Solar Energy

 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.

 Solar Photovoltaic Cell –

 When light falls on the photoemissive metals like Silicon, Germanium etc., they start emissing electrons, thereby creating the flow of current. This is known as photovoltaoic effect. This electricial energy can be stored in the form of charged batteries and can be used further for so many requirements. Present research on photovoltaic is immensely growing, still the cost of photovoltaoic cells has been remained more or less constant only.