Renewable energy sources

1. Unit 2 Solar Energy
ByMr.K.Stalin
AP/MECH/AMET UNIVERSITY

2. What is meant by Solar Energy?
Solar energy is the energy obtained by capturing heat and light from the
Sun. Energy from the Sun is referred to as solar energy.
Solar energy is an important, clean, cheap and abundantly available renewable energy.
It is received on Earth in cyclic, intermittent and dilute form with very low power
density 0 to 1 kW/m2.
Units of solar power and solar energy:
• In SI units, energy is expressed in Joule(W/m2 or kW/m2).
• Other units are angley and Calorie where 1
1 angley = 1 Cal/cm2.day 1Cal = 4.186 J
• For solar energy calculations, the energy is measured as an hourly or monthly
or yearly average and is expressed in terms of kJ/m2/day or kJ/m2/hour.

3. Energy from the sun:
The sun radiates about 3.8 x 1026 W of power in all the directions.
Out of this about 1.7 x 1017 W is received by earth.
The average solar radiation outside the earth’s atmosphere is 1.35 kW/m2 varying from
1.43 kW/m2 (in January) to 1.33 kW/m2 (in July).

4. Essential subsystems in a solar energy plant:
1. Solar collector or concentrator:
It receives solar rays and collects the energy. It may be of following types:
a) Flat plate type without focusing
b) Parabolic trough type with line focusing
c) Paraboloid dish with central focusing
d) Fresnel lens with centre focusing
e) Heliostats with centre receiver focusing
2. Energy transport medium:
Substances such as water/ steam, liquid metal or gas are used to transport the thermal
energy from the collector to the heat exchanger or thermal storage. In solar PV systems
energy transport occurs in electrical form.

5. 3. Energy storage:
Solar energy is not available continuously. So we need an energy storage medium for
maintaining power supply during nights or cloudy periods. There are three major types of
energy storage:
a) Thermal energy storage;
b) Battery storage;
c) Pumped storage hydro-electric plant.
4. Energy conversion plant:
Thermal energy collected by solar collectors is used for producing steam, hot water, etc. Solar
energy converted to thermal energy is fed to steam thermal or gas-thermal power plant.
5. Power conditioning, control and protection system:
Load requirements of electrical energy vary with time. The energy supply has certain
specifications like voltage, current, frequency, power etc.
6. Alternative or standby power supply: The backup may be obtained as power from electrical
network or standby diesel generator.

6. SOLAR COLLECTORS
SOLAR COLLECTORS
Solar collectors are used to collect the solar energy and convert the incident radiations into
thermal energy by absorbing them. This heat is extracted by flowing fluid (air or water or
mixture with antifreeze) in the tube of the collector for further utilization in different
applications.
The collectors are classified as;
• Non concentrating collectors
• Concentrating (focusing) collectors

7. • Flat- Plate collector
• Air heater collector
• Concentrating collector
Solar Collectors:

8. Flat-plate collector:
A typical flat-plate collector is a metal box with a glass or plastic cover
(called glazing) on top and a dark-colored absorber plate on the bottom.
The sides and bottom of the collector are usually insulated to minimize
heat loss.

9. The main components of FLAT-PLATE SOLAR COLLECTOR:
Black surface - absorbent of the incident solar energy
Glazing cover - a transparent layer that transmits radiation to the absorber, but prevents
radiative and convective heat loss from the surface
Tubes containing heating fluid to transfer the heat from the collector
Support structure to protect the components and hold them in place
Insulation covering sides and bottom of the collector to reduce heat losses

10. LIQUID FLAT PLATE SOLAR COLLECTORS

11. The flat-plate systems normally operate and reach the maximum efficiency within the
temperature range from 30 to 80oC ,however, some new types of collectors that employ
vacuum insulation can achieve higher temperatures (up to 100oC). Due to the introduction
of selective coatings, the stagnant fluid temperature in flat-plate collectors has been shown
to reach 200oC.
Transport fluid options:
Water is one of the common options as liquid fluid due to its accessibility and good
thermal properties:
• It has a relatively high volumetric heat capacity
• It is incompressible (or almost incompressible)
• It has a high mass density (which allows using small tubes and pipes for transport).
One disadvantage of water is that it freezes during winter, which can damage the collector
or piping system. Antifreeze mixtures such as ethylene glycol or propylene glycol can be
used instead of pure water to alleviate the problems.

12. Open and Closed Loop System

13. • Solar flat plate collectors usually range in size from about 32 square feet (4
x 8 ft) or 3 square metres.
• Weigh more than 100 kilo-grams each.
• One square foot (1000 cm2) heats about 10 litres of water per day to well
over 70oC.
• Therefore a single panel of 20 to 30 square feet will heat about (300 litres)
of water which is about the size of a standard hot water storage tank.

14. The main advantages of flat plate collectors are:
• Easy to manufacture
• Low cost
• Collect both beam and diffuse radiation
• Permanently fixed (no sophisticated positioning or tracking equipment is required)
• Little maintenance
Disadvantages
• Large heat losses by conduction and radiation because of large area.
• No tracking of sun.
• Low water temperature is achieved.