(Transcript of the lesson commentary.)

Solar energy

The sun is a giant sphere of hot hydrogen at the centre of our solar system. Inside it, thermonuclear fusion takes place, releasing huge amounts of energy. Only a small fraction of it reaches the Earth. Approximately 1,360 watts of energy per square metre falls on the Earth’s upper atmosphere. This value is called the solar constant. But some of the radiation is absorbed by the atmosphere or reflected back into space. Most of the radiation that reaches the Earth’s surface is ultraviolet, visible and infrared. It delivers an incredible 3,400,000 exa joules of energy per year, more than 7,500 times humanity’s current energy consumption. The insolation values vary in different regions. Because the Earth is round, most light falls per square metre at the equator, with the amount decreasing towards the poles. The values also vary throughout the year. They are higher in summer, when the hemisphere is inclined towards the sun, and lower in winter. Regional conditions, such as the slope of the terrain and the presence of clouds, have a further influence on insolation.

Passive solar systems

The sun’s rays transfer energy to the objects they hit, heating them up. The oldest use of solar energy is therefore heating. Simple systems that use solar heating are called passive systems because they contain no electromechanical components. These are greenhouses, solar collectors or simply suitably oriented houses. In the case of a south-facing house, the sun’s rays shine into the interior and warm it up. This way of harnessing the sun’s energy has been used since antiquity.

With the invention of glass, the possibilities of utilising the sun's heat have improved. Glass lets the sun’s rays in, but no longer lets the infrared rays from heated objects out. The heat inside the greenhouse increases. The warmer environment in the greenhouse has allowed more delicate crops to be grown, higher yields, as well as the ability to quite simply enjoy exotic tropical plants. Simple devices called solar collectors work on the greenhouse principle. The sun heats the inside of the glassed collector, which is fed with air, water or a heat transfer medium. Solar collectors are used for interior heating, domestic hot water or as part of a heat pump.

Active solar systems

Active solar systems increase the efficiency of energy harvesting by using various electromechanical devices. Thermal solar systems work on a principle similar to a magnifying glass. They concentrate the sun’s rays into a single point, a focal point. The high concentration of energy at one point allows for more efficient conversion of solar energy into industrially usable heat or electricity. Thermal solar power plants are divided into Central Tower Solar Power Plants, Parabolic Trough Collectors and Parabolic Dish Concentrators. Photovoltaic panels work on a completely different principle. They convert incident photons of sunlight directly into electricity. Unlike thermal solar power plants, they contain no moving mechanical parts. In 2022, the installed capacity of thermal solar power plants worldwide was 542 GW. Together they produced 440 TWh of heat. The installed capacity of all photovoltaic power plants was 1,177 GW, producing 1,300 TWh of electricity.

Concentration thermal solar power plants

A Central Tower Solar Power Plant concentrates the sun’s radiation into a single point using heliostats. Flat mirrors are arranged around a central tower and turned by a computer to reflect the sun’s rays into a receiver on the tower at every moment of the day. The high temperature reached at the focal point heats the heat transfer medium. This energy can be further used in technological processes that need heat or converted in the steam cycle into electricity.

A Solar Trough Collector consists of a system of parabolic troughs. At their focal point is a long pipe through which the heat transfer medium flows. The troughs follow the sun to harness the maximum of its energy.

Parabolic concentrators consist of a large parabola that is rotated with the sun. Temperatures of up to 700 degrees are reached at the parabola’s focal point. This energy is often harnessed by a Stirling engine. Smaller versions of parabolic concentrators can be used to heat food.

Thermal solar power plants have the advantage that the heat energy produced can be stored, for example in a molten salt tank, and used later, for example at night.

Photovoltaic power plants

A photovoltaic power plant utilises the energy of incoming photons and converts it directly into electricity. Its main unit is the photovoltaic cell. Its basis is most often a silicon semiconductor. It consists of two layers doped with different elements. Where they come into contact, a P-N junction is formed. The incident photons knock electrons out of the crystal lattice which accumulate in the N-layer, creating a voltage between the P and N-layers. Flat electrodes are attached to both layers of the semiconductor to allow the resulting current to be dissipated.

To prevent damage, the PV cell is sandwiched between a solid substrate and a cover glass. Depending on how much voltage and current is required, the PV cells are assembled into PV panels, and these are connected in larger arrays. Small PV cells can charge garden lights or mobile phones. Panels mounted on the roofs of houses cover a large part of the household consumption. Vast fields of photovoltaic panels produce vast amounts of electricity.

Pros and cons of solar energy

The great advantage of solar energy is that it is available all over the world. It is an inexhaustible and environmentally friendly resource. Devices that convert the sun’s rays into electricity are silent, so they can be installed virtually anywhere without disturbance. They produce no waste or emissions during their operation.

Because the Earth is spinning, the sun’s rays only hit the solar power plants during the day. However, the output of a solar power plant can fluctuate greatly during the day depending on the weather. In addition, the output of the plant varies considerably according to the season. In winter, when insolation is lower, it produces noticeably less than in summer. Furthermore, the closer to the equator the power plant is located, the more efficient it is. The same plant will have a lower output in higher latitudes.

The fluctuating and difficult-to-predict output of solar power plants leads to the need to store their output so that energy is available even when the solar power plant is not operating. Large battery storage systems are used. Thermal solar plants often store their daily output in large molten salt storage tanks. Anyway, solar energy has a large potential because the Earth’s surface receives 7,500 times more solar energy than is the consumption of the entire human civilization.