OPEN

Received: 27 January 2015 accepted: 12 June 2015
Published: 31 July 2015
White butterflies as solar photovoltaic concentrators
Katie Shanks1, S. Senthilarasu1, Richard H. ffrench-Constant2 & Tapas K. Mallick1

Man’s harvesting of photovoltaic energy requires the deployment of extensive arrays of solar panels. To improve both the gathering of thermal and photovoltaic energy from the sun we have examined the concept of biomimicry in white butterflies of the family Pieridae. We tested the hypothesis that the V-shaped posture of basking white butterflies mimics the V-trough concentrator which is designed to increase solar input to photovoltaic cells. These solar concentrators improve harvesting efficiency but are both heavy and bulky, severely limiting their deployment. Here, we show that the attachment of butterfly wings to a solar cell increases its output power by 42.3%, proving that the wings are indeed highly reflective. Importantly, and relative to current concentrators, the wings improve the power to weight ratio of the overall structure 17-fold, vastly expanding their potential application. Moreover, a single mono-layer of scale cells removed from the butterflies’ wings maintained this high reflectivity showing that a single layer of scale cell-like structures can also form a useful coating. As predicted, the wings increased the temperature of the butterflies’ thorax dramatically, showing that the V-shaped basking posture of white butterflies has indeed evolved to increase the temperature of their flight muscles prior to take-off.

Solar concentrators use mirrors and lenses to capture light and direct it towards smaller areas of photo- voltaic (PV) material where the solar energy is converted into electricity1. In this way the cost of the over- all system is reduced by decreasing the area of photovoltaic material required which is typically the most expensive part of a PV solar panel1,2. However, the introduction of these optical devices to focus light onto these solar cell(s) can result in very bulky systems. Although solar concentrators can reduce solar energy costs and improve efficiencies, their weight and size therefore often limits their deployment3,4. Current solar concentrators vary widely in design and even the simple polishing of metal can result in a reflective mirror finish but such polished surfaces are very heavy and specific curved shapes are difficult and therefore expensive to manufacture5,6. Reflective film adhered to plastic mirrors is a second option but this setup often has low reflectivity when applied to complex surfaces6. Polymer mirror films are a more recent third method to gain reflectance values of >90% but require specially designed structures to gain the appropriate shapes for a given application7,8. Vacuum metalizing is therefore the current best option but this process is highly dependent on the material and surface quality it is bonded with in order to ensure a high quality mirror finish5,9. Given the limitations of all existing systems, further study into possible lightweight reflective materials and structures is important. The benefits of a lightweight, easily applied reflective material or coating would not only improve the development of solar concentrator technologies but may also be beneficial to many other disciplines where lightweight highly reflective coatings are desirable.
The white butterflies of the genus Pieris take flight before other butterflies on cloudy days when solar inputs to flight muscle warming are limited. This ability to heat up quickly on cloudy days has been anecdotally suggested to relate to the V-shaped posture they adopt whilst basking in cloudy conditions, a process we here term ‘reflectance basking’. These white butterflies do indeed show high wing reflectance based upon a unique display of pterin containing nano-beads within their individual wing scales as

1Environment and Sustainability Institute, Biosciences,