Everything you need to know about solar power and solar heat

What is solar energy? What are the best locations for solar energy generation plants? How do you generate solar electricity? How do you generate solar heat? How does a solar power system work? How does a solar heating system work?
How can solar electricity be stored? How can solar heat be stored? Is it worthwhile operating a solar power system? Is it worthwhile to operate a solar heating system?
These and many other questions about solar energy are answered here. At the end, you can decide whether you want to enter the solar business in the future as a solar energy producer or as an investor.

What is solar energy?

Solar energy is energy that lands on the earth with the rays of the sun and is
        - as electricity (solar electricity),
        - as heat (solar heat)
        - or as chemically stored solar energy in the form of biomass, which plants build up thanks to photosynthesis,
          can be harnessed.
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These are electromagnetic rays that are generated on the surface of the sun, where temperatures are around 5,500 degrees Celsius (°C). The reason for this radiation, also known as blackbody radiation, is nuclear fusion processes inside the sun, also known as hydrogen burning.
According to current scientific knowledge, the sun will continue to provide energy for another five billion years. This makes the sun currently the largest renewable (regenerative) source of energy that mankind can harness.
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In the course of the energy turnaround - away from fossil energy sources to renewable ones - the term solar energy has taken on a very concrete interpretation: Solar energy refers to the two forms of energy that we obtain from the radiation delivered by the sun:
        - solar heat
        - and solar electricity.
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In other words, in everyday use we tend to call the products we generate from solar energy solar energy. For both forms of energy, we have developed technologies (solar plants) to convert solar radiation into thermal energy (heat) and electrical energy (electricity).

What is a solar system?

Solar systems are technical systems with the help of which we obtain usable energy from solar energy. This involves energy conversion from one form of energy (electromagnetic energy) to another (thermal energy or electrical energy).
Depending on how solar systems work (working principle) and which form of energy they ultimately produce, a distinction is made between three types of solar systems:
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- Solar thermal systems generate usable heat
   Solar thermal systems (also called thermal solar systems or solar heating systems) are solar systems that use collectors to    "collect" and absorb solar radiation, generating heat in the rather low temperature range. The heat is either transferred directly to    heat consumers via a suitable heat transfer medium (gaseous or liquid) and consumed by them, or it is first transferred to a heat    storage tank to be consumed at a later time. The payback period for a solar thermal system is on average 15 to 20 years.

   Solar thermal systems are available in small, medium and large sizes: To supply a single household with heat for hot water and/or    central heating backup, a few square metres of collectors can be installed on the roof of the house or on the façade, or set up and    operated freely on the property.
   Medium-sized systems supply entire apartment buildings, hotels or sports facilities with solar heat. Large solar thermal systems    with a collector area of up to several thousand square metres are also called ground-mounted systems (FFA) and are installed on    suitable sites.
   They supply heat directly to a heating network (local heating network), which either supplies a village community or an urban    district with heat, or to a heating network of a heat supplier (district heating network).
   Alternatively, the heat from medium-sized and large plants can also be used as process heat for various commercial or industrial    processes.

- Solar thermal power plants generate heat and usable electricity from it
   So-called solar thermal power plants concentrate the sun's rays with the help of mirrors to generate high-temperature heat, which    they use to heat water vapour or a so-called thermal oil to drive turbines that use generators to produce electricity. A distinction is    made between
        - parabolic trough power plants
        - and solar tower power plants.  
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   A parabolic trough power plant works like this: curved mirrors bundle the incident solar radiation in a so-called focal line. A thin    absorber tube runs along this line, in which water vapour or a thermal oil circulates. The heat generated by absorbing the solar    energy is transferred to the steam or thermal oil - we are talking about temperatures of up to several hundred degrees Celsius.    Then the hot steam or oil is led to turbines.
   The trough-shaped mirrors of the parabolic trough power plant are usually adjusted to the position of the sun in order to harvest a    maximum of solar radiation throughout the day.
   A solar tower power plant also makes use of the fact that solar radiation can be concentrated. Here, however, hundreds of    computer-controlled burning mirrors direct it to the top of a tower. There is a comparatively small absorber in which temperatures of    up to 1,000 degrees Celsius are generated. This heat is used to generate steam that drives turbines with generators.
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- Photovoltaic systems generate usable electricity
   A photovoltaic system consists of interconnected solar cells that have been assembled into modules. When solar radiation hits the    modules, it causes an electrical voltage (direct current). An inverter turns this into alternating current, which can be fed directly to    the consumer, temporarily stored in a battery for electricity storage or fed into the electricity grid. Photovoltaic systems also come in    small, medium and large sizes. Small ones, for example, supply a household partially or completely with electricity, medium-sized    ones supply entire apartment buildings as well as other medium-sized electricity consumers, and large systems installed on open    spaces feed their large amounts of electricity into the grid.
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   According to the Federal Environment Agency, photovoltaic systems pay for themselves in terms of energy after one to two years    of operation. After this time, a system will have generated as much energy as is needed for its production, operation and disposal.
   Most solar cells consist of the semiconductor material silicon, i.e. quartz sand, which is highly purified and crystallised under    oxygen deprivation. A crystalline solar cell has three layers:
        - The top layer consists of silicon atoms that are "contaminated" with foreign atoms such as phosphorus. This is called a           negatively doped layer. The individual atoms are saturated here, so that free electrons can be found in this layer.
        - The lowest silicon layer, on the other hand, is enriched with boron atoms, which have too few electrons.
          It is also called a p-doped layer.
        - The so-called boundary layer contains saturated silicon atoms. Via this layer, the excess electrons from the upper layer           migrate to the lower layer to attach themselves to the boron atoms.
          Thanks to the electron movements, an internal electric field is formed, which is also called a p-n junction.
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   When solar radiation hits the solar cell, the electrons detach again from the boron atoms in the boundary layer and migrate to the    now unsaturated atoms in the negative layer. Metal contacts made of aluminium or silver are placed on the top and bottom of the    solar cell and connected to each other via a cable.
   They conduct the electrons and make them flow through the cable - an electric circuit is created. In addition to the crystalline solar    cells described above, there are also monocrystalline and polycrystalline solar cells, non-crystalline (amorphous) thin-film cells and    organic solar cells (thin-film modules made of organic plastics).
   Solar electricity can be consumed or marketed. Exciting financing models arise here for large-scale plant operators or investors.    Likewise for land leasers for the installation of a solar park.

Is the solar radiation sufficient for the economic operation of a solar system?

A prerequisite for the operation of a solar system to be economically viable is sufficient solar radiation. How much solar radiation reaches a spot on earth depends largely on the weather and the position of the sun in the sky (position of the sun).
Since both vary, the intensity of the solar radiation that hits the earth's surface also varies. Due to the eccentricity of the earth's orbit around the sun alone, it varies by about seven percent during the course of the year. On average, the intensity of solar radiation at the boundary of the Earth's atmosphere is about 1,367 watts per square metre (W/m²) - a value also known as the solar constant.
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Part of the energy of the solar radiation is scattered and reflected by the Earth's atmosphere. For example
        - from solid suspended particles such as ice crystals and dust particles,
        - liquid suspended particles
        - or from gaseous components of the atmosphere.

The atmosphere absorbs another part of the radiation energy and converts it directly into heat. The rest of the radiation radiates through the atmosphere and hits the earth's surface. There it is also partly reflected and partly absorbed and converted into heat. Ultimately, all of the sun's energy is released back into space in the form of reflected light and thermal radiation.
How much of the radiation is reflected and absorbed and how much is transmitted depends on the current state of the atmosphere.

Climatic factors such as humidity, cloud cover and also the length of the path that the rays take through the atmosphere influence the whole thing. It is calculated that about 30 percent of the radiation is lost as it passes through the atmosphere. The remaining 70 percent make up the global radiation and can be divided roughly 50:50 into direct and diffuse radiation.
The daily average of the radiation reaching the earth's surface (based on 24 hours) is about 165 W/m², with considerable fluctuations depending on latitude, altitude and weather conditions.
It should be known that the total amount of energy reaching the earth's surface is more than five thousand times greater than that required by humans. 
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Depending on the location, you can expect 1,300 to 1,900 hours of sunshine for a solar system in Germany. The average is 1,550 hours of sunshine per year. Global radiation in Germany averages 1,050 kilowatt hours per square metre per year (kWh/m2/a). In northern Germany you can expect values of less than 1,000 kWh/m2/a and in southern Germany values of more than 1,200 kWh/m2/a. This means that both solar thermal systems and photovoltaic systems can be operated economically in this country.
Provided that the location also meets the following requirements:
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The perfect location for a solar system
Solar systems should be oriented so that they receive the maximum amount of solar radiation. For systems located in Germany, this means that you should orient them towards the south. If the orientation deviates from this, you must expect a loss of yield. It is assumed that
        - about 5 percent less yield if the system is oriented towards the southeast or southwest,
        - about 20 percent less yield if the system is oriented towards the east or west.
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In addition, the angle of inclination of the collectors and modules influences how much solar yield the system generates. Here, an angle of inclination of 30 to 40 degrees (°) is considered to maximise the yield. Steeper angles are recommended for northern Germany and flatter angles for southern Germany.
In addition, the shading of the solar collectors and solar modules also plays a role when it comes to the maximum yield of the systems. Mountains, tall buildings, tall trees and bushes are all potential sources of shade. Whereby you should always plan ahead when planning a solar system and also take future buildings into account.

How are solar installations doing in Germany? - Current facts and figures

The following facts and figures on solar installations in Germany, solar thermal installations as well as photovoltaic installations, come from the German Solar Industry Association (BSW Solar).
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Solar thermal systems in Germany (as of the end of 2021)
According to BSW Solar, 2.5 million solar thermal systems were installed in Germany at the end of 2021. Together, they would result in a solar collector area of 21.6 million square metres (m2). 81,000 new solar thermal systems were added in 2021, with a total solar collector area of 0.64 million m2.
According to the industry association, the total solar thermal capacity generated by solar thermal systems in 2021 was 15.1 gigawatt-thermal (GWTh). A solar thermal capacity of 450 megawatt-thermal (MWTH) was newly installed in the year.
In total, solar thermal systems in Germany generated 8.8 terawatt hours of thermal solar heat (TWhTH) in 2021. This avoided greenhouse gas emissions in CO2 equivalents of 2.4 million tonnes.
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Photovoltaic systems in Germany (as of May 2022)
The industry association puts the number of photovoltaic systems installed in Germany at the end of 2021 at 2.2 million. Together, these would have a total installed photovoltaic capacity of around 60 gigawatt peak (GWP). In 2021, 235,600 photovoltaic systems were newly installed and a photovoltaic capacity including PPA systems of 6 GWP was newly reported.
In 2021, gross electricity generation from photovoltaic systems amounted to 50 terawatt hours (TWhP). The share of photovoltaic systems in net electricity generation for public electricity supply was 10 per cent.
In total, the photovoltaic systems operating in Germany in 2021 avoided 34.4 million tonnes (t) of greenhouse gas emissions in CO2 equivalents.

What are the advantages and disadvantages of solar energy?

The advantages and disadvantages of solar energy are quickly listed:
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Advantages of solar energy
        - The generation of solar energy, solar heat and solar electricity, as well as their use, do not release any air pollutants.
        - Greenhouse gases are also not emitted when generating and using solar energy.
        - Solar energy is delivered free of charge by the sun to the generation plant.
        - It replaces fossil fuels and thus reduces dependence on oil and gas imports.
        - Renewable solar energy is available in practically unlimited supply.
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Disadvantages of solar energy
        - In order for solar energy to provide a constant or demand-responsive energy supply, appropriate storage technologies are           needed.
        - Large solar plants (ground-mounted systems) require land. The demand for land competes with other interested parties for           land (agriculture and settlement/transport).

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