1) Solar energy comes from nuclear fusion reactions in the sun. Some of this energy reaches Earth where it can be converted to electricity or heat through various technologies. 2) Photovoltaic cells directly convert sunlight into electricity by freeing electrons when photons are absorbed. PV cells are made of materials like crystalline silicon or thin films and connected in panels and arrays. 3) Concentrating solar power plants use reflectors to concentrate sunlight and convert it to high-temperature heat, which is then used to power steam turbines and generate electricity. Types of CSP plants include parabolic troughs, power towers, and parabolic dishes.

1. SOLAR SYSTEM
Energy from the sun is called solar energy. The Sun’s energy comes from nuclear
fusion that takes place deep in the Sun. The energy from these reactions flow out from the
sun and escape into space.
The output of sun is 2.8×10
23
KW. The energy reaching the earth is 1.5×10
18
KWH/year. When light travels from outer space to earth, solar energy is lost because of
following reasons:
- Atmospheric conditions including clouds, humidity, atmospheric density, and dust
- Time of day (rotation of the Earth)
- Season (location of the Earth in its orbit around the sun)
- Latitude (distance from the equator)
- Orientation of the collector‘s surface
Solar energy conversion
To use solar energy means that we must converting parts of the electromagnetic energy
spectrum to two other forms:
– Electricity
– Heat (thermal energy)
The amount of heat or electricity produced depends on the technology used and its
efficiency.

2. Photovoltaic Systems
A photovoltaic cell is a semiconductor device (photo for light, voltaic for electricity),
often abbreviated as PV, that directly converts sun light into electricity. Light consists of
energy packets called photons. When light is incident on the cell, some of the photons are
absorbed in the region of the junction and energy is transferred to the semiconductor,
freeing electrons in the silicon. If the photons have enough energy, the electrons will be able
to overcome the opposing electric field at the junction and move freely through the silicon
and into an external circuit. As these electrons stimulate current flow in the external circuit,
their energy can be converted into useful work.
Panels form an array. Each PV cell is rated for 0.5 – 0.7 volt and a current of
30mA/cm2. Based on the manufacturing process they are classified as:
Mono crystalline: efficiency of 12-14 %.
Poly crystalline: efficiency of 12%
Amorphous: efficiency of 6-8%
PV Cell Materials
The most common PV cells are made from crystalline silicon wafers. Other types of
materials include thin films like Cadmium Telluride (CdTe), Copper-Indium- Gallium-
Diselenide (CIGS), amorphous silicon (a- Si).The main goals for manufacturers are to
minimize the amount of materials and maximize efficiency. Today, the best crystalline
silicon cells are about 15% efficient; the best thin films are about 8% efficient.

3. PV Cells, Modules and Arrays
Photovoltaic systems for specific applications are produced by connecting individual
modules in series and parallel to provide the desired voltage and current. Each module is
constructed of individual solar cells also connected in series and parallel.
PV systems
A complete PV system may also include a device to convert DC to AC power
(inverter), batteries to store energy, and a back-up generator. PV systems can be connected
to the electric utility and can be used to reduce the amount of electricity purchased from the
local utility without using batteries or generators.
PV modules are rated based on the maximum power produced in Watts when the
amount of sunlight is 1,000 Watts/m2.PV systems are rated based on the maximum
combined power output of the PV modules. Since the amount of sunlight changes, the power
output of the system will vary.

4. Solar thermal power plants
The two main types of solar thermal power plants are
1. Concentrating Solar Power (CSP) plants.
2. Solar Chimneys
1. Concentrating Solar Power (CSP) plants
Solar thermal power plants generally use reflectors to concentrate sunlight into a
heat absorber. Such power plants are known as Concentrating Solar Power (CSP) plants.
Concentrating solar power plants produce electric power by converting the sun's
energy into high-temperature heat using various mirror configurations. The heat is then
channeled through a conventional generator. The plants consist of two parts, one that
collects solar energy and converts it to heat, and another that converts heat energy to
electricity.
Concentrating solar power systems can be sized for village power (10 kilowatts) or
grid-connected applications (up to 100 megawatts). The amount of power generated by a
concentrating solar power plant depends on the amount of direct sunlight.
Types of CSP plants
a. Parabolic Trough Systems
The sun's energy is concentrated by parabolically curved, trough-shaped reflectors
onto a receiver pipe running along the inside of the curved surface. This energy heats oil
flowing through the pipe and the heat energy is then used to generate electricity in a
conventional steam generator.
Fig: Principle of a Parabolic Trough Solar Power Plant

5. A collector field comprises many troughs in parallel rows aligned on a north-south
axis. This configuration enables the single-axis troughs to track the sun from east to west
during the day to ensure that the sun is continuously focused on the receiver pipes.
Individual trough systems currently can generate about 80 megawatts of electricity.
b. Parabolic dish systems
Parabolic dish systems consist of a parabolic-shaped point focus concentrator in the
form of a dish that reflects solar radiation onto a receiver mounted at the focal point. These
concentrators are mounted on a structure with a two-axis tracking system to follow the sun.
Fig: Principle of a Dish–Stirling System
The collected heat is typically utilized directly by a heat engine mounted on the
receiver moving with the dish structure. Stirling and Brayton cycle engines are currently
favored for power conversion. Projects of moduar system have been realized with total
capacities up to 5 MW. The modules have max sizes of 50 kW and have achieved peak
efficiencies up to 30% net.
c. Power Tower System
The technology utilizes many large, sun-tracking mirrors (heliostats) to focus
sunlight on a receiver at the top of a tower. A heat transfer fluid heated in the receiver is
used to generate steam, which, in turn, is used in a conventional turbine-generator to
produce electricity.
Fig: Power Tower System

6. Early power tower utilized steam as the heat transfer fluid; current designs
(including Solar Two, shown in fig) utilize molten nitrate salt because of its superior heat
transfer and energy storage capabilities. Current European designs use air as heat transfer
medium because of its high temperature and its good hand ability. Individual commercial
plant will be sized to produce anywhere from 50 to 200 MW of electricity.
2. Solar chimneys
A solar chimney is a solar thermal power plant where air is passes under a very large
agricultural glass house (between 2 and 30 kilometers in diameter); the air is heated by the
sun and channeled upwards towards a convection tower. It then rises naturally and is used
to drive turbines, which generate electricity.
A solar chimney is an apparatus for harnessing solar energy by convection of heated
air. In its simplest form, it simply consists of a black-painted chimney. During the daytime,
solar energy heats the chimney and thereby heats the air within it, resulting in an Updraft of
air within the chimney.
The suction this creates at the chimney base can also be used to ventilate, and
thereby cool, the building below. In most parts of the world, it is easier to harness wind
power for such ventilation, but on hot windless days such a chimney can provide ventilation
where there would otherwise be none. This principle has been proposed for electric power
generation, using a large greenhouse at the base rather than relying on heating of the
chimney itself. The main problem with this approach is the relatively small difference in
temperature between the highest and lowest temperatures in the system. Carnot's theorem
greatly restricts the efficiency of conversion in these circumstances.

7. Advantages
 sunlight is free
 quick to install
 easy to add on to the system
 no pollution from energy production
 little disturbance of land
 photovoltaic cells last for several Decades
Disadvantages
 high costs at present
 need access to the Sun about 60 percent of time
 needs energy storage system
 may need energy backup system
 some homeowners do not like solar panels’ appearance
 takes 40–50 years for energy savings to make up initial cost
 manufacturing produces hazardous silicon wastes.
Prepared by
R. RamaRaj, KCET
J. S. Sakthi suriya raj, KCET
K. Selva Narayanan, KCET