Patent Publication Number: US-2022228772-A1

Title: Heat storage system

Description:
FIELD 
     The present invention relates to a heat storage system. 
     BACKGROUND 
     Patent Literature 1 described below discloses a heat pump water heater including a hot water storage tank. FIG. 10 of Patent Literature 1 discloses that, after activation of the compressor, the operating frequency of the compressor is linearly increased to a target operating frequency. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] JP 2005-134070 A 
       
    
     SUMMARY 
     Technical Problem 
     If the temperature of the compressor and the temperature of the heat exchanger are still low after activation of the compressor, the temperature of hot water flowing out of the heat pump falls below a target value. As the amount of such low-temperature hot water flowing into the hot water storage tank increases, the energy consumption efficiency of the heat accumulating operation tends to decrease. In the technique of Patent Literature 1, the amount of low-temperature hot water flowing into the hot water storage tank at the start of the heat accumulating operation is likely to increase, which causes a problem in which the energy consumption efficiency is likely to decrease. 
     The present invention has been made to solve the problem as described above, and an object of the present invention is to provide a heat storage system advantageous for preventing a decrease in energy consumption efficiency of a heat accumulating operation. 
     Solution to Problem 
     A heat storage system according to the present invention includes: a compressor for compressing refrigerant; a heat storage tank for storing a heating medium; heat exchange means for heating the heating medium using heat of the refrigerant compressed by the compressor, the heat exchange means being provided outside the heat storage tank; a heat accumulating circuit including a feed path for feeding the heating medium flowing out of the heat storage tank to the heat exchange means, a return path for returning the heating medium heated by the heat exchange means into the heat storage tank, and a pump for circulating the heating medium; and control means capable of executing an initial operation configured to control an operating frequency of the compressor at a beginning of a heat accumulating operation in which the heating medium heated by the heat exchange means is accumulated in the heat storage tank. The initial operation includes a first operation configured to maintain the operating frequency at a first frequency and a second operation after the first operation. The second operation is configured to maintain the operating frequency at a second frequency higher than the first frequency. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide the heat storage system advantageous for preventing a decrease in energy consumption efficiency of the heat accumulating operation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a heat storage system according to Embodiment 1. 
         FIG. 2  is a graph illustrating an example of temporal changes in a compressor frequency, an outflow-hot-water temperature, heating power, a water flow rate, and power consumption, at a beginning of a heat accumulating operation. 
         FIG. 3  is a graph illustrating an example of temporal changes in an inflow water temperature, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and a COP, at an end of the heat accumulating operation. 
         FIG. 4  is a graph illustrating an example of temporal changes in the compressor frequency, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and an outlet refrigerant temperature, at the beginning of the heat accumulating operation according to Embodiment 1. 
         FIG. 5  is a graph illustrating an example of temporal changes in the compressor frequency, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and an outlet refrigerant temperature, at a beginning of a heat accumulating operation according to Embodiment 2. 
         FIG. 6  is a graph illustrating an example of temporal changes in the compressor frequency, the discharged refrigerant temperature, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and the outlet refrigerant temperature, at a beginning of the heat accumulating operation according to Embodiment 4. 
         FIG. 7  is a flowchart illustrating processing executed by a controller in Embodiment 5. 
         FIG. 8  is a flowchart illustrating processing executed by the controller in Embodiment 6. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. Elements common or corresponding to each other in the drawings are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted. 
     Embodiment 1 
       FIG. 1  is a view illustrating a heat storage system according to Embodiment 1. As illustrated in  FIG. 1 , a heat storage system  1  according to Embodiment 1 includes a controller  10 , a heat pump unit  11 , and a tank unit  12  having a hot water storage tank  2 . The heat storage system  1  according to Embodiment 1 is suitable for being used as a hot-water-storing hot water supply system. The heat pump unit  11  is provided outdoors. The tank unit  12  may be provided outdoors or may be provided indoors. The heat pump unit  11  and the tank unit  12  are connected via piping and an electric cable (not illustrated). 
     The controller  10  corresponds to control means that controls operations of the heat storage system  1 . Each of actuators and each of sensors included in the heat storage system  1  are electrically connected to the controller  10 . The controller  10  may have a timer function for managing time of a day. The controller  10  may also have a calendar function for managing year, month, and day. The controller  10  may be capable of communicating with a user interface device (not illustrated). In the illustrated example, the controller  10  is provided in the heat pump unit  11 . 
     Each of functions of the controller  10  may be implemented by processing circuitry. The processing circuitry of the controller  10  may include at least one processor  10   a  and at least one memory  10   b . The at least one processor  10   a  may read and execute a program stored in the at least one memory  10   b  to implement each of the functions of the controller  10 . The processing circuitry of the controller  10  may include at least one dedicated hardware. The configuration is not limited to a configuration in which the operations are controlled by the single controller  10  as in the illustrated example. The configuration may be a configuration in which the operations are controlled by a plurality of controllers cooperating with each other. For example, a configuration may be employed in which a first controller installed in the heat pump unit  11  and a second controller installed in the tank unit  12  cooperate with each other to control the operations of the heat storage system  1 . 
     The hot water storage tank  2  stores hot water heated by the heat pump unit  11 , i.e., high-temperature water, and low-temperature water before being heated by the heat pump unit  11 . Due to the difference in water density caused by the temperature difference, a temperature stratification in which the upper side has a high temperature and the lower side has a low temperature is formed in the hot water storage tank  2 . The hot water storage tank  2  corresponds to a heat storage tank that stores a heating medium. In the present embodiment, an example of using liquid water as the heating medium will be described. However, in the present disclosure, the heating medium may be a liquid other than water, for example, a calcium chloride aqueous solution, an ethylene glycol aqueous solution, a propylene glycol aqueous solution, or alcohol. The hot water storage tank  2  is covered with a thermal insulator (not illustrated) for preventing heat dissipation. The shape of the hot water storage tank  2  is not particularly limited and may be, for example, a cylindrical shape whose vertical direction is in the axis direction. 
     The heat pump unit  11  includes a refrigerant circuit in which a compressor  3 , a water heat exchanger  4 , a decompressor  5 , and an air heat exchanger  6  are annularly connected via refrigerant piping. The heat pump unit  11  further includes a blower  7  that sends outdoor air to the air heat exchanger  6 . The heat pump unit  11  performs an operation of a refrigeration cycle, i.e., a heat pump cycle, using the refrigerant circuit. The compressor  3  compresses refrigerant gas. The refrigerant may be, for example, any of carbon dioxide, ammonia, propane, isobutane, fluorocarbons such as HFC, HFO-1123, and HFO-1234yf. Using carbon dioxide as the refrigerant is more advantageous in terms of suppressing the influence on global warming. 
     The water heat exchanger  4  has a refrigerant flow path through which the refrigerant compressed by the compressor  3  flows, and a water flow path through which the water from the hot water storage tank  2  flows. The water heat exchanger  4  exchanges heat between the refrigerant flowing through the refrigerant flow path and the water flowing through the water flow path to heat the water. The water heat exchanger  4  is provided outside the hot water storage tank  2 . The water heat exchanger  4  corresponds to heat exchange means that heats the heating medium using heat of the refrigerant compressed by the compressor  3 . 
     The heat exchange means in the present disclosure is not limited to an example like the water heat exchanger  4  that directly exchanges heat between the refrigerant compressed by the compressor  3  and water. Although not illustrated, the heat exchange means may be, for example, one that heats water via a relay heating medium from refrigerant. In this case, the heat exchange means may include a first heat exchanger (not illustrated) that exchanges heat between the relay heating medium and the refrigerant compressed by the compressor  3 , a second heat exchanger (not illustrated) that exchanges heat between the relay heating medium heated by the first heat exchanger and water, and a pump (not illustrated) that circulates the relay heating medium between the first heat exchanger and the second heat exchanger. 
     The decompressor  5  expands and decompresses high-pressure refrigerant that has passed through the water heat exchanger  4 . The decompressor  5  may be an expansion valve whose opening degree is adjustable. The air heat exchanger  6  exchanges heat between outdoor air and low-pressure refrigerant that has passed through the decompressor  5  to evaporate the low-pressure refrigerant. Low-pressure refrigerant gas that has passed through the air heat exchanger  6  flows into the compressor  3 . 
     A lower part of the hot water storage tank  2  and an inlet of the water flow path of the water heat exchanger  4  are connected via a feed path  13 . A water pump  8  is connected to an intermediate portion of the feed path  13 . An outlet of the water flow path of the water heat exchanger  4  and an upper part of the hot water storage tank  2  are connected via a return path  14 . In the illustrated example, the water pump  8  is provided in the tank unit  12 . Instead of the illustrated example, the water pump  8  may be provided in the heat pump unit  11 . 
     A water supply pipe  15  is connected to the lower part of the hot water storage tank  2 . A hot water supply pipe (not illustrated) for supplying hot water from the hot water storage tank  2  is connected to the upper part of the hot water storage tank  2 . When the heat storage system  1  is used, low-temperature water supplied from a water source such as a waterworks passes through the water supply pipe  15  and flows into the lower part of the hot water storage tank  2 , so that the hot water storage tank  2  is always filled with water. At the time of hot water supply, pressure from the water supply pipe  15  causes hot water in the hot water storage tank  2  to flow out to the hot water supply pipe. Accordingly, the same amount of the low-temperature water flows from the water supply pipe  15  into the lower part of the hot water storage tank  2 . 
     The water pump  8 , the feed path  13 , and the return path  14  form a heat accumulating circuit. When the water pump  8  is operated, water circulates in the heat accumulating circuit. The heat storage system  1  is capable of executing a heat accumulating operation. The heat accumulating operation is an operation in which water is circulated in the heat accumulating circuit and high-temperature water heated by the water heat exchanger  4  is accumulated in the hot water storage tank  2 . The heat accumulating operation is performed as follows. The water pump  8  and the heat pump unit  11  are operated. Water that has flowed out of the lower part of the hot water storage tank  2  is sent to the water heat exchanger  4  through the feed path  13 . The high-temperature water heated by the water heat exchanger  4  returns to the tank unit  12  through the return path  14  and flows into the upper part in the hot water storage tank  2 . By performing the heat accumulating operation as described above, the high-temperature water is gradually stored in the hot water storage tank  2  from the upper part to the lower part, and a temperature boundary layer between the high-temperature water and the low-temperature water gradually moves downward. 
     In the following description, the temperature of the water flowing out of the water heat exchanger  4  is referred to as an “outflow-hot-water temperature.” The outflow-hot-water temperature corresponds to an outlet temperature which is a temperature of the heating medium flowing out of the water heat exchanger  4 . An outflow-hot-water temperature sensor  16  provided in the return path  14  detects the outflow-hot-water temperature. The outflow-hot-water temperature sensor  16  corresponds to outlet temperature detection means. 
     In the following description, the temperature of the refrigerant discharged from the compressor  3  is referred to as a “discharged refrigerant temperature.” A discharge temperature sensor  17  provided in a refrigerant path between an outlet of the compressor  3  and the water heat exchanger  4  detects the discharged refrigerant temperature. The discharge temperature sensor  17  corresponds to refrigerant temperature detection means. An outdoor air temperature sensor  18  detects a temperature of the outdoor air flowing into the air heat exchanger  6 . 
     A plurality of stored hot water temperature sensors  19  is attached to the hot water storage tank  2 . These stored hot water temperature sensors  19  are positioned at different heights. A temperature distribution of the hot water in the hot water storage tank  2  in the vertical direction can be detected by the stored hot water temperature sensors  19 . The controller  10  can calculate a hot water remaining amount and a heat storage amount in the hot water storage tank  2  using the temperature distribution. The stored hot water temperature sensors  19  correspond to temperature distribution detection means that detects a temperature distribution of the heating medium stored in the hot water storage tank  2 . 
     In the following description, a target value of the outflow-hot-water temperature is referred to as a “target outflow-hot-water temperature,” and an outflow-hot-water temperature detected by the outflow-hot-water temperature sensor  16  is referred to as an “actual outflow-hot-water temperature.” The target outflow-hot-water temperature may be, for example, a value within a range of 65° C. to 90° C. 
     In the heat accumulating operation, the controller  10  uses the water pump  8  to control the flow rate of the water flowing through the heat accumulating circuit so that the actual outflow-hot-water temperature becomes equal to the target outflow-hot-water temperature. This control is hereinafter referred to as “water flow rate feedback control.” The operating speed of the water pump  8  is hereinafter referred to as a “pump speed.” The pump speed may be, for example, the rotation speed of the water pump  8 . In the following description, the flow rate of the water flowing through the heat accumulating circuit is simply referred to as a “water flow rate.” 
     The water flow rate feedback control is performed as follows, for example. When the actual outflow-hot-water temperature is lower than the target outflow-hot-water temperature, the controller  10  reduces the pump speed to thereby reduce the water flow rate. When the actual outflow-hot-water temperature is higher than the target outflow-hot-water temperature, the controller  10  increases the pump speed to thereby increase the water flow rate. The water flow rate feedback control can cause the actual outflow-hot-water temperature to approach the target outflow-hot-water temperature. 
     In the following description, the amount of heat given by the refrigerant to the water in the water heat exchanger  4  per time is referred to as a “heating power.” The unit of the heating power is, for example, watt. 
     The controller  10  controls an operation of the compressor  3 . The operating speed of the compressor  3  is variable. The controller  10  can vary the operating speed of the compressor  3  by varying the operating frequency of an electric motor included in the compressor  3  by inverter control. As the operating frequency of the compressor  3  is higher, the operating speed of the compressor  3  is higher. As the operating speed of the compressor  3  is higher, the refrigerant circulation amount is higher, and the heating power is higher. The operating frequency of the compressor  3  is hereinafter referred to as a “compressor frequency.” 
     As the outdoor air temperature is higher, the amount of heat absorbed by the refrigerant from the outdoor air in the air heat exchanger  6  is higher. If the compressor frequency is made constant, as the outdoor air temperature is higher, the heating power is higher. 
     The controller  10  may determine a target value of the operating speed or operating frequency of the compressor  3  in the heat accumulating operation according to a target value of the heating power and the outdoor air temperature detected by the outdoor air temperature sensor  18 . 
     The opening degree of the decompressor  5  is hereinafter referred to as a “decompressor opening degree.” Furthermore, a difference between a target value of the discharged refrigerant temperature and the discharged refrigerant temperature detected by the discharge temperature sensor  17  is referred to as a “refrigerant temperature deviation.” The controller  10  corrects the decompressor opening degree in response to the refrigerant temperature deviation. For example, the controller  10  corrects the decompressor opening degree so that the discharged refrigerant temperature detected by the discharge temperature sensor  17  becomes equal to the target value. 
     Energy consumption efficiency of the heat pump unit  11  is represented as a ratio of the heating power to the power consumption of the heat pump unit  11 . In the present embodiment, a coefficient of performance (COP) is used as the energy consumption efficiency. In the following description, the power consumption of the heat pump unit  11  is simply referred to as “power consumption.” 
     The temperature of the water flowing from the feed path  13  into the water heat exchanger  4  is hereinafter referred to as an “inflow water temperature.” As the inflow water temperature is higher, the heating power decreases, so that the COP of the heat pump unit  11  decreases. 
     As described above, unheated low-temperature water flows into the lower part of the hot water storage tank  2  from the water supply pipe  15 , and high-temperature water heated by the heat pump unit  11  flows into the upper part of the hot water storage tank  2 . A medium-temperature water layer having a temperature of, for example, about 30° C. to 50° C. may be generated between the high-temperature water layer and the low-temperature water layer in the hot water storage tank  2 . This medium-temperature water layer gradually moves downward in the hot water storage tank  2  during execution of the heat accumulating operation. At the end of the heat accumulating operation, once the medium-temperature water layer reaches a lowermost part in the hot water storage tank  2 , the medium-temperature water flows into the water heat exchanger  4  through the feed path  13 . As a result, the inflow water temperature increases, so that the COP decreases. As the medium-temperature water in the hot water storage tank  2  increases, the inflow water temperature easily increases at the end of the heat accumulating operation, so that the COP easily decreases at the end of the heat accumulating operation. Thus, the increase in the medium-temperature water in the hot water storage tank  2  is not preferable. 
     The beginning of the heat accumulating operation is right after activation of the compressor  3 , and thus the temperature of the compressor  3  and the temperature of the water heat exchanger  4  are low. Thus, at the beginning of the heat accumulating operation, hot water at a temperature lower than the target outflow-hot-water temperature flows out of the water heat exchanger  4 . This state is hereinafter referred to as a “low-temperature outflow.” As the water flow rate in the low-temperature outflow is higher, the medium-temperature water in the hot water storage tank  2  easily increases, so that the COP easily decreases at the end of the heat accumulating operation. 
     In the present embodiment, the controller  10  is capable of executing an initial operation that controls the compressor frequency at the beginning of the heat accumulating operation. The initial operation includes a first operation that maintains the compressor frequency at a first frequency and, after the first operation, a second operation that maintains the compressor frequency at a second frequency higher than the first frequency. According to the present embodiment, executing the initial operation can reduce the water flow rate in the low-temperature outflow. As a result, the amount of the medium-temperature water in the hot water storage tank  2  can be reduced, so that the decrease in the COP at the end of the heat accumulating operation can be reduced. The reason for this will be described below. 
     A water-side heating power Qw is the amount of heat received by the water in the water heat exchanger  4  per time. The water-side heating power Qw can be calculated by the following formula, where the outflow-hot-water temperature is Two, the inflow water temperature is Twi, the water flow rate is Vw, and the specific heat of the water is Cpw. 
         Qw =( Two−Twi )× Vw×Cpw   (1)
 
     A refrigerant-side heating power Qr is the amount of heat lost by the refrigerant in the water heat exchanger  4  per time. The refrigerant-side heating power Qr can be calculated by the following formula, where the enthalpy of the refrigerant flowing into the water heat exchanger  4  is Ein, the enthalpy of the refrigerant flowing out of the water heat exchanger  4  is Eout, and the refrigerant circulation amount is Rc. 
         Qr =( E in− E out)× Rc   (2)
 
     Here, when the amount of heat dissipated from the water heat exchanger  4  to ambient air and the like is ignored, the water-side heating power Qw is equal to the refrigerant-side heating power Qr. Reducing the compressor frequency reduces the refrigerant circulation amount Rc, so that the refrigerant-side heating power Qr decreases based on the above formula (2). The water-side heating power Qw also decreases accordingly. Here, the inflow water temperature Twi is determined according to the environmental condition and thus can be considered constant. The outflow-hot-water temperature Two is controlled by the pump speed as described above. Accordingly, when the water-side heating power Qw decreases, the water flow rate Vw decreases based on the above formula (1). 
     When the compressor  3  is activated, the controller  10  executes the first operation of the initial operation. In the first operation of the initial operation, the controller  10  keeps the compressor frequency at the first frequency lower than the second frequency. Thus, in the first operation of the initial operation, the refrigerant-side heating power Qr is lower, and accordingly the water-side heating power Qw is lower and the water flow rate Vw is lower. In this way, the water flow rate Vw in the low-temperature outflow can be reduced. As a result, the amount of the medium-temperature water in the hot water storage tank  2  can be reduced, so that the decrease in the COP at the end of the heat accumulating operation can be reduced. 
       FIG. 2  is a graph illustrating an example of temporal changes in the compressor frequency, the outflow-hot-water temperature, the heating power, the water flow rate, and the power consumption, at the beginning of the heat accumulating operation.  FIG. 3  is a graph illustrating an example of temporal changes in the inflow water temperature, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and the COP, at the end of the heat accumulating operation. In these graphs, the solid lines correspond to an example according to the present embodiment, and the broken lines correspond to a comparative example. 
     As illustrated in  FIG. 2 , the controller  10  in the present embodiment executes the first operation of the initial operation between a time t 0  to a time t 1  and executes the second operation of the initial operation after the time t 1 . The controller  10  switches the compressor frequency from the first frequency to the second frequency at the time t 1 . 
     The comparative example is an operation that maintains the compressor frequency at the second frequency from the start without execution of the initial operation. As illustrated in  FIG. 2 , according to the present embodiment, the heating power, the water flow rate and the power consumption, at the beginning of the heat accumulating operation, are lower than those of the comparative example. According to the present embodiment, the water flow rate at the beginning of the heat accumulating operation is lower than that of the comparative example, so that the amount of the medium-temperature water accumulated in the hot water storage tank  2  is smaller than that of the comparative example. 
     As illustrated in  FIG. 3 , in the comparative example, at the end of the heat accumulating operation, the inflow water temperature starts to increase from a time t 2 . In association with the increase in the inflow water temperature, the heating power and the COP decrease. In contrast, according to the present embodiment, the inflow water temperature starts to increase from a time t 3  later than the time t 2 . Thus, according to the present embodiment, the timing at which the inflow water temperature starts to increase at the end of the heat accumulating operation can be more delayed than that of the comparative example. This is because the amount of the medium-temperature water in the hot water storage tank  2  is smaller than that of the comparative example. According to the present embodiment, the decrease in the COP at the end of the heat accumulating operation can be more reduced than that of the comparative example. 
     The value of the second frequency corresponds to a value that makes it possible to obtain a target heating power when the heat accumulating operation reaches a stable state. According to the present embodiment, the second operation that maintains the compressor frequency at the second frequency is executed in the initial operation, so that when the heat accumulating operation reaches a stable state, the target heating power can be obtained. 
     The controller  10  may determine the value of the second frequency in response to the outdoor air temperature detected by the outdoor air temperature sensor  18 . For example, the controller  10  may determine the second frequency such that as the outdoor air temperature is lower, the value of the second frequency is higher. 
     The controller  10  may complete the first operation on the condition that the actual outflow-hot-water temperature has reached a target temperature. In this case, in response to the resolution or the like of the outflow-hot-water temperature sensor  16 , when the actual outflow-hot-water temperature enters an allowable range of, for example, about ±2° C. with respect to the target outflow-hot-water temperature, the controller  10  may determine that the actual outflow-hot-water temperature has reached the target temperature and complete the first operation. This allows to complete the first operation at a more appropriate timing. 
     Embodiment 2 
     Next, with reference to  FIGS. 4 and 5 , Embodiment 2 will be described focusing on differences from Embodiment 1 described above. Elements common or corresponding to the elements described above are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted. 
     In the following description, the temperature of the refrigerant flowing out of the water heat exchanger  4  is referred to as an “outlet refrigerant temperature.”  FIG. 4  is a graph illustrating an example of temporal changes in the compressor frequency, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and the outlet refrigerant temperature, at the beginning of the heat accumulating operation according to Embodiment 1.  FIG. 4  corresponds to  FIG. 2  with the graph of the outlet refrigerant temperature added thereto. 
     As illustrated in  FIG. 4 , in Embodiment 1, the outlet refrigerant temperature may greatly increase right after the start of the first operation of the initial operation. This cause is as follows. Right after the start of the first operation, the actual outflow-hot-water temperature is lower than the target outflow-hot-water temperature, so that the water flow rate is suppressed at an extremely low value by the water flow rate feedback control. As the water flow rate is lower, the outlet refrigerant temperature is higher. As the outlet refrigerant temperature is higher, the difference between the enthalpy of the refrigerant flowing into the water heat exchanger  4  and the enthalpy of the refrigerant flowing out of the water heat exchanger  4  is smaller, so that the heating power is lower. As a result, the COP decreases. In this way, in Embodiment 1, the great increase in the outlet refrigerant temperature right after the start of the first operation may result in the decrease in the COP. 
     In Embodiment 2, to reduce the decrease in the COP right after the start of the first operation, the water flow rate right after the start of the first operation is made higher than that of Embodiment 1.  FIG. 5  is a graph illustrating an example of temporal changes in the compressor frequency, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and the outlet refrigerant temperature, at the beginning of the heat accumulating operation according to Embodiment 2. 
     As illustrated in  FIG. 5 , the water flow rate right after the start of the first operation in the present embodiment is higher than that of Embodiment 1 but lower than that of the comparative example. Thereafter, the water flow rate of the first operation in the present embodiment decreases at the same level as in Embodiment 1. According to the present embodiment as described above, right after the start of the first operation, the increase in the outlet refrigerant temperature is smaller than that of Embodiment 1, so that the decrease in the COP can be more reduced than that of Embodiment 1. 
     In the following description, the difference between the target outflow-hot-water temperature and the actual outflow-hot-water temperature is referred to as an “outlet temperature deviation.” In the water flow rate feedback control, the controller  10  corrects the pump speed in response to the outlet temperature deviation. 
     As a method of making the water flow rate right after the start of the first operation higher than that of Embodiment 1, at least one of the following examples 1 to 3 may be employed. 
     (Example 1) The controller  10  sets the target outflow-hot-water temperature such that the target outflow-hot-water temperature increases over time during execution of the first operation. For example, the controller  10  may set the target outflow-hot-water temperature to a low value right after the start of the first operation and, thereafter over time, increase the target outflow-hot-water temperature stepwise. Thus, the outlet temperature deviation right after the start of the first operation is smaller than that of Embodiment 1, so that the action of correcting the water flow rate in the decrease direction by the water flow rate feedback control is weaker than that of Embodiment 1. As a result, the water flow rate right after the start of the first operation is higher than that of Embodiment 1. 
     (Example 2) In the following description, in the water flow rate feedback control, the sensitivity at which the outlet temperature deviation is reflected in the decrease in the pump speed is referred to as a “first sensitivity,” and the sensitivity at which the outlet temperature deviation is reflected in the increase in the pump speed is referred to as a “second sensitivity.” For example, the first sensitivity corresponds to the gain of the water flow rate feedback control when the actual outflow-hot-water temperature is lower than the target outflow-hot-water temperature. Furthermore, the second sensitivity corresponds to the gain of the water flow rate feedback control when the actual outflow-hot-water temperature is higher than the target outflow-hot-water temperature. The controller  10  causes the first sensitivity to be lower than the second sensitivity in the first operation. Thus, right after the start of the first operation, the action of correcting the water flow rate in the decrease direction by the water flow rate feedback control is weaker than that of Embodiment 1. As a result, the water flow rate right after the start of the first operation is higher than that of Embodiment 1. 
     (Example 3) The controller  10  causes the sensitivity at which the outlet temperature deviation is reflected in the change in the pump speed in the first operation to be lower than the sensitivity at which the outlet temperature deviation is reflected in the change in the pump speed in the second operation. For example, the controller  10  causes the gain of the water flow rate feedback control in the first operation to be lower than the gain of the water flow rate feedback control in the second operation. Thus, right after the start of the first operation, the action of correcting the water flow rate in the decrease direction by the water flow rate feedback control is weaker than that of Embodiment 1. As a result, the water flow rate right after the start of the first operation is higher than that of Embodiment 1. 
     Embodiment 3 
     Next, Embodiment 3 will be described focusing on differences from the embodiments described above. Elements common or corresponding to the elements described above are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted. 
     The pressure of the refrigerant discharged from the compressor  3  is hereinafter referred to as a “refrigerant high pressure.” Furthermore, the difference between the enthalpy of the refrigerant flowing into the water heat exchanger  4  and the enthalpy of the refrigerant flowing out of the water heat exchanger  4  is hereinafter referred to as a “refrigerant enthalpy difference.” 
     The refrigerant circulation amount in the first operation in which the compressor  3  is operated at the first frequency, which is relatively low, is smaller than that in the second operation in which the compressor  3  is operated at the second frequency, which is relatively high. Thus, when it is assumed that the sensitivity of the throttle of the decompressor  5  is the same, the refrigerant high pressure hardly increases in the first operation. As a result, the refrigerant enthalpy difference is smaller, so that the heating power is lower. 
     In the present embodiment, the controller  10  causes the sensitivity at which the refrigerant temperature deviation is reflected in the change in the decompressor opening degree in the first operation to be higher than the sensitivity at which the refrigerant temperature deviation is reflected in the change in the decompressor opening degree in the second operation. This provides the following effects. In the first operation, the refrigerant high pressure easily increases even when the refrigerant circulation amount is small, so that the refrigerant enthalpy difference is larger than that of Embodiment 1, and the heating power is larger than that of Embodiment 1. Furthermore, the discharged refrigerant temperature increases more easily than that of Embodiment 1. From these, the outflow-hot-water temperature can be increased earlier than that of Embodiment 1. 
     Typically, when the sensitivity of the throttle of the decompressor  5  is increased, the refrigerant high pressure tends to increase excessively. In contrast, according to the present embodiment, the sensitivity of the throttle of the decompressor  5  is increased in the first operation in which the refrigerant circulation amount is small, so that there is no possibility that the refrigerant high pressure increases excessively. 
     Embodiment 4 
     Next, with reference to  FIG. 6 , Embodiment 4 will be described focusing on differences from the embodiments described above. Elements common or corresponding to the elements described above are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted.  FIG. 6  is a graph illustrating an example of temporal changes in the compressor frequency, the discharged refrigerant temperature, the outflow-hot-water temperature, the heating power, the water flow rate, the power consumption, and the outlet refrigerant temperature, at the beginning of the heat accumulating operation according to Embodiment 4. 
     The controller  10  in the present embodiment executes a shifting operation between the first operation and the second operation in the initial operation. In the shifting operation of the initial operation, the controller  10  controls the compressor frequency so that the compressor frequency gradually increases from the first frequency to the second frequency. 
     In the example illustrated in  FIG. 6 , the controller  10  executes the first operation between the time t 0  to the time t 1 , executes the shifting operation between the time t 1  to the time t 2 , and executes the second operation after the time t 2 . In this example, the heating power, the water flow rate, the power consumption, and the outlet refrigerant temperature, in the shifting operation gradually increase in association with the increase in the compressor frequency. Although in the example of  FIG. 6 , the compressor frequency in the shifting operation increases continuously, the compressor frequency in the shifting operation may increase stepwise in multiple stages. 
     The present embodiment provides the following effects. Executing the shifting operation between the first operation and the second operation can more reliably prevent the state of the refrigeration cycle from changing rapidly. As a result, the decrease in the COP can be reduced more reliably, and the excessive increase in the refrigerant high pressure can be prevented more reliably. In addition, the refrigerant enthalpy difference can be secured more stably, and the decrease in the heating power can be prevented more reliably. 
     Embodiment 5 
     Next, with reference to  FIG. 7 , Embodiment 5 will be described focusing on differences from the embodiments described above. Elements common or corresponding to the elements described above are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted.  FIG. 7  is a flowchart illustrating processing executed by the controller  10  in Embodiment 5. 
     In the following description, a target heat storage amount accumulated in the hot water storage tank  2  by the heat accumulating operation is simply referred to as a “target heat storage amount,” and the temperature distribution in the hot water storage tank  2  in the vertical direction which is detected by the stored hot water temperature sensors  19  is referred to as a “stored hot water temperature distribution.” In the present embodiment, at the time of starting the heat accumulating operation, the controller  10  determines whether to execute the initial operation, in response to the target heat storage amount and the stored hot water temperature distribution. When it is determined that the initial operation is not executed, the controller  10  performs control right after the start of the heat accumulating operation so that the compressor frequency is maintained at the second frequency. 
     When the target heat storage amount is small, the heat accumulating operation is completed before the medium-temperature water in the hot water storage tank  2  reaches the lowermost part in the hot water storage tank  2 , so that the increase in the inflow water temperature at the end of the heat accumulating operation does not occur. In such a case, it can be said that the advantage of executing the initial operation is relatively small. Thus, in the present embodiment, at the time of starting the heat accumulating operation, the controller  10  predicts whether the increase in the inflow water temperature at the end of the heat accumulating operation occurs, based on the target heat storage amount and the stored hot water temperature distribution. When it is predicted that the increase in the inflow water temperature at the end of the heat accumulating operation does not occur, the controller  10  does not execute the initial operation. This allows omission of the initial operation when the advantage of executing the initial operation is relatively small. 
     The controller  10  executes the processing of the flowchart of  FIG. 7  at the time of starting the heat accumulating operation. In step S 1  of  FIG. 7 , the controller  10  detects the stored hot water temperature distribution using the stored hot water temperature sensors  19 . From the stored hot water temperature distribution, the controller  10  can acquire information such as the current hot water remaining amount and the current heat storage amount in the hot water storage tank  2  and the position of the medium-temperature water in the hot water storage tank  2 . 
     Next, in step S 2 , the controller  10  calculates the target heat storage amount. Although not illustrated, the heat storage system  1  may include a supplied hot water temperature sensor and a supplied hot water flow rate sensor, for detecting a hot water supply load. The controller  10  may calculate the target heat storage amount using information obtained by learning the past hot water supply load. For example, the controller  10  may store the hot water supply load for the past two weeks in the memory and calculate the target heat storage amount based on data obtained by statistically processing the hot water supply load for the past two weeks. 
     Subsequently, in step S 3 , the controller  10  predicts the inflow water temperature at the completion of the heat accumulating operation. For example, the controller  10  predicts the inflow water temperature at the completion of the heat accumulating operation by calculating, based on the target heat storage amount, the target outflow-hot-water temperature, and the target heating power, how far the medium-temperature water in the hot water storage tank  2  will move downward by the end of the heat accumulating operation. 
     Next, in step S 4 , the controller  10  compares the inflow water temperature at the completion of the heat accumulating operation which has been predicted in step S 3  with a reference. When the predicted inflow water temperature at the completion of the heat accumulating operation is higher than the reference, in step S 5 , the controller  10  starts the heat accumulating operation in association with execution of the initial operation. In contrast, when the predicted inflow water temperature at the completion of the heat accumulating operation is equal to or less than the reference, in step S 6 , the controller  10  starts the heat accumulating operation without execution of the initial operation. 
     The heating power is lower during operation of the compressor  3  at the first frequency than during operation of the compressor  3  at the second frequency. Thus, when a large hot water supply load occurs while the hot water remaining amount in the hot water storage tank  2  is small and the compressor  3  is operated at the first frequency, the hot water in the hot water storage tank  2  may be depleted. According to the present embodiment, when the advantage of executing the initial operation is relatively small, the initial operation is omitted, and the compressor  3  is operated at the second frequency right after the start of the heat accumulating operation. Thus, even when a large hot water supply load occurs right after the start of the heat accumulating operation, depletion of the hot water in the hot water storage tank  2  can be prevented more reliably. 
     Embodiment 6 
     Next, with reference to  FIG. 8 , Embodiment 6 will be described focusing on differences from the embodiments described above. Elements common or corresponding to the elements described above are denoted by the same reference numerals, and repetitive descriptions thereof are simplified or omitted.  FIG. 8  is a flowchart illustrating processing executed by the controller  10  in Embodiment 6. 
     The controller  10  stores a start time and a completion time during nighttime hours. The nighttime hours may be, for example, hours from 23:00 to 7:00 next morning or hours from 22:00 to 6:00 next morning. The nighttime hours may be hours in which the unit price of the electric power charge is cheaper. Furthermore, the start time and the completion time during the nighttime hours may be set by a user. The nighttime hours can be considered hours having a high possibility that the user is asleep. Accordingly, it can be considered that the possibility that the user uses hot water supply is relatively low during the nighttime hours. 
     The hours other than the nighttime hours during a day is hereinafter referred to as “activity hours.” The activity hours can be considered hours having a high possibility that the user is not asleep. Accordingly, it can be considered that the possibility that the user uses hot water supply is relatively high during the activity hours. 
     In the present embodiment, the controller  10  executes the initial operation when the heat accumulating operation is performed during the nighttime hours and does not execute the initial operation when the heat accumulating operation is performed during the activity hours. When the initial operation is not executed, the controller  10  performs control right after the start of the heat accumulating operation so that the compressor frequency is maintained at the second frequency. 
     The controller  10  executes the processing of the flowchart of  FIG. 8  at the time of starting the heat accumulating operation. In step S 11  of  FIG. 8 , the controller  10  determines whether the current time is in the nighttime hours. When the current time is in the nighttime hours, in step S 12 , the controller  10  starts the heat accumulating operation in association with execution of the initial operation. In contrast, when the current time is in the activity hours, in step S 13 , the controller  10  starts the heat accumulating operation without execution of the initial operation. 
     A large hot water supply load may occur in the activity hours. When a large hot water supply load occurs while the hot water remaining amount in the hot water storage tank  2  is small and the compressor  3  is operated at the first frequency, the hot water in the hot water storage tank  2  may be depleted. According to the present embodiment, in the activity hours, the initial operation is omitted, and the compressor  3  is operated at the second frequency right after the start of the heat accumulating operation. Thus, even when a large hot water supply load occurs right after the start of the heat accumulating operation, depletion of the hot water in the hot water storage tank  2  can be prevented more reliably. 
     In contrast, in the nighttime hours, the possibility that a large hot water supply load occurs is low. Thus, when the heat accumulating operation is performed during the nighttime hours, the possibility that a large hot water supply load occurs while the compressor  3  is operated at the first frequency is low, so that depletion of the hot water in the hot water storage tank  2  can be prevented reliably. 
     It should be noted that, of the plurality of embodiments described above, two or more combinable ones may be combined and implemented. 
     REFERENCE SIGNS LIST 
     
         
           1  heat storage system 
           2  hot water storage tank 
           3  compressor 
           4  water heat exchanger 
           5  decompressor 
           6  air heat exchanger 
           7  blower 
           8  water pump 
           10  controller 
           10   a  processor 
           10   b  memory 
           11  heat pump unit 
           12  tank unit 
           13  feed path 
           14  return path 
           15  water supply pipe 
           16  outflow-hot-water temperature sensor 
           17  discharge temperature sensor 
           18  outdoor air temperature sensor 
           19  hot water temperature sensor