Flat plate collector
A certain type of solar collector is called a flat plate collector . The main feature is the alignment of a flat, level absorber surface to the sun (no curvature, no concentration). Flat-plate collectors are used to heat swimming pool water , mixtures of water and frost protection and air.
distribution
On the German market , by far the largest in Europe , flat-plate collectors with insulation material dominate compared to vacuum tube collectors in a ratio of around 9: 1. Vacuum flat collectors are a niche product. Technically, the 'normal' collectors differ from the evacuated collectors in terms of thermal insulation and the related details of the cover, frame and frame.
construction
cover
The task of the cover is to allow the sun's rays - even with a flat angle of incidence - as completely as possible into the interior of the collector and to prevent the escape of heat in the form of flow and radiation as far as possible.
The most frequently used material is solar glass , which is sometimes also given an anti-reflective treatment . Inexpensive bare garden glass is often used for self-construction projects.
Plastics have largely disappeared from the market due to their limited resistance to UV radiation, heat, frost and others. Only in the area of swimming pool absorbers (the flat-plate collector type shrunk onto the central component) are there still applications with black-colored polypropylene and EPDM .
absorber
Today's flat- plate collectors are most often designed as copper absorbers . In the 1980s, aluminum absorbers were also common; these could not prevail in the long term, mainly for electrochemical reasons: Due to mixed installations with copper pipes, special heat transfer fluids were used. After approx. 10 years of operation, however, damage became apparent due to errors during commissioning and maintenance. Future aluminum-based absorbers are under development.
The aluminum absorbers predominantly used in the Mediterranean region (partly with aluminum, partly with copper piping) are usually coated with so-called stovepipe or thermal paints. Such paints are heat-resistant up to 400 ° C. The lower efficiency is largely compensated for by the higher average sunshine duration and the ambient temperature in these areas.
Surface coating
The coating decides next to the insulation and the properties of the solar glass on the performance of the collector. It should be highly selective, i.e. absorb significantly more than 90% of the incident short-wave radiation (approx. 300-3500 nm wavelength) through absorption , but release less than 10% of the energy absorbed again in the form of long-wave radiation through emission .
Historically, for galvanically by chromating applied a black coating, the so-called black chromium . This layer contains hexavalent chromium , which is considered to be carcinogenic . For this reason, this type of coating only has a very small market share for product liability reasons . Newer coatings not only allow higher levels of efficiency, but are also considered more environmentally friendly from a production and recycling point of view.
The most widespread today is a blue-colored titanium- based layer applied by cathode sputtering , which achieves significantly lower emission values than black chrome .
This process - known as sputtering - is a physical process in which atoms are detached from a solid (target) by bombarding them with high-energy ions (predominantly noble gas ions) and then transitioning into the gas phase in order to subsequently deposit on the body to be coated.
The first series-ready coatings of this type were developed in Germany in the form of titanium-nitride-oxide coatings and brought onto the market by the company TiNox . Another new development is a coating from Alanod-Sunselect . It is a ceramic-metal structure (presumably also based on titanium), which, like the titanium-nitride-oxide coatings, is applied in a vacuum sputtering process and also has a bluish black sheen. Since 2005, BlueTec has offered a new copper coating that has meanwhile been used by leading manufacturers.
Assembly
There are several options for connecting the absorber sheet to the copper pipe register.
Depending on the frequency of use, these procedures are common:
- Ultrasonic welding
- soldering
- Plasma and laser welding
Laser welding is currently leading the way in terms of technology. Usually a full surface is connected to the entire register. Alternatively, an absorber strip (so-called fin) can be connected to a copper tube. Full-surface absorbers now dominate compared to fin absorbers in a ratio of around 4: 1.
Another manufacturing process is roll bonding .
Thermal insulation
The thermal insulation should keep heat losses from the collector on the side facing away from the sun (= rear wall) as low as possible.
Due to the high temperatures when idling, only temperature-resistant insulation materials can be used in solar collectors . Are particularly suitable, for. B. mineral wool or foam glass ; Wood fiber insulation boards can also be used to a limited extent . Polyurethane foam and others can only be used in the less stressed area. Depending on the material thickness, gas-filled insulation is somewhat less insulating than a vacuum, which is why somewhat larger collector surfaces are required in order to achieve comparable performance values, especially at very low ambient temperatures or if there is a high temperature difference between the outside air and the desired useful temperature. From an economic point of view (price / performance), the flat-plate collectors insulated with insulating material are nevertheless significantly more efficient.
frame
The frame has the task of holding the components together (side and rear wall), as well as stabilizing them during transport and wind loads.
In addition to aluminum and plastic profiles - z. B. in collectors that are used as a roof replacement - wood is also used.
frame
The frame ensures the connection with the location. These constructions should be wind, storm and weather-resistant, especially with freely installed collectors, but also with elevated collectors.
In addition to aluminum and stainless steel, galvanized steel and titanium zinc (especially as sheet metal) are used. Wood can also be used as a static load-bearing material in a blended form.