BLACK PAINTED SURFACE
The absorption value of the flat black painted surface is very high (i.e. 90 – 98 %). However, its radiation reflection value is also very high (i.e. 85 – 92 %). Flat black paints are generally polyester, acrylic and epoxy resin based. The filing materials used within these paints and the organic roots within the binders, prevent the surface to acquire selective features. There are special paints that give selective surface features other than flat paints. These paints can absorb the solar radiation quite well when applied in the form of very thin film layers such as 0.0013 – 0.0025 mm thick, and they can also act like a transparent material against thermal radiation. But because it is very difficult to apply these paints as very thin layers, they are not very common in the market.
SELECTIVE SURFACE
Selective surface is the surface which keeps almost all of the short wave radiation (i.e. 0.2 – 2.5 µm) and reduces the reflection of long wave radiation (i.e. 2.5 – 50 µm) to the minimum level. Thus, by increasing the temperature of the absorber plate, more heat transfer to the fluid is acquired. Selective surfaces reflect less radiation in case of an increase in the temperature; and as a result the collector efficiency increases. According to the values of absorption (α) and reflection (ε), selective surfaces are classified as follows:
Selective surface: [ α > 0.90 ve 0.00 ≤ ε ≤ 0.20 ]
Semi selective surface : [ α > 0.90 ve 0.20 ≤ ε ≤ 0.50 ]
Non selective surface : [ α > 0.90 ve 0.50 ≤ ε ≤ 1.00 ]
FACTORS EFFECTING ON COLLECTOR EFFICIENCY
- Intensity of Solar Radiation
- Environmental Temperature ı
- Wind Speed
- Construction of the Collector
- Properties of Transparent Cover (Glass)
- Radiation Absorption and Reflection Values of Absorber Surface
- Thickness of the Absorber Surface and Thermal Conductivity
- Type, Thickness and Thermal Conductivity of the Isolation Material
COLLECTOR EFFICIENCIES WITH RESPECT TO ABSORBER SURFACES
G : 800 W/m2 ( Solar Radiation Value)
Td : 20 °C ( Outer Environment Temperature)
Tm : 60 °C ( Average temperature of the fluid in the collector)
Nominal Process Point ΔT / G = ( 60 – 20 ) / 800 = 0.05
Selective Surface Collector Efficienc : % 56
Black Painted Collector Efficiency : % 34
Selective Surface Collectors are approximately 65 % more efficient
1 m2 selective surface collector ≈ 1.65 m2 flat black painted collector