Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application hereby incorporates by reference the application Ser. No. 14/133,739 titled “Intermittent Absorption Refrigeration System Equipped With A Waste Energy Storage Unit”. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates to an intermittent absorption refrigeration method and an economizer for use with an intermittent absorption refrigeration system. 
     2. Description of the Related Art 
     The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention. 
     Energy consumption due to air-conditioning and refrigeration applications is significant. The coincidence of maximum cooling loads with the period of highest solar irradiance makes solar energy an excellent candidate for powering refrigeration and air conditioning systems, thereby conserving electrical energy. Absorption chillers can operate even with relatively low-quality heat sources such as exhaust gases from industrial processes or solar radiation. In this regard, absorption chillers have the potential to directly use solar energy to produce refrigeration. 
     Typical absorption cooling systems utilize a heat source to generate refrigerant vapor out of a strong absorbent-refrigerant solution. The pressurized desorbed liquid refrigerant is then condensed by rejecting heat to the ambient environment. The condensed refrigerant is then used for evaporative cooling by evaporating it under lower pressure, whereby ambient heat is absorbed from the refrigerated space. The evaporated refrigerant is then absorbed back into the weak solution, resulting in a rich solution, thereby enabling the process to be repeated. 
     Absorption chillers are basically classified into two categories: continuous operation systems and intermittent operation systems. The basic difference between continuous and intermittent systems is their mode of operation. In continuous systems, both generation and absorption of the refrigerant take place at the same time in a continuous manner. However, in intermittent systems, generation and absorption do not take place at the same time; rather, they intermittently follow each other during the operation of the system. 
     Historically, the coefficient of performance of intermittent systems has typically been much lower than that of continuous systems. This is largely because a continuous system is able to employ a recuperator-type solution heat exchanger, wherein hot and cold fluids flowing past one another in adjacent channels exchange thermal energy. In this manner, waste heat generated in one portion of the system can be utilized to provide heat required by another portion of the system, thereby increasing the overall coefficient of performance of the system. 
     In a typical absorption cooling system, the generation process requires thermal energy to vaporize refrigerant out of a liquid absorbent-refrigerant solution, while on the other hand, the absorption process releases thermal energy as refrigerant vapor is absorbed into absorbent-refrigerant solution. In a continuous absorption system, the generation and absorption processes occur simultaneously, thus, both hot and cold solutions are continuously present during the operation of the system. Since both hot and cold solutions are present, a recuperator-type solution heat exchanger allows the system to recover thermal energy released by the absorption process and to use that recovered energy to help drive the generation process. 
     For intermittent systems, by contrast, it is not possible to use a recuperator-type heat exchanger for waste energy recovery, since hot and cold solutions are not available at the same time. Thus, the coefficient of performance of intermittent systems has been limited. 
     SUMMARY 
     The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 
     In an embodiment, the coefficient of performance of a solar powered intermittent absorption system is improved by introducing an economizer into the system. 
     In another embodiment, the economizer is a regenerative type solution heat exchanger. 
     In another embodiment, the economizer is a temporary energy storage unit containing the same fluid as that flowing through a solar collector. 
     In another embodiment, the economizer stores some amount of energy that is rejected by an absorber during a depressurization process. 
     In another embodiment, the stored energy in the economizer is returned back to the intermittent system during a pressurization process in a generator. 
     In another embodiment, the economizer reduces the total amount of solar energy input into the generator, thereby increasing the coefficient of performance of the intermittent system. 
     In another embodiment, a heat-transfer fluid stored in a economizer is used to pressurize and depressurize an absorbent-refrigerant solution in a generator/absorber. 
     In another embodiment, an intermittent absorption refrigeration system uses water as an absorbent and ammonia as a refrigerant. 
     In another embodiment, a dephlegmator is used to separate the refrigerant vapor from the absorbent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, wherein: 
         FIG. 1  is a schematic diagram of a solar absorption refrigeration system equipped with an economizer. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawing,  FIG. 1  shows an embodiment having a solar collector  01 , generator/absorber unit  07 , economizer  02 , dephlegmator  12 , condenser unit  14 , and evaporator unit  18 . 
     Solar collector  01  converts energy from sunlight into thermal energy that can be used to perform mechanical work on a fluid. Solar collector  01  is a type of thermal collector, which may include any of various configurations of heat-exchange media, such as water, saline, or oil, for example, and structures adapted for use with various heat sources, such as sunlight, exhaust gas, or geothermal heat, for example. Solar collector  01  may have one or more of various geometries including a flat plate, arc, or compound parabolic curve, for example. Likewise, solar collector  01  may exploit optical or other properties of sunlight, including absorption, reflection, or refraction, for example, to harness useable energy from sunlight. 
     Generator/absorber unit  07  plays the role of both a generator and an absorber. Generator/absorber unit  07  may use a combination of absorbent and refrigerant such as aqua-ammonia or lithium-bromide, for example. Generator/absorber unit  07  may take the form of a pressure vessel constructed of a material such as steel or aluminum that can withstand pressure and that is compatible with the particular refrigerant-absorbent combination used in the system. 
     Economizer  02  is a regenerative-type heat exchanger, or in other words, a temporary energy storage unit containing a heat-transfer fluid such as water, saline, or oil, for example. In an embodiment, the heat-transfer fluid in economizer  02  can be the same fluid as that used in solar collector  01 , or in another embodiment, it can be another fluid in a separate circuit. The main objective of economizer  02  is to store some amount of energy that is rejected by generator/absorber unit  07  when generator/absorber unit  07  is depressurized following the generation process. The stored energy in economizer  02  is returned back to the intermittent system during the pressurization process in generator/absorber unit  07 . As a result, economizer  02  reduces the total amount of solar energy input into generator/absorber unit  07 , thereby increasing the coefficient of performance of the intermittent system. Economizer  02  may be constructed of a material such as metal or plastic suitable to store the heat-transfer fluid. A thermal insulator such as one or more of a metal foil, fiberglass, or foam, surrounding economizer  02  can help maintain the temperature of fluid stored in economizer  02 . 
     Dephlegmator  12  is a device arranged for the partial condensation of the absorbent-refrigerant vapor. In dephlegmator  12 , absorbent condenses while refrigerant remains as a vapor. Dephlegmator  12  may have the form of a pipe with a heat exchanger or heat-sink to draw thermal energy out of the absorbent. Heat rejection may be achieved in dephlegmator  12  by means of a heat exchanger circuit having a coolant fluid flowing in it, or by free convection to the ambient air, for example. Dephlegmator  12  may be constructed of a material such as metal, plastic, or glass, that is suitable for use with a given absorbent-refrigerant combination. 
     Condenser unit  14  is a device arranged for the condensation of refrigerant. Condensed liquid refrigerant formed in the condenser flows into evaporator unit  18 . As with dephlegmator  12 , heat rejection in condenser  14  may be achieved by means of a heat exchanger circuit having a coolant fluid flowing in it, or by free convection to the ambient air, for example. Condenser  18  may be constructed of a material such as metal, plastic, or glass, that is suitable for use with a given refrigerant. 
     Evaporator unit  18  is where a cooling effect is generated by evaporation of the refrigerant. The cooling effect occurs in the evaporator, but the cooling can be made to be felt remotely via a heat exchanger circuit. The evaporator unit includes a pressure vessel that can be constructed of steel or aluminum or another material suitable to withstand pressure and that is compatible with the refrigerant. 
     Because of the intermittent behavior of the system, a single generator/absorber unit  07  functions as a generator during the daytime and as an absorber at the nighttime. The generator/absorber unit  07  thus is composed of a heat exchanger  08  for heating purposes when functioning as a generator and another heat exchanger  10  for cooling purposes when functioning as an absorber. The solar collector  01  and the economizer  02  are both connected to the generator/absorber unit  07 . 
     The economizer  02  is a temporary energy storage unit that, in an embodiment, contains the same fluid that also flows through the solar collector  01 . The energy stored in the economizer  02  is a function of the operating temperature range of the economizer  02  and the heat storage capacity of its energy storing medium. The stored energy in the economizer  02  is returned back to the intermittent system during the pressurization process in the generator/absorber unit  07 . As a result, the economizer  02  reduces the total amount of energy input into the generator/absorber unit  07  thus increasing the coefficient of performance of the intermittent system. 
     In an embodiment, the refrigeration process utilizes water as the absorbent and ammonia as the refrigerant. The process starts with the pressurization of strong aqua-ammonia solution in the generator/absorber unit  07  during the daytime. The pressurization process is initiated by the heat exchange between the economizer  02  and the strong aqua-ammonia solution in the generator/absorber unit  07  through heat exchanger unit  08  keeping the solar collector  01  isolated. The isolation of solar collector  01  is obtained by closing valve  03  and valve  05  while keeping valve  04  and valve  06  opened. As a result, the temperature of the strong solution will rise whereas the temperature of the economizer  02  will drop. Ideally the heat exchange continues until the temperature of the economizer  02  becomes equal to the temperature of the strong aqua-ammonia solution in the generator/absorber unit  07 . However, practically, the heat exchange will continue until a minimum temperature difference is maintained between economizer  02  and the strong aqua-ammonia solution in the generator/absorber unit  07  such that the temperature of the economizer  02  is higher than the temperature of the strong solution. Hence, during this process, economizer  02  cooling takes place and partial heating of strong aqua-ammonia solution takes place. After this, the economizer  02  is disconnected from the generator/absorber unit  07  with the help of the valve arrangement and the solar collector field  01  is connected to the generator/absorber unit  07 . This is achieved by closing valve  04  and valve  06  while opening valve  03  and valve  05 . 
     Pressurization continues to take place by gaining heat from the solar collector field  01 . As a result of this pressurization process, the temperature of the strong aqua-ammonia solution in the generator/absorber unit  07  rises. The generation process at constant pressure takes place in the generator/absorber unit  07  through heat from heat exchanger circuit  08 . The temperature of the solution increases as generation takes place during this process. As a result of this generation process, aqua-ammonia vapor is generated and the concentration of strong aqua-ammonia solution drops causing a further increase in the temperature. As generation continues to take place, the water content inside the generated aqua-ammonia vapor increases with the increase in temperature of generator/absorber unit  07 . 
     Removal of water content from the aqua-ammonia vapor is carried out by the dephlegmator  12 . Dephlegmator  12  is required to rectify the aqua-ammonia vapor for operation at evaporating temperatures below the freezing point of water. So, at the top of the generator/absorber unit  07  is located a dephlegmator  12 . The dephlegmator  12  also consists of a shell  12  and a heat exchanger circuit  11  for cooling purposes. The binary mixture of aqua-ammonia vapor generated in the generator/absorber unit  07  is cooled by the heat exchanger circuit  11  inside the dephlegmator shell  12  above the condenser temperature. This results in the rectification of ammonia vapor as all the water vapor is condensed inside the dephlegmator  12 . The aqua-ammonia condensate from the dephlegmator  12  moves back into the generator/absorber unit  07  whereas the rectified ammonia vapor moves to the condenser shell  14  while passing through valve  13 . The generator/absorber unit  07  is separated from the condenser  14  and evaporator units  18  with the help of a ball valve  13  and a throttling valve  09 . Hence, throughout the daytime operation of the intermittent system, the generator/absorber unit  07  acts as a generator as heat is added to the system through heat exchanger circuit  08 . The purified ammonia vapor then moves to the condenser  14 , where it is condensed by rejecting heat to the coolant inside the heat exchanger circuit  15  and stored as a saturated liquid refrigerant inside the evaporator  18  during the daytime. 
     The generation process is followed by the depressurization of weak aqua-ammonia solution in the generator/absorber unit  07 . By the start of the nighttime, the solar collector field  01  is isolated from the system and the economizer  02  is reconnected into the system by closing valve  03  and valve  05  while opening valve  04  and valve  06 . The depressurization process is initiated by heat exchange between the economizer  02  and the weak aqua-ammonia solution in the generator/absorber unit  07 . As a result, the temperature of the weak solution drops whereas the temperature of economizer  02  rises. Ideally, heat exchange continues to take place till the temperature of economizer  02  becomes equal to the temperature of the weak aqua-ammonia solution in the generator/absorber unit  07 . However, practically, the heat exchange will continue until a minimum temperature difference is maintained between economizer  02  and the weak aqua-ammonia solution in the generator/absorber unit  07  such that the temperature of the weak solution is higher than the temperature of the economizer  02 . Hence, during this process, heating of economizer  02  takes place and partial cooling of weak aqua-ammonia solution takes place. After this, the economizer  02  is again disconnected from the generator/absorber unit  07  by closing valve  04  and valve  06 . With both the solar collector field  01  and the economizer  02  isolated from the system, heat exchanger circuit  10  is connected to the generator/absorber unit  07 . 
     Throughout the nighttime operation of the system, the generator/absorber unit  07  behaves as an absorber for the system. Depressurization of the system continues to take place by heat rejection circuit  10  from the absorber. During depressurization, the saturated liquid refrigerant is kept isolated from the system by ball valve  16  and throttling valve  09 . After depressurization is completed, throttling valve  09  is opened which reduces the pressure inside evaporator  18  producing a refrigeration effect in the heat exchanger circuit  17  due to the evaporation of refrigerant. The heat exchanger circuit  17  inside the evaporator  18  is therefore required to be filled with a brine solution if the evaporation temperature is below the freezing point of water to avoid choking the heat exchanger circuit  17  by freezing inside it. The refrigerant vapor then moves from the evaporator  18  to the generator/absorber unit  07  through the throttling valve  09  to be absorbed into the weak solution in the generator/absorber unit  07 . Hence, strong solution is produced inside the generator/absorber unit  07  by rejecting the heat of absorption to cooling heat exchanger circuit  10 . Coolant is provided to the heat exchanger circuit inside condenser  15  and dephlegmator  11  during the daytime and to the heat exchanger circuit inside the absorber  10  during the nighttime. 
     The overall coefficient of performance for an intermittent system is basically the ratio of total energy of evaporation, i.e., the refrigeration effect, to the total energy of generation. The total energy of generation is the sum of energy required for pressurizing the generator/absorber unit  07  and the energy required to generate aqua-ammonia vapor. The economizer  02  contributes during the pressurization process only, reducing the energy required by solar input for pressurization and hence reduces the overall energy of generation required to run the intermittent system. However, the economizer  02  does not affect the energy of evaporation, i.e., the refrigeration effect, at all. Thus the increase in coefficient of performance is a direct result of energy conservation due to economizer  02 . The increase in the coefficient of performance of the system corresponds to a decrease in the required collector area for a particular load requirement. Since a solar collector may be the most expensive component of such a refrigeration system, this will result in a considerable decrease in the capital cost of such a system. The cost of the economizer  02 , which can be an insulated tank with some piping, can be comparatively low. Also, since the temperature in the economizer  02  need not be very high, a moderate level of insulation can be used, thereby reducing cost. 
     A refrigeration system according to another embodiment is limited to having a single condenser. A refrigeration system according to yet another embodiment is limited to having a single evaporator. In another embodiment, an evaporator is in a heat exchange relationship with an external cooling load circuit. In another embodiment, the external cooling load circuit uses water as the coolant. 
     In an embodiment, an economizer device is utilized for heating the refrigerant in the generator without operating the solar collector field. In an embodiment, pressurization takes place by isochoric heating of the generator, without the aid of a pump. In another embodiment, a refrigeration effect is achieved without utilizing a refrigerant heat exchanger. In another embodiment, a throttling process is achieved without utilizing a jet ejection mixer. In another embodiment, waste heat is captured and used by the system wherein an economizer is a regenerator type heat exchanger rather than a recuperator type heat exchanger. 
     In an embodiment, the regenerator is separated from the solar collector. In another embodiment, a refrigeration effect is achieved using a single absorption effect. In another embodiment, an economizer provides indirect heat exchange between a hot solution and a cold solution by first storing thermal energy within a third medium from the hot solution and then supplying the stored thermal energy to the cold solution. In another embodiment, a refrigeration system does not include a recuperator type heat exchanger. 
     Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, define, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

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