The sun is the closest star to us and, directly or indirectly, the source of almost all available energy on Earth. Solar energy comes from nuclear reactions at its center, where the temperature reaches 15 million ° C. It is a fusion, in which the fusion of hydrogen atoms produces helium, with the release of a large amount of energy. Every second, about 600 million tons of hydrogen are converted into helium in this way, with a mass of some 4 million tons of hydrogen being converted into energy.
This energy in the form of light and heat spreads into space, so a small part of it reaches the Earth. Under optimal conditions, 1 kW / m2 can be obtained on the Earth’s surface, and the actual value depends on the location, season, time of day, weather conditions, etc. In Croatia, the average value of daily insolation on a horizontal surface is 3-4.5 kWh / m2 .
The map showing the insolation level shows that Europe is not in a very suitable area for exploitation, but despite this, the direct use of solar energy in Europe is on the rise. This is mostly the result of the policies of individual countries that subsidize the installation of elements for the conversion of solar energy into a usable form of energy. The main problems of exploitation are low energy flux density, large oscillations of radiation intensity and high investment costs.
We have always used the energy of the Sun, ever since the human race existed. The sun is a celestial body, the star closest to the Earth, without it the survival of life on our planet is impossible. We use solar energy in different ways every day; for example, when drying laundry.
Plants grow with the help of solar radiation and thus produce food that animals eat. As described in the chapter on fossil fuels, oil, gas and coal were formed from plants and animals, which decomposed hundreds of millions of years ago without the presence of air. In other words, the fossil fuels we use today are in fact long-stored solar energy. Directly or indirectly, all the energy we harness comes from the Sun or other stars. Nuclear energy is no exception: uranium atoms used in nuclear energy were formed during the explosion of a star – a new one.
Let’s look at the ways we use solar energy.
In the past, solar energy was widely used to heat the water needed in the household. However, when affordable fossil fuels appeared on the market, heating water with gas or heating oil supplanted the traditional use of solar energy.
Today, interest in the use of solar energy in the preparation of domestic hot water is growing again due to the rising price of fossil fuels as well as due to the growing awareness of the need to preserve the environment. Modern equipment for the production of hot water – solar heat collectors – is installed on the roof of the house. They contain pipes with water that is heated by the sun’s rays.
Solar thermal power plants
Solar energy can also be used to produce electricity.
Some solar thermal power plants use curved mirrors that direct solar radiation to a tube in the focus of the mirror. Water flows through the pipe, which is heated and turned into steam under the influence of focused radiation. This steam is used to start the turbine and produce electricity.
The main problem with solar power plants is that they only work while the sun is shining, while on cloudy days or during the night they cannot produce electricity. Therefore, in some plants, the so-called hybrid technology – during solar periods they use solar energy, and at other times steam is produced using fossil fuels, so the power plant can be constantly in operation. Another version of the solar thermal power plant is a power plant with a central tower. In this case, a field of mirrors is placed around the large fluid reservoir, the central tower, which reflects the solar radiation and directs it towards the reservoir. The heated fluid is used to produce steam that drives the turbine and generator. During the day, mirrors follow the position of the Sun, so we call them heliostats.
By using solar cells, the energy of solar radiation is directly converted into electricity. Solar cells are also called photovoltaic cells (abbreviated: PV), and are often used on low-power consumers, such as PDAs.
When the Sun’s rays illuminate a PV cell, part of its energy is transferred to the electrons and they are released and move towards the cell surface, causing an imbalance in the number of electrons between the upper and lower sides of the cell. When the foreign cells are connected by a conductor, current will flow through it. Individual cells are connected into photovoltaic panels, and they into fields. Some fields are placed on devices that monitor the movement of the Sun, so they tilt to adjust to the angle of incident solar radiation. Electricity from PV cells can be used for lighting, for the operation of household appliances, or stored in batteries. Cars that use PV cells for propulsion are also in development. However, when the use of PV cells is mentioned, most people think of space satellites.