Solar Energy
Solar energy is energy recovered from direct and scattered (reflected) solar radiation. The maximum power that can be generated directly from solar energy per one square meter is called the solar constant, amounting on average to 1,367 W/m2. It is the power of solar radiation reaching the outermost strata of the atmosphere. Because a part of energy is reflected or absorbed by the atmosphere, we can use only up to 1,000 W/m2 near the surface of the Earth.
The characteristic features of solar radiation in a given region determine the possibilities and directions of using the solar energy. Solar radiation is suppressed when coming through the atmosphere as a result of reflection, scattering and absorption by dusts and gases. The part of radiation that reaches the surface of the Earth without hindrance is called direct radiation. Radiation reaching a defined surface from all directions is called scattered (diffused) radiation.
The power industry recognizes several important quantities connected with the amount of solar energy reaching the surface of the Earth:
- Total solar radiation (W/m2), being a sum of density of the direct and scattered radiation energy flux;
- Exposure (J/m2) reflecting energy falling on a unit of surface within a determined period of time;
- Insolation (h) reflecting the number of hours with a direct, visible solar operation.
At present there are two main methods of using energy from solar radiation:
- Converting it into electrical energy with the use of photo-voltaic cells (photo-voltaic conversion);
- Using it as thermal energy for the production of electricity or for heating purposes (mainly solar energy collectors heating up water).
The photo-voltaic cell technology is widely used in our daily life. The cells are used in many every-day use products, such as calculators, watches, etc. Their invention has facilitated the development of satellite technologies and cosmos studies, with the cells being installed in objects sent into the outer space. Photo-voltaic cells are elements that generate electrical voltage directly from the falling rays of sunshine. Each cell has two layers: a positive (+) and a negative (-) layer. When rays of sunshine fall on the cell, voltage is generated between those two layers. As a rule voltage generated in one cell has ca. 0.5V and 2W. Therefore, to obtain more usable voltage and higher power, cells are combined in modules. One module (panel) includes from a few to several or even several dozens of cells. Over 95% of presently used solar cells are built based on silicon (Si).
The today’s, mass-used solar cells have 10%-15% efficiency. There are intensive studies carried out to use other substances, apart from silicon, to increase the efficiency of photo-voltaic cells and to facilitate their use in less favourable solar exposure conditions.
The use of solar energy in Poland is presently on a trace level only. It can be partly explained by a relatively high cost of production of electricity using solar energy and relatively low, compared to areas situated closer to the equator, energy potential of the sun on our latitude. The annual density of solar radiation in Poland on a horizontal plane is ca. 950 - 1250 kWh/m2, and the average insolation is ca. 1,600 hours per year. Our latitude is also characterized by strongly uneven distribution of solar radiation throughout the year. 80% of total annual insolation falls for the six months of spring and summer. The duration of solar operation is equally variable, changing from 16 hours in summer to 8 hours in winter.
Annual solar radiation in Poland in kWh/m2
It should be emphasized that the peak quantity of electric energy that can be produced through the use of solar radiation falls on periods weakest for the wind power plants, and vice versa. It means that any hybrid installations, using both solar energy and wind power, can provide a much more stabile source of electrical energy than any of the systems working alone. It proves that alternative sources of energy can be used on a much wider scale than it is generally believed.