Solar thermal collectors are mainly used for water and space heating. They capture incident solar radiation, convert it to usable thermal energy, and transf er the energy into a heat transfer fluid. All of this should be accomplished economically with minimum energy loss. One of the most important components of the solar thermal collector is the solar absorber. To be effective, the absorber should exhibit wavelength selectivity, i.e. have maximum solar absorptance, minimum solar reflectance and thermal infrared emittance. A high solar absorptance is needed to collect as much of the incident solar radiation as possible and a low thermal infrared emittance is needed to minimize radiant energy losses. A lightweight materials (copper or aluminium) having a high thermal conductivity is needed to transport the absorbed energy from surface to the fluid which has to be heated.
Recent R & D activities in PLASMA are:
-Spectral selective coatings. These coatings are composed of islands of metal embedded in a three-dimensional matrix of dielectric. Recent research on mixtures of titanium nitride and titanium oxide at the PLASMA – Center for Plasma Technologies has yielded spectral selective coatings with a solar absorptance > 0.95 and an emittance epsilon, < 0.06. These TiNxOy based coatings are manufactured by Physical Vapour Deposition process – combination of magnetron sputtering and cathodic arc evaporation deposition;
- Enhanced construction solutions for better solar heat transmission from the absorber surface to the medium using the different absorber designs: compact Al extruded with selective coating, Al wing with selective coating ultrasonic welded with Al tube, Al wing with selective coating ultrasonically welded with Cu tube and Cu wing with selective coating ultrasonically welded with Cu tube.
The heart of the solar heating system is the solar collector. In order to enable effective solar energy utilization, absorption of incident solar radiation should be maximized and thermal losses from the collector minimized.The solar absorbing surface is one of the key components of a solar collector, and its optical properties and quality influence both the heat losses and gains.
High absorptance and low emittance accompanied with economical manufacturing cost, large production volumes and long-term durability are often the key driving factors of the R&D and industrial development work on PLASMA’s solar absorbers.
TiNxOy absorber surfaces
TiNxOy film possesses excellent absorptance towards visible light, in which most of solar energy lies, and good transmittance for the infrared.
This “blue” coating, based on Titanium, marked a significant increase in performance and represented the first environmentally friendly alternative to the so far commonly used “Black-Chrome”- and “Black-Nickel”- coatings.
The TiNxOy samples are preparing in a Physical Vapour Deposition – PVD chamber. In this case, the TiNxOy coating was deposited by magnetron sputtering and cathodic arc evaporation deposition process in ambience of Ar/N2/O2 onto aluminium and copper substrates. The substrates were cleaned by ultrasonic bath, then rinsed by isopropanol and dried in air.
Furthermore, Plasma in cooperation with several institutes working on dimensioning and disaining of absorbers and heat transfer in the absorbers.,
Theoretical efficiency curves for collectors with compact Al and Cu absorbers
ABSORBER EFFICIENCY COEFFICIENTS FOR TWO DIFFERENT COMPACT AL EXTRUDED ABSORBER
The values of the absorber efficiency coefficients can be classified as good values comparable to good products of competitors From the Table1 and Table 2, it can be seen that all the values of the absorber efficiency coefficients η’ (including heat transfer from the sheet to the fluid) are near to η. That means, with water and turbulent flow there is a very good heat transfer to the fluid. But if one change to glycol mixture, there will be laminar flow and η’ drops down. But at higher temperatures there will be turbulent flow and η’ will be higher.
The special PLASMA-coating does not only have a very high absorption but reduces dramatically the irradiation in the infrared spectrum what makes the heat remain inside the absorber. In other words, collectors equipped with a PLASMA-coated absorber work far more efficiently, even if the weather is cloudy.
Recent R & D Activities
Selective PVD sputtering coatings on Al, Cu or stainless tubes
These tubes are applied in medium or high temperature solar thermal collector
PVD selective coatings on glass tubes