Solar cooker, Sonnenkocher, Hot-Spot Cooker 2011-06-22

Solar cooker, Sonnenkocher, Hot-Spot Cooker 2011-06-22

Fresnel lens solar cooker using TV lens

Fresnel lens solar cooker using TV lens

Solar cooking using my fresnel lens solar cooker made using a 2'x4' fresnel lens from a rear projection TV. I show it cook minced pork, burn a leaf within seconds and how to find the focal length

Solar Powered Insinkerato

Experimenting with a Solar Powered Insinkerator at 14,000 feet in Nepal



On our second Blackstone Ranch/National Geographic Explorers Innovation Challenge Grant we brought an Evolution 200 Insinkerator (courtesy of Emerson Electronics) to the village of Dingboche (4400 meters) in the Himalayas of Nepal, en route to Mt. Everest. There, on the roof of the Khumbu Alpine Conservation Center, with team leader Dr. Alton Byers of the Mountain Institute, Carolyn Howe of India's Loop Environmental Solutions, and Debbie Marchinkowsky of the GAVI campaign, we installed 400 Watts of Photovoltaic power and two Chinook 200 Watt wind generators along with a vacuum tube solar hot water system. We brought the Insinkerator (and the kitchen sink!) to test running it on the solar electric system, using the solar hot water to heat ground up food waste from the trekking lodges for use, with a heat providing traditional Nepalise composting toilet, with a biogas digestor. The plan is a good one for alpine regions where the trekking industry has provided a wealth of food waste that can be turned into clean, affordable, climate friendly cooking fuel. Yaks traditionally ate the food waste, but the explosion of tourism and the decline of yak herding have created a situation where there is now more food waste than the ecosystem can absorb. At the same time, increased demand for cooking fuel for the lodges has created a situation where the soil binding juniper shrub ecosystem is being degraded and the only alternatives are expensive and hard to transport fossil fuels (kerosene and bottled gas). The insinkerator/food-waste-biodigestor/composting toilet combination gives an alternative to all of these problems. The only problem is that the insinkerator we brought kept overloading both the 1000 watt inverter we brought and a neighboring lodges 1500 inverter. The amperage draw and a possible bad overload circuit breaker caused a resistor in the Insinkerator circuit board to burn out and also burned two capacitors and a relay in the inverter. The lesson learned is to use bigger inverters (a 2000 watt inverter should be sufficient) and to always bring spare parts and circuit boards for servicing and repairing insinkerators in the field. Since Solar CITIES now installs insinkerators with our biodigestors in remote parts of the world, building capacity in Insinkerator repair and in basic electronics and mechanics is a critical piece of sustainable development, and one that we are pursuing.

Low Budget Parabolic Solar Cooker

A first approach to a self made, low budget (hopefully) solar cooker.

This article is my attempt at documenting my experience making a parabolic solar cooker. I have
been reading a lot about solar technologies on the internet and have had the great chance of working
with someone who owns a company that sells and designs solar thermal and photovoltaic solutions.
I wanted to get into a hands-on DIY project for a long time and the moment came when my father
moved to a new house. The old owners left their directv dish mounted on the roof, and since it was
no longer useful I decided it would be the perfect opportunity to take my first steps into the world of
solar heating, with a simple technology: solar cooking.
Before I start to describe my progress so far, I´d like to introduce myself and make a few comments
on the objective of this article. My name is Juan Sebastián Estrada, I’m a 27 year old environmental
engineer from Medellín, Colombia, where I currently live. Although I have a bachelor´s degree in
engineering and some understanding of mathematics and physics, I don’t have much experience in
mechanical engineering or anything of that sort, so for this kind of project I prefer to think of
myself as a simple amateur who is just starting his first solar project. Having said this, I hope that
this article will provide at least some useful information and encouragement for anyone who wants
to try and make their own solar cooker. There are lots of articles and information out there, a small
part of which I have read, and which are very useful, but I believe that every attempt should be
documented and discussed so we can learn from each other. Also, since I live in a Latin American
country, I will write the article in Spanish because I have found that there is a great lack of
information in my native language.

Read more , Low Budget Parabolic Solar Cooker

Solar Cookers in India

Solar Cookers in India news report

Hundreds of children attend the schools of the Muni Seva Ashram in the Western Indian state of Gujarat everyday. Their school meals used to be prepared over wood fires. Now a new project harnesses the power of the sun for the school canteen.

Solar Cooking (Lesson Plan)

Solar Cooking (Lesson Plan)

(Warming Up to the Properties of Solar Radiation & Its Uses in Our Homes)
Suggested Grade Level  6-8

Overview 
Students experiment with a virtual solar cooker to discover the mathematical relationships among reflection, transmission and absorption. They won’t stop there, though! Students then apply their knowledge to building and testing a solar cooker of their own invention. In an extension, students investigate how these principles can be used as sustainable energy sources for homes in Pennsylvania through passive solar heating. Approximately two to three (2-3) 50-minute class periods are required for this lesson.

Download Solar Cooking (Lesson Plan)

Solar Cooking

Solar Cooking

Background: The sun gives us energy. It provides the earth with heat and light. It helps grow food, makes
weather, and keeps living things alive. We can use the sun’s energy in many ways. When we burn wood in our fireplaces, we are
releasing the sun’s energy. The wood stores the energy from the sun and releases it as heat. We can use the sun’s energy to
cook food.

Read More Solar Cooking

Build A Solar Oven School Science Project

Build A Solar Oven

LESSON THEME  

An engineering Design Challenge to design and build a solar box cooker, and test it out to see if it works well enough to make 
S ‘mores.  

OBJECTIVES  

Students will:   Engage in the Engineering Design Process to complete a team challenge building a simple solar powered oven.

Download Build A Solar Oven School Science Project

School Science Project for Energy

Solar energy can be used to heat our homes, heat water, cook our food, and power our lights.

These science projects will help you learn about solar energy and how it works. The first three projects focus on different ways to use solar thermal  (or heat) energy. The fourth project focuses on solar electric energy.
Each project is broken into several parts:

• Z The purpose of the experiment
• Z The materials and equipment you will need to do the experiment
• ZWhere to find some of the materials
• Z How to assemble and conduct the experiment
• ZWhat you may see during the experiment
• Z How the specific energy type works.
• Some of the experiments may require help from an adult.

Download  School Science Project for Energy

Solar Cooker By Pakistan science club Junior Scientist

Solar Cooker By Pakistan science club



Solar Cooker by Pakistan science club Junior Scientist Aimen,Fatima, Neha

HotPot Simple Solar Cooker

Solar Cooker

HotPot Simple Solar Cooker

Solar cookers require direct sunlight to function properly. Shadows, clouds and inclement weather limit their effectiveness. In general, you can solar cook when the length of your shadow on the ground is shorter than your height. This is an indicator that the sun is high enough in the sky to cook.

You can typically solar cook two meals per day — a noontime meal and an evening meal. The sun is most intense between 10am and 2pm, which is when breads and pastries should be baked.


Global Sun Cooker

Light-Weight, Easy-To-Carry: The Global Sun Oven weighs 9.5 kg, folds up like a suitcase, and is equipped with a handle for easy transport.

One-Piece Collapsible Reflectors: Set up for use or taken down for storage in a matter of seconds. The reflectors literally fall into place at an angle that allows you to maximize the power of the sun. The reflectors are made of highly polished, mirror-like anodized aluminum that can be cleaned quickly and easily with glass cleaner, and they will never oxidize or rust.

Spill-proof Levelator: There is never any need to worry about your food spilling in a Global Sun Oven. While cooking, your food rests on a shelf that self-adjusts to always stay level as you refocus.

Self-Contained Leveling Leg: As the sun is at different points on the horizon the Global Sun Oven can readily be adjusted to follow it. A simple adjusting leg allows you to choose from 9 angled positions. All that is required is to watch the shadows created by the oven. When the shadows are even on all sides, the cooker is directly focused. A good rule of thumb is to plan to readjust every 30mins to maintain maximum heat.

At noon, the sun is high in the sky and moves quickly past the maximum focus point, creating the need to refocus more often. Later in the day, you will not need to refocus as often. Many meals can be cooked without refocusing. Global Sun Oven users often will put their ovens outside, focused for the mid-day sun, with their dinner in it when they leave for work in the morning.

Dark, thin-walled pots with lids work best. Dark pots change the light from the sun into heat energy. Lids are important because they hold steam in the pot. Shiny aluminum pots and pans cause light to be reflected out, thereby reducing the oven's temperature. Add 10-15mins of cooking time for each time the oven door is opened.

Shayla & Chelsea Solar Oven

Shayla & Chelsea Solar Oven

DIY Solar Oven

DIY Solar Oven



This is my new solar oven.
The other day I baked 5 potatoes in it and the taste is incredible.
It was 60 deg outside and within 2 minutes the temp rose to 100 deg.
After fifteen minutes the temp was 180 deg and within an hour it reached 240 deg!
I was very excited to break the 212 temp.

This design is called the 60/40 solar oven. 40 degrees is the angle of the side reflectors (The ones in between the straight edges.) 60 degrees is the angle that the reflectors sit in reference to the oven itself. NO aerosol paint used inside the oven, only heavy foil and Elmers natural glue.

I'm always happy to hear from anyone who wants to write me.

Walters Solar Cooker

Walters Solar Cooker



This is my Grandpas Solar Cooker you can tell it works very well because the temperature is the same as it is in the summer. If you are interested PM me and i will give you details.

Pakistani Solar cooker

Pakistani Solar cooker



Benefits of Solar Cookers

In terms of water, sanitation and ecological costs:

•It boils 4 kilograms of water within 20 minutes, killing germs and bacteria which cause diseases like Diarrhoea and Kidney related problems.
•There are zero carbon emissions as there is no burning of fossil fuels or any other substances e.g. wood and LPG.
•The solar cooker will allow the conservation of forests that are vital to the eco-system, especially endangered Mangrove Forests.
In terms of economical benefits:

•The solar cooker saves a significant amount of money for poor households as they will not need any sort of firewood or fossil fuels, especially with the rising fuel prices and inflation.
•There is no monetary maintenance of the cookers -- only general cleaning is required to ensure maximum efficiency.
•Due to the boiling of water, there will be far less contaminants in the water and that will reduce the healthcare bill for the families.
•Lastly, this will economically benefit the government as well, since lesser people will be a burden on the public healthcare system.
Formal specifications are as follows:

•Reflector Material: 0.6MM carbon steel covered Vacuum Aluminium Film
•Reflector Diameter: 1.5M
•Focus: 700MM
•The Height of Pot Stand: 110CM
•Bear-Loading: 25KG
•Power: 1000W

Solar Bikini with a Linear Fresnel Lens Cooking

Solar Bikini with a Linear Fresnel Lens Cooking

Solar Ovens - Part Two

Solar Ovens - Part Two

Image above: Prototype A. Greenhouse style solar oven in action. All photos by Juan Wilson.

As mentioned in Part One of this series, my goal was to build an insulated container that could be heated by solar radiation and hold that heat to provide slow cooking like a crock-pot. I wanted an oven that would last many seasons in a variety of weather conditions - this drove me away from using cardboard and aluminum foil construction material, even though the expense would be minimal and the the construction more complicated than just using a boxcutter and duct-tape. it also would make the oven heavier and bulkier.

The "heavier-and-bulkier" aspect was part of the reason I chose to build an oven that would only require horizontal rotation to follow the sun, and not have vertical adjustment as well. This was a major decision. It would make the oven less efficient at some period of the day, but make its construction and operation easier.

If the unit was to have a fixed vertical angle to the sun, what should that angle be on Kauai? For the highest yield of solar energy the conventional rough rule of thumb was to set the angle of the opening equal to your latitude. In other words, here in Hanapepe Valley that would be 22º above horizontal. The problem with this angle is that it is set to perform best at noon when the sun is highest in the sky. you get the most sun then but that angle will perform very badly early in the day (and more importantly), later in the day. It will also perform better in the summer than in the winter.

Another criteria is to set the fixed angle of the solar oven opening to the sun at a pitch that will collect the most energy throughout the day as well as throughout the year. That ends up with quite a different result. My conclusion was that is particularly true if when you want a hot cooked meal most is in the evening, in the winter. I looked to maximize first the winter performance. I found a site that suggested seasonal optimum orientation of solar voltaic panels (http://www.macslab.com/optsolar.html) with tables for various situations. it stated:

Optimum Tilt for Winter

The winter season has the least sun, so you want to make the most of it. To calculate the best angle of tilt in the winter, take your latitude, multiply by 0.89, and add 24 degrees. The result is the angle from the horizontal at which the panel should be tilted. This table gives the angle for some latitudes:

Latitude	Angle	% of optimum
25° (Key West, Taipei)	46.3°	81%
30° (Houston, Cairo)	50.7°	82%
35° (Albuquerque, Tokyo)	55.15°	84%
40° (Denver, Madrid)	59.6°	85%
45° (Minneapolis, Milano)	64.1°	86%
50° (Winnipeg, Prague)	68.5°	88%

For Kauai that formula results in an angle of 44º. For simplicity of cutting parts to fir I adjusted that angle of the opening to 45º. This would maximize performance for mid to late afternoon when it is most likely I would be cooking.

My tools, and readily available material at hand, guided me towards constructing a solar oven made of wood. I had read of problems people had with the taste of food when plastics (and some paints) were used inside the heated space of the oven. Initially decided to build a pine board box as the inner enclosure of the solar oven. The Ace Hardware a mile away had pine boards for use as shelves. They come in a variety of sizes. I determined I needed a couple of 1"x8"x8' and a 1"x12"x4'. Pine boards for this purpose are plumb and true. With a little effort boards with no knot holes, warps, or major imperfections can be selected.

My plan was to make an oven enclosure of about 14"x14"x14". This was in part because of the size of the pine boards. It also would comfortably fit our black cast-iron "Dutch" oven pot.

I like to use a portable electric drill and philip-head screws to put such projects together. Unlike hammering, which can be quite violent to a small job, using screws can allow you to "undo" a bad connection. Once a nail head is buried it can be a mess to retrieve it.



Image above: Pine board oven liner (14"x14"x14") held together with 1"x3" strips cut to 21".

Because I was building an insulated box I needed to connect the pine boards and provide separation between the inner oven and the outer "skin". I used 1.5" recessed philip-head screws.

I used 1"x3"x8' douglas fir lumber all cut to length of about 21". these would both hold together the pine boards as well as separate them from the outer skin (for the insulation space) and provide the structural support for that skin.

That outer skin was 3/8" plywood. It was also screwed into the 1"x3" framing. The resulting squareness and accuracy of the outer skin was highly dependent on how true the pine and fir were assembled. I covered the outside joints of the pine boards with 1/4"x1" furring strips.

The 1"x3" framing was laid out to make it easy to insulate the oven with a roll of 3" batt fiberglass. This material is cheap and easy to cut to size. Thinner and better performing material are available as well. As I was closing up the outer skin of the oven I stapled the paper of the insulation to the box.


Image above: Plywood oven skin screwed into 1"x3" framing. Note installed batt insulation on left and right walls.

I needed an insulated window for the open face of my oven. At the Hanapepe Trnsfer station I found an recycled small beverage refrigerator with a insulated two pane tempered glass door with rubberized seal. I removed the door from the unit and brought it home. This was my "Prototype A" solar oven. Itwas finished in late January of 2011.
On February 1st 2011 I tested the oven. See photo at top. It began sunny, but turned to occasional clouds between 9:30 and 11:00am. After an hour and a half my oven thermometer never got above 165º... not enough to warm a muffin. This oven worked miserably.
I quickly learned three things

• This particular insulated tempered glass filtered out too much light.
• The raw pine boards neither absorbed and held heat, or reflected it on the target.
• The oven opening was not gathering enough light. it needed relecttors.

Prototype B and C quickly followed. See part three of this series on solar ovens.

See also:
Ea O Ka Aina: Solar Ovens - Part One 5/13/11
.

Solar Ovens - Part One

Solar Ovens - Part One

POSTED: 5/13/2011 EDITOR: Juan Wilson

SUBHEAD: There are many kinds of solar ovens. Selecting one is determined by your purposes in having it.

By Juan Wilson on 13 May 2011 for Island Breath -


Image above: Greenhouse style solar oven prototype built from scratch codenamed "Hibiscus". Photo by Juan Wilson.

Around the beginning of the year I became interested in building a solar cooker. I was inspired by two articles posted by John Michael Greer at the end of 2010 in the ArchDruid Report.

The Haybox Factor
(http://islandbreath.blogspot.com/2010/12/haybox-factor.html)

The Tarpaper Shack Principle
(http://islandbreath.blogspot.com/2011/01/tarpaper-shack-prnciple.html)

The first was a way of slow cooking in an insulated box after heating the cooking vessel on a stove, and the other was about using the direct radiation of the sun to cook food in a box heat trap. My reaction was to build a two-in-one haybox/solar-oven. Specifically, the goal was to build an insulated container that could be heated by solar radiation and hold that heat to provide slow cooking like a crock-pot.

I knew going in that this was an experiment and that there would likely be missteps along the way. It would be more sensible to simply buy a commercially manufactured solar oven and try and insulate it or, on the other hand, buy a crock-pot and run it off an inverter off a battery fed by a couple of solar panels. As it turned out those alternatives would have been in the first case cheaper, in the other more effective.

None the less the unit I built does to an extent work. It took three major overhauls and several minor modifications to get it to where it is today. along the way I learned a lot - so much that I really need to re-build the unit from scratch. But more on that later.

Crucible or Greenhouse
If you are considering building (or buying) a solar oven there are two major types that are quite different. In one case the sunlight is trapped in an enclosure with a "Greenhouse" effect. Often flat reflective surfaces are used to multiply the effect of the sun. In the other case curved (parabolic) reflective surfaces focus sunlight in a "Crucible".

Today when I searched YouTube for "How to build solar oven" I go 545 matches (http://www.youtube.com/results?search_query=how+to+build+solar+oven).

You can build a parabolic reflector "crucible" that will focus sunlight on a small area and cook food like you might fry an ant with a magnifying glass. These cookers are effective generating high temperature in a small space. It's like cooking over a very small hot charcoal grill. The materials for highly reflective curved surfaces is quite expensive. The temperatures achieved will set lumber on fire in short order. Even a short period of bright sun can grill meat, however, cleanup can be difficult.

Example of Crucible

Video above: Grilled cheese sandwich in a minute over parabolic mirror. From (http://www.youtube.com/watch?v=aJ22QCAqFCc).

You can build a flat reflector "greenhouse" that will capture sunlight ins a cooking space like melting ice-cream in a closed car in the sun. These cookers are effective in holding medium temperature in a large space. It's like cooking in an oven set to 250º. The materials can be as cheap as tinfoil and cardboard. The temperatures achieved make it hard to boil water unless the sun is bright an the unit efficient. It will take hours to cook a casserole or soup. It is best to use glass mirrors to multiply light into the box.

Example of Greenhouse

Video above: Baking potatoes with tinfoil and glass in a box. From (http://www.youtube.com/watch?v=tt1DgZp0n2g).

For a wider spectrum of cooking options you might build a Crucible and a Greenhouse. That is what what one women did with tinfoil and cardboard.

A Bit of Both

Video above: Cooking vegetables in the Greenhouse and meat in the Crucible. From (http://www.youtube.com/watch?v=8JOSGSGM0KA&NR).

The biggest short coming (besides efficiency) is what happens if it rains. Both the Crucible and Greenhouse solar ovens tend to be bulky and large. They will likely spend a lot of time outdoors, and need to be weatherproof to get through a year.

I chose to build a Greenhouse oven. With these types of ovens you need to plan several hours in advance when preparing a meal. Those of you who have had crock-pots know the drill. You have to know before you go to work what you want for dinner - and do something about it.


Image above: Demonstration of how to build high performance Greenhouse oven. From (http://www.omick.net/solar_ovens/current_solar_oven.htm).

The link above is an example of a high performance Greenhouse solar oven that is rugged and high performing (400ºF). I think it's about the best design for a do-it-yourselfer I've found, although it's quite a bit of work to construct. I used some of what these folks learned in making my solar oven. But, before you try and make your own oven, look at some of the commercial products out there.

Commercial Products
There are many commercial solar ovens available online. One of sites with the widest selection is www.solarovens.net. They have have many videos of products in action.

Here's a video of a commercial parabolic oven that is available for about $700.

Video above: A high performance parabolic "Crucible" oven. From (http://www.youtube.com/watch?v=yzbKXZQeTTA).

Here's a video of a popular reflector box oven that is available for about $200.

Video above: Demonstration of popular "Greenhouse" style Sun Oven product. From (http://www.youtube.com/watch?v=VKPlrVLtgxM).

In Part Two of this series I will detail building my first "Greenhouse" solar oven and its performance.

Solar Cooker in Pakistan

 Solar Cooker in Pakistan

We are seeking sponsors to undertake sponsoring of solar cookers for a pilot project consisting of 25 solar cookers to be setup in hub area where we are already operating.
Benefits of Solar Cookers
In terms of water, sanitation and ecological costs:

• It boils 4 kilograms of water within 20 minutes, killing germs and bacteria which cause diseases like Diarrhoea and Kidney related problems.
• There are zero carbon emissions as there is no burning of fossil fuels or any other substances e.g. wood and LPG.
• The solar cooker will allow the conservation of forests that are vital to the eco-system, especially endangered Mangrove Forests.

In terms of economical benefits:

• The solar cooker saves a significant amount of money for poor households as they will not need any sort of firewood or fossil fuels, especially with the rising fuel prices and inflation.
• There is no monetary maintenance of the cookers – only general cleaning is required to ensure maximum efficiency.
• Due to the boiling of water, there will be far less contaminants in the water and that will reduce the healthcare bill for the families.
• Lastly, this will economically benefit the government as well, since lesser people will be a burden on the public healthcare system.

Formal specifications are as follows:

• Reflector Material: 0.6MM carbon steel covered Vacuum Aluminium Film
• Reflector Diameter: 1.5M
• Focus: 700MM
• The Height of Pot Stand: 110CM
• Bear-Loading: 25KG
• Power: 1000W

Cost per unit will be US $ 100 which includes all transportation and administrative costs associated with this pilot project.
Naeem Ul Hassan

How to Make Solar Cooker/Cooler

How to Make  Solar Cooker/Cooler

by Steven E. Jones, Professor of Physics at Brigham Young University (BYU), with Colter Paulson, Jason Chesley, Jacob Fugal, Derek Hullinger, Jamie Winterton, Jeannette Lawler, and Seth, David, Nathan, and Danelle Jones.
Introduction
A few years ago, I woke up to the fact that half of the world's peoples must burn wood or dried dung in order to cook their food. It came as quite a shock to me, especially as I learned of the illnesses caused by breathing smoke day in and day out, and the environmental impacts of deforestation -not to mention the time spent by people (mostly women) gathering sticks and dung to cook their food. And yet, many of these billions of people live near the equator, where sunshine is abundant and free. Ergo...
As a University Professor of Physics with a background in energy usage, I set out to develop a means of cooking food and sterilizing water using the free energy of the sun. First, I looked at existing methods.
The parabolic cooker involves a reflective dish that concentrates sunlight to a point where the food is cooked. This approach is very dangerous since the sun's energy is focused to a point which is very hot, but which cannot be seen. (BYU students and I built one which will set paper on fire in about 3 seconds!) I learned that an altruistic group had offered reflecting parabolas to the people living at the Altiplano in Bolivia. But more than once the parabolas had been stored next to a shed -- and the passing sun set the sheds on fire! The people did not want these dangerous, expensive devices, even though the Altiplano region has been stripped of fuel wood.
The box cooker: Basically an insulated box with a glass or plastic lid, often with a reflecting lid to reflect sunlight into the box. Light enters through the top glass (or plastic), to slowly heat up the box. Problems: energy enters only through the top, while heat is escaping through all the other sides, which have a tendency to draw heat away from the food. When the box is opened to put food in or take it out, some of the heat escapes and is lost. Also, effective box cookers tend to be more complicated to build than the funnel cooker.
While studying this problem, I thought again and again of the great need for a safe, inexpensive yet effective solar cooker. It finally came to me at Christmastime a few years ago, a sort of hybrid between the parabola and a box cooker. It looks like a large, deep funnel, and incorporates what I believe are the best features of the parabolic cooker and the box cooker.
The first reflector was made at my home out of aluminum foil glued onto cardboard, then this was curved to form a reflective funnel. My children and I figured out a way to make a large card-board funnel easily. (I'll tell you exactly how to do this later on.)
The Solar Funnel Cooker is safe and low cost, easy to make, yet very effective in capturing the sun's energy for cooking and pasteurizing water -> Eureka!
Later, I did extensive tests with students (including reflectivity tests) and found that aluminized Mylar was good too, but relatively expensive and rather hard to come by in large sheets. Besides, cardboard is found throughout the world and is inexpensive, and aluminum foil is also easy to come by. And individuals can make their own solar cookers easily, or start a cottage-industry to manufacture them for others.
Prototypes of the Solar Funnel Cooker were tested in Bolivia, and outperformed an expensive solar box cooker and a "Solar Cookit" - while costing much less. Brigham Young University submitted a patent application, mainly to insure that no company would prevent wide distribution of the Solar Funnel Cooker. BYU makes no profit from the invention. ( I later learned that a few people had had a similar idea, but with methods differing from those developed and shown here.) So now I'm trying to get the word out so that the invention can be used to capture the free energy of the sun - for camping and for emergencies, yes, but also for every day cooking where electricity is not available and even fuel wood is getting scarce.
How it Works
The reflector is shaped like a giant funnel, and lined with aluminum foil. (Easy to follow instructions will be given soon.) This funnel is rather like the parabolic cooker, except that the sunlight is concentrated along a line (not a point) at the bottom of the funnel. You can put your hand up the bottom of the funnel and feel the sun's heat, but it will not burn you.
Next, we paint a jar black on the outside, to collect heat, and place this at the bottom of the funnel. Or one can use a black pot, with a lid. The black vessel gets hot, fast. But not quite hot enough to cook with... We need some way to build up the heat without letting the air cool it. So, I put a cheap plastic bag around the jar -- voila, the solar funnel cooker was born! The plastic bag, available in grocery stores as a "poultry bag", replaces the cumbersome and expensive box and glass lid of the solar box ovens. You can use the plastic bags used in American stores to put groceries in, as long as they let a lot of sunlight pass. (Dark- colored bags will not do.)
I recently tested a bag used for fruits and vegetables, nearly transparent and available free at American grocery stores, that works great. This is stamped "HDPE" for high-density polyethylene on the bag (ordinary polyethylene melts too easily). A block of wood is placed under the jar to help hold the heat in. (Any insulator, such as a hot pad or rope or even sticks, will also work.)
A friend of mine who is also a Physics Professor did not believe I could actually boil water with the thing. So I showed him that with this new "solar funnel cooker," I was able to boil water in Utah in the middle of winter! I laid the funnel on its side since it was winter and pointed a large funnel towards the sun to the south. I also had to suspend the black cooking vessel -- rather than placing it on a wooden block. This allows the weaker sun rays to strike the entire surface of the vessel.
Of course, the Solar Funnel works much better outside of winter days (when the UV index is 7 or greater). Most other solar cookers will not cook in the winter in northern areas (or south of about 35 degrees, either).
I thought that a pressure cooker would be great. But the prices in stores were way too high for me. Wait, how about a canning jar? These little beauties are designed to relieve pressure through the lid -- a nice pressure cooker. And cooking time is cut in half for each 10º C we raise the temperature (Professor Lee Hansen, private communication). I used one of my wife's wide-mouth canning jars, spray-painted (flat) black on the outside, and it worked great. Food cooks faster when you use a simple canning jar as a pressure cooker. However, you can also put a black pot in the plastic bag instead if you want. But don't use a sealed container with no pressure release like a mayonnaise jar -- it can break as the steam builds up! (I've done it.)
How to Build Your Own Solar Funnel Cooker
What You will Need for the Funnel Cooker:

1. A piece of flat cardboard, about 2 feet wide by 4 feet long. (The length should be just twice the width. The bigger, the better.)
2. Ordinary aluminum foil.
3. A glue such as white glue (like Elmer's glue), and water to mix with it 50-50. Also, a brush to apply the glue to the cardboard (or a cloth or paper towel will do). Or, some may wish to use a cheap "spray adhesive" available in spray cans. You can also use wheat paste.
4. Three wire brads-- or small nuts and bolts, or string to hold the funnel together.
5. For a cooking vessel, I recommend a canning jar ("Ball" wide-mouth quart jars work fine for me; the rubber ring on the lid is less likely to melt than for other jars I've found. A two-quart canning jar is available and works fine for larger quantities of food, although the cooking is somewhat slower.).
6. The cooking jar (or vessel) should be spray-painted black on the outside. I find that a cheap flat-black spray paint works just fine. Scrape off a vertical stripe so that you have a clear glass "window" to look into the vessel, to check the food or water for boiling.
7. A block of wood is used as an insulator under the jar. I use a piece of 2"X4" board which is cut into a square nominally 4" x 4" by about 2" thick. (10cm square x 5cm thick.) One square piece of wood makes a great insulator.
8. A plastic bag is used to go around the cooking-jar and block of wood, to provide a green-house effect. Suggestions:

• Reynolds™ Oven Bag, Regular Size works great: transparent and won't melt. (Cost about 25 cents each in U.S. grocery stores.)
• Any nearly-transparent HDPE bag (High-density Polyethylene). Look for "HDPE" stamped on the bag. I've tested HDPE bags which I picked up for free at my grocery store, used for holding vegetables and fruits. These are thin, but very inexpensive. Tested side-by-side with an oven bag in two solar funnels, the HDPE bag worked just as well! (Caution: we have found that some HDPE bags will melt should they contact the hot cooking vessel. For this reason, we recommend using the oven-safe plastic bag wherever possible.)
• An idea attributed to Roger Bernard and applied now to the BYU Funnel Cooker: place a pot (having a blackened bottom and sides) in a glass bowl, and cover with a lid. Try for a tight fit around the bottom to keep hot air trapped inside. The metal pot or bowl should be supported around the rim only, with an air space all around the bottom (where the sunlight strikes it). Put a blackened lid on top of the pot. Then simply place this pot-in-bowl down in the bottom of the funnel - no plastic bag is needed! This clever method also allows the cook to simply remove the lid to check the food and to stir. I like this idea - it makes the solar cooker a lot like cooking over a fire. See Photographs for further details.

Construction Steps
Cut a Half-circle out of the Cardboard

Cut a half circle out of the cardboard, along the bottom as shown below. When the funnel is formed, this becomes a full-circle and should be wide enough to go around your cooking pot. So for a 7" diameter cooking pot, the radius of the half-circle is 7". For a quart canning jar such as I use, I cut a 5" radius half-circle out of the cardboard.
Form the Funnel

To form the funnel, you will bring side A towards side B, as shown in the figure. The aluminum foil must go on the INSIDE of the funnel. Do this slowly, helping the cardboard to the shape of a funnel by using one hand to form creases that radiate out from the half-circle. Work your way around the funnel, bending it in stages to form the funnel shape, until the two sides overlap and the half-circle forms a complete circle. The aluminum foil will go on the INSIDE of funnel. Open the funnel and lay it flat, "inside up", in preparation for the next step.
Glue Foil to Cardboard

Apply glue or adhesive to the top (inner) surface of the cardboard, then quickly apply the aluminum foil on top of the glue, to affix the foil to the cardboard. Make sure the shiniest side of the foil is on top, since this becomes your reflective surface in the Funnel. I like to put just enough glue for one width of foil, so that the glue stays moist while the foil is applied. I also overlap strips of foil by about 1" ( or 2 cm). Try to smooth out the aluminum foil as much as you reasonably can, but small wrinkles won't make much difference. (If even cardboard is not available, one can simply dig a funnel-shaped hole in the ground and line it with a reflector, to make a fixed solar cooker for use at mid-day.)
Join side A to side B to keep the funnel together.

The easiest way to do this is to punch three holes in the cardboard that line up on side A and side B (see figure). Then put a metal brad through each hole and fasten by pulling apart the metal tines. Or you can use a nut-and-bolt to secure the two sides (A & B) together.
Be creative here with what you have available. For example, by putting two holes about a thumb-width apart, you can put a string, twine, small rope, wire or twist-tie in one hole and out the other, and tie together.
When A and B are connected together, you will have a "funnel with two wings". The wings could be cut off, but these help to gather more sunlight, so I leave them on.
Tape or glue a piece of aluminum foil across the hole at the bottom of the funnel, with shiny side in. 
This completes assembly of your solar funnel cooker.
For stability, place the Funnel inside a cardboard or other box to provide support. For long-term applications, one may wish to dig a hole in the ground to hold the Funnel against strong winds.
Final Steps
At this stage, you are ready to put food items or water into the cooking vessel or jar, and put the lid on securely. (See instructions on food cooking times, to follow.)
Place a wooden block in the INSIDE bottom of the cooking bag. I use a piece of 2X4 board which is cut into a square nominally 4"X4" by about 2" thick. Then place the cooking vessel containing the food or water on top of the wooden block, inside the bag.
Next, gather the top of the bag in your fingers and blow air into the bag, to inflate it. This will form a small "greenhouse" around the cooking vessel, to trap much of the heat inside. Close off the bag with a tight twist tie or wire. Important: the bag should not touch the sides or lid of the cooking vessel. The bag may be called a "convection shield," slowing convection-cooling due to air currents.
Place the entire bag and its contents inside the funnel near the bottom as shown in the Photographs.
Place the Solar Funnel Cooker so that it Faces the Sun
Remember: Sunlight can hurt the eyes: Please wear sunglasses when using a Solar Cooker! The Funnel Cooker is designed so that the hot region is deep down inside the funnel, out of harm's way.
Put the Solar Funnel Cooker in the sun pointing towards the sun, so that it captures as much sunlight as possible. The design of the funnel allows it to collect solar energy for about an hour without needing to be re-positioned. For longer cooking times, readjust the position of the funnel to follow the sun's path.
It helps to put the Solar Funnel Cooker in front of a south-facing wall or window (in the Northern Hemisphere) to reflect additional sunlight into the funnel. A reflective wall is most important in locations farther from the equator and in winter. In the Southern Hemisphere, put the Solar Funnel Cooker in front of a North-facing wall or window to reflect additional sunlight into your cooker.
After Cooking
Remember that the cooking vessel will be very hot: Use cooking pads or gloves when handling! If you are heating water in a canning jar, you may notice that the water is boiling when the lid is first removed - it gets very hot!
Open the plastic cooking bag by removing the twist-tie. Using gloves or a thick cloth, lift the vessel out of the bag and place it on the ground or table. Carefully open the vessel and check the food, to make sure it has finished cooking. Let the hot food cool before eating.
Helpful Hints

1. Avoid leaving fingerprints and smudges on the inside surface of the cooker. Keep the inner surface clean and shiny by wiping occasionally with a wet towel. This will keep the Solar Funnel Cooker working at its best.
2. If your funnel gets out-of-round, it can be put back into a circular shape by attaching a rope or string between opposite sides which need to be brought closer together.
3. For long-term applications, a hole in the ground will hold the Funnel Cooker securely against winds. Bring the funnel inside or cover it during rain storms.
4. The lids can be used over and over. We have had some trouble with the rubber on some new canning-jar lids becoming soft and "sticky." "Ball canning lids" do not usually have this problem. Running new lids through very hot water before the first use seems to help. The lids can be used over and over if they are not bent too badly when opened (pry off lid carefully).
5. The jar can be suspended near the bottom of the funnel using fishing line or string (etc.), instead of placing the jar on a block of wood. A plastic bag is placed around the jar with air puffed inside, as usual, to trap the heat. The suspension method allows sunlight to strike all surfaces of the jar, all around, so that heats faster and more evenly. This suspension method is crucial for use in winter months.
6. Adjust the funnel to put as much sunlight onto the cooking jar as possible. Look at the jar to check where the sunlight is hitting, and to be sure the bottom is not in the shadows. For long cooking times (over about an hour), readjust the position of the funnel to follow the sun's path. During winter months, when the sun is low on the horizon (e.g., in North America), it is helpful to lay the funnel on its side, facing the sun.

Tests in Utah
I have personally used the Solar Funnel Cooker to cook lunches over many weeks. My favorite foods to cook are potatoes (cut into logs or slices) and carrot slices. Vegetables cook slowly in their own juices and taste delicious. I also make rice, melted cheese sandwiches, and even bread in the Solar Funnel Cooker. I usually put the food out around 11:30 and let it cook until 12:45 or 1 pm, just to be sure that it has time to cook. I've never had any food burn in this cooker.
I have also cooked food in the mountains, at an altitude of around 8,300 feet. If anything, the food cooked faster there - the sunlight filters through less atmosphere at high altitudes.
I find that people are surprised that the sun alone can actually cook food. And they are further pleasantly surprised at the rich flavors in the foods which cook slowly in the sun. This inexpensive device does it!
Students at Brigham Young University have performed numerous tests on the Solar Funnel Cooker along with other cookers. We have consistently found much faster cooking using the Solar Funnel Cooker. The efficiency/cost ratio is higher than any other solar cooking device we have found to date. Mr. Hullinger also performed studies of transmissivity, reflectivity and absorptivity of alternate materials which could be used in the Solar Funnel Cooker. While there are better materials (such as solar-selective absorbers), our goal has been to keep the cost of the Solar Cooker as low as possible, while maintaining safety as a first priority.
Tests in Bolivia
The BYU Benson Institute organized tests between the Solar Funnel Cooker and the "old-fashioned" solar box oven. The solar box oven cost about $70 and was made mostly of cardboard. It took nearly two hours just to reach water pasteurization temperature. The Bolivian report notes that "food gets cold every time the pots are taken from and into the oven." The solar box oven failed even to cook boiled eggs. (More expensive box cookers would hopefully work better.)
An aluminized-mylar Solar Funnel Cooker was also tested in Bolivia, during the Bolivian winter. Water pasteurization temperature was reached in 50 minutes, boiled eggs cooked in 70 minutes, and rice cooked in 75 minutes. The Bolivian people were pleased by the performance. So were we! (La Paz, Bolivia, August, 1996)
I also donated two dozen solar funnel cookers for people in Guatemala. These were taken there by a group of doctors going there for humanitarian service. The people there also liked the idea of cooking with the sun's free energy! For an aluminized-Mylar Solar Funnel Cooker kit, please contact CRM (licensed manufacturer) at +1 (801) 292-9210.
Water and Milk Pasteurization
Contaminated drinking water or milk kills thousands of people each day, especially children. WHO reports that 80% of illnesses in the world are spread through contaminated water. Studies show that heating water to about 65º - 70º C (150º F) is sufficient to kill coliform bacteria, rotaviruses, enteroviruses and even Giardia. This is called pasteurization.
Pasteurization depends on how hot and how long water is heated. But how do you know if the water got hot enough? You could use a thermometer, but this would add to the cost, of course. When steam leaves the canning jar (with lid on tight) and forms "dew" on the inside of the cooking bag, then the water is probably pasteurized to drink. (The goal is to heat to 160º Fahrenheit for at least six minutes.) With a stripe of black paint scraped off the jar, one can look through the bag and into the jar and see when the water is boiling - then it is safe for sure.
Think of all the lives that can be saved simply by pasteurizing water using a simple Solar Cooker! (See also Recent Advances in Solar Water Pasteurization)
Safety
Safety was my first concern in designing the Solar Funnel Cooker, then came low cost and effectiveness. But any time you have heat you need to take some precautions.

• The cooking vessel (jar) is going to get hot, else the food inside won't cook. Let the jar cool a bit before opening. Handle only with gloves or tongs.
• Always wear dark glasses to protect from the sun's rays. We naturally squint, but sunglasses are important.
• Keep the plastic bag away from children and away from nose and mouth to avoid any possibility of suffocation.

Cooking with the Solar Funnel Cooker
What do you cook in a crock pot or moderate-temperature oven? The same foods will cook about the same in the Solar Funnel Cooker -- without burning. The charts below give approximate summer cooking times.
The solar cooker works best when the UV index is 7 or higher. (Sun high overhead, few clouds.)
Cooking times are approximate. Increase cooking times for partly-cloudy days, sun not overhead (e.g., wintertime) or for more than about 3 cups of food in the cooking jar.
Stirring is not necessary for most foods. Food generally will not burn in the solar cooker.
Vegetables (Potatoes, carrots, squash, beets, asparagus, etc.)
Preparation: No need to add water if fresh. Cut into slices or "logs" to ensure uniform cooking. Corn will cook fine with or without the cob.
Cooking Time: About 1.5 hours
Cereals and Grains (Rice, wheat, barley, oats, millet, etc.)
Preparation: Mix 2 parts water to every 1 part grain. Amount may vary according to individual taste. Let soak for a few hours for faster cooking. To ensure uniform cooking, shake jar after 50 minutes. CAUTION: Jar will be hot. Use gloves or cooking pads.
Cooking Time: 1.5-2 hours
Pasta and Dehydrated Soups
Preparation: First heat water to near boiling (50-70 minutes). Then add the pasta or soup mix. Stir or shake, and cook 15 additional minutes.
Cooking Time: 65-85 minutes
Beans
Preparation: Let tough or dry beans soak overnight. Place in cooking jar with water.
Cooking Time: 2-3 hours
Eggs
Preparation: No need to add water. Note: If cooked too long, egg whites may darken, but taste remains the same.
Cooking Time: 1-1.5 hours, depending on desired yolk firmness.
Meats (Chicken, beef, and fish)
Preparation: No need to add water. Longer cooking makes the meat more tender.
Cooking Time: Chicken: 1.5 hours cut up or 2.5 hours whole; Beef: 1.5 hours cut up or 2.5-3 hours for larger cuts; Fish: 1-1.5 hours
Baking
Preparation: Times vary based on amount of dough.
Cooking Times: Breads: 1-1.5 hours; Biscuits: 1-1.5 hours; Cookies: 1 hour
Roasted Nuts (Peanuts, almonds, pumpkin seed, etc.)
Preparation: Place in jar. A little vegetable oil may be added if desired.
Cooking Time: About 1.5 hours
MRE's and prepackaged foods
Preparation: For foods in dark containers, simply place the container in the cooking bag in place of the black cooking jar.
Cooking Times: Cooking time varies with the amount of food and darkness of package.

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How to Use the Solar Funnel as a Refrigerator/Cooler
A university student (Jamie Winterton) and I were the first to demonstrate that the BYU Solar Funnel Cooker can be used - at night - as a refrigerator. Here is how this is done.
The Solar Funnel Cooker is set-up just as you would during sun-light hours, with two exceptions:
1. The funnel is directed at the dark night sky. It should not "see" any buildings or even trees. (The thermal radiation from walls, trees, or even clouds will diminish the cooling effect.).
2. It helps to place 2 (two) bags around the jar instead of just one, with air spaces between the bags and between the inner bag and the jar. HDPE and ordinary polyethylene bags work well, since polyethylene is nearly transparent to infrared radiation, allowing it to escape into the "heat sink" of the dark sky.
During the day, the sun's rays are reflected onto the cooking vessel which becomes hot quickly. At night, heat from the vessel is radiated outward, towards empty space, which is very cold indeed (a "heat sink").
As a result, the cooking vessel now becomes a small refrigerator. We routinely achieve cooling of about 20º F (10º C) below ambient air temperature using this remarkably simple scheme.
In September 1999, we placed two funnels out in the evening, with double-bagged jars inside. One jar was on a block of wood and the other was suspended in the funnel using fishing line. The temperature that evening (in Provo, Utah) was 78º F. Using a Radio Shack indoor/outdoor thermometer, a BYU student (Colter Paulson) measured the temperature inside the funnel and outside in the open air. He found that the temperature of the air inside the funnel dropped quickly by about 15 degrees, as its heat was radiated upwards in the clear sky. That night, the minimum outdoor air temperature measured was 47.5 degrees - but the water in both jars had ICE. I invite others to try this, and please let me know if you get ice at 55 or even 60 degrees outside air temperature (minimum at night). A black PVC container may work even better than a black-painted jar, since PVC is a good infrared radiator - these matters are still being studied.
I would like to see the "Funnel Refrigerator" tried in desert climates, especially where freezing temperatures are rarely reached. It should be possible in this way to cheaply make ice for Hutus in Rwanda and for aborigines in Australia, without using any electricity or other modern "tricks." We are in effect bringing some of the cold of space to a little corner on earth. Please let me know how this works for you.
Conclusion: Why We Need Solar Cookers
The BYU Funnel Cooker/Cooler can:

• Cook food without the need for electricity or wood or petroleum or other fuels.
• Pasteurize water for safe drinking, preventing many diseases.
• Save trees and other resources.
• Avoid air pollution and breathing smoke while cooking.
• Use the sun's free energy. A renewable energy source.
• Cook food with little or no stirring, without burning.
• Kill insects in grains.
• Dehydrate fruits, etc.
• Serve as a refrigerator at night, to cool even freeze water.

(Try that without electricity or fuels! See also Balancing the Scales.)
The burden for gathering the fuel wood and cooking falls mainly on women and children. Joseph Kiai reports from Dadaab, Kenya: "Women who can't afford to buy wood start at 4 am to go collecting and return about noon... They do this twice a week to get fuel for cooking... The rapes are averaging one per week." From Belize: "Many times the women have to go into the forest dragging their small children when they go to look for wood. It is a special hardship for pregnant and nursing mothers to chop and drag trees back to the village... they are exposed to venomous snakes and clouds of mosquitoes." (Anna K.) (Quoted in newsletters by Solar Cookers International.)
And the forests are dwindling in many areas. Edwin Dobbs noted in Audubon Magazine, Nov. 1992, "The world can choose sunlight or further deforestation, solar cooking or widespread starvation..."
Americans should be prepared for emergencies, incident to power failures. A Mormon pioneer noted in her journal: "We were now following in their trail traveling up the Platte River. Timber was sometimes very scarce and hard to get. We managed to do our cooking with what little we could gather up..." (Eliza R. Snow) Now there's someone who needed a light-weight Solar Cooker!
Here's another reason to use a solar cooker. Many people in developing countries look to see what's being done in America. I'm told that if Americans are using something, then they will want to try it, too. The more people there are cooking with the sun, the more others will want to join in. A good way to spread this technology is to encourage small local industries or families to make these simple yet reliable solar cookers for others at low cost. I've used this cooker for three summers and I enjoy it. Cooking and making ice with the funnel cooker/cooler will permit a significant change in lifestyle. If you think about it, this could help a lot of people. The BYU Solar Funnel Cooker uses the glorious sunshine -- and the energy of the sun is a free gift from God for all to use!

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Answers to commonly-asked questions
Will the cooker work in winter (in the United States)? As the sun moves closer to the southern horizon in the winter, the solar cooker is naturally less effective. A good measure of the solar intensity is the “UV index” which is often reported with the weather. When the ultraviolet or UV index is 7 or above– common in summer months– the solar cooker works very well. In Salt Lake City in October, the UV index was reported to be 3.5 on a sunny day. We were able to boil water in the Solar Funnel Cooker during this time, but we had to suspend the black jar in the funnel so that sunlight struck all sides. (We ran a fishing line under the screw-on lid, and looped the fishing line over a rod above the funnel. As usual, a plastic bag was placed around the jar, and this was closed at the top to let the fishing line out for suspending the jar.)
The solar “minimum” for the northern hemisphere occurs on winter solstice, about December 21st each year. The solar “maximum” occurs six months later, June 21st. Solar cooking works best from about March 20 - October 1 in the north. If people try to cook with the sun for the first time outside of this time window, they should not be discouraged. Try again when the sun is more directly overhead. (One may also suspend the jar in the funnel, which will make cooking faster any time of the year.)
It is interesting to note that most developing countries are located near the equator where the sun is nearly directly overhead all the time. Solar Cookers will then serve year-round, as long as the sun is shining, for these fortunate people. They may be the first to apply fusion energy (of the sun) on a large scale! And they may accomplish this without the expensive infrastructure of electrical power grids that we take for granted in America.
How do you cook bread in a jar?
I have cooked bread by simply putting dough in the bottom of the jar and placing it in the funnel in the usual way. Rising and baking took place inside the jar in about an hour (during summer). One should put vegetable oil inside the jar before cooking to make removal of the bread easier. I would also suggest that using a 2-quart wide-mouth canning jar instead of a 1-quart jar would make baking a loaf of bread easier.
What is the optimum “opening angle” for the funnel cooker?
A graduate student at Brigham Young University did a calculus calculation over two years ago to assess the best shape or opening angle for the Solar Funnel. Jeannette Lawler assumed that the best operation would occur when the sun’s rays bounced no more than once before hitting the cooking jar, while keeping the opening angle as large as possible to admit more sunlight. (Some sunlight is lost each time the light reflects from the shiny surface. If the sunlight misses on the first bounce, it can bounce again and again until being absorbed by the black bottle.) She set up an approximate equation for this situation, took the calculus-derivative with respect to the opening angle and set the derivative equal to zero. Optimizing in this way, she found that the optimum opening angle is about 45 degrees, when the funnel is pointed directly towards the sun.
But we don’t want to have to “track the sun” by turning the funnel every few minutes. The sun moves (apparently) 360 degrees in 24 hours, or about 15 degrees per hour. So we finally chose a 60-degree opening angle so that the cooker is effective for about 1.2 hours. This turned out to be long enough to cook most vegetables, breads, boil water, etc. with the Solar Funnel Cooker. We also used a laser pointer to simulate sun rays entering the funnel at different angles, and found that the 60-degree cone was quite effective in concentrating the rays at the bottom of the funnel where the cooking jar sits.

Construction of Parabolic Solar Cooker photo gallery

Construction of Parabolic Solar Cooker photo gallery

http://www.paksc.org/pk/parabolic-solar-cooker.html

Principles of Solar Box Cooker Design

Principles of Solar Box Cooker Design

By Mark Aalfs, Solar Cookers International
e-mail: aalfs@yahoo.com
The purpose of this paper is to summarize the basic principles that are used in the design of solar box cookers.
People use solar cookers primarily to cook food and pasteurize water, although additional uses are continually being developed. Numerous factors including access to materials, availability of traditional cooking fuels, climate, food preferences, cultural factors, and technical capabilities, affect people's approach to solar cooking.
With an understanding of basic principles of solar energy and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device. This paper outlines the basic principles of solar box cooker design and identifies a broad range of potentially useful construction materials.
These principles are presented in general terms so that they are applicable to a wide variety of design problems. Whether the need is to cook food, pasteurize water, or dry fish or grain; the basic principles of solar, heat transfer, and materials apply. We look forward to the application of a wide variety of materials and techniques as people make direct use of the sun's energy.
The following are the general concepts relevant to the design or modification of a solar box cooker:

HEAT PRINCIPLES

The basic purpose of a solar box cooker is to heat things up - cook food, purify water, and sterilize instruments - to mention a few.
A solar box cooks because the interior of the box is heated by the energy of the sun. Sunlight, both direct and reflected, enters the solar box through the glass or plastic top. It turns to heat energy when it is absorbed by the dark absorber plate and cooking pots. This heat input causes the temperature inside of the solar box cooker to rise until the heat loss of the cooker is equal to the solar heat gain. Temperatures sufficient for cooking food and pasteurizing water are easily achieved.
Given two boxes that have the same heat retention capabilities, the one that has more gain, from stronger sunlight or additional sunlight via a reflector, will be hotter inside.
Given two boxes that have equal heat gain, the one that has more heat retention capabilities - better insulated walls, bottom, and top - will reach a higher interior temperature.
The following heating principles will be considered first:

A. Heat gain

Greenhouse effect:  This effect results in the heating of enclosed spaces into which the sun shines through a transparent material such as glass or plastic. Visible light easily passes through the glass and is absorbed and reflected by materials within the enclosed space.

	The light energy that is absorbed by dark pots and the dark absorber plate underneath the pots is converted into longer wavelength heat energy and radiates from the interior materials. Most of this radiant energy, because it is of a longer wavelength, cannot pass back out through the glass and is therefore trapped within the enclosed space.The reflected light is either absorbed by other materials within the space or, because it doesn't change wavelength, passes back out through the glass. Critical to solar cooker performance, the heat that is collected by the dark metal absorber plate and pots is conducted through those materials to heat and cook the food.
	Glass orientation:  The more directly the glass faces the sun, the greater the solar heat gain. Although the glass is the same size on box 1 and box 2, more sun shines through the glass on box 2 because it faces the sun more directly. Note that box 2 also has more wall area through which to lose heat.
	Reflectors, additional gain:  Single or multiple reflectors bounce additional sunlight through the glass and into the solar box. This additional input of solar energy results in higher cooker temperatures.

B. Heat loss

The Second Law of Thermodynamics states that heat always travels from hot to cold. Heat within a solar box cooker is lost in three fundamental ways: Conduction, Radiation, and Convection
Conduction:
The handle of a metal pan on a stove or fire becomes hot through the transfer of heat from the fire through the materials of the pan, to the materials of the handle. In the same way, heat within a solar box is lost when it travels through the molecules of tin foil, glass, cardboard, air, and insulation, to the air outside of the box.

The solar heated absorber plate conducts heat to the bottoms of the pots. To prevent loss of this heat via conduction through the bottom of the cooker, the absorber plate is raised from the bottom using small insulating spacers as in figure 6.Radiation: Things that are warm or hot -- fires, stoves, or pots and food within a solar box cooker -- give off heat waves, or radiate heat to their surroundings. These heat waves are radiated from warm objects through air or space. Most of the radiant heat given off by the warm pots within a solar box is reflected from the foil and glass back to the pots and bottom tray. Although the transparent glazings do trap most of the radiant heat, some does escape directly through the glazing. Glass traps radiant heat better than most plastics. Convection: Molecules of air move in and out of the box through cracks. They convect. Heated air molecules within a solar box escape, primarily through the cracks around the top lid, a side "oven door" opening, or construction imperfections. Cooler air from outside the box also enters through these openings.

C. Heat storage:

As the density and weight of the materials within the insulated shell of a solar box cooker increase, the capacity of the box to hold heat increases. The interior of a box including heavy materials such as rocks, bricks, heavy pans, water, or heavy foods will take longer to heat up because of this additional heat storage capacity. The incoming energy is stored as heat in these heavy materials, slowing down the heating of the air in the box.
These dense materials, charged with heat, will radiate that heat within the box, keeping it warm for a longer period at the day's end.