Solar mirrors have numerous uses in the current building and energy environment – including concentrating solar power (CSP) for generating electricity, solar cooling, and heat generation for industrial uses – often utilizing heliostats or parabolic troughs. For each application and environment, different materials and coatings have been developed to maximize efficiency in often harsh conditions, with required lifespans typically considered to be approximately 30 years with minimal performance drops. As this not a practical timeline to test new materials within, a standardized method for accelerated aging needs to be developed to determine long term performance and minimize maintenance requirements – a key hurdle to economic viability. To that end, samples developed over the last 20 years, with a wide range of materials and exposure times, have been identified to resume experiments on; this includes beginning exposure on samples representing the current state of the art. This will allow performance and degradation mechanisms to be studied for different types of solar mirrors over a longer timeline – some samples having over 15 years of natural aging time - compared to other experiments which can be limited in timeframe, usually to around 3 years of outdoor exposure, and allow for a more accurate picture of the long term performance of solar mirrors to be derived.

 

The outdoor exposure campaign for this project began at NREL in 2018 and involves characterizing samples at periodic intervals. Decades worth of measurements on thousands of samples are also being cataloged, providing a large data pool to draw correlations from. And by resuming exposure and characterization on samples with large historical data sets, continuity can be established between older and modern equipment. This will eventually help to establish, in addition to a large publicly accessible database, more accurate guidelines for accelerated aging techniques (thermal and humidity cycling, irradiance, salt sprays, etc.) as well as using machine learning techniques to find novel trends amongst the data. For this first year, new data is relatively limited, but at least one trend has already been identified – 4mm glass solar mirrors have shown to have a slight performance advantage over 3mm glass mirrors of identical material in less than 2 years of exposure.

 

Characterization for each sample currently includes hemispherical spectral reflectance measurements using a PerkinsElmer Lambda 1050 spectrophotometer, and specular reflectance measurements with a Device and Services (D&S) Portable Specular Reflectometer. The exposure campaigns will also evaluate new characterization techniques, primarily as of now image analysis for surface corrosion. By introducing imaging techniques, it may be possible to consider the entire surface area, if in a more limited capacity, of a solar mirror compared to current measurements which would be cost- and time-prohibitive to consider a large portion of the surface.  
 

In the paper, the current status of NREL’s outdoor test campaign of solar mirrors will be reported and provide a valuable reference to interested researchers in the related areas.


Outdoor Degradation Test of Solar Mirrors at Nrel

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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