A solar collector or solar collector ( Latin sol " sun " and colligere "collect"), also thermal solar collector ( thermal solar collector ), converts solar radiation into thermal energy . The heat can be used for heating, cooling, water disinfection, desalination, etc. Solar collectors are part of a thermal solar system .
Solar collectors can be installed both on building roofs and as open-space systems. Open-space systems are used in particular in the form of large-scale systems as a core element of solar district heating systems. A liquid heat transfer medium (oil, water) is usually heated in the collectors. Solar collectors that heat the air are called air collectors .
To the solar collectors also are sometimes solar ponds counted, in which solar radiation heats an aqueous salt solution, which is below a fresh layer. In contrast, devices for generating electrical energy in photovoltaic systems are not referred to as solar collectors, but rather as solar modules .
Principle of the thermal solar collector
Thermal solar collectors achieve relatively high levels of efficiency when utilizing solar radiation - typically between 60 and 75%. In Europe, depending on the season and the position of the sun, between 200 and 1000 W / m² fall in sunshine (see also solar constant ).
The central component of the collector is the solar absorber, which converts the radiant energy of the sun into heat and transfers it to a heat transfer medium flowing through it . With the help of this heat carrier, the heat is removed from the collector (e.g. via a heat exchanger ) and then used or stored directly.
To reduce the inevitable heat losses, is a good thermal insulation of the absorber from the environment necessary. A distinction is made according to the insulation technology
- Flat-plate collectors using conventional insulation material;
- Evacuated tube collectors , which achieve insulation through a vacuum , but are more expensive to purchase; and
- Vacuum flat-plate collectors , d. H. Flat design, good gross / net area ratio and vacuum insulation.
- Single absorbers that are used as low-temperature collectors for heating swimming pools: They are usually made of plastic and are usually not additionally insulated at all.
- The simplest design is a dark, water-filled container. In the sunshine , suitable containers heat up to almost boiling temperature in a few hours , which has been used in the south for centuries. Even in Central Europe , an ordinary garden hose can reach water temperatures of over 60 ° C in summer. For reasons of hygiene, this technique should not be used for drinking water.
These collectors absorb the radiation almost evenly from all directions, they do not have to track the sun and still deliver a certain amount of power even when it is cloudy. There are also concentrating collectors that work on the principle of the focal mirror and achieve significantly higher temperatures. Parabolic trough collectors in solar thermal power plants reach temperatures of around 400 ° C, by which a steam power plant can be operated. Such procedures are only worthwhile when there is strong direct sunlight (without cloud cover). While in the past only firmly established and anchored collectors used, there are now also systems that the direction to the sun tracked are.
The solar collector is the central component of a thermal solar system and was mostly only used for hot water preparation until the beginning of the nineties, the energy is increasingly being used in space heating. In connection with a low-energy house and a seasonal heat storage system , the room can even be heated entirely using solar collectors.
If several solar collectors are connected in parallel, they must be connected according to the Tichelmann system in order to ensure the most even flow possible.
A thermosiphon collector works without a pump according to the gravity circulation principle: In the collector, water is heated and rises upwards, when it cools down it sinks again (natural circulation). Conversely as with gravity heating using the same principle , the storage tank must therefore be located above the solar collector. The thermosiphon collector has often already integrated a hot water storage tank and thus represents a complete, simple solar system. Such systems can be found on many roofs , especially in southern countries ( Greece , Turkey , Israel , Australia ), because the storage tank exposed above the roof cladding would be in cool down too quickly in colder countries. In addition, for reasons of weight, it is difficult to mount a storage tank on the roof whose capacity is sufficient in countries with seasonally low sunshine duration and intensity.
The thermosiphon system should not be confused with the thermosiphon storage tank, in which the thermosiphon principle is used to charge a hot water storage tank with optimal temperature stratification.
The scheme shows the basic structure of a flat-plate collector with the most important components. The rays of the sun falling through a glass plate hit a solar absorber. When the sun's rays strike, almost the entire spectral range of the light is absorbed. The heat released should not be lost, which is why the collector is thermally insulated on all sides. The convective heat dissipation to the front is reduced by one or two panes of glass. In the case of vacuum collectors, it is completely prevented.
Heat, which is radiated again by the absorber due to its own temperature, can at least be retained by the glass pane, since glass is not transparent for the higher wavelength (wavelength-selective transparency, greenhouse effect) - a radiation equilibrium is formed. Special solar glass is often used for solar collectors ; it is more transparent than window glass, withstands temperature inhomogeneities better and is less degraded by ultraviolet light and aging.
The absorber can have wavelength-selective absorption, especially in vacuum collectors, so that on the one hand there is a high level of absorption for sunlight and on the other hand there is a low emissivity in the mid-infrared and ensures that less thermal radiation is emitted.
The heated absorber transfers the heat to a heat transfer fluid (water / glycol, brine, oil, so-called solar fluid ) flowing through copper or aluminum pipes that are permanently connected to the absorber . It transports the heat to a consumer or a heat store. There are solar collectors with an open liquid circuit in which the water to be heated flows directly through the absorber (especially in thermosiphon systems ). In regions with a greater risk of frost, however, separate fluid circuits are usually used. A substance is added to the self-contained solar circuit, also known as the primary circuit, which lowers the freezing point - for example the non-toxic propylene glycol . The heat is then passed through a heat exchanger e.g. B. transferred to the heating water or the water of the heat storage tank.
In evacuated tube collectors, the thermal insulation is improved by an evacuated space inside the glass: thermal energy can only be released back to the colder environment through radiation, but not through convection or conduction. Round glass tubes are used to withstand the compressive forces.
The solar absorber is a main component of a thermal solar collector. It serves to absorb solar radiation.
The solar absorbers are located on sheets made of aluminum or copper. Supported by a selective coating, this absorber heats up in sunlight; this heat is absorbed by a fluid ( solar fluid or air) flowing in pipes on the absorber and transported to use or to a heat store . The solar collector must have good thermal insulation (by means of a vacuum or suitable insulation materials) in order to avoid heat loss and to maintain a higher working temperature.
Roof tile absorber
The roof tile absorber is an absorber design that should not impair the appearance of the roof. It is an open aluminum full-surface absorber in the form of a roof tile. The absorbers dissipate the heat via heat conducting plates to a previously installed pipe system, which is located under the roof tiles on the roof battens and through which a heat transfer fluid flows. During assembly, the roof tile absorbers only need to be clicked onto the roof tile using the guide plate. Roof tile absorbers are robust due to the lack of a cover, but cannot achieve the same level of absorption or insulation as other collectors, which is why they are not very efficient.
Swimming pool absorbers / absorber mats
So-called swimming pool absorbers are mats made of UV- resistant black plastic that are laid out or set up near swimming pools or outdoor pools . The mats consist of hoses or a plate radiator-like shape made of polyethylene , through which the swimming pool water is pumped directly - this makes a heat exchanger superfluous. Compared to other collectors, such absorber mats only achieve moderate increases in temperature, but this is of no consequence for the stated purpose .
Surface or plate absorbers
The absorber has the shape of a plate ( flat plate collector ). The heat transfer fluid is passed through copper pipes that are connected to the plate absorbers by soldering, welding or seaming in order to ensure thermal conductivity. Another design consists of profiled copper plates placed on top of one another and connected to one another, through which the carrier fluid flows directly through the profile space (strip absorber , also known as sunstrip ). They can be produced easily and efficiently on an industrial scale.
Tube absorbers are those designs in which the tube through which the heat transfer fluid flows itself serves as an absorber or is only supplemented by relatively narrow additional lateral absorber surfaces. Tube absorbers can be found, for example, in certain types of vacuum tube collectors , in which a water-carrying absorber tube runs in the vacuum- sealed glass tubes, possibly supplemented by a narrow absorber plate that is soldered on. Swimming pool absorbers (see above) are also sometimes referred to as tube absorbers if they consist of hoses running close together.
PV / T or PVT systems combine photovoltaics (PV) with thermal (T) use of solar energy. However, the PV cells - especially those made of crystalline silicon - have a decreasing efficiency as the temperature rises. Therefore, low temperature systems are particularly suitable for PVT.
It is also possible to use air as a heat transfer medium. One then speaks of an air collector . The heated air is usually pumped directly into the room to be heated and is used for both ventilation and heating.
Solid absorbers are usually part of the building . A massive wall, wall or roof surface is heated by solar radiation and ambient air. Pipelines with heat transfer fluid run inside the component. The thermal energy stored in the liquid is usually brought to a higher temperature level by a heat pump .
In order to achieve the highest possible absorption of solar radiation, the surface of the absorber facing the sun ideally appears black . Black color is suitable for this, but a selective absorber is better, which absorbs the energy of the sun, which mainly radiates in the visible spectral range, as well as possible and only poorly emits the longer-wave heat radiation of the absorber. For this purpose, the emissivity or the degree of absorption (they are equivalent to each other) for light must be as large as possible (close to one) and in the wavelength range relevant for the emission (according to Planck's law of radiation and Wien's law of displacement at 100 ° C by 8.5 µm)
For a long time, black nickel or black chrome , galvanically applied layers of a certain structure, were used, which have an emissivity of 10 ... 18% in the mid-infrared. To put it very simply, the structure consists of microscopic metal hairs that trap the sunlight between them, but due to their small size, emit little at longer wavelengths.
Selective coatings such as Tinox (titanium nitride oxide coating), Sunselect , Mirotherm and eta plus (cermet coating), and others usually have a bluish shimmering color. With 91… 96% absorption for light, they achieve similarly high values as the previously used black chrome coating, but at the same time significantly lower infrared emissivities (around 5%), so they lose less heat through radiation. As a result, they achieve a significantly higher level of efficiency than absorbers painted in black and also as absorbers coated in black chrome.
Absorber layers must be heat and UV resistant over the long term. Aluminum and copper are used as carrier metals.
Selective thin-film absorbers are also considered to be more environmentally friendly because they do not use galvanic processes, lower manufacturing energy requirements per area and unproblematic recycling.
In hot countries, absorbers are often used that are only coated with so-called solar paint. This black lacquer is very heat-resistant, but the emissivity in the mid-infrared is very high, as with all lacquers - part of the captured heat is therefore radiated again. Tinox and cermet coatings can only be applied to copper so far. A nickel oxide coating has been developed for aluminum sheet.
Is the temperature that the collector reaches with standard irradiation of 1000 W / m² when idling without solar fluid. The level of the stagnation temperature of the collector depends on its quality. Most of the time, the certificates of collectors show temperatures between 170 and 230 degrees Celsius; for some collectors this temperature is given as over 300 ° C. The better a collector is insulated, the higher this temperature is. Every collector must be designed in such a way that it can withstand these extreme temperatures without damage. However, accelerated aging occurs more or less always, depending on the design and make. Copper manifolds scale with repeated stagnation. There are also collectors with stainless steel manifolds. Is one in stagnation befindlicher collector with cold solar fluid filled, the sudden cooling may cause damage. Refilling should therefore take place with the collector covered or in the early morning hours or in the evening.
The principle of solar thermal energy has been in use for a long time: Burning and concave mirrors have existed since ancient times. The use of solar energy goes back to the Greek mathematician and inventor Archimedes of Syracuse (285–212 BC), who allegedly set the Roman fleet on fire with the help of burning mirrors.
In the 18th century, the naturalist Horace-Bénédict de Saussure invented the forerunners of today's solar collectors. In the 18th century he built a simple wooden box with a black bottom and glass cover. With this first solar collector, it reached a temperature of 87 ° C.
In the middle of the 19th century, the French Augustin Mouchot developed the de Saussures solar collectors and combined them with burning mirrors. In 1878 he presented a solar steam engine at the Paris World Exhibition . He suggested using these steam engines to convert solar energy into electricity .
Areas of application: household to industry
The best known and most common application of solar heat is the hot water preparation in the household . With a suitable design of collector and storage volume it extends in Central Europe throughout the summer season for washing and bathing. Theoretically, the solar heat can cover the needs of a household all year round, but then the system will either be much larger and deliver a lot more heat in summer than can be used, or a seasonal heat store is required . Efficient systems can supplement conventional heat sources even in the winter months. The share of a solar system in the supply of hot water is between 50 and 60% over the year, which corresponds to approx. 14% of the heating energy requirement.
The first large-scale applications since the energy crisis of the 1970s were the heating of public and increasingly private swimming pools . Another upswing in the spread of hot water collectors in Germany was achieved not least through various federal and state funding programs. Also industrial plants use solar radiation for a long time as process heat . So is u. a. the heating of biomass cultures - such as for the generation of biogas - ready for production . If higher process temperatures are required, parabolic trough collectors can be used.
Larger collector systems are useful for room heating. With standard heating, it can contribute double-digit percentages to heating energy on an annual average and therefore noticeably reduce heating costs. If you also use a seasonal heat store, it is even possible to store enough heat in the summer half-year so that the heating energy demand can be covered all year round. There are only restrictions if the collector surface that can be installed is too low in relation to the annual heating energy requirement, for example in multi-storey houses. Seasonal heat storage systems use the heat capacity of water, gravel or concrete or the latent heat of brine or paraffin. Houses with passive solar construction or solar collectors and seasonal heat storage are also referred to as solar houses .
In order to ensure sufficient hot water even on cloudy and rainy days, a hot water storage tank with a heat exchanger function is built into the solar thermal system , which for individual households - depending on the number of people (family size) and usage behavior - has a water filling capacity of around 300 to 1500 liters. For larger residential units, hospitals, hotels, etc., which can have relatively cheap amortization periods due to their size and significantly more continuous use, adapted industrial storage systems are often used. To a higher demand for heat or to compensate for lack of an overcast sky heat from the collector, in the hot water tank is either a heater built in, or the memory is on another integrated heat exchanger with the heating boiler connected to the house.
The black cover of a solar balloon usually consists of thin, light plastic foil. Inside the solar balloon there is normal ambient air, the density of which decreases due to the warming. As a result, the solar balloon is a subspecies of the hot air balloon . The resulting buoyancy is typically around 100 grams per cubic meter, rarely higher.
Solar collector systems are generally particularly attractive in terms of their low operating costs, since they only cause low running costs without a fuel requirement. A maintenance check is also required every two years. In contrast to passive solar architecture , which already affects the design of the building envelope, collector systems can often be easily integrated into existing buildings, which is why economic considerations in old buildings often only take place between a solar collector system or other active forms of heating. In such a comparison, the environmental impacts should also be included. The system is also very easy to use, since z. B. no residual ash has to be removed, as is the case with many pellet heating systems .
Since the heating energy demand can already be influenced by the building insulation, a decisive question is whether the available funds are generally invested in a larger heating system or, instead, in better thermal insulation. The answer depends on the existing insulation and the structural options for attaching a larger collector surface or the use of other forms of heating, including completely dispensing with heating in new buildings with passive solar architecture.
When designing a heating system, a distinction must be made between the sole use with a certain season of heat storage and the combined use with another form of heating. The choice of technology for seasonal intermediate storage has a decisive influence on the profitability of the overall system. The classic limitation of a collector system to "DHW heating and heating support" can be fundamentally wrong, provided that the acquisition costs for the seasonal heat storage are low enough. Here you have to start from the specific product price for the respective overall system, as well as the service life and running costs. Often it is precisely through the technically comparatively simple seasonal intermediate storage of heat, for example with largely loss-free thermochemical heat storage systems , large or at least well-insulated buffer heat storage systems , or also low-loss latent heat storage systems, that low overall costs can be achieved. A possible adjustment of the collectors or a change in the installation angle towards winter can also affect the price-performance ratio.
In Germany, solar collectors with a total area of 900,000 m² were newly installed in 2014, in Austria the total newly installed area in 2013 was 150,000 m².
Solar collector systems do not cause any direct emissions during operation and reduce CO 2 and particulate matter emissions compared with conventional heating systems . In just a few months, a collector has supplied the same amount of energy to the heating system that had to be used for the production etc. of the collector. Depending on the location (i.e. annual solar radiation) and the technology used (glazed and unglazed collectors), the energetic amortization period is between 2 and 12 months, the carbon dioxide payback time is 1–2 months for unglazed collectors and 12 and 30 months for glazed collectors.
- Norbert Schreier et al .: Optimal use of solar heat . Wagner & Co Verlag, 1980–2005, ISBN 3-923129-36-X .
- Ulrich Fox: Solar collectors - thermal solar systems . Kohlhammer, Stuttgart 1998, ISBN 3-17-015009-X .
- John A. Duffie et al .: Solar engineering of thermal processes . Wiley, Hoboken 2006, ISBN 0-471-69867-9 .
- Ursula Eicker : Solar Technologies for Buildings. Basics and practical examples. 2nd, completely revised edition. Vieweg + Teubner, Wiesbaden 2012, ISBN 978-3-8348-1281-0 .
- Martin Kaltschmitt , Wolfgang Streicher, Andreas Wiese (eds.): Renewable energies. System technology, economy, environmental aspects . Springer Vieweg, Berlin / Heidelberg 2013, ISBN 978-3-642-03248-6 .
- Volker Quaschning : Regenerative Energy Systems. 9th edition. Hanser, Munich 2015, ISBN 978-3-446-44267-2 .
- * Viktor Wesselak , Thomas Schabbach , Thomas Link, Joachim Fischer: Handbook Regenerative Energy Technology. 3rd, updated and expanded edition. Berlin / Heidelberg 2017, ISBN 978-3-662-53072-6 .
- Achmed AWKhammas: The Book of Synergy available online, accessed on October 15, 2011.
- http://www.spf.ch/fileadmin/daten/publ/OTTI2002_Solarglas.pdf Stefan Brunold, Ueli Frei: What is solar glass? , Publication by the Institute for Solar Technology SPF , accessed on October 22, 2017
- the company website Oekotech solar collectors GmbH to sunstrip -Kollektoren, accessed on 22 October 2017
- http://www.fvee.de/fileadmin/publikationen/tmp_vortraege_jt2007/th2007_11_gombert.pdf Andreas Gombert, Rolf Reineke-Koch, Karsten Fenske, Thomas Hofmann: Optical coatings for solar collectors - technologies and quality assurance , lecture at the FVS annual conference 2007 , accessed on Oct. 22, 2017
- http://www.bine.info/fileadmin/content/Publikationen/Projekt-Infos/1999/Projekt-Info_05-1999/projekt_0599internetx.pdf BINE Informationsdienst 5/99 of the Fachinformationszentrum Karlsruhe (Society for Scientific and Technical Information mbH), Page 3, accessed Oct. 22, 2017
- Stiftung Warentest: Test of solar systems - test 03/2008
- Statistical figures of the German solar heating industry ( Memento from January 29, 2016 in the Internet Archive ), Bundesverband Solarwirtschaft , 06. 2015
- solar thermal market collapses ( Memento from February 3, 2016 in the Internet Archive ), Wirtschaftsblatt , July 25, 2014
- Gabriele Comodi et al .: Life cycle assessment and energy-CO2-economic payback analyzes of renewable domestic hot water systems with unglazed and glazed solar thermal panels . In: Applied Energy 64, (2016), pp. 944-955, doi : 10.1016 / j.apenergy.2015.08.036 .