Introduction To Solar Energy :-
Solar energy has greatest
potential of all the sources as renewable energy and if only a small amount of
this form of energy could be used, it will be one of the most important
supplies of energy, especially when other sources in the country have depleted.
This energy comes
to the earth from the sun. This energy keeps the temperature of the earth,
above that in colder space causes current in the atmosphere and in oceans,
causes the water cycle and photosynthesis in plants.
The solar power
where sun heats the atmosphere is 10e17 Watts,
whereas solar power on the earth’s surface is 10e16 Watts.
The total worldwide power demand of all needs of civilization is 10e13 Watts. Therefore sun gives us 1000 times more power than
that we need. If we use 5 %of this energy, it will be 50 times what the world
will require. The energy radiated by the sun on a bright sunny day is
approximately 1 kW/m², attempts have been made to make use of this energy in
raising steam, which may be used in driving the prime movers for the purpose of
electrical energy generation.
Advantages
of solar energy:-
1. Solar energy is
the very large, inexhaustible source of energy. The power from the sun,
intercepted by the earth is approximately 1.8 x 10e11 MW, which is many
thousand times larger than the present energy consumption rate on the earth of
all commercial energy sources..
2. It is one of the
most promising unconventional sources of energy.
3. Unlike fissile
fuels and nuclear power, it is an environmentally clean source of energy.
4. It is free and
available in adequate quantities in almost all parts of the world.
Disadvantages :-
1)It is the dilute
source of energy. Even in the hottest region, solar radiation flux available
rarely exceeds 1 kW/m².
2)Large collecting
areas are required in many applications.
3)Availability
varies with time. Variation in availability occurs daily due to day -night
cycles and seasonally because of earth’s orbit around the sun
4)variations occur at
a specific location because of local weather conditions.
The energy collected
when the sun is shining must be stored for the use during periods when itis
not available.
5)The needs for
storage also add significantly to the cost of any system.
6)The real challenge
in utilizing solar energy as an energy alternative is of an economic nature.
7)One has to strive for the development of cheaper methods of collection and
storage solar that large initial investment required art present in most
applications are reduced.
Classification of methods for solar
energy utilization:-
Solar energy utilization
Direct Methods
Indirect Methods
Thermal Photovoltaic 1) Water power
2)
Wind
3)
Biomass
4)
Wave energy
5)OceanTemp.
Differences
Devices for collection and storage: -
Principle of collection device-
To expose dark surface to solar radiation so that the is absorbed. The part of absorbed surface is then transferred to fluid like air or water.
Devices for collection-
1) Liquid Flat Plate
Collector
2) Cylindrical
Parabolic Concentrating Collector
3) Paraboloid
Concentrating Collector
·
Liquid Flat Plate Collector:-
A
schematic diagram of a liquid flat plate collector is shown.
It
consists of an absorber plate on which solar radiation falls after coming
through one or more transparent covers (usually made up of brass). The absorbed
radiation is partly transferred to a liquid flowing through tubes which are
fixed to the absorber plate.
This energy is transfer is useful gain. The remaining part of
radiation absorbed in the absorber plate is lost by convection and reradiation to the surrounding from the top surface, and by conduction through back and edges. The liquid mostly used is water.
Thermal applications-
1) Water Heating
2) Space Heating
3) Power Generation
4) Space Cooling and
Refrigeration
5) Distillation
6) Drying, and
7) Cooking
1)
Water Heating –
Solar water heating is one of the most attractive solar thermal applications from economic standpoint. Diagram of a simple, small capacity, natural circulation system, suitable for domestic purpose is shown
The main components of system are liquid flay plate collector and the storage tank, the tank being located above the level of collector. As water in the collector is heated by solar energy, it flows automatically to the top of water tank and it is replaced by cold water from bottom tank. Whenever this is done, cold water automatically enters the bottom.
Advantages –
1) Thermodynamic
efficiency based on considerations of availability of energy can be shown to be
higher in case of solar water heating system than water heating system using
natural gas or electricity.
2) Most of the
systems have capacities of 100 or 200
litres per day and use one or two flat plate collectors having a face area of 2
m² each.
3) The installed cost
is about Rs. 110 per litre per day and temperature of hot water delivered
ranges from 50º to 70º.
2)
Space Heating –
A
space heating system is illustrated in fig.
Water is heated in the solar
collector (A) and stored in tank (B). Energy is
transferred to air
circulating in the house by means of water to air heat exchanger (E). The pumps
(C) provide forced circulation between collector and tank, and between the tank
and heat exchanger. Provision is also made for auxiliary heat.
3) Power Generation –
Generation
of electrical power is one of the most important applications
of the energy source.
Solar thermal power cycles are classified as:
1) Low temperature cycles
–
Capacity upto
about 50 kW.
Installed cost is
about Rs. 30000/- per kW for 6 – 8 hrs. for daily
operation.
2) Medium temperature
cycles-
Capacity about 14
MW to 80 MW.
80 MW plant has
cost Rs. 90000/- per kW.
3) High temperature
cycles –
Capacity above 80
MW to 100 MW.
Cost approximately
Rs. 600000/- per kW.
4)
Space Cooling and
Refrigeration-
One of the interesting thermal
applications of solar energy is the
purpose of
cooling. Cooling is required mostly in summer. Diagram of solar operated
refrigeration system is shown.
5)
Distillation –
The principle of solar
distillation is simple and can be explained with
reference to
the following fig.
Principle:-
Evaporation
of water with the help of concentrated sunrays.
It
consists of shallow air tight basin lined with a black impervious material
which
contains saline water. A sloping
transparent cover is provided at the top. Solar radiation is transmitted to the
cover and is absorbed in the black lining. It heats up the water by about 10 to
20º to evaporate. The resulting vapour rises, condenses as pure water at the
underside of the cover and flows into condensate collection channels on the
side. An output about 3 lit/m²
with an associated efficiency of 32 to 35 % can be obtained in a well designed
still on a good sunny day.
In India, a plant
having capacity of 5000 litres a day was built in the village of Awania
near Bhavnagar
in Gujrat in 1978.
6)
Drying -
One of
the traditional uses of solar energy has been for drying of
agricultural product.
The drying process removes moisture and helps in preservation of products.
The disadvantage of drying is that
the process is slow and that dust and insect get mixed with the product.
7)
Cooking -
An
important domestic thermal application is cooking. Box type cooker
with one reflector
is shown in fig.
A single glass reflector whose
inclination can be varied easily is used as a box type cooker. The addition of
mirror helps in achieving enclosure temperatures which are higher by about 15 -
20º C. As a result, the cooking time is reduced. Cookers with reflectors on all
four edges are also built.
SOLAR RADIATION MEASUREMENT :-
Instruments :-
1) Pyranometer -
It is
an instrument which measures either global or diffuse radiation over
hemispherical
field of view.
2) Pyrheliometer -
It is
an instrument, which measures only beam radiation.
3) Sunshine Recorder -
A
sunshine recorder is shown in fig. The rays of sun are focused by a
glass
sphere to a point on a curved strip held in a groove in a spherical bowl
mounted concentrically with the sphere. When there is a bright sunshine, the
image formed is intense enough to burn a spot on a curved strip. Through the
day, as the sun moves across the sky, the image moves along the strip. Thus a
burnt trace, whose length is proportional to the duration of sunshine is
obtained on the strip.