1-What Is Solar Energy

Solar Energy:
The Ultimate Renewable

Resource
What is Solar Energy?
 Originates with the
thermonuclear fusion
reactions occurring in
the sun.
 Represents the entire
electromagnetic
radiation (visible light,
infrared, ultraviolet, x-
rays, and radio waves).
Advantages and Disadvantages
 Advantages
 All chemical and radioactive polluting byproducts of
the thermonuclear reactions remain behind on the
sun, while only pure radiant energy reaches the
Earth.
 Energy reaching the earth is incredible. By one
calculation, 30 days of sunshine striking the Earth
have the energy equivalent of the total of all the
planet’s fossil fuels, both used and unused!
 Disadvantages
 Sun does not shine consistently.
 Solar energy is a diffuse source. To harness it, we
must concentrate it into an amount and form that we
can use, such as heat and electricity.
 Addressed by approaching the problem through:
1) collection, 2) conversion, 3) storage.
How much solar energy?
The surface receives about 47% of the total solar

energy that reaches the Earth. Only this amount
is usable.
Putting Solar Energy to Use:
Heating Water
 Two methods of heating water:
passive (no moving parts) and
active (pumps).
 In both, a flat-plate collector is
used to absorb the sun’s energy
to heat the water.
 The water circulates
throughout the closed system
due to convection currents.
 Tanks of hot water are used as
storage.
Heating Water: Active System
Active System uses antifreeze so that the liquid does not

freeze if outside temp. drops below freezing.
Heating Water—Last Thoughts
 Efficiency of solar heating system is always less than 100%
because:
 % transmitted depends on angle of incidence,
 Number of glass sheets (single glass sheet transmits
90-95%), and
 Composition of the glass
 Solar water heating saves approx. 1000 megawatts of energy a yr,
equivalent to eliminating the emissions from two medium sized
coal burning power plants.
 By using solar water heating over gas water heater, a family will
save 1200 pounds of pollution each year.
 Market for flat plate collectors grew in 1980s because of
increasing fossil fuels prices and federal tax credits. But by 1985,
when these credits were removed and fossil fuel prices were low,
the demand for flat plate collectors shrunk quickly.
 While solar water heating is relatively low in the US, in other
parts of the world such as Cyprus (90%) and Israel (65%), it
proves to be the predominate form of water heating.
Solar-Thermal Electricity:
Power Towers
 General idea is to collect the light from many reflectors
spread over a large area at one central point to achieve
high temperature.
 Example is the 10-MW solar power plant in Barstow, CA.
 1900 heliostats, each 20 ft by 20 ft
 a central 295 ft tower
 An energy storage system allows it to generate 7 MW of
electric power without sunlight.
 Capital cost is greater than coal fired power plant, despite
the no cost for fuel, ash disposal, and stack emissions.
 Capital costs are expected to decline as more and more
power towers are built with greater technological advances.
 One way to reduce cost is to use the waste steam from the
turbine for space heating or other industrial processes.
Power Towers
Power tower in Barstow, California.

Solar-Thermal Electricity:
Parabolic Dishes and Troughs
 Focus sunlight on a smaller receiver for each device; the
heated liquid drives a steam engine to generate electricity.
 The first of these Solar Electric Generating Stations
(SEGS) was installed in CA by an Israeli company, Luz
International.
 Output was 13.8 MW; cost was $6,000/peak kW and
overall efficiency was 25%.
 Through federal and state tax credits, Luz was able to
build more SEGS, and improved reduced costs to
$3,000/peak kW and the cost of electricity from 25 cents to
8 cents per kWh, barely more than the cost of nuclear or
coal-fired facilities.
 The more recent facilities converted a remarkable 22% of
sunlight into electricity.
Parabolic Dishes and Troughs
Collectors in southern CA.
Because they work best under direct sunlight, parabolic

dishes and troughs must be steered throughout the day in
the direction of the sun.
Direct Conversion into
Electricity
Photovoltaic cells are capable of

directly converting sunlight into
electricity.
 A simple wafer of silicon with wires
attached to the layers. Current is
produced based on types of silicon
(n- and p-types) used for the layers.
Each cell=0.5 volts.
 Battery needed as storage
 No moving partsdo no wear out,
but because they are exposed to the
weather, their lifespan is about 20
years.
Solar Panels in Use
 Because of their current costs, only
rural and other customers far away
from power lines use solar panels
because it is more cost effective than
extending power lines.
 Note that utility companies are
already purchasing, installing, and
maintaining PV-home systems
(Idaho Power Co.).
 Largest solar plant in US, sponsored
by the DOE, served the Sacramento
area, producing 2195 MWh of
electric energy, making it cost
competitive with fossil fuel plants.
Efficiency and Disadvantages
 Efficiency is far lass than the  Cost of electricity from coal-
77% of solar spectrum with burning plants is anywhere b/w
usable wavelengths. 8-20 cents/kWh, while
photovoltaic power generation
 43% of photon energy is is anywhere b/w $0.50-1/kWh.
used to warm the crystal.  Does not reflect the true costs of
 Efficiency drops as burning coal and its emissions
temperature increases (from to the nonpolluting method of
24% at 0°C to 14% at the latter.
100°C.)  Underlying problem is weighing
 Light is reflected off the efficiency against cost.
front face and internal  Crystalline silicon-more
electrical resistance are other efficient, more expensive to
factors. manufacture
 Amorphous silicon-half as
 Overall, the efficiency is efficient, less expensive to
about 10-14%. produce.
Final Thought
 Argument that sun provides power only during the
day is countered by the fact that 70% of energy
demand is during daytime hours. At night, traditional
methods can be used to generate the electricity.
 Goal is to decrease our dependence on fossil fuels.
 Currently, 75% of our electrical power is generated
by coal-burning and nuclear power plants.
 Mitigates the effects of acid rain, carbon dioxide, and
other impacts of burning coal and counters risks
associated with nuclear energy.
 pollution free, indefinitely sustainable.
The End

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Solar Energy:

The Ultimate Renewable

The surface receives about 47% of the total solar

Active System uses antifreeze so that the liquid does not

Power tower in Barstow, California.

Collectors in southern CA.

Because they work best under direct sunlight, parabolic

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