# Solar energy

Solar energy available worldwide. The colors in the map show the local solar radiation on the earth's surface averaged over the years 1991–1993 (24 hours a day, taking into account the cloud cover determined by weather satellites).
The areas marked by dark panes would be sufficient to cover the current global demand for primary energy through solar power alone (with an efficiency of 8%).
A laundromat in California , USA that uses solar energy to heat its hot water

As solar energy or solar energy is called the energy of solar radiation , which can be used industrially in the form of electricity, heat or chemical energy. Solar radiation is the electromagnetic radiation that arises as black body radiation on the solar surface due to its temperature of approx. 5500 ° C , which is ultimately due to nuclear fusion processes in the sun's interior ( hydrogen burning ).

Solar energy can be used both directly (e.g. with photovoltaic systems or solar collectors ) and indirectly (e.g. by means of hydropower plants , wind power plants and in the form of biomass ). The use of solar energy is an example of modern backstop technology .

## intensity

The solar radiation hitting the earth has been almost constant since it was measured. There is also no evidence of significant fluctuations in historical time. The average intensity of solar radiation at the boundary of the earth's atmosphere is around 1367  W / . This value is also known as the solar constant . Part of the radiated energy is scattered and reflected by the atmosphere from solid (e.g. ice crystals, dust) or liquid suspended particles as well as from the gaseous components . Another part is absorbed by the atmosphere and already there converted into heat. The rest passes through the atmosphere and reaches the surface of the earth . There it is partly reflected and partly absorbed and converted into heat. This energy is made usable in photosynthesis , photothermics and photovoltaics , among other things . The percentage distribution of the radiation on reflection, absorption and transmission depends on the respective state of the atmosphere. The humidity , the cloud cover and the length of the path the rays travel through the atmosphere play a role here. The radiation hitting the earth's surface is around 165 W / m² worldwide on a daily average (based on 24 hours) (with considerable fluctuations depending on latitude, altitude and weather). The total amount of energy hitting the earth's surface is more than five thousand times greater than the energy needs of mankind . Ultimately, all of the sun's energy is released back into space in the form of reflected light and thermal radiation.

## Use of solar energy

In terms of quantity, the largest area of ​​use of solar energy is the warming of the earth, so that biological existence in the known forms is possible in the near-surface area, followed by photosynthesis by algae and higher plants . Most organisms, including humans, are dependent on solar energy either directly (as herbivores) or indirectly (as carnivores). Fuel and building materials also come from it. The solar energy is also responsible for the fact that there are air pressure differences in the atmosphere, which lead to wind. The earth's water cycle is also powered by solar energy.

In addition to these “natural” effects, there is increasing technical use, especially in the area of ​​energy supply. Since solar energy is a regenerative energy source , its use is promoted in many countries, in Germany for example by the Renewable Energy Sources Act (EEG) .

With the help of solar technology , solar energy can be used both directly and indirectly in various ways:

Direct forms of use include:

Solar energy is used indirectly:

## Storage of solar energy

The solar irradiation is subject to daily and seasonal fluctuations from zero to the maximum value of the irradiance of around 1000 W / m². In order to guarantee the necessary security of energy supply, additional measures such as energy storage , control technology or additional systems such as a fuel- operated boiler are therefore always necessary.

In March 2011, the first solar gas system in Germany went into operation in the Morbach energy landscape . Solar energy is converted into synthetic natural gas and stored in gas form.

Thermal solar systems use different types of heat storage . In the case of devices for hot water, this is usually sufficient for a few days so that - at least in the summer half-year - sufficient heat can also be provided at night and during periods of bad weather. Long-term storage systems that transfer summer heat into winter are technically possible, but still relatively expensive.

In solar thermal, electrical power plants, mirrors use concentrated solar radiation to evaporate liquids and generate electricity using steam turbines . Heat storage (e.g. liquid salt tanks) can also store part of the heat (with low losses) during the day in order to compensate for short-term fluctuations in demand or to drive the steam turbine at night.

In photovoltaic power plants, electricity is generated using semiconductor effects. The direct current produced in this way is either used as such as part of a decentralized power generation in a stand-alone power grid ( buffering, for example, by batteries ) or fed into an existing alternating current grid via inverters . Storage via decentralized batteries and conversion to hydrogen and methane and subsequent storage in the natural gas network is possible there. The existing natural gas storage facilities in Germany would be sufficient to cover the storage requirements of a power supply that is mainly based on generation from photovoltaic and wind power plants.

## Potential of solar energy

DESERTEC : Sketch of a possible infrastructure for a sustainable power supply in Europe, the Middle East and North Africa

As the largest source of energy, the sun delivers an amount of energy of around 1.5 · 10 18 kWh per year to the earth's surface. This amount of energy corresponds to more than 10,000 times the global energy demand of mankind in 2010 (1.4 × 10 14  kWh / year).

The composition of the solar spectrum , the duration of the sunshine and the angle at which the sun's rays fall on the surface of the earth depend on the time, season and latitude. This also means that the radiated energy differs. This is, for example, around 1,000 kWh per square meter per year in Central Europe and around 2,350 kWh per square meter per year in the Sahara . There are various scenarios of how a renewable energy supply for the EU can be realized, including by means of energy conversion in North Africa and high-voltage direct current transmission . For example, satellite-based studies by the German Aerospace Center (DLR) have shown that with less than 0.3 percent of the available desert areas in North Africa and the Middle East, thermal solar power plants provide enough energy and water for the increasing demand in these countries and for Europe can be created. The Trans-Mediterranean Renewable Energy Cooperation , an international network of scientists, politicians and experts in the fields of renewable energies and their development, advocates such a cooperative use of solar energy. A publication from the USA called Solar Grand Plan suggests a comparable use of solar energy in the USA.

There was also thought to capture solar energy via satellite and transmit it to the earth. The advantage would be a higher energy density on the ground and the avoidance of day-night fluctuations. Due to the great effort required for this, far above all previous space technology, none of these projects has been implemented so far.

## Dependence of the radiation power on the angle of incidence

Solar radiation on the earth's surface is the main factor influencing weather conditions and the regional and global climate . The radiant flux density (irradiation ), i.e. the radiant energy per area and period of time, depends on the angle of solar radiation. At a shallow angle, fewer photons per area hit the ground and heat it less strongly than at perpendicular incidence. This is expressed by the following formula:

${\ displaystyle J = J_ {0} \ cdot \ sin (\ beta)}$

Here the radiant power refers to the radiant power at a perpendicular angle of incidence and the angle of incidence with respect to the horizon. ${\ displaystyle J}$${\ displaystyle J_ {0}}$${\ displaystyle \ beta}$

The effect is reinforced by the longer path that light has to travel through the atmosphere at shallow angles.

## Evaluation of the use of solar energy

• Use of solar energy does not release any air pollutants, such as B. Fine dust.
• The use of solar energy does not release any greenhouse gases and is therefore climate-friendly.
• The use of solar energy saves imports of fossil or nuclear fuels and thus reduces dependence on possible trouble spots and international conflicts, such as in the Middle East region.
• Solar energy is available in unlimited quantities according to human judgment.

• Due to the weather, day and season-dependent solar radiation, solar energy does not enable a constant or demand-based energy supply.
• Due to its low energy density, the use of solar energy requires a relatively large amount of space and the use of photothermal or photovoltaic systems can compete with the agricultural use of land.