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A Story Illustrating the Application of Solar Cooker Design Principles in a Field Project

One doesn't have to think long about solar cooking to realize its potential as a solution to a host of serious problems--hunger, contaminated water, deforestation, and health to name a few. Although the concept is versatile, the actual application and solar cooker design may vary markedly depending on specific needs.

This following account, although purely fictitious, is told to illustrate the general concepts relevant to solar cooker design. While elements of the design process may vary, this story poses scenarios that are likely to be common to many projects.

I'd finally arrived. Everything was pretty hazy through the film of dust on my glasses. Crowded in with a half-dozen villagers, I'd just spent the last four hours perched on the two-inch wide top of the tailgate of the Datsun truck that deposited us in that small town in Central America - my Peace Corps assignment.

My purpose was to work with the local people to help create solutions to cooking fuel shortages, health problems, and use of time issues involving cooking. Before I arrived, we worked with the townspeople to assess the cooking fuel alternatives. The available fuels were wood, some propane, and sunlight. Wood was becoming very scarce, and propane was expensive, when available. I took the position there because people wanted to develop ways to make direct use of the sunlight.

There were a number of sociological factors that were critical in this resource development process. The people who would be using the cookers played a major role is assessing how it could best be integrated into their lives. That's another story, however. The subject of this article is a sequential design process created to avoid the numerous pitfalls often encountered with cross-cultural resource development projects. This is an example of how the Principles of Solar Box Cooker Design might be applied in a real-life situation.

In our initial solar assessment we consulted solar maps of the area and concluded that the sunlight available is more than enough for solar cooking. The primary meal is late in the afternoon after the villagers have completed their field work for the day. The women were extremely interested in finding alternatives to the 16 hours per week they foraged for firewood. People had heard that others in Central America were cooking with the sun. We had formed a design team and were ready to go.

The principle central to most use of solar energy -- solar cooking, grain drying, water pasteurization and distillation, domestic water heating, and building space heat -- is the greenhouse effect. Understanding the workings of this basic solar engine is key to peoples' ability to develop solar tools that meet their particular needs. The material that creates this effect -- transparent or translucent material such as glass or plastic -- is the essential element in the majority of solar applications. The first task for the design team was to determine what window material, based on price and availability, would best serve their needs. Although glass was not cheap, it was available enough in the area to use in the cooker design.

Once people understood the capabilities of a window material to trap the heat, we discussed the next important concept --insulation--the dynamic of holding that trapped heat in a container of some sort. Although wood itself is a fair insulator, the box heat loss could be reduced further, saving more energy for cooking, if the box were better insulated. Some materials such as feathers, dry grasses, crumpled paper, and others, are good insulators. Heavier materials such as rock and brick are not good insulators. In order to be functional and durable, the container must have a certain structural integrity. The people were familiar with woodwork and decided they could get many times more BTUs of energy by using their wood to trap solar energy instead of burning it in their cookfires. The design team decided to saw their planks thin, make one wooden box within another wooden box, and fill the space in between with dried grass that was plentiful and typically used for animal feed.

The designers had considered other materials for the cooker. Lighter materials, cardboard, and heavier materials such as adobe bricks have worked well for others. However, these people preferred the durability and weather resistance of wood in this humid climate. They also preferred the adjustability of the wood cooker compared to the immovable adobe cookers.

The designers next needed to determine the size and proportion of the new solar cooker. We considered the food containers that they were presently using over the cook fires. They were dark earthenware, further blackened by the fire, somewhat heavy, and varied in shape, some being low and wide while others were taller than wide. We decided that the low, wider pots would absorb more sunlight and therefore work best in the solar cooker. The team figured the average family size, the number of courses of food required, including weaning food for the children, and determined the necessary size of the cooker -- approximately two by three feet and eight inches deep. The cookers were designed to be just deep enough to accommodate the cookware. Cookers that are deeper than necessary have more surface area through which to lose heat and therefore do not cook as efficiently.

For people to design good solar ovens, they must have a clear understanding of how the heat gets to the food. In most solar ovens, the food is simultaneously heated from the top and bottom. The sunlight heats the tops of the food containers. The tops radiate heat down to the food in the container. The sunlight is also absorbed by the dark conductive plate at the bottom of the solar oven. The heat is conducted through this metal plate to the bottoms of the food containers. If the oven is filled with containers, most of the cooking is done from the top. If there are only one or two containers of food, more heat is conducted to the bottoms of the containers through the dark absorber plate.

We discussed the possibility of adding more thermal mass -bricks, tiles, or stones. Additional mass would extend the afternoon cooking further into the evening. To achieve this, the oven would require more time to heat up at the beginning of the day. The designers decided that the evening meal was early enough so additional mass would not be required.

The proportion of the cooker is also a design factor that affects how often the cooker must be adjusted to face the sun. The longer the east/west axis of the cooker compared to the north/south axis, the less often the cooker must be adjusted to face the sun. This factor was not critical since someone was home in most households for a good part of the day doing other necessary work. They were there and able to adjust the cooker every two hours or so.

Access to the food was another design consideration. Although it involved extra construction work, the design group decided on a door on the front of the oven for ease of access to the food. Top access would have been more difficult and would have increased the risk of glass breakage and possible injury to children. Furthermore, it was easier to keep air leakage at a minimum with an oven door compared to access from the top.

Next, we discussed the options for maximizing the cooking temperatures by bouncing more sunlight into the chamber with a reflector. The team could see that the reflector was particularly effective later in the day when the sun was at a low angle.

Another consideration was the placement of the cooker. Most of the women were interested in having the cooker on a table in the courtyard, near enough to the cook fire or wood cookstove so that food preparation could all be done in the same area. They cook the vegetables, bread, and time - and energy -intensive beans in the solar oven. The tortillas are prepared over the fire in the early morning and early evening. The meals have more courses and less of the scarce wood fuel is used. Many households, that before solar cooking had no way to bake, now enjoy breads on a regular basis.

Building on the successes of the solar cooking, the people are considering other ways they can make use of the sun. Typically, coffee is spread out on stone or concrete slabs to dry in the sun. The solar designers are considering the application of solar grain drying techniques to drying coffee. Traditionally, domestic water has been heated over the wood cookfires. The designers are considering dark containers on the roofs in the sun to provide gravity-fed running hot water. At night, especially higher in the hills, the homes become quite cold. People are sketching out plans for solar window placement and grass insulation in the walls to keep the homes warmer at night.

For more information, contact Mark Aalfs at aalfs@yahoo.com

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