PhotovoltaicsEfficiencyShines 01






Photovoltaics’ Efficiency Shines

(June 2003)Japan converts more sunlight

As the business and technology of photovoltaics have unfolded in the past few years, the leadership shown by Japan has continued to dominate or match both the installed base and technological improvements which make solar energy more familiar and competitive in the world market. This past April 21, 2003, Sharp Corporation added to the technology with a new single crystal photovoltaic module which boasts a 17.4% efficiency. The way the performance is specified, in module efficiency rather than higher (and less practical) ‘cell’ efficiency, speaks volumes about Sharp’s no-nonsense leadership position in PV. This module with all mechanical mountings and electrical interconnects produces 17.4% for the total in-place assembly. A module of 47 inches by 31.6 inches by 2.2 inches thick produces a nominal 167 Watts. At a suggested selling price of $1017, this module’s output is $ 6.09/Watt.

Not content with terrestrial records, Sharp, in a presentation at the Third World Conference of Photovoltaics Energy Conversion in May, disclosed a 36.5% efficient cell, considered to be the highest efficiency produced outside the laboratory environment. The cell is seven mm square  and 400 micrometers thick. When configured in a module with mechanical and electrical connections, the total efficiency drops to 22% efficiency, making the above mentioned ‘residential’ module quite respectable in performance. The most likely initial applications for the assumedly more ‘pricy’ cell will be in space vehicle power, but such technology may lead to lower cost production of commercial PV.

Another Japanese based firm, Kyocera, introduced in May of this year  its highest efficiency PV module dubbed the d.Blue which provides 167 Watts in a 50.8” by 39”  area. This is 1981 square inches, a 33% greater area than the Sharp 167 Watt module. This would place the  efficiency in the ballpark of 12%. Despite  the lower efficiency, the module’s cost and final installed cost per Watt would be the most important comparisons for any of the modules.

Kyocera provides a 25 year power output warrantee which implies that the module will perform to some percentage of initial rating for 25 years. To make truly competitive monetary comparisons of dollars per installed Watt, it is necessary to have output degradation figures factored in, and add replacement costs using present worth calculations. PV might gain cost benefits if annual maintenance costs were included, too, since PV should not require much maintenance compared to wind generators or turbine based generators.

USA stays in the race

One might think that all the good work is coming from Japan, but SunPower (coincidently of Sunnyvale, CA)  announced on May 12th that they have a new low-cost cell which has been verified by the National Renewable Energy Laboratory at 20.4% efficiency. The comparison to the Sharp PV module becomes gray because the SunPower efficiency is at the cell level. This may indicate that a full module efficiency will be something less than 20%. How much less is not reported, but there are some pieces of information which can help clear the view. The SunPower cell does not use front side interconnections which would reduce module efficiency. Rather, with a patented process, all the contacts are on the rear surface. Does this mean that the module is 20% efficient? Not yet! A SunPower module designated the A-300 delivers 3 kW in a space less than 17 square meters. By comparison, the Sharp NT-167AK requires 17.3 meters to generate 3 kW. Since the Sharp module is rated at 17.4% efficiency, we can surmise that the SunPower Module is  greater than 17% at the module level.  Since there are no specifics on the A-300, it could actually be very close to the 20% efficiency at the module level.

Adobe Photoshop ImageThe new Sharp single crystal photovoltaic (PV) module achieved a conversion efficiency of 17.4%, claiming to be the world’s most efficient module for residential applications. Named the ‘Sun Vista, a single panel generates a nominal 167 Watts at 41 Volts in 1500 square inches..

Behind the expertise of SunPower is Cypress Semiconductor Corp. which is a semiconductor manufacturer with experience in manufacturing silicon and depositing conductors plus doped materials on that silicon. Cypress has a 57% equity holding in SunPower which almost makes SunPower a division of Cypress. Whatever the business connections, the technology connection bodes well for the future of PV at SunPower.

State of the art experience is not new to SunPower. In selecting the PV for the Helios solar powered aircraft built by AeroVironment Inc. for NASA, the SunPower cells were used to power Helios to a record of 96,863 feet in 2001. More is to be expected as the craft is being rebuilt to include closed cycle hydrogen generation with fuel cell power during dark periods.

While billed as low-cost, there are  not yet published prices on the A-300, so panel and installed cost per Watt is yet to be determined. Production is scheduled to be 2 megaWatts (per year?) at the pilot plant in Round Rock, Texas, with additional production being considered near the Cypress high-volume assembly and test facility in the Philippines.

Does efficiency drive success?

By implication, the opposite of ‘highly efficient’ implies ‘highly  wasteful,’ thereby implying that higher efficiency would cost less. An auto with a more efficient engine, should travel farther on a gallon of gasoline which simultaneously reduces pollutants emitted per mile. If the more efficient car only holds four people and you have to carry six, that means both the costs per passenger mile and total time invested may favor a higher priced, lower efficiency vehicle. The total picture has to be evaluated .

The complexity in efficiency extends to PV. For satellite power, PV hardware on the launch pad requires many thousands of dollars per pound to get it into orbit. If the PV array is more efficient, the reduction in launch costs may pay for more expensive and more efficient PV. Then too, the size of the array impacts  manufacturing costs and support structure costs. A smaller, more efficient array  may require less corrective actions from stored fuel, leading to longer vehicle life. Reliability, degradation, plus effects on vehicle dynamics affect the total cost for a given unit of performance.

But on the earth, size and weight may not affect the economics to the benefit of higher efficiencies. What does matter is the total installed cost for a given system, plus degradation  (which will dictate replacement costs,) plus maintenance costs. If the buying power of a dollar were to remain constant, we would expect grid electricity to increase in the next 50 years because of greater scarcity of organic fuels. If PV costs remained constant, there would be a point where PV would become one of the least expensive power sources. In today’s market with better efficiencies, PV has not broached the $5/W in a panel, and the balance of system, a combination of construction and electrical equipment, does not offer hopes for noticeable reductions per Watt, even though the number of installations continues to expand because of world government subsidies. The competitiveness of PV merely has to wait for the price of oil to triple with falling supply. Nuclear power costs, when waste problem costs are included, will continue to escalate, and if the health related costs of airborne pollutants were added to the costs of petroleum, coal, and natural gas generated electric, PV might now be competitive. Brushing aside the limited life of our sun at 5 billion years, PV is the most environmentally responsible closed system solution.

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The SunPower single crystal, 125 mm A-300 cell generates three Watts.Using a patented rear contact design, the total efficiency is maximized and assembly plus interconnect is simplifed. The 20. 4 % verified efficiency may lead to lower cost installed PV in industrial and consumer applications. (Photo is from SunPower Corp.).

Time itself may be favorable for PV to become the principle provider of electricity. In 2000, the USA used 3,800 billion kWhof electricity. During that time PV only accounted for 850 million kWh. Even wind at 5 billion kWH contributes much more. It would be difficult to ask PV to ramp up to 4 trillion kWh in a few years when companies like SunPower are producing 2 megaWatts/year. Even if the PV arrays operated for 10 hour, in 300 days, the output would only add 6 million kWh each year. The gap between needed kWh and producible PV kWH has too many zeros. With time frames more in the neighborhood of 50 years, the availability of PV has a better chance of living up to its responsibilities.

That requirement only accounts for the grid electrical demand. To consider having PV generate the hydrogen to replace the oil needed for transportation places additional responsibility for to PV meet total future demands. Already other renewables are taking their place in energy production and may always be a significant alternative to PV since they also ultimately derive their energy from the sun. The dark horse in this entire scenario is fusion which has neither a usable timetable or unit energy cost. PV is here today and will be available for a lot longer than we need consider.

Don’t confuse the role of fuel cells

Many following the popular press, would wonder why BD’s support for solar energy leaves out the miraculous fuel cell. Isn’t is supposed to save this planet form the ravages of ...well something?

No, the fuel cell is merely a device which takes in one form of energy and converts it into electricity. With organic laden fuel input ot produce hydrogen, its reformer ejects dirty  emissions into the air. Pure hydrogen inputted to fuel cells results in only  electricity and clean water production. But, the hydrogen had to come from somewhere. Even hydrogen produced from ‘cleaner’ natural gas or coal fired generators produces harmful pollutants.

But fuel cells do not create energy, they only convert it from one form to another. How environmentally friendly the fuel cell opeates depends on the constituents of the energy source.

PV doesn’t create energy either, but it converts energy from extraterrestrial solar rays. Not only does PV generate electric from the extraterrestrial source, but it doesn’t produce pollutants in doing such.

The good news is that the fuel cell powered with hydrogen fuel produced from PV electricity, can be a happy marriage. Getting new electrical enerby via the PV from ‘beyond the earth each day,’ can  provide stationary load electricity and generate clean hydrogen to power the clean fuel-celled autos for almost 5 billion years,...unless?

PV and hydrogen-more than one way to skin a cat

The need for hydrogen, either for portable/transportation or for use at dark and inclement times when the PV is not receiving solar energy, is a critical factor in the future of PV energy utilization. Present methods for generating hydrogen utilize the ordinary output electricity from a PV system to operate an electrolyzer which separates hydrogen and oxygen from water. This is not a bad approach because the installed PV system can be called upon to supply electricity for connected loads or to make hydrogen.

Adobe Photoshop ImageThe SunPower single crystal, 125 mm A-300 cell generates three Watts.Using a patented rear contact design, the total efficiency is maximized and assembly plus interconnect is simplifed. The 20. 4 % verified efficiency may lead to lower cost installed PV in industrial and consumer applications. (Photo is from SunPower Corp.).
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The evolution of planar photoelectrodes for PEC systems:

a.) Original single crystal p-type silicon has OER catalyst on the back surface.  
b.) Front surface is coated with Shottky-barrier metal followed by HER catalyst layers thin enough for optical transmission, replacing the semiconductor-liquid junction with a rectifying semiconductor/metal junction.  
c.) It uses a triple junction amorphous silicon diode. HER and OER thin-film sputter-deposited catalysts provide long life in KOH. Efficiencies to 7.8% were reported.  
d.) The external wiring of ‘figure c’ was eliminated with a stainlesss steel substrate for  a triple junction amorphous diode. Perfromance showed a reduction in efficiency and greater optical losses.  
e.) The fifth generation adds a highly transparent and corrosion-resistant encapsulation film that does not need to be conductive. Only high stability surfaces are exposed to electrolyte.Prediction of conversion efficiencies to 15% using encapsulated photoelectrode design with multijunction copper-indium-gallium-diselenide (CIGS) cells are stacked side-by-side. (Graphic is courtesy of the Department of Energy and the University of Hawaii, facilitated by Richard Rocheleau.).
But, suppose a PV system were designed with its only function being to generate hydrogen? Combining the ability of PV to generate significant Voltage with the requirement of water to dissociate at 1.23 Volts and the concept of the PV generated Voltage at the material surface directly applied to water results in hydrogen production without an external apparatus. This direct conversion is Photoelectrochemical (PEC) hydrogen production. The Department of Energy (DOE) is funding research in PEC at the University of Hawaii, for work documented for almost a decade. The DOE has identified goals which state that PEC must achieve solar-to-hydrogen efficiencies of 10% or greater and  demonstrate long lifetimes in corrosive aqueous electrolyte environment. (Ed note: It sounds like the challenges of batteries which include electrodes, electrolytes and separators, but  with sunlight added!)

Efficiency in PEC must include both the electrical generating efficiency and the hydrogen dissociation efficiency. If a cell had a 20% electrical conversion efficiency and 50% dissociation efficiency, the solar-to-hydrogen efficiency would be 10%.

Research was initiated in many stages. The first configuration used a single-crystal p-type silicon with hydrogen evolution reaction (HER) catalyst at the light-impinging surface and oxygen evolution catalyst OER on the dark side. The electrolyte is potassium hydroxide which has good conductive features and light absorbing and corrosive negative features. Later configurations have expanded to triple layer  amorphous silicon with solar-to-hydrogen efficiencies up to 7.8%. A more recent fifth generation configuration uses triple-junction alpha silicon with a highly transparent and corrosion-resistant encapsulation film at the light receptive surface which does not have to be conductive. This will allow higher efficiency materials such as copper-indium-gallium-diselenide (CIGS) to be used, leading to achievement of the 10% efficiency goal while isolating the material from corrosive effects. A detailed description of the status is available from the DOE as document NREL/CP-610-32405, titled: Photoelectrochemical Production of Hydrogen.

Another contribution to direct hydrogen production involves developing catalysts which provide a reaction surface which lowers the activation energy. With such catalysts, either the PEC or external current methods may be able to operate at acceptable costs.  In the case of PEC, the question of surface material compatibility would have to be answered. Researchers at the Tokyo University have described a new catalyst that splits water under UV radiation. The material was made by adding lanthanum to an existing catalyst which led to a nine fold increase in catalytic activity.1

Whether PV is called upon to produce electricity or hydrogen, the path of success with increased efficiency and lower costs continues to follow the perceived possibilities because world government funds are fueling research and applicable environments to provide the ever expanding population with a brighter energy future, simultaneously with cleaner, healthier air. With niche markets such as remote locations already being filled with cost effective, reliable and convenient PV, its presence in energy production accelerates.

Reference
1. HIGH TECH MATERIALS, CHINA CORNER. 01 April 2003. page 5. See www.RareEarthsMarketPlace. com
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