GEOLUX® is a specialist engineered material designed with a 30-year service life, to increase the bifacial energy yield through increasing the front and rear-incident irradiance by increasing the natural albedo of the ground. GEOLUX can increase the ground albedo to approximately 75%, offering significant increases in yield and a reduction in the Levelized Cost of Energy (LCOE). One common question is whether GEOLUX will provide a high albedo increase over the full life of the project.

Solar Energy Reflectance
ASTM E903-20 is a standard test method developed by ASTM International for measuring the solar absorption, reflectance, and transmittance of materials using spectrophotometers equipped with integrating spheres. GEOLUX has been tested under these conditions and has achieved the following results:

Test conditions
Method A:
Using a spectrophotometer
Apparatus used:
Agilent Cary 5000 UV/VIS/ NIR spectrophotometer,
Spectral range:
350–2500 nm
Solar energy reflectance is calculated as per par 8.3.1.1
Date of test:
November 2, 2021
These same samples of material were then introduced to advanced UV using the methodology from NF EN 12224-2000, a European standard that specifies methods for determining the resistance of geotextiles and geotextile-related products to weathering conditions. This methodology helps ensure that these materials can withstand the weathering conditions they are likely to encounter in their intended applications. GEOLUX was retested for Solar Energy Reflectance (ASTM E903-20) after set periods of advanced UV aging to determine how the reflectivity performed in the long-term. The following chart plots the results of the Average Direct Normal Irradiance and as shown after a cumulative 900MJ/m2 (7740 hours) between 290-400 nm irradiance (The UV spectrum wavelength between 300 nm and 400 nm appears to be the most aggressive in damaging geosynthetic polymers which occurs on a molecular structure level) there has been no decrease in reflectivity, with any changes within a standard deviation of measurement.
How this relates to the real world depends on many site specific factors such as height of the solar panels, whether they are fixed or tracker, and where GEOLUX is placed in relation to the panels, as shading will reduce the UV exposure. Secondly, the actual location of the site, and therefore the differing UV Irradiance levels will greatly impact upon this.
How long it would take for a certain amount of solar radiation to accumulate to 900MJ/m² would depend on the intensity of the solar radiation at your specific location and time of year. For example, if we consider the annual UV radiation in a place like Florida, which is approximately 350 MJ/m² per year, it would take approximately 2.57 years (900/350) to accumulate 900MJ/m². However, as the peak irradiance is during the middle of the day, which is also when GEOLUX will typically be in the shade, it is reasonable to deduce that the testing to date in an area of high UV is equivalent to at least four years with no noticeable decrease in reflectivity.

Material composition and skin coating
Pigment

The white pigmentation that is widely used to create the reflective surface on geomembranes for UV protection is titanium dioxide (TiO2) and this compound is itself susceptible to UV degradation when exposed to sunlight. Knowing this, the white surface of GEOLUX is packaged with UV stabilizers to prevent the degradation of TiO2 when exposed to UV light. As such, the white surface on GEOLUX provides maximum protection against UV degradation which extends the lifespan considerably. Figure 2 shows a comparison between an industry standard titanium dioxide with none or poor-quality UV stabilizers and titanium dioxide with high quality UV stabilizers following 1,600 hours of UV exposure. As shown significant surface cracking has developed on the sample with no UV stabilizers.

Further to this the Paper titled “QUV Accelerated Weathering Study: Analysis of Polyethylene Film and Sheet Samples” by Wagner and Ramsey assesses the effects of Titanium Dioxide on UV resistance. Two key points can be taken from this report:
That no significant difference is seen in the rate of decay between black liner with 0.08–0.10 mm of a white surface and black liner with 0.13–0.15 mm of a white surface. Deterioration occurs at generally the same rate, as shown in Figure 3.
In general, the more TiO₂, the more opacity with respect to transmission of UV radiation, and therefore the more effective it is in minimizing the effects of UV damage. Therefore, it is desirable to achieve an optimum quantity of TiO₂. The deterioration of three sets of specimens- each with different levels (2%, 5%, and 10%) of TiO₂ in the white layer (which is 0.13–0.15 mm thick) of 1.5 mm thick black/ white liner. No real difference is seen between 5% and 10% TiO₂ samples. However, the sample with only 2% TiO₂ deteriorated at a much faster rate.
With these two points in mind, it should bring comfort to know that the White Reflective layer on GEOLUX is 0.13–0.15mm thick and contains a TiO₂ concentration of approximately 20%.


Lifetime expectancy
When we discuss the lifetime of GEOLUX, we are discussing the time it takes for a mechanical property to reach its half-life. It is important to understand for environmental and performance reasons that when GEOLUX reaches its service life the geomembrane will not disintegrate or break up.
For a lifetime expectancy of geomembranes, “GRI White Paper # 6 - Geomembrane Lifetime Prediction: Unexposed and Exposed Conditions” is frequently cited. It is shown that in exposed conditions an HDPE geomembrane hasan expected half-life of > 36 years.
As the service life is a half-life and implemented to create certainty that chemicals can be contained by a geomembrane, in the unique albedo enhancement application where there is no requirement for such certainty, the actual life span of GEOLUX could be significantly longer. Also worth considering is that GEOLUX will be shaded by solar panels for certain period of the day, increasing life further.
Monitoring the service-life
The service-life of GEOLUX is dependent on the formulation and on the conditions of use and the environmental impact.
GEOLUX will be influenced during its service-life by UV weathering, cleaning and could also be impacted by wind stress if not sufficiently anchored. Solmax considers that GEOLUX will be installed in a relaxed state with sufficient anchoring, so that mechanical damage and prestressing can be excluded from further consideration.
As such, UV aging and thermal aging will be considered further. During the aging process stabilizers will be consumed to protect the polymer (known as the Antioxidant depletion time). If the polymer is not protected by stabilizers, the polymer will be affected by aging (known as the induction time) and after a further period chain scission will take hold and GEOLUX becomes brittle. Resistance to aging can therefore be measured by monitoring the stabilizer consumption and the change of the tensile properties.
Solmax would suggest that sampling and testing can be executed at a Solmax laboratory after 10 years, and then every five years forth to perform tensile testing with a focus on break elongation, and in addition to monitor the consumption of UV stabilizers.
Monitoring the development of the HP-OIT/OIT values:
Oxidative Induction Time acc. to ASTM D 3895
High Pressure Oxidative Induction Time acc. to ASTM D 5885
These values present the stabilization (Antioxidant and UV-stabilizer consumption/content) of GEOLUX. With antioxidants and UV-stabilizers present the polymer structure is protected. After their consumption, the polymer is not protected and after a further service period becomes brittle.
The end of service life is reached when the geomembrane is brittle, which is not after the consumption of stabilizers, but only when the polymer chains break and therefore mechanical properties change.
GEOLUX is brittle when the polymer structure can no longer elongate. Thus, when reaching only 50 % elongation at break in the tensile test GEOLUX will have aged strongly. This value is the monitoring value to determine that the acc. to ASTM D 6693 service life has been reached.
With testing throughout the project design life, it can be determined when the geomembrane will reach the end of its service life and can be replaced securely. Monitoring should be executed at the most severe location where the highest UV radiation occurs. For testing purposes material can be removed and then additional material can be installed.