A Parabolic Solar Cooker for Unattended Cooking
Li-Yan Zhu and Yun K. Kim
Parabolic solar cookers are very efficient. However conventional parabolic cookers need frequent adjustment to track the sun. They are also expensive. In this article, we describe a parabolic cooker for unattended cooking. It cooks at substantially constant power for two hours without adjustment. Optimized for robustness, it can be made of low-grade material, and very wide tolerance. Without moving parts, it is extremely reliable and very easy to use. Being efficient, it cooks without a greenhouse (e.g., plastic bag) year-round, even in temperate regions.
For clarity an ideal form is described in this article. Variations can be made to suit locally available material. In fact all six prototypes we made are different, each out of a different set of scrap material. Three of the prototypes have been used extensively for two years in San Jose, California (37.6° N), all with excellent results.
The first step in the design is to select a pot. Approximately 50 mm (2 inches) deep of dense food (e.g., soup or rice) can be cooked unattended. The pot diameter depends on the desired capacity. As a concrete example, a pot 200 mm in diameter is chosen for this article. It cooks 1.5 liters of food unattended.
The pot depth should be approximately 60% of its diameter. The depth is needed to catch focused sunlight. The pot will be less than half-full most times.
All other linear dimensions, such as pot thickness, focal length, reflector radius, frame height, and base width, are directly proportional to the pot diameter.
The pot should be dark in all exterior surfaces, and preferably dark on the inside as well. The dark surface may be hard-anodized, nonstick, or of any other nontoxic, heat-resistant, and wear-resistant coating. At least 3 mm of aluminum is preferred, to ensure uniformity in the food temperature. The lid should fit well. It should also be aluminum dark on both sides. However it may be thinner than the pot.
Handles of the pot and of the lid should not be too large, to minimize heat loss. They shouldn’t be shiny, for eye protection. For the same reason nothing within the reflector should be shiny, except for the parabolic surface. Detachable handles are preferred, since they can stay cool outside of the cooker. Fixed handles become very hot and should be handled through oven gloves.
The reflector is parabolic. Its focal length is 178 mm. It is optimized for angular tolerance, when the sun is at 50° elevation . This angle will be called the default elevation. Projection of the reflector in the direction of its principal optical axis is an ellipse (fig. 1). Major axis of the ellipse is 810 mm. Minor axis of the ellipse is 700 mm. The sunlight collecting area is 0.445 m². The gain of the cooker is 14.
Principal optical axis of the reflector intersects major axis of the ellipse, 15 mm off the center of the ellipse. The rim (i.e., outer edge) of the reflector resembles the edge of a bent potato chip. When viewed in the direction of the minor axes of the ellipse, the rim appears as a parabola approximately ¼ as deep as the reflecting surface (fig. 1). The rim is also slightly tilted with respect to the directrics of the parabola.
For mass production, the reflector should be molded out of Styrofoam or other types of plastic. For a prototype it can be made out of a sheet-material by cutting and folding . Sheet-metal is difficult to work with. Cardboards contract significantly when dried up, thus causing wrinkles on the aluminum foil. Fiberglass-filled epoxy sheets, commonly used in patios and gazebos, are preferred. They are inexpensive, durable, and easy to work with. Aluminum foil, commonly used in the kitchen, should be applied to the inner surface after the parabolic reflector is formed. The aluminum foil should have its shiny side facing the sun.
Fig. 1 A side-view and a “top-view” of the reflector, supported by a frame and a spacer on a flat base
The reflector is held by a rigid frame, which is attached to a flat base. The shortest diameter of the rim of the reflector is parallel to the narrow sides of the base. Principal optical axis of the reflector is inclined with respect to the base, at the default elevation. A cross-section through the longest diameter of the rim of the reflector and the principal optical axis is shown in fig. 2.
Fig. 2 Relative position of essential components, shown in a cross-section
The frame may be cut out of a jumbo-size tomato-cage. Most reflectors are stiff enough that one circular support by the frame, at approximately half of the rim diameter, is adequate (fig. 1). The base may be a piece of rectangular plywood (approximately 690×490mm). The closest point on the reflector is 10 mm above the base by design, to allow for manufacturing tolerance. The closest point is near the rim. After the reflector is fixed (through the frame) to the base, a spacer may be inserted in the gap to provide additional support (fig. 1).
Legs of the frame are not necessarily uniformly distributed around the circular contact, especially if the spacer is counted as a reliable additional support. If all legs are of the same thickness, longer legs are flimsier. They should be closer to each other. Shorter legs are aided by the spacer. They can be farther apart.
The cooker in this example is designed for 50° default elevation. It is adjustable for 25° in either direction. Then the range of achievable elevation is between 25° and 75°. This is adequate at temperate area, where the sun never reaches 90° elevation.
To achieve the desired ±25° range, the wedge should stand on its shorter side. It should also be pushed toward the center of the base. For stability the wedge is only pushed in for about 30% of the base length. In this case the wedge length needs to be at least 211 mm. For convenience let the wedge be 220 mm long.
To achieve ±1° resolution, the tip of the wedge should be no more than 12 mm thick. For convenience let the tip be 10 mm thick.
The wedge height should be approximately 70% of the wedge length. Let it be 150 mm tall (fig. 3). The wedge width can also be 150 mm. The wedge can be hollow. It can be made of any locally available material.
Fig. 3 Elevation adjustment using a wedge
The potholder consists of two semicircular rails 182 mm in diameter (fig. 4). The plane of each semicircle is perpendicular to the base. The planes are parallel to each other, and 141 mm apart. Four end points (two from each rail) define a 182×141 mm rectangle. Longer (shorter) sides of the rectangle are parallel to length (width) of the base. The center of this rectangle coincides with the focal point of the reflector. Regardless of the base inclination, the pot can be placed effortlessly in the cooker—level and in-focus.
To avoid scratching the pot, the rails should be stiff. The rails can also be wrapped with white or light-colored cotton cloth. The rails are held together by a rigid structure, which penetrates the reflector, and is secured to the base (fig. 5). Alternatively the rails can be secured to the frame of the reflector. However the configuration shown in fig. 5 is preferred, because it casts very little shade on the pot. In either configuration, the potholder should be made of metal. It should not deflect noticeably under the weight of food and cookware, at the maximum tilt (25°).
Fig. 4 Front, side, and top-View of a pot resting on a pair of semicircular rails
Fig. 5 A self-centering potholder mounted on the base
The cooker should withstand locally “reasonable” wind, at maximum tilt. Obviously a larger base is more stable. However for economy, and for the ease of shipping and storage, the base shouldn’t be too large. Stability can be enhanced by adding weights (e.g., bricks, sand, soil, or rocks) to the base. Optimal size and location of the base (with respect to the potholder) are best determined through experimentation. Forces due to the wind may be simulated by pushing the reflector with your hand.
The base need not be a solid rectangle. Holes are allowed where solid surface is not required to accommodate the wedge, or the alignment guides. The base should incorporate a hook, so that the cooker can be hung under the ceiling when not in use. The base may also include mechanisms for elevation adjustment, if they are reliable, easy to use, and not significantly more expensive than the wedge.
A pinhole is drilled on the reflector, 100 mm above the base. Warning: Do NOT look directly into the sun! When the principal optical axis points directly toward the sun, a bright spot appears on the base. Name and mark this spot M. Alignment can be made in the future without looking at the sun. Simply adjust azimuth and elevation of the base to bring the bright spot onto this mark. The maximum cooking power is achieved instantly.
This mark M can be generated without ever looking at the sun. First level the base. Then run a plumb line through the pinhole. The line intersects the base at a point P. The mark M is located 84 mm from P, away from the side of the base closest to the reflector.
For unattended cooking, the cooker should be aligned approximately one hour ahead of the sun. Movement of the spot is easily predicted using a chart attached to the base . The proper spot location is identified by the present time and desired advance (fig. 6).
Fig. 6 An alignment chart for 40°N latitude at winter solstice (Dec. 23). Hours shown are present local (astronomical) time. Minutes shown are desired advance cooking time. The spot moves along selected trace. Intersection of the traces is the mark M, which corresponds to perfect alignment.
Each chart is good only for a specific season and latitude. Compare two extremes in the season (winter and summer solstices) at the same latitude (40°N, see figs. 6 and 7). Also compare charts for the same season (summer solstice) at two latitudes (40°N and 10°N, see figs. 7 and 8). Thus the chart should be easily replaced on the base.
Fig. 7 An alignment chart for 40°N latitude at summer solstice (June 21).
Fig. 8 An alignment chart for 10°N latitude at summer solstice.
Place the cooker on a sunny, flat ground, away from strong wind. Identify desired spot location on the alignment chart. Turn the base on the ground until the spot attains the correct abscissa. Then adjust the wedge position until the spot attains the correct ordinate.
Place a pot of food on rails of the potholder. There is no need for any type of greenhouse, not even a plastic bag. Cooking begins, at almost full power. There is no need to observe or adjust the cooker. Most types of food require no stirring. Rice and soup will boil. However they will not overflow, because heat is received uniformly in all directions. Thus there will be no mess to clean up after cooking. The cooker is designed so that food will not burn, if the pot is filled to designed capacity. However food can be browned, or burnt, by reducing the load. We made well-done ribs this way.
The spot travels along a selected trace toward the intersection M, while the cooking power increases slightly. Cooking power reaches its maximum when the spot reaches the intersection M. Then the spot moves away from the intersection. Cooking power drops slightly. Since the cooker is designed for two-hour unattended cooking, its power drops drastically outside of this two-hour duration. It keeps the food warm, but not dried out.
If you want to cook for less than two hours, simply reduce the advance. For example if you begin cooking by aligning the spot on the intersection M, full-power cooking will last only one-hour. If you wish to cook for two hours at reduced power, you may either cover up part of the reflector, or intentionally let the spot miss the correct trace.
A parabolic solar cooker suitable for unattended cooking is described. It features a short focal length, an oversized dark pot, a self-centering potholder, an alignment guide, and a low-profile frame. The cooker contains no moving parts. It is insensitive to misalignment, thus can be manufactured with wide tolerance. With ample power, it requires no greenhouse in sunny days. (A plastic bag can be used as greenhouse in windy or cloudy days.)