Which CSP Solar Mirror is More Profitable, Annealed or Tempered?

concentrating solar thermal plant, California
Which type of glass is best for concentrating solar mirrors?
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According to calculations by DLR (German Aerospace Center) the increase in yield resulting from annealed parabolic mirrors over a period of 25 years amounts to more than seven million Euros. This is based on independent figures and on the company’s practical experience with more than seven million solar mirrors that have been installed in the field over the past 30 years.

In the industry it is debated whether CSP solar mirrors made from tempered or annealed glass achieve a better overall performance. Manufacturers of tempered mirrors advertise lower costs for mirror replacement. However, this calculation fails to take into account the higher plant yield that solar mirrors made of annealed glass achieve owing to their better precision collector interception.

The key parameters for the yield of a CSP plant are the bending properties of the mirrors. The basis for assessing the bending properties of a solar mirror is the focus deviation (FDx). This measures the frequency distribution of the real focus deviation in transversal (x) direction of the reflected beam from the ideal focal line of the collector parabola. Consequently, the lower the FDx value, the higher the plant yield. According to the calculations of the DLR, each millimeter of decreased scattering loss amounts to an additional plant yield of 11.25 million Euros(1) over a period of 25 years.

Based on these data, FLABEG Holding GmbH, the market leader in the field of glass finishing and the leading international manufacturer of solar mirrors has made the following calculation:

Yield and cost summary for a 50 MW plant with 23,000 tubes and 200,000 mirrors

Factor Annealed Tempered
Additional yield due to better precision collector interception (FDx) + 7,875.000 Euros(1) 0
Costs for mirror replacements due to glass breakage – 650,000 Euros – 42,500 Euros
Costs for absorber tube replacements due to glass breakage – 65,000 Euros 0
Total + 7,160.000 Euros – 42,500 Euros

Additional yield of the plant with annealed solar mirrors after 25 years: 7,117,500 Euros

The result shows that during long-term observation, the annealed solar mirrors are markedly more profitable: More than seven million Euros of additional plant yield is achieved over the period of 25 years.

Axel Buchholz, CEO of FLABEG Holding GmbH, is able to confirm this calculation based on independent figures from his own practical experience: “Competitors advertise with extrapolations. In contrast, our solar mirrors have been in the field for 30 years already and have proven their durability and constantly high performance”.

Another item of practical experience to be borne in mind: “Glass breakage generally arises only at the corners or edge of a mirror. Tempered glass shatters into a thousand individual pieces in this case, which may result in considerable damage to other components in the field. Our annealed mirrors on the other hand continue to remain operational”.

FLABEG, founded in 1882, is an independent company with 13 facilities in Europe, America and Asia. FLABEG is active in the commercial sectors of automotive mirrors, solar mirrors and technical glass; specializing in all processes of glass finishing. As the pioneer of solar technology with a worldwide network, FLABEG produces mirrors for all CSP-applications – Parabolic Trough, Dishes, Tower and Linear Fresnel. Due to latest coating technologies our solar mirrors are characterized by a reflectivity of more than 95%. FLABEG sets technological standards in terms of the company’s core competencies – bending and coating.

Basis of Calculation:

Average mirror breakage rate: annealed 0.20 %(2), tempered 0.01 %(3)

Average absorber tube breakage rate: annealed 0.025 %(2), tempered 0 %(3)

Precision rate / focus deviation: annealed FDx < 8 mm(4), tempered FDx < 8.7 mm(3)


  1. Mean values according to calculations by the German Aerospace Center, reference values: plant capacity 50 MW, period of observation 25 years, nominal glass thickness 4 mm.
  2. Average values according to KJC/Harper Lake, period of observation 10 years, nominal glass thickness 4 mm.
  3. Tempered glass manufacturer’s indications.
  4. Current average focus deviation of FLABEG parabolic solar mirrors.

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