Thermal Retention Device

1. Field of the Invention

This invention pertains to thermal retention devices for storing thermal energy and releasing the thermal energy over time. More particularly, this invention pertains to a thermal retention device which utilizes a phase change material as a medium for storing and releasing thermal energy.

2. Description of the Prior Art

The prior art includes so-called phase change materials for storing thermal energy. A phase change material is a material which includes a substance which changes phase (for example, from solid state to liquid state or from a liquid state to a gaseous state) upon the application of thermal energy to the material. The process of changing phase results in energy (referred to as latent heat) being stored in the material. When the phase change reverses, the latent heat is released.

Phase change materials can be provided with a wide variety of phase change temperatures (i.e., the temperature at which the phase change material changes phase). An example of a phase change material is described more fully in U.S. Pat. No. 5,211,949 which describes a hydrocarbon dispersed within a finely divided silica. The phase change material stays in a powder form above and below the melting point of the hydrocarbon. The hydrocarbon melting point can be selected to be any one of a number of different temperatures. The phase change material results in an effective storing of thermal energy with energy being released at about the melting point of the hydrocarbon.

Numerous devices containing phase change materials are known in the prior art. For example, seat cushions are formed which will include a foam pad impregnated with a phase change material. The seat cushion is contained within an envelope of plastic such as vinyl or the like. The entire cushion may be placed in a source of thermal energy (for example, a conventional microwave oven). Energy is applied to the phase change material and the cushion will retain an elevated temperature for a substantial period of time (commonly one to four hours).

Other means, in addition to phase change materials, are known for storing energy. For example, U.S. Pat. No. 5,300,105 to Owens dated Apr. 5, 1994 teaches a foam pad vacuum sealed in a plastic envelope. The envelope also contains a liquid which is heated by application of microwave energy. The '105 patent teaches several uses for the invention including therapeutic pads, toys and cushions.

While prior art microwave thermal retention devices are acceptable for many uses, there are certain applications where such devices have limitations. For example, the user of such a device must have access to a source of the energy. In the case of a microwave thermal retention device, the user must have access to a microwave oven to supply the necessary energy to cause the phase change material or other heat retention means to store the energy. For many users, access to a microwave oven is not practical. Also, for many users, there may be a substantial period of time between which the energy is applied and the device is to be used. An example of such an inconvenience is where a thermal retention device is to be used as a cushion for either outdoor activity or for stadium use at sporting events and the like. In such cases, there may be a substantial period of time between the application of energy to the device and the use of the device.

According to a preferred embodiment of the present invention, a thermal retention device is disclosed which includes an envelope for defining an enclosed volume. An electrically resistive element is disposed within the envelope and terminates at an electric coupler which is adapted for coupling to an external source of electric power. A heat retention material is contained within the envelope in thermal conductivity with the resistive element.

FIG. 1 is a perspective view of a first embodiment of the present invention in the form of a heating pad incorporating the invention of the present invention and shown with an internal envelop removed from a cloth cover;

FIG. 2 is a cross-sectional view of the assembled heating pad of FIG. 1;

FIG. 3 is an end view of a first end of the heating pad of FIG. 1 taken along lines 3--3 of FIG. 1;

FIG. 4 is an opposite end view of the heating pad of FIG. 1;

FIG. 5 is a plan view of a resistive heating element of the heating pad of FIG. 1 shown in plan view;

FIG. 6 is a schematic representation of the heating coil shown in FIG. 5;

FIG. 7 is a plan view of an alternating current power cord for energizing the pad of FIG. 1;

FIG. 8 is a plan view of a direct current power cord for energizing the pad of FIG. 1;

FIG. 9 is a side cross-sectional view shown in schematic of an alternative embodiment of the present invention; and

FIG. 10 is a still further embodiment of the present invention shown in side cross-section schematic.

With initial reference to FIG. 1, a heating pad 10 is shown incorporating the teachings of the present invention. It will be appreciated while the present invention is disclosed with reference to a heating pad such as a cushion or the like, that the teachings of the present invention are readily adapted to a wide variety of products and uses. For example, the present invention can be used in clothing (e.g., footwear, mittens, etc.), food warming devices (e.g., beverage containers, serving dishes, bread warmers, etc.), toys to be warmed as comforters and therapeutic pads.

A. First Embodiment

The heating pad 10 is substantially rectangular in shape. The pad 10 includes an outer cloth covering 12 which is open at its opposite ends 14, 16. Contained within cloth covering 12 is a sealed vinyl or plastic envelope 18. Envelope 18 is completely sealed to define an internal volume. Contained within the volume is an electric resistive element (FIGS. 2, 5 and 6) in the form of a plurality of wire coils 22.

The resistive element 22 in a preferred embodiment includes a first resistive coil 24 and a second resistive coil 26. The coils 24, 26 terminate at a coupling 28 which includes three female connectors 31, 32 and 33. Connector 31 and connector 33 are electrically connected to opposite ends of coil 24. Connector 32 and connector 33 are connected to opposite end of coil 26 as best shown schematically in FIG. 6. Each of coils 26, 24 are connected to connector 33 through a thermal switch 30 which will be more fully described. Also, as will be described, coil 24 is intended for use in an alternating current circuit and coil 26 is intended for use in a direct current circuit. A coupling 28 for terminating the coils 24, 26 is shown and described in U.S. Pat. No. 2,980,881.

The coils 24, 26 are disposed sandwiched between two layers of open cell foam material 34, 36 (FIG. 2). The foam material 34, 36 is also contained within the volume of envelope 18. The coupling 28 is exterior to the envelope 18 passing through and sealed at the envelope 18 at location 19. Also contained within envelope 18 is a microwave receptive heat retention mass 20. While the heat retention mass need not be a phase change material (as will be described with reference to alternative embodiments), in the embodiment of FIGS. 1-6, the thermal retention mass 20 is a phase change material.

In the specific embodiment shown, a preferred phase change material 20 is sold under the name "Michemlube 270R" available through Mechelman, Inc., 9080 Shell Road, Cincinnati, Ohio, Order No. A56141. Such a phase change material is selected to have a phase change temperature of about 140.degree. F. and is a paraffin based emulsion.

With the coils 24, 26, foam 34, 36 and phase change material 20 contained within envelope 18, the envelope 18 (which is flexible plastic) has a vacuum applied to its interior and the peripheral edges of the envelope 18 are completely sealed while the vacuum is applied. Commonly assigned U.S. Pat. No. 5,300,105 teaches sealing a phase change liquid and a foam pad within a flexible plastic envelope.

With the envelope 18 and its contents constructed as thus described, the envelope 18 is contained within the cloth cover 12. The ends 16, 14 of cloth cover 12 are initially open. As best shown in FIG. 3, end 16 is stitched shut substantially along its length but is open at a corner 16A. The opening of corner 16A is sized to freely pass coupling 28. Opposing surfaces of the edge 16 at opening end 16A are provided with hook-and-loop fasteners 40, 41 (such as well-known Velcro.TM. fasteners) to permit the end 16A to be closed as snugly as possible around the cord extending from coupling 28. The opposite end 14 is also provided with hook-and-loop fasteners 42, 44 to permit end 14 to be closed.

To energize the resistive elements 24, 26, power cords 50, 60 are provided as shown in FIGS. 7 and 8. Power cord 50 is an alternating current power cord having a standard male plug end 51 which may be plugged in any conventional wall outlet and terminating at a coupling 52 having male connectors 53, 54, 55 disposed to be received and mate with the female connectors 31, 32, 33 of coupling 28. Only connectors 53, 55 are connected to plug 51 (connector 54 is not connected to a power source or ground). Power cord 60 is a direct current power cord having a plug end 61 which is a conventional plug to be received within a conventional cigarette lighter in automobiles. Plug 60 terminates at a coupling end 62 provided with male connectors 63, 64 disposed to engage and mate with female connectors 32, 33 of coupling 28. Plug 61 is adapted for a 12 volt DC battery and plug 51 is adapted for insertion into a standard 120 volt AC outlet.

In a preferred use as a cushion or the like, it is desirable to provide energy to the coils 24, 26 at a sufficient rate to enable as rapid heating as possible of the pad 10. In the preferred embodiment, direct current coil 26 is selected to have a resistive value of about 0.08 ohms per foot and with a length of about 18 feet. Resistive coil 24 is selected to have a resistance of 11 ohms per foot and length of about 13 foot. If faster heating is desired, higher resistive valves can be used.

As previously noted, both of resistive elements 24, 26 terminate at neutral connector 33 across a thermal switch 30. Thermal switches are well known in the art and preferably, thermal switch 30 is selected to be a normally closed switch which opens in response to a sensed temperature of 150.degree. F. (i.e., 10.degree. above the phase change temperature of the preferred phase change material).

With the construction thus described, the heating coils 24, 26 can be individually heated by coupling 28 being secured to either of plugs 50, 60. With the use of either direct current or alternating current, the resistive element 22 heats in response to the electrical energy. The heat of the resisting element heats the phase change material 20 such that the phase change material 20 will undergo a phase change. When heating is complete, the cords 50, 60 may be removed and the pad 10 will retain its energy for a substantial period of time of about two to three hours. As a result, the pad can be heated through either direct current or alternating current. It may be heated in a user's home or in a user's car or place of business. In addition to the foregoing, the vinyl plastic 18 is a microwave transparent material as is cloth 12. The particular phase change material 20 thus described is microwave receptive. Accordingly, the entire pad 10 may be placed in a conventional consumer microwave oven and the necessary energy to heat the phase change material may be applied through microwave energy. For microwave applications, the user may open opening 16A by separating fasteners 40, 41 and folding the coupling 28 into the interior of the cloth covering 12. This prevents the coupling 28 and its metallic elements from being directly exposed to the interior of a microwave oven.

B. Alternative Embodiments

FIGS. 9 and 10 show alternative embodiments of the present invention. In FIG. 9, a pad 10' is shown having a microwave transparent outer plastic envelope 18' which is sealed to define a sealed interior 20'. Two packets 30' of identical construction are contained within the interior 20'.

Each packet contains a layer of open cell foam material 32' and a liquid 33'. The liquid (e.g., water) is impregnated within the foam 32' and the liquid and foam 32' are surrounded by a microwave transparent plastic pouch 34' which completely surrounds the foam 32' and liquid 33' and which is sealed. In the forming process, the pouch 34' may be partially evacuated to partially compress the foam 32'. Each pouch 30' may be of the construction and method of manufacture such as that disclosed in commonly assigned U.S. Pat. No. 5,300,105.

Positioned between the pads 30' is a heating coil shown schematically as a resistor 22' accessible at an external electrical plug 28'. As mentioned, ends 18a' and 18b' of envelope 18' are sealed. In the schematic view of FIG. 9, end 18b' is shown open simply to illustrate that coil 22' extends through sealed envelope 18' to a plug 28' accessible on the exterior of envelope 18'.

As with the previous embodiment, the plug 28' permits the coil 22' to be heated electrically through either an AC or a DC source. The heating of the coil 22' heats the packets 30' which retain the heat after the plug 28' is disconnected from a source of electrical power. Alternatively, the pads 30' may be heated through microwave energy. The pads 30' may contain a phase change material but need not. Instead, the pads 30' may contain a liquid 33' as disclosed in the aforementioned U.S. Pat. Nos. 5,300,105 or 5,339,541.

FIG. 10 shows a still further embodiment for a pad 10" containing a microwave transparent outer envelope 18" sealed at ends 18a" and 18b" and which includes internal dividing walls 20" separating the interior of outer envelope 18' into three stacked and separately sealed volumes 21", 22" and 23". Layers 21", 23" may be filled with any suitable heat retention medium such as a phase change fluid 33" as previously described or a heat packet such as packets 30' of FIG. 9. A coil 122" is contained within chamber 22' and accessible through an external plug 28" which may be plugged into a wall outlet or to a source of DC power. Accordingly, the design permits the heat retention mediums within volumes 21", 23" to be heated either electrically or through microwave radiation.

With the foregoing specification, it has been shown how a thermal heating device has been attained in a preferred embodiment of a heating pad. The device may be heated through either microwave energy, alternating current through a standard wall outlet or direct current through an automobile cigarette lighter or the like. While the foregoing description has been made with reference to a preferred embodiment of a heating pad, it will be appreciated that a thermal heating device according to the present invention can be provided in a wide variety of uses and applications such as clothing (footwear, mittens or the like), beverage or food containers or warmers or other devices which are desired to be heated and retain heat for a substantial period of time. Accordingly, it is intended that the scope of the claims of the present invention not be limited by the preferred embodiment but shall include such modifications and equivalents as shall occur to one of ordinary skill in the art having the benefit of the teachings of the present invention.