Presentation Dialogue



SOLAR ENERGY. All life on Earth depends on energy from the sun. Solar

energy is the source of energy for photosynthesis. It provides the warmth

necessary for plants and animals to survive. The heat from the sun causes

water on the Earth's surface to evaporate and form clouds that eventually

provide fresh rainwater.



Solar energy is the result of thermonuclear fusion reactions deep

within the sun. These reactions produce so much energy that they keep the

surface temperature of the sun at about 10,300B0F (5,700B0C). Even though

solar

energy is the largest source of energy received by the Earth, its intensity

at the Earth's surface is actually very low due to the large distance betwee

n

the Earth and the sun and the fact that the Earth's atmosphere absorbs and

scatters some of the radiation. Even on a clear day with the sun directly

overhead, the energy that reaches the Earth's surface is reduced about 30

percent by the atmosphere. When the sun is near the horizon and the sky is

overcast, the solar energy at ground level can be negligible. It also varies

from one point to another on the Earth's surface.



Nevertheless, in the 20th century, the sun's energy has become an

increasingly attractive source for small amounts of direct power to meet

human needs. A number of devices for collecting solar energy and converting

it into electricity have been developed, and solar energy is used in a

variety of ways. Solar energy is used to heat houses, and in many countries

specially designed solar ovens are used for cooking. The sun also supplies

energy to electric generators that provide power for weather and

communications satellites and for radio and television equipment.



Because the intensity of the sun's radiation at the surface of the

Earth is so low, collectors designed to capture solar energy must be large.

In the sunniest parts of the continental United States, for example, in orde

r

for a collector to gather enough energy to serve one person for one day, the

area of the collector's surface must be about 430 square feet (40 square

meters). The actual energy that can be used depends on the efficiency of the

collector and of the device that converts the radiation into usable energy.



Flat-plate collectors. The most common flat-plate collectors consist

of a dark metal plate, covered with one or two sheets of glass, that absorbs

heat. The heat is transferred to air or water, called carrier fluids, that

flows past the back of the plate. This heat may be used directly or it may b

e

transferred to another medium. Flat-plate collectors are used for home and

hot-water heating . Flat-plate collectors typically heat carrier fluids to

temperatures ranging from 150B0 to 200B0F (66B0 to 93B0C). The efficienc

y of such

collectors varies from 20 to 80 percent.



Concentrating collectors. When higher temperatures are required, a

concentrating collector is used. These collectors reflect and concentrate

sunlight from a wide area. One such device, called a solar furnace, was

installed in the Pyrenees in France and has several acres of mirrors focused

on a single target. The energy concentrated at the target is 3,000 times tha

t

received by any single mirror, and the unit produces temperatures of up to

3,630B0F (2,000B0C). Another structure, the so-called "power tower" plant

near

Barstow, Calif., generates 10,000 kilowatts of electricity. Here, the furnac

e

acts as a boiler and generates steam for a steam turbine-electric generator

power plant.



In sophisticated concentrating collectors such as the California

tower, each mirror is rotated by a heliostat that directs the sun's rays fro

m

the mirror to the target. Positioning motors, drives, and controllers make

such systems expensive. Less costly collectors can produce temperatures lowe

r

than those of more advanced concentrating collectors but higher than those o

f

flat-plate collectors. For example, parabolic reflectors that concentrate

sunlight on black pipes can produce fluid temperatures of about 400B0 to 55

0B0F

(200B0 to 290B0C) and can concentrate the solar energy up to 50 times its

original strength.





Small Stand-Alone DC System



The small stand-alone system is an excellent replacement for propane

or kerosene lights in a remote cabin, a recreational vehicle or a boat. The

size of the photovoltaic (PV) array and battery will depend upon individual

requirements. The actual sizing methods are discussed elsewhere. The PV arra

y

charges the battery during daylight hours and the battery supplies power to

the loads as needed. The charge regulator terminates the charging when the

battery reaches full charge. The load center may contain meters to monitor

system operation and fuses to protect wiring in the event of malfunction or

short circuit in the house.



PV - Generator Combination



The PV - Generator Combination system may be an economical

alternative to a large stand-alone PV system, because the PV array does not

have to