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Current Status of Solar Photovoltaic Technology Platforms, Manufacturing Issues and Research

Sep 27, 2012
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Questions, Answers & Comments from the Webinar

Current Status of Solar Photovoltaic Technology Platforms, Manufacturing Issues and Research

Thursday, Sept. 27, 1 p.m. EDT

Dr. Steven S. Hegedus
Institute of Energy Conversion
University of Delaware

In his talk, Dr. Hegedus, a 30-year solar cell research veteran, provides an overview of the existing status of today's solar technology platforms and manufacturing issues, as well as provide viewers with his perspective looking 3 to 5 years into the future. He discusses cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) thin-film technology. He also provides up-to-date results for advanced crystalline silicon (c-Si) high efficiency cell technology concepts such as the amorphous/c-Si heterojunction, all-back-contact cells, selective emitters and laser-fired contacts. Finally, he briefly describes his lab's current work on addressing critical issues in CIGS and c-Si cell technology.

Dr. Hegedus has been a member of the research staff at IEC at the University of Delaware, the world's oldest photovoltaic research laboratory, since 1982. He co-edited the 1st and 2nd editions of the "Handbook of Photovoltaic Science and Engineering" (Wiley 2003, 2011) and is a co-editor of the journal "Progress in Photovoltaics."

Questions & Answers from the Webinar:

How does the theoretical efficiency limit of single modulePV cell compare to that of plants (Chlorophyl)?

Conversion of energy viaphotosynthesis is 0.1-0.3% efficient in converting sunlight to ‘energy’(calories, or biomass). PV is 10-30% or 100 times more efficient. Electricitycan be used directly, while the calories or biomass then have to be converted again to electricity or heat.

How does the theoretical efficiency limit change with heat if at all?

All the theoretical calculations assume the cell is at standard 25°C. But all solar PV cells have a negative temperature coefficient. They produce less energy as the operating temperature increases. Their operating temperature is almost always greater than 25°C. Standard c-Si solar modules lose about 0.5% (relative) per degree C while advanced Si (HIT, IBC) and thin films have lower losses, only 0.2-0.4%/°C. So a c-Si module rated at 200 W at 25°C produces 175 W at 50°C. Remember that the operating temperature is typically 20-30°C above the ambient air temperature.

Triple Junction efficiency depends heavily on cell irradiance (500 to 1000 suns). How does the efficiency of TF-PV vary with irradiance throughout the solar day. And what is the useful solar day or cost-effective solar angles for fixed arrays?

Effect of tilt is over-rated. Annual output for a fixed tilt module changes by 5% as the tilt changes from 0 (horizontal, flat roof) to 55 degree (fairly steep). Optimum tilt is usually same as the latitude. Seasonal maximum depends on tilt, but annual output averaged over a year rather insensitive to tilt over wide range. TF or c-Si have same dependence on tilt.

What are the realistic lifetimes that one can expect fromTF-PVs? Is there a half-life criterion where the output falls to 50% of initial? What are the failure mechanisms/modes?

Modules are being guaranteed for 25+years. Actual rating spec says they will produce 80% of initial rated power after 25 years. TF PV has unique degradation modes, but many are short term. Degradation can be short term or only appear after years of exposure. There have been a-Si and CdTe modules (none from any company in business today) on outdoor test at NREL or SANDIA National Labs for decades.

In roof-top applications how does PV affect the heat-loading into the building compared to reflective roofing designed to shed or reduce insolation heating?

PV modules protect the roof from heat and UV damage. They either reflect or absorb the light. So long as there is room for air circulation between the roof and module the roof will be cooler than if it was exposed directly.

For different technologies - various thin film technology and crystalline Si, how much percentage is the cost of manufacturing vs other costs?

Costs are not publicly known for CIGS or CdTe. Not sure what you mean ‘other costs’. For installed price, module is about half, but recent price reductions in module have not been matched by reduction in installation, permitting, finance. In fact, financing costs are significant for module and for large installation projects.

Is the reason for d(Voc)/d(Eg) < 1 for CIGS alloys understood?

Much effort has gone into this at R&D level. It seems consensus is developing around a defect in the bandgap which moves closer to midgap (way from the conduction band) as the bandgap widens. (defect energy is fixed). It changes from being a trap to a more serious recombination center.

While there is a quite a bit of R&D for increasing the efficiency of solar photovoltaics, the batteries or storage devices will be a major bottleneck … would you please comment?

Most experts agree. Electric storage is critical to increase the fraction of renewables.

What about the toxicity of CeTe? At the end of life, the module will likely end up in the dump broken where acidic groundwater leaches the Cd into the environment.

This is a well researched and highly controversial topic. I cover it in detail in a slide at the end of my talk (among the back-up slides). There is a much greater perceived risk than real risk. PV competitors in the Si industry have used it to create fear of their strongest competitor, and well meaning but uninformed environmental groups have come out against it too. Manufacturing risks are well managed. Installation and disposal is handled by First solar with a cradle-to-grave tracking, insurance and recycling policy. Many papers available on real risks : go to www.nrel/gov/cdte.

How the Raman spectroscopy is used for the characterization of PVs?

Primarily to characterize the fraction crystallinity in nanocrystalline Si films.

What about thin film GaAs...there are companies working on this?

Alta Devices (Silicon Valley spinoff from Caltech) is new kid on the block, has record efficiency for single junction cell.

Why not laser interconnection on full stack cells?

They do, all multijunction a-Si andnc-Si devices are laser scribed .

Recently I purchased a 20 KW natural gas generator? Was I wrong?

Depends what you want it for. The optimimum off-grid system in terms of reliability nd cost is a combined PV/fossil fuel generator ‘hybrid’. Optimum seems to be meeting 90% of demand with PV and remainng 10% with diesel or gas generator.

Do you think that the market for CdTe and CdSe - based solarcells will be limited by new laws which prohibit them because of healthy issues? Is it worth researching in something that is poisonous and therefore could not be allowed anymore?

Could be limited since EU has twice granted exception for CdTe in PV after they banned Cd in NiCd batteries. In US, EPA has determined the CdTe modules are not toxic waste. Some CIGS manufacturers trying to get away from even the 20-30 nm thick CdS layer so they can also promote their product as not having toxic heavy metals. Again, these are largely political and psychological issues, not strictly medical or technical. People hear ‘cadmium terlluride’ and they think ‘cadmium’. One is the element, another a compound. Most of the toxicity studies are on Cd. We sprinkle NaCl on our food but Na and Cl are toxic. We use H2O to extinguish fires but H2 is flammable. Remember, CdTe is NOT Cd.

What is the future for solar with many companies going bankcrupt recently, including some big ones?

Any reasonable projection of a sustainable future and livable planet requires massive amounts of renewable clean energy to be installed in next 10-20 years. Many studies conclude that PV is the most widely available, and easily integrated. But this requires storage. Question should be ‘if we don’t massively install PV, what will we be doing for electricity in 30 years’.

Other Comments:

Was Sharp once in the top 6 in 2006 has announced a closure of all panel making?

Yes, in the 90s they were number 1. Now they are trying to sell their c-Si and thin film Si manufacturing.

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Other Comments:

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