By: Tina Casey
From the wonderful world of biomimicry comes a solar energy breakthrough based on the posture at rest of a small butterfly called the Cabbage White. Who knew that voguing is still a thing? Apparently, this stylin’ butterfly forms a uniquely angled “V” with its wings, which according to new research from the UK’s University of Exeter indicates a new pathway for developing lighter, more efficient solar energy harvesting systems.
Butterflies & Solar Energy
To get why the new Exeter solar energy research is significant, let’s take a step back and consider that butterfly wings are powerful examples of Mother Nature’s engineering skills. Here’s the rundown according to the Exeter team:
Butterfly wings are in fact surprisingly complex as butterflies not only have pairs of wings that are effectively linked in flight (and overlap at rest) but the scale cells on their wings also show dramatically different morphologies and orientations. Further, these scale cells can exist as complex overlapping layers therefore potentially conferring complex overall optical properties on the whole wing…
The key to powering all this sophisticated equipment is solar energy. Butterflies need to heat up their flight muscles before they can go fluttering around. They do it by basking in the sun, so on cloudy days it takes a little longer for them to get off the ground.
Somewhere back in time researchers began to notice that on cloudy days, the Cabbage White typically went airborne before other types of butterflies. Naturally, that touched off a series of investigations leading to an explanation of the Cabbage White advantage. According to the Exeter team:
This ability is thought to be due to the v-shaped posturing, known as reflectance basking, they adopt on such days to maximise the concentration of solar energy onto their thorax, which allows for flight.
Loosely speaking, solar energy is concentrated as it bounces back and forth down the V of the wings, and the Cabbage White is especially good at it.
In terms of biomimicry, the butterfly wings resemble v-shaped concentrated solar systems, in which special mirrors reflect and concentrate sunlight onto photovoltaic cells.
That’s not all. The unique substructures in the wing of the Cabbage White make the reflection itself highly efficient, which means that solar energy reaches the flight muscles more quickly.
Skipping ahead to the final result of a lengthy study (you can find the whole thing in Scientific Reports), the researchers found that at an optimal wing angle of approximately 17 degrees, the Cabbage White could increase its body temperature by 7.3 degrees. That’s compared to when the wings are held flat, which unfortunately was probably not very comfortable for the butterfly.
That’s still not all. When the team attached butterfly wings (ouch!) to a conventional solar cell, they achieved a 42% increase in power output, increasing the power-to-weight ratio of the overall structure 17-fold.
Of course, attaching thousands if not millions of Cabbage White butterfly wings to solar cells is not exactly an effective way to boost solar cell efficiency in real life.
However, tearing a page from the book of graphene research, the Exeter team used sticky tape to lift a single layer of cells from the butterfly’s wings. The simple monolayer achieved a similarly high level of reflectivity, indicating that a synthetic version could be an effective coating to boost efficiency in concentrated solar systems, without having to replicate the complex sub-structures of butterfly wings:
…we further speculate that nano-fabrication of a layer of ovoid pigment containing beads will also form a reflective and light weight mimic of a pierid scale cell, provided that the nano-beads are presented in their correct orientation.
The Exeter team notes that reflecting systems required for concentrated solar energy are typically heavy and bulky. In terms of final costs, the expense of this equipment undercuts the efficiency gains of the solar cells. All else being equal, a lighter, more compact system would provide the same or better efficiency with less expense.
More & Better Biomimicry
The Exeter research apparently marks the first solar energy study of the Cabbage White butterfly, but other types of butterfly wings have also been studied for solar applications.
One example is a 2009 biomimicry study undertaken by a research team from China and the US, which replicated the microstructure of butterfly wings to improve the efficiency of dye-sensitized solar cells
In 2012, CleanTechnica took note when a research team from Shanghai released its findings on heat absorption and generation by butterfly wings, possibly leading to solar energy applications.
Butterfly references also pop up in graphene-assisted magnetic energy research, in the form of “Hofstadter’s Butterfly,” a repetition of the same pattern in steadily decreasing scale (or increasing, depending on how you look at it).