unit can elevate those temperatures as needed to effectively meet the heating unit requirements. (See e.g.  FIG. 1  and  FIG. 2 .) 
     While the embodiments have been described in terms of various specific embodiments, those skilled in the art will recognize that the embodiments can be practiced with modification within the spirit and scope of the claims.