Cooling System With Integral Thermal Energy Storage

Abstract

A cooling system is provided which includes a primary heat exchanger including heat exchange pipes filled with a phase change material comprising water. The heat exchange pipes are in heat transfer relation with a coolant fluid, which, upon cooling, is transferred to a separate liquid to air heat exchanger for conversion into cool air, for example, for use in air conditioning or refrigeration.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a cooling system, and more particularly, to a cooling system which utilizes phase change materials for the storage and release of thermal energy.

[0002] It is known that a large amount of electric power is consumed by the cooling of residual and commercial buildings, especially during daylight hours. Overall, a large imbalance in electric power usage exists during daylight hours, due primarily to the amounts of power consumed by industry, businesses, and public transportation. To compensate for the extensive day time use of electric power, utility companies provide generating capacity sufficient to supply day time usage, leaving unused capacity available for the night hours.

[0003] A need has arisen in the art for a cooling system which can provide more efficient cooling and which can make effective use of energy during off-peak hours. Cooling units are known which utilize phase change materials to provide more effective cooling. A cooling unit utilizing a phase change material is described in my U.S. Pat. No. 5,765,389. The cooling unit includes heat exchange conduits including a coolant fluid therein which are positioned in heat transfer relationship with a phase change material such as a melt mix polymer or linear crystalline alkyl hydrocarbons.

[0004] However, there is a need for an improved cooling system which can reduce commercial and residential energy costs through the use of more effective phase change materials.

SUMMARY OF THE INVENTION

[0005] Embodiments of the invention meet that need by providing a residential and commercial cooling system such as an air conditioning unit which provides cooling by using a phase change material contained in a plurality of heat exchange pipes which are in heat transfer relation with a coolant fluid.

[0006] According to one aspect, a residential and commercial cooling system is provided which comprises a primary heat exchanger comprising an outer shell, and a plurality of heat exchange pipes positioned in the shell, where the pipes contain a phase change material therein. The system further includes at least one cooling element in the shell. Preferably, the cooling system includes first and second cooling elements positioned at the top and bottom of the primary heat exchanger.

[0007] The cooling system further includes a coolant fluid in contact with the heat exchange pipes, and an inlet and an outlet for the transfer of the coolant fluid to and from the heat exchanger.

[0008] The system further includes a separate liquid to air heat exchanger including an inlet and outlet for the transfer of coolant fluid to and from the liquid to air heat exchanger; and an inlet and outlet for the transfer of air to and from the liquid to air heat exchanger.

[0009] In one embodiment, the phase change material comprises water/ice. The phase change material may further comprise from about 1 to about 5% by weight polyvinyl alcohol.

[0010] In an alternative embodiment, the phase change material comprises a water/urea phase change material comprising from about 60 to 80% by weight water and from about 20 to 40% by weight urea.

[0011] The coolant fluid for use in the system is selected from the group consisting of ethylene glycol, propylene glycol, and glycerine. The coolant fluid is contained in the cavity of the shell such that it comes into contact with the heat exchange pipes filled with phase change material.

[0012] The heat exchange pipes comprise a metal selected from copper, stainless steel, and glass-coated steel. The heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches, and include a closure such as a cap at each end such that the phase change material is sealed therein.

[0013] The cooling system preferably has a three-dimensional rectangular or cubic configuration with generally flat sides. A layer of insulation may be included on the exterior surface of the shell. The insulation is preferably vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.

[0014] The coolant fluid is cooled by the cooling element(s) which may be filled with a refrigerant such as freon. The cooling system is preferably operated so that the coolant fluid (and the phase change material) is cooled from the bottom up. As the coolant is cooled, cooling of the phase change material is accomplished via direct contact of the coolant fluid with the metal heat exchange pipes containing the phase change material contained therein. When the cooling system is not in use, energy stored in the phase change material is transferred back to the coolant fluid such that the temperature of the fluid lowers and may be transferred out from the heat exchanger to the separate liquid to air heat exchanger positioned externally from the primary heat exchanger, where the coolant fluid will be used to cool air, for example, for air conditioning. Alternatively, the cooled coolant fluid could be used in a refrigeration unit.

[0015] In one embodiment, the cooling system may include a solenoid valve in conjunction with the inlet or outlet to provide pulsatile flow of the coolant fluid and improve cold transfer.

[0016] Accordingly, it is a feature of the invention to provide a cooling system which employs a primary heat exchanger comprising a plurality of heat exchange pipes including a phase change material therein. These, and other features and advantages of the invention, will become apparent from the following drawings, detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1A is a partial cross-sectional view of one embodiment of the cooling unit;

[0018] FIG. 1B is a top view of embodiment of the cooling unit shown in FIG. 1A; and

[0019] FIG. 2 is a perspective view of a single heat exchange pipe including a phase change material therein.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Embodiments of the cooling system use water as a phase change material where the water in liquid form provides sensible heat and the water/ice combination provides a latent heat of about 80 calories per gram. Water provides advantages over the use of other conventional phase change materials such as linear crystalline alkyl hydrocarbons as it has a higher latent heat and higher sensible heat, is much lower in cost, and is readily available.

[0021] Referring now to FIGS. 1A and 1B, one embodiment of the cooling system 10 is shown. As shown in FIG. 1A, the cooling system 10 comprises a shell 12 including a primary heat exchanger 14 therein which comprises a plurality of heat exchange pipes 16 including a phase change material 18 therein. The heat exchanger 14 is used in conjunction with a separate liquid to air heat exchanger 32 which is capable of supplying the cooling requirements of a residential home.

[0022] The shell 12 is preferably rectangular or cubic in configuration and should be of a sufficient height to be able to accommodate up to about 75 gallons of coolant fluid. The shell 12 may be comprised of copper, stainless steel, or glass-coated steel. It should be appreciated that the shell and the heat exchange pipes should comprise the same material. For example, if the heat exchange pipes comprise stainless steel, the shell should also be comprised of stainless steel to provide optimum heat transfer and to avoid creating a battery effect between the two metals in solution.

[0023] As shown, the shell 12 has a flat exterior surface 20 which is surrounded by an insulation material 22. The insulation material 22 preferably covers substantially the entire exposed exterior surface 20 of shell 12. Preferably, the insulation material 22 has an "R" value of at least about 50 to 60 per inch. Vacuum panel insulation suitable for use includes vacuum panel insulation available from AcuTemp under the designation ThermoCor.RTM.. The coolant fluid is supplied to the shell during manufacture of the heat exchanger and is filled close to the top of the shell as indicated by fluid level 48 as shown in FIG. 1A. As the shell is a closed unit, there is no need to add additional coolant fluid.

[0024] Suitable coolant fluids for use in the cooling unit include ethylene glycol, propylene glycol, and glycerine, which are preferably mixed with water. A preferred coolant fluid is a mixture of ethylene glycol and water in approximately equal amounts.

[0025] An outlet line 26 allows cooled fluid to flow from the heat exchanger 14 to a separate heat exchanger 32 for conversion to cooled air as will be explained in further detail below. If desired, the outlet line 26 may include a programmable solenoid valve 50 as shown. The solenoid valve may be partially closed at regular intervals to provide pulsatile flow of coolant fluid to improve cold transfer. The solenoid valve can be programmed to vary both the amplitude and frequency in which the valve is partially closed to provide the desired pressure drop. Variations of frequency from 5 to 60 cycles per minute, and more preferably, from about 15 to 30 cycles per minute are desirable. The valve closure is preferably regulated so as to create a pressure drop of at least 5 psi, and preferably up to about 50% of the available fluid pressure.

[0026] The cooling unit 10 further includes cooling elements comprising coils 28 and 30 positioned at the top and bottom portions of the shell. The cooling coils 28, 30 preferably contain a refrigerant therein such as freon or fluorocarbon gas. These materials are preferably selected to have a boiling point which is desirable to initiate freezing of the phase change material. In operation, evaporation and cooling of the refrigerant material takes place in the cooling coils similar to a freon type refrigeration or air conditioning system. The cooling unit further includes a compressor 34 positioned externally from the heat exchanger 14 which includes a condenser for returning the freon to liquid form for reuse as in a conventional refrigeration unit. The freon or other refrigerant is pumped through the cooling coils. As the refrigerant evaporates, it cools the coil, which then cools the coolant fluid in the primary heat exchanger. The evaporated coolant gas is then returned to the compressor, where the gas is converted to a liquid which is directed to the coils in the primary heat exchanger 14.

[0027] The cooling unit 10 may also include a timer (not shown) connected to a power supply (not shown) to control the power usage of the heat exchanger during designated time periods, e.g. turning off the power supply during peak power usage hours, thus reducing energy costs. Suitable power sources include photovoltaic or wind chargers.

[0028] The heat exchanger 14 further includes a plurality of heat exchange pipes 16 including the phase change material 18 therein. As shown, the heat exchange pipes 16 are positioned vertically in the heat exchanger. The heat exchange pipes are preferably configured in the shell as shown in the top view of the water heater depicted in FIG. 1B and are preferably held in position by a perforated metal screen (not shown) with holes therein which fit around the heat exchange pipes to hold them together as a unit.

[0029] Prior to being filled with the phase change material, the pipes are hollow and are comprised of a heat conducting material. The heat exchange pipes include a cap at each end for sealing the phase change material, which will be described in more detail below. While the pipes and phase change material are shown in cylindrical form, it should be appreciated that both the pipes and phase change material may also vary in shape. For example, the pipes and corresponding phase change material may be square or rectangular in shape.

[0030] A preferred phase change material for use in embodiments of the invention is water, which has a melting/freezing temperature of 0.degree. C. and a latent heat capacity of 80 calories/gram. Preferred for use in the invention is pure water obtained by distillation or reverse osmosis.

[0031] Another suitable phase change material is a water/urea mixture having a melting/freezing temperature of about -12.degree. C. and a latent heat capacity of about 70 calories/gram. Such a water/urea mixture preferably comprises about 70% water and 30% by weight urea. In this embodiment, a lower temperature refrigerant would be required for use in the coolant coils to freeze the phase change material. Suitable lower temperature refrigerant materials include Freon materials having varied fluorine contents and molecular weights, such as, for example, Freon.RTM. 22 commercially available from Dupont.

[0032] Upon melting and freezing, the phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point. The phase change materials may be repeatedly converted between solid and liquid phases to utilize their latent heats of fusion to absorb, store, and release heat during the phase conversions at about 0.degree. C. for pure water or about -12.degree. C. for water/urea mixtures.

[0033] The phase change material may further include from about 1 to 5% by weight of a polymeric thickening agent such as polyvinyl alcohol which raises the viscosity of the water phase change material, reducing the likelihood of leakage of water during the multiple cycles of melting and freezing that take place during operation of the cooling unit.

[0034] The phase change material may further include nucleating agents such as silicon dioxide dry powders or silver iodide. Such agents may be included in amounts of about 1 to 10% by weight silicon dioxide or about 0.05 to 0.5% by weight silver iodide to prevent super cooling in the system.

[0035] Referring now to FIG. 2, a single heat exchange pipe 16 for containment of the phase change material 18 is shown which is initially hollow in form and may be formed from metals including, but not limited to, stainless steel, copper, and glass-coated steel. The outer diameter of the pipes may range from about 0.5 to 2.5 inches, more preferably, about 1 to 2 inches, and most preferably about 1.5 inches. The wall thickness of the pipe should be sufficient to withstand normal pressures during operation and is preferably from about 0.030 to 0.125 inches, and more preferably, about 0.060 inches. The pipes 16 are preferably provided in lengths of about 27 inches, while the phase change material 18, once placed into the pipes, is about 24 inches in height, which allows at least about 3 to 4 inches of empty space in the pipe for expansion of the phase change material when it freezes.

[0036] Prior to providing the phase change material 18 in the pipe, an end cap 40 is applied to one end of the pipe and adhered thereto by a high temperature thermosetting adhesive, by providing mating threads on the pipe and cap, or by soldering (where copper pipes are used).

[0037] The open end of the empty pipe 16 is then filled with a source of inert gas such as nitrogen or argon such that most of the oxygen in the pipe is purged. The phase change material is then poured into the pipe sealed at the bottom with cap 40 such that the water level is about 4 inches below the top of the pipe. A second end cap 42 is then placed over the open end of the pipe 16 and secured thereto in a conventional manner as described above. The second end cap 42 includes a hole 44 which has been drilled in the center of the cap which allows residual gas inside the heat exchange pipe to be vented as the phase change material freezes and expands for the first time. This avoids the buildup of high pressure from the expansion of the phase change material which could potentially cause swelling and/or rupture of the pipes.

[0038] The initial freezing and expansion of the phase change material should take place prior to final assembly of the cooling system, i.e., prior to placing the heat exchanger inside the shell. After the venting process, the cap 42 is then permanently sealed. The cap may be sealed with a threaded metal screw comprised of the same metal as the pipe, by the use of a high temperature thermosetting adhesive or by soldering (where copper pipes are used).

[0039] The filled heat exchange pipes are then assembled in a compact configuration consisting of approximately 24 rows with about 24 heat exchange pipes in each row (arranged in a rectangular or square configuration). A perforated metal screen 36 having openings to accommodate the pipes may be used at the top and bottom of the pipes to maintain them in proper position. A thin metal strip may also be attached to the bundle of pipes to keep the pipes in place. The bundle of pipes including the phase change material therein is then inserted into the shell of the cooling unit.

[0040] The coolant fluid contained in the cavity of the heat exchanger provides a fluid transfer medium which comes into contact with the water-filled heat exchange pipes for cooling. The water phase change material is in direct heat transfer contact with the inner surfaces of heat exchange pipes 16 so that, during operation, as the coolant fluid surrounds the heat exchange pipes, heat can be transferred from the phase change material 18 to the coolant fluid and vice versa.

[0041] The thermal energy supplied from the cooling system is delivered on a plateau of nearly constant temperature until the latent heat capacity is exhausted and electric power is supplied. If desired, lower cost off-peak electricity or a green source of energy such as solar photovoltaic or wind driven devices can be used to supply the energy required to "charge" the phase change material, resulting in significant cost savings for consumers.

[0042] Referring again to FIG. 1A, the cooling system 10 operates in the following manner. The cooling coils 28 and 30 positioned at the top and bottom portions of the shell include a refrigerant material therein such as freon which is supplied from compressor 34. The temperature of the refrigerant material is controlled by a thermostatic valve (not shown) which controls the rate of coolant flowing through the top and bottom coils such that a desired temperature differential is maintained between the two coils and to allow freezing of the phase change material from the bottom up. By freezing the phase change material from the bottom up, the phase change material can expand into the empty space at the top of the heat exchange pipe without a build-up of pressure.

[0043] As the coolant fluid in the cooling system 10 is cooled by the cooling coils (via evaporation of refrigerants in the coils), energy from the coolant fluid is transferred from the metal heat exchange pipes 16 to the water phase change material 18 contained therein, which begins to freeze. Once the water reaches the freezing point (0.degree. C.), the ice continues to cool the system until heat is added to the system at which point the coolant fluid becomes warmer than the (ice) phase change material and a melt cycle begins.

[0044] Thus, when the cooling unit 10 is not in operation, e.g., during peak times of power usage, the phase change material in the heat exchanger 14 cools the coolant fluid, i.e., heat energy from the coolant fluid is transferred to the phase change material which lowers the temperature of the coolant fluid.

[0045] The cooled coolant fluid is then transferred from the primary heat exchanger 14 to the liquid to air heat exchanger 32 via outlet line 26. The heat exchanger 32 is a conventional liquid to air heat exchanger and includes a return line 66 for the return of coolant fluid to the heat exchanger. The liquid to air heat exchanger 32 further includes a warm air inlet 68 and cold air outlet 70. As cooled coolant fluid enters the liquid to air heat exchanger 32, air entering the heat exchanger 32 is cooled by transport over the cooled pipes of the heat exchanger 32 such that the temperature of the air is reduced. The rate of heat transfer depends on the rate of air flow, the surface area of the pipe, and temperature differentials. The cool air then exits through air outlet 70.

[0046] While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and apparatus disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. A residential and commercial cooling system comprising: 1) a primary heat exchanger comprising: an outer shell; a plurality of heat exchange pipes positioned in said shell, said pipes containing a phase change material therein; at least one cooling element in said shell; a coolant fluid in contact with said heat exchange pipes; an inlet and an outlet for the transfer of said coolant fluid to and from said primary heat exchanger; 2) a liquid to air heat exchanger comprising: an inlet and outlet for the transfer of said coolant fluid; and an inlet and outlet for the transfer of air.

2. The cooling system of claim 1 including first and second cooling elements positioned at the top and bottom of said primary heat exchanger.

3. The cooling system of claim 1 wherein said phase change material comprises water.

4. The cooling system of claim 3 wherein said phase change material further comprises from about 1 to about 5% by weight polyvinyl alcohol.

5. The cooling system of claim 1 wherein said phase change material further comprises a water/urea phase change material comprising from about 60 to 80% water and from about 20 to 40% by weight urea.

6. The cooling system of claim 1 wherein said coolant fluid is selected from the group consisting of ethylene glycol, propylene glycol, and glycerine.

7. The cooling system of claim 1 further including insulation on the exterior surface of said outer shell.

8. The cooling system of claim 7 wherein said insulation is vacuum panel insulation having an R value of about 50 to 60 per inch of thickness.

9. The cooling system of claim 1 wherein said heat exchange pipes comprise a metal selected from copper, stainless steel, and glass-coated steel.

10. The cooling system of claim 1 wherein said heat exchange pipes have an outer diameter of from about 0.5 to about 2.5 inches.

11. The cooling system of claim 1 wherein each of said heat exchange pipes include a cap at each end such that said phase change material is sealed therein.

12. The cooling system of claim 1 having a rectangular configuration.

13. The cooling system of claim 1 having a cubic configuration.