Solar energy is a renewable resource that comes from the sun. It can be used to generate electricity through methods like photovoltaics or concentrated solar power, or to heat and cool buildings. While solar energy has many benefits, its widespread adoption faces challenges related to intermittent availability, storage, and the large land and material resources required. Future prospects include new concentrating and luminescent solar technologies as well as potentially placing solar arrays in space to collect energy unaffected by Earth's conditions.

1. CHAPTER 12 – Solar Energy
Introduction
Solar energy is a renewable resource that cannot be depleted (at least for
millions of years).
Solar energy is radiant energy that is produced by the sun.
Where does this energy come from?
The sun’s extremely high pressure and temperature cause nuclear fusion.
Four hydrogen nuclei fuse to become one helium atom, a process that is
accompanied by a loss in mass. This loss in mass results in the emission of
radiant energy.

2. Introduction
It takes millions of years for the energy in the sun’s core to make its way to the
solar surface, and then just a little over 8 minutes to travel the 93 million miles
to Earth. The solar energy travels to the Earth at a speed of 186,000 miles per
second (mps), which is the speed of light.
Solar energy application can be divided into two groups—heating and cooling,
and electricity generation.
In terms of energy strength, the sun constantly delivers 1.36 kW (1360 J/s) of
power per square meter (430 Btu/h·ft2 or 123 W/ft2) to the Earth. However,
some of this power is absorbed by the atmosphere so that, even under ideal
conditions, a receptor on Earth may receive less than 1,000 W/m2 .

3. Early History
As early as the seventh century BCE, people used simple magnifying glasses to
concentrate the light of the sun into beams so hot they would cause wood to
catch fire.
Solar radiation has also been used by man since the beginnings of time for
heating homes, agriculture, and personal comfort.
A solar distillation unit was built in Chile in 1872 to produce fresh water from
salt water.
A 40-kW solar engine was built in Cairo, Egypt, in 1913. Smaller solar-powered
steam engines were built during the period 1930 to 1960, but it was difficult to
market them at that time since they were competing with engines running on
inexpensive gasoline.

4. Early History
Commercial concentrated solar power plants were first developed in the 1980s.
Recently, with the increased cost of fossil fuels (particularly crude oil) and the
depletion of natural resources, interest has further increased in the harnessing
of solar energy for:
1- Heating and cooling
2- The generation of electricity
3- Other purposes

5. Availability
Photovoltaic power is the generation of an electromotive force at the junction
of two dissimilar materials in response to radiant energy.
Solar thermal power is defined as the process of taking sunlight and using it to
heat water or other fluids.
Concentrated solar power systems use mirrors to concentrate a large area of
sunlight onto a small area and then use that energy to drive a generator.

6. Distribution
The average amount of solar radiation flux striking the Earth’s surface is
approximately 630 W/m2.
The solar energy received also varies significantly with latitude, as well as with
time of year.
The radiant energy received by Earth is distributed as follows:
7.82 %: ultraviolet spectrum
47.33 %: visible spectrum
44.85 %: infrared spectrum

7. Characterization
The speed of electromagnetic radiation is approximately 3 × 108 m/s in a
vacuum. This velocity is given by the product of the wavelength (λ) and the
frequency (v) of the radiation—that is,
c = λv
There is a wide range of electromagnetic radiation. Each is characterized by
wavelengths that cover some portion of the electromagnetic spectrum. The
range varies from long-wave radio waves (with wavelengths hundreds of
meters long) to gamma rays, with wavelengths down in the vicinity of 10–13 m.
The so-called solar portion covers only a small fraction of these wavelengths;
they are in the 2.0 × 10–7 to 25.0 × 10–7 m range. The visible portion of the
spectrum ranges from about 4.0 × 10–7 m to 8.0 × 10–7 m.

8. Extraction
The speed of electromagnetic radiation is approximately 3 × 108 m/s in a
vacuum. This velocity is given by the product of the wavelength (λ) and the
frequency (v) of the radiation—that is,
The design and operation of solar collectors for commercial, industrial, and
institutional service are, of course, similar in principle.
For example, the flat-plate solar collector for air or water heating consists of a
heat-conducting plate painted with nonreflecting, heat-absorbing paint on the
inside or coated with a selective coating.

9. Extraction
Units that have been employed in the past include:
1. Solar thermal devices
2. Star Rankine-cycle conversion
3. Solar concentrators
4. Reflecting surfaces
5. Heliostats
6. Parabolic mirrors
7. Parabolic mirror troughs

10. Extraction
There are two general types of solar space heating systems: passive and active;
hybrid systems are a combination of passive and active systems.
In a passive solar home, the whole house operates as a solar collector.
A passive house does not use any special mechanical equipment such as pipes,
ducts, fans, or pumps to transfer the heat that the house collects on sunny
days.
A passive solar home relies on properly oriented windows.
A passive solar home converts solar energy into heat.
Passive buildings are quiet, peaceful locations in which to live.
A passive solar home can often get 50 to 80 percent of the heat it needs from
the sun.

11. Extraction
When special equipment is added to a passive solar home, the result is called a
hybrid system.
An active solar home uses mechanical equipment such as pumps and blowers
and an outside source of energy to assist the heating of the house when solar
energy is not sufficient.
For solar cooling (air conditioning), the collector system is almost identical
with that for solar space heating. The absorption-type refrigeration system is
used.
A heat exchanger supplies heat to evaporate ammonia from a water–ammonia
solution. The cooling coils absorb heat at low temperature from the air being
circulated through the building. It is like a mechanical air-conditioning system.

12. Extraction
A thermal power plant is somewhat like a traditional power plant.

13. Processing
The topic of processing will solely address concentrating of solar power.
Concentrating solar power (CSP) systems use lenses or mirrors and tracking
systems to focus a large area of sunlight into a small beam.
A wide range of concentrating technologies exists; the most widely developed
are the parabolic trough, the concentrating linear Fresnel reflector, the Stirling
dish, and the solar power tower.
A parabolic trough consists of a linear parabolic reflector that concentrates
light onto a receiver positioned along the reflector’s focal line. The receiver is a
tube positioned right above the middle of the parabolic mirror that is filled
with a working fluid. The reflector is made to follow the sun during the
daylight hours by tracking along a single axis.

14. Transportation/Transmission
Solar energy is not available at night, making energy storage an important
issue in order to pro- vide the continuous availability of energy.
Solar power is thus an intermit- tent energy source, requiring that all available
output should be received when it is available and either stored for when it can
be used, or transported, over transmission lines, to where it can be used.
Solar energy can be stored at high temperatures using molten salts.
Off-grid PV systems have traditionally used rechargeable batteries to store
excess electricity.

15. Transportation/Transmission
With grid-tied systems, excess electricity can be sent to the transmission grid.
Net metering programs give these systems a credit for the electricity they
deliver to the grid. This credit offsets electricity pro- vided from the grid when
the system cannot meet demand, effectively using the grid as a storage
mechanism. Credits are normally rolled over month to month and any
remaining surplus is settled annually.

16. Environmental Issues
Collection devices covering a large area are necessary for solar energy systems,
and therefore large quantities of material are required for the construction of
these devices.
Environmental effects will differ somewhat for three basic solar energy
technologies:
Solar photovoltaic energy
Solar thermal energy conversion
Solar heating and cooling

17. Environmental Issues
One of the most obvious environmental effects of a photovoltaic energy system
is the large land requirement and the resulting visual (aesthetic) impact. Some
farmland or grazing land may be taken out of use or production by central-
station photovoltaic solar plants or thermal conversion plants.
There is some air pollution as a result of the energy used in the production of
solar equipment and related materials.
Quantities of particulates SOX, and NOX released to the atmosphere result from
the production of the power required and the production of materials (such as
carbon steel), glass glazing, and aluminizing back panels.

18. Environmental Issues
Primary aluminum production results in emissions of particulates and gaseous
fluorides.
Air pollutants produced in photovoltaic solar cells are cadmium sulfide,
silicon, and gallium arsenide.
There are three potential areas of significant water pollution caused by the
production of solar photovoltaic materials:
1.Surface water degradation and groundwater disturbance due to coal,
zinc, copper, and bauxite mining
2.Water pollution related to ore roasting, smelting, and refining
3.Wastewater discharges in CdS, Si, and GaAs production and device
fabrication

19. Environmental Issues
Many of the environmental effects of solar thermal energy conversion systems
are the same as or like those discussed for photovoltaic systems. In addition,
heat discharges are like those of conventional fossil fuel plants, and hence the
same cooling tower problems will arise, such as blowdown and the use of
cooling tower chemicals.

20. Environmental Issues
Anticipated environmental impacts of solar heating and cooling appear to be
relatively minor.
One drawback of using water as a storage and heat transfer fluid is the
problem of corrosion. This problem is solvable, but the chemical inhibitors
being used can later cause environmental problems. Glare from solar collectors
might well present a problem to people on the ground or in nearby buildings.

21. Future Prospects and Concerns
Solar energy technologies can have serious environmental implications in the
future.
New systems are being developed. For example, concentrated photovoltaic
(CPV) systems employ sunlight concentrated onto photovoltaic surfaces for
the purpose of electrical power production.
Luminescent solar concentrators are useful as they can improve performance
of PV-solar panels drastically.

22. Future Prospects and Concerns
Finally, space-based solar power (SBSP) is a theoretical design currently for the
collection of solar power in space for use on Earth. SBSP differs from the usual
method of solar power collection in that the solar panels used to collect the
energy would reside on a satellite in orbit, often referred to as a solar power
satellite (SPS) rather than on Earth’s surface.
In space, collection of the sun’s energy is unaffected by the day/night cycle,
weather, seasons, or the filtering effect of Earth’s atmospheric gases.
Average solar energy per unit area outside Earth’s atmosphere is on the order
of 10 times that available on Earth’s surface.