Photovoltaics (PVs) will be a key pillar of our future sustainable energy system, and 1:1:1 for wind, solar, and others (hydro, biomass, geothermal) is a reasonable expectation—according to Prof. Eicke R. Weber, director of the Fraunhofer Institute for Solar Energy Systems ISE, and professor for physics/solar energy at the Faculty of Mathematics and Physics and at the Faculty of Engineering at the Albert-Ludwigs-University of Freiburg, Germany.

Crystalline silicon will remain the dominant PV technology. Classical thin-filmhas to show lower prices or comparable efficiencies. Highly-efficient concentrated photovoltaics (CPVs) will take up a rapidly increasing niche market, competing with concentrating solar power (CSP).

Prof. Weber, along with S. Janz and S. Glunz from the Fraunhofer-Institute for Solar Energy Systems ISE and Albert-Ludwigs, University, Freiburg, Germany, presented his thoughts on ‘Photovoltaics: Pillar of our Future Sustainable Energy System,’ at the recently held Intersolar Europe 2012 in Munich, Germany.

The technology challenge in PV will be to generate innovations in efficiency and cost reduction fast enough to maintain a profitmargin; only players in the XGW-range will survive in the long term. State support for investments in XGW plants, e.g., credit guarantees, might be necessary to maintain globally a level-playing field for PV production.

Creating market barriers will lead to higher prices and less pressure for cost reduction and innovation, ultimately hurting the economies that adopt them.

Bright future for PV
Globally, the required power is 16 TW today, and it will reach at least 30 TW by 2050. PV is offering at least 10 per cent of this requirement. Optimistically, it should meet 30-40 per cent of the global energy needs. 3TW power corresponds to 12 TW or 12,000 GW of PV capacity.

PV capacity globally installed till 2009 was 20 GW. It was +17 GW in 2010 and increased to +25 GW in 2011. To reach 12,000 GW, we would need almost 500 years at this rate! The PV market will move from a $50-billion market to hundreds of billion dollars in a few years. This will be accompanied by a drastic cost reduction, making PV one of the most inexpensive ways to produce energy, in the range of 5 cts/kWh, comparable with hydro and onshore wind but less than nuclear and fossil fuels by 2030 or earlier!

For a 100 per cent renewable energy scenario, at least three components or technologies are required in a ratio of 1:1:1. These are solar energy (PV and solar thermal), wind and hydro, and geothermal and biomass.

The maximal sum of PV and wind production was 7.6 TWh in January 2012. The minimal sum was 5.6 TWh in February 2012. The total annual electricity need of Germany is about 600 TWh. The global market forecast is said to be 30 GW in 2014 and 110 GW in 2020. The annual growth rate should be in the range of 20-30 per cent.

If you look at the global PV production development by technology, by 2011, thin filmhad accounted for 3204 MWp, ribbon-Si 120 MWp, multi-Si 10,336 MWp and mono-Si 9114 MWp, respectively.

Efficiencies in the solar cell market
Efficienciesin the solar cell market range from 1 to 5 per cent for organic, dye and nanostructure cells. PV technologies of interest in the next one to two decades are:
1. 6-11 per cent: Thin-film cells (a-Si, microcrystalline-Si, CIS, CIGS, CdTe)
2. 14-18 per cent: mc-Si, umg-Si, simple c-Si cells
3. 20-24 per cent: High-efficiencyc-Si cells
4. 36-41.1 per cent: High-efficiencyIII/V tandem cells for concentrators with 25-30 per cent module efficienc

The price learning curve for all c-Si PV technologies indicated that with each doubling of cumulative production, price went down by 20 per cent. So thin-filmtechnologies must ramp up fast enough to maintain a clear cost advantage at lower efficiencies!

Prof. Weber cited examples: Solar cells built with 100 per cent umg-Si have a conversion efficiencyof less than 20 per cent. Silicor plans a umg-Si plant with a capacity of 16,000 tpa at a cap-ex of $600 million. The median efficiencyat CaliSolar is now 16.6 per cent. At least 11 per cent of the cells have a conversion efficiencyof above 17 per cent with the highest at 17.7 per cent. Q-cells has achieved 18.2 per cent efficiencyusing umg-Si cell with backside contacts.

The advantages and future requirements for mc-Si include a mature process and no scalability limit. Quality needs to be high enough for 20 per cent efficien solar cells. Diffusion length should be greater than 500 μm at cell thickness of 150 μm. Impurities should exhibit low activity/good gettering behaviour. There should be monocrystals, leading to easy texturing. Some other requirements include increase of yield (less low-quality areas), processes for umg-Si feedstock and cost below 0.30/Wp.

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