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Harnessing the sun: we won't be going back to the horse and buggy for power, but an even older energy source is worth a review. (Article).

Photovoltaic cells convert light energy directly into electrical power (Figure 1) for applications ranging from wristwatches to electric utilities to space vehicles. One may produce cells of just a few square centimetres, or combine modules into arrays of unlimited size. The cells are silent, produce no emissions, have no moving parts. PV units offer reliable economical power where grid power is not readily available [1-4]. Without energy storage, PV systems are limited to daytime applications that can tolerate intermittent and variable power output. Storage batteries have allowed small and midsize PV systems to work in on-demand or 24-hour applications with high reliability. Some battery manufacturers are tailoring their products to fit the specific needs of PV applications. The batteries and charging systems must tolerate variable charging and discharging rates, survive exposure to outdoor conditions, and demonstrate high reliability with little maintenance in order to complement the strengths of PV power s ystems. Battery technology has advanced much slower than has PV technology, and batteries represent a growing portion of PV system costs in remote applications with energy storage [1].

Applications for Electrical Utilities

Central Station Utility Power -- Energy via photovoltaics is still too costly to compete with utility-supplied electricity or with fossil-fuel boilers scaled for utilities and large industries. The current focus for photovoltaics is to bring it to a level where it is cost-effective in providing intermediate load power [5]. Today, PV modules are sold at about $3 to $5/[W.sub.p]. PV systems are priced at about twice this figure (dollar values refer to U.S. dollars in 1995). Various studies suggest that systems prices in the range of about $1 to $3/[W.sub.p] for a PV system will be needed in the competition for the most energy-significant markets. The U.S. Department of Energy cost goal for Thin Film PV technologies is about $0.33/[W.sub.p], which is based on a module efficiency goal of about 15% and module manufacturing costs of about $50/[m.sup.2].

The United States has indicated its commitment to PV technologies in the utilities business. PVUSA, for example, has received more than $15 million in funding commitments and is testing a number of arrays in Davis, GA. Figure 2 shows that the world shipments increased to a record of more than 200 MW in 1999, which correspond to a 500% increase since 1989. To accelerate the development of PV and ensure the U.S. technology and global market leadership, the U.S. Department of Energy provided $72.2 million for photovoltaics in 1999: 47% for R&D, 26% for technology development and 27% for systems engineering and applications.

Grid-Connected Distributed

Applications -- The primary gridconnected applications are roof-mounted arrays on residences, mobile homes, two-storey apartments, low-rise offices, small retail shops, educational and government facilities, light industry, barns, and parking garages. These systems can save energy and allow a degree of independence from a utility while enjoying the security of the grid as a backup.

Flexibility for Utilities -- Utilities are beginning to learn that PV power can play a useful role for them long before PV generating costs are competitive with those at conventional plants [6]. In many applications, studies indicate, a PV system can contribute value well beyond that of the power it produces. A major U.S. utility found that distributed PV systems could extend the life of overloaded transmission circuits and delay the need for capacity increases. Utilities worldwide are examining the potential benefits of using roof-mounted systems in demand-side management programs. U.S. utilities are beginning to offer PV systems to some users requesting line extensions and plan to market systems to remote users.

In 1995-1996, U.S. utility companies allocated over $100 million for PV purchases at over 400 installation sites. In 1993, a California utility (PG&E) began testing the use of a 500 kW PV array to reduce peak loads on distribution equipment (Figure 3). Sacramento Municipal Utility District's (SMUD) 2-MW plant produces enough power to serve 660 Sacramento-area homes.

Remote Power Energy Systems

Photovoltaic technology is now at the point where it can be a competitive energy source for stand-alone power needs; that is, for facilities not connected to a utility grid, especially in remote locations and in underdeveloped countries [6].

A 4 kW PV system, for example, will supply the electricity for a typical U.S. home; furthermore, the annual amount of carbon dioxide saved by the PV system is approximately equal to that emitted by typical family car. Normally, the power of PV modules varied from 90 [W.sub.p] (thick film) to 130 [W.sup.p] (mono-crystalline) and PV units can be available from 2 kW to 50 kW, which can be applied for different installations such as PV water pumping systems, PV-powered lighthouses, PV wind-asoline hybrid systems, rooftop PV arrays, satellites, etc.

Currently Available PV Systems and Their Economics

A summary of the characteristics and economics of the various PV technologies available today is presented in Table 1.

Future Breakthrough Technologies New Thin-Film Materials -- New thin-film materials are showing signs that they can be both inexpensive and efficient [7, 9], and have created "a sense of tremendous excitement in the technological side of the field," says Ken Zweibel, who heads thin-film solar cell research at the National Renewable Energy Laboratory (NREL) in Golden, CO.

At a 1996 photovoltaic (PV) specialists' conference in Arlington, VA, Zweibel and other participants were energized by a report that "one material -- a mixture of copper, indium, gallium, and selenium (CIGS) -- has been made into prototype cells that convert nearly 18% of incoming sunlight to electricity, a performance approaching that of the best crystalline silicon cells." Hans Schock, a thin-film solar cell expert at the University of Stuttgart in Germany, says this result and others on display at the meeting "really give us a chance to bring the cost down."

A solar battery (Figure 5) has been developed by U.S. Army [9] by combining photovoltaic modules with thin film polymer battery. Based on the Gratzel cell principle, the nanoparticle based organic solar cell part of the solar battery (Figure 6) works as follows:

How far down? Current crystalline solar cells can be built for manufacturing costs of $3.50 to $4 per watt generated. Many researchers expect the new thin films -- GIGS and one other, a blend of cadmium and tellurium (CdTe) -- to do better. "If researchers can overcome nagging manufacturing and marketing problems, new devices could produce power for less than $0.50 per watt, low enough to make the cost of PV-generated electricity competitive with gas g enerators", says Zweibel. Figure 4 snows the laboratory results of two thin films (CIS and CdTe) over the past 25 years.

* charge generation from the adsoption of light by the organic dyes;

* charge transfer to semiconductor nanoparticules from the dyes;

* charge collection by the electrodes from the semiconductor nanoparticules via a polyelec-trolyte.

This power generating and storage device gives actually around 5% Eff compared to the 7.1-7.9% Eff for the Gratzel cell [10]. However, developments in nanomaterials and solid-state organic photovoltaics offer unexplored and potentially exciting new opportunities to reach high efficiency with a low cost material ([TiO.sub.2], plastic, conducting polymer, natural pigment, etc).

New Solar Turbines -- New solar turbines are an exciting area of research [11]. Although not a PV device, a prototype solar device that produces enough hot air from the sun's rays to drive the turbines of a 50 kW power station has brought the prospect of cheap, solar-generated electricity a step closer. The solar energy collector has been successfully tested by Israeli researchers at the Weizmann Institute of Science.

Many modern power stations use hot gas to drive the turbines that produce electricity. The gas must be between 1200[degrees]C and 1350[degrees]C and must reach pressures of between 10 and 30 bar. Existing solar collectors, which focus the sun's rays on tubes of air, cannot do this. The best they can achieve is about 700[degrees]C at normal atmospheric pressure.

The new device focuses the Sun's rays through a quartz window to heat ceramic pins around which air flows. The array of pins -- which researchers Jacob Karni, Abraham Kribus and Rahamim Rubin have nicknamed the 'porcupine' -- absorbs solar energy and transfers it to the air. Because the ceramic pins have a large surface area they transfer heat to the surrounding air very efficiently.

A funnel-shaped device concentrates the energy using internal mirrors that channel the rays. This can concentrate the energy to 10,000 kilowatts a square metre. "That makes it the hottest thing in the solar system other than the Sun," says Karni. This concentration of solar rays then passes through the quartz window. Quartz is used because it is transparent and is strong enough to withstand the pressure of the gas inside. The pins absorb the Sun's energy and reach 1800[degrees]C. Air flow around the pins is carefully controlled to prevent them overheating.

In principle, this technology could be used in any size of power station, from small industrial generators to large stations feeding a national grid. Karni estimates that "Israel's peak-hour electricity demand of 6,000 megawatts could be provided by solar power stations collecting sunlight from an area of 2,000 hectares." This is considerably less than the area currently taken up by Israel's fossil-fuel power stations.

Roof-Top PVs: ECD Technology -- The new ECD technology was shown off at the Olympic Games in Atlanta, GA, where the U.S. Department of Energy had built a solar-powered house. On the roof were 110 solar shingles supplied by Energy Conversion Devices (ECD) of Detroit, MI. "From the ground, it's hard to tell the difference between the asphalt shingles and the solar ones," said company chairman Robert Stempel (Figure 7).

ECD's manufacturing arm, United Solar Systems Corporation, begins with a roll of stainless steel sheet 800 m long and 35 cm wide. The steel is fed through a series of rolling machines, "much like newspaper production," according to company vice-president and electronics engineer Subhendu Guha. The machines deposit nine layers of amorphous silicon on the steel, followed by a protective layer of transparent polymer. Finally, a coloured top coating ensures that the shingle blends in with surrounding roof tiles. The shingles work in the same way as conventional cells. Photons knock electrons from the valence band into an energy level called the conduction band, where they form an electric current.

A combination of higher utility costs and government subsidies means that Japanese and German households are likely to be the first major customers. Beginning next year, thousands of Japanese residents will be eligible for a government subsidy that pays for half the cost of installing the photovoltaic system of their choice. Germany is considering a similar program [12].

Roof-Top PVs: Power-Light Technology -- PowerLight Corporation has completed a 40 [kW.sub.AC] PowerGuard [TM] building-integrated (BI) system at the Elverta Maintenance Facility of the Western Area Power Administration (WAPA) to function as both a roof and solar electric photo-voltaic power plant (13). Located just north of Sacramento, this PowerGuard [TM] system (Figure 8) is believed to be the largest roof-integrated PV system in the USA.

The Elverta PowerGuard [TM] project was funded by the Sacramento Municipal Utility District (SMUD), with WAPA contributing funding commensurate with the saving of conventional roofing materials. SMUD and WAPA were the first to demonstrate the PowerGuard [TM] technology in 1994. Co-funding for this 40 [kW.sub.AC] system was provided by the Utility Photovoltaic Group (UPVG) through project TEAM-UP with support from the United States Department of Energy. The Elverta project is the third completed TEAM-UP PowerGuard [TM] system.

The PowerGuard [TM] PV tiles were used to reroof the maintenance building, saving conventional roofing materials as well as the cost to tear off the old roof. The system featured 420 PowerGuard [TM] tiles, incorporating high-efficiency polycrystalline PV cells from Solarex Corporation. The project was installed in a period of seven days during mid-May despite high winds and an unseasonably rainy week.

Photo-electrochemical Systems -- A considerable amount of research is being done in the conversion of solar energy to hydrogen fuel by means of photoelectrochemical cells (Figure 9) (14), although possible commercial systems would need development work over many years. Similar work is also being carried out in electrochemical photovoltaic cells (15).

Conclusions

Reliable and economically competitive PV technologies are already available for low-power applications in isolated, remote applications. For certain utility applications, especially in the U.S. where the electrical rates are much higher than in Canada, PV technologies are on the verge of being economically competitive in certain specialized situations, e.g., to extend the life of overloaded transmission circuits by delaying the need for capacity increases. However, for central station utility power, the cost of the PV power sources must be reduced by a factor of four in order to compete with the conventional sources of power generation, in some areas. There are encouraging signs that this will be achieved within the next decade.

The generation capacities of power stations may ultimately decide the fate of PV technologies. For instance, in Japan, Germany and some parts of the U.S., subsidized roof-top PV tiles are being installed in order to delay the need for increases in the generation capacities.

In the meantime, new manufacturing technologies as well as fundamental materials research are paving the way for the large scale PV applications in the future.

[FIGURE 2 OMITTED]

[FIGURE 5 OMITTED]
Table 1. A comparison of photovoltaic cell characteristics [1, 5, 7, 8].

Cell Material Eg (1) (eV) [Eff.sub.theo] (2) (%)

Single-crystal silicon 1.1 30
 with concentrator 1.1 37

Polycrystalline silicon 1.1 25

Amorphous silicon 1.75 17
 (including Si-C-F-H alloys)

Polycrystalline thin films
 Cadmium telluride 1.44 27
 Copper indium diselenide 1.0 19

Gallium arsenide 1.43 28
 with concentrator 1.43 39

Cell Material [Eff.sub.labo] (3) (%)

Single-crystal silicon 20-23.5
 with concentrator 28.2

Polycrystalline silicon 13-18

Amorphous silicon 10-13
 (including Si-C-F-H alloys)

Polycrystalline thin films
 Cadmium telluride 15.8
 Copper indium diselenide 18.8

Gallium arsenide 27.6
 with concentrator 29.2

Cell Material [Eff.sub.com] (4) (%)

Single-crystal silicon 12 to 14
 with concentrator 13 to 15

Polycrystalline silicon 11 to 13

Amorphous silicon 4 to 8
 (including Si-C-F-H alloys)

Polycrystalline thin films
 Cadmium telluride 8 to 10.6
 Copper indium diselenide -

Gallium arsenide -
 with concentrator -

Cell Material Cost (5) ($/[W.sub.p])

Single-crystal silicon 4 to 7
 with concentrator 5 to 8

Polycrystalline silicon 4 to 7

Amorphous silicon 3 to 5
 (including Si-C-F-H alloys)

Polycrystalline thin films
 Cadmium telluride -
 Copper indium diselenide -

Gallium arsenide -
 with concentrator -

(1) Band gap is the amount of energy (in electron volts [eV]) necessary
to generate an electron-hole pair. Materials with 1.4 eV can absorb a
broader part of the solar spectrum more efficiently than can those with
a lower eV level.

(2) Theorical efficiencies are figured at AM1 (air mass 1) -- terestrial
level at a latitude of 45[degrees]N -- and 20[degrees]C.

(3) Laboratory efficiency is the ratio of the electric energy that a
solar cell produces under full sun conditions to the energy from
sunlight incident on the cell.

(4) The ratio of the electric energy that a solar cell produces under
full sun conditions to the energy from sun-light incident on the cell,
for commercially-produced modules.

(5) Cost refers to the module cost and does not reflect the cost of the
balance of the system, which in most cases approximately doubles the
total cost.

Note: Because of differing claims for surface areas, conditions,
materials, structures, and other nonstandard factors, to come up with
totally consistent figures for the different solar cell types is
impossible. This chart comprises information from the Institute of
Electrical and Electronics Engineers (IEEE) 18th Photovoltaic
Specialists' Conference, Fundamentals of Solar Cells (A. Fahrenbruch and
R. Bube), and the Solar Energy Research Institute, Golden, CO.

Source: Fundamentals of Solar Cells; NREL; SRI International


References

(1.) Weick, B., "Technology Profile: Photovoltaics', TechMonitoring [TM], SRI International, Menlo Park, CA, May 1996.

(2.) Leng, G., L. Dignard-Bailey, G. Tamizhmani, E. Usher and J. Bragagnolo, 'Overview of the worldwide Photovoltaic Industry', EDRL 96-41-A1 (TR), CANMET Energy Diversification Laboratory (CEDRL), Varennes, QC, June 1996.

(3.) Bruno, E-E., Ed., 'Implementing Agreements on Photovoltaic Systems', IEA-PVPS Ex. Co. 1996. 5, Annual Report (1995), International Energy Agency, Vienna, Austria, 1996.

(4.) PVUSA Project Team, '1995 PVUSA Progress Report: Photovoltaics for Utility Scale Applications', available from EPRI, California, March 1996.

(5.) http://www.nrel.gov/ncpv/ (February 2002); http://www.nrel.gov/pvmat (February 2002).

(6.) http://www.eren.doe.goe/pv/ (February 2002); http://cedrl.mets.nrcan.gc.ca (February 2002) http://www.powerlight.com/powertracker.html (February 2002).

(7.) Service, RE., 'New Solar cells Seem to Have Power at the Right Price', Science, 272:1744. 1996.

(8.) Ladouceur, M. 'Electrodeposition de materiaux semi-conducteurs en couche minces pour la fabrication de panneaux photovoltaiques', IREQ Report: IREQ-98 105, 1998.

(9.) Samuelson, Lynne, 'Conducting Polymers and ConformalPhotovoltaic Devices', VII International Seminar of Conductive Polymers, Niagara-on-the Lake, ON, June 2001.

(10.) O'Reagan, B. and M. Gratzel, 'A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Ti[O.sub.2] Films', Nature, 353:737-739, 1991.

(11.) Watzman, H., 'Solar Power Turns on the Heat', New Scientist, 150(2029):21, 1996.

(12.) Manning, E., 'Heat on a Hot, Thin Roof', New Scientist, 151(2038):19, 1996.

(13.) PowerLight Corporation, Communique of July 25, 1996; Contact D. Shugar (tel: 510-540-0550; fax: 510-540-0552).

(14.) Vijh, A.K., "The New Dawn in Electrochemistry: Modern electrochemistry is the result of market pull and the technological push', Canadian Chemical News, 52(4):24 2000.

(15.) Vijh, A.K. 'Electrochemical Physics: The Cutting Edge of Modern Electrochemistry', Canadian Chemical News; 36(3):10 (1984).

Michel Ladouceur, PhD, is a scientist at the Institut de Recherche d'Hydro-Quebec (IRE Q), Varennes, QC. His research interests are in electrochemistry photoelectrochemistry, electroanalytical chemistry and corrosion. Ashok K. Vijh, PhD, O.C., C.Q., FCIC, FRSC is maitre-de-recherche at IREQ. He is one of the most prominent Canadian electrachemists and is the recipient of numerous major distinctions, both nationally and internationally
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Title Annotation:use and application of solar cells
Comment:Harnessing the sun: we won't be going back to the horse and buggy for power, but an even older energy source is worth a review. (Article).(use and application of solar cells)
Author:Ladouceur, Michel; Vijh, Ashok K.
Publication:Canadian Chemical News
Geographic Code:1USA
Date:Apr 1, 2002
Words:3032
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