Solar Enrgey Essay Research Paper THE SOLAR

Solar Enrgey Essay, Research Paper

THE SOLAR RESOURCE

SIMPLE APPLICATIONS OF SOLAR ENERGY

The Sun’s rays have long been used as the heat source for evaporating and distilling water. Solar evaporation has always been an important salt-production process. Salt water is pumped into shallow ponds that are open to the Sun. As the water evaporates, its salts form crystals that settle at the bottom and are eventually collected.

Producing drinkable water from brine is accomplished in a solar still, where salty water is evaporated. The salt becomes concentrated in the bottom of the still basin, while the water vapor rises, condenses on the still cover as fresh water, and is drawn off.

The solar cooking stove has become an important device in tropical countries where firewood is in short supply. The stove may be simply a hot box, an insulated container, perhaps with a mirrored cover that intensifies the Sun’s heat. More complex stoves have reflectors that focus sunlight directly on the cooking area, or they use separate heat-collector devices that transfer heat to water inside a steam cooker.

Solar heat is also used throughout the world to dry agricultural crops, fruits, and vegetables.

SOLAR ENERGY USE IN BUILDINGS

Both water and space heating are among the most successful small-scale applications of solar energy. Thousands of systems of these types have been installed throughout the world.

Solar Collectors

Solar collectors are devices that absorb solar energy and produce heat. They are mounted on the roofs of buildings or in other areas that are open to direct sunlight, and they are used for space heating and cooling and for heating water.

The flat-plate collector is made of a copper, aluminum, or steel heat-absorber plate, the surface of which is blackened to make the plate more efficient in absorbing solar heat. A heat-transfer liquid–usually a water-and-antifreeze solution–circulates through a set of tubes and removes heat from the plate. The tubes may be attached to the plate, or passageways for liquid or air may be incorporated into the plate itself, or the heat-transfer medium may simply flow across the surface of the plate.

To minimize convective and conductive heat losses into the atmosphere, two layers of glass or transparent plastic, separated by an air space, are placed above the plate. The cover layers also minimize reradiation from the collector. To reduce heat losses further, the back and sides of the collector are heavily insulated, as are all pipes and ducts leading to and from the heat-storage area.

Evacuated-tube collectors consist of two glass tubes, one within the other, with a vacuum between the tubes to minimize heat loss. Because the tubes are round and are backed with reflecting material, this type of collector can absorb more sunlight and has a significantly higher overall efficiency than the flat-plate collector. Evacuated-tube collectors are used in northern climates where light intensities are low.

Concentrating, or focusing, solar collectors focus the Sun’s rays on a tube (trough type), a point (dish type), or a concave mirror to provide higher temperatures for special purposes, such as industrial-process heat. Such collectors must be able to move both vertically and horizontally in order to track the Sun across the sky in all seasons.

Solar Water Heating

Solar water heating is an old and simple application of solar heat, and an inexpensive system for many buildings. The most common system consists of a collector located outside the building and tilted at an angle that favors uniform yearlong solar input. (The tilt angle is approximately equal to the local latitude.) In addition to the collector, there is a small fractional-horsepower pump for water circulation and a tank to store the heated water for later use. A simple controller that compares tank and collector temperature operates the pump. Whenever the collector is warmed to a temperature greater than that of the tank, the pump is turned on.

The size of collector needed can be approximately determined by the rule of thumb that states that the collection area, in sq ft, should be the same as the number of gallons of hot water needed per day. In the United States each resident in a home uses between 15 and 20 gal (57 to 76 liters) of hot water per day. A four-person family would therefore need 60 to 80 ft(2) (5.6 to 7.4 m(2)) of solar collector.

If poor weather reduces the amount of available sunlight, the solar system will produce no hot water. Then the conventional water-heating system will take over the task of providing domestic hot water.

An alternative to the pumped system is the “thermosiphon” system, where fluid circulation is produced by a density difference between hot fluid in the collector and cold fluid located above the collector in a tank. The lighter, warm fluid will tend to rise, causing cold fluid to replace it in the collector. The performance of these systems is good; the only problem is the need for the tank to be located in a position above the collector.

Active Solar Space Heating

Most of the heating energy used in residences is for space heating, that is, for providing the heat needed to maintain comfort within a building. Solar energy is a good match for this heating task because it is able

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