Patent Publication Number: US-2018054919-A1

Title: Refrigerant supply device, phase-change cooling apparatus equipped with the same, and method of supplying refrigerant

Description:
TECHNICAL FIELD 
     The present invention relates to a refrigerant supply device, a phase-change cooling apparatus equipped with the same, and a method of supplying refrigerant and, in particular, relates to a refrigerant supply device used for a cooling apparatus that transport and discharge heat by refrigerant in a cycle of evaporation and condensation, a phase-change cooling apparatus equipped with the same, and a method of supplying refrigerant. 
     BACKGROUND ART 
     In recent years, data centers, in which servers and network equipment are concentrated in one place, have been playing an increasingly important role as the internet and other services expand. The electricity consumption by data centers has been increasing as more and more data is processed. In data centers, the electric power consumed by air conditioners for cooling the electronic appliances is especially large, accounting for nearly a half of the total electricity consumption by data centers. It is hence desired to reduce the electric power consumed by data centers. As a means for satisfying this need, attempts have been made to utilize techniques of directly transporting heat discharged from the rack containing electronic appliances to the outside of the building and discharging the heat into the open air, without the help of an air conditioner. 
     Methods of transporting heat discharged from the rack include, apart from a method employing circulated cold water, a method utilizing the phenomenon of phase changes of refrigerant. This method utilizes refrigerant in a cycle of evaporation and condensation for transporting and discharging heat and is characterized by transportation of a large amount of heat, enabled by the utilization of latent heat at the time of phase changes of refrigerant between liquid phase and gas phase. The technique is considered promising as a means for reducing electricity consumption by the air conditioners used in data centers. 
     PTL 1 discloses an example of such a cooling device based on refrigerant circulation cycle utilizing phase changes of refrigerant. 
     The cooling system for electronic appliances according to a related art disclosed in PTL 1 has an evaporator disposed near the server. A cooling coil is provided in the evaporator and the refrigerant liquid flowing in the cooling coil evaporates due to the heated air discharged by the server and absorbs heat of evaporation from the environment during this gasification. The evaporator is provided with a temperature sensor to measure the temperature of the air heated and discharged by the server and cooled by the evaporator. At the inlet of the cooling coil, an expansion valve is provided for adjusting the flow rate of the refrigerant supplied to the cooling coll. The degree of opening of the expansion valve is automatically adjusted based on the temperature measured by the temperature sensor. 
     This configuration allows the opening of the expansion valve to be narrowed to reduce the flow rate of the refrigerant being supplied when the temperature of the air cooled by the evaporator gets lower than a preset temperature. The cooling system for electronic appliances according to the related art thus enables the efficient cooling at a small running cost of electronic appliances emitting a large amount of heat, according to the inventors. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2012-146331 (paragraphs [0021] to [0026]) 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, the cooling system for electronic appliances according to the related art disclosed in PTL 1 is configured to adjust the amount of refrigerant supply in response to the load. This is for supplying the refrigerant at such a flow rate that the latent heat of the supplied refrigerant matches the amount of heat discharged from the rack. This is because, when the flow rate is not large enough for providing the required latent heat, refrigerant liquid will be in short supply in the downstream of the refrigerant path in the heat reception unit, which precludes phase changes and heat absorption from happening. When the flow rate is more than enough to provide the required latent heat, refrigerant liquid is in excessive supply, resulting in a liquid cooling by sensible heat, which causes a temperature rise in the downstream, the magnitude of the temperature rise being determined by the heat capacity of the refrigerant liquid. This lowers the heat exchange efficiency and hinders sufficient heat absorption. 
     The above-described cooling system for electronic appliances according to the related art has a disadvantage in dealing with a plurality of electronic appliances with changing loads, such as servers. When the load on the servers contained in the rack changes, the heat absorbing performance of the system deteriorates. Increasing the load on the air conditioning. The cause of this is as follows. 
     The cooling system for electronic appliances according to the related art is configured to supply refrigerant to server racks containing a plurality of servers, adjusting the flow rate of the refrigerant for individual racks. It follows that the refrigerant is supplied at a flow rate suitable for ensuring required latent heat only after the amount of load on the whole rack is recognized. However, since refrigerant to provide the latent heat is supplied at a small flow rate, the amount of refrigerant supply cannot be timely adjusted when the load on the servers changes. This results in a deterioration of heat absorbing performance. 
     The development of energy saving technology has led to the processors and the like in a server configured to widely change the load, and hence electricity consumption, in response to changes in the amount of data processing. In the cooling system for electronic appliances according to the related art, the heat absorbing performance deteriorates every time the load changes because of the reasons described above. As a result, the load on the air conditioner in the server room increases. 
     As described above, phase-change cooling apparatuses cooling a plurality of beat-emitting bodies have a disadvantage in that the cooling performance deteriorates due to changes in the amount of heat emission by a plurality of heat-emitting bodies. 
     An object of the present invention is to provide a refrigerant supply device that solves the above-described problem, i.e., the problem of deteriorating cooling performance of a phase-change cooling apparatus cooling a plurality of heat-emitting bodies due to changes in the amount of heat emission by the plurality of heat-emitting bodies, as well as to provide a phase-change cooling apparatus equipped with the same and a method of supplying refrigerant. 
     Solution to Problem 
     A refrigerant supply device according to the present invention includes: a first reservoir for storing refrigerant liquid caused to flow by a drive pump; and a refrigerant liquid amount adjustment means for adjusting the flow rate of the refrigerant liquid flowing out of the first reservoir to a heat reception unit, wherein the reservoir comprises a branch outlet, wherein the branch outlet is provided in a position higher than the refrigerant liquid amount adjustment means, and wherein refrigerant liquid stored in the first reservoir flows out of the branch outlet to a second reservoir disposed in a position lower than the first reservoir. 
     A phase-change cooling apparatus equipped with refrigerant supply devices according to the present invention includes: a plurality of heat reception units containing refrigerant and disposed in a vertical direction; a heat discharge unit for discharging heat that the refrigerant receives in the heat reception units, refrigerant liquid flowing out of the heat discharge unit; a drive pump to cause the refrigerant liquid to flow; and a plurality of refrigerant supply devices that respectively supply the refrigerant liquid to the plurality of heat reception units, wherein each of the refrigerant supply devices includes: a first reservoir for storing the refrigerant liquid caused to flow by the drive pump; and a refrigerant liquid amount adjustment means for adjusting a flow rate of the refrigerant liquid flowing out of the first reservoir to a heat reception unit, wherein the first reservoir includes a branch outlet, wherein the branch outlet is provided in a position higher than the refrigerant liquid amount adjustment means, and wherein refrigerant liquid stored in the first reservoir flows out of the branch outlet to a second reservoir disposed in a position lower than the first reservoir. 
     A method of supplying refrigerant according to the present invention includes: retaining reserve refrigerant liquid, which is refrigerant liquid caused to flow by a drive pump and stored; controlling a flow rate of circulating refrigerant liquid, which is part of the reserve refrigerant liquid and flows to a heat reception region: and causing part of the reserve refrigerant liquid to flow downward in a vertical direction from a vicinity of the liquid surface of the reserve refrigerant liquid. 
     Advantageous Effects of Invention 
     A refrigerant supply device of the present invention, a phase-change cooling apparatus equipped with the same, and a method of supplying refrigerant prevent the deterioration of cooling performance even when a plurality of heat-emitting bodies are cooled and the amount of heat emission by the plurality of heat-emitting bodies changes. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross sectional view illustrating a configuration of a refrigerant supply structure according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a schematic view illustrating an outline of a phase-change cooling apparatus according to a second exemplary embodiment of the present invention, disposed in a data center building. 
         FIG. 3  is a perspective view of an outlook of a rack containing a heat reception unit module included in the phase-change cooling apparatus according to the second exemplary embodiment of the present invention and electronic appliances. 
         FIG. 4  is a front view schematically illustrating a configuration of a heat reception unit module included in the phase-change cooling apparatus according to the second exemplary embodiment of the present invention. 
         FIG. 5  is a schematic view for illustrating the circulation of the refrigerant in a heat reception unit module according to the second exemplary embodiment of the present invention. 
         FIG. 6  schematically illustrates the temperature distribution of the air discharged from a heat reception unit included in the phase-change cooling apparatus according to the second exemplary embodiment of the present invention. 
         FIG. 7  illustrates the relation between the degree of opening of valves and the position of valves in a heat reception unit module of a related art. 
         FIG. 8  illustrates the relation between the degree of opening of valves and the position of valves in a heat reception unit module according to the second exemplary embodiment of the present invention. 
         FIG. 9  is a front view schematically illustrating a configuration of a heat reception unit module included in a phase-change cooling apparatus according to a third exemplary embodiment of the present invention. 
         FIG. 10  is a front view schematically illustrating another configuration of the heat reception unit module included in the phase-change cooling apparatus according to the third exemplary embodiment of the present invention. 
         FIG. 11  is a schematic view illustrating an outline of a phase-change cooling apparatus according to a fourth exemplary embodiment of the present invention, disposed in a data center building. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     With reference to the attached drawings, embodiments of the present invention will be described hereinbelow. 
     First Exemplary Embodiment 
       FIG. 1  is a cross sectional view illustrating a configuration of a refrigerant supply device according to a first exemplary embodiment of the present invention. The refrigerant supply device (refrigerant supply structure)  10  according to the present exemplary embodiment includes a reservoir  11  and a refrigerant liquid amount adjustment means  12 . 
     The reservoir  11  stores refrigerant liquid caused to flow by a drive pump  21 . The refrigerant liquid amount adjustment means  12  controls the flow rate of the refrigerant liquid flowing out of the reservoir  11  to a heat reception unit  22 . The reservoir  11  includes a branch outlet  11   c,  and the branch outlet  11   c  is disposed in a higher position than the refrigerant liquid amount adjustment means  12 . Refrigerant liquid stored in the reservoir  11  flows out of the branch outlet  11   c  to another reservoir  11 X disposed in a lower position than the reservoir  11 . 
     The branch outlet  11   c  may be connected with a branch pipe  13  for transporting refrigerant liquid in the reservoir  11  from the vicinity of the liquid surface of the refrigerant liquid in the reservoir  11  to another reservoir  11 X. More specifically, the branch pipe  13  is for transporting refrigerant liquid in the reservoir  11  from the vicinity of the liquid surface of the refrigerant liquid in the reservoir  11  to another reservoir  11 X provided in a position lower than the reservoir  11  in a vertical direction. 
     Although  FIG. 1  illustrates a configuration in which the refrigerant liquid amount adjustment means  12  is provided for a pipe connected to the bottom surface of the reservoir  11 , the refrigerant liquid amount adjustment means  12  may be configured in a differently manner and may be provided for a pipe connected to a lower end of a side surface of the reservoir  11 . 
     The heat reception unit  22  contains refrigerant and receives heat from heat-emitting bodies, and the refrigerant liquid evaporates because of the heat. The refrigerant supply structure  10  according to the present exemplary embodiment has a configuration in which the reservoir  11  stores refrigerant liquid, and the refrigerant liquid is supplied from the reservoir  11  via the refrigerant liquid amount adjustment means  12  to the heat reception unit  22 . The reservoir  11  hence serves as a buffer to a change in the amount of refrigerant liquid in the heat reception unit  22 . Therefore, even with a sudden change in the amount of heat emitted by the heat-emitting bodies, the heat reception unit  22  will have no excess or shortage in the amount of the refrigerant liquid. 
     The refrigerant supply structure  10  according to the present exemplary embodiment has a configuration in which the refrigerant liquid in the reservoir is transported through the branch pipe  13  to another reservoir  11 X located in a lower position. Because of this, even when refrigerant liquid is supplied to a plurality of heat reception units  22  corresponding to a plurality of heat-emitting bodies, changes in the amount of heat absorbed by a part of the heat reception units  22  will not cause any excess or shortage in the amount of the refrigerant liquid supplied to the other heat reception units  22 . 
     As described above, the refrigerant supply structure  10  according to the present exemplary embodiment prevents deterioration of cooling performance even when a plurality of heat-emitting bodies are cooled and the amount of heat emitted by the plurality of heat-emitting bodies changes. 
     The reservoir  11  may be configured to include an inlet  11   a  into which the refrigerant liquid flows, an outlet  11   b  of which the refrigerant liquid flows out toward the heat reception unit  22 , and a branch outlet  11   c  connected with a branch pipe  13 . The refrigerant liquid amount adjustment means  12  is connected to the outlet  11   b.  The reservoir  11  may be configured to have a capacity large enough at least to contain a volume of the refrigerant liquid such that the total amount of heat of evaporation of the volume of the refrigerant liquid is equal to the maximum amount of heat receivable by the heat reception unit  22 . 
     The refrigerant liquid amount adjustment means  12  may be configured to control the flow rate of the refrigerant, liquid in such a way that the heat reception unit  22  has substantially equal cooling characteristics at different points along a direction in which the refrigerant liquid flows. Specifically, a cooling characteristic of the heat reception unit  22  may be the temperature of the air discharged from the heat reception unit  22 . 
     Typically, the refrigerant liquid amount adjustment means  12  is a variable flow valve. 
     The refrigerant liquid amount adjustment means  12  may be a pipe connecting the heat reception unit  22  with the reservoir  11 . In such a case, the pipe may include a part having an inner diameter different from the inner diameter of another pipe connecting the reservoir  11 X with another heat reception unit. The refrigerant liquid amount adjustment means  12  may be configured in a different manner as long as different pressure losses are provided for refrigerant liquid. More specifically, for example, the pipes may have different lengths, different radii of curvature, or different coefficients of friction for the inner walls. 
     Next, a method of supplying refrigerant according to the present exemplary embodiment will be described. 
     The method of supplying refrigerant according to the present exemplary embodiment is implemented by, first, retaining reserve refrigerant liquid, which is refrigerant liquid caused to flow by a drive pump and stored, and controlling the flow rate of circulating refrigerant liquid, which is a part of the reserve refrigerant liquid and flows to a heat reception region. Further, part of the reserve refrigerant liquid is caused to flow downward in a vertical direction from a vicinity of the liquid surface of the reserve refrigerant liquid. 
     The flow rate of circulating refrigerant liquid may be controlled in such a way that the heat reception region has substantially equal cooling characteristics at different points along a direction in which the circulating refrigerant liquid flows. Further, the circulating refrigerant liquid may be caused to flow toward each of a plurality of heat reception regions disposed in a vertical direction, wherein a control is performed in such a way that a heat reception region disposed in a lower position in a vertical direction has a greater pressure loss for the circulating refrigerant liquid flowing into the heat reception region and that the lower the position is, the greater the pressure loss is. 
     As described above, the refrigerant supply structure  10  and the method of supplying refrigerant according to the present exemplary embodiment prevent deterioration of cooling performance even when a plurality of heat-emitting bodies are cooled and the amount of heat emitted by the plurality of heat-emitting bodies changes. 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment of the present invention will be described. As for the present exemplary embodiment, a phase-change cooling apparatus equipped with refrigerant supply structures  10  according to the first exemplary embodiment will be described. Hereinbelow, descriptions will be made, as an example, as to a phase-change cooling apparatus  100  equipped with refrigerant supply structures  10 , which is contained in a server rack disposed in a data center (DC) or the like. In the following descriptions the “phase-change cooling apparatus  100  equipped with refrigerant supply structures  10 ” will be simply referred to as the “phase-change cooling apparatus  100 ”. 
       FIG. 2  is a schematic view illustrating an outline of a phase-change cooling apparatus  100  according to the present exemplary embodiment, disposed in a data center building. 
     The phase-change cooling apparatus  100  according to the present exemplary embodiment includes a heat reception unit module  110 , a heat discharge unit  120 , and a drive pump  130 . The heat reception unit module  110  includes a plurality of heat reception units containing refrigerant and disposed in a vertical direction and a plurality of refrigerant supply structures that respectively supply refrigerant liquid to the plurality of heat reception units. The heat discharge unit  120  discharges heat that the refrigerant receives in the heat reception unit and refrigerant liquid flows out of the heat discharge means. The drive pump  130  causes the refrigerant liquid to flow toward the heat reception unit module  110 . 
     In a server room  500  of a data center or the like, a housing (a rack)  510  containing a plurality of electronic appliances  511  is disposed and data processing is performed. The electronic appliances  511  generate heat because of the load such as data processing and the heat is discharged by means of air to the outside of the rack  510 . 
     The phase-change cooling apparatus  100  according to the present exemplary embodiment is configured in such a way that the plurality of heat reception units are disposed in a housing (rack)  510  containing the electronic appliances  511  as objects to be cooled. In other words, the heat reception unit module  110  included in the phase-change cooling apparatus  100  is disposed on the air discharging side of the rack  510 , for example, on the side of the door from which the air is discharged. The heat reception unit module  110  is connected via a liquid pipe  140  and a vapor pipe  150  with a heat discharge unit  120 , which is disposed in, for example, a machine room  520  located outside of and adjacent to the server room  500  or outdoors. The above-described drive pump  130  is disposed in the flow path of the liquid pipe  140  and transports refrigerant liquid between the heat discharge unit  120  and the heat reception unit module  110 .  FIG. 3  illustrates an outlook of a rack  510  installed with a heat reception unit module  110  according to the present exemplary embodiment and the electronic appliances  511 . 
     The heat generated in the electronic appliances  511  is discharged from the heat discharge unit  120  directly to the outside of the server room  500 . This reduces the amount of heat that the air conditioner in the server room  500  disposes of in the cooling process and thereby reduces the load on the air conditioner. The arrows in  FIG. 2  indicate the transportation of the heat generated in the electronic appliances  511 . 
     The refrigerant to be used for the phase-change cooling apparatus  100  may be, for example, a refrigerant with a low boiling point, such as hydrofluorocarbon (HFC) or hydrofluoroether (HFE). By removing the air by evacuation after filling the refrigerant, the refrigerant is used in an environment under the saturated vapor pressure. 
     The refrigerant in the heat reception unit module  110  changes phase from liquid to gas by the presence of heat discharged from the electronic appliances  511 , the heat being absorbed for heat of evaporation. The evaporated refrigerant, or refrigerant vapor transports heat through the vapor pipe  150  to the heat discharge unit  120 . In the heat discharge unit  120 , the refrigerant vapor discharges heat by heat exchange to the open air or to cold water and changes phase again to liquid, becoming refrigerant liquid. The refrigerant liquid is caused to flow through the liquid pipe  140  back to the heat reception unit module  110  by the driving force of the drive pump  130 . 
       FIG. 4  illustrates a configuration of the heat reception unit module  110 . 
     The heat reception unit module  110  includes a plurality of heat reception units  111 , and each of the heat reception units  111  is provided with a reserve tank  112  serving as a reservoir, a valve  113  serving as a refrigerant liquid amount adjustment means, and a branch pipe  114 . 
     The heat reception units  111  exchange heat between the heated air discharged by the electronic appliances  511  and the refrigerant. The reserve tanks  112  serve as buffers by temporarily storing refrigerant liquid. The valves  113  are disposed between the heat reception units  111  and the reserve tanks  112  and adjust the flow rates of the refrigerant liquid. 
     Next, an operation of the phase-change cooling apparatus  100  according to the present exemplary embodiment will be described. 
     First, with reference to  FIG. 5 , circulation of the refrigerant in the heat reception unit module  110  will be described. 
     Each of the reserve tanks  112  includes an inlet into which the refrigerant liquid transported from the drive pump  130  flows, an outlet of which the refrigerant liquid flows out toward the heat reception unit  111 , and a branch outlet connected with a branch pipe  114 . From the outlet a portion of refrigerant liquid flows out, which is herein referred to as a heat reception unit refrigerant liquid flow  211 , to be supplied to the heat reception unit  111  connected with the reserve tank  112 . From the branch outlet a portion of the refrigerant flows out, which is herein referred to as a branch refrigerant liquid flow  212 , when a certain volume of the refrigerant liquid is stored in the reserve tank  112 , to be supplied through the branch pipe  114  to another reserve tank disposed in a lower position in a vertical direction. 
     The reserve tank  112  may be configured to have a capacity large enough at least to contain a volume of the refrigerant liquid such that the total amount of heat of evaporation of the volume of the refrigerant liquid is equal to the maximum amount of heat receivable by the heat reception unit  111 . In other words, the reserve tank  112  may have a capacity large enough at least to store (reserve) an amount of refrigerant liquid calculated by dividing the maximum amount of heat to be exchanged in the heat reception unit  111  by the latent heat of the refrigerant. This allows the refrigerant liquid to be supplied in an appropriate amount in response to a change in the load on the electronic appliances  511 . 
     The refrigerant liquid  221  in the heat reception unit  111  exchanges heat with the heated air discharged by the electronic appliance  511 , changes phase to become refrigerant vapor  222 , and flows out to the vapor pipe  150 . In  FIG. 5 , the discharged air flows in a direction perpendicular to the drawing plane. As illustrated in the drawing, the refrigerant liquid  221  flows into a lower part of the heat reception unit  111 , changes to refrigerant vapor  222  and flows out from an upper part of the heat reception unit  111 .  FIG. 6  schematically illustrates the temperature distribution of the air discharged from a heat reception unit  111 . 
     In  FIG. 6 , as in  FIG. 5 , the discharged air flows in a direction perpendicular to the drawing plane. The refrigerant liquid  221  flows into a lower part of the heat reception unit  111 , changes to refrigerant vapor  222 , and flows out from an upper part of the heat reception unit  111 . 
     By supplying the heat reception unit  111  with the refrigerant liquid at a flow rate at which the latent heat of the refrigerant liquid supplied is equal to the heat to be exchanged in the heat reception unit  111 , all the refrigerant liquid will have changed phase to gas to become refrigerant vapor by the time it flows out of the heat reception unit  111 . 
     When the flow rate of the refrigerant liquid is smaller than in the above-described state, with the valve  113  at a smaller degree of opening, the discharged air temperature T out  at the downstream side of the heat reception unit  111  illustrated in  FIG. 6  becomes higher than the discharged air temperature T in  at the upstream side because the refrigerant liquid that is to change phase is not sufficiently supplied. When the flow rate of the refrigerant liquid is greater than in the above-described state, with the valve  113  at a greater degree of opening, the refrigerant liquid that remains unevaporated but heated flows downstream and a temperature rise by sensible heat of the refrigerant liquid will occur in the downstream, the magnitude of the temperature rise being determined by the heat capacity of the refrigerant liquid. Hence, the discharged air temperature T out  at the downstream side becomes higher than the discharged air temperature T in  at the upstream side also in this case. As described above, when the flow rate of the refrigerant liquid is not optimal, the heat exchange performance deteriorates more badly in the parts further downstream of the heat reception unit  111 . To avoid this, the degree of opening of the valve  113  is to be adjusted in such a way as to achieve the optimal flow rate of the refrigerant liquid by monitoring load information with respect to the electronic appliance  511  and temperature information with respect to the heat reception unit  111 . 
     Next, with regard to a case in which the heat reception unit module  110  is provided with a plurality of heat reception units  111 , a method of adjusting the degree of opening of valves  113  will be described in detail. 
       FIG. 7  illustrates an example of the relation between the degrees of opening of valves and the positions of valves in a heat reception unit module of a related art with no reserve tank. In this example, the heat reception unit module of a related art is provided with four heat reception units. In  FIG. 7 , the degrees of valve opening are laid out along the vertical axis whereas the vertical positions of valves are along the horizontal axis, lower positions being plotted further away from the origin. In other words valves V 1 , V 2 , V 3 , and V 4  are disposed from top to bottom in a vertical direction in this order. W in the drawing indicates the range of adjustment of the degree of opening of each valve, and the hollow circles indicate the degree of opening of each valve at a time when the discharged air temperature T out  at the downstream side of the heat reception unit is equal to the discharged air temperature T in  at the upstream side (T out =T in ), in other words, when the refrigerant liquid flows at an optimal flow rate. 
     When the load on an electronic appliance  511  changes, the refrigerant liquid no longer flows at an optimal flow rate at which the latent heat of the refrigerant liquid supplied is equal to the heat generated. In other words, as described above, when the load decrease, the flow rate becomes too great and the discharged air temperatures of the heat reception unit  111  will be T out &gt;T in . When the load increases, the flow rate becomes too small and the discharged air temperatures of the heat reception unit  111  will be T out &gt;T in . To avoid this, adjustment is to be made in such a way as to bring the degree of opening of each valve to the point indicated by the hollow circle (∘) to achieve T out =T in . More specifically, the degree of opening of the valve is reduced when the load decreases, and the degree of opening of the valve is increased when the load increases, to achieve T out =T in . 
     When the refrigerant liquid is supplied from an upper part of the rack  510 , the refrigerant liquid flows from the upstream to the downstream. i.e., downward in a vertical direction as a portion of the refrigerant liquid changes phase to gas in each of the heat reception units  111 . Thus smaller amounts of refrigerant liquid are supplied to the heat reception units  111  disposed in lower positions in a vertical direction. The degrees of opening of the valves are accordingly smaller in lower positions in a vertical direction. In addition, the degrees of opening of upstream valves need to be adjusted in response to the changes in the load on all the electronic appliances that exchange heat with the heat reception units disposed in the downstream. Because of this, a valve disposed further in the upstream needs a greater range of adjustment of the degree of opening, and the further in the upstream it is, the greater the range needs to be. In other words, a valve disposed further in the upstream has a smaller tolerance for degree of opening in responding changes in the load on the electronic appliances, and the further in the upstream it is, the smaller the tolerance is. 
     In contrast, the phase-change cooling apparatus  100  according to the present exemplary embodiment includes reserve tanks  112  respectively disposed in the upstream of the valves  113 , the reserve tanks  112  serving as buffers by storing refrigerant in an amount sufficient for coping with changes in the load on the electronic appliances  511 . This mitigates the above-described adverse effects of disposing plurality of heat reception units in a vertical direction. 
       FIG. 8  illustrates an example of the relation between the degrees of opening of valves and the positions of valves in a heat reception unit module  110  included in the phase-change cooling apparatus  100  according to the present exemplary embodiment. In  FIG. 8 , the degrees of valve opening are laid out along the vertical axis whereas the vertical positions of valves arc along the horizontal axis, lower positions being plotted further away from the origin. W in the drawing indicates the range of adjustment of the degree of opening of each valve, and the hollow circles indicate the degree of opening of each valve at a time when the discharged air temperature T out  at the downstream side of the heat reception unit is equal to the discharged air temperature T out  at the upstream side (T out =T in ), in other words, when the refrigerant liquid flows at an optimal flow rate. 
     As described above, the heat reception unit module  110  according to the present exemplary embodiment is provided with reserve tanks  112  respectively disposed in the upstream of the valves  113 . Each of the reserve tanks  112  at least stores (reserves) an amount of refrigerant liquid that corresponds to the maximum amount of heat to be exchanged in the heat reception unit  111  and the refrigerant exceeding this reserve amount overflows to another reserve tank  112  disposed in the downstream. As this configuration allows refrigerant liquid to be supplied respectively from the reserve tanks  112  to heat reception units  111 , valves disposed upstream have a large tolerance for degree of opening in responding to changes in the load on the electronic appliances, as illustrated in  FIG. 8 . Unlike with the configuration with no reserve tank as illustrated in  FIG. 7 , it is not the case that smaller amounts of refrigerant liquid are supplied to the heat reception units  111  disposed in lower positions. This mitigates dependency of the degrees of opening of the valves on the position of the valves. 
     As a result, even when the load on the electronic appliances abruptly changes, the supply of refrigerant can be adjusted in a short time, not deteriorating the heat exchange performance. In addition, the reliability of the cooling system is improved as the degrees of opening of the valves are adjusted to a smaller extent and less frequently. 
     As described above, the phase-change cooling apparatus  100  according to the present exemplary embodiment reduces the load on the air conditioner in the server room. This is because the refrigerant in the reserve tanks serves as a buffer to load changes on the servers, allowing for greater tolerance in response to load changes on the servers. Because of this, even when the load on the servers changes abruptly, there is no excess or shortage in the amount of supply of the refrigerant liquid, which prevents deterioration of heat absorbing performance. 
     Further, the phase-change cooling apparatus  100  according to the present exemplary embodiment enables improvement of the reliability of the cooling system. This is because, as described above, a large tolerance in response to load changes in the servers allows the adjustment of the degrees of opening of the valves to be done to a smaller extent and less frequently. This contributes to reducing the risk of failure of the driving components and prolonging the life of the valves. 
     Third Exemplary Embodiment 
     Next, a third exemplary embodiment of the present invention will be described. 
       FIG. 9  illustrates a configuration of a heat reception unit module included in the phase-change cooling apparatus according to the present exemplary embodiment. As illustrated in  FIG. 9 , the heat reception unit module  110  according to the present exemplary embodiment includes valves  113  serving as refrigerant liquid amount adjustment means as well as heat reception unit liquid pipes  340  serving as pipes respectively connecting heat reception units  111  with reserve tanks  112  serving as reservoirs. The heat reception unit liquid pipes  340  are configured to include parts formed in such a way that the more downward a heat reception unit liquid pipe is located in a vertical direction, the smaller inner diameter the part of the heat reception unit liquid pipe has. More specifically, the heat reception unit liquid pipes  340  are configured to have consistently different inner diameters that depend on the vertical positions of the heat reception unit liquid pipes  340  or to include parts having such different inner diameters. 
     Such a configuration allows greater pressure losses in pipes disposed in lower positions in a vertical direction when the refrigerant liquid flowing from the liquid pipe  140  flows through the pipes. Therefore, the degrees of opening of the valves  113  can be made nearly uniform from the upstream to the downstream. This results in a further increased tolerance for the degree of valve opening in response to changes in the load on the electronic appliances  511 , which allows simplification of the control system for adjusting the degrees of opening of the valves. 
     As illustrated in  FIG. 10 , the heat reception unit liquid pipes  340  may be provided without valves  113 . Such a configuration allows greater pressure losses in pipes disposed in lower positions in a vertical direction when the refrigerant liquid flowing from the liquid pipe  140  flows through the pipes. In this case, pressure losses in the heat reception unit liquid pipes  340  take values respectively fixed for the heat reception units  111 . Still, in cases in which the load conditions are predictable, for example, when the range of changes in the load on the electronic appliances  511  is small, operations without adjusting the degree of valve opening are feasible since load changes can be dealt with to a certain extent by the reserve tanks  112 . 
     As described above, the phase-change cooling apparatus according to the present exemplary embodiment enables reduction of valve costs and cost of a control system for adjusting the degree of valve opening. 
     Fourth Exemplary Embodiment 
     Next, a fourth exemplary embodiment of the present invention will be described. 
       FIG. 11  is a schematic view illustrating an outline of a phase-change cooling apparatus  400  according to the present exemplary embodiment, disposed in a data center building. The phase-change cooling apparatus  400  according to the present exemplary embodiment is configured to have a plurality of heat reception units  111  disposed away from a housing containing an object to be cooled. In other words, the phase-change cooling apparatus  400  has a heat reception unit module  410  disposed, for example, on a wall of a server room  500 , wherein the heat reception unit module  410  includes a plurality of heat reception units  111  and a plurality of refrigerant supply structures. The heat reception unit module  410  receives on the wall of the server room  500  heat discharged by electronic appliances  511 , and the heat discharge unit  120  disposed outside the server room  500  discharges heat. The arrows in the drawing indicate the transportation of heat generated in the electronic appliances  511 . 
     This configuration eliminates the need of providing a heat reception unit module for each of the racks  510 . This reduces the initial investment cost for the cooling apparatus. 
     With the phase-change cooling apparatus according to the above-described embodiments, heat discharged from a rack containing a plurality of severs with changing loads, for example, in a data center can be transported to the outside of the server room by forced circulation, which allows reduction of power consumption by the air conditioner. 
     The present invention has been described above with above-described exemplary embodiments as exemplary examples. The present invention, however, is not limited to the above-described exemplary embodiments. In other words, various aspects of the present invention that a person skilled in the art can understand may be applied within the scope of the present invention. 
     The present application claims priority based on Japanese Patent Application No. 2015-051064 filed Mar. 13, 2015, the disclosure of which is herein incorporated by reference in its entirety. 
     REFERENCE SIGNS LIST 
       10  refrigerant supply structure 
       11 ,  11 X reservoir 
       11   a  inlet 
       11   b  outlet 
       11   c  branch outlet 
       12  refrigerant liquid amount adjustment means 
       13 ,  114  branch pipe 
       21  drive pump 
       22 ,  111  heat reception unit 
       100 ,  400  phase-change cooling apparatus 
       110 ,  410  heat reception unit module 
       112  reserve tank 
       113  valve 
       120  heat discharge unit 
       130  drive pump 
       140  liquid pipe 
       150  vapor pipe 
       211  heat reception unit refrigerant liquid flow 
       212  branch refrigerant liquid flow 
       221  refrigerant liquid 
       222  refrigerant vapor 
       340  heat reception unit liquid pipe 
       500  server room 
       510  rack 
       511  electronic appliance 
       520  machine room