Household Aluminum Foil Matte And Bright Side Reflectivity Measurements: Application To A Photobioreactor Light Concentrator Design

Household aluminum foil has a wide range of applications in engineers and researchers day to day lives, from a means of protecting the food for lunch, to a handy way to shade photosensible samples from light. Indeed, these foils are mechanically robust, water-proof, light weight, long lasting and affordable.

Our interest in this material emerged from our project to develop a new photobioreactor to study the impact of lighting conditions on microalgae growth. To do so, we designed a flat panel photobioreactor [1], [2], [3] with the aim of providing an homogeneous light field to the culture. In order to achieve such conditions, the reactor has to be very thin (width × length × thickness 5 × 12 × 0.6 cm3) and the incident light field should be uniform. The light source should also be flexible in terms of power and light/dark cycles. Hence, we choose a dimmable LEDs panel as light source.

There are two ways of obtaining an uniform light field on the surface of the reactor:

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• place the LEDs panel in contact with the reactor. It would be very simple and yield a high luminous flux on the reactor surface. Sadly, it would lead to an overheating of the photobioreactor and the loss of the culture

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• place the LEDs panel further away from the reactor and use a light concentrator.

The second option was chosen. The foreseen design is pictured in Fig. 8. The LEDs panel would be placed at one end of a rectangular shaped concentrator, while the photobioreactor would be at the other end. The inner faces of the concentrator would be coated with aluminum foil as it is unexpensive and highly reflective. Yet this choice raised the question of which side of the foil would be best suited. To addressed this question, we first started by an extensive literature survey. Among the studies openly reporting the use of household aluminum foil, mostly as light reflector, one can note its employment to:

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• increase photosynthetically active radiation on growing plants [4], improving the production of fruits per plant by 48%, seeds weight by 57%, total biomass by 50%,

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• enhance bacteria removal in low-tech, low-cost, water purifier [5], lowering the disinfection time by 46%,

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• to inexpensively reflect more solar radiation onto a photovoltaic module [6], [7], boosting its production by 14%, build a concentrated solar power thermal system [8] or a solar cooker [9]

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• to protect heat sensor from unwanted radiation [10].

Yet their use remains under reported in the literature.

From a manufacturing perspective, aluminum foils are defined as aluminum sheets with a thickness below 200 μm. Usually household foils is 16 μm thick, up to 24 μm for heavy duty. Industrially such low thicknesses are achieved by successively rolling foils between twin rolls mills [11]. During the latest rolling stages, in order to prevent the foils from shredding, two foils are rolled together. As a result household aluminum has two sides: a bright mirror-like one (in contact with the rolls) and a matte one (in contact with the other foil).

Sadly after this survey, we found that no scientific article dealing with this question. Only that out of common knowledge [12], it is admitted that both sides would have the same reflectivity, even though an obvious difference exists. Furthermore, the few authors admitting using aluminum foil did not precise which side of the foil was used. Had they done it, it could have been used as guideline. The lack of published articles dealing with household aluminum foils reflectivity does not mean that aluminum was not studied as a reflecting material. On the contrary, given its high reflectivity and low absorptivity [13], it is commonly used to produce mirrors. Two production techniques exist:

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• Physical vapor deposition (PVD) is the technique yielding the best reflectivity, usually higher than 90% [14], [15], [16]. It consists in vaporizing aluminum under a low pressure atmosphere before depositing it as a thin film on an optically polished surface. As it is an expensive technique, it is reserved to high end applications such as: internal mirrors for spectrophotometers, lasers, astronomers instruments [14], [15], [17], [18], [19], radiative heat shields for space applications [16], rust-proof coating [13], [20] and, of course, high quality solar mirrors [21].

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• Rolling consists in thinning of an aluminum sheet between twin rolls right after the casting stage. This process can produce mirror-like surfaces at a much lower cost than PVD. Its products are usually used when one has to keep costs in mind and mechanical robustness is sought after. To this regard, aluminum has been shown to be 5.6–81% more efficient than stainless steel, depending on the application [6], [8]. Numerous authors have used and investigated the capabilities of rolled aluminum sheets in the context of solar energy recovery [22], [23], [24], [8], or in a more general manufacturing context, such as car shininess [13], [25], wastewater treatment [26] or even goat milk pasteurization [27]. Furthermore the rolls passings are known to leave stripes on the aluminum foil [11]. These stripes tend to orient reflected light and make light reflection non-isotropic [23], [28], [29]. More generally, aluminum sheets reflectivity is affected by the alloy purity [30] and surface roughness. As a good rule of thumb, the higher the roughness – or number of roll passings – the lower the total reflectivity and the higher the diffuse component of this reflectivity [21], [29], [31]. Nevertheless, if need be, aluminum sheets reflectivity can be improved by chemical etching or electrochemical polishing [14], [25].

All those studies give valid points of comparison for new results. Furthermore, mirrors manufacturers data should not be disregarded. They have been reviewed by Harisson [16].

In the absence of literature dealing with the optical properties of household aluminum foils, we decided to undertake optical characterization of this material. To do so, both bright and matte sides of a commercially available aluminum foil were analyzed using an integrating sphere, yielding specular, diffuse and total reflectivity of the material. Those measurements are reported in this article and available as additional materials. Going one step further, Soltrace [32], an open source raytracing software was used to, in silico, design our lab scale light concentrator and decide which side of the foil would we use.