Why should one use solar panels

Solar collector: a good alternative to photovoltaics?

Solar collectors are in use in an increasing number of German households. They absorb the sun's energy and convert it into heat. Regardless of whether it is used to generate hot water or to support heating: A solar collector has many areas of application. We explain to you which different collectors there are, how they work and what costs you have to reckon with.

Solar collector: a good alternative to photovoltaics?
Solar collector: a good alternative to photovoltaics?

What does a solar panel do?

Solar collectors (solar thermal) and solar cells (photovoltaics) can store the sun's energy and convert it into heat (solar thermal) or electricity (photovoltaics). This is a great achievement, because our sun is a huge source of energy: every year the sun delivers an amount of energy of around 1.5 times 10 to the power of 18 kilowatt hours to the earth's surface - around 10,000 times more energy than we need.

Although the use of solar energy began in antiquity and the forerunner of today's solar collectors was invented in the 19th century, the real use of thermal solar technology in Europe only began during the 1973 oil crisis then the first solar collector came onto the market. Most of the time, however, the collectors were too expensive and technically unreliable.

But a lot has happened in the last 45 years. Solar panels can now be found on many house roofs. No wonder: a solar collector is a good way to generate additional heat and to use solar energy properly.

How is a solar collector constructed?

A solar thermal system is required to use the sun's energy to heat water or for heating. The heart of this solar system is the solar collector. Inside there is an absorber that captures solar radiation and heats up in the process. This creates heat, which in turn is transferred to a heat transfer medium. The heat transfer medium is a liquid (solar liquid), usually consisting of water and propylene glycol. The main purpose of the propylene glycol is to prevent the water from freezing in winter and too hot in summer. The heated liquid is pumped through the pipes through the absorber and then into the solar storage tank of the solar system. Most of the time, the absorber is insulated on the side and underneath to prevent heat loss.

Is a solar collector an alternative to solar cells?

Experience has shown that a solar collector has a significantly higher degree of efficiency than a solar system with photovoltaic modules. A solar collector achieves efficiencies of around 90 percent, with heat losses of just under 50 percent. However, this is still better than the efficiency of a photovoltaic system with an efficiency of only 20 percent.

The reason for this is that sunlight covers a wide spectrum. A solar collector can convert any type of radiation into heat. However, photovoltaic systems or mini-photovoltaic systems can only convert a small part of the radiation into energy - but this solar system is also worthwhile.

Solar collector: what is the yield?

The annual yield of a solar collector depends on several factors:

  • Climatic conditions: There can be considerable differences between individual locations within Germany. There are locations with over 1,000 kilowatt hours per square meter per year and locations with less than 900 kilowatt hours per square meter per year. This factor also affects the yield of a solar collector.
  • Alignment: How much solar energy a solar collector can convert into heat also depends on its orientation. It is best to align the collector to the south and at an angle of about 45 degrees.
  • Surroundings: If there are tall buildings or trees next to the solar collector, these shadows may appear on the collector. The solar system then receives less sun and can no longer convert its energy into heat as well.
  • Pollution: Soiling on the solar collector is another factor that affects the yield of the collectors. Rain and snow carry away a large part of the dirt, but sometimes that's not enough. It is best to use gentle detergents and a soft brush against stubborn stains. Under no circumstances should you clean a solar collector with a high-pressure cleaner. Instead, follow our tips for cleaning a photovoltaic system.


In general, a second heat generator is always necessary because, especially in winter, the energy generated by the solar thermal system is not sufficient, especially with flat-plate collectors. Once a solar collector is covered in snow, it usually no longer delivers any yield. Often, however, there is no yield on these days anyway, because the solar radiation is usually too low. Expensive but the only alternative: You save the sun's heat for the winter with huge tanks. With tube collectors, however, a good yield is still possible on clear, cold days, so removing the snow can make sense.

The yield of a solar collector is therefore dependent on many factors - above all on the type of collector and the size of the collector surface. Further down in the article we will tell you how large the collector surface should be for your household and what costs you have to reckon with for the solar collector.

What kind of solar panels are there?

Depending on the installation space and budget, you can choose between two basic models for a solar collector: the flat-plate collector and the tube collector.

Flat plate collector as a solar collector

  • Absorber: The flat-plate collector is used very frequently. It contains a flat, continuous absorber - for example made of blackened metal - on the back of which the water pipes are attached so that they conduct heat. How many sun rays are absorbed by the absorber depends on the material, the shape, the arrangement of the tubes and the coating. The panels absorb the sun's rays best when the material is as black, thin and thermally conductive as possible.
  • Heat transfer medium: In the flat-plate collector, the circulating water is enriched with antifreeze agents such as propylene or ethylene glycol so that temperatures below zero do not burst the pipes. However, additives should be environmentally neutral.
  • Collector housing: The absorber rests in a tightly closed box that keeps moisture and dust away. The housing must be made of corrosion-resistant material.
  • Insulation layer: The housing is lined with a light but strong material. It must have a low U-value and withstand the high temperatures in the flat-plate collector. Hard foam panels in connection with mineral wool mats are widespread, but sheep's wool and rock wool are also used as an insulating layer on flat-plate collectors. However, caution is advised with outgassing ingredients: They form a precipitate on the inside of the glazing and reduce the incidence of light.
  • The glass cover: The collector box is covered with a glass plate. The so-called degree of transmission is important for the selection of the glass. It indicates how much light radiation the glass lets through. The glass should reach a value of 0.9. For this purpose, toughened glass that is low in iron is used. It is usually structured so that it doesn't dazzle. For a number of years there has been anti-reflective glass that has a higher permeability than structured glass. However, they cost a little more.

Read more interesting articles here

Vacuum tube collector as a solar collector

The vacuum tube collectors consist of a large number of vacuum tubes surrounded by metallic reflectors. The sunlight is thus reflected by the reflectors into the tube. These contain a dark absorber. Because the tubes are evacuated, the loss of heat is minimized. In addition, higher temperatures occur on a smaller area. The collector works effectively even in diffuse sunlight. In this way, tube collectors generate as much hot water on 3.5 square meters as a flat plate collector on an area of ​​five square meters. With a lot of sun in summer, this is almost completely enough to heat the water of a family of four. Over a year, 60 percent of the family's hot water needs are met. The disadvantage: a tube collector costs twice as much as a flat-plate collector.

With the tube collectors it should be noted that there are two different types, which we present in the following:

  • Heat pipe principle: Each closed tube in the solar collector forms a closed circuit. The rays of the sun that hit the absorber in the tube cause a special liquid to evaporate. At the top of the tube, the steam cools down on a heat exchanger block. The steam liquefies again. The liquid in the tube flows back down - the evaporation starts again. The heat released in the process heats up the circulation fluid that flows past the heat exchanger. The heat pipe system requires a minimum angle of inclination of around 30 percent for the circuit to function.
  • Tubes with direct flow: In this system, the sun directly heats the heat transfer fluid in the absorber tube in the middle of the tube. The heat transfer medium runs through several tubes. A great advantage of this system is that the tubes in the solar collector can be installed vertically and horizontally.

How much does a solar collector cost?

Solar collector for water heating

Regardless of whether you use solar cells or a solar collector: The yield and the costs depend on the size of the solar system. Anyone who uses flat-plate collectors needs 1.5 square meters of collector surface per person in the household to heat the water. For the more powerful tube collectors, one square meter per head is sufficient.

Rule of thumb for the size of the water storage tank: Calculate 40 liters of 45 degrees Celsius warm water per person and day times two. A four-person household can get by with six square meters of flat or four square meters of tube collectors and a storage tank with around 300 liters. The costs for the solar system then amount to around 3,500 euros.

Heating with solar energy

In order to use solar heat for space heating, calculate with three to four square meters of collector surface per person in houses with good thermal insulation. The storage tank should hold around 60 liters per square meter of collector surface, slightly larger for tube collectors. The costs start here at around 6,000 euros. In a well-insulated house, the solar heating system can provide up to 25 percent of the energy for heating and hot water.