Real outdoor conditions are so variable that this qualification doesn't mean very much. Where, at what altitude, under what level of sun, at what angle, with what air temperature and what wind speed, how clean are the mirrors and so on? And, it seems that you are expecting an 'accurate' answer -- may I ask why? The temperatures are going to be so far below that which would damage a mirror as to be negligible.

 Thank you for your consideration, Pr Maine. The environmental parameters (like altitude, wind speed, cleaning, etc) can be neglected. I would like to have an idea of the front glass temperature of (primary and secondary) reflectors in the same way that the literature gives an operating temperature, at the absorber, of 400°C for 2D-concentrating system or 800°C for 3D-concentrating system.

Many works [1-3] suggest that reflectors show long-term degradations involving reflectance decrease (glass/silver interface degradation or surface damage). To evaluate the natural aging effects on glass-based reflectors, we have to know the average surface temperature of glass-based reflector according to the concentrating system (2D or 3D). If you find scientific papers providing some information about the topic, I would appreciate it. Thank you once again for your assistance.

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 (1) A. García-Segura et al. Durability studies of solar reflectors: A review, Renewable and Sustainable Energy Reviews 62 (2016) 453–467.

(2) M. Karim et al. Study of the surface damage of glass reflectors used in Concentrated Solar Power Plants, Energy Procedia 69 (2015) 106–115.

(3) M. Brogren et al. Analysis of the effects of outdoor and accelerated ageing on the optical properties of reflector materials for solar energy applications, Solar Energy Materials and Solar Cells 82 (2004) 491–515.

Dear Vincent, The investigations that examine the behaviour of the thermal absorber in a solar thermal system are not really applicable to solar reflectors, since the energy density at the reflector is just sunlight while at the absorber it is sunlight concentrated many times. You will more likely find data useful to your research on the behaviour of windows. I can't see why you consider that the environmental conditions can be neglected. The heat loss from a mirror in sunlight will depend a lot on local ambient wind speed, temperature and maybe humidity, as it will (if it is a good mirror like solar thermal systems need) reflect at least 80% of the solar energy incident upon it - so that leaves a heat input of about 300W/m2 remaining in the mirror. The temperature of the mirror will depend a lot on ambient conditions with this low level of heat input.

I suggest you also have a look at aging of glass in papers on subjects such as architecture and civil engineering - in my view they are also applicable to what you will get in a solar reflector system.