Solar Energy

The Sun is star in our closest proximity and provides almost all available energy on Earth. Solar energy originates from nuclear reactions in core of the sun, where the temperature reaches 15 million Celsius degrees. It is all about fusion, where hydrogen atoms merge and as a result helium is produced with large amounts of energy released. Every second about 600 million tons of hydrogen are being converged into helium, wherein the weight of about 4 million tons of hydrogen is being transformed into energy.

This energy in form of light and heat radiates, spreading itself all over the universe with one small amount of coming to surface of the Earth. It is estimated that nuclear fusion on Sun is about 5 billion years old, and it can be calculated that it will continue for another 5 billion years based on available hydrogen reserves. Under optimal conditions, the earth's surface can be gained 1 kW / m2, and the actual value depends on the location, season, time of day, weather conditions, etc. The main problems are exploiting the small energy flow, huge oscillations of radiation intensity and large investment costs.

On the map, one can see insulation levels for all the continents. It can be clearly observed that Europe, Canada and Russia are not in favorable situation when it comes to the sun radiation. However, direct exploitation of solar energy in these countries is in constant increase, due to the policies of individual countries that subsidize the installation of elements for converting solar energy into a usable form of energy.

 Basic principles of direct exploitation of solar energy are:

- photovoltaics - the direct conversion of solar energy into electricity
- solar collectors - preparing hot water and space heating
- Concentrating solar power - use in large power plants

Photovoltaic (Photovoltaic cells)

Photovoltaic cells are semiconductor devices that directly convert solar energy into electricity. Main disadvantage of these devices is low efficiency. For instance, for photovoltaic cells based on amorphous silicon efficiency is as low as 10%, which are considered to be cheap versions of this technology. However, some expensive units can reach up to 25%. Due to the low efficiency and high starting price per kW, photovoltaic cells are still economically unprofitable.

 

 Photovoltaic cells can be used as the sole source of energy or as an additional source of energy. As an independent energy source is used for example on traffic signs, satellites, and distant objects that are demanding long-lasting energy source. One option is to use photovoltaic cells ss supplementary energy source, that can be connected to the power grid, however it is not economically profitable at the moment.

Solar Collectors (Thermal Heat)

 Solar collectors transform solar energy into thermal energy of water. Water heating systems can be open or closed, depending on the application. In case of open collectors, water to be heated goes directly through a roof collector. Closed collectors are filled with liquid that do not freeze (eg, antifreeze) and can be used even at temperatures below zero. If the weather is nice, the water can be heated only in collectors. In case of bad weather, collectors help in water heating thus reducing the electricity consumption. Solar collectors also find convenient application for heating swimming pools. In this case, open heating systems is applied due to the low water temperature. In contrast of solar collectors for heating water, there are collectors that directly heat the air. These systems circulate the air through collectors and transfer large part of the energy in the air. The air is later returned to the room to be heated and therefore the temperature is maintained in the room. One can achieve significant savings if combination of air and water heating is applied.

In the European Union, there is a trend of significant increase of the amount of installed solar panels systems for heating water and apartments (houses). Germany and Austria are leaders in using solar energy for heating. At the first glance, this seems ironically, given the fact that nor Germany nor Austria are in suitable geographical position when it comes to the solar radiation. However, policies in these countries organize campaigns to promote solar energy, which as a result has efficient use of solar energy.

Concentrating Solar Power

Concentrating solar power is used for large generators or heating installations. Focusing is achieved by many lenses or more often using mirrors arranged in a plate or tower configuration. "Power Tower" configuration is based on computer controlled mirror's field for focusing solar radiation on the central tower, which then drives the main generator. This type of system can provide output power above 10 MW. Even during the night or in case of bad weather, these systems store hot liquid in very efficient container

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One version of concentrating solar systems is "Dish" systems, which track the movement of the sun and thus concentrating solar power. We should mention also "Trough" system of concentrating solar power, which is characterized with high efficiency. The main disadvantage of the focusing systems is that large space is required for the plant. In addition to this, high price of mirrors and concentrating solar power systems is also an obstacle.