SOLAR THERMAL

In the past, humans have used the heat from sunlight directly to cook food and heat water and homes. Today, solar collectors can gather solar thermal energy in almost any climate to provide a reliable, low-cost source of energy for many applications including hot water for homes, residential heating, and hot water for industries such as laundry and food processing. In recent years, utilities have begun to use solar thermal energy to generate electricity by boiling water and using the steam to drive a turbine that generates electrical power.

Solar thermal describes technologies that use the heat of the sun for heating water or producing electric power. Small scale water heating systems use flat plate collectors to capture heat from the sun, while solar thermal electric power plants use various concentrating devices to focus sunlight and achieve the high temperatures necessary to produce steam for power.

Flat plate collectors transfer the heat of the sun to water either directly or through the use of another fluid and a heat exchanger.

Flat Plate Collector

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Solar thermal electric generating plants produce electricity by using solar radiation to heat a fluid and produce steam to drive turbine generators. When the sun heats the fluid it passes through a heat exchanger to transfer its energy to water to produce the steam.

There are several advantages to solar thermal electricity generation:

(1) Electricity and hot water can be provided at the same time

(2) Plants can be scaled to match the application

(3) Utilities can match power production to daytime electricity demand peaks

(4) There are few pollutants associated with solar thermal power production

(5) Permits can be issued and plants built in as little as 18 months, which is much faster than conventional fossil fuel or nuclear plants.

Solar thermal electric plants use several different methods for concentrating sunlight, and vary significantly in size.

Central receiver systems are large-scale plants where mirrors focus sunlight on a single collector mounted on a central tower. The mirrors are heliostats, meaning they track the sun in order to maintain focus on the receiver.

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Troughs are long, curved mirrors that focus sunlight onto tubes filled with a circulating heat transfer fluid. The fluid reaches very a high temperature, for instance as high as 400 deg. C (750 deg. F) in the case of the Solar Electric Generating Systems in Southern California.

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Dish systems use a parabolic reflector to focus solar radation on a receiver mounted at the focal point of the dish. Fluid circulates through the receiver and powers a central turbine generator. Sunlight can be concentrated from 100 to 2000 times using this method.

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Applications of Solar Thermal

1.Residential Hot Water

It is often economical to install solar hot water heaters in residential homes. Standard tanks for consumer use in the U.S. range from 60 gallons to 120 gallons, and homes can often be easily retrofitted with a solar system. Most solar thermal systems have a backup-heating element to compensate for cloudy weather or prolonged hot water use. Even taking these conditions into account, the solar system can satisfy 60 to 80% of hot water needs.

2. Commercial and Industrial Hot Water

About 250,000 commercial and industrial buildings in the United States use solar thermal systems to provide hot water or space heating. Industries in which these systems are most common include laundry, food service, food processing, metal plating, and textiles.

3. Swimming Pool Heating

Swimming pools can be heated using a simple solar collector system. Most pool heaters are designed to raise the pool temperature 8 to 10 !F, and to extend the swimming season three or four months. Pool systems differ from residential hot water systems in that the pool serves as a storage tank, and the collector is black plastic tubing with no glazing. The pool pump may serve to circulate water through the system

4. Utility Power Generation

Using trough, dish, or central receiving collectors, solar thermal electric power plants concentrate sunlight to achieve temperatures in excess of 600 deg. C. There are many commercial plants now in service, and several larger ones being planned or constructed.