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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Solar ovens. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources The successful use of solar ovens (cookers) was noted in Europe and India as early as the 18th century. Solar cookers and ovens absorb solar energy, converting it into heat, which is stored inside an enclosed space. The absorbed heat is used for cooking, frying and baking. The temperature in the solar oven can reach 200 degrees Celsius. Solar ovens come in many shapes and sizes. Here are some examples: oven, concentrator oven, reflector, solar steamer, etc. With all the variety of models, all ovens capture heat and keep it in a heat-insulated chamber. In most models, sunlight directly affects food. Box solar ovens Box solar ovens consist of a well-insulated box, painted black on the inside, in which black pots of food are placed. The box is covered with a two-layer "window" that lets solar radiation into the box and keeps the heat inside. In addition, a lid with a mirror on the inside is attached to it, which, being folded back, enhances the incident radiation, and when closed, improves the thermal insulation of the furnace.
The main advantages of box solar ovens:
They are relatively inexpensive (compared to other types of solar ovens). Of course, they also have some disadvantages:
Due to their advantages, box solar ovens are the most common type of solar ovens. They are of different types: industrial production, handicraft and home-made; the shape can resemble a flat suitcase or a wide low box. There are also stationary stoves made of clay, with a horizontal lid (in tropical and subtropical regions) or an inclined one (in temperate climates). For a family of five, standard models with an aperture area (entrance area) of about 0,25 m2 are recommended. On sale there are also larger versions of furnaces - 1 m2 or more. Recommendations for the construction of solar ovens Since the heat absorbed by the inner surface of the box must be transferred to the pots, the best material for the box is aluminum, which has a high thermal conductivity. In addition, aluminum does not corrode. For example, a steel box, even with a galvanized coating, cannot withstand the hot and humid environment inside the oven for a long time during the cooking process. Sheet copper is too expensive. Do not attach metal parts to the outside of the box that could create thermal bridges. Glass, synthetic wool or some natural material (peanut, coconut, rice, corn, etc.) can serve as a heat-insulating material. Whatever material is used, it must remain dry. The furnace cover can consist of one or two glasses with an air layer. The distance between two layers of glass is usually 10-20 mm. Studies have shown that the use of a transparent honeycomb material that divides the interior into small vertical cells can significantly reduce the heat loss of the oven, thus increasing its efficiency. The interior glass is thermally stressed, so tempered glass is often used; or both layers may consist of ordinary glass with a thickness of about 3 mm. The outer cover of the solar oven is a reflector that amplifies the incident radiation. The reflective surface can be an ordinary glass mirror, a plastic sheet with a reflective coating, or an unbreakable metal mirror. As a last resort, you can use foil from cigarette packs. The outer box of a solar oven can be made of wood, fiberglass or metal. Fiberglass is lightweight, inexpensive, and water resistant, but not very durable in continuous use. Wood is stronger, but heavier and more susceptible to deterioration due to moisture. Aluminum sheets in combination with wooden fasteners form the highest quality surface, resistant to mechanical stress, temperature changes and humidity. An aluminum-reinforced wooden box is the most durable, but it is more expensive and quite heavy, and it takes time to make. The performance of a standard solar oven with an aperture area of 0,25 m2 reaches about 4 kg of food per day, i.e. Enough for a family of five. The peak temperature inside the solar oven can reach over 150°C on a sunny day in the tropics; it's about 120°C above ambient temperature. Since the water contained in food does not heat up above 100°C, the temperature inside the filled oven will always be correspondingly lower. The temperature in the solar oven drops sharply when dishes with food are placed in it. It is also important that the temperature remains well below 100°C most of the cooking time. But the boiling point is 100°C is not needed for most vegetables and cereals. The average cooking time in a solar oven is 1-3 hours in good sunny conditions and a moderate load. The use of thin-walled aluminum pans significantly reduces the cooking time compared to stainless steel cookware. In addition, the following factors also influence:
Mirror stoves with reflector The simplest mirror oven is a parabolic reflector and a pan stand located at the focus of the oven. If the stove is exposed to the Sun, then the sunlight is reflected from all the reflectors to the central point (focus), heating the pan. The reflector may be a paraboloid made of, for example, sheet steel or reflective foil. The reflecting surface is usually made of polished aluminium, mirror metal, or plastic, but may also consist of many small flat mirrors attached to the inner surface of the paraboloid. Depending on the desired focal length, the reflector can be in the form of a deep bowl into which the pan with food is completely immersed (short focal length, the dishes are protected from the wind) or a shallow plate if the pan is installed at the focal point at a certain distance from the reflector.
All reflector ovens use only direct solar radiation, and therefore must constantly turn towards the sun. This complicates their operation, as it makes the user dependent on the weather and the control device. Advantages of mirror ovens:
These advantages are accompanied by some disadvantages:
The difficulty of handling these ovens, combined with the fact that the cook has to stand in the sun, is the main reason for their lack of popularity. But in China, where cooking traditionally requires high heat and power, they are widespread. Heat output of solar ovens The heat output of a solar oven is determined by the amount of solar radiation, the working absorbing surface of the oven (typically between 0,25 m2 and 2 m2) and its thermal efficiency (typically 20-50%). The table compares typical values for area, efficiency and power for a box oven and a reverberatory oven. Standard values for area, efficiency and output of box oven and reverberatory oven
As a rule, reflector ovens have a much larger working surface than box ovens. Consequently, they are much more powerful, able to boil more water, cook more food, or process comparable quantities in less time. On the other hand, their thermal efficiency is lower because the dishes cool down under the influence of the atmosphere. In tropical and subtropical countries, you can expect clear weather and normal daily light almost all year round. Around noon, when the total solar irradiance reaches 1000 W / m2, it is quite realistic to count on a thermal output of 50-350 W, depending on the type and size of the stove. The amount of radiation in the morning and in the daytime is naturally lower and cannot be fully compensated by the solar tracking system. For comparison, burning 1 kg of dry wood produces approximately 5000 watts times the thermal efficiency of the stove (15% for a primitive hearth and 25-30% for an improved stove used in developing countries). The heat output actually reaching the dishes is therefore 750-1500 watts. The amount of solar radiation is sharply reduced when cloudy and during the rainy season. In conditions of lack of direct radiation, a solar oven is unsuitable for anything other than keeping cooked food warm. The weak point of solar ovens (regardless of their type) is that on cloudy and rainy days (2-4 months per year for most developing countries) food has to be cooked using conventional means: wood, gas or kerosene burner. Solar radiation and stoves The main prerequisite for the successful use of a solar oven is adequate illumination with a small number of cloudy days throughout the year. The duration and intensity of solar radiation must allow the use of the solar oven for extended periods. While in Central Europe solar cooking is possible on a sunny summer day, a minimum solar energy of 1500 kWh/m2 per year is desirable for a solar oven (corresponding to an average daily insolation of 4 kWh/m2). But annual averages can sometimes be misleading. An essential condition for the suitability of a solar oven is stable summer weather, i.e. regular, predictable periods of cloudless days. Resources of solar energy in different countries differ significantly even within the tropical belt in third world countries. For example, solar radiation in most regions of India is considered very good in terms of solar energy utilization. The average amount of solar energy is between 5 and 7 kWh/m2 per day, depending on the region. In much of the country, illumination is at its lowest during the rainy season and almost as low during December and January. The climate and solar potential of Kenya are favorable for the use of solar ovens. Kenya is located close to the equator and therefore has a tropical climate. In the capital, Nairobi, the amount of solar energy ranges from 3,5 kWh/m2 per day in July to 6,5 kWh/m2 per day in February, while in other areas it remains almost unchanged (6,0 - 6,5 kWh/m2 per day in Lodwar province). Solar radiation in Nairobi allows cooking with solar energy nine months of the year (except June-August). On the other hand, on cloudy or foggy days you have to rely on traditional fuels. However, in the province of Lodwar, solar ovens can be used all year round. Solar ovens for developing countries The purpose of using solar ovens is undoubtedly to save energy in the face of a double energy crisis: a crisis of the poor, consisting in a growing shortage of firewood, and a national energy crisis, increasing pressure on its balance of payments.
Compared to other countries, developing countries consume very little energy. For example, the rate of energy consumption per capita in India in 1982 - 7325 GJ - was one of the lowest in the world. But the country's energy consumption is growing almost twice as fast as its gross national product. The same is happening in other developing countries. Most people in developing countries get the bulk of their energy from non-commercial sources: from traditional local energy resources, through their physical labor. They simply cannot afford to buy the right amount of commercially produced energy. The logical consequence of this is the relative lack of fuel for the poor, whose standard of living is further degraded as a result. Solar ovens are a step towards improving their living conditions. Of all the "poor majority" of the inhabitants of the third world countries, solar ovens should be used primarily by the rural population. How much energy do you need to cook food The daily need for fuel depends on what kind of food is cooked and on its quantity. A resident of a developing country burns, on average, 1 ton of firewood per year. A typical Indian family needs 3-7 kg of firewood per day; in cooler regions, the daily amount of firewood for one family is almost 20 kg in winter and 14 kg in summer. In the south of Mali, the average family (of 15 people) burns about 15 kg of firewood per day. A study conducted at an Afghan refugee camp in Pakistan showed that the daily need for firewood there is up to 19 kg per family. More than half of the wood in a typical household is used for baking bread, the rest for cooking other food. In winter, of course, more firewood is required. Even though the amount of energy needed for cooking varies, solar cookers offer significant energy savings. The primary goal of solar ovens is to reduce the need for firewood, which is still the most important fuel for cooking. The problem is that wood is inexpensive compared to kerosene, bottled gas and electricity. Increasing uncontrolled cutting of trees for own use and for sale is the main cause of deforestation, desert expansion, soil erosion, lowering of groundwater levels, and has a long-term adverse impact on the ecological balance. The scarce remnants of forests in Pakistan and rampant deforestation in Kenya are proof that fears about this are not exaggerated. If deforestation in Sudan does not slow down, there will be nothing left of it by 2005. In general, solar ovens are unlikely to make a big contribution to the national energy. However, they can significantly improve the living conditions of the poor, help them overcome their personal energy crisis.
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