Finer System Level Details for the Comparison of Photovoltaic Technologies

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by Joseph McCabe, PE

In our last article  “Metrics for Thin Film Solar CIGS Company Comparisons,” we alluded to finer system level details in the comparison of photovoltaic (PV) technologies and promised this follow up article on the subject.

System level details begin with the PV modules themselves. Band gap, temperature corrections  and fill factor are just some of the finer technology details, all slightly related in that they can produce system performance differences when comparing similar PV technologies.

Band gap is the quantum-level point where the PV technology absorbs photons. Think of the last time you saw a rainbow with its many colors. These colors are reflecting the visible wavelengths of sunlight. Band gap is akin to the measurement of the number of wavelengths, or colors, that can be absorbed; even ones you can’t see. The higher the band gap, the more the PV technology is able to grab the power contained in our terrestrial sunlight. Locations in higher latitudes have different wavelengths than at the equator, so knowing the band gap can be an important performance factor, especially in high latitude locations.

The higher the band gap, the lower the temperature correction. Temperature correction was covered in the previous article, but to summarize again, the lower the temperature correction the better the PV system performance, especially in places like Phoenix Arizona. So higher band gaps mean greater potential energy capture, and better temperature corrections.

Fill factor is the ratio of the technology’s actual ability to capture available energy to the energy that is theoretically available. More mature PV technologies like single crystalline silicon have higher fill factors, newer thin films like CIGS have lower fill factors which will be increasing as the technologies mature.  Higher fill factors means more power out of the relatively same cost of manufacturing (more voltage and or amperage and thus more maximum power from the same surface). When comparing PV manufacturing companies’ technologies consider current, and future band gaps, temperature corrections and fill factors. 

Over time, PV performance is typically reduced due to weatherization, packaging and PV cell material degradation. Comparison of different PV technologies should include annual performance degradation. For example, lower grade silicon feed stocks will have higher annual performance degradation. There is the potential for new, high quality PV materials and packaging that have no annual degradation. Certain certification tests attempt to simulate performance degradation over time, like IEC 61215 for crystalline silicon and IEC 61646  for thin films. However these certification procedures do not have the ability to expose the modules to sunlight for the years and years needed to evaluate actual annual degradation. My brethren in the industry might be upset with this suggestion, but under nondisclosure you should ask manufactures for the historical paper trail and results from all certification tests. Huge investments in PV manufacturing companies as well as PV projects should know the documented test results for bankability assurances.

Another finer system level detail in comparing PV technologies is whether it is deposited on glass or a flexible substrate. Glass needs to be held in place with a structure that insure it will not fly away, fall down or break. Flexible PV modules promise to be integrated into building materials, similar to the way United Solar, a division of Energy Conversion Devices (ENER), laminations have been used in single ply roofing and standing seam metal roofing. When a PV technology can reduce the structural balance of systems (BOS) cost there is an economy for the installation due to the lack of glass and the potential for true building integration. Look for CIGS companies like Miasolé, Global Solar Energy, Ascent Solar (ASTI), and Nuvosun to follow in SoloPower’s footsteps in certifying the long-term performance and safety of high efficiency flexible PV modules for building integrated (BIPV) and other flexible applications. Perhaps structural and electrical BOS components can be the subject of a future AltEnergyStocks article on the potential for further reduction of PV system costs.

Two approaches in the PV industry can help assure the potential long term performance of a PV system.  One approach is to do internal due diligence, risk analysis and bankability evaluations including all the finer system level details some of which are discussed above. Another approach is to obtain project performance insurance, possibly combined with an acceptable level of internal analysis. Performance insurance is a new aspect to the PV industry which can help to assure project financial performance overtime. Chartis (formerly AIG), Zurich Insurance Group, The Hartford Financial Services group, ACE Limited, JP Morgan, Chubb Group of Insurance Companies, Munich RE and others are beginning to step into this PV performance insurance function. The insurance industry is developing new risk management products for the maturing PV industry like business interruption Insurance, which can be used in lieu of internal assurances. These new insurance products do not eliminate the need to ensure bankability of the modules, systems, and quality of installation, but they can make the job easier.

Joseph McCabe is a solar industry veteran with over 20 years in the business and degrees in Mechanical Engineering, Masters of Nuclear and Energy Engineering and an MBA. He is an American Solar Energy Society Fellow, a Professional Engineer, and is internationally recognized as an expert in thin film PV, BIPV and Photovoltaic/Thermal solar industry activities. Joe can be reached at energy [no space] ideas at gmail dotcom.


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