Distributed solar collector

 

It is distributed system of individual solar collector modules interconnected to a single power producing unit by the means of a heat transfer medium. This is also known as Parabolic Trough Systems.

In distributed receiver power plants, parabolic trough collectors with line focus are most commonly used. The sun rays are reflected by parabolic or cylindrical troughs. The reflected rays are focused on linear conduit (pipe) located along the axis of the trough.

Parabolic Trough Systems use a linear parabolic concentrator with a mirrored surface to focus solar radiation on an absorber pipe running along the focal line of the parabola. The absorber pipe contains the Heat Transfer Fluid or HTF which is heated and pumped to the steam generator. The steam generator is in-turn connected to a steam turbine. For change of the daily position of the sun perpendicular to the receiver, the trough tilts east to west so that the direct radiation remains focused on the receiver. However, seasonal changes in the in angle of sunlight parallel to the trough does not require adjustment of the mirrors, since the

Figure 2.12 - Solar thermal Electric power plant using distributed solar collectors

light is simply concentrated elsewhere on the receiver. The receiver can reach temperatures of 400 °C and generates live steam to drive the steam turbine, which coupled with generator.

Solar Pond Electric Power Plant

 

A solar pond can be used to generate electricity by driving a turbine using an organic fluid. Even low temperatures heat that is obtained from solar pond can be converted into electric power. The conversion efficiency is limited due to its low operating temperatures (70-100ºC). Because of low temperature, the solar pond power plant requires organic fluid which have low boiling points such as halo-carbons (like Freons) or hydrocarbons (such as propane).

The figure shows solar pond electric power plant. The heat trapped in solar pond can be used to evaporate organic fluid in evaporator. This organic fluid vapour is expanded in turbine, which is coupled with electric generator. The organic fluid is condensed in condenser and pumped to evaporator to continue the cycle.

 

Figure 2.11 –Solar Pond Electric Power Plant

Advantages of Solar pond

 

1. The heat storage is massive, so energy can be extracted day and night - hence it is a source of 'base load' solar power  - no batteries or other storage needed

2. Solar ponds can have very large heat collection area at low cost.

3. The major production potential is during peak electrical power demand (and price) in mid summer

Solar pond

 

A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy. A solar pond can be used for various applications, such as process heating, desalination, refrigeration, drying and solar power generation.

Figure 2.10 – Solar Pond

Solar pond collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.

A solar pond has three distinctive zones.

· The top layer is the surface zone that has a low salt content and is at atmospheric temperature. It is also called the upper convective zone (UCZ)

· The bottom layer has a very high salt content and is at a high temperature, 70°C-90°C. This is the zone that collects and stores solar energy in the form of heat and it is called the lower convective zone (LCZ).

· There is an intermediate insulating zone with a salt gradient. It establishes a density gradient that prevents heat exchange by natural convection, and hence it is called the nonconvective zone (NCZ). In this zone, salt content increases with depth, creating salinity.

When solar energy is absorbed in the water, its temperature increases, causing thermal expansion and reduced density. If the water were fresh, the low-density warm water would float to the surface, causing a convection current.

The temperature gradient alone causes a density gradient that decreases with depth. However the salinity gradient forms a density gradient that increases with depth, and this counteracts the temperature gradient, thus preventing heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C.

The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive an organic Rankine cycle turbine or Stirling engine for generating electricity.

Solar distillation

 

There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available.

Solar distillation is one of the "simplest' of applications of solar energy. Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water vapour from dissolved matter, which is condensed as pure water.

Solar water distillation is a solar technology with a very long history and installations were built over 2000 years ago, although to produce salt rather than drinking water. Documented use of solar stills began in the sixteenth century. An early large-scale solar still was built in 1872 to supply a mining community in Chile with drinking water. Mass production occurred for the first time during the Second World War when 200,000 inflatable plastic stills were made to be kept in life-crafts for the US Navy.

The energy required to evaporate water is the latent heat of vaporisation of water. This has a value of 2260 kilojoules per kilogram (kJ/kg). This means that to produce 1 litre (i.e. 1kg since the density of water is 1kg/litre) of pure water by distilling brackish water requires a heat input of 2260kJ. This does not allow for the efficiency of the heating method, which will be less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.

It should be noted that, although 2260kJ/kg is required to evaporate water, to pump a kg of water through 20m head requires only 0.2kJ/kg. Distillation is therefore normally considered only where there is no local source of fresh water that can be easily pumped or lifted.

Figure 2.9 – Solar Distillation

 

In the simplest arrangement, untreated water is placed in an air tight solar collector with a sloped glazing material, and as it heats and evaporates, distilled water condenses on the collector glazing, and runs down where it can be collected in a tray. Solar distilled water is proven to be free from bacteria, microbes of all kinds, harmful /toxic chemicals and heavy metals. 

Solar Greenhouse

 

A greenhouse is a building in which plants are grown. These structures range in size from small sheds to very large buildings. A miniature greenhouse is known as a cold frame.

A greenhouse is a structure with different types of covering materials, such as a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming visible solar radiation (for which the glass is transparent) from the sun is absorbed by plants, soil, and other things inside the building.

 

Figure 2.8 – Solar Green house

Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. In addition, the warmed structures and plants inside the greenhouse re-radiate some of their thermal energy in the infrared spectrum, to which glass is partly opaque, so some of this energy is also trapped inside the glasshouse.

However, this latter process is a minor player compared with the former (convective) process. Thus, the primary heating mechanism of a greenhouse is convection. Thus, the glass used for a greenhouse works as a barrier to air flow, and its effect is to trap energy within the greenhouse. The air that is warmed near the ground is prevented from rising indefinitely and flowing away.Although heat loss due to thermal conduction through the glass and other building materials occurs, net energy increases (and therefore temperature) inside the greenhouse.

Greenhouses can be divided into glass greenhouses and plastic greenhouses. Plastics mostly used are PEfilm and multiwall sheet. Commercial glass greenhouses are often high-tech production facilities for vegetables or flowers. The glass greenhouses are filled with equipment such as screening installations, heating, cooling, lighting, and may be automatically controlled by a computer.

Because greenhouses allow certain crops to be grown throughout the year, greenhouses are increasingly important in the food supply of high latitude countries.

Solar furnace

 

A solar furnace is any device that creates heat by concentrating solar radiation through the use of reflectors. Solar furnace is a structure that captures sunlight to produce high temperatures, usually for industry. This is done with a curved mirror (or an array of mirrors) that acts as a parabolic reflector, concentrating light (Insolation) onto a focal point. The temperature at the focal point may reach 3,500 °C and this heat can be used to generate electricity, melt steel, make hydrogen fuel or nanomaterials

The heat produced by large solar furnaces can even melt rock, steel, or used to produce hydrogen fuel.

 

Figure 2.7 – Solar furnace

The operation principle of a solar furnace is very simple requiring the use of mirrors called heliostats. The mirrors are angled at a focal point, which increased the intensity of sunlight to approximately threefold. The reason for this is the fact that the focal point is the concentrated light from the sun as well as the two mirrors.

Solar Absorption air-conditioning system

 

This system has been the basis of most of the experience to date with solar air- conditioning. To begin with, the solar energy is gained through the collector and is accumulated in the storage tank. Then, the hot water in the storage tank is supplied to the generator to boil of water vapor from a solution of lithium bromide+water.

The water vapor is cooled down in the condenser and then passed to the evaporator where it again is evaporated at low pressure, thereby providing cooling to the required space. Meanwhile, the strong solution leaving the generator to the absorber passes through a heat exchanger in order to preheat the weak solution entering the generator. In the absorber, the strong solution absorbs the water vapor leaving the evaporator. Cooling water from the cooling tower removes the heat by mixing and condensation. Since the temperature of the absorber has a higher influence on the efficiency of the system than the condensing temperature, the heat-rejection (cooling water) fluid, is allowed to flow through the absorber first and then to the condenser.

An auxiliary energy source is provided, so that the hot water is supplied to the generator when solar energy is not sufficient to heat the water to the required temperature level needed by the generator. In the solar powered absorption air-conditioning system, it is very much essential to have a hot water storage . It serves as a buffer reservoir to have nearly constant heat input. Similar to the hot water storage tank, a chilled water storage tank is often used in the solar powered air-conditioning system.

 

Figure 2.6 - Solar Absorption air-conditioning system

In the heating season, the hot water is directly provided from the hot water storage to the fan-coil of the air-conditioned space, or/and to places where the heat is used for bathing or other domestic applications.

Solar Refrigeration and Air conditioning system

Solar Refrigeration and Air conditioning system

Solar air conditioning refers to an air conditioning (cooling) system that uses solar power, power from the heat obtained from Sun’s rays.

The technologies that are being developed for gas cooling systems are the same ones being developed for active solar space cooling systems. Desiccant cooling systems and advanced absorption systems are the primary technologies that are used.

1. Desiccant system

A moisture absorbing material (desiccant) is located in the air stream going into the living space. As the air passes through the desiccant, which is usually located on a wheel that slowly rotates into the air stream, moisture is removed from the air, dropping the humidity level in the air stream to the point that an evaporative cooler can then cool the air. The desiccant is dried by the heat generated by the solar collectors as it rotates out of the air stream.

2. Absorption system

Heat from solar collectors separates a low boiling refrigerant in a generator which receives the pressurized refrigerant from an absorber. Solar heat can also be used in the evaporation stage of the cycle.

Advantages and Disadvantages of Solar Water Heater

 

Advantages:

1. Safe operation

2. It conserves energy

3. It protects enviroment

4. It can be used as auxiliary heater in electric heaters

5. Solar water heater can be used around the clock throughout the year.

Disadvantages

1. Installation complicated

2. Maintenance more troublesome

3. Initial investment required

4. Solar water heaters are weather dependent.

Solar water heater

 

Solar water heating systems use free heat from the sun to warm domestic hot water. It absorbs the energy from the sun in the collector panels located on the roof of home.

The sun's energy is then transferred either directly or indirectly (depending on the system) to the water stored in the hot water cylinder. When there is not sufficient energy from the sun to heat the water in the cylinder a booster system (either electric, gas or wetback) is used to heat the water to the required temperature.

A solar water heating system consists of three main parts:

1. Solar hot water collector - which are located on the roof

2. Hot water tank which can be also located on the roof or on the ground (a split system)

3. Pumps and controller

There are two types of solar water heating systems:

1. Active system which have circulating pumps and controls (Forced flow type)

2. Passive system, which don't have circulating pumps and controls (Thermosyphon type)

Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.

There are two main types of solar water heater collectors, either flat panel or evacuated tube. Flat panel systems collect the suns rays via a metal plate with a dark coating. They work best with direct sunshine. Evacuated tube systems collect the suns rays in glass tubes with a vacuum inside. They are generally more efficient than flat panel systems because they trap heat even in low sunlight.

 

 

Geographic location, sunshine hours, home orientation and roof angle must be taken into account when choosing a suitable solar water heating system.

1. Active System

Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are also more efficient.

2. Passive system

Passive systems move water through the system without pumps. Passive systems have no electric components to break. This makes them generally more reliable, easier to maintain, and possibly longer lasting than active systems. Passive systems can be less expensive than active systems, but they can also be less efficient.

A thermosiphon system relies on warm water rising, a phenomenon known as natural convection, to circulate water through the collectors and to the tank. In this type of installation, the tank must be above the collector. As water in the collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system. The storage tank is attached to the top of the collector so that thermosiphoning can occur.

Merits and Demerits of solar cooker

 

Merits

1. No attention is needed during cooking as in other devices.

2. No fuel is required.

3. Negligible maintenance cost.

4. No pollution.

5. Vitamins of the food are not destroyed and food cooked is nutrition and delicious with natural taste.

6. No problem of over flowing of food.

Demerits

1. One has to cook according to the sunshine, the menu has to preplanned.

2. One cannot cook at short notice and food cannot be cooked night or during cloudy days.

3. It takes comparatively more time.

4. Chapaties are not cooked because high temperature for baking is required.

Box type Solar Cooker

 

The principal of operation of box type solar cooker is illustrated in fig 2.4. The solar rays penetrate through the glass covers and absorbed by a blackened metal tray kept inside the solar box. The solar radiation entering the box are of short wave length. Two glass covers are provided to minimize the heat loss. The loss due to convection is minimized by making the box air tight by providing a rubber strip all around between the upper lid and the box.

 

Figure 2.4 – Box Type Solarcooker

Insulating materials like glass wool, saw dust or any other material is filled in the space between blackened tray and outer cover of the box. This minimize heat loss due to conduction with type of cooker is placed in the sun the blackened surface starts absorbing sun rays and temperature inside the box starts rising. The cooking pots, which are also blackened are placed inside with food material get heat energy and food will be cooked in a certain period of time depending upon the intensity of solar radiation and material of insulation provided. The amount of solar radiation intensity can be increased by provided mirror or mirrors.

The solar cooker is made up of inner and outer metal or wooden box with double glass sheet on it. The top cover contains 3mm thick two plain glasses fixed on wooden or metal frame, keeping about 25mm distance between the two. The entire top cover can be made tight with padlock hasp. Neoprene rubber sealing is provided around the contact surfaces of the glass cover and hinged on one side of the glass frame. A mechanism (guide for adjusting mirror) is provided to adjust the reflector at different angles with the cooker box when the reflector is adjusted to shine in the cooker box, 115^oC to 125^oC.Temperature is achieved inside the cooker box. Addition of the reflector is useful in cooking earlier particularly in winter. The solar cooker is able to cook about 1.25kg dry food materials, which is enough for a family of 5 to 7 persons. The total weight of the cooker is about 22kgs. Overall dimensions of a typical model are 60x60x20cm height.

Types of Solar Cooker

 

Basically there are three designs of solar cooker

1. Flat plate box type solar cooker with or without Reflector.

2. Multi Reflector type solar cooker.

3. Parabolic disc concentrator type solar cooker.

Flat plate box type design is the simplest of all the designs. This cooker allows solar radiation to enter through a double walled glass cover placed inside a blackened box which is well insulated and made airtight. Maximum no load temperature with a single reflector reaches up to 160^oC. In Multi Reflector type, four square or triangular or rectangular reflectors are mounted on the oven body. They all reflect the solar radiation into the cooking zone in which cooking utensils are placed. Temperature obtained is of the order of 200^oC. The maximum temperature can reach to 250^oC if the compound cone reflector system is used.

In parabolic type cooker, parallel sun’s rays are made to reflect on a parabolic surface and concentrated on a focus on which the Utensils for cooking are placed. The temperature of the order of 450^oC can be obtained in which solar radiation are concentrated on to a focal point

Solar Cooker

 

In our country energy consumed for cooking shares a major portion of the total energy consumed in a year. In villages 95% of the consumption goes only to cooking. Variety of fuel like coal, kerosene, cooking gas, firewood, dung cakes and agricultural waste are used. The poor of the developing countries who have been using dry wood, picked up from the fields and forests as domestic fuel, have been affected in their own way, due to scarcity of domestic fuel in the rural areas. The supply of wood is fast depleting because of the indiscriminate felling of trees in the rural areas and the denudation of forests. There is a rapid deterioration in the supply of these fossil fuels like coal, kerosene or cooking gas. The solution for the above problem is the harnessing of solar energy for cooking purpose.

The most important is that the solar cooker is a great fuel saver. It is calculated that a family using a solar cooker 275 days a year would save 800kgs of fire wood or 65 liters of kerosene. Similarly an industrial Canteen or a Hostel mess using the larger community solar cooker which can cook for 20 to 25 people could save 400kgs of fire wood or 335 liters of kerosene per year.

Comparison Between Flat Plate collector and concentrating type collector

 

Sl.no	Flat Plate collector	Concentrating type collector
1	It is less efficient solar collector	It is the most powerful type of collector.
2.	Maximum Temperature of fluid is 300oC	Fluid temperatures up to around 5000oC can be achieved
3	It can be used in water heating.	It can be used in solar furnaces and solar power plants.

Concentrating (focusing) type solar collector

 

Focusing collector or concentrating type solar collector is a device to collect solar energy with high intensity of solar radiation on the energy absorbing surface. Such collectors generally use optical system in the form of reflectors or refractors. A focusing collector is a special form of flat-plate collector modified by introducing a reflecting or refracting surface between the Solar Radiation and the absorber. In these collectors radiation falling on a relatively large area is focused on to a receiver or absorber of considerably smaller area. As a result of the energy concentration, fluids can be heated to temperatures of 5000 ^oC or more.

 

Figure 2.3 - Concentrating (focusing) type solar collector

A Schematic diagram of a typical concentrating collector is shown in fig. The collector consists of a concentrator and a receiver. The concentrator shown is a mirror reflector having the shape of a cylindrical parabola. It focuses the sunlight on to its axis where it is absorbed on the surface of the absorber tube and transferred to the fluid flow through it.

A concentric glass cover around the absorber tube helps in reducing the convective and radiative losses to the surroundings. In order that the sun’s rays should always be focussed on to the absorber tube, the concentrator has to be rotated. This movement is called tracking in the case of cylindrical parabolic concentrators, rotation about a single axis is generally required. Fluid temperatures up to around 5000 ^oC can be achieved in cylindrical parabolic focussing collector system.

Flat plate collectors

 

Flat plate collectors are used for heating water and nonfreezing aqueous solutions. They are made in rectangular panels from about 1.7 to 2.9 sq.m, in area, and are relatively simple to construct and erect. Flat plates can collect and absorb both direct and diffuse solar radiation, They are consequently partially effective even on cloudy days when there is no direct radiation.

It basically consists of a flat surface with high absorptivity for solar radiation called the absorbing surface. Typically a metal plate, usually of copper, steel or aluminum material with tubing of copper in thermal contact with the plates are the most commonly used materials. The absorber plate is usually made from a metal sheet 1 to 2 mm in thickness, while the tubes, which are also of metal, range in diameter from 1 to 1.5cm. They are soldered, brazed or clamped to the bottom of the absorber plate with the pitch ranging from 5 to 15 Cm, In some designs, the tubes are also in line and integral with the absorber plate.

The primary function of the absorber is to absorb maximum radiation reaching it through the glazing, to lose maximum heat upward to the atmosphere and down ward through the back of the container and to transfer the retained heat to the working fluid. Black painted absorbers are preferred because they are considerably cheaper and good absorbers of radiation.

 

Figure 2.2 –Flat plate solar collector

Heat is transferred from the absorber plate to a point of use by circulation of fluid (usually water) across the solar heated surface. Thermal insulation of 5 to 10cm thickness is usually placed behind the absorber plate to prevent the heat losses from the rear surface. Insulation materials is generally mineral wool or glass wool or fiber glass.

The front covers are generally glass that is transparent to incoming solar radiation and opaque to the infra-red re-radiation from the absorber. The glass covers act as a convection shield to reduce the losses from the absorber plate beneath. The glass thickness of 3 and 4 mm are commonly used. The usual practice is to have 2 covers with specific ranging from 1.5 to 3cm.

Advantages of second glass which is added above the first one are

(i) Losses due to air convection are further reduced. This is important in windy areas.

(ii) Radiation losses in the infra-red spectrum are reduced by a further 25%, because half of the 50% which is emitted out wards from the first glass plate is back radiated.

Solar Collectors

 

A solar collector is a device for collecting solar radiation and transfer the energy to fluid passing in contact with it. Utilization of solar energy requires solar collectors.

These are generally of two types.

(i) Non- concentrating (or) flat plate solar collector.

(ii) Concentrating (focusing) type solar collector

Solar Energy applications

 

1. Heating and cooling of residential building.

2. Solar water heating.

3. Solar drying of agricultural and animal products.

4. Salt production by evaporation of seawater.

5. Solar cookers.

6. Solar engines for water pumping.

7. Solar Refrigeration.

8. Solar electric power generation.

9. Solar photo voltaic cells, which can be used for electricity.

10. Solar furnaces.

Pyrheliometer

A pyrheliometer is an instrument for direct measurement of solar irradiance. Sunlight enters the instrument through a window and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal voltage is converted via a formula to measure watts per square metre. It is used with a solar tracking system to keep the instrument aimed at the sun. A pyrheliometer is often used in the same setup with a pyranometer.

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Pyranometer

 

A pyranometer is a device used to measure global solar radiation. The "pyranometer" is basically a flat plate (covered with a transparent dome) that is coated with an extremely absorptive surface. As the sun strikes it, the surface gets hot. The temperature of the surface is measured with a thermopile, giving an output voltage related to the amount of solar radiation striking the surface.

The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates a voltage output signal that is proportional to the solar radiation.

Solar Radiation measurements

 

Measuring the sun’s radiation output has a variety of useful purposes: it allows for the development of solar energy devices and permits scientists to predict rates of future global warming on a grander scale.

The measurement of solar radiation is based on a rate of kilowatts per square meter, represented as W/m2. This is the measurement standard for scientific data, designed for generating a direct estimate of solar energy – basically, how much sunlight is hitting a particular part of the Earth at any given time. Measurements taken for the purpose of energy production via solar panels and other photovoltaic equipment may be calculated in kilowatt-hours per square meter, or kWh/m2, designed to represent the amount of energy being generated by that sunlight.

Several different tools can be used to measure kilowatts and kilowatt-hours per square meter.

They are

1. Pyranometer

2. Pyrheliometer

Solar Radiation at the Earth’s Surface

 

The solar radiation that penetrates the earth’s atmosphere and reaches the surface differs in both amount and character from the radiation at the top of the atmosphere. Part of the radiation is reflected back in to the space, especially by clouds. Furthermore, the radiation entering the atmosphere is partly absorbed by molecules in the air. Oxygen and Ozone (o3), absorb nearly all the Ultraviolet radiation, and water vapour and carbon dioxide absorb some of the energy in the infrared range. In addition, part of the solar radiation is scattered (i.e. its direction has been changed) by droplets in clouds by atmosphere molecules, and by dust particles.

Solar Radiation that has not been absorbed or scattered and reaches the ground directly from the sun is called “Direct Radiation” or Beam Radiation. Diffuse radiation is that Solar Radiation received from the sun after its direction has been changed by reflection and scattering by the atmosphere. Because of the Solar Radiation is scattered in all directions in the atmosphere, diffuses radiation comes to the earth from all parts of the sky. The sum of the beam and diffuse radiation flux is referred to as total or global radiation.

Solar Constant

 

The sun is a large sphere of very hot gases, the heat being generated by various kinds of fusion reactions. Although the sun is large, it subtends an angle of only 32 minutes at the earth’s surface. This is because it is also a very large distance. Thus the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its center to its edge. However for engineering calculations, it is customary to assume that the brightness all over the solar disc in uniform. As viewed from the earth, the radiation coming from the sun appears to be essentially equivalent to that coming from a back surface at 5762 ^O k.

“The rate at which solar energy arrives at the top of the atmosphere is called solar constant”. This is the amount of energy received in unit time on a unit area perpendicular to the sun’s direction at the mean distance of the earth from the sun. Because of the sun’s distance and activity vary throughout the year, the rate of arrival of solar constant is thus an average from which the actual values vary up to 3 percent in either direction.

The National Aeronautics and Space Administration’s (NASA) standard value the solar constant, expressed in three common units, is as follows:

(i) 1.353 kilowatts per square meter

(ii) 116.5 Langleys per hour (1 langely being equal to 1cal/cm² of solar radiation received in one day)

(iii) 429.2 Btu per Sqr.ft. per hour.

The distance between the earth and the sun varies a little through the year. Because of this variation, the extra – terrestrial flux also varies. The earth is closest to the sun in the summer and farthest away in the winter.

Solar energy

Energy from the sun is called solar energy. The Sun’s energy comes from nuclear fusion reaction that takes place deep in the sun. The energy from these reactions flow out from the sun and escape into space.Solar energy is sometimes called radiant energy. There are different kinds of radiant energy emitted by sun. The most important are

· Light infrared rays.

· Ultra violet rays, and

· X- Rays.

The beam radiation received from the sun on the earth is reflected in to space, another 15% is absorbed by the earth atmosphere and the rest is absorbed by the earth’s surface. This absorbed radiation consists of light and infrared radiation without which the earth would be barren.

All life on the earth depends on solar energy. Green plants make food by means of photosynthesis. Light is essential from in this process to take place. This light usually comes from sun. Animal get their food from plants or by eating other animals that feed on plants. Plants and animals also need some heat to stay alive. Thus plants are store houses of solar energy. The solar energy that falls on India in one minute is enough to supply the energy needs of our country for one day. Man has made very little use of this enormous amount of solar energy that reaches the earth.