US20150135743A1 - Air conditioning system having supercooled phase change material - Google Patents

Abstract

An air conditioning system 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.

Description

BACKGROUND
• 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.
SUMMARY
• 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.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 depicts an air conditioning system in an exemplary embodiment.
• FIG. 2 depicts a condenser coil assembly in an exemplary embodiment.
• FIG. 3 depicts an actuator in an exemplary embodiment.
• FIG. 4 is an plot of temperature versus time, illustrating phase change material state changes in an exemplary embodiment.
• FIG. 5 is a flowchart of a control process in an exemplary embodiment.
DETAILED DESCRIPTION
• FIG. 1 depicts an air conditioning system in an exemplary embodiment. A chiller system includes acompressor10, afirst heat exchanger12, anexpansion device14 and asecond heat exchanger16. Thefirst heat exchanger12 may be used as a condenser coil, and may be located outside of a building or space to be conditioned. Thesecond heat exchanger16 may be used as an evaporator coil. As known in the art, refrigerant is subjected to a vapor compression cycle throughcompressor10,condenser12,expansion device14 andevaporator16. Heat is absorbed atevaporator16 and heat is discharged atcondenser12.
• The system ofFIG. 1 may be a water chiller system.Evaporator16 is in thermal communication with a heat exchanger18 (e.g., a coil) that carries a fluid coolant, e.g., water. Asupply pump20 circulates coolant fromheat exchanger18 cooled byevaporator16 to asupply valve22.Supply valve22 supplies chilled water (about 45° F.) to a local zone terminal where a fan draws air over a coil to chill a space as known in the art. Areturn valve24 receives fluid returned from the local zone terminal and provides the return fluid toheat exchanger18. It is understood that embodiments of the invention may be used with other types of air conditioning systems (e.g., forced air) and embodiments are not limited to water chiller systems.
• Condenser coil12 is in thermal communication with aphase change material26.Condenser coil12 may be fully embedded in thephase change material26 or the phase change material may be in a housing containingcondenser coil12. Alternatively, a portion of the condenser coil may be exposed to ambient air. Afan28 may draw air through thephase change material26 to aid in cooling thephase change material26. In exemplary embodiments,phase change material26 is a material that achieves a supercooling state. Acontroller32 then initiates the transition of thephase change material26 from supercooled liquid to solid. Anactuator30 is used to initiate the transition of thephase change material26 from supercooled liquid to solid when thephase change material26 is in a supercooled state, as described in further detail herein.
• Acontroller32 controls operation of the system.Controller32 may be implemented using a general purpose microprocessor executing computer code stored in a storage medium for performing the functions described herein.Controller32 receives a phase change material temperature signal from a phasechange material sensor34 in thermal contact withphase change material26.Controller32 also receives an ambient temperature signal from anambient temperature sensor36.Ambient temperature sensor36 may monitor the outside air temperature in the vicinity ofcondenser12.Controller32 may send control signals tocompressor10,pump20,supply valve22,return valve24,fan28 andactuator30. Operation of the system is described in further detail herein with reference toFIG. 5.
• FIG. 2 depicts a condenser assembly in an exemplary embodiment.Condenser coil12 is in thermal communication withphase change material26. In the embodiment ofFIG. 2, thecondenser coil12 is embedded inphase change material26. In other embodiments, a portion ofcondenser coil12 may extend beyond thephase change material26, such that theentire condenser coil12 is not embedded in thephase change material26.Air paths40 are formed in thephase change material26 to allow air to be drawn through thephase change material26 byfan28. The air paths may be arranged in a variety of configurations, and embodiments are not limited to the arrangement shown inFIG. 2.Controller32 may turnfan28 on when additional ambient airflow is needed to cool thephase change material26.
• Acoolant supply line42 is also in thermal communication with thephase change material26, and may be embedded in thephase change material26 as shown inFIG. 2. In situations where the ambient temperature is insufficient to adequately cool the phase change material26 (e.g., to a supercooled state),controller32 may direct chilled coolant fromsupply valve22 to thephase change material26 and back to returnvalve24.Controller32 activatescompressor10 to produce chilled coolant incoil18.Controller32sets valves22 and24 to route chilled coolant to thephase change material26 andpump20 is activated circulate the chilled coolant to thephase change material26.
• FIG. 3 depicts anactuator30 in an exemplary embodiment.Actuator30 is controlled bycontroller32 to initiate a transition from supercooled, liquid phase change material to solid phase change material. Supercooled, as used herein, refers to thephase change material26 being in a liquid state and at a temperature below the phase change material freezing temperature. The actuator inFIG. 3 includes atube50 of phase change material and athermoelectric cooler52 that maintains the phase change material intube50 in a frozen or solid state under all conditions. Valve54 connects thetube50 to the main reservoir ofphase change material26.Controller32 opensvalve54 to trigger freezing of the main phasechange material reservoir26. Whenvalve54 is opened, some of the unfrozen liquid inphase change material26 flows toward the frozen volume intube50 and begins to freeze. The freeze front moves outward from the valve area to the rest of thephase change material26 as the latent heat is released. Another embodiment ofactuator30 includes an ultrasonic emitter to produce ultrasonic sound waves to trigger transition of thephase change material26 from supercooled liquid to solid.
• As described in further detail herein, thephase change material26 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.FIG. 4 illustrates an exemplary diurnal temperature versus time profile, along with states of thephase change material26. In the example ofFIG. 4, the outside air temperature ranges from about a 95° F. day time high to about a 70° F. night time low. If the transition temperature of thephase change material26 is selected to be about 75° F., then thephase change material26 will be frozen (or recharged) at night and then melt (or discharge) during the day. During the day, the frozenphase change material26 absorbs energy from thecondenser coil12, improving the efficiency ofcondenser12 when the chiller system is running and increasing efficiency and capacity of the chiller system.
• FIG. 5 is a flowchart of a control process executed bycontroller32 in an exemplary embodiment. The process begins at100 where thecontroller32 obtains climate zone data for the system. The climate zone data may be indicated by known climate maps and programmed intocontroller32 upon installation of the air conditioning system. At102,controller32 monitors the ambient air temperature throughambient temperature sensor36 and may store multiple ambient temperatures over time. At104controller32 obtains the phase change material temperature from phase changematerial temperature sensor34, and may store multiple phase change material temperatures over time.
• At106,controller32 predicts a nighttime temperature profile based on the climate zone data and one or more ambient air temperature readings over time.Controller32 may be preloaded with predicted nighttime temperature profiles indexed by climate zone data and daytime ambient air temperatures.
• At108,controller32 determines if the predicted nighttime temperature profile will be sufficient to supercool thephase change material26, based on one or more phase change material temperature measurements. For example, if thephase change material26 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., thencontroller32 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 thephase change material26, flow proceeds to110 where the controller determines a trigger time to transition the supercooledphase change material26 from liquid to solid.Controller32 attempts to trigger this transition when the ambient temperature is at or near a minimum value, so that the heat released by thephase change material26 is more rapidly absorbed by the ambient air. At112, controller sends a trigger signal toactuator30 at the trigger time to initiate transition of the supercooledphase change material26 to a solid.
• If at108 the predicted nighttime temperature profile is insufficient to supercool thephase change material26, flow proceeds to114 where it is determined if runningfan28 to draw ambient air through thephase change material26 will result in supercooling of thephase change material26. This determination may be made bycontroller32 determining, based on the current phase change material temperature, that only a small temperature decrease is needed to supercool thephase change material26. If so,fan28 is turned on at116 and flow proceeds to110 and112 as described above.
• If at114controller32 determines that thefan28 will not supercool the phase change material, flow proceeds to118 wherecontroller32 runs the chillersystem including compressor10 andpump20. At120,supply valve22 and returnvalve24 are set bycontroller32 to direct coolant fromcoil18 to thephase change material26. Flow proceeds to110 and112 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 inFIG. 4. The chiller may be off during most of the setback period during unoccupancy.
• The advent of less expensive phase change materials that have a choice transition temperature (T_m) means that systems can be designed to pick the better of the OAT and T_mto 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 T_min 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.

Claims (20)

1. An air conditioning system comprising:

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.

2. The air condition system ofclaim 1 wherein:

the condenser is embedded in the phase change material.

3. The air condition system ofclaim 1 further comprising:

an ambient temperature sensor providing an ambient temperature signal to the controller.

4. The air condition system ofclaim 1 further comprising:

a phase change material sensor providing a phase change material temperature signal to the controller.

5. The air condition system ofclaim 1 wherein:

the phase change material includes an internal airway for allowing ambient air to flow through the phase change material.

6. The air condition system ofclaim 5 further comprising:

a fan for drawing air through the airway.

7. The air condition system ofclaim 1 wherein:

the phase change material includes a coolant supply line in thermal communication with the phase change material, the coolant supply line coupled to the chiller system.

8. The air condition system ofclaim 1 wherein:

the actuator generates a sound wave to initiate changing the phase change material from the supercooled state to the solid state.

9. The air condition system ofclaim 1 wherein:

the actuator includes frozen phase change material, the frozen phase change material being placed in contact with the phase change material to initiate changing the phase change material from the supercooled state to the solid state.

10. The air condition system ofclaim 9 wherein:

the actuator includes a valve separating the frozen phase change material from the phase change material, the controller opening the valve to initiate changing the phase change material from the supercooled state to the solid state.

11. The air condition system ofclaim 1 wherein:

the controller predicts a nighttime temperature profile in response to climate zone data and ambient temperature; and

the controller determining if the phase change material will supercool in response to the nighttime temperature profile.

12. The air condition system ofclaim 11 wherein:

the controller providing the trigger signal to the actuator in response to the nighttime temperature profile.

13. The air condition system ofclaim 11 wherein:

the phase change material includes an internal airway for allowing ambient air to flow through the phase change material;

when the controller determines that the phase change material will not supercool in response to the nighttime temperature profile, the controller activating a fan for drawing air through the airway to supercool the phase change material.

14. The air condition system ofclaim 11 wherein:

the phase change material includes a coolant supply line in thermal communication with the phase change material, the coolant supply line coupled to the chiller system;

when the controller determines that the phase change material will not supercool in response to the nighttime temperature profile, the controller activating the chiller system to cool the coolant supply line to supercool the phase change material.

15. 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 comprising:

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.

16. The method ofclaim 15 wherein:

triggering the actuator includes determining a trigger time in response to the ambient temperature profile and triggering the actuator at the trigger time.

17. The method ofclaim 15 further comprising:

running a condenser fan in response to determining that the ambient temperature profile will not result in supercooling of the phase change material.

18. The method ofclaim 17 further comprising:

running the chiller system in response to determining that the ambient temperature profile will not result in supercooling of the phase change material and the running the condenser fan will not result in supercooling of the phase change material.

19. The method ofclaim 18 wherein:

running the chiller system includes cooling the phase change material with coolant.

20. The method ofclaim 15 wherein:

determining whether the ambient temperature profile will result in supercooling of the phase change material includes predicting a nighttime temperature profile in response to climate zone data, daytime temperature and a phase change material temperature.

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