2010-07-04.  Newly-emerging Solar Photo-Voltaic (PV) technologies are striving to make considerable improvements in performance levels.  The ever-increasing demand for higher-efficiency Crystalline-Silicon is resulting in lower cost solar panels, forcing thin-film manufacturers to re-think where purchasing decisions are headed, and what they need to do next as Solar PV market prices steadily continue to decline.  Recent price reductions in crystalline-silicon PV modules are exerting significant margin pressure on some thin-film solar PV producers, lowering thin-film market demand and reducing factory utilization to very low levels.  Grid-tie inverter electronics with DSPs, integral anti-islanding & power disconnects, batteries and charge controllers are also benefiting from new nanotechnology materials and cost-effective improvements in electrical design and manufacturing.

According to a recent statement by SMA Technology AG (the largest world producer of Inverters), current world-wide iGBT & semiconductor manufacturing cut-backs and market shortages are slowing electronic Inverter deliveries and affecting solar installations. Micro-Inverters using DSPs directly built into the solar panels are suggesting a better way to go, although there are significant economic considerations.  Using a Micro-Inverter system approach dramatically increases the performance of solar power systems, while adding to the Balance-of-System cost (BOS refers to all the system components … except the solar PV modules).  The BOS components and installation labor can account for more than 50-60% of the rising total system cost, and unfortunately most of the system maintenance.

The Micro-Inverter shifts the DC-to-AC system design approach from using large, centralized inverters to employing compact integral units attached directly to each solar module in the power system, thereby reducing limitations of string design, co-planarity, and matched modules.  Additional modules can simply be added as additional power demand increases, without requiring the costly replacement of large centralized inverters.  Rho Ventures led a $15M VC round with Battery Ventures in micro-inverter startup SolarBridge, which is marketing at the OEM-level through “strategic partnerships with top-ten module companies.”  SolarBridge claims they eliminate some of the BOS cost by removing the junction box and bypass diodes, and the need for a separate centralized Inverter.  Battery Ventures believes that power electronics in residential PV systems will eventually become integral to the PV module.

 GreenTech Media looked at the pros & cons of micro-inverters, because centralized Inverters have characteristically been the weak link in Solar PV systems, where the Levelized-Cost-of-Electricity (LCOE) increases if Inverters need to be replaced every five or ten years.

 The Enphase Enlighten monitoring system provides visibility into the performance of each individual solar module in the system.  The Enphase “Maximum Power Point Tracking” (MPPT) algorithm works at each solar module in an installation and achieves greater than 99.6% tracking accuracy.  This enables it to maximize energy harvest at all times, even during variable daylight conditions.  The MPPT algorithm is used to calculate and respond to temperature and light changes detected on a solar power system, and to determine how much power to draw from the module.  In contrast, a centralized inverter MPPT algorithm sees the entire solar power system as a single module, and responds to the lowest production numbers it detects.  Tests indicate that systems using Enphase Micro-Inverters increase energy harvest by as much as 25% over systems using traditional inverters.

 Traditional centralized-inverter implementations create a potential single-point-of-failure for solar power systems.  If the inverter fails, then the entire system is disabled.  Enphase Micro-Inverters convert power independently at each solar module.  Therefore, if one micro-inverter fails, the rest continue to operate as usual.  Also, if a micro-inverter is damaged or fails, it can be easily replaced during routine maintenance or whenever convenient, further reducing maintenance costs.  The Enphase Micro-inverter is CSA Listed per UL1741 and can withstand surges of up to 6kV.  Enphase Micro-inverter Systems undergo rigorous testing including HALT and HASS, ensuring reliability.  Independent testing by Relex, a leading third-party reliability expert, estimates a Mean-Time-Between-Failure (MTBF) of 365 years, which is certainly comforting to know.

Thin-film Amorphous Silicon (a-Si) cell materials perform better at higher temperatures than Crystalline Silicon, but have lower average output efficiency (8-10%).  However, they are also less sensitive to variations in overcast daylight conditions, and the aesthetics of thin-film a-Si PV modules are preferable to those of 'metal-flake" Crystalline Silicon.  Thin-film a-Si modules are substantially lower in cost, both in terms of $/Watt and $/Unit Area, as compared to mono-crystalline and poly-crystalline modules. 

 The value of a solar installation is reduced to the cost of each Watt of power the panel can produce over its useful lifetime, and Applied Materials (AMAT, Santa Clara, CA) claims they are bringing down the cost with thin-film solar cell manufacturing equipment that produces huge panels the size of “garage doors” (5.7m²), and therefore requires less wiring and mounting hardware (BOS) and labor to install and maintain.  Their weight:  100 Kg (220 lbs) each.  These are best considered for ground-mounted Commercial utility-scale solar farms, rather than roof-top installation.


Figure 2.  Technicians at Applied Materials test a laser scribe machine, part of the company’s equipment line for making the world’s largest solar panels.  [Photo Credit: Jen Siska]

 The primary factor that determines whether a solar system going to be installed is the cost of the system - represented in dollars per watt of solar produced power ($/W).  This can also be expressed as dollars per meter square ($/m²) divided by watts per meter square (W/m²).  Thus, a solar manufacturer can reduce the cost of a solar product in two ways: 1.) reduce the cost of the production materials (wafers, metal contacts, glass, etc.) and other area related costs (labor, overhead, etc); and/or 2.) increase the wattage of the product.  PPG recently announced a new double-sided anti-reflective glass to be used for Solar PV manufacturing.  Through supply chain optimization, manufacturing scale, yield-management and other initiatives, solar manufacturers have been able to significantly reduce area-related costs.  Now, they are looking at the denominator, power output, to continue their impressive cost reduction trajectory.

 SunPower (San Jose, CA), the current market leader in high-performance silicon-based solar panels, announced the highest conversion ratio of 24.4%, nearly double the general market average of 15%, while the best performing Cadmium-Telluride (CdTe) cells produced by FirstSolar are about 9%.  China-based SunTech recently boosted their Pluto cell efficiency to 18.8%, the company's highest-efficiency multi-crystalline solar cell.  Laboratory test efficiency refers to the best cells the company produces, instead of the average efficiency of cells coming off the production line (which is generally lower).  Crystalline PV will probably come down to $1.00 - $1.10 per Watt within the next year or so, while additional investments in China will undoubtedly help to make Thin-Film more competitive in the long-run.

 Solar manufacturers appear to have many options to choose from … if their sole objective is to produce higher-efficiency solar cells.  Unfortunately, the options can actually increase the manufacturing cost, since the area-related cost increase does not offset the wattage-related cost savings.  Therefore, the real objective for a solar manufacturer is to maximize the efficiency improvement while minimizing the additional area-related costs.

 But high-efficiency isn’t cheap.  Notwithstanding higher cost, the highest efficiencies achieved to-date using concentrated sunlight (CPV) is Boeing’s Spectrolab, attaining an astonishing 41.6% efficiency using a triple-junction approach.  The new cell is an advanced version of a lattice-matched triple-junction technology being produced in high volumes for space and terrestrial applications.  The cell incorporates multiple improvements in wafer processing to reduce metal grid shadowing and series resistance, raising overall cell efficiency (not to mention higher heat generated by concentrating solar energy).  SolFocus (Mountain View CA) use 2-axis solar tracking CPV arrays with mini-parabolic reflector modules to concentrate sunlight more than 500 times onto Gallium Arsenide (GaAs) “chips” that use 1/1,000^th of semiconductor material needed by traditional PV cells.

 New semiconductor compounds are being tested in R&D centers using materials that are electrically-reactive to the broad invisible Infra-Red and Ultra-Violet regions of the light spectrum.  Infra-Red materials are less affected by overcast daylight conditions than conventional silicon compounds, since they are more reactive to heat.  Materials reactive to Ultra-Violet tend to be semi-transparent in the visible light spectrum, and can suitably be considered for use as tinted windows or solar shades.

 Additionally, PV products that combine abundantly available low-cost materials with stabilized high-volume manufacturing processes have the potential to offset balance-of-system costs.  Companies developing large-scale production technology for coating flexible inexpensive substrates with thin-film solar paint or solar ink-jet films (CIGS), printed like the daily newspaper, could be the lowest net-cost producers, competing with traditional solar panels based on amorphous-silicon and crystalline technologies.

 Two companies to watch are NanoSolar and InnovaLight (Silicon Valley, California), both attempting to reduce manufacturing cost to mere pennies-per-watt (FirstSolar achieved the lowest manufacturing cost per watt in the industry, breaking $1 per watt in 2008).  But Thin-Film is facing bigger problems.  With variable efficiencies from different producers, there are lingering questions whether it can be manufactured consistently, economically cost-effective, and perform reliably in the long-term “as advertised.”

 InnovaLight says its nano ink-jet technology “improves the economics of solar cell manufacturing, allowing crystalline silicon solar cell manufacturers to increase output capacity and solar cell performance, while reducing costs.”  Both the NREL and the Fraunhofer’s Institute for Solar Energy Systems (ISE) verified 18% efficiency of InnovaLight’s thin-film CIGS solar ink cells.  NREL fabricated the highest performing CIGS cells, achieving 19.4%.  InnovaLight is intent on achieving more than 20% efficiency.

 InnovaLight’s Cougar™ Platform enables solar cell manufacturers to produce mono-crystalline solar cells with claimed efficiencies up to 19%.  Based on a set of patented technologies, the Cougar Platform in combination with Innovalight™ Silicon Ink can be easily retrofitted on existing solar cell manufacturing lines.  By adding a single Silicon Ink screen printing step to a conventional cell manufacturing line, solar cells can be produced with higher conversion efficiency at lower cost per watt.

 The proprietary material is comprised of silicon nano-particles dispersed in an environmentally-friendly blend of chemicals.  Additionally, the material was formulated to be compatible with low-cost industry standard screen printers – the same tool cell manufacturers use for printing metal contacts.  To meet the demand of the Cougar Platform, Innovalight is planning to ramp Silicon Ink production to support over 1,000 MW of Cougar Cells by the end of 2010.

 With the formulation of Innovalight Silicon Ink for the Cougar Platform commercialized, Innovalight is now altering the material properties for use with other high efficiency screen printer based platforms in line with Innovalight’s roadmap.   In addition to altering the material for novel solar cell designs, the material can also be modified for use with a variety of printing equipment.  In fact, a lower viscosity material has been developed for use with inkjet printing equipment.

 InnovaLight’s high-volume capable Development Line:

Architecture:      Homogenous, Selective Emitter, Others
Substrate Type:  Mono-crystalline & Multi-crystalline 
Substrate Size: 125 mm & 156 mm
Texturization:     Acidic & Alkaline
Diffusion:         In-line & Tube
Junction Isolation:  Laser, Plasma & Wet Chemistry 
Equipment:       Rena, Despatch, Roth and Rau, Baccini & others 
Cell Characterization:  IV testing, IQE, QSSPC, CoreScan, SEM & Luminescence Imaging
Reliability Testing:  Damp Heat 85/85, Thermal Cycling & Light Soaking

 In closing, to quote a famous American:

 "Genius is one-percent Inspiration, ninety-nine percent Perspiration."

 "I'd put my money on the Sun, and solar energy.  What a source of Power!
I hope we don't have to wait ‘till Oil and Coal run out before we tackle that." 
                                 -  Thomas Alva Edison (1847-1931)