Glass a key material in STE parabolic trough plants
Solar thermal energy generates electricity from the heat of the sun, concentrating solar radiation to generate steam that activates a turbine.
To complete this process is a requisite to use quality mirrors. In comparison with annealed glass, widely used in the solar thermal industry, tempered glass offers more and more security, efficiency and productivity.
” The treatment of the glass is a key process to guarantee the quality of the mirrors.
The solar field in parabolic trough solar thermal plants is formed mainly by three components: reflective mirrors, absorbing tubes and metallic structures. Among these elements, the mirrors is the one which covers a larger surface of the solar field so having greater control over them leads to a better performance, efficiency and security of the solar field.
Parabolic trough shaped mirrors are made of a 96 % of glass among other materials, and, in a smaller proportion, of a silver layer, a copper layer and three layers of protective paint. For this reason, the nature of the glass and its treatment are critical factors to guarantee the quality of the final mirror.
From the same initial ultra-clear glass, different results can be obtained depending on the thermal treatment. In the STE industry, tempering and annealing are the main treatments.
Tempered glass has been processed to increase its resistance. During the tempering process, the glass is heated above 600 ºC and is then cooled sharply, inducing permanent tensions and achieving a higher resistance to efforts.
Annealed glass is slightly heaten above its softening temperature (585 ºC) and then put under a controlled cooling process that avoids residual tension.
Due to its high resistance, tempered mirrors can have a reduced thickness.
Reflectivity of the mirrors is directly linked with the efficiency of the plant. Using a glass of 5mm reduces the reflectivity of the mirror in a percentage between 0.7 % and 0.8 % and, thus, the efficiency of the plant.
• Focal Deviation (FDx):
Parabolic shaped mirrors present a deviation when focusing the sun rays onto the receiver tube, since they are not a perfect parábola. Even though there sould not exist differences in the optical precission of both types of glass, the tempering process permits to achieve a higher replicability.
• Wind resistance:
Tempered mirrors can stand heavier wind loads than annealed mirrors. Tempered mirrors can stand winds of up to 200 km/h in any position while annealed mirrors break at 110 km/h in stand-by position.
In the event of breakage, tempered glass breaks into small fragments, which makes this glass ideal for its use in industries with strong safety requirements, reducing risks in the operation and maintenance of plants.
• Thermal resistance:
Tempered mirrors bring dimensional mechanical stability when facing sharp changes of temperature while other mirrors would be affected by thermal tension, thus breaking.