Solar radiation energy, or solar energy, is the basis for almost all other energy sources on Earth. Fossil fuels (coal, oil and natural gas) are an accumulated form of solar energy. Wind energy, hydropower and biomass energy are indirect forms of solar energy. In energy generation, the term “solar energy” means the direct exploitation of solar radiation energy to generate thermal energy and electricity.
Thermal energy is usually generated from solar radiation in solar collectors for the preparation of hot service water. It is also possible to heat residential, commercial or industrial premises in this way or use the converted thermal energy in certain industrial processes. A key issue with the use of solar energy for the heating of buildings is that it is least available in the winter, when it is needed most, and vice versa, there is an abundance of it in the summer, when the demand for thermal energy is lowest. Modern thermal energy storage systems can successfully bridge this challenge. In addition, trigeneration technology has been available for quite some time, although it is not seen as competitive in most cases due to its complexity and low efficiency.
The simplest way to generate electricity is to use photovoltaic panels and this type of transformation has virtually become synonymous with solar power plants. However, solar thermal power plants also figure prominently in the development of solar energy. They are usually suitable only for areas with intensive solar radiation and large swaths of infertile, desert land. The cost of electricity generated in large solar thermal power plants is competitive with the cost of power generated in photovoltaic facilities.
At its core, solar energy is the energy of the Sun’s electromagnetic radiation which reaches the surface of the Earth. The total spectrum of solar electromagnetic radiation covers a wide range of wavelengths, from 0.1 nm to the order of 1 km. Wavelengths between 360 and 700nm, which we visually recognise as light, carry the highest amount of energy. This is also the main source of energy on Earth and the driver of the climate system in the atmosphere, which extends from the ground to an altitude of about ten kilometres. On the ground, this radiation is transformed and re-emitted back into the cosmos as long-wave (infrared) radiation. The difference between the received radiation and the re-emitted radiation (the radiation balance) is the main energy component of the climate in any spot on the Earth’s surface.
According to the Renewables 2018 Global Status Report, capacities for hot water preparation using solar energy reached 472 GW in 2017. Of particular interest are solutions which enable thermal energy accumulation over an extended period, ranging from a couple of months to half a year, thus allowing for the use of accumulated thermal energy during wintertime. One of the better examples for this is Demark, which has been successfully using solar energy for centralised heating of towns and cities notwithstanding its cold climate. In 2017, the surface area of solar collectors in the country’s central heating systems exceeded 1.5 million m2, while construction of new capacities has continued increasing at an exponential rate.
Photovoltaic power plants are the most promising renewable energy source. In 2017, more photovoltaic power plants were commissioned than coal-powered, natural gas-powered and nuclear power plants combined. Having contributed new 101 GW, accounting for 55% of newly-installed renewable energy power plants in 2017, photovoltaic power plants have had an absolutely dominant position in the energy sector across the world. At the end of 2017, more than 400 GW of solar plants of this type were connected to the power system. The snowball effect of declining cost of photovoltaic energy and its increasingly widespread use since 2010 has brought the cost installing a solar power plan to less than 1000 euros per installed kilowatt. The cost of electricity generated by these capacities is already lower than 5 eurocents per kilowatt-hour.
Interestingly, solar thermal power plants, as complex and seemingly expensive facilities, are increasingly attracting investors. These facilities have three main advantages over photovoltaic plants. According to the statistics of the International Renewable Energy Agency, virtually all solar thermal power plants of this type constructed in the world generate more than 2000 kWh/m2, with the most recent ones even generating up to 3000 kWh/m2, while photovoltaic power plants cannot produce more than 300 kWh/m2 even with under the most optimal conditions. Secondly, with appropriate thermal energy accumulation, solar thermal power plants successfully bridge the greatest shortcoming of photovoltaic power plants – the simultaneity of power generation and presence of solar radiation. Most solar thermal power plants can extend their operation into the evening hours, when electricity consumption is still high, with some of them managing to extend their operation by more than 8 hours. Thirdly, with a sufficiently high heat storage capacity, solar thermal power plants have a very high rate of efficiency of utilisation of their installed capacity (more than 60%) and can be used to balance the energy system. Although the cost of technology is high (from 6000 to 12000 USD/kW), the cost of electricity generated by these power plants is expected to drop from 0.1-0.3 USD/kWh to below 0.1 USD/kWh. Although the capacity of the currently installed solar thermal power plants is the world is only 4.9 GW, the declining trend of costs and improved performance will gradually put solar thermal power plants in a position where they can compete with all other technologies.