(174f) Concentrated-Light Aging Techniques for High-Temperature and Solar-Energy Materials: Preliminary Results

Energy supply may be the biggest economic issue that present-day society is facing. Although, most of the (fossil) energy resources currently exploited will not be depleting “soon”, humanity and environment are calling for a sustainable model. Thus, green buildings and renewable energies are becoming worthwhile solutions [1]. Among them, solar thermal collectors [2], solar thermal electricity (STE) [3] seem to play a decisive role in the renewable energy mix, apart from photovoltaic technologies (PV) [4]. In any case, all technologies demand for more efficient [5] and durable materials [6], while the expected lifetime of most related components should exceed 20 years. In this context, state agencies declared materials to be a key enabling technology to help society lead into a resource-efficient, low carbon economy and highlighted that the absence of more complete testing methods, e.g. accelerated aging , inhibit and delay further improvements [7]. Aging generally refers to the degradation of given abilities over a certain period of time. These abilities depend on the technology and cover a wide range. For solar absorbers/receivers are absorption of solar radiation and heat transfer [8], for solar collectors/reflectors is reflectivity of solar radiation [6], and for photovoltaics is the optoelectric conversion [9]. Meeting and assuring of lifetime goals for all components is important for the success of solar energy technologies and for its competitiveness with other energy production technologies, especially with conventional ones that have already proved their durability and resilience.

In accelerated aging studies (accelerated tests-AT), generally, information from experiments at high levels of one or more accelerating variables (e.g. temperature, use rate or radiation) is extrapolated, through a physically reasonable statistical model, to obtain estimates of life or long-term performance at lower, normal levels of the accelerating variable(s) [10]. Indoor ATs for light induced degradation (photodegradation) approximate natural sunlight using, initially, Carbon-arc (since 1918) and, later, Xenon-arc (since 1960’) light sources [11]. Concentrated sunlight devices have been developed as part of the outdoor ATs (weathering facilities, e.g. EMMAQUA [12]). Although, concentrated light is been used in outdoor ATs since 1980’ [12] as a way to accelerate photodegradation by five to ten times, it was only recently introduced at higher rates. Organic photovoltaic materials (OPV) are extremely sensitive to light and especially UV, so they were among the first materials to be tested under concentrated light; first up to 27 suns (i.e. 27x the normal sunlight radiation) Tromholt et al. [20] and later up to 10,000 suns Katz et al. [21]. Note that, AT acceleration rates depend on the number of suns (also called concentration ratio) but it not exactly proportional. Recently, the combined effect of both temperature and radiation, acceleration variables, on OPV was also studied [22]. However, all these studies were conducted using concentrated sunlight that limits the operational time and control over the radiation, while some of the observed differences could be attributed to processes occurring during the dark periods [22]. Concerning other materials, Boubault et al. [8] used, again, homogenized concentrated sunlight to investigate an AT for a solar absorber material (Inconel625 coated with anon-selective paint coating Pyromark 2500) but by controlling only the total radiation and low concentration ratios, relative to the standard operation of solar absorbers. Cotfas et al. [9] presented an AT for photovoltaic cells (multijunction Emcore and monocrystalline) using concentrated artificial light but with no homogenization, thus, very poor control on the radiation levels. Most recently, our group displayed the thermal and optical evaluation of advanced solar receiver/reflector materials (MAX phase ceramics [23]) using a well-controlled environment (both temperature and radiation) under homogenized, artificial, and concentrated light [24].

The current problem of high-temperature and solar energy technologies, that our work deals with, is how to make up the scarcity of test procedures for performance evaluation and prediction-of-service-life of new materials. Following a rigorous analysis of the state-of-the-art, some of the most important limitations of current ATs, for solar energy materials, are the omission of the service-life light induced degradation and their low acceleration rates for this type of degradation, whenever included. Moreover, there is a lack of high-temperature AT for solar energy materials. (e.g. absorbers) and of facilities able to provide controlled AT environment using concentrating light. A situation even more problematic for research stage materials that typically are in need of faster screening, in other words ATs with higher acceleration rates. Here, we investigate the hypothesis that an accelerated aging technique based on concentrated light can address the aforementioned technology gaps.

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