Abstract:
Methods for optimizing a thermocline in a thermal energy storage fluid within a thermal energy storage tank are disclosed. The methods comprise identifying a thermocline region in the fluid, adding thermal energy to a fluid stream extracted from the thermocline region, and returning the fluid stream to the tank at a plurality of locations above the thermocline region. The methods further comprise regulating the temperature of the fluid returned to the tank at a set point temperature by modulating the flow rate of the fluid stream and by changing the location from where the fluid is extracted from the tank.

Description:
TECHNICAL FIELD 
     The present invention relates to a thermal energy storage system. More specifically, the invention relates to maintaining and optimizing a thermocline in a thermal energy storage fluid contained within a thermal energy storage tank. 
     BACKGROUND 
     A thermal energy storage system comprises a thermal energy storage fluid contained within a tank. In operation, thermal energy is stored within the fluid by extracting the fluid from the tank, pumping the extracted fluid through a heat exchanger wherein thermal energy in the form of heat is added to the fluid thereby raising its temperature, and returning the heated fluid to the thermal energy storage tank. As is well known in the art, the density of a fluid decreases as the temperature of the fluid increases. As such, a fluid at a relatively lower temperature will have a density which is relatively higher than the density of the same fluid at a relatively higher temperature. Accordingly, the density of the heated fluid returned to the tank will be relatively lower than the density of the fluid extracted from the tank. Therefore, the relative higher temperature and relatively less dense fluid will stay above the relatively lower temperature and relatively more dense fluid. As can be seen, the fluid at the highest temperature will stay near the top of the thermal storage tank while the fluid at the lowest temperature will stay towards the bottom of the tank. This separation of the relatively warmer and relatively less dense fluid from the relatively cooler and relatively more dense fluid is known as thermal stratification or thermocline. In a perfectly stratified thermal storage tank, only the fluid at the highest temperature will be at the top of the tank, and only the fluid at the coldest temperature will be at the bottom of the tank. However, in a typical thermal storage tank, there exists a transition layer between the hot and cold regions. 
     One shortcoming of thermal energy storage systems is that it is relatively difficult to maintain a thermocline in a fluid within a thermal storage tank. The process of repeatedly extracting the fluid at a relatively cooler temperature from near the bottom of the tank and returning the fluid at a relatively higher temperature to near the top of the tank increases the width of the transition layer resulting in a degradation of the thermocline in the fluid within the thermal storage tank. The ratio of the difference between the height of the fluid in the tank and the width of the transition layer to the total height of the fluid in the tank is known in the art as the utilization factor. The utilization factor provides an indication of the amount of useful thermal energy stored in the tank. Accordingly, as the width of the transition layer increases, the utilization factor, and therefore the thermal storage capacity, decreases. 
     As is well known in the art, the thermocline in a thermal storage tank degrades due to a number of factors such as conduction between the layers of fluid at different temperatures, mixing of the fluid due to turbulence, an increase in the velocity of the fluid, etc. Additionally, thermocline degradation becomes more severe where there is an insufficient amount of thermal energy available. The repeated process of partially charging the tank, i.e., adding thermal energy to the tank, and not completely discharging the tank, i.e., removing thermal energy from the tank, further degrades the thermocline in a thermal storage tank. One approach for minimizing thermocline degradation and improving the utilization factor is to periodically discharge the tank completely to eliminate the thermocline and then re-establish a thermocline by fully charging the tank. However, this approach is inefficient, cumbersome, and not operationally practical. 
     In order to more efficiently store thermal energy in a fluid contained within a thermal energy storage tank, it is desirable to maintain a region of relatively warmer fluid separate from and above a region of relatively cooler fluid. As such, it is desirable to maximize the utilization factor by minimizing the width of the thermocline region. 
     Accordingly, it is an objective of the present invention to provide a thermal energy storage tank for optimally managing a thermocline in a thermal energy storage fluid contained within the thermal energy storage tank. 
     SUMMARY 
     An embodiment of the invention comprises a method for optimally managing a thermocline in a thermal energy storage fluid contained within a thermal energy storage tank. Temperatures of the fluid within the tank are measured at a plurality of locations along the vertical height of the tank, and are used for identifying the thermocline region within the fluid and for computing the average temperature of the fluid in the thermocline region. Fluid is extracted from the tank at a location whereat the temperature of the fluid within the tank equals the computed average temperature of the fluid in the thermocline region. The extracted fluid is pumped through a heat exchanger wherein thermal energy is added to the fluid to increase its temperature, and the heated fluid is returned to the tank at a location above the thermocline region. The temperature of the fluid exiting the heat exchanger and returned to the tank is maintained at a set point temperature by modulating the flow rate of the fluid extracted from the tank and pumped through the heat exchanger. 
     A thermal energy storage tank in an embodiment of the invention comprises a thermal energy storage fluid contained within the tank and a plurality of temperature sensors for measuring the temperatures of the fluid at a plurality of locations along the vertical height of the tank. The tank further comprises a plurality of valves at a plurality of locations along the vertical height of the tank for extracting the fluid from the thermocline region. An inlet of each one of the plurality of valves is in fluid communication with the fluid within the tank and an outlet of each one of the plurality of valves is in fluid communication with a header. A pump in fluid communication with the header extracts fluid from the tank and pumps the extracted fluid through a heat exchanger. In the heat exchanger, thermal energy is added to the extracted fluid pumped through the heat exchanger and the heated fluid is returned to the tank at a location above the thermocline region. The operation of the pump and the operation of each one of the plurality of valves is controlled by a controller in accordance with an embodiment of the method of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a thermal energy storage system in accordance with an embodiment of the invention; 
         FIG. 2  is a flowchart of a method for identifying an extraction location for the fluid from the tank in accordance with an embodiment of the invention; 
         FIG. 3  is a flowchart of a method, in accordance with an embodiment of the invention, for regulating the fluid exiting the heat exchanger at a set point temperature; 
         FIG. 4  is a flowchart of a method for setting an extraction location for the fluid from the tank in accordance with another embodiment of the invention; 
         FIG. 5  is a flowchart of a method for identifying an extraction location for the fluid from the tank and computing the fluid flow rate in accordance with yet another embodiment of the invention; 
         FIG. 6  is a flowchart of a method for identifying an extraction location for the fluid from the tank at the minimum flow rate in accordance with another embodiment of the invention; 
         FIG. 7  is a flowchart of a method for identifying an extraction location for the fluid from the tank at the maximum flow rate in accordance with yet another embodiment of the invention; 
         FIG. 8  is an exemplary application of the thermal energy storage system of  FIG. 1 ; and 
         FIG. 9  is another exemplary application of the thermal energy storage system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     While multiple embodiments of the instant invention are disclosed, still other embodiments may become apparent to those skilled in the art. The following detailed description describes only illustrative embodiments of the invention. It should be clearly understood that there is no intent, implied or otherwise, to limit the invention in any form or manner to that described herein. As such, all alternative embodiments of the invention are considered as falling within the spirit, scope and intent of the disclosure. 
       FIG. 1  is an illustration of thermal energy storage system  100  in accordance with an embodiment of the invention. Thermal energy storage system  100  comprises thermal energy storage fluid  102  contained within thermal energy storage tank  104 . 
     In an embodiment of the invention, thermal energy storage fluid  102  is a single phase fluid in the form of a liquid which does not undergo a change in phase. In an alternate embodiment of the invention, thermal energy storage fluid  102  is a phase change fluid which undergoes a change in phase between the liquid and solid phases. In another embodiment of the invention, fluid  102  is a slurry comprising both liquid and solid phases. In yet another embodiment, tank  104  contains a solid such as rocks or pebbles submerged in fluid  102  wherein fluid  102  is a liquid such as oil or fluid  102  is a phase change fluid which undergoes a change in phase between the liquid and solid phases. In an alternate embodiment, tank  104  contains an encapsulated phase change material submerged in fluid  102  wherein fluid  102  is a liquid such as oil or fluid  102  is a phase change fluid which undergoes a change in phase between the liquid and solid phases. 
     Tank  104  includes a plurality of temperature sensors  106  at a plurality of locations along a vertical height of the tank. While four temperature sensors  106  are shown in  FIG. 1 , it should be understood that there is no intent to restrict the total number of temperature sensors  106  to four. Temperature sensors  106  are used for measuring the temperature of fluid  102  within tank  104  and therefore for identifying a thermocline region in fluid  102 . Accordingly, in alternate embodiments of the invention the total number of temperature sensors  106  can be less than four or more than four. In the embodiment of the invention illustrated in  FIG. 1 , temperature sensors  106  are located inside tank  104  and submerged in fluid  102 . In an alternate embodiment, temperature sensors  106  are within a plurality of thermowells extending into fluid  102  through a surface of tank  104 . In another embodiment, temperature sensors  106  are in contact with an inside surface of tank  104 . In yet another embodiment, temperature sensors  106  are in contact with an outside surface of tank  104 . 
     Tank  104  further includes a plurality of valves  108 . While four valves  108  are shown in  FIG. 1 , it should be understood that there is no intent to restrict the total number of valves  108  to four. In alternate embodiments the total number of valves  108  can be less than four or more than four. Valves  108  are used for extracting fluid  102  from a plurality of locations along the vertical height of tank  104 . Each one of the plurality of valves  108 , for example valve  108 A in  FIG. 1 , includes valve actuator  110  for operating valve  108 A to an open position, to a closed position, or to any position between the open and closed positions. Each one of the plurality of valves  108 , again for example valve  108 A in  FIG. 1 , comprises inlet  112  with flow path  114  providing fluidic communication with fluid  102  at a plurality of locations along the vertical height of tank  104 . Each one of the plurality of valves  108 , again for example valve  108 A, further comprises outlet  116  with flow path  118  providing fluidic communication with header  120 . 
     Flow path  122  provides fluidic communication between header  120  and an inlet of pump  124 . Flow path  126  provides fluidic communication between an outlet of pump  124  and inlet  128  of heat exchanger  130 . In an embodiment of the invention, pump  124  is a variable speed pump for modulating the flow rate of the pumped fluid between a minimum flow rate and a maximum flow rate. As such, pump  124  can be operated to maintain a minimum flow rate or to maintain a maximum flow rate or to modulate the flow rate between the minimum and maximum flow rates. In another embodiment, pump  124  is a constant speed pump providing a fixed flow rate. 
     Flow path  132  within heat exchanger  130  provides fluidic communication between inlet  128  and outlet  134  of heat exchanger  130 . Flow path  136  provides fluidic communication between outlet  134  of heat exchanger  130  and fluid  102  within tank  104 . Sensor  138  measures the temperature of the fluid returned to tank  104  along flow path  136 . In an embodiment of the invention, sensor  138  measures the temperature of the fluid at outlet  134  of heat exchanger  130 . 
     Thermal energy storage system  100  further comprises controller  140  for maintaining an optimal thermocline in fluid  102 . Signals from sensor  138  and from the plurality of temperature sensors  106  are transmitted to controller  140  and, in accordance with an embodiment of the invention as described herein below, controller  140  transmits command signals to pump  124  and to each valve actuator on each of the plurality of valves  108 , for example valve actuator  110  on valve  108 A. 
       FIG. 2  is a flowchart of a method for identifying an extraction location for fluid  102  within tank  104  in accordance with an embodiment of the invention. In block  202 , the plurality of temperature sensors  106  are used for measuring the temperatures of fluid  102  within tank  104  at a plurality of locations along the vertical height of tank  104 . In block  204 , the measured temperatures from block  202  are used to identify the thermocline region in fluid  102  within tank  104 . The temperatures of fluid  102  within the thermocline region are used in block  206  to compute the average temperature of fluid  102  within the thermocline region. In block  208 , the first location along the vertical height of tank  104  is identified as the location whereat the temperature of fluid  102  within tank  104  equals the computed average temperature of fluid  102  in the thermocline region as computed in block  206 . From the plurality of valves  108 , block  210  identifies the extraction valve as the valve closest in proximity to the first location as identified in block  208 . At connecter  212  the method for optimizing the thermocline in fluid  102  within tank  104  continues as described herein below with reference to  FIG. 3 . 
       FIG. 3  is a continuing flowchart of the method for optimizing the thermocline in fluid  102  within tank  104  in accordance with an embodiment of the invention. At connector  302 , the method continues as follows. At block  304  the extraction valve is opened and all other valves are closed. At block  306 , fluid  102  from tank  104  is extracted through the extraction valve and the extracted fluid is pumped through heat exchanger  130  and returned to tank  104  at one or more locations above the thermocline region. The temperature of the fluid exiting heat exchanger  130  is measured at block  308  and compared with the exit set point temperature in decision blocks  310  and  312 . 
     At decision block  310 , if the temperature of the fluid exiting heat exchanger  130  is less than the exit set point temperature, then the fluid flow rate is decreased at block  314  and in decision block  316  the decreased flow rate is compared to the pre-specified minimum flow rate. If the flow rate has decreased to the minimum value, then at block  318  the extraction valve for fluid  102  is changed to a different extraction valve along the vertical height of tank  104  and the method repeats at block  304 . If the flow rate has not decreased to the minimum value, then the extraction valve is not changed and the method repeats at block  304 . 
     At decision block  310 , if the temperature of the fluid exiting heat exchanger  130  is not less than the exit set point temperature, then decision block  312  checks whether the temperature of the fluid exiting heat exchanger  130  is greater than the exit set point temperature. At decision block  312 , if the temperature of the fluid exiting heat exchanger  130  is greater than the exit set point temperature, then the fluid flow rate is increased at block  320  and in decision block  322  the increased flow rate is compared to the pre-specified maximum flow rate value. If the flow rate has increased to the maximum value, then at block  318  the extraction valve for fluid  102  is changed to a different extraction valve along the vertical height of tank  104  and the method repeats at block  304 . If the flow rate has not increased to the maximum value, then the extraction valve is not changed and the method repeats at block  304 . 
       FIG. 4  is a flowchart for another embodiment of a method for identifying an extraction location for fluid  102  within tank  104 . In block  402 , the plurality of temperature sensors  106  are used for measuring the temperatures of fluid  102  within tank  104  at a plurality of locations along the vertical height of tank  104 . In block  404 , the measured temperatures from block  402  are used to identify the thermocline region in fluid  102  within tank  104 . In block  406 , the first location along the vertical height of tank  104  is identified as the location below the thermocline region. From the plurality of valves  108 , block  408  identifies the extraction valve as the valve closest in proximity to the first location as identified in block  406 . At connecter  410  the method for optimizing the thermocline in fluid  102  within tank  104  continues as described herein above with reference to  FIG. 3 . 
       FIG. 5  is a flowchart for yet another embodiment of a method for identifying an extraction location for fluid  102  within tank  104  and for computing the flow rate for the extracted fluid. In block  502 , the plurality of temperature sensors  106  are used for measuring the temperatures of fluid  102  within tank  104  at a plurality of locations along the vertical height of tank  104 . In block  504 , the measured temperatures from block  502  are used to identify the thermocline region in fluid  102  within tank  104 . The temperatures of fluid  102  within the thermocline region are used in block  506  to compute the average temperature of fluid  102  within the thermocline region. In block  508 , the first location along the vertical height of tank  104  is identified as the location whereat the temperature of fluid  102  within tank  104  equals the computed average temperature of fluid  102  in the thermocline region as computed in block  506 . Block  510  computes the amount of thermal energy available at heat exchanger  130  for transfer to the fluid flowing through heat exchanger  130 . As will be more apparent from the description herein below in reference to  FIGS. 8 and 9 , the thermal energy available at heat exchanger  130  can be computed and/or measured in alternate embodiments of the invention. Alternatively, any one or more energy source can be configured to provide the thermal energy for transfer to the fluid flowing through heat exchanger  130 , and the amount of thermal energy available at heat exchanger  130  can be computed and determined by means well known in the art. 
     The flow rate for the fluid flowing through heat exchanger  130  is computed at block  512  as a function of the amount of thermal energy available at heat exchanger  130  as computed at block  510 , the average temperature of fluid  102  within the thermocline region as computed at block  506 , and the exit set point temperature for the fluid exiting heat exchanger  130 . From the plurality of valves  108 , block  514  identifies the extraction valve as the valve closest in proximity to the first location as identified in block  508 . At connecter  516  the method for optimizing the thermocline in fluid  102  within tank  104  continues as described herein above with reference to  FIG. 3 . 
       FIG. 6  is a flowchart for another embodiment of a method for identifying an extraction location for fluid  102  within tank  104  at the minimum flow rate for the extracted fluid. Block  602  computes the amount of thermal energy available at heat exchanger  130  for transfer to the fluid flowing through heat exchanger  130 . As will be more apparent from the description herein below in reference to  FIGS. 8 and 9 , the thermal energy available at heat exchanger  130  can be computed and/or measured in alternate embodiments of the invention. Alternatively, any one or more energy source can be configured to provide the thermal energy for transfer to the fluid flowing through heat exchanger  130 , and the amount of thermal energy available at heat exchanger  130  can be computed and determined by means well known in the art. The inlet temperature for the fluid entering heat exchanger  130  is computed at block  604  as a function of the amount of thermal energy available at heat exchanger  130  as computed at block  602 , the pre-specified minimum flow rate for the fluid flowing through heat exchanger  130 , and the exit set point temperature for the fluid exiting heat exchanger  130 . In block  606 , the plurality of temperature sensors  106  are used for measuring the temperatures of fluid  102  within tank  104  at a plurality of locations along the vertical height of tank  104 . In block  608 , the first location along the vertical height of tank  104  is identified as the location whereat the temperature of fluid  102  within tank  104  equals the inlet temperature for the fluid entering heat exchanger  130  as computed at block  604 . Block  610  sets the flow rate for the fluid flowing through heat exchanger  130  to the pre-specified minimum flow rate value. From the plurality of valves  108 , block  612  identifies the extraction valve as the valve closest in proximity to the first location as identified in block  608 . At connecter  614  the method for optimizing the thermocline in fluid  102  within tank  104  continues as described herein above with reference to  FIG. 3 . 
       FIG. 7  is a flowchart for yet another embodiment of a method for identifying an extraction location for fluid  102  within tank  104  at the maximum flow rate for the extracted fluid. Block  702  computes the amount of thermal energy available at heat exchanger  130  for transfer to the fluid flowing through heat exchanger  130 . As will be more apparent from the description herein below in reference to  FIGS. 8 and 9 , the thermal energy available at heat exchanger  130  can be computed and/or measured in alternate embodiments of the invention. Alternatively, any one or more energy source can be configured to provide the thermal energy for transfer to the fluid flowing through heat exchanger  130 , and the amount of thermal energy available at heat exchanger  130  can be computed and determined by means well known in the art. The inlet temperature for the fluid entering heat exchanger  130  is computed at block  704  as a function of the amount of thermal energy available at heat exchanger  130  as computed at block  602 , the pre-specified maximum flow rate for the fluid flowing through heat exchanger  130 , and the exit set point temperature for the fluid exiting heat exchanger  130 . In block  706 , the plurality of temperature sensors  106  are used for measuring the temperatures of fluid  102  within tank  104  at a plurality of locations along the vertical height of tank  104 . In block  708 , the first location along the vertical height of tank  104  is identified as the location whereat the temperature of fluid  102  within tank  104  equals the inlet temperature for the fluid entering heat exchanger  130  as computed at block  704 . Block  710  sets the flow rate for the fluid flowing through heat exchanger  130  to the pre-specified maximum flow rate. From the plurality of valves  108 , block  712  identifies the extraction valve as the valve closest in proximity to the first location as identified in block  708 . At connecter  714  the method for optimizing the thermocline in fluid  102  within tank  104  continues as described herein above with reference to  FIG. 3 . 
       FIG. 8  is an exemplary application of the thermal energy storage system of  FIG. 1  wherein like elements are identified by like numerals. As illustrated in  FIG. 8 , and in accordance with an embodiment of the invention, solar receiver  802  is a heat exchanger wherein thermal energy is transferred to a fluid stream flowing along flow path  804  through solar receiver  802 . As illustrated, flow path  804  provides fluidic communication between an outlet of pump  124  and inlet  806  of solar receiver  802 . Flow path  808  within solar receiver  802  provides fluidic communication between inlet  806  and outlet  810  of solar receiver  802 . Flow path  812  provides fluidic communication between outlet  810  of solar receiver  802  and fluid  102  within tank  104 . Sensor  814  measures the temperature of the fluid returned to tank  104  along flow path  816 . In an embodiment of the invention, sensor  814  measures the temperature of the fluid at outlet  810  of solar receiver  802 . 
       FIG. 9  is another exemplary application of the thermal energy storage system of  FIG. 1  wherein like elements are identified by like numerals. As illustrated in  FIG. 9 , and in accordance with an embodiment of the invention, heat exchanger  130  is in fluidic communication with solar receiver  902 . A fluid stream flowing along flow path  904  transports the thermal energy from solar receiver  902  to heat exchanger  130 . In heat exchanger  130 , thermal energy from the fluid stream flowing along flow path  904  is transferred to the fluid stream flowing along flow path  132 . 
     In embodiments of the invention comprising a solar receiver, such as solar receivers  802  and  902  in  FIGS. 8 and 9 , respectively, the amount of thermal energy available for transfer to a fluid stream, such as fluid streams flowing along flow paths  808  and  904  in  FIGS. 8 and 9 , respectively, can be calculated or measured with means well known in the art. 
     Various modifications and additions may be made to the exemplary embodiments presented hereinabove without departing from the scope and intent of the present invention. For example, while the disclosed embodiments refer to particular features, the scope of the instant invention is considered to also include embodiments having different combinations of features different from and/or in addition to those described herein. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as falling within the scope and intent of the appended claims, including all equivalents thereof.