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Guide to Solar Energy

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The sun is a natural source of energy: in under an hour, the sun radiation that permeates through the earth's atmosphere accounts for more energy than consumed by mankind in one calendar year. At the source, in one complete second, the sun emits more energy than people have consumed since the beginning of time! And as much as it seems a more recent alternative to other sources of energy; anyone who ever owned or used a pocket calculator might remember a small rectangular panel that looked like a strip of plastic. That rectangular panel is a simple solar cell, also known as a photovoltaic cell.

Simply Sun
Shining since roughly 5 billion years, the sun will continue burning for another 5 billion years. As we all know, the sun is a massive ball of burning gases; hydrogen and helium. The huge mass of the sun has understandably strong gravity. At the core of this enormous gaseous ball, the gravitational pull causes intense pressure resulting in nuclear fusion. Hydrogen atoms collide into each other with such colossal force that results in breakaway atomic particles. These particles fuse together to create helium and this process releases energy that accounts for roughly 80 percent of the suns energy.
The sun emits electromagnetic radiation which can travel 93 million miles through the vacuum of space and reach the earth's atmosphere. Solar energy to the magnitude of 1400W/m2 is released outside the earth's atmosphere; only roughly 350W/m2 permeates through the atmospheric barriers protecting the earth. The rest is absorbed by the atmosphere, causing winds to blow and some of it is reflected back into space. As an alternative to fossil fuels, the sun seems almost inexhaustible. This is exemplified by the fact that in a single day, the solar energy reaching the USA is sufficient to cover the energy requirements of the country for over a year!

Types of Solar Energy
There are many ways to benefit from the advantages of clean sustainable energy that we can derive from the sun.
A diagrammatic explanation of solar energy, its history and types is presented by the National Energy Education Development Project . For a more detailed scientific exposition, UCSUSA's How Solar Energy Works provides substantial information and analysis on solar energy.

Passive Solar Building Design
Thousands of years ago, Native Americans, employed passive solar heating techniques that would naturally utilize the sun and wind in their cliff-side residences in Colorado. The Greeks and the Chinese incorporated environmental variables such as sun and wind direction, ventilation and shade into their architectural designs. Socrates description of the Megaron House clearly delineates how construction could be planned according to the movement of the sun. Today these considerations are taken a few steps further by many eco-conscious architects:

  • The position of the area according to latitude
  • The path the sun would take over the construction site
  • Difference and variation in temperature between day and night
  • Variations in seasonal solar heating and cooling across the year
  • Humidity
  • Land topography and vegetation
  • Obstructions to solar gain or breeze
  • Excessive shade by natural or artificial means

In terms of actual construction, solar energy is manipulated to ensure efficacy in passive solar designs. This can be through means of direct solar gain, which would control the amount of solar radiation entering the living space. Another means is through thermal mass: using specific materials that have high density and high heat storage capacity. Thermal mass allows heat to be captured, stored and released through conduction or convection in order to balance temperatures. Isolated solar gain is another concept that aids in harnessing solar power. This technique uses water or air convection to move heat to and from the living space from sources such as sunspaces, conservatories or windows facing the equatorial side. Proper insulation insures that heat does not escape. Passive cooling prevents over heating through means of ventilation, shade, or earth cooling tubes. Landscaping can also be a powerful tool to lower the temperature when needed via adequate shade, ponds and land contouring.
Current passive solar designs for construction are becoming more ambitious with the aim to not only encourage the construction of buildings that require little energy to run, but to construct buildings that provide a net gain of energy!

Photovoltaic or Solar Cells
As the name implies, photovoltaic cells convert radiant energy into electricity: photo means light and voltaic refers to volt, a measurement of electricity. Single units are known as cells while multiple cells connected electrically together on a flat grid are known as a module or solar panel.
Photovoltaic cells are made of two layers of semiconductors, usually crystalline silicon. One layer carries a positive charge, while the other a negative charge. The resultant electric field allows the energy absorbed from radiation to flow in a specific direction; creating a current. This current then travels through the electric circuitry to power a calculator or an electricity utility grid. For more sensitive and complex applications that are powered through solar panels, the photovoltaic cells are made of gallium arsenide.
This system of garnering and generating energy is not very effective: only 10 to 20 percent of radiation is converted into electrical energy. The cost of solar panels is usually only seen as a worthwhile investment when used in standalone situations such as to power a remote village where providing connections to the city grids would be too costly. In recent times, however, cost effective solar panels have been produced to suit installation in houses. Roughly 60 percent of a household's electricity consumption can be met 6 months in a year using a domestic solar panel. Alternatively, solar cells can also be used on appliances like door bells, night lights and burglar alarms.
The US Department of Energy provides an easy to follow guide to photovoltaic cell production.
Jacob Silverman at 'howstuffworks' postulates how a nanostructure semiconductor will help improve solar panel efficacy.

Concentrating Solar Energy
Thermal power plants that concentrate solar energy are capable of producing very high temperatures. Using different shaped mirrors, solar energy can be collected and redirected into thermal receivers that absorb, collect and transfer it to a generator that will produce electricity. This intense heat can also be used in industrial procedures. The biggest advantage of the electricity generated through large solar thermal systems is the ability to store this solar energy to produce electricity even when the sun is down! Given the right scale, with proper storage systems, a solar plant that concentrates radiant energy can provide continuous electricity and function as a reliable source of baseload electricity. Future solar plants can compete with or replace the energy output of traditional fossil or nuclear plants.
There are three types of concentrating solar thermal systems:

  • Linear concentrators or parabolic troughs - These use long curved mirrors that concentrate the energy on tubes carrying liquid to a central collector where the produced steam powers a turbine that generates electricity.
  • The parabolic dish This focuses solar energy to a single point and uses a thermal receiver to a heat engine. One kind is the Stirling engine, which provides an alternative to burning gasoline to create mechanical power to move pistons.
  • A power tower or central receiver This design concentrates solar energy using a large field of mirrors on top a tower focusing on the receiver. The heat conductive fluid generates steam causing the turbine to create electricity.

Solar Thermal
Besides producing electricity, solar energy can be used to actively heat water, spaces and to produce cooling.
Gathering solar thermal energy is a simple two part process:

  • Step 1 - large flat boxes are installed on the roof or sides of buildings, painted black on the inside for absorption and covered with glass.
  • Step 2 - Tubes run through this box, heating and carrying the heat transfer fluid, either water or a combination of antifreeze and corrosion inhibitor to the building where heat is dispersed by passing the hot liquid through radiators. Alternatively this can be is used to heat water using large water tanks. Copper coils of the heat transfer fluid are submerged in the tank to heat the water.

For the water heating system to have constant optimal temperature, often an alternative fossil fuel dependant energy source is present. The auxiliary system ensures the temperature does not fall beyond the set point.
Desiccant evaporators or dehumidifiers also use solar thermal energy collectors to cool air by removing moisture or humidity. The moist air is expelled outside and the remaining air is drier and cooler.

Inexhaustible and cultivatable: where does the catch lie?
It's been established that the sun will not only be shining for another 5 billion years, the energy it produces is virtually inexhaustible. The production of solar radiation is not the result of harmful chemical reactions, but nuclear fusion that takes place naturally. The generation of solar energy is essentially clean, with no waste or toxic emissions. So if everything works in the favor of solar energy then why is the world still fighting over oil wells and the price of the oil barrel? Harnessing solar radiation is not as simple it sounds!

  • Solar energy systems are expensive to produce and install.
  • Cloud cover and pollution can interfere with radiation energy efficacy.
  • Large thermal plants can cause harm to the natural ecosystem as highly concentrated sunbeams can kill birds and insects.
  • Production of photovoltaic cells produces certain toxic waste materials as a result of the solvents and chemicals used.

Partha Das Sharma's blog on 'Solar power ' Sustainable green energy to protect our economy and environment' talks in detail about the different solar technologies out there and their advantages as well as their negative impact on the environment. It also presents research and future possibilities of using solar energy. Another blog on solar energy discusses in detail the advantages and disadvantages of using solar power.

Get the facts about solar energy use here in this handy info site.

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