Month: November 2018

Crystalline silicon solar PV is the technology of choice in today’s commercial solar PV electricity generation

It is undoubted that solar photovoltaic is the most likely-looking green energy technology in the 21st century and is the best technology to exploit largely untouched idle energy source: Solar energy. It is having great potential for cutting fossil fuel consumption and harmful emissions. And it is armed with all the necessary tools and implements needed to reverse the fast-maturing change in the climate system. But… However, it is unfortunate that we have not yet been using it adequately.

To illustrate, take a look at the Following pie chart. It displays the world’s electricity generation in 2015, by energy sources.

Figure 01: World electricity generation by source in 2015 [1]

The cumulative electricity generation in 2015 was 24 310 TWh. As illustrated by the chart, coal, gas and oil have generated 40.2%, 21.3%, and 4.3% of the electricity demand, respectively while solar PVs generated merely 2%. In a nutshell, almost 65.8% of electricity demand has been met by burning fossil fuels. It is 30 times greater than the solar electricity generation. What does it imply?

Assuming no change in the electricity demand, we need to double the solar electricity generation each year if we want to completely replace fossil fuels with solar electricity within five years (25= 32). Sound interesting!

But… Keep in mind!

Demand for electricity will never remain unchanged or will never be decreasing. It will be constantly increasing. Obviously!

So… As we can see it, we have a lot to do if we really want to make the Prussian blue, orange and yellow colored segments dark-blue colored! Of cause, wind and other renewable energy sources can also play a big role. But still… as the big brother in the renewable energy industry with the greatest energy potential, solar PV technology must shoulder the key responsibility.

So,

In order to cut fossil fuel consumption…

In order to reverse the greenhouse effect…

In order to fight against the climate change…actively, convincingly with enthusiasm…

We need to spread out solar PVs over the rooftops, across the regions, territories and the world. This requires manufacturing more and more solar PVs and thus needs a copious amount of raw materials. Simply, the raw materials need to be abundant in nature!

Silicon which is the key raw material in commercialized crystalline photovoltaic industry is such material available in the form of silicon dioxide in sand and quartz. And it is one of the most abundant elements in the Earth’s crust (29.5%) as we discussed earlier [2].

Silicon offers many other benefits over other semiconducting materials. Hazardous materials coming with electronic devices, pesticides, fossil fuels and, etc. causes a range of detrimental environmental effects and health problems whilst our planet has terribly been contaminated with a lot of toxic materials. Therefore, we should take every possible step to prevent the already stressed-earth from contaminating. Unlike many other photovoltaic materials, silicon is a non-toxic material which does not impose any known adverse effect on the environment or health. This property makes it an ideal material for mass production of solar PVs.

References

[1] Breyer, C., Bogdanov, D., Aghahosseini, A., Gulagi, A., Child, M., Oyewo, A. S. & Vainikka, P. (2017). Solar photovoltaics demand for the global energy transition in the power sector. Progress in Photovoltaics: Research and Applications.

[2] Yaroshevsky, A. A. (2006). Abundances of chemical elements in the Earth’s crust. Geochemistry International, 44 (1), 48-55.
Ctd

In the previous article, we realized that crystalline silicon solar cells were more realistic and cost-effective in commercial solar electricity generation compared to GaAs solar cells although crystalline silicon solar cells were less efficient than GaAs solar cells. In this article, we are going to discuss the biography of crystalline silicon solar cell technology and the latest trends in crystalline silicon cell industry.

Depending on the degree of crystallinity, we have a variety of silicon allotropes namely, single-crystalline silicon (s-Si), polycrystalline silicon (pc-Si), nanocrystalline silicon and amorphous silicon. While amorphous silicon and nanocrystalline silicon are used in thin film solar cell technologies (2nd generation solar cells), both pc-Si and s-Si are used to fabricate 1st generation crystalline silicon solar cells.

Genesis of crystalline silicon solar cells

The first c-Si solar cell was developed by Bell laboratories in 1953 with an efficiency of 4.5 % and they improved the efficiency to 6% in 1954 [1, 2]. It was an impressive efficiency and amazing beginning at the time and astounded the scientific community all around the world attracting much attention from researchers leading to incredible advances in terms of $/W (system cost), $/kWh (cost of electricity), efficiency, and system durability.

Soon after the mind-blowing invention of efficient c-Si solar cells, researchers started to think of its potential applications and finally, they outmatched.

What did they focus on?

No…. They did not think of developing solar cells to pump water, to light up cities, or to keep the refrigerators cool but to power space satellites outside the Earth.

Nobody tried to install solar PVs on a house or factory.

Nobody invested in a solar PV park.

….as solar PV were too much expensive.

Anyway, growing interest urged to develop solar PVs for terrestrial applications. Sharp Solar was one of the frontrunners in research and development. It was founded as a subsidy of Sharp Electronics in 1959. Braking the ice, they started manufacturing commercial solar modules in 1964. Although it was not initially economical to illuminate a single village at night, the company could install solar panels on as many as 256 lighthouses by 1972. [3] And it showcased the potential of solar PV technology to be an alternative to black gold!

Oil embargo happened in 1973 was also a salutary blessing in the development of solar PV technology. As other companies like Sharp Solar and Phillips came into play, the price of solar modules started to drop dramatically.

Solar PVs are no longer big-tickets. They are now much affordable, as never before, even for average electricity customers who wish to make their energy bill zero or negative. Crystalline silicon solar cells are the most efficient solar PV technology available for commercial electricity generation purposes. They have never lost their battle to retain their influence in terrestrial solar PV applications. While crystalline solar cell technology has been dominating in the market, s-Si and pc-Si technologies have also been competing each other. It could be observed that pc-Si solar cells have outmaneuvered their s-Si twin brothers in the current market. s-Si solar cells represent only 33% of the current PV market while pc-Si solar cells make up 53% of the market [4]. Anyway, their cumulative contribution to the terrestrial solar PV market is remarkable and is about 86%. While two technologies are challenging each other, they are closely working together against climate change and the greenhouse effect. And both are now increasingly exerting an unprecedented pressure on the fossil fuel market leading to a downward trend in the fossil fuel prices.

References

[1] G. L. Pearson, 18th IEEE Photovoltaic Specialists Conference, PV founders award luncheon (1985).

[2] D. M. Chapin , C. S. Fuller , G. L. Pearson , J. Appl. Phys. 25 , 676 (1954).

[3] Green, M. A. (2005). Silicon photovoltaic modules: a brief history of the first 50 years. Progress in Photovoltaics: Research and applications, 13 (5), 447-455.

[4] Tao M. Inorganic Photovoltaic Solar Cells: silicon and beyond. The Electrochemical Society Interface 2008:30–5

Being an ever-evolving technology, solar photovoltaic technology has been giving birth to a number of different species of solar photovoltaics. The efficiency, durability, technical/ economic feasibility, payback time…. All depends on the type of solar cells.

Some are more efficient than others but not cost-effective. They are, of cause, ideal to power space satellites but not to offset your electricity bill. If used in terrestrial applications, it would cost an arm and a leg. Simply, they do not fit your rooftop or wallet. Out of hundreds, only a few types of photovoltaics have been commercialized because most of other PV technologies are still not technologically/ economically viable.

Figure 01: Market share (%) of different photovoltaic technologies in 2015 [1]

Above pie chart demonstrates the current market share of the most dominant photovoltaic technologies available in today’s market. As it can be seen, silicon-based solar photovoltaics make up almost 93% of the market whilst all other types of photovoltaics represent merely 7% of the market. No any other type of photovoltaics has come to win a minute or at least a second from the above chart!

Depending on the key materials used and level of commercial maturity of the technology, photovoltaic technologies are classified into three generations namely first, second, and third generations [2].

First generation solar cells

The first generation solar photovoltaics are well-matured in terms of their technology, and fabrication process. They represent the oldest commercially available photovoltaics technologies. Typically, they are made of either crystalline silicon (c-Si) or GaAs wafers.

GaAs is a direct bandgap semiconductor material with a bandgap of 1.43 eV at 300 K and exhibits remarkable optical properties compared to silicon. Silicon, on the other hand, is an indirect bandgap material whose bandgap is about 1.12 eV at 300 K [3]. Therefore, silicon solar cells require comparatively thicker silicon wafers in order to absorb incoming sunlight sufficiently.

Advantages of GaAs over c-Si

• Having a wide bandgap and outstanding optical properties, GaAs solar cells outperform much efficiently their crystalline silicon counterpart.
• GaAs solar cells require much less amount of semiconductor material than silicon solar cells do. This ability paves the way to manufacture lightweight, high-efficient solar cells.
• Performance of any photovoltaic technology drops with increasing temperature. But GaAs solar cells perform well even at higher temperatures since their temperature coefficient (A measure of performance loss with increasing temperature relative to 25˚ C) is impressively low. This distinctive characteristic makes them an ideal option for hot countries to minimize the energy loss of PVs.
• Unlike other photovoltaic technologies, GaAs solar cells can endure harsh environmental conditions like UV radiation and moisture.
• Electrons run much faster through the GaAs structure than they do through silicon. Simply, electron mobility in GaAs is much greater than in Si.
• GaAs solar cells possess a smaller short-circuit current density, a greater open-circuit voltage and a superior conversion efficiency than c-Si solar cells do.

c-Si or GaAs solar cells?

It is true that GaAs solar cells offer unparalleled benefits with the highest conversion efficiency of any photovoltaic material [4]. But still…They have not yet been chosen for commercial solar electricity generation purposes.

Why?

What are the factors hindering the commercial terrestrial applications of GaAs solar cells?

Well

Firstly, the GaAs solar cell manufacturing process is much expensive. Secondly, GaAs is a an expensive material than silicon since gallium and arsenic are a very rare materials on the earth’s crust. Another key point is that mass production of GaAs solar cells requires more and more gallium/ arsenic. Note that natural abundance of Si, Ga and As are 29.5%, 0.0019% and 0.00017%, respectively [5]! It is a big challenge to maintain a continuous supply in large-scale since both gallium and arsenic are rare/ expensive materials. Following chart clearly explains how rare materials arsenic and gallium are.

Figure 02: Natural abundance of Si, Ga and As [5]

Take a look at the above chart and see if you can observe any Ga or As.

You would see…

You can observe neither Ga nor As. You can observe Si, though.

It is evident that silicon is one of the most abundant element and thus is the best material for large-scale implementation. Moreover, arsenic has been identified to be a carcinogen. Inorganic arsenic (GaAs is partly dissociated in vivo into inorganic arsenic) can induce lung, skin, and bladder cancers [6]. In addition, it has been observed that arsenic is having a link with cardiac disorders, neurological diseases and even diabetes [7]. Some studies also suggest its relationships with some chronic diseases like acute kidney failure. Hence, large-scale implementation of GaAs solar cells might be a disaster unless they are not recycled properly after their end-of-life.

c-Si solar cells are less efficient than GaAs solar cells. But other advantages offered by c-Si solar cells far outweigh any drawbacks.

Conclusion

• Si is one of the most abundant elements on the Earth’s crust while Ga and As are rare elements. No doubt! c-Si solar cell industry will not have a raw material problem in future. Finding raw materials for GaAs solar cells, however, would be a huge problem in mass production lines.
• c-Si solar cells release a much smaller amount of toxic materials during and after their lifecycle than GaAs solar cells do.
• The last fact is the most important thing. c-Si solar cells are less efficient than GaAs solar cells but are much less expensive and economically viable. That is why we have never seen GaAs solar panels at least on a single rooftop while thousands of rooftops are using c-Si solar panels to generate clean electricity.
• c-Si solar cell technology is a proven, cost-effective way of generating electricity. Being a much expensive technology, GaAs solar cells cannot compete with the current c-Si solar cell technology.

Will GaAs solar cells be cost-effective? If so, when?

Nobody knows.

Anyway… We know one important thing!

c-Si solar cell technology has already proven to be cost-effective even in some snowy countries. And it is a God’s gift for Sunbelt countries!

References

[1] Tao M. Inorganic Photovoltaic Solar Cells: silicon and beyond. The Electrochemical Society Interface 2008:30–5

[2] Ranabhat, Kiran, et al. “An introduction to solar cell technology.” Journal of Applied Engineering Science 14.4 (2016): 481-491.

[3] Sze, S. M., & Irvin, J. C. (1968). Resistivity, mobility and impurity levels in GaAs, Ge, and Si at 300 K. Solid-State Electronics, 11 (6), 599-602.

[4] Jean, J., Brown, P. R., Jaffe, R. L., Buonassisi, T., & Bulović, V. (2015). Pathways for solar photovoltaics. Energy & Environmental Science, 8 (4), 1200-1219.

[5] Yaroshevsky, A. A. (2006). Abundances of chemical elements in the Earth’s crust. Geochemistry International, 44 (1), 48-55.

[6] Omura, M., Tanaka, A., Hirata, M., Zhao, M., Makita, Y., Inoue, N., & Ishinishi, N. (1996). Testicular toxicity of gallium arsenide, indium arsenide, and arsenic oxide in rats by repetitive intratracheal instillation. Toxicological Sciences, 32 (1), 72-78.

[7] Hong, Y. S., Song, K. H., & Chung, J. Y. (2014). Health effects of chronic arsenic exposure. Journal of preventive medicine and public health, 47 (5), 245.