The subject matter disclosed herein generally relates to air conditioning systems, and in particular relates to an air conditioning system utilizing supercooled phase chase material to store thermal energy.
Existing air conditioning systems employ phase change materials to improve capacity and/or efficiency of the system. Exemplary air conditioning systems include energy storage systems which freeze a phase change material when energy costs are relatively low (e.g., non-peak rates). The phase change material is then used to absorb thermal energy during other modes of operation to improve efficiency and/or capacity of the air conditioning system.
One embodiment is an air conditioning system which includes a chiller system including a compressor, a condenser, an expansion device and an evaporator; a phase change material in thermal communication with the condenser; an actuator coupled to the phase change material; and a controller providing a trigger signal to the actuator to initiate changing the phase change material from a supercooled state to a solid state.
Another exemplary embodiment is a method for operating an air conditioning system having a chiller system including a compressor, a condenser, an expansion device and an evaporator, a phase change material in thermal communication with the condenser, and an actuator coupled to the phase change material, the method including determining whether an ambient temperature profile will result in supercooling of the phase change material; and in response to the determining, triggering the actuator to initiate changing the phase change material from a supercooled state to a solid state.
Other exemplary embodiments and features are described herein.
The system of
Condenser coil 12 is in thermal communication with a phase change material 26. Condenser coil 12 may be fully embedded in the phase change material 26 or the phase change material may be in a housing containing condenser coil 12. Alternatively, a portion of the condenser coil may be exposed to ambient air. A fan 28 may draw air through the phase change material 26 to aid in cooling the phase change material 26. In exemplary embodiments, phase change material 26 is a material that achieves a supercooling state. A controller 32 then initiates the transition of the phase change material 26 from supercooled liquid to solid. An actuator 30 is used to initiate the transition of the phase change material 26 from supercooled liquid to solid when the phase change material 26 is in a supercooled state, as described in further detail herein.
A controller 32 controls operation of the system. Controller 32 may be implemented using a general purpose microprocessor executing computer code stored in a storage medium for performing the functions described herein. Controller 32 receives a phase change material temperature signal from a phase change material sensor 34 in thermal contact with phase change material 26. Controller 32 also receives an ambient temperature signal from an ambient temperature sensor 36. Ambient temperature sensor 36 may monitor the outside air temperature in the vicinity of condenser 12. Controller 32 may send control signals to compressor 10, pump 20, supply valve 22, return valve 24, fan 28 and actuator 30. Operation of the system is described in further detail herein with reference to
A coolant supply line 42 is also in thermal communication with the phase change material 26, and may be embedded in the phase change material 26 as shown in
As described in further detail herein, the phase change material 26 is selected so that the phase change material transitions from liquid to solid when cooling demand on the chiller system is low or non-existent. This may occur in the evening, when ambient temperatures are lower.
At 106, controller 32 predicts a nighttime temperature profile based on the climate zone data and one or more ambient air temperature readings over time. Controller 32 may be preloaded with predicted nighttime temperature profiles indexed by climate zone data and daytime ambient air temperatures.
At 108, controller 32 determines if the predicted nighttime temperature profile will be sufficient to supercool the phase change material 26, based on one or more phase change material temperature measurements. For example, if the phase change material 26 transition temperature is about 75° F., the current phase change material temperature is about 80° F. and the predicted nighttime temperature profile indicates four hours of ambient air temperature of about 72° F., then controller 32 may determine that the predicted nighttime temperature profile will result in the phase change material supercooling before the next chiller cycle is initiated (i.e., before the space being conditioned requires cooling). This determination will be affected by factors such as the amount of phase change material, its temperature transition characteristics, etc.
If the ambient temperature alone is sufficient to supercool the phase change material 26, flow proceeds to 110 where the controller determines a trigger time to transition the supercooled phase change material 26 from liquid to solid. Controller 32 attempts to trigger this transition when the ambient temperature is at or near a minimum value, so that the heat released by the phase change material 26 is more rapidly absorbed by the ambient air. At 112, controller sends a trigger signal to actuator 30 at the trigger time to initiate transition of the supercooled phase change material 26 to a solid.
If at 108 the predicted nighttime temperature profile is insufficient to supercool the phase change material 26, flow proceeds to 114 where it is determined if running fan 28 to draw ambient air through the phase change material 26 will result in supercooling of the phase change material 26. This determination may be made by controller 32 determining, based on the current phase change material temperature, that only a small temperature decrease is needed to supercool the phase change material 26. If so, fan 28 is turned on at 116 and flow proceeds to 110 and 112 as described above.
If at 114 controller 32 determines that the fan 28 will not supercool the phase change material, flow proceeds to 118 where controller 32 runs the chiller system including compressor 10 and pump 20. At 120, supply valve 22 and return valve 24 are set by controller 32 to direct coolant from coil 18 to the phase change material 26. Flow proceeds to 110 and 112 as described above.
Embodiments employ a phase change material that meets cost objectives but has a transition temperature high enough so that the nighttime temperature drops below the transition temperature each night. A phase change material is chosen that has a high propensity for supercooling. As the nighttime temperature drops below the starting temperature of the phase change material, sensible cooling takes place according to the heat capacity of the phase change material. When the nighttime temperature is near a minimum, the supercooled phase change material is triggered to release its latent heat quickly.
The temperature of the phase change material rises according to the heat capacity driven by the latent heat release until the limit of the melting temperature is reached. This provides a higher temperature difference between the outdoor air and the phase change material and the heat transfer from the phase change material to the outside air can occur at faster rates. The heat exchanger design can be optimize to take advantage of this rapid heat release. An example of a candidate phase change material with a transition temperature in the right region which is known to exhibit supercooling and which may be inexpensive enough is natural coconut fatty acid mixture.
Embodiments harness supercooling for positive uses by permitting the daytime heat captured in a medium to be released over a shorter period of cooler night air than otherwise would be possible. The difference in temperature created in the phase change material between the outdoor air temperature and the melting point temperature permits faster heat release to the environment and downsizing of the associated heat exchanger. This increases the viability of thermal energy storage from a cost/benefit perspective.
Chillers normally reject heat into hot outside air (95° F. rating T) during periods of occupancy. The “lift” from the chilled water temperature (CWST) to the outside temperature (OAT) governs chiller efficiency, as illustrated in
The advent of less expensive phase change materials that have a choice transition temperature (Tm) means that systems can be designed to pick the better of the OAT and Tm to reject heat during periods of occupancy, lowering the chiller lift and increasing chiller efficiency when electric rates are typically highest. The phase change material discharges throughout the day. During periods of unoccupancy at night when the chiller runs infrequently, the phase change material is recharged by cooler night air after the night air temperature drops below Tm in an “economizer” mode. Embodiments use a phase change material that exhibits supercooling so that when triggered, the phase change material temperature rises relative to the night air and the recharge goes faster. If necessary, the chiller system can assist so as to complete the recharge of the phase change material before morning occupancy. If the chiller system is needed, it will operate at lower lift than it would have during the day and use cheaper electricity.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
Filing Document | Filing Date | Country | Kind |
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PCT/US2013/027745 | 2/26/2013 | WO | 00 |
Number | Date | Country | |
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61642072 | May 2012 | US |