WO1995016175A1

WO1995016175A1 - Thermal storage apparatus

Info

Publication number: WO1995016175A1
Application number: PCT/US1994/013858
Authority: WO
Inventors: William J. Longardner; Alexander P. Rafalovich; Gilbert P. Keller; Thomas C. Schmidter; Joseph A. Gustin
Original assignee: Store Heat and Produce Energy Inc
Current assignee: Store Heat and Produce Energy Inc
Priority date: 1993-12-10
Filing date: 1994-12-09
Publication date: 1995-06-15
Anticipated expiration: 1996-06-10
Also published as: AU1334395A

Abstract

A thermal storage apparatus (20) is provided including a housing (30) and a plurality of elongated sealed containers (60) each containing phase charge materials and each preferably having corrugations (78) on their exterior surfaces. Advantageously, containers (60) may be stacked in an array in such a way as to allow for flow of a heat exchange fluid therebetween, the flow of the heat exchange fluid being regulated by the expansion and contraction of the containers (60).

Description

THERMAL STORAGE APPARATUS

Background and Summary of the Invention

The present invention relates to a thermal storage apparatus. More particularly, the present invention relates to a thermal storage apparatus including a plurality of thermal storage containers configured to be incorporated into the air distribution system of a vehicle or the like. Phase change materials ("PCMs") store heat during phase transition, typically liquid/solid phase transitions. For example, salts and salt hydrates have notably high energy densities over temperature ranges of practical significance. A large amount of thermal energy can be stored as latent heat of fusion during the melting of an appropriate salt or salt hydrate. The stored heat can then be extracted from the liquid PCM by cooling it until it crystallizes.

Various attempts have been made to incorporate PCMs into heating and air conditioning systems, including heat pump systems, solar collection systems, and more conventional heating and air conditioning systems. For example, U.S. Patent No. 5,054,540 to Carr describes a cool storage reservoir positioned in the air duct of a vehicle or the like. A plurality of elongate sealed containers is positioned in the cool storage reservoir, each of the sealed containers being filled with a gas/water medium capable of forming a gas hydrate.

Gas hydrates, however, may possess a variety of disadvantages. Gas hydrates suffer from the development of significant pressures during decomposition and may be subject to excessive supercooling. They may also require specific devices to initiate nucleation.

Another example is the "heat battery" designed to provide "instant" heating to a vehicle cabin. (Automotive Engineering, Vol. 100, No. 2, February, 1992). The core of the heat battery includes a series of flat, sheet metal PCM envelopes in spaced-apart relationship. The heat battery and an electric coolant pump are installed in a coolant line running from the engine to the cabin heater, forming a closed circuit capable of very rapidly heating the cabin when the engine is turned on.

While such a design possesses certain advantages in typical passenger vehicle applications, there remains a need for thermal storage system designs which can be operated more flexibly. For example, there remains a particular need for thermal storage system designs which can provide space conditioning to an enclosed space when the engine is off. According to the present invention, a thermal storage apparatus is provided which comprises a housing defining an interior region and a plurality of sealed containers positioned in the interior region for containing phase change materials. Advantageously, each of the sealed containers has a corrugated surface to enhance heat transfer characteristics.

According to another aspect of the present invention, a thermal storage apparatus is provided which comprises a housing and a plurality of flexible containers positioned in the interior region for containing phase change materials. The flexible containers are stacked in an array in such a way that a flow passageway for a heat exchange fluid is provided between the containers of the array. Advantageously, expansion and contraction of the individual sealed containers regulates the size and geometry of this flow passageway, and thus regulates flow of the heat exchange fluid along the flow passageway.

Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.

Brief Description of the Drawings The detailed description particularly refers to the accompanying figures in which:

Fig. 1 is a schematic view of a space conditioning system for a vehicle or the like incorporating a thermal storage apparatus; Fig. 2 is a perspective view with portions broken away showing a thermal storage apparatus in accordance with the present invention;

Fig. 3 is an enlarged perspective view of a PCM container in accordance with the present invention including a plurality of rings on its exterior surface; and

Fig. 4 is a plot of temperature vs. time showing performance of a thermal storage apparatus constructed in accordance with the present invention.

Detailed Description of the Drawings

A thermal storage device constructed in accordance with the present invention is adapted for integration into a space conditioning system used for climate control of an enclosed space. Thermal storage devices as described herein are particularly suited for use in connection with the heating and cooling systems of a vehicle, but can also be used with a variety of other heating and air conditioning systems, including those for heating and cooling buildings and the like. A thermal storage apparatus in accordance with the present invention is illustrated schematically in Fig. 1 integrated into a space conditioning circuit for a typical vehicle. As shown, a space conditioning circuit 10 typically includes the vehicle's engine 12, a radiator 14, and a space conditioner 16 arranged in a closed loop. Space conditioner 16 may be a standard cabin heater or a standard vehicle air conditioner (with a compressor 17 connected thereto) familiar to those of ordinary skill in the art. Ducting 18 is connected to space conditioner 16 to distribute conditioned air (or other heat exchange fluid) to a plurality of discharge vents (not shown) to discharge conditioned air into the vehicle cabin in a conventional manner.

To integrate a thermal storage unit 20 into circuit 10, a bypass duct 22 is connected between ducting 18 and an inlet 24 of thermal storage unit 20. A damper 26 is provided in bypass duct 22. Air discharge outlets 28 are provided on apparatus 20.

Such a circuit can advantageously be operated to provide space conditioning not only during vehicle operation, but also when the vehicle is parked with the engine off. This is particularly advantageous for truck cabs, motor homes, buses, and other vehicles in which it is desirable to control the temperature of a sleeping compartment or living space when the vehicle is parked over an extended period of time (e.g., overnight).

Circuit 10 can provide such temperature control by cycling between a charging cycle (when the vehicle is operating with the engine on) and a discharge cycle (when the vehicle is parked with the engine off) . For example, in a heating application, when the vehicle is operating with the engine running, coolant circulates through the radiator 14, through engine 12, and to space conditioner (heater) 16. From there the air flows through ducts 18 to the vehicle cabin.

A portion of the hot air will flow through bypass duct 22 to thermal storage unit 20. Damper 26 is placed in an open position to allow air flow through bypass duct 22. In thermal storage unit 20, the heat from the air will be transferred to the PCM inside thermal storage unit 20 as heat of transition, causing solid PCM to melt and "storing" the heat in the PCM. This "charging" cycle continues until the PCM is fully melted, at which point additional heat is stored in the form of sensible heat. When the vehicle is parked and the engine is shut off, the temperature of the conditioned space (e.g., the sleeping compartment or vehicle cabin) begins to approach ambient conditions. When the temperature falls to a predetermined level, as optionally measured by a thermostatic control or other controller, the "discharging" cycle commences. Damper 26 is closed so that no air flow from duct 18 enters bypass duct 22. When damper 26 is moved to the closed position, an associated port (not shown) in bypass duct 22 is opened so that unconditioned air from the cabin flows into thermal storage unit 20. As described further below, one or more fans can be provided in thermal storage unit 20 to induce airflow.

As the relatively cool air passes the liquid PCM, heat is discharged from the PCM to the air, raising the temperature of the air stream. As the cycle continues, warm air is discharged from thermal storage unit 20 to the vehicle cabin. In addition, the PCM begins to crystallize. Eventually, the PCM must be again liquefied in another charging cycle. It will be appreciated that an optional control system may be implemented for use in conjunction with thermal storage unit 20 in circuit 10. Controls may be provided to protect the system from minimum and maximum temperature potentials, to regulate the supply of heat exchange fluid, and to open and close damper 26 and its associated port, to activate fans in outlet 28, and to control other parameters.

It is evident that these same operating procedures can be applied to "cool storage" by selecting the appropriate PCM and connecting thermal storage unit 20 to the vehicle's space conditioner 16 (operating as an air conditioner) . In the "charging" cycle for cool storage, cool air or other heat transfer fluid circulating in the air conditioning system causes liquid PCM in thermal storage unit 20 to crystallize. In the discharging cycle, relatively warm cabin air passes in heat transfer relationship with the PCM, transferring heat to the PCM. The air stream thus exits thermal storage unit 20 at a reduced temperature, cooling the cabin. It will be appreciated that sources of heat exchange fluid other than those described in connection with Fig. 1 may be used to "charge" PCMs in a thermal storage unit such as unit 20. For example, a dedicated electric resistance heater may be provided. Such a dedicated heater would be designed so that the electrical load would not affect engine operation and could be designed with a controller so that the "charging" cycle commences automatically upon engine start. A dedicated heater could be located either outside or inside unit 20. Another alternative would be to provide a water-to-air heat exchanger plumbed directly into engine coolant circuit 10. A heat pump might also be used.

A variety of designs for thermal storage unit 20 are contemplated as being within the scope of the present invention. One preferred embodiment particularly suitable for use in a sleeping compartment of a truck is shown in Fig. 2. As shown, thermal storage unit 20 in accordance with the present invention includes a housing 30 defining an interior region 32. The geometry of housing 30 may vary, although for application in sleeping compartments and the like, limited space is typically allowed and a design as shown having a relatively small footprint is usually desirable. A tray or the like (not shown) may be rigidly secured to the floorboard of the vehicle cabin and housing 30 may in turn be bolted or otherwise secured to the tray to insure that housing 30 is held in place during vehicle movement. A layer of suitable insulation 34 (shown best in Fig. 3) is positioned on all inner surfaces of housing 30 to fully insulate interior region 32 from ambient conditions.

Housing 30 may be constructed from a variety of commonly available lightweight and durable metals and plastics. Housing 30 will be fabricated using known construction techniques. Housing 30 includes a first wall 36 which is formed to include inlet 24 for receiving air flow or flow of other heat exchange fluids from bypass duct 22 (shown in Fig. 1) . The location of inlet 24 can be varied according to design constraints. Inlet 24 may be formed on other walls of housing 30, or may be formed in a bottom surface of housing 30.

Housing 30 also includes a second wall 40 (typically the top wall) which is formed to include at least one outlet 28. Typically, second wall 40 will be removable to allow access to interior region 32. At least one fan 44 is provided adjacent to each outlet 28 to induce air flow from interior region 32 through outlet 28. The at least one fan 44 can be wired to the vehicle's electrical system or to other independent power sources. Safety grills (not shown) are positioned in outlets 28 to prevent inadvertent contact with fans 44.

Baffles 46 and 48 may be provided in interior region 32 proximate to inlet 24 to direct incoming air flow uniformly throughout interior region 32. Baffle 46 includes a horizontal piece 50 and an angled piece 52.

Horizontal piece 50 splits the air stream at inlet 24 into a first portion 54 and a second portion 56. First portion 54 impinges upon angled piece 52 and is directed into that portion of interior region 32 nearer inlet 24. Second portion 56 of the air stream flows underneath horizontal piece 50 of first baffle 46 and impinges upon an angled piece 58 of second baffle 48. Second portion 56 of the air stream is thus directed into that portion of interior region 32 further from inlet 24. It will be appreciated that other baffle arrangements may be used (depending upon the location of inlet 24 and other factors ) to ensure uniform airflow throughout interior region 32.

One important feature of the design of any thermal storage unit is the arrangement of the PCMs within the unit. Although a variety of configurations are contemplated within the scope of the present invention, one preferred design provides PCMs 82 (see Fig. 3) in sealed, elongated containers 60 placed in a stacked array in interior region 32. Upper and lower gratings or racks 62, 64 are provided to ensure that the array of containers 60 is maintained in tight-packed relationship even during vehicle movement. Containers 60 are arranged so that their long axes 66 are perpendicular to the direction of air flow through interior region 32. Of course, other arrangements may be desirable depending upon design considerations.

Indeed, it may be desirable to arrange the containers 60 so that their longitudinal axes 66 are parallel to the air flow.

It is anticipated that containers 60 may be subject to thermal expansion during solid to liquid phase change.

Radial expansion may be constrained somewhat by the grids 62, 64, tightly packing the array together. However, expansion along axis 66 is expected to occur, and some clearance must be provided between the ends 68 of containers 60 and walls 36, 70 to allow for such expansion. Unfortunately, such a clearance space would allow a passageway for air flow around the ends of containers 60. Air flowing in such passageways would bypass the array and reduce heat transfer efficiency between the air stream and the PCMs. This problem may be solved in a variety of ways in accordance with the present invention. Walls 36, 70 may be constructed of flexible diaphragm material which will flex when containers 60 expand but will at other times remain flush with the ends of containers 60.

Another possibility is to provide a foam pad 72 or the like between the ends of containers 60 and insulation 34. Pad 72 blocks air flow when containers 60 are in their normal condition and also allows for containers to expand along longitudinal axis 66.

One preferred embodiment of a container 60 in accordance with the present invention is shown in Fig. 3. Container 60 is a flexible, elongated cylinder made of plastic which defines an internal chamber 74 for containing PCMs. The type of plastic will vary depending upon temperature requirements, but high density polyethylene (HDPE) manufactured by OEM Miller is suitable for many applications. Metal containers may be needed where applications require the use of PCMs having particularly high melt temperatures (such as magnesium chloride, for example) .

The dimensions of containers 60 will vary with the application. However, it has been found that HDPE containers having a 1 1/2 inch (3.8 cm) outer diameter and a 1 1/4 inch (3.2 cm) internal diameter are suitable.

Likewise, the number of containers 60 in the array will vary depending upon design considerations well known to designers of heat transfer equipment. An 8 X 6 array of HDPE containers 60 of the dimensions described above was found to provide heat transfer rates and pressure drops within prescribed limits in one application.

Containers 60 are sealed at their ends 68 by a variety of means. The ends may be heat sealed or may be otherwise compressed or crimped. Preferably, end caps 76 are provided. End caps 76 may also be made from HDPE and may be spin-welded to the ends of containers 60. As caps 76 are spin-welded into place, the outer surfaces of caps 76 and the inner surface of container 60 partially melt, bonding and hardening when caps 76 cool and thus providing a hermetic seal.

While smooth-walled flexible containers may be used, containers 60 are preferably corrugated containers 60 as shown in Fig. 3. The corrugated rings 78 on the exterior surface of each container 60 may vary in size, shape, and spacing. When multiple containers 60 are packed together in a stacked array as has been described herein, the corrugated rings 78 of adjacent containers 60 will be in contact, advantageously leaving the interstitial spaces 80 open to form a passageway for airflow through the array as shown best in Fig. 3.

It is contemplated, then, that when containers 60 expand during phase change, corrugations 78 will tend to flatten, restricting the passageway for air through the array and reducing the rate of heat transfer. This will advantageously allow for slow initial discharge rates during a heat discharge cycle, with increasing rates later in the cycle. The opposite is likely to occur in cooling discharge cycles. That is, the passageway for air through the array will be relatively unrestricted during the initial phase of the cooling discharge cycle, and will gradually become more restricted as container 60 expands and corrugations 78 flatten out. Thus, corrugations 78 provide, in effect, a means for self-regulating the airflow passageway through interstitial spaces 80. Of course, these effects are likely to vary depending upon the type of PCM 82 which is used.

The use of corrugated containers 60 provides numerous additional advantages. The corrugated containers 60 have a greater external surface area for heat transfer than do smooth-walled containers of the same dimensions. In addition, corrugations 78 assist crystal growth in the PCM by providing increased probability of surface imperfections which can provide points for crystal nucleation. Corrugations 78 provide constantly changing contours preventing large linear crystal growth that might otherwise puncture or damage containers 60.

Corrugations 78 may also provide a deterrent to PCM stratification during phase change transition. It is thought that the additional surface area offered by corrugations 78 will promote equal distribution of solid PCM particles throughout the entire interior volume of container 60, avoiding the accumulation of a high concentration of solid PCM at the bottom of containers 60. Additionally, the corrugations 78 provide superior structural properties as compared to smooth walled containers. This is particularly important in the present application where leakage of PCMs may corrode or otherwise cause damage to thermal storage unit 20. A wide variety of PCMs 82 may be used in connection with the present invention. PCMs are typically chosen based upon their latent heat characteristics, but may also be selected for their additional qualities, such as congruent melting, minimal supercooling. Typical classes of PCMs usable in accordance with this invention include paraffin waxes, eutectic mixtures of salts, salt hydrate solutions, and water.

It may be desirable to include a first series of containers 60 containing "heat storage" PCMs and a second series of containers 60 containing "cool storage" PCMs. Another alternative is to use the first series of containers during the winter months and then change the system over to the second series of containers in the summer months. Preferred PCMs are calcium chloride hexahydrate solutions of the type described in U.S. Patent Nos. 4,272,390; 4,613,444; and 4,412,931, relevant portions of which are hereby incorporated by reference. Such solutions have excellent latent heat characteristics, low electrical conductivity, and outstanding fire retardant capability. Other salt hydrate solutions contemplated as being useful in accordance with the claimed invention include CaBr₂.6H₂0 (e.g., as described in U.S. Patent No. 4,690,769), mixed calcium halide hexahydrates (e.g., as described in U.S.

Patent No. 4,637,888), magnesium nitrate hexahydrate (e.g., as described in U.S. Patents Nos. 4,272,391; 5,271,029; and 4,273,666) , magnesium chloride hexahydrate (e.g., as described in U.S. Patents Nos. 4,338,208 and 4,406,805), mixtures of magnesium nitrate hexahydrate and magnesium chloride hexahydrate (e.g., as described in U.S. Patents Nos. 4,272,392; 4,329,242; and 4,402,846), mixtures of magnesium nitrate hexahydrate and ammonium nitrate (e.g., as described in U.S. Patent No. 4,283,298), and certain gelled PCMs (e.g., as described in U.S. Patent No. 4,585,572) .

An example of the operation of a thermal storage unit 20 in accordance with the present invention is illustrated in Fig. 4. The sleeping compartment 21 (see Fig. 1) of a truck was fitted with a standard insulation package and subjected to testing in a cold room at 0 °F (-17.8°C) . The temperature inside sleeping compartment 21 was recorded over time. The results are shown as a dashed-line plot in Fig. 4. In a separate test conducted for further comparison, a sleeping compartment 21 of a truck was fitted with a superior insulation package and was subjected to testing in the cold room at 0 °F (-17.8°C) . Temperature was again measured over time and the results were plotted as a solid-line plot in Fig. 4. Next, a thermal storage unit 20 was installed in sleeping compartment 21 of the truck having the standard insulation package referred to above. A plot of temperature over time for cold room testing of this setup is shown as a dotted line plot in Fig. 4. As illustrated, thermal storage unit 20 showed a considerably greater capacity to maintain temperature at comfortable levels over an extended period of time than did the standard insulation package alone or the superior insulation package. Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.

Claims

CLAIMS :

1. A thermal storage apparatus comprising a housing defining an interior region and including a first wall formed to include at least one inlet for flow of a heat exchange fluid from a space conditioning system into the interior region and a second wall formed to include at least one outlet for flow of a heat exchange fluid out of the interior region into a space to be conditioned, a plurality of elongated sealed containers positioned in the interior region for containing phase change materials, each of the sealed containers having a corrugated exterior surface.

2. The apparatus of claim 1, further comprising a baffle extending into the interior region from the inlet to split the flow of heat exchange fluid through the inlet to the interior region into first and second portions so that heat exchange fluid flows uniformly to the interior region.

3. The apparatus of claim 2, further comprising a second baffle downstream of the first baffle to direct the second portion of the heat exchange fluid to the interior region.

4. The apparatus of claim 1, wherein the sealed containers are positioned in a stacked array with their longer axes perpendicular to the direction of flow of heat exchange fluid.

5. The apparatus of claim 1, further comprising a damper positioned adjacent the inlet and movable between a open position forcing heat exchange fluid from the space conditioning system to enter the interior region through the inlet and an closed position allowing air from the conditioned space to enter the interior region through the inlet.

6. The apparatus of claim 1, wherein the housing further includes at least one flexible wall adjacent to the ends of the sealed containers to allow for thermal expansion of the containers.

7. A thermal storage apparatus comprising a housing defining an interior region and including a first wall formed to include at least one inlet for flow of a heat exchange fluid into the interior region and a second wall formed to include at least one outlet for flow of a heat exchange fluid out of the interior region, a plurality of flexible sealed containers positioned in the interior region for containing phase change materials, the containers being fabricated of a material permitting expansion and contraction of the containers, the containers being stacked in an array to provide a flow passageway for heat exchange fluid therebetween so that expansion and contraction of the containers regulates the flow of heat exchange fluid through the flow passageway.

8. The apparatus of claim 7, further comprising a baffle extending into the interior region from the inlet to split the flow of heat exchange fluid through the inlet to the interior region into first and second portions so that heat exchange fluid flows uniformly to the interior region.

9. The apparatus of claim 8, further comprising a second baffle downstream of the first baffle to direct the second portion of the heat exchange fluid to the interior region.

10. The apparatus of claim 7, wherein the sealed containers are positioned with their longer axes perpendicular to the direction of flow of heat exchange fluid.

11. The apparatus of claim 7, further comprising a damper positioned adjacent the inlet and movable between a open position forcing heat exchange fluid from the space conditioning system to enter the interior region through the inlet and an closed position allowing air from the conditioned space to enter the interior region through the inlet.

12. The apparatus of claim 7, wherein the housing further includes at least one flexible wall adjacent to the ends of the sealed containers to allow for thermal expansion of the containers.

13. A container for encapsulating a phase change material, the container comprising an elongated tube having a corrugated exterior surface and defining an internal chamber for containing phase change materials.

14. The apparatus of claim 13, wherein the tube is flexible.

15. The apparatus of claim 13, further comprising first and second end caps for sealing the ends of the tube.

16. A process for encapsulating a phase change material in a corrugated container, the process comprising the steps of filling the corrugated container with phase change materials, and spin-welding end caps to the ends of the container.