Questions Frequently Asked by NSTTF Visitors About Solar Energy

How do Central Receiver power plants produce electricity from the heat of the sun?

Briefly, a central receiver system consists of a field of individually guided mirrors, called heliostats, that re-direct the sun's energy to a receiver mounted on top of a tower. In the receiver, the radiant solar energy is absorbed in a circulating fluid and then is either transferred to a storage system for use during a later period or used directly to power a conventional steam turbine/generator.

How much electriclty could the NSTTF produce?

The NSTTF field contains 222 heliostats that can direct up to 5 megawatts of solar radiation onto the receiver or other experimental objects. If we were to convert the 5,000,000 watts of heat to electricity, we could get about 1,500,000 watts, enough to power about 750 homes.

How big is the NSTTF tower?

The NSTTF tower is 200 feet tall, and its 8-foot-thick foundation is 50 feet below ground. A total of 150 truckloads of concrete was required to complete the structure. The tower's large elevator can lift test items weighing up to 200,000 pounds to the top or to any of the north-facing test bays.

How much land does the NSTTF occupy?

About 9 acres--the heliostats are situated on about 8 acres (the size of 7 football fields), and the tower and its utilities occupy about 1 acre.

How much did the NSTTF cost?

When completed in 1978, the NSTTF cost just over $21 million. In a Central Receiver power plant, the heliostats and their computer control systems account for almost half the total cost; the tower, solar receiver, and thermal storage systems for about one fourth; and the electric power plant for the remainder. Specifically, the CRTF heliostats, including the computer control systems, cost about $35 thousand each. Costs have decreased, and will continue to decrease, as further technical improvements are made and as production quantities increase. Heliostats for the Solar II power plant near Barstow, California, for example, cost about $15 thousand each. Similarly, the computers, which accounted for about 5% of the total cost of the CRTF, will represent only about 1% of plant costs in future central receiver systems.

Is a Central Receiver power plant economical?

Not yet. For a Central Receiver power plant to become competitive with other types of power plants, costs for heliostats, receivers, and other solar components must be further reduced. With sufficient utility and industry interest, such facilities should be economical before the turn of the century.

Is the US the only country developing a Central Receiver system?

France, Spain, Italy, Switzerland, Israel, Germany, Japan and Russia are also working on central receiver concepts. Most of these countries are involved in the design and operation of test facilities like the CRTF and in conducting experiments associated with the facilities.

What are the solar receivers made of?

The receivers normally consist of a large number of metal tubes that contain a flowing fluid--water, air, liquid metals, molten salts, etc.-that picks up the heat. The outer surfaces of the tubes are black to assure that the light is absorbed and converted to heat. The metals used for the tubes are the same as those used in other high-temperature, nonsolar processes.

How do the heliostats move?

The mirrors (facets) are mounted on individual frames that are tipped up and down and rotated east to west by small motors much like those used in electric clocks. The motors are controlled by a computer which determines how to position each heliostat so that its reflection hits the receiver at any time of the day and any day of the year.

What are the NSTTF mirrors made of?

The mirrors are made of two layers of glass with reflective silver between the glass layers. The quality of the glass is like that in your windows at home. The silver in one heliostat (25 mirrors-in one frame) weighs only about 1 ounce.

Does rain, snow, dust, or hail hurt the mirrors?

Rain, snow, and other natural forms of moisture actually help keep the mirrors clean by washing away accumulated dust. Hail and dust storms have not harmed the mirrors. Only hail over 1 inch in diameter is likely to break the mirrors.

How hot is the solar beam at the CRTF?

If we put an uncooled object in the beam, we can generate temperatures of over 4000°F. The metal tubes in a solar receiver reach only 1000°F to 1500°F because the fluid inside carries most of the heat away.

Can the solar beam be used as a "star wars" weapon?

No! We are unable to target on fast moving objects, and it is impractical to highly focus the sun at very long ranges. Even if we could overcome these technical difficulties, we would have only a "fair weather" weapon.

Has any testing at the NSTTF had practical results?

Yes, the receiver and heliostats tested at the NSTTF were used at Solar One , a 10-megawatt electric solar power plant near Barstow, California. The plant, now called Solar Two, is being modified to include the molten salt working fluid tested for receivers, energy storage, and steam generation at the NSTTF.

National defense programs have benefitted from the use of the CRTF solar beam to inexpensively simulate heating effects that can otherwise be generated only with very costly electrical heating equipment.

Studies on critical nuclear reactor components and instruments have contributed to the reliable use of these devices in postulated accident conditions.

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