CHAPTER 2
BASIC THEORY AND STRATEGIES FOR USAGE
In practice, all one needs to produce good solar cooked meals is a common sense understanding of a solar box cooker. Solar cooking a few familiar recipes soon makes using an SBC easy. Since it helps for solar box cooker designers and teachers to have a more detailed understanding of solar cooking, solar radiation, solar heat production, heat traps and heat transfer, these items will be covered at length in this chapter.
"Solar box cooker" is used in this book as a generic term which covers any of many designs of solar cooker characterized by a large, insulated, horizontal cooking chamber with a glazed window on the top to allow sunlight into the insulated box. Usually there is one reflector stabilized by an adjustable prop to reflect additional sunlight through the window. Customarily the reflector is part of the lid. Solar box cookers may be constructed of any serviceable, nontoxic materials. The inner box and glazing must also be heat resistant.
Three BASIC NATURAL LAWS are employed in making and using solar box cookers. One natural law is that when solar radiation (sunlight) strikes a dark surface it changes to infrared radiation (heat). A second natural law is that when light falls on light-colored or shiny surfaces it reflects and so can be directed to where it is needed. A final natural law is that solar radiation (sunlight) passes through a transparent window easily, but infrared radiation (heat) does not, so heat can be trapped. "Sunlight, the energy resource that powers photosynthesis and drives the earth’s weather, is the foundation for all life on earth. Energy resources such as coal, oil, and gas, formed from solar grown organic matter, are not renewable and will be consumed in a tiny fraction of the millions of years required for their formation. Although solar energy is a diffuse resource, its effective use has been documented since the time of the Greeks, the Romans, and the ancient cliff dwellers of the North American Southwest. As we deplete our non-renewable energy resources, we must continue to develop ways to use solar energy directly.
"Throughout history, humans have applied solar energy principles to provide for a variety of needs....As we approach the next millennium, the problems of world hunger, contaminated drinking water, deforestation, and the fuelwood crisis, are becoming more acute. The invention and use of simple, inexpensive, and low-tech solar box cookers, have resulted in the emergence of solar cooking as a solution to this variety of problems.
"With an understanding of basic principles of heat flow and access to simple materials such as cardboard, aluminum foil, and glass, one can build an effective solar cooking device...."8
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....
"The basic heating dynamics are:
A. Heat gain
B. Heat loss
C. Thermal mass
A solar box cooker cooks because the interior of the box is heated by the energy of the sun. Given this heat input, the temperature inside of a solar box cooker will continue to rise until the heat loss of the cooker is equal to the solar heat gain.
Given two boxes that have equal heat loss, 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 less heat loss—better insulated walls, bottom and top—will reach a higher interior temperature.
The most efficient use of solar radiation by SBCs is determined by the location of heat production both directly and through the use of reflection and by the competence of the insulation retaining the heat thus produced. Both direct and reflected sunlight passing through the glazing onto a dark pot produce heat on the pot sides which flows directly into food since heat naturally seeks to equalize temperatures by moving from hotter areas into colder areas. Sunlight also falls on the reflective oven sides and the dark bottom of the oven. Reflective sides in an SBC throw additional light onto the dark pots in the center as well as adding to the sunlight on the dark drip tray.
A reflective bottom will reflect sunlight up the sides and back out through the glass without it ever becoming heat. For this reason the bottom of solar box cookers is customarily dark. Then even if there is no dark pot in the oven, heat from the dark tray rises to preheat the oven air. A totally reflective oven will not get very hot unless there is a dark object inside.
Cold air is excluded and the heat produced is trapped by insulation in the sides and bottom of the SBC as well as by the glazing and the close fitting lid. The hot oven air transfers additional heat to the pots. The temperature range an SBC will achieve is determined by a combination of all these factors, as well as by others discussed in Chapter 4.
Don’t get overwhelmed with all these details which are provided to give background for trouble-shooters, educators, researchers and designers. Actual cooking consists of putting the food in a dark, lidded pot, pointing the SBC at the sun and giving it sufficient time to cook. That is all most people need to consider when using their SBC.
The MAXIMUM TEMPERATURES for empty SBCs focussed in full sunlight range from 220 F (104 C) to 300 F (149 C) and occasionally above. Temperatures always temporarily drop when the ovens are opened and particularly when cold food is put inside. They immediately begin to heat the food which keeps the temperatures naturally low until the food is hot. Cooking occurs at any point above 190 F (88 C); boiling temperature is 212 F (100 C) at sea level, lower at higher altitudes. Any SBC which reaches 250 F (121 C) or above is considered a good model. Ovens peaking below that level take longer to cook and do not brown food but may be very serviceable.
COOKING TIMES are effected by a number of other factors in addition to the caliber of SBC. An SBC’s function varies with sun angles, sun conditions, location including latitude and altitude, weather and mass. Temperatures are a result of a combination of all these varying conditions so cooking times in written materials are always approximations.
SUN ANGLES are effected by daily changes and seasonal changes as well as differences in sun angles dependent on the latitude. During the early morning and late afternoon with their lower sun angles, foods clearly cook more slowly than during mid-day. When the shadows are long, cooking times will be longer and it will not be possible to finish hard to cook foods, or large quantities of food without special care. It is not necessary to wait for optimum sun angles to utilize an SBC. Preheating of food can be done by the low angles of the morning sunlight, while food that is already hot and cooking will continue to cook in late afternoon at low sun angles.
The most frequent special approach to SBC cooking with a low functioning SBC and/or low sun angles is to heat food briefly before putting it into the SBC for the remainder of cooking time.
Angles also change slowly as the daily arc of the sun swings higher or lower across the sky over the seasons, with best cooking capacity in summer and reduced cooking capacity in winter. With regard to seasonal changes in sun angles, we are revising our expectations as we hear from SBC users. Individuals can use SBCs for single servings when family quantities might be impossible. As winter comes on, the solar cook begins to feel when solar cooking of the foods and amounts needed can no longer be done.
Areas of 20 LATITUDE or less have year-round solar box cooking angles and proportionally less as one goes further from the equator. By about 30 latitude, there is a noticeable drop in sun angles in the winter but easy to cook dishes can be prepared any sunny day. At about 40 latitude, cooking is easily possible 7 to 8 months of the year in Sacramento (Sea level 38.5 N. Latitude). In Seattle (Sea level, 47.5 N. Latitude), they have found that reduced quantities of food can be cooked even in January. This is at a far higher latitude than our early experience led us to expect winter cooking. A 5 pound (2.25 kg) chicken was cooked in Seattle on November 2, 1989, which was a windless, sunny day. They were using a homemade SBC of the SBCI Eco-design with double oven cooking bag in double glazing. There is also a report of cooking a full meal in mid-February in Seattle. Neither of these cooks was using any special cooking methods.
Thus, for well over half the year, assuming solar cooking in the south latitudes is comparable, the area of potentially effective solar box cooking reaches from lower Canada in the northern hemisphere almost to the tip of the South American continent in the southern hemisphere. In brief, geographically this covers all of Afghanistan, all of the African continent, all of the Arabian peninsula, all of Australia, all of Central and South America except the very most southern parts of Chili and Argentina, most of China, southern portions of France and Italy, all of the India peninsula, the Hawaiian Islands, all of Japan, Indonesia, Mexico, most of the Middle Eastern countries north to the level of Bucharest, parts of New Zealand, Pakistan, the Philippines, southern portions of the Soviet Union, all of Spain, Sri Lanka, and Thailand, as well as all of the United States. This list is only representative of the areas of the globe for reference by people from various locations and does not represent all the territory where SBCs could be used. Also, this list does not consider local factors such as persistent cloud cover, severe air pollution, etc., which will eliminate some areas.
ALTITUDE, as opposed to latitude, has little effect except perhaps to provide somewhat better sun conditions as the atmosphere and pollution thins at higher altitudes. Yet, because water boils at lower temperatures, cooking some foods may be slower at higher altitudes. It has been reported that these factors seem to balance each other out and solar cooking proceeds at high altitudes much like anywhere else. Solar box cooking definitely has been accomplished at 12,000’ altitude, 15 latitude in March. On the Bolivian altiplano, rice was reported to cook more rapidly than it would in Sacramento with the same sun angle. Their small potatoes as well as their fish were delicious.
SUN CONDITIONS also effect cooking time. Bright, clear sunlight is obviously the most powerful. With the solar box design, cooking can continue but more slowly when there is some reduction in sunlight due to cloud cover or overcast conditions, high humidity, and air pollution including smog and dust. Sometimes cooking can be improved by simply moving the SBC away from the side of a road, to where it is sheltered from traffic pollution by a building or a tree. During harmattan, cooking was successful in Sierra Leone in January.
MARGINAL WEATHER is not a great problem. A full sunny day is not required for cooking in a solar box oven, in contrast to primarily reflective solar cookers. Sunlight must be strong enough for a fairly distinct shadow. SBCs use indirect as well as direct radiation and so can cook, although more slowly, under somewhat overcast conditions. Likewise, when there are intermittent clouds, but with full sunlight for roughly 30% or more of the time, cooking will proceed in a solar box, although more slowly. This wide range of radiation used by solar box cookers is one reason the designs are particularly suitable for serious solar cooking. They can be depended on to produce cooked food for a large portion of the year in most of the world’s temperate and tropical zones. On a completely cloudy day or one with less than 30% sunshine, traditional methods can be used.
On a semi-cloudy day, easy to cook foods may be chosen or food may cut into small pieces, or divided into several pots. If necessary, foods can be started with hot water, or even brought to a boil before placing in the SBC. If miscalculation or changing conditions leave the food not quite done, a little conventional fuel can finish cooking the meal.
MONSOON solar cooking may be possible simply by identifying the bright hours and adapting to the length of cooking time available. For instance, during the monsoon season in Arizona, there are usually about two hours of bright early morning sun before clouds close in. Easy to cook foods started early can be well begun or often completely cooked in this limited sunny time. Moderately hard or hard to cook foods may brought to a boil by other fuels before being placed in the SBC. Or just as the clouds arrive, pots which are fully heated and cooking in an SBC can be converted to retained heat techniques by packing insulated cushions around the pots inside the cooker. See the Retained Heat section in this Chapter.
When it is WINDY AND COLD but sunny, SBC cooking is slowed somewhat by chilling as wind moves across the glazing. On a bright, very cold, still day, one need only allow extra time for cooking. When it is cold AND windy, a second layer of glass or other glazing ¼ to ½ inch (6 to 12 mm) above the first glazing is essential. It creates a layer of still hot air to insulate the window. Sometimes an SBC lid and prop must be tied on, or the whole oven braced with rocks to withstand gusting winds and yet it will still cook. Water condensed on the inside of an SBC window may simply be wiped off periodically with a soft cloth. Condensation on the window is more often due to cooking without a tight lid than to weather conditions. Thin glazings that flutter in the wind are less effective than rigid glazing, either glass or plastic. For more about windows and glazings, see the glazing section in Chapter 4.
The MASS within the oven insulation—walls, tray, pots, and food—all make a difference in cooking time. The well-insulated oven, lined with foil-covered cardboard, having a light metal, dark colored tray has very little mass as compared to a wooden or metal oven lined with sheet metal or having a heavy metal tray. Both will heat, but the thick sheet metal will require longer to heat initially and will cool correspondingly more slowly when sunshine is reduced. The same considerations apply to pots. Thin-sided, darkened stainless steel or graniteware pots start to cook quicker than heavier ceramic, tinted or painted glass, earthenware or cast iron containers. However, due to heat stored in the thermal mass, once the food is cooking in the heavier cast iron and ceramic pots, it will continue to cook through intermittent clouds better than thinner pots. Once the mass is hot, all pots cook similar foods in about the same time.
These construction and equipment details cause some of the variations in the cooking ability of various stoves. They also make it clear why under most circumstances adobe, brick or ceramic walls do not make a good interior siding for solar box cookers...too much heat goes into the walls of the stove rather than into food.
In addition to the mass of the oven and the pots, another important consideration is how much food mass is being cooked and the shape of the mass. Large mass, say 10 pounds (4.5 kg), in one pot will cook more slowly than the same mass divided into two or three pots. Whole potatoes or meat in a large solid piece cook more slowly than the same foods cut into smaller pieces. For cooking regularly under good solar conditions, it does not matter much how heavy the pot or how the mass is distributed—heavier and thicker simply takes more time. But if cooking conditions are marginal, a thin pot with food cut into small pieces may cook when more massive configurations do not. Although aluminum pots transfer heat quickly and are available around the world, I hesitate to recommend them because as yet scientific studies have not been done to establish if significant amounts of aluminum compounds enter the food when cooking for the lengths of time required by SBCs.
Extra mass is sometimes temporarily added to an oven to store heat for a rapid start, as when cooking breads. Additional mass also provides stabilizing heat during days with variable clouds and stores additional heat for holding foods after sundown. Bottles of water may appear to be a better choice than rocks, adobe bricks, railroad spikes or cast iron pans for heat storage because water can store the most BTUs. However, the maximum temperature unpressured water can reach is the boiling point, 212 F (100 C) at sea level. Although it takes about 5 pounds (2.25 kg) of bricks to equal the heat storage capacity of 1 pound (0.45 kg) of water, bricks can store heat up to the maximum temperature of the SBC. This provides a higher temperature and greater quantity of stored heat which is available for breads or other foods when they are first placed in the SBC.
The strategy of using mass should be varied depending on the needs in a given situation. If there is ample sun and ample time, putting food and extra mass in all at once is a labor saving combination of steps. However, heat forming in the oven will go equally into all available mass. If food is put in alone first, all available heat will flow into it, which starts the food cooking more quickly. Mass added after food is up to temperature utilizes excess heat not going into the food and so does not delay cooking.
RETAINED HEAT cooking goes hand in hand with solar box cooking. In traditional retained heat cooking, a fairly large amount of food in a pot with a tight lid is brought to a boil and simmered for 5 to 30 minutes depending on the type of food being cooked. (See "Simmering Times," page 13). After simmering, the food is quickly moved to a well insulated box and packed with light, clean pillows of insulation. Care is taken not to disturb the lid so steam does not escape. Cooking levels of heat will be retained for up to four hours. In two to three times the usual stove time, food is cooked by the retained heat. This technique works well with 3 pints (1.4 liters) or more of food mass, but small recipes may not retain heat for sufficiently long periods to complete cooking and may have to be reheated after an hour or so.
When combining retained heat and solar cooking, if food has gotten thoroughly hot in an SBC, but clouds arrive before the food is finished cooking, a switch from solar to retained heat cooking should be made before the oven temperature drops below the boiling point. For large recipes this may be accomplished by simply closing the reflective lid on the pots of cooking foods. For smaller recipes, the solar oven is opened,taking care not to allow steam to escape from under the lids, pots are pushed close together along with any heated additional mass. Insulating pads or soft cushions are tucked closely around the pots and well heated mass. The SBC lid is then closed. This effectively makes the transition from solar to retained heat cooking for small quantities. The cooker lid remains closed until shortly before serving time, when the food is tested. If not completely done, a very little conventional fuel will usually finish the job. As a caution, if the mass is not well heated to the core, it may absorb some of the heat from the food and so quickly lower the overall temperature and delay cooking.
Usually solar/retained heat cooking is done right where the SBC is located. However, a lightweight portable SBC can be moved temporarily indoors for its retained heat cooking time if the sun clouds over or if it rains. It may also be brought inside more or less permanently during the off season or at night and function as an insulated box for retained heat cooking. Used in this way the SBC continues to save fuel rather than simply being stored until conditions are right for solar cooking.
Some of the ADVANTAGES OF RETAINED HEAT cooking over conventional cooking are that it uses reduced amounts of fuel, reduces the amount of attention directed toward cooking, and food can be prepared any time, day or night under any weather conditions. This procedure is well worth exploring in different areas of the globe and works very well in conjunction with solar cooking.
FOOD SAFETY with regard to retained heat cooking has been studied by Dr Kirschner. She states, "Cooking with retained heat in a fireless cooker means that the food cooks while the temperature drops very slowly. I have checked innumerable times, using many different amounts and types of foods and sizes of pots, and found that for up to 4 hours—and often much longer—food stays steaming hot in the cooker, and the temperature is at or above 145 F (63 C) .... Should you want to leave the food in the cooker for longer periods ...bring it back to the boiling point and simmer it for 5 minutes to be absolutely sure it won’t spoil."
Food safety for food cooked by any method requires meeting specific rigid conditions. Cooked food at temperatures between 125 F (52 C) and 50 F (10 C) can grow harmful bacteria. This temperature range is known as the danger zone. To protect against food poisoning, microbiologists and home economists strongly recommend that food be kept either above or below these temperatures. These precautions are the same whether food is cooked with gas, electricity, microwaves, wood fire, or solar heat as well as foods cooked by retained heat, crock pot, barbecue pit or any other method. In cooked food held at room temperature, there is a chance of Bacillus cereus food poisoning, a major intestinal illness. Worse, if the food is not thoroughly reheated before consumption, there is a chance of deadly botulism poisoning or salmonella. Even if it is reheated, when cooked food has been in the danger zone for three to four hours, there remains a risk of food poisoning3 in solar cooked food as in food cooked by any other method.
It has been carefully documented with regard to solar box cookers that it is safe to place raw refrigerated or frozen food, even chicken or other meat, in an SBC in the morning several hours before the sun begins to cook it. Refrigerated food placed in an SBC remains sufficiently cold until the sun starts to heat the SBC. Once the full sun is on the oven, the heating of food proceeds quickly enough so that there is no danger of food poisoning.2 Uncooked grains, beans and other dried raw foods can also be placed in an SBC in advance. Both of these methods facilitate absentee cooking.
There are three main points at which caution is required: it is dangerous to keep cooked food more than three or four hours in an unheated or cooling SBC unless both the SBC and food have been cooled rather quickly to below 50 F (10 C) in which case the SBC is serving as a cool box; it is dangerous to let cooked food remain overnight in an SBC unless it is likewise cooled; and it is dangerous for food to partially cook and then remain warm in the SBC when temperatures are not sustained as might occur on a poor solar cooking day, at the end of the day or when clouds move in. Cooked or partially cooked food should either be cooled to below 50 F (10 C) or cooking should be finished with an alternate fuel. If food has remained in the temperature danger zone for 3 to 4 hours it should be considered spoiled and should be discarded. Reheating the food does not correct the problem as heat does not inactivate all toxins.
Food does not have to be visibly spoiled in order to be toxic and cause illness evidenced by nausea, vomiting and diarrhea. Even if food has not been at the incubating temperatures of the danger zone for the full 3 to 4 hours, absolutely discard food that is bubbling, foaming, has a bad smell, is becoming discolored, or gives any other indication of spoilage. Discard it out of reach of animals and children and thoroughly wash the pot. Discard it without tasting it as even small amounts can make an adult very sick.
If temperatures below 50 F (10 C) cannot be obtained, it is still valuable to drop food temperatures as low as possible and as quickly as possible rather than allowing food to remain warm since bacteria grow more slowly at lower temperatures.
An alternative method of holding cooked food is to reliably maintain the temperature of the entire food mass above 125 F (53 C). This can be achieved by first heating the food to boiling, simmering for a few minutes to allow heat to penetrate to the center of each particle and for a pocket of steam to collect under the lid. Then proceed as for retained heat cooking. This provides the level of temperature needed throughout the food, whereas leaving a pot of food on a very small flame may allow food at the edges to remain in the danger zone. Where neither of these methods can be used, it is best to cook amounts of food that will be consumed in one meal relatively soon after being cooked.
ADVANTAGES OF SOLAR BOX COOKING over both conventional cooking methods and retained heat cooking are that it uses only sunlight most days. SBCs can cook large or small amounts of mass. A wider range of pots can be used in an SBC, since an absolutely tight lid is not required, although recommended. And SBCs can produce delicious boiled, simmered, baked, or roasted and lightly browned food with a more varied texture.
FUEL SAVINGS with either solar cooking or retained heat cooking as well as with the solar/retained heat combination can be significant. In India, families using SBCs regularly reduce use of conventional fuels or firewood by about half. In Arizona the fuel saving is more than that. It has not been formally calculated for many places but families using solar box cookers as their major stoves become adept at using what sunlight is available to reduce their use of other conventional fuels such as firewood, gas or charcoal to a minimum.
SBCs have been well received in a great many areas. The solar box cooked food often duplicates or at least closely approaches the traditional flavors, but some regional foods may take careful trials to match customary textures and flavors. Acceptance of SBCs sometimes depends on discovering the best solar box cooking method for favorite local foods. A very small group, or even one individual, can pioneer solar cooking in an area. This can be a social pleasure as well as test a lot of recipes.
In addition to cooking traditional foods, the SBC offers opportunities for cooking foods in different ways in more versatility, so that new recipes may be used. Such recipes may or may not be traditional, but certainly can be very good. Baking breads, cakes and cookies can be a particular delight where baking is not customarily possible. Experimenting can be fun. Most Americans enjoy Chinese, Japanese, Mexican, Middle Eastern and other different foods as well as any of the foods encountered while they grew up. The popularity being experienced by American fast food chains as they expand into other countries highlights the ability of traditionally habituated palates to accept new tastes.
Flavoring is a delicate process. Some of the differences in taste can come from cooking methods. Browning onions, garlic and other spices prior to adding the remainder of the ingredients induces a different flavor, subtle but important in some stews and casseroles. Rice and other grains must have the familiar texture as well as the right flavoring to satisfy—that is, to taste "right." Since people in each locality can taste the subtle differences in their foods, local pioneer solar cooks are in the best position to test different methods. Experimental cooking can be done, using any of at least seven different approaches and the results compared for the best flavor.
- Put all ingredients into pot at the beginning...get it on early...don’t worry about overcooking. This is the SBC standard, trouble-free method and usually works well.
- Heat the pan perhaps with a little oil, then add the uncooked ingredients... Good for steaming fresh, moistened leafy greens according to some; others like their greens only when in a bath of delicious cooking water. Preheating the SBC and pans is also good for cooking most breads and starting meats.
- Bring water to simmering, then add air temperature food—raw grain or meal or heavy vegetables. Stir mushes, grits and porridges well initially and stir again after a half an hour or so. Stirred at those two points, then they can cook unattended as long as necessary without burning or sticking. Vegetables like broccoli, cauliflower or green beans cook well dropped into steaming hot water for 5 to 10 minutes, then drained and kept in a pot in the SBC for another half an hour or so. Soups or drinks may be made with the cooking water to preserve nutrients.
- Bring water to simmering while separately heating the dry ingredients in the SBC. When both are hot, combine them and cook for reduced amounts of time. The consistency of pasta is greatly improved by this method, particularly if a little oil is stirred into the pasta before heating. Sauces can be prepared in this way for dishes which are simply not the same if all cooked in a single pot. Sadza, the staple made from ground white corn in Zimbabwe, may be made this way. Hot corn meal (Mealy meal) is added to near boiling water in proper proportions, and vigorously mixed for about a minute to achieve the proper texture. Some rice that becomes mushy if cooked from the beginning in water, comes out with separate grains when heated separately and combined to cook for a shorter period.
- Briefly precook selected ingredients over a small fire...Saute garlic and other spices and herbs before combining with the rest of the dish and putting it in the SBC. If a difference in taste is due to the absence of browned flavor or of smoke, this may provide just the needed touch.
- Add some ingredients near the end of cooking...grated cheese, some spices or herbs, butter, vinegar, etc.
- Prepare new recipes for solar cooking using traditional ingredients.
If certain traditional foods cannot be duplicated, reserve those recipes for cloudy days when former cooking methods must be used.
Food safety for food cooked by any method requires meeting specific rigid conditions. Cooked food at temperatures between 125° F and 50° F (52° C - 10° C) can grow harmful bacteria. This temperature range is known as the danger zone. To protect against food poisoning, microbiologists and home economists strongly recommend that food be kept either above or below these temperatures. These precautions are the same whether food is cooked with gas, electricity, microwaves, wood fire, or solar heat as well as foods cooked by retained heat, crock pot, barbecue pit or any other method. In cooked food held at room temperature, there is a chance of Bacillus cereus food poisoning, a major intestinal illness. Worse, if the food is not thoroughly reheated before consumption, there is a chance of deadly botulism poisoning or salmonella. Even if it is reheated, when cooked food has been in the danger zone for three to four hours, there remains a risk of food poisoning in solar cooked food as in food cooked by any other method.
