Parabolic reflective solar heater

Abstract

This product is a solar cooker that combines portability with the precision of a mathematically-designed parabolic reflector. It comprises a rigid framework and a semi-rigid reflector surface. The framework has a cross-shaped footprint. The upper surface of the framework is a parabolic dish roughly two feet in diameter. The contour of the dish may be a simple paraboloid, compound paraboloid, or semi-compound paraboloid. The framework is lightweight and may be hollow and/or deconstructible for ease of portability. An alternative embodiment allows the dish to incline at various angles. The reflector surface is formed from sheets that fit atop the framework and assume the shape of the parabolic dish. Securing means on the upper surface of the framework hold the reflector sheets in place in a precisely parabolic configuration.

Description

1. FIELD OF THE INVENTION

This invention is in the field of solar heaters, specifically paraboloidal reflective solar heaters and cookers.

2. BACKGROUND AND OBJECTS OF THE INVENTION

In the context of this invention, the term “solar” refers to the direct use of sunlight to heat objects. It does not suggest the use of photovoltaic panels to generate electricity.

Solar cooking can be accomplished either by trapping sun heat in an enclosure, much like an oven, or by focusing it in one spot, much like a flame grill. In practice, it is often done by a combination of these two methods. The present invention focuses sunlight by means of a reflective surface.

The geometry of a reflective solar cooker is crucial for its effectiveness. A cooker in the shape of a parabolic dish has the property of focusing all incoming sunlight to one focal point. It is this concentration of energy that allows for temperatures much higher than the ambient environment. When a vessel of food is placed at the focal point, it can be cooked properly.

Although several solar cookers on the market achieve a nearly perfect parabolic shape, they do so only by a relatively complicated scaffold made from metal tubes or wood. This makes them heavy, cumbersome, and difficult to assemble. On the other hand, more lightweight and portable cookers only achieve approximations of a parabolic shape, leading to a highly decreased efficiency due to the inevitable scatter of sunlight.

The present invention overcomes the disadvantages of both kinds of cookers, achieving greater efficiency, portability and ease of assembly. The device can be carried in a hiking backpack. Using only a small number of simple parts, it can be assembled in minutes, and it is very sturdy.

To generalize the scope of this invention, it is not restricted to the purpose of cooking. Solar heaters can be used for a variety of purposes, such as boiling water, melting wax, or starting a fire. I envision cooking as the best mode of this invention, so I will usually refer to it as a “cooker.”

3. DESCRIPTION OF RELATED TECHNOLOGY

It is well known that a mirror in the shape of a paraboloid (parabolic dish) will reflect incoming sunlight to a focus. The concentrated solar energy can be used to heat materials or to cook food at that focus. To be effective, a solar cooker must have a parabolic dish about two feet in diameter or larger. The typical parabolic dish is made and sold in one piece. Such a product is bulky and can be heavy. It is difficult to take a parabolic dish on a camping or hiking trip.

The reflective surface of a parabolic dish can be a lightweight sheet of flexible material such as aluminum foil, vinyl, or Mylar. The “Copenhagen Cooker,” invented by Sharon Bush-Clausson, makes use of four square vinyl panels and a central cardboard base plate. Each panel is tied to one edge of the base plate with bootlaces. The panels are then curved upward and clipped together with clothespins. The result is a bowl-shaped reflective surface. The Copenhagen Cooker is extremely lightweight and portable. Its limitation is that its shape is only a rough approximation of a paraboloid. It is not nearly as effective as a solidly constructed reflector.

One challenge associated with solar cooking is the fairly limited range of directionality. In a typical reflector, the image of the sun will cross the entire cooking vessel in an hour, whereas effective cooking could take several hours. The reflector must be continually monitored and adjusted. Variances in season and latitude are secondary concerns. The sun's height in the sky is very different in the summer than in the winter. The sun is also higher in the sky at lower latitudes. A reflector that is designed for use in the tropics may be inefficient in temperate climates.

The directionality problems may be mitigated with a compound paraboloid design. A compound paraboloid is formed by two parabolas with the same focus and co-planar axes of symmetry. Each parabola is revolved around its axis of symmetry. The intersection of their solids of revolution is the compound paraboloid. Stated heuristically, the compound paraboloid is formed by “half of one paraboloid and half of another,” with a cusp point at the boundary. As the sun moves across the sky, its energy will be directly incident first on one half of the reflector and then the other. The energy is always directed to the same focus. This allows the reflector to track the sun for a greater duration of time with no need for manual adjustments.

The compound paraboloid shape of solar reflector was first introduced in U.S. Pat. No. 4,002,499, invented by Winston and assigned to the United States Energy Research and Development Administration (the ERDA Patent). The ERDA Patent was intended primarily for the industrial-scale collection of solar energy for purposes of providing power. It includes an “energy receiver” for absorbing the sunlight. My invention does not include an energy receiver. It directs energy to a container of food or other material provided by the individual using the device.

U.S. Pat. No. 4,741,610 (Dudley) discloses a compound parabolic reflector in which each surface is part of a parabolic cylinder, ie parabolic along one axis. The device utilizes two reflective surfaces that are not contiguous. The surfaces face each other, supported in a very particular arrangement by a rigid framework. One surface reflects energy onto the other surface, which then focuses the radiation to a point. The solar cooker constructed according to these principles is shown in FIG. 3 of the Dudley patent.

The disadvantage to the ERPA and Dudley patents is that they are large, heavy, bulky products. They are not portable, thus making them unavailable to hikers and campers.

What is needed is a lightweight, portable solar cooker with a precise paraboloid or compound-paraboloid configuration.

4. SUMMARY OF THE INVENTION

My cooker consists of two main elements: a rigid framework and a set of flexible reflector sheets.

The rigid framework has a cross-shaped base. The rigid framework may be one solid piece. Alternatively, it may be formed by fitting together 2-4 spines. The advantage to the multiple-spine construction is portability.

The upper surface of the rigid framework has the shape of a parabolic dish. Its design is computer-assisted, in order to achieve a very precise parabolic profile. The rigid framework may be mathematically designed to form a simple paraboloid or a compound paraboloid, as described in the previous section.

When the cooker is set up for use, the flexible sheets are set atop this dish so that they assume the contour of a parabolic mirror. The upper surface of the rigid framework has supporting structures to hold the reflector sheets in place. A cooking vessel support may then be situated on the structure.

Symmetry of the design allows the reflector sheets to variously form a perfect paraboloid, a compound paraboloid, or even a semi-compound paraboloid (that is, forming a parabola on only one of the two axes, in order to either increase the angle of incoming sunlight, or the time of exposure). Given the modular design of the cooker, a semi-compound parabola can easily replace the standard parabola within less than a minute, should the weather or the type of cooking require it.

An alternative embodiment of the design allows the cooker to be tipped, so that it can face the sun at a variety of angles of inclination.

5. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the rigid framework.

FIG. 2 is a perspective view of the rigid framework in the embodiment when the rigid framework consists of two spines. The upper spine is shown sliding downward onto the lower spine.

FIG. 3 is a close-up view of the central upper surface of the rigid framework.

FIG. 4 shows the reflector sheets positioned on the rigid framework to form a parabolic dish. The cooking vessel support is also shown in this figure.

FIG. 5 is close-up view of the reflector sheets fitted into the upper surface of the rigid framework.

FIG. 6 demonstrates the geometry of a compound parabola.

FIG. 7 is a perspective view of an embodiment of the solar cooker with variable inclination.

FIG. 8 is a side-plan view of an embodiment of the solar cooker with variable inclination.

FIG. 9 shows the cooking vessel support.

6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Terms in quotation marks are terms of art in this patent application. They are to be constructed in the claims as defined or used in the specification.

My cooker consists of two main elements: a rigid “framework” (11) and a set of flexible “reflector sheets” (14).

The framework can be made from aluminum, plastic, reconstituted wood/MDF or even cell foam. It is lightweight, with portability in mind. The framework can be made hollow and designed to hold sand or water to provide stability while the cooker is in use.

The framework has a cross-shaped base. It may be constructed of one solid piece. In the best embodiment, it is formed by fitting together two or more “spines.” For example, in FIG. 2, an “upper spine” (17) is shown sliding downward to interlock with a “lower spine” (18). The advantage of the multiple-spine construction is portability. When separated, the two spines are less bulky and easier to carry than a unitary dish.

The upper surface of the rigid framework has the shape of a parabolic dish. Its design is computer-assisted, in order to achieve a very precise parabolic profile. The rigid framework may be mathematically designed to form a simple paraboloid or a compound paraboloid, as described previously.

The reflector sheets are backed with a semi-rigid material such as heavy paper/light cardboard or vinyl. They are coated on the front side with a highly reflective surface such as aluminum or Mylar.

When the cooker is set up for use, the flexible reflector sheets (14) are set atop the parabolic dish so that the reflector sheets assume the contour of a parabolic mirror (see FIG. 4). Greater precision is further achieved by using “securing means” to secure the reflector sheets to the paraboloidal upper surface of the framework. Suitable examples of securing means include a “ridge” (12) down the middle of each spine, or a precise line of protruding “studs” (13) that ensure the correct lateral tension of the reflector sheets. FIG. 3 shows how each ridge (12) has a slightly “elevated lip” above the upper surface of the rigid framework. In FIG. 5, the reflector sheets (14) are fitted between the upper surface of the framework and the elevated lips of the ridges. This tight fit keeps the reflector sheets firmly in place. FIG. 5 also shows studs (13) fitted through small holes in the reflector sheets (14), again to anchor the reflector sheets in place. The use of studs in various locations maintains a tension in the reflector sheets. This helps the reflector sheets maintain their proper parabolic shape.

When the cooker is assembled, it will appear as in FIG. 4. A “cooking vessel support” (15) is attached to the surface of the rigid framework. The cooking vessel is then set atop the cooking vessel support. Although the reflector sheets may not cover all area of the surface beneath the cooking vessel, that central region is in the cooking vessel's shadow, so it does not receive any incoming sunlight anyway.

To allow the cooker to fully face the sun at various angles, the rigid framework can be modified to the configuration shown in FIGS. 7 and 8. In this configuration, the spines are rounded at the bottom. One of the spines has a full-length base. The other spine is V-shaped. The framework (11) can then be tipped to almost any inclination. It is supported by one or more wedges (16). This configuration allows easy adjustment to match the altitude of the sun in different locations and at different times of the day and the year.

In the variable-inclination configuration of FIGS. 7 and 8, the cooker can also be rotated more easily because it has less surface area in contact with the ground. Rotary adjustments can be further facilitated by installing wheels on the cooker.

FIG. 6 demonstrates the geometry of a compound parabola. On the left, two-dimensional parabola (a) is shown intersecting with parabola (b). The two parabolas have different axes of symmetry. Each parabola is most effective at focusing incoming sunlight from near the direction of its axis of symmetry. The two parabolas have a common focal point (f). Angle (t) is the planar region between their axes of symmetry. The figure on the right shows the common interior of the two parabolas, which is bound by curve (c). When each parabola is rotated about its axis of symmetry, the result is a three-dimensional compound paraboloid. When the reflector surface assumes the shape of a compound paraboloid, it is effective at focusing sunlight incoming from anywhere within angle (t). As the sun's position in the sky progresses through angle (t), it remains concentrated near focus (f). This allows the cooker to be effective 2-3 times as long as a simple paraboloidal reflector without being manually adjusted.

The symmetry and modularity of the cooker design allow the variability to form a perfect paraboloid, a compound paraboloid, or even a semi-compound paraboloid (that is, forming a parabola on only one of the two axes, in order to either increase the angle of incoming sunlight, or the time of exposure). A semi-compound parabola can easily replace the standard parabola within less than a minute, should the weather or the type of cooking require it.

Claims

1. A portable reflective solar heater, comprising:

a lightweight rigid framework with a cross-shaped base and a paraboloidal upper surface with diameter between 12 and 36 inches;

reflector sheets shaped and sized to fit onto the dish-shaped upper surface of the framework, said reflector sheets having a highly reflective front surface and a semi-rigid backing material;

securing means to secure the reflector sheets tightly against the upper surface of the framework;

2. The solar heater as disclosed in claim 1, wherein the paraboloidal upper surface of the framework has a shape selected from the set of simple paraboloid, compound paraboloid, and semi-compound paraboloid.

3. The solar heater as disclosed in claim 1, wherein said securing means include at least one of the following structures:

ridges running radially along the upper surface of the framework, so that the edges of the reflector sheets can be secured beneath the elevated lips of the ridges;

studs protruding from the upper surface of the framework, which can fit through small holes in the reflector sheets and maintain tension in the sheets.

4. The solar heater as disclosed in claim 1, wherein the framework is hollow and can be filled with sand or water to keep the cooker steady when in use.

5. The solar heater as disclosed in claim 1, wherein the framework comprises two spines that fit together at right angles and can be easily detached for ease of portability.

6. The solar heater as disclosed in claim 5, wherein

one of the spines has a full-length horizontal base to make full contact with the ground;

the other spine is V-shaped so that the cooker may be tipped to face the sun at any inclination;

each spine is slightly rounded at bottom to allow for easy tipping of the framework;

one or more wedges is included with the cooker, to support the V-shaped spine so that it can maintain a constant inclination.

7. The solar heater as disclosed in claim 1, further comprising a cooking vessel support mounted on the framework and positioned at the focal point of the paraboloid.

8. The solar heater as disclosed in claim 7, wherein the paraboloidal upper surface of the framework has a shape selected from the set of simple paraboloid, compound paraboloid, and semi-compound paraboloid.

9. The solar heater as disclosed in claim 7, wherein said securing means include at least one of the following structures:

ridges running radially along the upper surface of the framework, so that the edges of the reflector sheets can be secured beneath the elevated lips of the ridges;

studs protruding from the upper surface of the framework, which can fit through small holes in the reflector sheets and maintain tension in the sheets.

10. The solar heater as disclosed in claim 7, wherein the framework is hollow and can be filled with sand or water to keep the cooker steady when in use.

11. The solar heater as disclosed in claim 7, wherein the framework comprises two spines that fit together at right angles and can be easily detached for ease of portability.

12. The solar heater as disclosed in claim 11, wherein

one of the spines has a full-length horizontal base to make full contact with the ground;

the other spine is V-shaped so that the cooker may be tipped to face the sun at any inclination;

each spine is slightly rounded at bottom to allow for easy tipping of the framework;

one or more wedges is included with the cooker, to support the V-shaped spine so that it can maintain a constant inclination.