This isn’t quite it…..
Our Goal:
We aim to provide an appropriately priced, yet efficient and sustainable system of sunlight concentration for developing countries to use. Our intent is for people in developing countries to be able to easily construct or purchase appropriate solar concentrators. We are working with a very low budget and aim to use our concentrators as renewable energy cooking facilities. To see what is currently done please visit the “Links to Relevant Content” section below.
San Pablo’s Needs: San Pablo is currently receiving free electricity but this will not last forever. In fact, it will be running out relatively soon. When this occurs, the people of San Pablo will need to find new, appropriate, creative ways to provide energy for all their needs. One such technology that should help in this regard is solar concentrators. Solar concentrators can cook food, heat houses, water, etc. They also provide many other miscellaneous opportunities if used with innovation.
Where we started:
We have picked up an ongoing project and are focusing on improving the dish feature. The current design we have is expensive and very complicated to make. It is bent, welded, and riveted pieces of aluminum sheeting and tubing. Aluminum is expensive and requires a high level of skill to work with (thus inappropriate). The sheeting alone cost $100 per square meter. Counting the cost of materials, skill level, and labor, the current dish we have is worth approximately $1000.
A discussion of possible materials for future dish construction was the conducted. To decide the best materials to be used, decision matrixes were compiled for the male mold, dish, and reflective surface.
Male Mold
| Weight | Sand | Foam | |
| Ease of Construction | 5 | 4 | 5 |
| Cost | 8 | 6 | 5 |
| Accessibility | 5 | 5 | 4 |
| Skill | 8 | 6 | 8 |
| Sustainable Materials | 6 | 5 | 4 |
| Longevity | 6 | 5 | 5 |
| Repairability | 6 | 4 | 4 |
| Total | 49 | 35 | 35 |
For an exact parabolic shape, we sought to use a 3-dimensional foam cutter located on the campus of Cal Poly San Luis Obispo. The automated shaping CNC is fed an equation (y=x^2) and then cuts the exact shape. This mold can then be fiber-glassed and smoothed; it can then be shipped to the location of use (San Pablo). There, the mold can then be used to produce more dishes. If the mold is broken, a new mold can be made using a dish as a female mold, thus garaunteeing the projects continuance.
Dish:
| Weight | Fiberglass | Aluminum | Ceramic | |
| Cost | 10 | 8 | 2 | 9 |
| Longevity | 10 | 7 | 10 | 8 |
| Repairability | 8 | 8 | 6 | 6 |
| Sustainable Materials | 6 | 4 | 2 | 5 |
| Tools Needed | 5 | 5 | 2 | 4 |
| Ease of Construction | 5 | 5 | 1 | 4 |
| Weight | 6 | 6 | 4 | 2 |
| Required Skill | 5 | 4 | 2 | 4 |
| Total | 55 | 47 | 29 | 42 |
Fiberglass was the decided material for future dish construction. Weight is a factor only because it influences the stand. The heavier the dish, then the bulkier, and thus more expensive, the stand must be. Fiberglass has a major advantage in this category. Also of note, is minimal skill required to use fiberglass. Aluminum’s primary failure is in the cost (which is significantly more expensive than fiberglass) and technical capabilities necessary for aluminum working. This skill is not as likely to be found in third world countries as fiberglass which is easy to use. Ceramic’s failure is not its cost, but rather its weight, which we mentioned earlier would require a much bulkier base to support.
Prototype 1
This dish was built to practice laminating techniques with the fiber glass. This design did not incorporate the pex ring and the male mold was simply a cooking wok. Reflective tape was then applied to the dried and hardened dish. Crinkling was minimal, however the tape did not reflect light efficiently due to absorption. Either a more reflective tape is found or the dish will not be efficient.
To watch Prototype 1 being constructed please visit the following links:
- http://www.youtube.com/watch?v=i7T-jYtdXDg
- http://www.youtube.com/watch?v=ShOC5dmE6_Q
- http://www.youtube.com/watch?v=KQteQNSJYRY
Prototype 2
Prototype 2 aimed for a one step production approach. Mylar was placed over the male mold (wok), then a dish was fiber-glassed on top. The end product was riddled with crinkles, however had many optically flat, and thus efficient, areas. The mylar was much more efficient in reflecting, though more dispersed due to the imperfections from application.
Lamination process is the same, however we incorporated the pex ring. We used two different techniques to try and maximize our outcomes. We cut the fiber glass into small rectangles and fiber-glassed around half of the edge and also tried leaving the overhanging, excess cloth alone and simply wetting and wrapping it.
This video demonstrates our attempt to thermoform the plastic to be optically smooth and eliminate wrinkling.
4. http://www.youtube.com/watch?v=DYG59y7_bSk
This next video show the result of that…temporarily failure…
5. http://www.youtube.com/watch?v=OLgXClxA328
Prototype 3
Prototype 3 was very similar to prototype 2. The main difference is that instead of simply placing the mylar over the wok before applying the resign, we spread honey over the wok before putting the mylar on it. We did this hoping that the mylar would stick better and we would have a flatter reflective surface with less crinkles and solar interference. Also, we cut slits in the mylar sheet and overlapped to reduce the crinkling. Overlaps were then taped. After applying the honey, we put the mylar over the wok just like the previous prototype and resined over it onto the cloth that went on top of the mylar. It has not dried yet; results to come.
Molds separated prior to honey removal and optical check
We are waiting to have the male mold done to make another dish so we can begin to test the optical properties of the reflective surfaces that we are integrating into our system. This should be done before the next lab.
Where we are going: The following section has been abandoned for lack or “appropriateness”.
We are going to construct a male mold this week. We planned on doing this via a hot wire CNC machine in the aerospace laboratory in the ATL on Cal Poly’s campus. This machine would create a male mold with an exact parabolic shape for our dish.
This will then enable us to quantify how efficient our reflective surfaces are that we will also be playing with this week. More specifically we will be taking sheets of mylar and trying to get it to stick to the male mold better than we did in prototype 2 and make another dish (Prototype 3). We are then going to make Prototype 4 using the male mold and adhesive strips for the reflective material. We have included a video demonstrating how to apply the adhesive reflective strips under the links to relevant content section below.
After in depth conversations, we realized that the CNC is impractical and too difficult to make our shape. Instead, we plan to rotate a parabolic shape over a mound of sand, shaving excess away to create the ideal parabolic shape. The parabolic shape is made from a plywood cutout mounted to electrical tubing in order to rotate. Once the sand mound has been shaped, it will be resined, thus making the long sought after male mold. Although this method will result in a less exact parabolic shape, it is a more practical method for the people of San Pablo, and thus the best option.
| Weight | Fiberglass | Aluminum | Ceramic | |
| Cost | 10 | 8 | 2 | 9 |
| Longevity | 10 | 7 | 10 | 8 |
| Repairability | 8 | 8 | 6 | 6 |
| Sustainable Materials | 6 | 4 | 2 | 5 |
| Tools Needed | 5 | 5 | 2 | 4 |
| Ease of Construction | 5 | 5 | 1 | 4 |
| Weight | 6 | 6 | 4 | 2 |
| Required Skill | 5 | 4 | 2 | 4 |
| Total | 55 | 47 | 29 | 42 |
|
|
Value | CNC | Sand | |
| Cost | 5 | 3 | 4 | |
| Ease in Construction | 10 | 2 | 8 | |
| Tools Needed | 10 | 4 | 7 | |
| Required Skill | 5 | 2 | 4 | |
| Practicality | 8 | 1 | 7 | |
| total | 38 | 12 | 30 |
Recent Male Mold Developments
This last week (1st-7th of March) we spent our time researching more reflective materials and concerned with making our male mold. We abandoned the use of the CNC hot wire machine because it isn’t an appropriate technology. Instead we constructed a basic device that could be used to shape a pile of sand or dirt into a parabolic shape and put fiberglass and resin on the mound to achieve the proper shape.
Completion/Breakdown of Parabolic Device
We had to first get the tool in the middle of the pile of sand.
We then took a 2”x4” and used a shovel to help the “appropriate” tool work.
As we perfected the shape the effectiveness of our tool increased dramatically!
With a little luck…or scientific/artistic abilities we achieved a parabolic mound!
Reflective Surface Conclusion
Reflective Surface Materials:
| Weight | Mylar | Al Sheets | Reflective Tape | |
| Cost | 10 | 10 | 2 | 7 |
| Longevity | 10 | 8 | 9 | 7 |
| Repairability | 8 | 8 | 5 | 6 |
| Sustainable Materials | 6 | 4 | 2 | 3 |
| Tools Needed | 5 | 5 | 2 | 5 |
| Ease of Contruction | 8 | 2 | 2 | 8 |
| Weight | 6 | 6 | 2 | 6 |
| Required Skill | 8 | 2 | 2 | 8 |
| Total | 61 | 45 | 26 | 50 |
We have concluded through building three prototypes, research, outcomes of decision matrix, and for safety and timeliness of construction that a reflective tape is the best way to have a reflective surface.
With recommendations for where to purchase this tape please visit the following link!
Conclusions
Over the course of this project, our group learned a lot, about the needs of developing countries, the difficulties in helping them provide for themselves, as well as technical properties and designs. Each successive prototype continued to improve. In the end, we believe we developed a mold capable of creating reproduceable appropriate solar concentrator dishes with the mold and learned a lot about how to do it. We also determined the best ways to apply a reflective surface to a dish. Mylar is flimsy and difficult to work with, but is also cheap and applicable. Reflective tape, if equally reflective is easier to apply to the surface if applied in horizontal strips and not too much more expensive. We showed that both options can work with relative effectiveness, but reflective tape creates less crinkles and thus, a more efficient surface.
Links to Relevant Content
Scheffler Solar Concentrator: http://www.solare-bruecke.org/infoartikel/Papers_%20from_SCI_Conference_2006/21_wolfgang_scheffler.pdf
Past Projects at CalPoly: http://apptechcalpoly2010des.wetpaint.com/page/Solar+Cooking
Ideas for light weight grout: http://www.greenhomebuilding.com/pumicecrete.htm#LWC
How to apply adhesive reflective strips to a dish: http://surviving-21st-december-2012.com/parabolic-mirror-paraboloid-homemade-solar-concentrator-p2-diy-do-it-yourself/
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