SOLAR ENERGY

CONTENTS

1 INTRODUCTION
2 CLASSIFICATION OF SOLAR POWER
(a) PASSIVE OR ACTIVE
(b) FOCUS TYPE
3 TYPES OF SOLAR POWER TECHNOLOGIES
(a) SOLAR DESIGN IN ARCHITECTURE
(b) SOLAR HEATING SYSTEMS
(i) compact system
(ii) pumped system
(iii) solar heating thermal collectors
(iv) solar thermal cooling
(c) PHOTO VOLTAIC CELLS
(d) SOLAR THERMAL ELECTRIC POWER PLANTS
(i) concentrating solar power (csp) plants
(ii) solar chimney
(e) SOLAR CHEMICAL
(f) SOLAR COOKING
(g) SOLAR LIGHTING

4 ENERGY STORAGE
5 CONCLUSION





INTRODUCTION

Solar power describes a number of methods of harnessing energy from the light of the Sun. It has been present in many traditional building methods for centuries, but has become of increasing interest in developed countries as the environmental costs and limited supply of other power sources such as fossil fuels are realized. It is already in widespread use where other supplies of power are absent such as in remote locations and in space.
As the Earth orbits the Sun, it receives approximately 1,400 W / m² of energy, as measured upon a surface kept normal (at a right angle) to the Sun (this number is referred to as the solar constant). Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy. The generally accepted standard is 1020 watts per square meter at sea level.
After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and ultraviolet light. Plants use solar energy to create chemical energy through photosynthesis. We use this energy when we burn wood or fossil fuels or when we consume the plants as a source of food.

















CLASSIFICATION OF SOLAR POWER



Solar power can be classified as :
(a) direct or indirect.
(b) Passive or active type
(c) Focus type
Direct solar power involves only one transformation into a usable form. For example:
• Sunlight hits a photovoltaic cell (also called a photoelectric cell) creating electricity.
• Sunlight hits the dark absorber surface of a solar thermal collector and the surface warms. The heat energy is carried away by a fluid circuit.
• Sunlight strikes a solar sail on a space craft and is converted directly into a force on the sail which causes motion of the craft.
• Sunlight strikes a light mill and causes the vanes to rotate, although little practical application has yet been found for this effect.
• Sunlight is focused on an externally mounted fibre optic cable which conducts sunlight into building interiors to supplement lighting.









Indirect solar power involves more than one transformation to reach a usable form. For example: Systems which close insulating shutters or move shades. Many other types of power generation are indirectly solar-powered. Some of these are so indirect that they are often excluded from discussion of solar power:
• Vegetation use photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity, see biofuel.
• Energy obtained from oil, coal and peat originated as solar energy captured by vegetation in the remote geological past and fossilised. Hence the term Fossil fuel. Though strictly solar power, the great time delay between the input of the solar energy and its recovery means these are not normally classified as such.
• Hydroelectric dams and wind turbines are indirectly powered by solar energy through its interaction with the Earth's atmosphere and the resulting weather phenomena.
• Energy obtained from methane (natural gas) may be derived from solar energy either as a biofuel or fossil fuel , but some methane derives from the primeval gas cloud which formed the Solar system and is therefore not solar in origin.
• Ocean thermal energy production uses the thermal gradients that are present across ocean depths to generate power. These temperature differences are ultimately due to the energy of the sun.












Passive or active :
Solar power can also be classified as passive or active:
• Passive solar systems are systems that do not involve the input of any other forms of energy apart from the incoming sunlight, although (in the case of solar heat through windows) there may be draperies or panels used to reduce nighttime heat losses and thermostatically or manually operated vents (but not fans) to prevent overheating. Some passive solar water heating systems use a thermosiphon to reduce nighttime heat loss and have no pumps. Other space heating systems use a thermal diode to similar effect.
• Active solar This usually refers to system which use additional mechanisms such as circulation pumps, air blowers or automatic systems which aim collectors at the sun.
Focus type :
Effective use of solar radiation often requires the radiation (light) to be focussed to give a higher intensity beam. Consequently, another scheme for classifying solar power systems is
• Point focus. A parabolic dish or a series of heliostats are used to concentrate light at a point (the focus). At the focus you might place high-concentration photovoltaic cells (solar cells) or a thermal energy 'receiver'. Solar One was an example of the latter.
• Line focus. A parabolic trough or a series of long narrow mirrors are used to concentrate light along a line. The SEGS systems in California are an example of this type of system.
• Non-focussing systems include solar domestic hot water systems and most photovoltaic cells. These systems have the advantage that they can make use of diffuse solar radiation (which can not be focussed). However, if high temperatures are required, this type of system is usually not suitable, because of the lower radiation intensity.
Types of solar power technologies
Most solar energy used today is harnessed as heat or electricity.
Solar design in architecture
Solar design is the use of architectural features to replace the use of grid electricity and fossil fuels with the use of solar energy and decrease the energy needed in a home or building with insulation and efficient lighting and appliances.
Architectural features used in solar design:
• South-facing (for the Northern Hemisphere) or north-facing (for the Southern Hemisphere) windows with insulated glazing that has high ultraviolet transmittance.
• Thermal masses -- any masses such as walls or roofs that absorb and hold the sun's heat. Materials with high specific heat like stone, concrete, adobe or water work best. See Trombe walls.
• Insulating shutters for windows to be closed at night and on overcast days. These trap solar heat in the building.
• Fixed awnings positioned to create shade in the summer and exposure to the sun in the winter.
• Movable awnings to be repositioned seasonally.
• A well insulated and sealed building envelope.
• Exhaust fans in high humidity areas.
• Passive or active warm air solar panels. Pass air over black surfaces fixed behind a glass pane. The air is heated by the sun and flows into the building.
• Active thermal solar panels using a heat transfer fluid (water or antifreeze solution). These are heated by the sun and the heat is carried away by circulation of the fluid for domestic hot water or building heating or other uses.
• Passive thermal solar panels for preheating domestic hot water.
• Photovoltaic systems to provide electricity.
• Solar chimneys for cooling.
• Planting deciduous trees near the windows. The leaves will give shade in summer but fall in winter to let the sunlight enter the building.

Solar heating systems
Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and usually a reservoir to stock the heat for subsequent use. The systems may be used to heat domestic hot water, to heat a swimming pool, to provide heat for a heating circuit (usually radiators or floor heating coils). The heat can also be used for industrial applications or as an energy input to other uses (such as cooling equipment).
In many climates, a solar heating system can provide a very high percentage of domestic hot water energy. In many northern European countries, combined systems (hot water and space heating) are used to provide 15 to 25% of home heating energy.
Residential solar thermal installations can be subdivided in two kind of systems: compact and pumped systems. Both include typically an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting(i. e. 50 ºC), so hot water is available always, even in rainy days.









Compact systems
Consist of a tank for the heated water, a few panels and pipes. Based on the thermo siphon principle, the water flows upwards when heated in the panel. When this water enters the tank (placed in the upper part) it expels some cold water from inside, so there is no need for pumps. A typical system for a 4 members home in a sunny region consists of a 300 liters tank and 2 panels (2 square meters each).
"Direct" compact systems are not suitable for cold climates, because at nighttime the remaining water in the panels can freeze and damage them. Besides, the tank is placed together with the panels, generally outside the house (even if the can be hidden beneath the tiles). Some compact systems have a “primary circuit”. This primary circuit includes the collectors and the external part of the tank. A graphical explanation of the thermosyphon principle can be found at this site Instead of water, some non-toxic antifreezing liquid is used. When this liquid is heated up, it flows to the external part of the tank, transferring the heat to the water placed inside. However, direct systems are slightly cheaper and more efficient.
A compact system can save up to 4.5 tonnes per annum of gas emissions. So, in order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in 6 years, and they achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems don't use any external source for power (as water flows due to thermosyphon principle).
Usually flat solar thermal collectors are used, but a few compact systems with vacuum tubes can be found.

Pumped systems
They are commonly used in bigger installations (hotels, gyms, and so forth) and the main difference is that the storage tank is placed inside the building, and thus require a controller that measures when the water is hotter in the panels than in the tank, and a pump for transferring water between the two. Most controllers also activate the pump when the outside temperature gets close to 0º C, in order to prevent the water from freezing and thus damaging the panels.
These systems can be controlled remotely, by means of the data logger and a modem-connection.
The most commonly used panel is the flat panel, but sometimes cheaper ones, like polypropylene panels (for swimming pools), or higher-performing ones like vacuum tubes are used.

Solar heating thermal collectors
There are three main kind of solar thermal collectors in common use:
• Formed Plastic Collectors (such as polypropelene, EPDM or PET plastics). These consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. Used for extending the swiming season in swiming pools. In some countries heating a open-air swiming pool with non-renewable energy sources is not allowed, and then these cheap systems offer a good solution. This panel is not suitable for year round uses like providing hot water for home use, mainly due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than than temperature of the fluid being heated.
• Flat Collector. It consists of a thin absorber sheet (usually copper, to which a selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with (usually) a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger, or to some other device for using the heated fluid. Flat-plate collectors for solar water heating had a popularity in Florida and Southern California in the 1920s. There was a resurgence of interest in them in North America in the 1970s. With various improvements, the collectors of this basic design have frequently been used in "off-grid" home situations (or in other sorts of buildings), but now they present in all most every city in the world. Naturally, like a lot of solar-heating strategies that have been available until recently, conventional flat-plate solar collectors were originally developed for use in sunny, warm climates. Benefits from this kind of collector are considerably diminished when colder or cloudy days present unfavorable conditions

Evacuated (or vacuum) tubes panel
• Evacuated tube collectors are made of a series of modular tubes, mounted parallel, whose number can be added to or reduced as hot-water-delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated-tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it, and in doing so the heat is transferred to a liquid flowing through the tube. The heated liquid circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated-tube collectors heat to higher temperatures. Even in some northern climates, this sort of system may capture excess heat which can also be used to supply room heat in winter. However they are more expensive and fragile than flat panels.
Solar thermal cooling
There are some new applications of thermal hot water, like air cooling, currently under development. The absorber machine works basically as a fridge; it uses hot water to compress a gas that once expanded will produce an endothermic reaction, cooling the air. The main problem right now is that the absorber machine works with liquid at 90ºC, a pretty high temperature to be reached with pumped solar pannels with no auxiliary power supply. Some commercial systems are expected to be relased soon.
The same pumped solar thermal installation can be used for producing hot water the whole year, cooling in summertime and partially heating the building in wintertime.


Photovoltaic cells

The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 Amps in full, direct sunlight
Solar cells (also referred to as photovoltaic cells) are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from the sunlight. Because of high manufacturing costs, their use has been limited until recently. One cost-effective use has been in very low-power devices such as calculators with LCDs. Another has been remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third has been to power orbiting satellites and other spacecraft.
However, the continual decline of manufacturing costs (dropping at 3% to 5% a year in recent years) is expanding the range of cost-effective uses. The average retail cost of a large solar panel declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - that is, systems with no battery that connect to the utility grid through a special inverter - now make up the largest part of the market. In 2004 the worldwide production of solar cells increased by 60%. 2005 is expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.
Solar thermal electric power plants
The two main types of solar thermal power plants are Solar Chimneys and Concentrating Solar Power (CSP) plants.
Concentrating solar power (CSP) plants


Solar Two, a concentrating solar power plant
Solar thermal power plants generally use reflectors to concentrate sunlight into a heat absorber. Such powerplants are known as Concentrating Solar Power (CSP) plants.
• Heliostat mirror power plants (power towers) use an array of flat, moveable mirrors to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One, Solar Two and the 15 MW Solar Tres plants. In South Africa a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².
• A parabolic trough power plant is another type of solar thermal collector. It consists of a series of troughs rather like rainwater guttering with a hollow tube running its length. Sunlight is reflected by the mirror and concentrated on the tube. Heat transfer fluid, oil in the Luz systems, runs through the tube to absorb heat from the concentrated sunlight and is used to power a steam turbine.
• A Parabolic Reflector power plant is rather like a large satellite dish but with the inside surface made of mirror material. It focuses all the sun's energy to a single point and can achieve very high temperatures. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator. Planned 850 megawatt Solar Stirling Condenser array [1] [2].
• A linear Fresnel reflector power plant uses a series of carefully angled plane mirrors to focus light onto a linear absorber. Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo).

solar chimney
A solar chimney is an apparatus for harnessing solar energy by convection of heated air.
In its simplest form, it consists of a black-painted chimney. During the daytime, solar energy heats the chimney, thereby heating the air within it, resulting in an updraft of air within the chimney. The suction this creates at the chimney base can be used to ventilate, and thereby cool the building below. In most parts of the world, it is easier to harness wind power for such ventilation, but on hot windless days such a chimney can provide ventilation where there would otherwise be none.


General concept of proposed solar chimney power station
This principle has been proposed for electric power generation, using a large greenhouse at the base rather than relying on heating of the chimney itself.
The main problem with this approach is the relatively small difference in temperature between the highest and lowest temperatures in the system. Carnot's theorem greatly restricts the efficiency of conversion in these circumstances.
Solar chemical
There have been experiments to harness energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis without using living organisms but no practical process has yet emerged.
A promising approach is to use focussed sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of metalic zinc.
Solar cooking
A solar box cooker traps the Sun's power in an insulated box; these have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner environment for the cooks. The first known western solar oven is attributed to Horace de Saussure
Solar lighting
The interior of a building can be lit during daylight hours using fibre optic light pipes connected to a parabolic collector mounted on the roof. The manufacturer claim this gives a more natural interior light and can be used to reduce the energy demands of electric lighting.
Energy storage
See main article at Grid energy storage
For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques: -
• Electrochemically in batteries,
• Hydrogen produced by electrolysis of water and then available for pollution free combustion (see direct solar thermal water splitting),
• Compressed air in a cylinder,
• Pumped-storage hydroelectricity
• Flywheel energy storage,
• Molten salt
• Superconducting magnetic energy storages.
• Cryogenic liquid air or nitrogen
Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night.