Concentrating Cooker - Current Design Approaches
Overview
Given that this year's design concept does not perform to specifications, creating a second design concept is an open question toward which any number of approaches may be taken. There is a large body of existing work concerning various methods of building concentrating solar cookers. The solar cooking wikia is one example of a resource that can be used (http://solarcooking.wikia.com). The Fall 2009 Final Report also discusses several examples of existing designs and the flaws and merits of various approaches. Although there are a large number of possible designs, our team believes that these characteristics should be retained if possible in the next design:
- An asymmetrical configuration that places the cook on the opposite side of the cooker from the sun, with the sunlight reflecting upwards toward the focus from below. This avoids shadowing the reflector array and provides a setup that is more similar to "conventional" cooking (See Figure 4).
- A countertop imposed between the cook and the reflector array. This provides extra safety in additional to convenience. (See Figure 4).
- Simple construction that is easy and inexpensive to build from basic materials and tools.
- Sufficient power to cook tortillas at a 450 F surface temperature (about 1000 W for a small cooking surface such as a 12" cooking pan)
- The cooking surface should be open to the air so that foods such as tortillas can be easily handled
A number of current ideas are suggested in the subsections below.
Figure 4 - Desired format for concentrating cooker
Alternative Secondary-Reflector Design
This year's research has determined that two parabolic troughs cannot be made to interact in manner that causes twice-reflected light to converge on a single point in 3D space. However, it may be possible that they could be arranged in a way that causes the non-convergent light to fall within a sufficiently small area to generate useful concentrated heating. For a solar cooker, it is only necessary for light to land somewhere on the surface of the pan, rather than a mathematically unique point in space. Light that is theoretically divergent may be practically convergent if the scatter falls within the desired "target." Using the same approach used to model the behavior of the Solar Fire prototype, one could experiment with different configurations of reflectors. The incentive for finding such a configuration is that it would spare the builder the task of building surfaces that are curved in three dimensions, such as a paraboloid. These complex shapes add to the expense and difficulty of creating concentrating solar cookers. The original appeal of the Solar Firedesign concept was that it used only simple 2D geometries and would thus be far simpler and less expensive to build.
Conducting Concentrating Cooker (El Barra del Sol)
To avoid the complexity of constructing a 3D parabola or using secondary reflectors, another alternative is to use an "off-center" 2D parabola to concentrate light on a black metal rod or rectangular plate which will conduct heat to the central heating pot. Nick Chisholm and Scott Johnson have begun exploring this possibility.
We are using a method for constructing the 2D parabola that is different from the one used in the Solar Fire prototype. Instead of forming the shape from sheet metal, we will bend a sheet of fiberboard into the proper shape. It will be coated with mylar as has been done previously. We believe that this fiberboard surface would be smoother than a metal surface because metal folds and wrinkles. Curved plywood is used similarly to make ramps for skateboarding.
The light will be reflected to a black bar which is in thermal contact with the cooking surface, for example a black pan. The top of the bar will be insulated with mineral wool to prevent contact with the wooden frame and minimize heat loss.
With approximations, defining the temperature profile of the system is a reasonably simple heat transfer problem. There will be a term for radiative heat transfer to the bar, radiative transfer from the bar, convective transfer from the bar to the air, and conduction through the bar to the cooking surface. Work is currently ongoing to derive a steady-state solution to the problem.
So far this semester, we have constructed the single reflector and replaced the old reflector in the prototype. Before the summer we plan to connect a bar or rod to the prototype and use thermocouples to test the performance of the conduction concentrating cooker without a pan in place. If all goes well, we plan to continue our research of this cooking method next semester.
Piece-Wise Reflectors
If the above design approaches do not prove successful, it may be necessary to resort to using a piece-wise reflector. This approach can be implemented in many different ways.
One design mounts flat reflective rectangles that are all independently angle to direct incoming light at the target. These pieces could be connected to a flat frame that swings on an axis, similar to the reflector assembly used in the Solar Fire prototype. They could either be fixed in position or connected by ball in socket joints. The ball and socket joints would allow for easy adjustments, but may be hard to implement.
Another design mounts flat pieces to a 3-D parabolic frame in a manner similar to the Devos described in the Fall 2009 report. Wood is the traditional construction material we have used in the past. It may be a good idea to investigate other building materials (metal wires, cloth, etc.) that may reduce the cost and/or weight of the cooker.
Yet another idea involves cutting half parabolas for the frame of the reflector. These parabolas are arranged into a star shape or clamshell. Wedges of fiberboard or a similar material could be bent to fit the shape of each section of the frame in order to create a paraboloid shape.
These are just a few of many possible ideas based on the concept of a piece-wise reflector. Given that piece-wise reflectors are the most common strategy used to implement concentrated solar cooking, it is worthwhile to do a careful analysis of existing designs before attempting to build an original design.
Laser Diagnostics and Parallel Lighting Setup
Lasers can be a useful tool for evaluating the reflector geometry of a built prototype and assessing how effectively it is directing light onto the target. They can provide information at a more detailed resolution than possible by simply taking a prototype into the sunlight and taking temperature readings. Temperature readings can be influenced by many factors not related to the geometry of the reflectors. If a prototype does not perform as expected, lasers can help determine whether this is due to a flaw in the shape of the reflector and what areas of the reflector are specifically at fault.
In order for lasers to provide accurate information, they must be able to approximate a set of parallel light rays. Alice Yu has constructed one system in which 20 standard key-chain laser pointers are mounted in a set of wooden boards that have been drilled with holes that fit the diameter of the pointers precisely. These holes were created using a drill press and should be almost perfectly perpendicular to the surface of the boards. These boards can be mounted on the same metal scaffold used to hold the lamps for box-oven testing and clamped in place to correct any warp that may exist in the boards. The result is an array of laser pointers whose beams are very close to parallel. These beams can be aimed at a prototype's reflectors and their trajectories observed.
Another approach would be to use a single powerful laser pointer that shines upon the prototype from very far away. The beam can be directed at different areas of the reflector in turn to observe its characteristics. Because of the large distance, the angular difference would be very small such that the incident angle of the laser beam with respect to the reflector would be virtually the same no matter what part of the reflector you are aiming for.
The team has also discussed the possibility of simulating parallel sunlight using a halogen lighting setup similar to the one used to conduct box oven testing. This would involve fashioning reflective inverted parabolic troughs that would sit inside the lamp casing above the rod-shaped light element and create parallel light in a manner similar to how the reflectors inside a car headlight turn a point source of light into a parallel beam. However, it is not yet certain whether the proposed system would generate truly parallel light or whether it would remain dispersive in one or more dimensions. We may reinvestigate the possibility of such a system if the need arises for repeated performance testing of a proven design under controlled conditions.
Future Work
Throughout the last week of finals and next semester we plan to continue researching the conducting rod design. We will test the basic design with temperature sensors. We also plan to build models and compare our tests with what the models predict. If the results of the test look promising, we plan to design a new cooker optimized for the use of a conducting rod to transport the heat to the pan.
We also plan to brainstorm alternative means of configuring a system of primary and secondary reflectors such that a larger portion of the incident radiation is captured and directed onto the target. All ideas would be modeled in MATLAB to determine whether they should be considered as the basis for a new prototype.
It is recommended that future students attempting to construct a solar concentrating cooker conduct a thorough design phase before beginning construction. The strengths and weaknesses of existing designs should be evaluated during the brainstorming process. The ideal cooker is powerful, simple to make, and safe to use. MATLAB modeling can help predict the viability of a design before it is constructed, and laser diagnostics and field testing can help characterize its performance after it is constructed. If an initial prototype satisfies basic criteria, it would be worthwhile to find ways to reduce materials to make the design less bulky while simultaneously reducing its cost.