Dakota Sager
Mr. Reid
DDP
May 14, 2014 Solar Cookers A solar cooker is a device which uses the energy of the sun’s rays to heat or cook food or drink. The vast majority of solar cookers presently in use are cheap. Because they use no fuel and cost nothing to operate, many nonprofit organizations are promoting their use worldwide in order to help reduce fuel costs (for poor people) and air pollution. Solar cooking is a form of outdoor cooking and is often used in situations where minimal fuel consumption is important, or the danger of accidental fires is high. A box cooker has a transparent glass or plastic top, and it may have additional reflectors to concentrate suns rays into the box. The top can usually be removed to allow dark pots with food in it to be placed inside. One or more reflectors of shiny metal or foil-lined material may be put in place to bounce extra light into the middle of the oven. Cooking containers and the inside bottom of the cooker should be dark-colored or black. Inside walls should be reflective to reduce radioactive heat loss and focus the light towards the pots and the dark bottom, which is in contact with the pots. The box should have insulated sides. Thermal insulation for the solar box cooker must be able to withstand temperatures up to 300 °F without melting. Crumpled newspaper, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker. Metal pots and bottom trays can be darkened either with black spray paint. The solar box cooker typically reaches a temperature of 300 °F. This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Parabolic’s are compound curves, which are more difficult to make with simple equipment than single curves. Although parabolic solar cookers can cook as well as a conventional oven, they are difficult to construct. Frequently, these reflectors are made using many small segments that are all single curves which together approximate compound curves. Although parabolic are difficult to make from flat sheets of solid material, they can be made quite simply by rotating open-topped containers which hold liquids. The top surface of a liquid which is being rotated at constant speed around a vertical axis naturally takes the form of a parabolic. Centrifugal force causes material to move outward from the axis of rotation until a deep enough depression is formed in the surface for the force to be balanced by the leveling effect of gravity. It turns out that the depression is an exact parabolic. If the material solidifies while it is rotating, the parabolic shape is maintained after the rotation stops, and can be used to make a reflector. This rotation technique is sometimes used to make parabolic mirrors for astronomical telescopes, and has also been used for solar cookers. Devices for constructing such parabolic are known as rotating furnaces. Parabolic reflectors generate high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China. A Scheffler cooker (named after its inventor, Wolfgang Scheffler) uses a large ideally parabolic reflector which is rotated around an axis that is parallel with the earth's using a mechanical mechanism, turning at 15 degrees per hour to compensate for the earth's rotation. The axis passes through the reflector's centre of mass, allowing the reflector to be turned easily. The cooking vessel is located at the focus which is on the axis of rotation, so the mirror concentrates sunlight onto it all day. The mirror has to be occasionally tilted about a perpendicular axis to compensate for the seasonal variation in the sun's declination. This perpendicular axis does not pass through the cooking vessel. Therefore, if the reflector were a rigid parabolic, its focus would not remain stationary at the cooking vessel as the reflector tilts. To keep the focus
