In our modern industrial society we consume vast amounts of energy to make our daily life more comfortable, productive and enjoyable. All of us use energy every day - for heat and light in living and working areas, cooking, transportation, manufacturing, and entertainment. The choices we make about how we use energy - turning machines off when we’re not using them or choosing to buy energy efficient appliances - impact our environment and our lives.

Energy comes from several different sources. These sources can be split into two main categories: non-renewable and renewable. Non-renewable types of energy include the three major types of fossil fuels – coal, oil and natural gas. Fossil fuels supply more than 90% of the world's energy. Oil leads with a share of about 40 percent of total world energy consumption, followed by coal (24 percent) and natural gas (22 percent). All of these are burned to produce power.

Fossil fuels are relatively easy to use to generate energy because they only require a simple direct combustion. However, a problem with fossil fuels is their environmental impact. Not only does their excavation from the ground significantly alter the environment, but their combustion leads to a great deal of air pollution. Carbon dioxide ( CO₂ ) from burning fossil fuels is the largest source of greenhouse gases from human activities. Extra greenhouse gases we are putting into the atmosphere are causing global warming and climate change. Besides, these are fuels that are being consumed more rapidly than they are being replaced. That means that someday we could run out of these fuels.

Another nonrenewable source is uranium. Uranium is an element that gives us nuclear energy by splitting an atom's nucleus, and this process is called fission. Nuclear energy is a better source of energy because it doesn't put carbon dioxide into the atmosphere. But like all industrial processes, nuclear power generation has by-product wastes: tremendous steam of radioactive products and heat. Hazardous wastes and the possibility of a nuclear disaster are the principal concerns for nuclear power.

With the growing concerns over the environmental problems today (air pollution, global climate change, massive flooding in river systems, etc.) and the price of non-renewable energy sources soaring, we have to take a closer look at the alternative energy sources. Alternatives to the fossil fuels and nuclear power are renewable sources of energy and they are considerably more attractive in many ways. Renewable sources are derived and replenish quickly from nature and usually do not pollute our environment when used to generate electricity. The five renewable sources used most often include hydropower (water), solar, wind, biomass, and geothermal.

In the context of renewable energy, solar power is associated with the harnessing of the sun's present emissions of heat or light. There are a variety of types of technologies that can do this. Solar energy is typically divided into three categories: passive solar, active solar, and photovoltaic (electrical) solar energy. All of them produce solar energy, but in very different ways.

In active solar category the solar energy is directly converted in the application form. One of the simplest and economical ways to utilize solar energy is through solar thermal systems. Solar thermal technology is employed for collecting and converting the sun energy to heat energy for application such as water and air heating, cooking and drying, steam generation, distillation, etc.

Solar thermal technologies include solar heat collectors (flat-plate collectors, evacuated-tube collectors) and solar concentrating collectors. Flat-plate collectors are the most commonly used type of collector today. A typical flat-plate collector consists of a box containing a sheet of metal painted black, which absorbs the suns energy. In the most common design, built in pipes in the box carry liquids that take the heat from the box and bring it into the building. This heated liquid, usually a water-alcohol mixture to prevent winter freezing, is used to heat water in a tank or is put through radiators to heat the air. Solar heat collectors sit on the rooftops of buildings and are generally used in hotels and homes.

Solar concentrating collectors (parabolic concentrators) use mirrors and lenses to concentrate and focus sunlight onto a receiver mounted at the system's focal point. The receiver absorbs and converts the sunlight into heat. This heat is then transported by means of a heated fluid (either water or molten salt) through pipes to a steam generator or engine where it is converted into electricity. There are also large centralized solar power plants, known as "power towers". Power tower is a large tower surrounded by small rotating (tracking) mirrors called heliostats. These mirrors align themselves and focus sunlight on the receiver at the top of tower, collected heat. This focused heat turns water into steam that is used to power a generator. Solar concentrating systems produce high temperatures that can be used for industrial purposes.

Photovoltaic (electrical) solar technologies directly convert solar radiation into electricity through the use of photovoltaic cells (PVs), also called solar cells. Photovoltaic or solar electric panels use semiconductor materials such as silicon to convert sunlight to electricity. Most solar cells are made from silicon because it is, so far, the most cost-effective material. Sunlight is composed of light energy in the form of photons. When these photons strike the cell, some electrons in the cell material absorb sufficient energy to break away from their atoms and flow through the material to produce electricity. This electricity can either be used directly as it is or can be stored in the battery. The stored electrical energy then can be used at night.

PV technology can be used to meet our electricity requirements. Most houses with good roofs have enough space to generate a total of about 1/4 to 1/2 of their yearly electricity use. Solar photovoltaic panels can be installed on the rooftops. They can be flush mounted as well as tilted up. PV systems can also provide electricity in remote places deprived of grid power.

PVs can be used in a variety of applications. The simplest photovoltaic systems power small consumer goods such as calculators and wrist watches. Other applications include water pumps, street and car-park lighting, garden footpath lighting, etc. Some experimental cars also use PV cells. They convert sunlight directly into energy to power electric motors on the car. Solar photovoltaic panels are applied in satellites and spacecrafts as well.

Passive solar energy is energy or warmth obtained without any mechanical intervention. The most common of the passive solar technologies is referred to as direct solar gain. A direct gain system includes south-facing large windows that allow the sun's rays to heat surfaces inside the building. The result is that in cold weather the surfaces absorbs solar energy and radiates heat throughout the room.

Of all the solar energy technologies, photovoltaics show the greatest promise for worldwide acceptance and application. Working photovoltaics are relatively simple in design, have no moving parts, need very little maintenance and are environmentally benign. They simply and silently produce electricity whenever they are exposed to light. In the developing world PVs are seen as a very attractive option. They are especially useful for rural electrification, vaccine refrigeration and water pumping.

Solar energy demand has grown at about 25% per annum over the past 15 years but it has clearly not reach its full potential. The main reason for the lack of mass exploitation of solar power technologies is economic. In order for widespread generation of electricity using solar panels to be feasible it needs to be economically advantageous. In order for solar panels to be an economically viable choice for the production of electricity, production costs must go down and efficiency of the final product must go up.

The hidden factor behind the lack of widespread solar power production is the absence of mass consumer demand for solar technologies. If there is a demand for a product, there will be people that will supply that product at a cost that fulfills that demand. As a result, economic and efficient solar power technologies will be developed and exploit more quickly.

Sources: www.crest.org/Fsolar/Fsolar_intro.html