What is Solar Energy? Solar Photovoltaic Cells | Solar Thermal Power Plant

Solar energy is the abundant (virtually unlimited) source of renewable energy. If harnessed properly it could meet the present global energy demand.

There is a gradual depletion of fossil fuels and sharply increasing demand of energy. So the solar energy could well become an alternative source of energy to meet the future demand as earth receives 1.8 x 10¹¹ MW of solar power while the present global energy demand is about 1.6 x 10⁷ MW. 

The geographical location of India is ideal for solar energy. The country receives about 300 clear days in a year, which could produce enough power for more than total energy consumption of India.

Solar energy can be utilised in two ways, namely, solar thermal and solar photovoltaic. Solar thermal energy is used for cooking, heating, drying, timber seasoning, distillation, cooling, refrigeration, cold storage, and also for electricity generation. Some of the solar thermal devices are solar cooker, solar collectors, solar hot water systems, solar pond, solar hot air system, solar driers, solar still, solar air-conditioners, etc.

Solar energy is used for generating electricity by photovoltaic systems. Solar photovoltaic cells convert sunlight directly into electricity. This electricity can either be used directly or can be stored in batteries. This stored energy in batteries can be used during night hours when there is no sunlight.

Solar photovoltaic (SPV) systems can be used for a number of applications such as, rural electrification, water pumping, desalination, solar lighting, etc.

SOLAR ENERGY SUPPLY

Solar energy comes from the sun. The distance of sun from earth is 1.5 x 10⁸ km. So, how energy from sun comes to earth? Sun is a huge sphere of gaseous matter of diameter 1.39 x 10⁹ m and has temperature varying from 8 x 10⁶ to 40 × 10⁶ degree Kelvin. All substances above absolute temperature emit energy in the form of electromagnetic waves. Due to very high temperature, sun emits energy in the form of electromagnetic wave called solar radiation. These waves are transmitted to earth in the form of photons at a speed of light, i.e., 3 x 10⁸ m/s Photons are electromagnetic waves having packet of energy. Total energy received on earth has three spectral regions according to wavelength (λ).

1. Ultraviolet    6.4%       (λ < 0.38 μm)

2. Visible          48%    (0.38 μm < λ < 0.78 μm)

3. Infrared      45.6%         (λ > 0.78 μm)

Energy received from sun before entering earth's atmosphere is constant. This is called extraterrestrial radiation. It is expressed as solar constant and has value of 1367 w/m².

Solar radiation then passes through earth's atmosphere where it is scattered and some parts are absorbed. Ultraviolet rays of short wavelength are absorbed by ozone layer. Infra-red rays of long wave length are absorbed by CO₂ and water vapour. Scattering is due to air molecules, dust particles and water droplets. Thus solar radiation decreases while passing through earths atmosphere. It is called attenuation of radiation. 

The solar radiation in earth's atmosphere has following radiation: 

1. Beam radiation (I_b): It is the radiation received on earth's surface without change in direction. It is also called direct radiation.

2. Diffuse radiation (I_d): Scattered radiation received is called diffuse radiation. 

3. Albedo radiation (I_a): Radiation reflected by the ground or other objects on the ground.

4. Global radiation (I_g): I_g = I_b + I_d + I_a 

Solar radiation passes through different distance through atmosphere depending on position of sun and corresponds to different solar radiation intensity as listed below:

Symbol	Position of Sun	Solar Radiation Received W/m²
AM0	Extraterrestrial Radiation	1367 W/m²
AM1	Sun is overhead (also called zenith)	1105 W/m²
AM1.5	Sun is at 48° from zenith	1000 W/m²
AM2	Sun is at 60° from zenith	894 W/m²

About 30% of total solar radiation energy is reflected back to space by reflection from cloud by scattering and by reflection from earths surface. This is called albedo.

The amount of solar radiation available at a given location and time depends on latitude, longitude of location, time of day, month. Solar radiation increases from sunrise to noon and then decreases from noon to sunset.

Alternatively, the energy received by the land in one hour

= 1105 x 10⁴ watt-hour

= 11050 kWh

SOLAR PHOTO-VOLTAIC CELLS

Construction of Solar Cell

The structure of a typical solar photo-voltaic cell is shown in the Fig. 1.

Fig. 1. Solar Photo-Voltaic Cells

The main components of a solar photo-voltaic cell are:

(i) p-type material

(ii) n-type material

(iii) p-n junction

(iv) front metallic grid 

(v) back metal contact

(vi) anti-reflective coating

The p-type material is made of silicon and has a thickness of about 100 to 300 microns depending on technology. A thin layer of n-type silicon is formed at the top of the surface of p-type material by diffusing with an impurity like phosphorous which forms a p-n junction.

The n-type layer has a thickness of about 0.2 micron. The top surface of n-type material has a metallic grid structure to collect the current generated by falling photons and forms the negative terminal of the solar cell. It is in the form of thin metallic grid to allow the maximum photons to strike. The back metal contact is continuous layer which forms the positive terminal of the cell. When sunlight falls on the front surface, a major portion is reflected back. To prevent this an antireflective coating is provided on the front surface of the cell so that most of the light is absorbed by the cell. The blue colour of the cell is due to the anti reflective coating.

Working Principle

A solar cell is a p-n junction diode under light illumination over the surface of the cell. When solar radiation is absorbed by p-n junction, electron-hole pairs are formed. The generation of electron-hole pairs will depend on solar radiation. The electrons from p-side will move to n-side and holes from n-side will move to p-side, so there is net positive charge on p-side and net negative charge on n-side.

This build-up of positive and negative charges produces a potential difference across the p-n junction due to light falling on it. This potential difference is called photo-voltage. The generation of photo-voltage due to light illumination is called photo-voltaic effect. Thus, a solar cell works on the principle of photo-voltaic effect. For a silicon p-n junction voltage generated is 0.5 to 0.6 volt.

I-V Characteristic

When there is no light falling on the solar cell there is no photo-voltage. When light falls on the solar cell photo-voltage is generated and current flows if a load is connected across the terminals of a solar cell. The I-V characteristic of a solar photo-voltaic cell is shown in the Fig. 2.

Fig. 2. I-V Characteristics

The four main parameters of the I-V characteristic of a solar cell are short-circuit current I_sc, open circuit voltage V_oc, maximum current I_m and maximum voltage V_m. V_m and I_m correspond to the maximum power point (MPP) P_m as shown in the Fig. 2.

I_sc is the current flowing through the solar cell when its terminals are short-circuited. V_oc is the potential difference across the open-circuited terminals of the solar cell. Ideal power (P_o) is the product of V_oc and I_sc.

P_o = V_oc × I_sc

Maximum power (P_m) that can be extracted by a solar cell is the product of V_m and I_m.

P_m = V_m × I_m

Fill Factor (FF) : It is defined as the ratio of maximum power P_m to ideal power P_o.

FF = P_m/P_o = V_m.I_m/V_oc.I_sc

Efficiency of a Solar Cell

The efficiency (η) of a solar cell is the ratio of output power (P_m) to input solar radiation power (P_rad).

η = P_m/P_rad

Standard solar radiation power (P_rad) is 1000 W/m² at a temperature of 25°C. Power ratings of the solar cells are based on this standard solar radiation. However, a solar cell will give less power because radiation falling on it is less than this standard.

Also the operating temperatures of a solar cell affects the performance of it. Solar cell produces less power as temperature increases beyond 25°C. A solar pv panel rated at 100 watts at standard temperature of 25°C will be an 83 watt panel at 43°C.

Comparison Between Solar Cell and Battery

Solar Cell	Battery
1. It converts solar radiation energy into electrical energy.	1. It converts chemical energy into electrical energy.
2. Cost of solar PV panel is higher than that of a battery.	2. Cost is lower.
3. Less maintenance required.	3. Regular maintenance required.
4. The voltage output is in the range of 0.5 to 0.6 volt	4. The voltage output is 1.2 V for alkaline batteries and 2.1 V for lead acid battery.
5. It is a semiconductor device having p and n-type materials forming a p-n junction.	5. It has two electrodes in a solution called electrolyte.
6. Size of solar cell I compact.	6. Size of battery is bulky.

Merits of Solar Photo-voltaic Cells

1. The energy of the solar radiation is directly converted into electrical energy.

2. It is a solid state semiconductor device and has no moving so no noise pollution.

3. It is easy to operate.

4. The maintenance cost is low.

5. It has long life span about 25 years.

Demerits of Solar Photo-voltaic Cells

1. Efficiency of a solar cell is very low and is about 15%.

2. Output from a solar photo-voltaic cell is d.c. so additional electronic device like inverter is required to convert it into a.c. 

3. The capital cost of a solar photo-voltaic system is high.

4. The disposal of solar photo-voltaic cell poses problem due to the presence of arsenic and cadmium.

5. Solar photo-voltaic cells cannot produce electricity when there is no sunlight as during night hours and cloudy days.

SOLAR THERMAL POWER PLANT

A solar thermal power plant utilises heat energy of solar radiation to generate electrical energy. The schematic diagram of a solar thermal power plant using a flat-plate solar collector is shown in the Fig. 3.

Fig. 3. Solar Thermal Power Plant

The main parts of the solar thermal power plant are:

• Flat plate solar collector
• Hot water tank
• Heat exchanger
• Turbine
• Generator
• Condenser
• Circulating pumps
• Cooling tower

When solar radiation falls on an array of solar collectors, it will heat the water passing through the water channel inside it. Cold water is fed to the collector from the water tank and hot water from the outlet of the collector is supplied to the tank. Thus the water is circulated through hot water tank and collector with the help of circulating pump. This increases the temperature of water in the hot water tank upto 90°C. The tank is air insulated to prevent heat loss.

The hot water from the tank passes through a heat exchanger with the help of a hot water circulating pump and gives its heat energy to the working fluid used in the turbine. The working fluid is methyl chloride (CH₂Cl) or butane (C₂H₁₀) which has a boiling temperature below 90°C. When the fluid gets heated in the heat exchanger the vapour is formed which flows to a turbine working on Rankine cycle. The vapour expands over the turbine which utilises heat energy of the vapour to produce mechanical energy. An ac generator is coupled with the turbine. The generator converts the mechanical energy of the turbine into electrical energy. The vapour is condensed at the outlet of the turbine with the help of condenser. This increases overall efficiency of the turbine. Finally the organic fluid is pumped back from the condenser to the heat exchanger and the is repeated. The overall efficiency of this plant is about 2%.

Such type of a power plant has electrical power upto 150 kW capacity. However, electrical power of 10 MW and higher can be obtained by adopting concentrated type of collector where temperature of 400°C is obtained.

SOLAR ENERGY UTILISATION TECHNIQUES

Solar energy can be utilised for different useful purposes by adopting different techniques. The main techniques involved are of two types, namely, solar thermal conversion system and solar photo-voltaic system.

In solar thermal conversion system, energy of solar radiation is converted into heat energy which can be utilised for different purposes. In photo-voltaic system the energy of solar radiation is converted directly, into electrical energy which can be used for power generation, lighting, heating, charging batteries, etc.

Solar Thermal Utilisation Techniques 

Applications of solar energy based on solar thermal conversion system are:

• Solar pond
• Solar water heating
• Space cooling
• Refrigeration
• Solar dryer
• Solar distillation
• Solar cooker
• Solar thermal power plant

(1) Solar Pond

It is an artificially constructed pond, which is designed for collecting and storing solar heat. In the lower regions of the solar pond significant temperature increase is achieved. 

So the solar energy is stored in the thermal form but in the low grade (60 to 100°C). This low temperature energy can directly be used for space heating or industrial process heat. For other applications Rankine cycle engines may be used.

The schematic diagram of a non-convective solar pond is shown in the Fig. 4. In this type of solar pond the bottom of the pond is blackened to serve as an absorber. The bottom layer of salty water above the blackened surface may attain the temperature as high as 70 to 85°C. while the top layer of fresh water is at the normal temperature (about 27°C). The non-convective layer in between the two layers acts as a thermal insulator.

Fig. 4. Non Convective Solar Pond

The salty water of the bottom layer heats up. This heated water does not rise and stays trapped below. This is due to the density of salty water at the bottom layer is higher than that of water above.

This hot water from the bottom is taken out from the pond and used to evaporate an organic working fluid in a heat exchanger and then supplied back to the pond. The organic working fluid produces mechanical power in a rankine cycle which then drives the turbine. Coupled to this turbine an ac generator produces electricity. Some solar pond sites are mentioned below:

(i) A solar pond of an area of 1200 m² is located in Bhavnagar. Gujrat which was built in 1973.

(ii) Worlds second largest solar pond with dimensions of 100 m (length) x 60 m (width) x 3.5 m (depth) is located in Bhuj, Gujarat. It provides 90,000 litres of hot water at temperatures about 80°C daily.

(2) Solar Water Heating

Solar water heating system is used for heating of water with the help of energy of solar radiation. The detailed description is already given in the chapter. Over 3000 solar water heating systems of different capacities in the range of 100 litres per day to 10,000 litres per day are in operation in different parts of the country. These systems have following benefits:

• Solar energy is locally available and is free of cost.
• Solar energy is pollution free.
• Solar water heating systems do not require extra space as they can be mounted on roof tops, windows, etc. 
• Their installation, operation and maintenance is easy.
• Solar water heating systems of capacity upto about 150 litres per day (lpd) do not require circulating pump.

(3) Solar Dryers

Solar dryer is a solar thermal device which uses solar energy for drying of foodgrains and other agricultural products. Natural convection solar dryer consists of an enclosure with a transparent cover on the top and the sides. The internal surfaces of the enclosure are blackened. The product to be dried is placed inside the enclosure. The evaporation of moisture from the product takes place due to direct absorption of solar radiation by the product and heating of internal surfaces by the radiation incident on them.

Solar dryers are hygienic, free from dirt, dust, insects and also unaffected by adverse weather conditions. Also they do not need any fuel or electricity for drying. Because of these advantages solar dryers are replacing the traditional (open to sun drying) method in some parts of the country.

(4) Solar Distillation

Production of potable water from the saline water by distillation is already practised. There are various commercial desalination plants which use either fossil fuels or electricity as the energy source. As better option we have to use distillation system using solar energy. So, solar water still, a device to produce potable water by utilising solar heat energy can be used.

Distillation of saline water is done by exposing thin layers of the saline water to solar radiation, and condensing the water vapour deposited on a transparent cover and then collected in receiving troughs.

Some of the solar distillation plants are listed below:

• Salt works, Bhavnagar is situated in Gujrat having capacity of 1000 m³ per day is mainly used for drinking purpose for workers.
• Bitra Island, Lakshadweep has a solar distillation plant having capacity of 2000 m³ per day in an evaporating area of 750 m².
• Awania village has a solar distillation plant near Bhavnagar, Gujarat having capacity of 5000 m³ per day. It supplies drinking water to the village where saline water has a TDS (total dissolved salt) of 4500 Parts Per Million (PPM) and fluoride 10 PPM.
• A solar distillation plant at Bhaleri in Churu district of Rajasthan has capacity of 8000 m³ per day. Saline water has TDS of 3800 PPM, nitrates 340 PPM, fluoride 5 PPM.

(5) Solar Thermal Power Plant

Solar thermal power plant utilizes heat energy of solar radiation to generate electrical energy. The detailed description of this plant is already discussed in Article 2.13.

Solar PV Utilisation Techniques

Application of solar energy based on solar photo-voltaic system are:

• Solar PV water pumping system.
• Solar lantern.
• Solar street light.
• Solar home lighting system.
• Solar PV power plant.
• Solar PV power in satellite.

MERITS AND DEMERITS OF SOLAR ENERGY

Merits

1. Solar energy is a renewable energy source. The solar energy systems use radiation from the sun which is inexhaustible.
2. Solar energy does not use any fuel, so there is no pollution of air and water. It is clean source of energy.
3. The areas where there is no supply of electricity particularly the inaccessible areas, remote areas without grid connectivity can utilise solar energy for useful purposes.
4. Source of solar energy is vast in nature. The solar power received by earth is about 1.8 × 10¹¹ MW.

Demerits

1. Solar power depends on sunshine. It is not available during night hours and in cloudy days.
2. Intensity of solar radiation varies during the daytime. Maximum solar radiation is available only for 5 to 6 hours in a day.
3. The disposal of solar photo-voltaic cells causes problem due to the presence of arsenic and cadmium.
4. Solar energy is distributed and not concentrated, therefore, it is dilute form of energy source.
5. Solar reflectors are hazardous to eyesight.