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Solar panels: warm water for free
Solar panels: warm water for free
Video: Solar panels: warm water for free
Video: FREE HOT WATER | Building a Solar Batch Water Heater 2023, February

An increasing number of German households are using solar collectors. They absorb solar energy and convert it into heat. Regardless of whether for the production of hot water or to support the heating - the areas of application are diverse and a step towards sustainable living.

Solar panels (solar thermal) and solar cells (photovoltaics) can store the sun's energy and convert it into heat (solar thermal) or electricity (photovoltaic). This is a great achievement because the star in the center of the solar system is a huge source of energy: about 1.8 x 1, 014 kilowatts hit the earth every day. We humans do not even need as much energy - an energy output of 1, 010 kilowatts already covers the global requirements, and we are supplied with around 10, 000 times more energy every day.

Although the use of solar energy already 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 in the time of the oil crisis in 1973. At that time, alternatives to heating oil were searched, so that a few years the first solar panels came onto the market. Most of them were too expensive and technically unreliable.

But a lot has happened in the past 45 years. Solar panels can now be found on many rooftops, and they are a great way to generate additional heat.

Häuser mit Sonnenkollektoren
Häuser mit Sonnenkollektoren

Thanks to their discreet appearance, solar collectors are now suitable for any architectural solution. The roof integration has been tried and tested over many years and offers protection against wind and weather.

Photo: Fotolia / Gyula Gyukli

Structure and function of the solar collector

A solar thermal system is required to use the sun's energy for heating water or for heating. But be careful: A second heat generator is always necessary, especially in winter, the energy generated by the solar thermal system is not sufficient. Expensive, but only alternative: you can save the sun's heat in the winter with huge tanks.

The heart of a solar system is the collector. Inside is an absorber that captures the sun's rays and heats up. 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 propylene glycol is to prevent the water from freezing in winter and becoming too hot in summer. The heated liquid is pumped through the pipes through the absorber and then into the solar storage tank. The absorber is usually insulated to the side and below, preventing heat loss.

Types of solar panels

Depending on the footprint and budget, you can choose between two basic models: the flat collector and the tube collector.

Flat plate collector


The flat collector is used very often. It contains a flat, continuous absorber made of blackened metal, for example, on the back of which the water pipes are attached with good heat conduction. How many sun rays are absorbed by the absorber depends on the material, the shape, the arrangement of the tubes and the coating. The plates absorb the sun's rays best when the material is as black, thin and heat-conductive as possible.

Heat transfer medium

The circulating water is enriched with antifreeze such as propylene or ethylene glycol so that freezing temperatures do not cause the tubes to burst. 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 collector. Hard foam boards are widespread in connection with mineral wool mats, but also sheep and rock wool. Caution 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 transmittance 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. Low-iron, tempered glass is used for this. It is usually structured so that it does not dazzle. Anti-reflective glasses that have a higher permeability than structural glasses have been available for some years. However, they cost a little more.

Vacuum tube collector

The vacuum tube collectors exist from a variety of vacuum tubes surrounded by metallic reflectors. Sunlight is thus reflected into the tube by the reflectors. These contain a dark absorber. The fact that the tubes are pumped empty of air minimizes heat loss. In addition, higher temperatures arise in a smaller area. The collector works effectively even in diffuse sunlight. For example, tube collectors on 3.5 square meters generate as much warm water as flat-plate collectors on an area of ​​five square meters. In summer, this is almost completely enough to heat the water of a family of four. In a year, 60 percent of the family's hot water needs are met. Disadvantage: the tube collector costs twice as much as a flat plate collector.

With tube collectors, it should be noted that there are two different types, which we will introduce below:

Heat pipe principle

Each closed tube forms a closed circuit. The sun's rays that hit the absorber in the tube let a special liquid 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 down again - the evaporation starts again. The heat released in this way heats up the circulation liquid that flows past the heat exchanger. The heat pipe system requires a minimum inclination angle of around 30 percent for the circuit to function.

Directly flowed tubes

With this system, the sun heats up the heat transfer fluid in the absorber tube directly in the middle of the tube. The heat transfer medium passes through several tubes. A great advantage of this system is that the tubes can be installed vertically and horizontally.


Vacuum tube collectors deliver optimal energy yields even in low sunshine and in the cold season.

Photo: Fotolia / cybercrisi

What conditions does the yield depend on?

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

  • The 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.
  • Orientation: The best is a south orientation and a tilt angle of around 45 degrees.
  • Shadows from other buildings or trees.
  • Pollution: Rain and snow mostly remove a large part of the dirt. It is best to use gentle cleaning agents and a soft brush to remove stubborn stains. Do not use high-pressure cleaners.

The size of the collectors for water heating

If you use flat-plate collectors, you need 1.5 square meters of collector area per person in the household to heat the water. For more powerful tube collectors, 1 square meter per head is sufficient. Rule of thumb for the size of the water storage: Calculate 40 liters of 45 degrees Celsius warm water per person per day times 2. A 4-person household can therefore cope with 6 square meters of flat or 4 square meters of tube collectors and a storage with 300 liters. Cost: about 3, 500 euros.

Heating with solar heat

To use solar heat for space heating, calculate 3 to 4 square meters of collector area per person in well-insulated houses. The storage tank should hold about 60 liters per square meter of collector area, slightly larger for tube collectors. Cost: from around 6, 000 euros. In a well-insulated house, the solar heating system can contribute up to 25 percent of the energy for heating and hot water.

Effect of snow

Once the solar panels are covered in snow, they usually do not produce any yield. Often, however, the yield is not available on these days anyway, because the sunlight is usually too low.

With tube collectors, a good yield is still possible on clear, cold days, so that removing the snow can make sense.

Solarkollektor mit Schnee bedeckt
Solarkollektor mit Schnee bedeckt

If the solar collector is covered with snow, it usually does not absorb sunlight and therefore cannot store energy in the form of heat.

Photo: istock / alexandrumagurean

The efficiency of solar collectors and solar cells in comparison

What is more - solar collectors or solar cells? Experience has shown that solar collectors are significantly more efficient than photovoltaic modules. In this way, solar collectors achieve efficiencies of around 90 percent, with heat losses only 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 range. Any type of radiation can be converted into heat. However, photovoltaic systems can only convert a small part of the radiation into electricity.

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