First post: PV cells

This is a blog on photovoltaic (PV) energy, with a primary focus  on photovoltaic cells and semiconductor technology. Other topics like PV systems and global energy trends will also be discussed.

The battle of solar energy with carbon based fuels has begun and could be a long-drawn one, but there is little doubt that the infinite energy of our home star can power all our energy needs till the end of time. The Earth is a speck in comparison to the sun (0.00007% of its volume) [1] and the sun’s energy powers everything on earth in various manifestations. It is present in plants through photosynthesis, heat that living beings require, blowing winds, flowing rivers, ocean tides as well as the fossil fuels we burn for our transportation, heating and machinery.

PV cells are one way of tapping into this energy. They can convert electromagnetic radiation to electricity. Electric energy is increasing becoming the preferred form of energy for human needs as it is easy to transport and store and can offer high efficiency. In 2012, electricity powered 18% of our energy needs [2] encompassing lighting, heavy machinery, computer technology, climate control systems, transportation and so on. This share is expected to rise to 40% by 2050. PV cells have the potential to directly meet this section of our energy demand.

Image courtesy: IEA.org [2]

PV cells have come a long way from the first practical cell in 1954 [3]. Research was boosted by space applications and developments in the semiconductor industry. Commercially available multi-crystalline Si solar cell now record an efficiency of 14%-19%, and encompass 90% of the PV market. The theoretical limit on efficiency (Schokley-Queisser limit) for crystalline Si lies at 29.4%., but it can be exceeded with the use of multi-junctions, light concentrations and several other techniques. The figure below is one of the most widely used progress charts for different PV technologies. The classification is in five broad categories: Multijunction cells, single-junction GaAs, Crystalline Si cells, thin-film technologies and emerging PV technologies. The highest efficiency achieved thus far is 46% . The focus of cell research is now shifting from power efficiency to application specific utilities such as low material wastage, flexibility, weight and production cost and ease of manufacturing.

 

Image courtesy: nrel.gov[4]

Additionally, PV cells have no harmful emissions or moving parts. Lifetimes can be as along as 25 years for crystalline silicon and energy payback time (EPBT)* is as low as three years [5]. All this means that the pricing of electricity through PV is now increasingly competitive. The levelised generation cost (LCOE)** for PV systems in Germany (2013) was 78-142 euro/MWh while the cheapest source, coal, was 38-80 euro/Mwh [6]. When viewed as a cost during the construction of a building, PV modules can cost 75 euro/m2 while brick-stones cost 40-70 euro/m2 and windows around 100-300 euro/m2! [7].

Swanson’s law*** predicts that the price of solar PV modules drop 20% for every doubling of  cumulative shipped volume. In the last decade, the annual growth of cell production has been greater than 40%. Thus, if current efforts continue, we can look forward to a cleaner, greener, star-powered future.


Important definitions:
* https://en.wikipedia.org/wiki/Crystalline_silicon#Energy_payback_time
** https://en.wikipedia.org/wiki/Cost_of_electricity_by_source#Levelized_cost_of_electricity
*** https://en.wikipedia.org/wiki/Swanson%27s_law



References:
1. “Solar System Exploration: Planets: Sun: Facts & Figures”. NASA. Archived from the original on 2 January 2008.

2. 2014 Key World Energy Statistics (1st January 2016) “http://www.iea.org/publications/freepublications/publication/KeyWorld2014.pdf” 

3.”April 25, 1954: Bell Labs Demonstrates the First Practical Silicon Solar Cell”. APS News (American Physical Society) 18 (4). April 2009.

4. National renewable energy laboratory (1st January 2016) “http://www.nrel.gov/ncpv/images/efficiency_chart.jpg”;

5.Vasilis Fthenakis and Erik Alsema (2005). “Photovoltaics Energy Payback Times, Greenhouse Gas Emissions and External Costs: 2004–early 2005 Status” (PDF).clca.columbia.edu. Archived from the original on 25 March 2015.

6. “Levelized cost of electricity renewable energy technologies” (PDF). FRAUNHOFER. 2013. Retrieved 6 May 2014

7. J. Werner,International symposium on Innovative Solar Cells, Tokyo, 2014 university of Stuttgart