Reasons for using solar radiation
Solar radiation is an easily available renewable energy source. Its use does not pollute the environment. An another advantage of solar radiation is its a more even distribution in comparison with the world's reserves of traditional and finite fuels such as oil, coal and natural gas. The disadvantage is the many times lower concentration of solar radiation, what results in a low efficiency of energy conversion. And therefore on the basis of present knowledge, solar radiation cannot yet be considered as equivalent to replace fossil sources.
As for vertical incidence of the sun's rays on the Earth's atmosphere, 1 m2 is hit on average by 1, 36 kW of radiation. It is referred to as so called “solar constant”. A part of the direct solar radiation is absorbed and scattered in the Earth's atmosphere by gases and aerosols and a part of it is reflected back into space, so in cloudless weather, the earth's surface is reached by radiation with an output of about 1 kW/m². Due to barriers in the atmosphere and on the Earth's surface, we register three basic kinds of solar radiation:
1. Direct: it falls from a clear sky directly on the surface,
2. 2. Scattered – diffused: it is created by scattering direct radiation at clouds and dirt in the atmosphere,
3. 3. Reflected: from the Earth´s surface and other objects.
In our geographical conditions, the overall duration of sunshine without clouds is about 1200 – 2000 hours per year, depending on the occurrence of fog and inversion.
The largest share in energy production by solar panels has direct and diffuse radiation. While diffuse radiation presents in summer 10 – 50% of the global radiation, its share in winter is due to cloudy weather significantly higher, making it about 60% of the annual average. The solar system works even if the sky is overcast – it uses diffuse and reflected radiation.
The intensity of sunlight varies during the year. The maximum sunlight in Slovakia is recorded in July and the minimum at the turn of December and January. The energy of the Sun can be actually used 7 – 9 months in the year for water heating, in spring and autumn months also for additional heating. During the day, the most radiation hits the ground at noon, when the position of the Sun in the sky is the highest and the route of the sunlight passing through the atmosphere is the shortest.
Annual value of global radiation in Slovakia in kWh/m2
Although the published map of solar radiation can give the impression that only southern Slovakia is the right place for using solar heat, it is not so. The darkest colour displays on the scale the value of 950 kWh/m² per year, so it does not present an unusable “dark area”, only seeks to distinguish the extent of deviations in Slovakia. The difference in the solar radiation intensity between the warmest and coldest regions of Slovakia is around 15%.