Solar thermal collector

A solar panel or solar panel (from the Latin: sol for sun and colligere "collect" ) is a device for collecting the energy contained in sunlight, which then energy for heating, cooling, water disinfection, etc. is used.

Traditionally, the term used for a thermal solar collector (also thermal solar collector called ) which is a transmission medium ( hot water ) heating up with the absorbed solar energy, with almost the entire radiation spectrum of sunlight is used in solar thermal systems with relatively high efficiency. Devices for producing electrical energy in photovoltaic systems are usually referred to as solar panels. Solar panels are generally to devices for the collection of solar energy contained in sunlight for technical utilization (see also: Solar power plant).

The solar panels but are also open solar ponds, which use solar energy heating up an aqueous salt solution, with an overlying stratified freshwater layer serves as an insulator.

Principle of thermal solar collector

Solar thermal panels reach in the utilization of solar radiation relatively high efficiencies - typically between 60 and 75 %. In Europe, covered in sunshine depending on the season and Sun between 200 and 1000 W / m² (also see solar constant ).

A key component of the solar collector absorber of light energy from the sun is converted into heat and this gives to a heat carrier flowing through it. With the help of that heat source the heat is removed from the collector (eg, via heat exchangers) and then used directly or stored.

In order to reduce the inevitable heat losses, good thermal insulation of the absorber relative to the environment is necessary. After the insulation technology differentiates

• Flat plate collectors that use conventional insulation material;
• Vacuum tube collectors, which reach the insulation by a vacuum, but are more expensive to purchase; and
• Vacuum flat solar panels, ie flat design, good gross / net area ratio and vacuum insulation.
• Simply absorbers, which are used as low-temperature collectors for heating swimming pools: They consist mostly of plastic and are not additional insulation in the rule.
• The simplest type is a dark, water-filled container. In sunshine, suitable container heat in a few hours almost to the boiling point, which is used in the south for centuries. Even in Central Europe, an ordinary garden hose in summer water temperatures reach above 60 ° C. For hygienic reasons, this technique can not be used for drinking water.

These panels take light approximately equally from all directions, it is not necessary to track the sun and provide even on cloudy days, a certain power. There are also concentrating collectors that work on the principle of the fuel level and achieve significantly higher temperatures. Parabolic trough collectors in solar thermal power plants can reach temperatures around 400 ° C, with which a steam power plant can be operated. Such processes are worthwhile only in strong direct sunlight. While previously used exclusively firmly established and anchored collectors, there are now systems that will track the direction of the sun.

The solar panel is the central component of a solar thermal system, to be used until the early nineties only for water heating, and increasingly use the energy in space heating takes place. In conjunction with a low-energy house and a seasonal thermal storage space heating can even be performed entirely by solar panels.

If multiple solar panels to be used in a system, they must be connected in accordance with Tichelmann system to ensure uniform as possible flow.

A simple feature is the Thermosiphonkollektor, which is designed for solar systems that operate without a pump by gravity - circulation principle: The collector water is heated and rises, cools it sinks back down ( " natural circulation "). Conversely, as the same principle -using gravity heating, the storage must therefore be located above the solar collector. The Thermosiphonkollektor has often already integrated hot water tank and thus represents a complete simple solar system; whereas this plant are found mainly in southern countries ( Greece, Turkey, Israel) on many roofs. Due to the low thermal insulation, these memory lose at night and on cold days very quickly her warmth.

The thermosiphon is not to be confused with the Thermosiphonspeicher in which the thermo-siphon principle is used to fill a hot water tank with optimum solar temperature stratification.

Construction scheme

The diagram shows the basic structure of a flat plate collector with the most important components. Making the incident through a glass panel sun 's rays onto a solar absorber. Upon impact, the sun rays of the entire spectral range of the light is almost absorbed. The released heat is not lost, so the collector is thermally insulated on all sides. The convective heat transfer to the front is reduced by one or two panes of glass. In vacuum collectors it is completely prevented.

Heat is radiated due to the residual temperature of the absorber of this by issuing again, can also be largely retained by the glass, since the glass for the higher wavelength is not transparent ( wavelength selective transparency). Thus it is trapped in the collector - the equilibrium of radiation leads to a higher temperature than without the plate. This is the effect that (better: greenhouse effect ) often with heat trap or greenhouse effect is described. In modern solar thermal collectors special solar glass is used.

The absorber may have in particular for vacuum collectors a wavelength- selective absorption, so that on the one hand a high absorption of sunlight and on the other hand is in the near infrared there is a low emissivity and ensures that less heat radiation is emitted.

The heated absorber transmits the heat to the current flowing in the absorber is fixedly connected to copper or aluminum, heat transfer fluid tubes. It transports the heat output to a consumer or a heat storage. There are solar panels with open liquid circuit, in which the absorber is passed through directly from the water to be heated (especially in thermosiphon ). In regions with a greater risk of frost, but separate liquid circuits are used in the rule. The self-contained solar circuit, also referred to as the primary circuit, while a substance is added which reduces the freezing point of - often a propylene glycol. The heat is then transferred via a heat exchanger on the water.

In vacuum tube collectors, the thermal insulation is improved by an evacuated space inside the glass; as in a thermos can be delivered to the colder ambient heat energy only by radiation, but not by convection or conduction again. In order to withstand the pressure forces, round glass tubes are used.

There is also a semi-spherical solar collector, otherwise can use as a flat plate collectors, solar radiation throughout their circulation well. This design has not yet established itself in the market.

Absorber technology

In solar technology is a solar absorber, just simply absorber, a component of a thermal solar collector. It is used to (absorption ) of energy of various radiation from the sun (see also: electromagnetic wave).

The solar absorber contained in solar collectors consist usually of one or more absorbent sheets of aluminum or copper. Supported by a selective absorber coating, this heated in the sunlight; this heat is absorbed by the absorber tubes flowing through " solar liquid " and then transported to a utility device and a heat storage. The rest of the solar collector is used for the thermal insulation of the absorber ( by vacuum or by means of suitable insulating materials ) in order to prevent heat losses and to enable a higher operating temperature.

Commercially, the term solar absorber is, however, primarily used for facilities that are used for low-cost solar swimming pool heating, and are therefore also referred to as swimming pool absorbers.

Types of absorbers

Roof tile absorber

The roof tile absorber is a new type of absorber, which no longer alter the appearance of the roof. It is an open aluminum full -area absorber. He is modeled in form and color to a normal roof tiles and can be easily installed using special tiles on the roof. Through this customized combination of form and color, the absorber box on the roof is nearly invisible. The absorbers dissipate the heat via a specially designed baffle to a previously set piping system, which is located under the tiles on the roof battens and flows through a heat transfer liquid. When mounting the roof tile absorber only need to be clicked by baffle on the roof tiles. An extremely simple system with good extraction capacity, the overheating and downtime has no problems with the open design.

Swimming pool absorber / absorber mats

With swimming pool absorbers are mostly mats made ​​of heat - and UV-resistant black plastic which are designed, for example, on the roofs of dressing rooms. In these mats hoses are embedded, by the most of the swimming pool water is pumped directly (this is a heat exchanger superfluous ). Compared to flat plate and vacuum tube collectors such absorbers mats only reach low temperatures from 20 to 40 ° C, the heat can not be saved. But they serve their purpose as they only have to work in the summer when the weather in outdoor swimming pools.

Surface or plate absorber

The absorber has ( for example, in flat panel ) to turn to a large surface area as possible with the smallest possible volume of the sun to the shape of a plate. The carrier fluid is guided through common systems through copper pipes to which the collector plate is brazed for optimal heat transfer. Another design are soldered together profiled copper plates, the profile of gap flows directly through the carrier medium. These so-called strip absorbers are also used increasingly rare - mainly in DIY collectors or when it comes to special geometric surfaces.

Tube absorber

As tube absorber such constructions are referred to, in which the through by the heat transfer fluid tube itself serves as an absorber and is complemented by only relatively small additional lateral absorber surfaces. Such tube absorbers are found for example in certain types of vacuum tube, in which the individual tubes containing only a single tube as an absorber, possibly supplemented by a narrow soldered collector plate. Also swimming pool absorbers (see above) are sometimes referred to as a tube absorber, if they consist of closely adjacent running hoses.

Air absorber

Also possible is the use of air as heat carrier. This is called an air collector. The heated air is pumped directly into the space to be heated and serves as both the ventilation and heating.

Hybrid absorber

PV/T- or PVT systems combine photovoltaic (PV ) with thermal (T) use of solar energy. The PV cells - particularly those made ​​from crystalline silicon - but have decreasing efficiencies with increasing temperature. Therefore, particularly low-temperature systems for PVT are suitable.

Coatings

To achieve the highest possible absorption of solar energy is that of the sun -facing surface of the absorber either dyed black or coated with a special coating that acts selectively, that is, coming from the outside shorter -wave solar energy receives as well as possible and the longer wavelength heat energy of the absorber poorly Agreement.

Modern coatings eg eta plus, Tinox ( Vaillant ), sunselect and others usually have a bluish shimmering color. You can reach 91 to 96 percent absorption similarly high values ​​as the ( shimmering black ) black chrome coating mainly used earlier, but at the same time significantly lower emissions, so lose less heat by radiation. This they achieve overall significantly higher power ratings than just black painted absorber, but also measurable and noticeable higher values ​​than black chrome - coated absorber.

The absorber is to direct and diffuse solar radiation field as well as possible and transformed to heat (absorption ). At the same time he is as little heat again in the form of radiation emit (emission of thermal radiation). Technically speaking, he should behave selectively against the respective characteristic wavelengths. We also speak of a solar selective absorber.

In addition, it must be heat - and UV-resistant even in the long term.

In hot countries absorbers are frequently used which are " covered" only with so-called solar paint. This solar paint is very heat resistant and usually black, so as to achieve the best possible absorption values ​​for solar radiation. At the same time, the emission levels in the mid-infrared are very high - a part of the trapped heat is therefore re-radiated.

To minimize the energy loss, so a so-called high - selective coating of the collectors is used. This will absorbance values ​​of about 94 % of the sunlight ( 0.4-0.8 micron wavelength) and emission levels of less than 6% for the result of the inherent temperature of the absorber re - emitted radiation (infrared with wavelengths around 7.5 microns ) achieved. This is achieved with a plasma edge.

One of the first coatings with selective absorption, which could be mass-produced, was the so-called black chrome coating. It was applied in an electroplating process to the copper or aluminum, consisting of the absorber sheet. Very simply, it consists of microscopic chromium hairs that capture sunlight between them, but emit little due to their small size at longer wavelengths.

Until about 1997, the black chrome coating was found dominant. But are now allowing newer coatings not only higher efficiency, but are - mainly because of the omission of galvanic processes - even under production and recycling aspects as environmentally friendly. A now no longer available on the market alternative to black chrome was a - also electroplated - nickel coating ( " Black Nickel" ).

Both coatings can be previously applied only to the absorber plates of copper; appropriate techniques for aluminum absorbers are on the market only recently. These aluminum absorber, however, use for heat removal by the " solar liquid " (see Solar thermal system ) piping made ​​of copper, which is connected by laser welding process with the absorber.

In addition to the absorber coating, different manufacturers differ in their basic structure. Often, full-surface absorbers that consist of a single absorber plate to encounter. The tubing is brazed in this serpentine or meander shape or harp at the back or welded. In addition, there are absorbent strips, which consist of individual fins 10-15 cm narrow strip, on the reverse side, respectively, a thin pipe is welded. The fins are then soldered at both ends in a collection tube, so that a kind of " Harp " arises. A third design, the cushion absorber. As full-surface absorbers consist of a single continuous absorber plate, on the back side but instead of a pipe, a compression molded second sheet is applied. The heat transfer fluid flows between these two plates.

Basically have full surface absorber on the best performance values ​​. Since the beginning of the manufacturers of the new, highly selective coating could only use copper plates that do not exceed a certain width, mainly absorber fins are used primarily in older collector models yet. Absorber plates are now available up to 1200 mm in width, which allows a great flexibility in the geometry of the absorber. In contrast absorber fins only allow the piping in harp form, on the other hand can be a simpler way Finns adjustments to the roof form to ( customized panels ).

History

The principle of solar thermal energy has long been used: burning and concave mirror existed already in antiquity. The use of solar energy goes back to the Greek mathematician and inventor Archimedes of Syracuse ( 285-212 BC), who sat allegedly with the help of burning mirrors the Roman fleet on fire.

In the 18th century naturalist Horace - Bénédict de Saussure invented the forerunner of today's solar panels. He built in the 18th century, a simple wooden box with a black bottom and glass cover. This first solar collector it reached a temperature of 87 ° C.

Middle of the 19th century developed the Frenchman Augustin Mouchot on the solar collectors de Saussure and combined them with burning mirrors. In 1878 he exhibited at the World Exhibition in Paris in front of a solar steam engine. He suggested that convert using these steam engines solar energy into electricity.

Applications: household to industrial

The best known and most common application of heat gained from solar energy is hot water in the household. With a suitable design of collector area and storage volume it ranges in Central Europe throughout the summer season for washing and bathing. Theoretically, the solar heat cover the whole year on the needs of a household, but then the system is much larger and supplies in the summer a lot more heat than can be used. The efficiency of such a plant is considerably deteriorated, and the high investment costs for this oversizing are rarely compensated by the savings in gas, oil or electricity - is to say: an investment would be unprofitable. However, economically designed systems can complement the additional conventional heat source in the winter months. The proportion of such investment on the hot water supply is seen throughout the year between 50 and 60 %, which represents about 14 % of the heating demand.

The first large-scale applications were since the energy crisis of the 1970s the heating of public and increasingly private swimming pools. A further upsurge in the spread of hot water collectors in Germany was not least achieved through various funding programs of the federal and state governments. Also industrial plants use solar energy for a long time, contributing to the process energy in the low temperature range. Thus, including the heating of biomass crops - long thrived for the market - about the production of biogas. If higher process temperatures are required, only a pre-heating the heat carrier is, however, possible with the conventional technique. Currently developed concentrating solar systems ( parabolic trough technology) Open this solar technology in the next decade another big market potential.

Binds to the solar energy is not only in the hot water, but also in space heating demand a, larger collector systems are useful. For standard heaters they may well contribute double-digit percentages for heating energy and therefore reduce heating costs considerably the annual average. Substituting also a seasonal heat storage one, it is even possible to store in the summer months so much heat that the heating energy demand can be met throughout the year. The only limitations apply at too low mountable collector in relation to the annual heating energy demand, such as multi- storey houses. Seasonal heat storage in the simplest case of a sufficiently large quantity of water or passive thermal mass. Houses with passive solar design or solar panels and seasonal heat storage are marketed under the term solar house.

To ensure even on cloudy and rainy days, enough hot water, a special hot water tank with heat exchanger function is built into the solar thermal system, the individual household - ranging from about 300 to 1500 liters of water filling - depending on the number of people ( family size ) and usage behavior. For larger units, hospitals, hotels, etc., which may have relatively favorable payback periods because of the size and the significantly more continuous use, often come custom industrial storage used. In conventional memories of the solar circuit heat up only the lower half of the boiler, which is heated due to convection, but then complete except for the maximum temperature ( 95 ° C). In order to compensate for a higher heat demand, sky overcast lack of heat from the collector, the hot water tank either an additional current-fed heat source ( heater ) is integrated, or the memory is connected via a further built-in heat exchanger to the boiler of the house.

To be distinguished from the thermal solar panels are the solar cells and solar modules, the solar energy into electrical energy (photovoltaic ).

See also: Solar thermal system, thermochemical heat storage, solar oven, solar chimney power plant

Economic Consideration

Solar panel systems are generally attractive especially in terms of low operating costs because they cause by the non-existent fuel supply difficulties running costs. The regular cost is limited to low power costs about 8 € per year for the operation of the electric circulation pump at 7 W power consumption, and 5300 operating hours per year, based on an average single-family house and an electricity price of 21.5 cents per kWh. In addition, falls every two years to a maintenance check. Unlike the passive solar architecture, which already affects the design of the building envelope, to collector systems can often be easily integrated into existing buildings, which is why the economic balance in older buildings often takes place only between a solar panel system or other active forms of heating. In such a comparison principle should also consider the environmental effects are included. Also, the system is very easy to handle, since eg no residual ash must be removed, such as in many pellet.

Since the heating requirements can already be affected by the building insulation but is a crucial question whether one generally invests available funds in a larger sized heating or instead of in better insulation. The answer depends on the existing insulation and the structural possibilities of mounting a major collector or the use of other forms of heating from, including the complete abandonment of heating in new buildings with passive solar architecture.

When designing a heating you have between the sole use certain season heat registers, and the combined use with another different form of heating (at best neutral ecologically as well). The choice of technology for seasonal caching is co-determines the efficiency of the overall system. The classical restriction of a collector system to " DHW heating and central heating backup " may be fundamentally wrong, as long as the cost of the season heat storage are small enough. Here you have to start from the concrete product price for the respective overall system, as well as take into account lifetime and running costs. Often can achieve low total cost by the very technically relatively simple seasonal temporary storage of heat, such as having a substantially lossless thermochemical heat storage, large or at least well-insulated buffer heat registers, or too low-loss latent heat storage. Also a possible tracking of the panels, or a change in the Aufstellwinkels as winter approaches can affect the price-performance ratio.

In deciding for or against a solar panel system, the promotion of different technologies plays a role. A decision for the better gefördertere photovoltaics beneficiary. In Germany there are 5.11 cents extra per kWh according to CHP Act plus the usual market price 3-8 cents, compared with 43.01 cents feed-in tariff in 2009 according to EEG in photovoltaics).

Ecological perspective

Solar panel systems cause during operation no direct emissions and decrease in comparison with conventional heating systems, CO2 and particulate matter emissions. In a few years, a collector has supplied the same amount of energy for heating, which had to be spent on the production, etc. of the collector.

Economically compete eg combined heat and power based on renewable fuels because of the necessary agricultural production of fuels with food production for arable land and cause the delivery of the fuel regularly an additional road traffic. With solar collector systems, this is in contrast generally not the case.