The sun rises over solar cells


The sun rises over solar cells

It’s no secret that solar energy has been one of the major discoveries of the world in recent times. While it addressed the crucial energy needs of many countries, it also laid the path for ‘clean and green’ energy. But when solar cells came with a heavy price tag, they were used mainly for low power or at isolated places. But a limitation is that solar cells work best at around 20% efficiency and the ones that can be used commercially, only at 15%. It goes without saying that improving the output of solar cells is of great importance.

At a large scale, this can save thousands of square kilometres of open land, the cost, and the pollution hazards of course. Thus, efforts are on to get more light to fall on solar cells, especially at correct wavelengths. This way, solar cells can make the most out of the light falling on them. Here is a peek into some of the efforts that have been undertaken in recent times with this purpose:

Reflectors for maximum energy
In order to maximise the light on solar cells, a variety of reflectors have been experimented with. One of the notable improvements was the closely-spaced rulings on the reflectors, which steer light coming in from different directions. However, this splits up the colours of light and only a small portion of the radiance falls on the cells. In contrast, the quasicrystal, whose structure is not a regular repetition, but has a long range uniformity, can funnel a range of frequencies. Reflectors with partially regular ruling are able to work with a range of frequencies. Creating these partially random patterns, however, is difficult and impossible in practical terms.

Then came the discovery of Blu-ray. Text matter stored on the Blu-ray undergoes repeated transformation, which renders the pattern on the disc almost, but not quite random. The pattern is found to be quasiregular which can focus on a range of frequencies. The pattern can be transferred on to reflectors for solar cells with the advantage that they can then be mass produced.

But of all the discoveries, one stands out in this scenario: it turns out that a particular species of butterfly has been using just this property all along. Butterflies have lightweight wings and limited muscle power and the muscles need to be warmed in the sun before the wings can be put to use. One kind of butterfly, the White Pieris, however, is able to get going before other butterflies, even when the sky is overcast.

Katie Shanks, S Senthilarasu, Richard H ffrench-Constant and Tapas K Mallick, at the University of Exter, UK, describe in the journal, Scientific Reports, that the reason for this is a nano-pattern of microscopic beads on the surface of the wings. The pattern has a quasiregular form and is able to funnel even diffused sunlight and prepare the muscles for flight. The pattern can be transferred on to lightweight reflectors for solar cells, with 43% increase in productivity and 17% improvement in the power to weight ratio.

We all know that the sun’s energy is more in the visible and ultraviolet (UV) range of the spectrum. Solar cells, however, use only the part towards the red end of the spectrum for generating electricity. The energy at the blue end and in the UV end is thus lost. This part is also more energetic and can cause damage to the solar cell.

Jingwen Ding, Jie He and Challa V Kumar, from the University of Connecticut, USA, at an exposition of the American Chemical Society, reported a low-cost, biodegradable sheath, which takes in the light at the higher frequencies and delivers it to the solar cell at lower frequencies that the cell can use. The action is somewhat like the common fluorescent lamp, where the material of the tube takes in UV light and emits visible light.

The present case is more complex, with energy absorbed by one atom being transferred to another atom, followed by emission at a lower frequency.  The material used, however, is a simple mixture of albumin, derived from animal serum, and fat from the common coconut, to create a gel that sets as a film when warmed. The cover is found to double the efficiency of solar cells from 15-30%. 

The solar cell warms while it works and this rise in temperature lowers its efficiency. Linxiao Zhua, Aswath P Raman, and Shanhui Fan at Stanford University, USA report a way to help the solar cell radiate heat at a special frequency that can escape out into space in the Proceedings of National Academy of Sciences.

The atmosphere absorbs much of the radiation that comes from the sun. It also absorbs the heat emitted by warm objects on the earth, and keeps the surroundings warm. But there is a particular frequency of radiation which the atmosphere does not absorb. If warm objects are radiated at this frequency, the heat would go out to space and the objects would cool.

The Stanford team found that silica, or common sand, is able to radiate at this frequency. A sheet of silica placed over a solar cell would thus collect the heat the cell radiates and send it out at the frequency that has a free passage out to space. The cover is found to cool the cell by 13 degrees Celsius, with a rise in efficiency by more than five per cent.

Ingenious solutions
The Karlsruhe Institute of Technology, in Karlsruhe, Germany and the Centre for Solar Energy and Hydrogen Research, Stuttgart, report in the journal, Advanced Optical Materials, a surface treatment that helps solar cells hold on to light that falls upon them, with a 13% rise in efficiency. Normal surfaces reflect a part of the light that falls on them and in the case of the solar cell, this part is lost. The natural world, which needs to keep reflection down, has found ingenious solutions.

For instance, the eyes of the common moth, a nocturnal insect, need to minimise reflection, to catch light and avoid predators. The surface of the moth’s eye has evolved to have tiny, conical protuberances, of dimensions of the wavelength of light. The reflection at a transparent surface is because the waves of light change speed when light moves from the air to any medium. A pattern, of the scale of the wavelength of light, on the surface, smoothes this transition and allows more of the light to pass through.

The team in Germany has found that many plants and leaves also have such a surface, of which the rose petal has the most effective one. They then found a way to transfer the pattern on to a glass surface that could be placed over the solar cell. In addition to preventing reflection, the surface bends light rays to make the cell still more efficient. The result is 13% improvement, which rises to 43% when the light is slanting.

Looks like many shortcomings of solar cells can be solved with some simple and ingenious solutions.

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