Indian Wildlife Club Ezine (August, 2016)

Rose petals for solar power

And yet again, nature shows the way to make the best use of sunlight, says S.Ananthanarayanan.

The solar cell takes in light energy to and turns it into electricity. Vegetation makes use of light energy too, but to drive chemical changes to build carbohydrates.  While the solar cell is a recent entrant, plants have been in the business since ages and have evolved to capture and use as much of the light that falls on them as possible.

Ruben Hünig, Adrian Mertens, Moritz Stephan, Alexander Schulz, Benjamin Richter,  Michael Hetterich, Michael Powalla, Uli Lemmer, Alexander Colsmann,  and Guillaume Gomard, from Karlsruhe Institute of Technology, in Karlsruhe, Germany and the Centre for Solar Energy and Hydrogen Research, Stuttgart, report in the journal, Advanced Optical Materials, that they have been able to isolate and reproduce the surface structure of the petals of the garden rose to create a surface that helps solar cells reduce how much light they lose to reflection, and hence a 13% increase in their efficiency.

The authors explain that an early instance of an anti-reflecting surface, which was studied, is that of the eye of the moth, which needs both to see in the dark as well as to stay out of sight of predators. The moth’s eye surface, or the cornea, is provided with an array of protrusions, of the dimensions of the wavelength of light, which make for a great part of light falling on the surface to pass through, in place of being reflected and lost.  While this feature of the moth is effective to admit more light and also show no reflection, plant surfaces can go a step further, by also rearranging the light waves that they pass without so that the waves can be better used by the underlying light mechanism, the authors say. 

Light, as we know, consists of a combination of electric and magnetic waves, each being the cause and also the effect of the other, as they pass through, vacuum, in space, or the air, or glass, or water, etc.  We can imagine that passage of the waves, particularly their speed, depends on how electric or magnetic fields behave in different media.  And in the case of all media, the effect is that the speed of light is less than what it is in vacuum, and for most materials, less than what it is in air. This is why a light beam bends when it enters glass or water, and it is thanks to this that we have cameras, telescopes and the magnifying glass.

Apart from affecting the speed and the direction of light, these properties of materials also affect how much of incident light would pass through or reflect off a surface.  For all materials, hence, a part of the light that falls on a surface is not transmitted, but is reflected. This is so for almost all the angles at which light strikes the surface, with more reflection when the angle is shallow.  Some of the light is thus always lost to reflection by the normal cornea of the eye, the leaf surface, which needs light to create sugars, and, what is vital now for industry, the surface of the photo cell. And hence the great interest and value in developing ‘anti-reflecting’ or AR surfaces, to capture all the light falls on them.

In taking up the study of AR capabilities in plants, the researchers first carried out a survey of different plant surfaces to see which one was the best. The result of the survey is shown in the graph in the top left in picture 1, which shows how the proportion of reflection drops as we move from a plane glass sheet to the surface of different kinds of leaf or petal, and reflection is the least, about 8%, in the case of the rose petal. 

 

As shown in the inset on the top right of the same picture, the surface of the rose petal has protrusions packed close together, some 19 microns high and 32 microns wide, or nearly a half wider than they are high. 

The experimenters copied the micro-pattern of the petals’ surface onto a silicone polymer mould.  The mould was then used to create the same pattern on the surface of a glass substrate, as in the lower half, picture 1, The reflection properties of this surface were then compared with those of a similar glass sheet with a simple cover with no micro-pattern. As shown in the graph in picture 2,  while the level of reflection is low and about equal when light fell directly on the surfaces,  this changes as the slant increases and, at glancing angles, the level of reflection is many times lower for the rose structure treated surface. The trials also showed that this was so over all colours of the spectrum, which makes the rose petal structure a very attractive template for light collection.

The study of the micro-pattern on the surface of the moth cornea, published in the year 2006, includes a discussion of the reason why the micro-pattern reduces the extent of reflection. As stated earlier, the reflection of light waves from transparent surfaces happens because of the difference of the speed of light in vacuum, or air, and the material whose surface it is. It is the steep change in the material properties, and hence the speed of light, when light moves from vacuum or air to other materials, that brings about reflection rather than passage through the transparent material. This is where the microstructure comes in.  As the structures are of the same dimensions as the wavelength of light, they are able to affect the speed of light, and as the waves move from the peaks of the micro-protuberances to their base, they experience a gradual change of speed, in place of the sudden change when they strike a plane surface. For the same reason, the pattern on the rose petal, now transferred to the experimental glass sheet, is able to ease the transition from air to glass and reduce the proportion of light that is reflected.

The study also showed that the rose petal structure, which behaved like an array of micro-lenses, had the effect of making light diverge within the underlying medium, a light focusing effect, which increased the extent of its path and the efficiency of conversion in a solar cell.  In trials that were carried out with organic solar cells, the experimenters report an efficiency rise of 13% when the rose petal structure was used. This was with light falling normally on the solar cell. At glancing angles, the rise in efficiency was as high as 44%. 

“While standard micro lens arrays show both AR and light trapping properties, the rose structure enhances these effects through auxiliary nanofoldings…” the authors of the paper say.  Further, with natural structures, higher ‘width to height’ ratios are possible, apart from the ease of fabrication by simple replication, they say. There is need to gain “further knowledge about the contribution of the nanofolding, in particular its impact on the broadening of the propagation angle distribution,” the authors say.

[the writer can be contacted at response@simplescience.in]

The moth, which forages at night, is an evolutionary predecessor of butterflies, which are colourful and active in daylight. Butterflies appear to have lost the light capturing feature of the moth cornea. But there is a species of butterfly that has a feature that is useful in the application of solar cells.

As the butterflies need to conserve weight, they have lightweight wings and limited muscular resources. The muscles used to take to flight hence need to warm in the sun before they can be used. The White Pieris butterfly, however, is found to get started, even on cloudy days, before other kinds of butterfly.

 

The reason is the angle at which it holds its wings, but mainly a nano-pattern that covers the surface of the wings. Striations with spacing of the order of the wavelength of light, ruled on a reflecting surface, can focus light. Each wavelength, however focuses at a different angle, and this may not be useful to collect sunlight, which has many wavelengths. And a random pattern would not be of any use at all.  But a ‘quasi-random’ pattern, which can be painstakingly generated on a computer, it is found, can focus a range of wavelengths. Now, the White Pieris butterfly has evolved to have just this kind of pattern on its wings. The wings thus focus all the use useful wavelengths of sun’s warmth on to the Pieris’ muscles and it takes to flight before others can.

The pattern on the wings have been peeled off and used with solar cells, resulting in huge increase in output, and also with addition of very little weight. 

Another development is that the patterns on Blu Ray discs, which are a complicated coding of text matter or images, also have a quasi random character. These have also been found good for focusing a range of wavelengths on to solar cells, with the advantage that they can be mass-produced.

While these are methods of getting more light to fall on the solar cell, the Karlsruhe/Stuttgart-rose petal advance is to help the solar cell make the best of the light that does.

Seabirds track ocean winds

Tracking the flight of birds is found to be a good way to map winds on the surface of the sea, says S.Ananthanarayanan.

The weather cock and the “single swallow that does not a summer make” represent the role that our feathered friends have traditionally played in climate science. It should not, hence, come as a surprise that following birds which fly above the sea turns out to be more accurate than satellites or anemometers to get a picture of low altitude winds over the oceans.

Yoshinari Yonehara,, Yusuke Goto, Ken Yoda, Yutaka Watanuki, Lindsay C. Young, Henri Weimerskirch,

Charles-André Bost, and Katsufumi Sato, from the Tokyo University, Universities of Nagoya and Hokkaido in Japan, in Honolulu and the Unversité de La Rochelle, in France, report in the Proceedings of the National Academy of Sciences (PNAS) that flight paths of soaring seabirds can complement existing sea surface wind data by providing very fine grained and rapid information of wind velocities.

The traditional devices are the windsock, which was once a familiar sight at airports, which were called aerodromes, and then the more sophisticated anemometers. While the windsock only showed the direction of the wind, there were other devices to measure its speed. The first was the cup anemometer, which was set spinning by the wind. The next was the vane anemometer, which was little windmill with a tail. The speed of the spin was converted to wind speed by a counter, or even a dial. And there is the hot wire anemometer, where how much a hot wire cools in the wind is measured by the change in the electrical resistance of the wire. 

 

The weatherman also tracked the changes in wind speed and direction by sending up a hydrogen or helium balloon with a metal plate hanging from it. The plate served to show up in a radar screen and indicate the wind speed and direction at different altitudes, as it rose up and up. Much more sophisticated is the laser Doppler anemometer, which depends on the change in frequency of laser light that is reflected by very small particles in the air. This is the method used to survey the wind distribution around a real, power generation wind turbine.

The PNAS authors explain that these are really methods only to sample the wind speed at a few locations that are far apart and the sampling is often not continuous. For recording the wind behaviour on the surface of the sea, which is important to understand the climate and also in coastal areas, the only means available was anemometers mounted on buoys that were distributed over a limited area being monitored. 

The modern method, which is used to study winds over the surface of the sea, is with the help of satellites and a device called a scatterometer. In this method, pulses of microwaves, which are very short wavelength radio waves, are sent down to the earth from a satellite, and the reflected pulses are detected. Winds create ripples on the surface of water, which can grow into large waves or swell, in the open ocean. While ripples are waves on the surface only, depending on the tendency of the surface not to be pierced by the wind, the weight of the water being displaced also comes into play, and the resulting waves are in equilibrium with the wind. The pattern of such waves on the water leads to alternately lower or higher points of reflection and this leads to mutual interference of the reflected radio waves, which can be related to the speed and direction of the wind. As the satellite goes round, it scans the entire earth and in conjunction with data from buoys stationed all over, which helps weather forecasts and the study of ocean dynamics.

But the PNAS authors point out that a satellite observes each area only twice a day and the buoys are also far apart. The picture created is thus not fine grained and details of changes in winds could be missed. Another difficulty, they point out, is that in coastal areas, where the wind and circulation features are significant, the topography affects the reliability of satellite data.

The use of animals that carry miniaturized instrumentation has proved to be a way out in many challenging situations. “The extensive movement range and locomotion ability of marine mammals and seabirds enable observations to be obtained in places and scales unresolved by conventional observations,” Yonehara and other say in the paper. “For example, instrumented seals have been providing temperature and salinity profiles in the Antarctic Ocean for more than 10 years, especially under sea ice coverage that was difficult to measure by conventional methods,” they say.

Coming to wind data over the sea, the authors have studied how effective and useful the movement of soaring seabirds can be. Lightweight Global Positioning System (GPS) units were strapped onto the backs of three species, the streaked shearwater (0.6 kg), the Laysan albatross (3.1 kg), and the wandering albatross (9.7 kg). Their flight path was then plotted by recording their position every second. We can see that this is sixty points for every minute of flight and would provide a fine grained picture of the flight trajectory. As GPS visualizes the actual movement with respect to the ground, the speed of the wind that is affecting the birds’ flight would be extracted from changes in direction and speed. 

 

The shearwater and the albatross are suitable because they carry out soaring flight which is largely wind dependent. The estimation of the wind speed is based on the fact that the ground speed is the highest with a tail wind and the least with a head wind, and in between when the wind is in other directions. The various levels of ground speed, in different directions, over a span of soaring, would thus be distributed in an ‘up and down’ way, as is shown in the picture. And from the shape of the variation of ground speed in terms of direction, the wind speed and the direction of the wind can be worked out.

 

The paper details how the wind speeds derived from the bird sensors was verified with reliable instances of data from. The data had been collected at the Funakoshi–Ohshima Island breeding colony in Japan, Ka’ena Point, Oahu Island breeding colony in Hawaii and at Possession Island, Crozet archipelago in the South Indian Ocean and the areas matched some of the ‘swathes’ covered by the satellite. In the area in the open sea, where the satellite gave good results, there was close agreement with the two sets of results, the paper says. 

While wind data from bird carried sensors could thus supplement that derived from the satellite and buoy mounted sensors, the paper says, and birds could be deployed to collect data in areas where satellite data is not possible, as near the coast. The attaching of instruments to birds could also be a powerful instrument to study the wind environments that birds encounter during migratory flights, the paper says.  Obtaining atmospheric and oceanographic data variables measured by seabird borne sensors is a unique platform to study conditions far out at sea, “by using seabirds as a fast-moving, living ocean buoy,” the paper says.

[the writer can be contacted at response@simplescience.in]