Patent Application
20140331991
This invention is in the field of solar heaters, specifically paraboloidal reflective solar heaters and cookers.
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.”
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.
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.
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
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
When the cooker is assembled, it will appear as in
To allow the cooker to fully face the sun at various angles, the rigid framework can be modified to the configuration shown in
In the variable-inclination configuration of
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.
