Why the efficiency of solar panels is limited?

29/8/2018

The term efficiency tells the proportion of output to that of input. The more the proportion, the higher is the output. The opposite of efficiency is the term “inefficiency”, meaning comparatively less output than desired to that of input.

The efficiency is a matter of size

I would say that the term inefficiency, in the residential solar context, is misunderstood by the people. Let us understand with an example, I am taking two 200-watt solar panels, one with 15% efficiency and the another is 20% efficient.

Is the one with higher efficiency provide me the more power?

No, both the solar panels work in the same way, giving me 200-watt power. The only difference is that the one which is 15% efficient is bigger in the size than that of 20% efficient panel. So, the efficiency in the context of the solar is based on the size of the solar panel.

"The efficiency is a matter of size, the less efficient solar panel will require slightly more space

Your solar panels are rated under standard conditions

The output power of the solar panel is the power under standard conditions when the cell temperature is 25°C and the solar radiance is 1000 W/m² (the change in either of the parameters changes the output of the panel). The output of the panels varies with the change in the temperature and the amount of the solar radiations falling on its surface. Thus, you have more output at the noon when the solar radiations are more and the output reduces in the morning, evening and in the cloudy weather as the solar radiations are less during these periods. The solar radiations vary during the day are shown as below:

You cannot harvest 100% efficiency in any technology and solar-photovoltaics is no exception

Not all the solar energy is converted into electricity by the solar panels, some of its get wasted as heat.

"The maximum limit of the solar cell efficiency is 33.7% by the laws of physics

In 1961, the two physicists Shockley and Quessier explained this fundamental law of Physics. It says that under the standard condition when the 1000 Watts of solar radiations is falling over 1m² of the area, only 33.7% (337 Watts/m²) is utilized by the solar cell and is converted to electricity.
Let us understand it in more detail, the solar cells are made of semiconductors like silicon, Gallium arsenide. The unique property of the semi-conductors is that there is a gap called the band gap between the valence band and the conduction band
In order to excite an electron and send it to the conduction band where it can move freely and start conducting electricity, enough energy equal to or more than the band gap energy is to be provided. The condition is that the energy must be higher than the band gap energy otherwise the electron cannot jump the gap.
The light comes from the sun have different frequencies, the lowest being the red colour and violet corresponds to the highest energy. The energy spectrum of visible light is in the range of 1.65 eV – 3.26 eV. See the chart below:

The Shockley-Quessier limit says that you cannot harvest more than 33.7% of the solar energy because not all the photons are able to excite electrons and make the solar cell to conduct (semiconductors do not respond to all the spectrum of the sunlight). The photons which have less energy than the silicon cell’s band gap energy pass through the cell without exciting the electrons, resulting in wastage of around 18% of the incoming solar energy. Moreover, the electrons will only absorb the energy equal to the band gap energy from the photons with higher energy and re-emit the surplus energy as a light and heat, resulting in additional loss of 49%. Thus, around 67% of the energy is lost and only 33% of the solar energy is left to be converted into electricity.
Let’s see the band gap of different semiconductor materials:

Silicon: 1.1 eV
Gallium arsenide: 1.43 eV
Cadmium Selenide: 1.73 eV

The highest efficiency of the mono-crystalline cell till now is around 24% while the theoretical maximum efficiency limit is 33.7%. Where is the remaining 9.7%. Well there are certain losses like:

Some light is always reflected back
Tiny shadow of wires over the cell
Some losses due to impurities remained in the cell because of the manufacturing process
Some energy is lost at the junctions between the cell and the wires to the load

Can we actually improve and increase the SQ limit?

The SQ limit of 33.7% applies to the single p-n junction solar cell. If somehow, we can use more than one p-n junction (multi-junction solar cell) each one adjusted to the different wavelength/frequency of the solar spectrum, we can increase the efficiency of the solar photovoltaic.
Concentrate the sunlight over solar cell: If we mount the solar cell at the focus of the Fresnel lens is able to concentrate the intensity of the solar radiations 500 times its normal intensity.
Scatter and spread quantum dots (Nano-crystals of the semiconductor) over the large semiconductor material, resulting in capture and absorption of more photon energy and increased efficiency.

Conclusion

If you ask my opinion, more investments are being made in the solar photovoltaics and it is a matter of time that we will witness solar cell efficiency around 40%-50% in the coming years.
Moreover, if we talk about the energy conversion efficiency then it should be in the context of the original source. Let’s say the efficiency of the coal plant is 75%, meaning the burning coal is converting 75% of its internal energy into electricity. But is coal the original source?
No, the coal comes from the debris of plants and animals buried inside the earth crust experiencing high temperature and pressure for millions of years. Now, if I calculate the efficiency, it would be less than 1%.
In this viewpoint, our solar panels are highly efficient as the source of energy is immediately available in abundance.

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