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Papasan Chair Solar Cooker

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Engr305 Appropriate Technology page in progress
This page is a project in progress by students in Engr305 Appropriate Technology. Please do not make edits unless you are a member of the team working on this page, but feel free to make comments on the discussion page. Check back for the finished version on May 15, 2008.

Ready to cook!


Concept

The word "Synergy" comes from the Greek words for “to work together”. This project combines the existing parabolic shape and comfortable sitting cushion of the Papasan chair with the natural-source energy utilization of the solar cooker. My hope was that, in the spirit of synergy, the whole would be greater than the sum of the parts and that people may find the idea of owning a solar cooker more practical if they can also sit and read a book in it when its not in use. Compared to the traditional mono-functional Parabolic Solar Cooker, this project serves as a chair when not in use, which saves space and is less of a fire hazard than the traditional Parabolic Solar Cooker. It is built mostly from salvaged materials as to not contribute to consumption.

Bart Orlando

Local Solar Cooking expert and Appropriate Technology innovator Bart Orlando inspired and co-created this project. Bart has been working with students in the Humboldt area for 15 years, building and designing pedal powered and solar powered technologies. Working with Bart means that this project can benefit from the innovative design that comes from years of experience. For more information on Bart's projects, please visit these pedal power and parabolic solar cooker web pages.

Literature Review

http://solarcooking.wikia.com/wiki/Compendium_of_solar_cooker_designs

http://solarcooking.org/ http://www.fsec.ucf.edu/en/research/solarthermal/solar_cooker/documents/reflectivematerialsreport.pdf

Criteria

This project was designed with the following Criteria in mind:

Time – Must be able to be designed and constructed within the timeframe of one college semester, or about 4 months of part time labor.

Budget – No set budget, however the more materials that can be salvaged or easily acquired by most people, the better.

Durability – The project must be able to withstand periodic exposure to the elements, especially sun, over the course or years. Also, sitting in the chair must not damage the reflectors.

Portability - The device should be lightweight so that one person could move it without toil, and compact enough to be towed via bike trailer. Rattan, the fiber that many Papasan chairs are made from, is reasonably light weight- making reflector material the key issue in terms of weight.

Design - Should incorporate new improvements or previously not thought of innovations to the solar cooker plan. Should be stable in mild wind.

Synergy/Integration - Should be a multi functional device, capable of easily purifying water as well as cooking food, igniting paper or grass when needed, and still be able to fit easily into an everyday household setting.

Level of embedded energy - Materials should be found or salvaged so that it can be shown that this can be built without industrial equipment.

Ease of use - If multiple parts are to be used, then they should be safe, employ intuitive design, and not require the use of tools for operation.

Design

Solar Cooker Background

A solar cooker is used to concentrate sunlight onto a pot or dish. It can be used to bake, fry, make popcorn, cook rice, light fire, sterilize and boil water. This has potentially important applications in sun soaked areas where fuel for cooking is scarce, or where women must travel farther and farther from their villages to gather wood as fuel becomes more scarce. If solar cooking catches on it, it would become a new style of outdoor recreation similar to the barbecue, but one that is directly connected to the world at large (ie the sun) and has no carbon footprint (if people reused their plates and silverware).

There are several shapes a solar cooker can take on, from box-like to parabolic, spherical, funnels and plane mirrors. Each form has different pros and cons in terms of construction, durability, materials, performance, etc. This Wikia page has pictures of all sorts of solar cookers.

So Why Parabolic?

Parabolic cookers seem to be heavier, and more expensive than the other types of solar cookers. Due to the fact that they focus light to a small area, they require constant tracking in relation to the sun to be effective(adjustments about every 15 minutes.) However, they have the advantage of being able to reach higher temperatures than the other cookers due to the intensity of the precise focus created. Other solar cookers, such as solar ovens use a greenhouse effect to trap heat in an enclosed area, whereas parabolic cookers actually concentrate the sunlight from a larger area into a smaller area.


Elements of a Parabolic Solar Cooker

Frame

This is the backbone of the cooker. It must be:

  • Strong enough to support the weight of the reflectors (and possibly the food, depending on the design)
  • Stable enough to face moderate winds without the risk of spilling hot/boiling food/liquid on the ground and people
  • Light enough to allow frequent adjustments to track the sun.
The frame can be constructed in a variety of ways; it can be cast from a mold, built from salvaged materials, such as satellite dishes, and even blackberry bushes, or be fabricated with new materials especially for the cooker. It can also utilize a pre-existing paraboloid (as in this case).
Papasan chair with cushion removed.
  • In this design, a used Papasan Chair provides a sturdy, lightweight, somewhat-stable backbone without the requisite of purchasing new materials. This design method is not available in all areas of the world, butRattan, the material this papasan chair's frame is made from is widespread throughout Asia, Africa, and Australia.



Reflector

The reflective surface itself can be a variety of materials. Sometimes a thin metal frame is built around a single piece of cast, polished metal. This table compares different aspects of potential reflectors for the papasan chair solar cooker:

Material Reflectivity Weight Internet price quote Advantages Disadvantages
Polished Anodized Aluminum ~95% Light ~$7/square foot Reflective, lightweight, can be salvaged, structurally durable, easily flexed and shaped Easily scratched, not super cheap
Mylar >98% Super light $30 for a 4'x'50 roll Super reflective, super light, super cheap Not good at standing up to the elements, forms 'bubbles' if glue starts to give, requires a rigid backing
Aluminum Foil 88% on bright side Super light $3.29 for 1.5'x70' roll Extremely cheap and widely available Not so reflective, corrodes when mixed with acidic juices, structurally weak, would only last one or 2 sessions
Can lids 70-80% (my estimate) Light FREE Widely Available, effective, salvageable, did I mention free?! Not super reflective, non-uniform shape is labor intensive to use
Acrylic Mirror 99% Medium/Heavy $78 for 1'x4' plane Very reflective, nearly unbreakable Comes in plane, likely very difficult to fit to parabola and still maintain reflectivity, super expensive
Glass Mirror 99% Very heavy At least $100/square foot Super reflective, widespread- meaning it could be salvaged Very expensive new, comes in rigid plane- could be broken to fit paraboloid in mosiac in exchange for a precise focus
Astro-foil 76% Light $37 for 1'x50' roll Relatively strong Not so relective


Source

  • Many potential materials with high reflectivity and strength are only available by special order from industrial companies. Though I have not tried to order or to ask for a donation of these fine products, the manufacturers usually deal in bulk.
In addition, I am not sure a special-order industrial super-reflector meets this projects criteria of salvageablility and wide availability. That said, a bulk roll of high quality reflective material may be a wise investment for the aspiring do-it-yourself solar technician.
Close up view of Bart's triangular aluminum reflectors, with carriage bolt.


  • This project ended up utilizing a previously-owned Anodized Polished Aluminum sheet donated by Bart Orlando. The sheet was previously cut into triangular facets with a plasma cutter for use in a different cooker some years ago. He purchased the 4'X 8' sheet new for $80, but professes that cheaper ones are periodically available at the local salvage yard.




Food holding device

The next part of the cooker is what is actually going to hold the food. This has to fit a few criteria. It must be :

  • Black, as to readily absorb the sun's rays.
  • Able to withstand intense energy without becoming toxic (no plastic pot-handles)
  • Rigged to either be suspended from a tripod from above or mounted across the rim-plane of the paraboloid.

Because the project is also functioning as a chair, we decided that a suspension would make more sense than a bar mounted across the parabola.

Some possibilities for holding the food that were considered in this project:

  • 12" bike wheel rim with 3 hooks extending outwards at 120 degree angles. Unpainted coat-hanger wire woven in a grid on the inside of the rim would provide a flat grill-space for the cooking receptacle. The receptacle could be a tea kettle, frying pan, or cooking pot.
  • A product called a "HotPot" by Gaiam. This pot is specifically designed to cook with the sun's rays. This exemplifies a good solar food holder because it is conductive, black, and contains an air-insulated layer so that wind will not cool the pot. We decided to forgo buying one of these in favor of a more salvageable design.
The HotPot cooking vessel consists of a dark pot suspended inside a clear pot with a lid
  • A gallon sized tin can painted with black tempura paint. A wire coat hanger with a paper base can be unhooked from the paper, revealing a perfect holder for a gallon sized can. 2 holes are punctured in the can at 180 degree angles 1" from the top, and the coat hanger is hooked inside. This proved to be a good method for testing the cooker, however the inside of the can may possibly off-gas small amounts of toxic chemicals. To avoid this, fill the tin can with water, placing your food in a small or medium sized mason jar (with lid) inside of the tin can, and then cover the tin can with a matching-sized pot lid (no plastic) or aluminum foil. This will use the double-boiler effect and not toxify your food. This is the most easily salvageable/reusable design.
Using a mason jar inside of a tempura painted can.
Tempura painted can with pot lid.
  • A black stainless steel or cast-iron cooking pot with lid and suspension hooks would also be appropriate. No plastic can be present on the pot.

Suspension

The suspension mechanism holds the food over the focus from above while the cooker is being used.

We wanted a suspension mechanism that could be:
Top end of suspension with inner tube tied around bamboo and chain.
  • Easily adjusted
  • Lightweight
  • Strong
  • Stable in wind and around dogs, kids, etc
  • Easy to make

We went with a tripod constructed of bamboo and a bike inner tube, with a 1/2" width chain for suspension. The bamboo should be a least 1" thick for durability and strength. In addition to being a fascinating organism, Bamboo is a choice building material for sustainability.

Fun Math stuff

  • In math, a parabola is a conic section with the graph (y=x²) on a Cartesian plane. If that 2 dimensional image were to be "spun" around the origin in the 3rd dimension, the resulting shape would be a paraboloid, the shape used by this cooker. Both parabolas and paraboloids have the effect of concentrating energy waves to a single point (or sphere, in the case of a paraboloid) known as the focus. There are numerous examples of parabolas in the natural world.
  • A deep focused parabola is one that contains the focus within the form. A shallow focus parabola projects the focus outside of the form. For this reason, deep focused parabolic cookers are much safer than shallow because they don't project balls of incendiary light outside of the rim plane and accidentally burn things or people.
  • I highly recommend checking out this web page is a to help visualize this concept, and to see how incoming rays of energy focus to a point on a parabolic graph. Notice if you decrease the coefficient "a" past a certain point, the focus is projected outside of the parabola. For the parabolic equation y=ax2, if "a" is set at .3, the focus will occur at the asymptote parabola's "arms". Decrease "a" beyond that, and the focus will be outside of the parabola. The papasan chair we used has a parabolic focus just inside of the rim plane, its graph is estimated to be y=.37x2

Calculations

  • On a typical day of full sun, it is commonly approximated that a flat surface facing the sun will receive 1000 Watts of power per square meter.

The chair is approximately 4.2 feet in diameter and 18 inches deep. The amount of sunlight that the parabola is exposed to is limited to the cross sectional surface area of the aperture.

Equation Model

  • Cross sectional surface area in meters *(1000 Watts * square meters-1) * Reflectivity Coefficient of anodized aluminum = Watts per papasan solar cooker on sunny day.

Crunchy Numbers

  • Pi*r2 =Area of a circle in m2

3.141*0.70104m2 = 1.53m2

  • Area of circle in m2 * 1000 watts/m2 = Ideal power output in W (m2 cancel out)

1530 m2 * 1000 W/m2 = 1530W

  • Ideal power output in W * Reflectivity coefficient of anodized aluminum = Reality-adjusted power output in W

1530W * .95 = 1453W

This means the papasan chair solar cooker can utilize over 1450W of power in full sun - not too shabby!

Factors unaccounted for -

  • Shadowing of cooker by cooking pot (not a huge difference)
  • Energy losses via imprecisely focused facets
  • Diffusion of light from scratched/dirty facets (Bart estimates that the reflectivity for this project is more along the lines of 90%)

Implementation

General advice - always over build.

Reflector

Suspension

Notes on cooking

Always over-build

Tied in back

Bike Rim Suspension for Frying Pan - Picture

Safety

  • As with any type of concentrated energy this has the potential to be dangerous and must be respected. Solar cookers can easily reach temperatures above 450 degrees Fahrenheit. Care must be taken around small children and animals, especially when hot food or liquid is suspended from a tripod.
  • Solar Cookers have the potential to start fires if left unattended in the sun. This is easily avoidable with this project, just put the cushion on when the Cooker is not in use.
  • Be SURE to use SUNGLASSES with UV protection when cooking/testing/adjusting your cooker! Concentrated sunlight is even more damaging to the eyes than the regular kind... And if you are wearing cheap sunglasses that only protect against visible light, you will be more comfortable than usual when staring at concentrated UltraViolet radiation that you can't see. This radiation is even more damaging to the eyes than visible light. UV protection!!

Cost

Quantity Material Source Cost ($) Total ($)
1 Ball of hemp twine Solutions on G Street $3.50 $3.50
3 3 Bamboo poles sized 8' by 1' Mad River Garden Supply $1.50 $4.50
1 Small container Citra-Solv natural de-greaser Co-Op $3.40 $3.40
4' Length of chain Ace Hardware $1/foot $4.00
1 1" long 5/16th bolt/nut Solar Rack Center Donated Free
1 Used bicycle inner tube Friend Salvaged Free
1 Pot Lid My house Borrowed Free
1 Gallon size tin can Jolly Green Giant Cafeteria Donated Free
1oz Black Tempura Paint HSU Child Development Center Donated Free
1 Set of anodized aluminum reflectors Bart Orlando Donated Free
1 Papasan Chair Craigslist Donated Free
1 Pair pliers Roommate Borrowed Free
1 Power drill Roommate Borrowed Free
Total Cost $15.40

Discussion

The Bamboo + bike tube = awesome stand Another question was if the chair was a perfect parabola. Its possible to graph a parabola and then overlay the graph on a photograph of the chair to see if they line up. However, one would have to guess the coefficient of the parabola. Bart says that a close enough to cook with. though focus is a little wide.

Because of foggy/cloudy Arcata life, testing opportunities were limited, and solar technology in general become more of a challenge. I don't think this is a bad thing! Maybe it means there are less realistic uses for solar up here, but it also means that we must refine the technology to get better results.

Does this meet the criteria?

Yes and no, but mostly yes. The hemp, the rattan, the tin can, the bamboo, the discarded bike tube- all super Apprppriate in my mind. These are all sustainable, salvageable or re-used. Then comes the Anodized Sheet of Aluminum Alloy fabricated with high powered industrial machinery. Is that Appropriate? In the absolute? No. In the context of an carbon saving device in a world mostly run on fossil fuels? Yes, I think so, it is Appropriate in that it is a definite step in the direction of do it yourself low-impact living.

Time – Done on time

Budget – Yes, total costs were $15. I was lucky enough to have both chair and sheet metal donated. If one were to purchase a used papasan chair and a used piece of metal that could be cut for free, it would have cost up to $50 more. Still relatively inexpensive.

Durability – Yes. Should last at least 5 years.

Portability - Yes. Easily carried short distances by one or two people, transportable by bike trailer or car.

Design - Yes. It works. Could use a little refining with time.

Synergy/Integration - Yes. Works great as a chair and as a cooker.

Level of embedded energy - Mostly yes. Sheet metal is used, but is a potential salvageable material. Plasma cutters use lots of energy, but not much plasma cutting is required.

Ease of use - Yes. People can be taught the basics in under an hour by somebody who knows what they are doing. Learning curve of a few cooking sessions.

Picture Gallery


External Links

http://solarcooking.wikia.com/wiki/Compendium_of_solar_cooker_designs An Excellent Overv[[Image:Example.jpg]]