photovoltaics: the present presages the future

John Thornton is a principal engineer for the Photovoltaics

Engineering and Applications Branch of the National Renewable Energy Laboratory (NREL), formerly the

Solar Energy Research Institute, in Golden, Colorado. He has been active

in the renewable energy field since 1963. Mr. Thornton holds a B.S. in

electrical engineering from Michigan State University and is a Registered Professional Engineer in Colorado.

Linda Brown received a B.S. in journalism with a specialization in

physics from the University of Colorado. She has been a staff science

writer at NREL since 1987.

Photovoltaics: The Present Presages the Future Recent advances in photovoltaics have made this promising technology cost-effective as an alternative to line extensions and as a power source for many small utility applications. As costs continue to decline and confidence increases, photovoltaics are expected to compete in ever-larger segments of the utility market.

John Thornton and Linda Brown

p hotovoltaic (PV) devices di- rectly transform the sun's en-

ergy into electricity. First brought into commercial use in the 1950s, the technology has since made great strides in efficiency and reliability. Having already proven it- self as a reliable power source in the consumer, space, and military markets, PV is now being recog- nized for many cost-effective utility applications.

PV systems offer many advan- tages to utilities. They are modu- lar, lightweight, portable, and highly reliable. They require short construction lead times and are easily erected in remote areas without expensive support equip- ment. Most remote systems are

relatively small (less than 1 kWe in size) and can be purchased di- rectly from local distributors. They are quiet, environmentally attractive, require no fuel or water, generate no air or water pollution, and eliminate the need for transmission lines. Perhaps most impressivel}~ PV systems' energy costs have dropped more than twenty-fold to the 25- 40c/kWh range in 1991, with further economic improvements likely.

Although the capital cost of a PV system is often higher than other options, relatively low oper- ation and maintenance (O&M) costs and high reliability translate to low life-cyde costs. The sim-

34 The Electricity Journal

plicity of PV, especially nontrack- ing flat-plate systems with no moving parts, provides a low-risk approach for utilities. Cost-re- lated risks are known prior to installation when they can be most easily controlled. Utilities are also relieved of problems such as fuel price escalation or new regula- tions concerning emissions.

Many companies, universities, and government laboratories are aggressively pursuing the development of PV technologies, with the goal of making PV competitive with conventional power plants early in the next c e n ~ As Bernard Gillespie, President of Mobil Solar Energy Corporation, put it, "It's not a question of whether photovoltaics will become a technology of choice it's only a question of when. "1

I. The Solar Resource

Sunlight is a diffuse source. The peak intensity of sunlight reach- ing the surface of the United States is about 1000 W/m2; the annual mean insolation (incident solar radiation) is about 5.3 kWh/m2/day. Because sunlight varies over the United States due to atmospheric conditions, the annual available insolation varies from about 3.3-6.7 kwh/m2/day . With the range of efficiencies available today from off-the-shelf PV technologies, utilities in rea- sonably sunny areas can expect to average about 5.7 kWhe daily for each square foot of PV module surface.

If we assume a solar-to-electric conversion efficiency of 10%, PV systems are capable of collecting

and converting the solar energy striking the United States to supply about 250 times our 1990 electrical demand of 2.8 trillion kwh! 2 This demand could have been supplied from about 10,000 square miles of our desert South- west, an area equivalent to about 9% of the land area of Nevada. Obviously, resource availability is not a limiting factor in using PV for electric power generation.

II. PV Cells and Systems

A. Thick- and Thin-Film Cells

PV cells are semiconductor devices that produce electricity when exposed to sunlight. Two or more semiconductor materials with different electronic proper- ties produce electric fields at a common interface. When sunlight is absorbed in a cell, electrons are released in the semiconductor layers. The electric field then drives the electrons through

an external circuit, producing current. 3

There are two basic types of PV cells: thick-film and thin-film. Thick-film single- or polycrystal- line-silicon cells, which are typically 300 microns thick, are readily available today and account for more than 80% of domestic PV production. Commer- dally available cells exhibit efficiencies as great as 14%; laboratory cells have achieved 23%. (See Figure 1). When packaged for use, these thick-film devices have demonstrated good reliability and stability under severe environmental conditions. They are well suited for today's utility applications.

Cells have also been developed that exhibit efficiencies of as much as 34% when exposed to concen- trated sunlight of up to 500 "suns" (500 times the intensity of normal sunlight). These concen- trator cells are designed to be

35

30

25

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8 lo

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~ , - 32"/o

23.2%

15.7%

I i I z I l l I I I l I 78 79 80 81 82 83 84 85 86 87 88 89

Y e a r

H igh-ef f ic iency concent ra to r cells

Flat-plate single-crystal sil icon cells

Flat-plate thin f i lms

Figure 1: The efficiencies of laboratory cells have increased steadily in all photovoltaic technologies.

April1992 35

used with tracking collectors, as

will be described later. Thin-film cells are based on ma-

terials such as cadmium telluride, gallium arsenide, copper indium

diselinide, and amorphous silicon. Typically, the semiconductor materials are deposited in layers only 1-2 microns thick. Manufac- turing these cells requires less energy and as little as 1% of the ma-

terial needed for thick-film devices, factors that can signifi-

cantly reduce cost. The efficienc~ reliability, and

stability of thin-film technologies are improving steadily. Recent

laboratory tests of cadmium-telluride cells resulted in efficiencies of 14.6%, up from about 11% a few years ago. But the great potential of thin films lies in the promise of

high-volume, automated, low- cost production.

B. The TI/SCE Spherical Cell

A unique type of cell is being de-

veloped by Texas Instruments (TI) and Southern California Edison.

According to TI, its manufacturing process requires only metallur-

gical-grade silicon costing $1/lb instead of the $65/lb, semiconductor-grade silicon needed for conventional crystalline or polycrys- talline cells. Each 4-in 2 unit

consists of 17,000 spherical cells deposited on an aluminum-foil substrate. Adaptable to large- scale manufacturing, this concept promises high stability and efficiency as well as low cost. The flexible substrate reduces break- age during assembly and permits very high yields. Although these cells are not yet available in corn-

mercial quantifies, TI started small-scale production in 1991 and soon expects to have modules ready for field testing. Test results will be used to design a

high-volume production facility. TI's targeted system cost is $1.50- 2.00/W, an achievement that will position this PV technology as a serious option for intermediate- load, or perhaps even baseload, generation. 4

C. PV Modules and Systems

Individual cells don't provide

significant amounts of power.

The TI/SCE spherical cell is aiming at a

$1.50-$2.00/Watt cost, which would

make it a serious economic contender

in many cases.

Rather, individual cells must be

interconnected in series and in parallel to achieve desired levels

of current or voltage. Groups of cells called modules can be environmentally packaged in con- venient size for use in the field. Modules are generally less than 1 m 2 in size and deliver between 50-

150 W of electric power. Modules are, in turn, assembled

into arrays. The simplest, most reliable, and least expensive type is a fixed-position, flat-plate array.

Flat-plate modules can also be

mounted on one- or two-axis trackers that follow the sun's mo- tion across the sky. Trackers are more expensive and complex than fixed arrays, but they can op- timize PV system performance.

The choice between tracking and non-tracking arrays is usually

based on cost versus energy production. It also depends on the

PV technology selected, the application, and the availability of sun-

Ught.

C oncentrating cells are fixed

at the focal point of optics, such as Fresnel lenses, to form

modules. These modules are then mounted on tracking arrays to fol-

low the sun. Sun-to-electricity conversion efficiencies may be greater than 30%. Again, the choice between concentrating and flat-plate arrays is most often an economic one. Because concentrating systems use direct sunlight, they are best suited for loca-

tions in the western or southwestern United States.

Today's PV systems are most commonly used for remote, stand-

alone applications, a role for which they are ideally suited and

for which they are often the most cost-effective option. But PV systems can also be connected to the grid, either in a central or distributed role, While there are some applications where the load pro-

file matches solar resource availability, in most cases some form of energy storage is required. Public Service Company of Colorado

(PSCo) is considering a 35-kW hy- brid PV system to power about 30 homes in the small mountain


town of White Pine. Battery storage and a backup propane genera- tor will ensure consistent service. Although this option is more expensive than replacing the town's aging feeder line, there's much interest in demonstrating the potential of this technology.

PV systems can also use the grid as a storage device in a "fuel saver" mode. Electricity is supplied from the PV system when- ever possible, while fossil fuels such as oil and gas are used when the sun isn't available. Systems of the future may use advanced storage techniques, such as com- pressed air, advanced batteries, superconducting magnetic energy storage, and hydrogen storage.

III. PV Economics and Utility Applications

Since PV was first seriously con- sidered for terrestrial use in the early 1970s, the federal government has contributed about $1 billion to PV-associated R&D, while industry has invested more than $2 billion. The results are encour- aging. Worldwide sales of PV ap- proached $300 million in 1990 and are growing at a compounded rate of about 15% per year. With the capacity of individual production lines ranging from 1-10 MWe annually, U.S. companies ac- counted for about 32% of world PV production (46.5 MWe) in 1990. More than 40% of the U.S. market share of 14.8 MWe was ex- ported. The largest single domestic application (19%) was for off- grid residences. About 3% of domestic production found its

way to grid-connected utility applications. 5

Today's PV modules range in price from $4.00-8.00/W depending on the size of the order as well as cell type and efficiency. For an installed system, the energy cost is not yet a bargain, at about 25- 4 0 c / k W h . 6 But even at these costs, PV is often the most economically attractive option for a variety of utility applications. More than two dozen U.S. utilities have reflected this fact with the installation of thousands of units ranging in size from a few watts up to about 8 kWe. Those already using or anticipating the use of

PV is often the most economically attractive option for a variety of utility applications.

PV systems include such well- known utilities as Pacific Gas and Electric Company (PG&E), Public Service Company of Colorado, Ar- izona Public Service, Georgia Power, New York Power Author- ity, Florida Power Corporation, Alabama Power, Niagara Mo- hawk, Virginia Power, and many others.

The majority of today's utility- owned PV systems are used for low-power, remote, stand-alone applications. A recent survey by

Ascension Technolog3~ Inc., a Mas- sachusetts-based company, identified 65 applications for which PV is being used by utilities. The most common uses are for signal- ing devices such as sirens, lights, and aircraft warning beacons as well as marine, highway, and rail- road signs. PV systems are also used to provide power for transmission and distribution (T&D) sectionalizing switches, gas-line and stream-flow monitors, micro- wave repeaters, emergency call boxes, cathodic protection, and lighting, 7 Off-the-shelf systems are available from a variety of distributors .8

A s capital and O&M costs associated with extension

lines and diesel generators in- crease, stand-alone PV systems are becoming a viable alternative to line extensions. Long distances and rugged terrain, especially in the western United States, make PV attractive because of its modu- larity and ease of installation. But the use of stand-alone PV systems in place of extension lines isn't re- stricted to remote areas. Some urban planners are finding it less expensive to install a self-suffi- cient PV/battery system than to trench and lay cable under an existing street to power lighting, switching, or sprinkler systems.

PG&E has launched an aggres- sive program under which more than 1100 PV systems have been installed throughout its service area instead of new extension lines to perform services for the utility. Hundreds more are planned for the near future. Most of these systems provide power

April 1992 37

for gas-flow monitoring comput-

ers, water-level sensors, and automated gas meters. Not all sys-

tems are small; PG&E has installed three cathodic protection

systems with PV arrays ranging in size from 1000-7200 W. The

company claims that PV was selected for these sites because it is more cost-effective than either die- sels or new line extensions. 9

p G&E is finding that PV systems may be an economical

choice for its customers as well. The utility installs about 2000 new

line extensions each year at an average system-wide cost of $53,000

per mile. A California Public Util- ity Commission regulation has mandated that new customers be given "distance credits" based

upon the type of load; PG&E is re- qui~zd to provide a free line exten-

sion equivalent to the credits.

Customers are required to pay for

feeder lengths in excess of this credit. Experience has shown

PG&E that a PV system will be more cost effective if the length of the line extension exceeds the free distance by more than 1/3 mile and if the load is 1.2 kW or less. A 4-kW system, approximately the

size required for a small resi- dence, becomes cost effective after a mi l e . 10

Figure 2, based on a study by

PG&E, compares the economics of PV with diesel engines and ex-

tension lines for a typical new residential customer. Although the

privately-owned diesel has the lowest capital cost, the life-cyde cost is substantially greater than either a privately-owned or a util-

ity-owned PV system, n Increasing O&M costs often

mean that PV systems can not

I~] Life-cycle cost

1 -mile !,~-mile 1/4-mile Private Utility Private line line line diesel PV PV

extension extension extension

R0ure 2: Pacific Gas and Electric Company's cost study showed that the life-cycle cost of stand-alone PV systems can be more economical for remote residences than conventional line extensions or diesel generators.

only compete successfully for

new service extensions, but can be economical to replace or upgrade existing services. The Electric Power Research Institute (EPRI) estimates that U.S. utilities install 1.5 million new and replacement

power poles each year at a cost of $1000 to $4000 each. 12 Tree trim-

ming costs the utility industry an estimated $500 million every year. 13 Replacement of lines and

transformers due to violent

storms or lightning strikes are other problems frequently en-

countered.

K~ . Electric Association, a

ral cooperative in east- ern Colorado, is considering PV to replace existing service. This cooperative services 4000 square

miles and 5500 customers with about 2700 miles of distribution

lines. Power is used for residential, commercial, industrial, and ir-

rigation purposes. During March 1988, an ice storm destroyed

nearly 1000 power poles. Repair costs reached nearly $1 million, about 10% of K.C. Electric's annual operating revenue.

The association has identified al- most 350 small agricultural appli-

cations, mostly livestock watering sites, that are serviced by its exten-

sion lines. Feeders to these sites range in length from 100 feet to 5

miles. A typical pump supplies from 1000-3000 gallons a day while consuming only 500-1000 kWh per year. The estimated value of the portion of K.C. Electric's distribution system required to supply these 343 sites is about $500,000, or 3% of the value

of the utility plant. The annual


revenue received from these sites is about $78,000, or 0.3% of annual sales. These sites require nearly 10 times the value of the distribution plant than sales actu- ally warrant.

Of the feeders that serve these sites, K.C. Electric identified nearly 90 miles that would cost an estimated $1 million to replace if destroyed in a storm. In that event, the utility has determined that PV will provide a more economical approach if replacement becomes necessar)a

T he cooperative is also considering stand-alone PV sys-

tems for ranchers instead of new line extensions. NEOS Corpora- tion of Denver, Colorado, compared conventional electric service with PV for a typical livestock watering application in the cooperative's area. The site was assumed to be one mile from an existing extension line. Both the feeder and PV system were sized to pump 1500 gallons per day from a 100-foot-deep well. The analysis showed the new feeder would have a capital cost of $11,737 compared with $4,705 for the PV system. The levelized annual cost for the feeder over a 30-year lifetime is $910. Even when O&M is factored in, the levelized annual cost of the PV system is only $420, amortized over a 20-year lifetime. The break- even distance from an existing line for this PV system to become cost effective is about 2000 feet) 4 A recent regulatory action has given a big boost to the use of PV in Colorado. In March 1991, the Colorado Public Utility Commis-

sion amended its line extension rules to require that regulated utilities offer their potential customers a cost comparison between PV and an extension. Ap- plicants supply the utility with an estimate of monthly energy use. If the ratio of monthly use (in kWh) to the required extension- line distance (in miles) is 1000 or less, the utility must supply the comparison free of charge. For ra- tios larger than 1000, the utility must notify the customer of the

availability of the comparison and may charge for it. The commission is evaluating the effective- ness of the new rule and expects to reach its conclusions by April 1992.15

Line extension costs in Colo- rado range from $10,000 to $50,000 per mile depending upon the terrain, type of service, and the number of service drops. The current cost of installing PV on a modern, energy-efficient house using about 150 kWh per month is about $15,000. The break-even

distance for a typical residential PV system is about 800 feet.

Not all PV systems demonstrated to date have been for small applications. ARCO Solar (now Siemens Solar) completed a grid-connected system at Carriza Plain (near Bakersfield, Calif.) in 1984, selling the power to PG&E. The system consisted of 756 two- axis trackers producing a peak power of 6.5 MWe. Although decommissioned because the PV modules now have a higher value in remote applications than they do in supplying power to PG&E's grid, the system provided data that will be used to design the next generation of large PV utility systems. ARCO Solar also con- structed a smaller, 860-kWe system near Hesperia, Calif. Power was sold to Southern California Edison.

T he largest grid-connected system operating today is

the 2-MWe array located near the Sacramento Municipal Utility District's Rancho Seco nudear power plant, about 35 miles from Sacramento, California. The PV array, built with modules from three suppliers, has been operating since 1986.

The City of Austin Electric Util- ity Department has tested two different 300-kWe grid-connected systems in Austin, Texas. The old- est plant (which consists of a single-axis tracking, fiat-plate array owned entirely by the city) has been operating since July 1986. The newest plant, commissioned in 1990, uses linear-Fresnel lens concentrators mounted on two- axis trackers. In addition to the

April1992 39

city, the project is jointly funded by DOE, the state of Texas, and the 3M Company.

p G&E is working with DOE, the California State Energy

Commission, EPRI, and other utilities to develop utility-scaleable PV systems, evaluate them for performance and reliability, assess O&M costs in a utility setting, and provide the results to the utilities and regulators. During Phase I of the project, known as Photovolta- ics for Utility Scale Applications (PVUSA), PG&E is testing eight 20-kWe arrays from seven U.S. manufacturers, two 200-kWe arrays, and one 400-kWe array Nearly I MWe of arrays is expected to be in place by the end of 1991. Phase II will give priority to field testing for newer PV technologies for use in both multi- kilowatt and multimegawatt applications.

The next domestic utility market in which PV will likely be cost competitive is support for distri-

bution feeders and subs ta t ions . 16

Instead of replacing conductors on a line, or upgrading a substation to handle increasing loads, a PV array can be located at the end of a feeder or next to a substation. By offsetting peak daytime loads, a grid-connected, distributed PV system can help prevent thermal aging or overloading of transformers and conductors. PV arrays would range in size from 500 kWe to 3 MWe, PG&E has already selected the Kerman Substa- tion in the San Joaquin Valley as a possible test site. Preliminary analysis indicates the benefits of distributed line support; further analysis will show whether PV is the most economic alternative. 17 PG&E and DOE are considering a prototype system at Kerman to compare analytical results with ac- tual field data. TM

IV. Long-Term Forecast

What is the long-term forecast for this clean, renewable technol-

Q

The Sun, a "free energy device."

ogy? Will PV costs ever drop to where the technology will become a significant utility option, substantially adding to the nation's energy supply?

Production costs are part of the problem. Economies-of-scale could be achieved with highly automated, large-scale manufacturing plants capable of producing hundreds of megawatts of PV capacity per year. To encourage this kind of development, DOE and NREL have initiated a five-year, $55-million technology transfer ef- fort called the Photovoltaic Manu- facturing Technology (PVMaT) initiative. Industry participants are selected through a multi- phased, competitive solicitation; winners share the cost of improving their own production lines and solving general industry problems. The result should be lower costs--and increased sales in the world market.

S ignificant penetration of the utility market depends on

other factors besides cost. Utili- ties must become more knowledgeable about PV's status and potential. They must be able to purchase well-designed, well- tested systems from established companies, complete with war- ranties and servicing. Utilities must also have the expertise to op- erate and maintain large PV arrays once they are installed.

Another factor, now becoming critical to the use of PV systems, is the involvement of the regulatory community. Regulators must also be knowledgeable about PV's potential and be willing to encourage its use. Substantial penetra-


tion of the power-generat ion mar-

ket will not take place wi thout the

active interest and participation of

regulatory bodies. 19

PV will also benefit f rom the in-

creased interest in least-cost plan-

ning and the environmental con-

sequences of energy-generat ing

technologies. A s tudy completed

by Pace Universi ty in 1990 indi-

cated that at least 31 states had

new least-cost planning rules in

force or u n d e r consideration; 29

had taken or were considering

taking some action to account for

environmental externalities. 2°

Following in Colorado's foot-

steps, several states m a y modi fy

their line extension rules to en-

courage PV and other sources of

renewable energy.

A s part of DOE's National

Photovoltaics Program,

both NREL and Sandia National

Laboratories in N e w Mexico are

working with universities, manu-

facturers, utilities, and regulatory

agencies to develop advanced PV

technologies to meet future en-

ergy needs. The t imely transfer of

new developments to utilities, as

well as manufacturers and regula-

tors, is essential if PV is to contrib-

ute substantially to power genera-

tion by the end of this century.

As costs cont inue to decrease

and confidence continues to

build, PV systems are expected to

compete in ever-larger segments

of the utility market. The near-

term goal of DOE's PV program

is to achieve 12-20c/kWh by the

mid-1990s. Expectations include

at least 1000 GWe of PV capacity

installed in the U.S. by 2000, wi th

an addit ional 500 1VIVVe interna-

tionally.

DOE's long-term goal is an elec-

tricity price equivalent to 5-

6 c / k W h (19915) by 2030. DOE

market studies suggest that as

much as 100 GWe of utility-re-

lated PV systems could be opera-

tional by 2030. Cumulat ive sales

of PV by that t ime m a y total more

than $200 billion, bringing sub-

stantial profits to indust ry as well

as electric power and jobs to

Americans. 21 •

~, r"~ 'f/ k . 1 • I.~( /,

Footnotes:

1. U.S. Dept. of Energy, Photovoltaic Division, National Photovoltaic Pro- gram, Photovoltaic Program Plan, FY91-95, forthcoming.

2. ENERGY INFO. ADMIN., ANNUAL EN- ERGY REVIEW 1990 (DOE/EIA- 0384(90), 1990).

3. K. ZWEIBEL, HARNESSING SOLAR

POWER: THE PHOTOVOLTAIC CHAL-

LENGE (Plenum Press 1990) provides a good introduction to PV principles, manufacturing, and applications.

4. Southern California Edison, South- ern California Edison and Texas Instru- ments Develop a Low Cost Solar Cell (news release), 1991.

5. SOLAR ENERGY RES. INST., PHOTOVOL-

TALCS - NEW OPPORTUNITIES FOR UTILI-

TIES (DOE/CH10093-113, Jul. 1991); U.S. DEPT. OF ENERGY, SANDIA NATL.

LABS., SYSTEMS FOR UTILITIES (SAND90- 1378, June 1990).

6. Note 1, supra.

7. T. Moore, On-Site Utility Applica- tions for Photovoltaics, EPRI J., Mar. 1991.

8. Solar Energy Industries Assn., Solar Applications and Directory of the US. Photovoltaic Industry, Wash., D.C., Jan. 1991.

9. C. Jennings, PG&E's Cost-Effective Photovoltaic Applications, 21st IEEE Photovoltaics Specialists Conference, Kissimmee, Fla., May 22, 1990.

10. J.M. Eyer, K. Firor, and D. S. Shugar, Utility-Owned Distributed Photovoltaic Systems, Pacific Gas and Electric Co., 20th IEEE Photovoltaic Specialists Conference, Las Vegas, Nev., Sept. 26-30, 1988.

11. Id.

12. M. Shepard, Managing America's Wood Pole Inventory, EPRI J., Sept. 1987.

13. R.O. Barry, Controlling Tree Growth, EPRI J., Jan.-Feb. 1983.

14. Final Report, Phase II Technical Assistance for K.C. Electric Associa- tion, prepared by NEOS Corporation, Lafayette, Cal., Jan. 1991.

15. Colo. Pub. Util. Comm'n, Amend- ment to Rule 31, Docket No. 90R-672, Jan. 23, 1991.

16. Note 13, supra.

17. Note 5, supra.

18. Id.

19. NATL. ASSN. OF REG. UTIL.

COMM'RS, RENEWABLE ENERGY AND

UTILITY REGULATION (Wash. D.C., Apr. 10, 1991).

20. PACE U. CTR. FOR ENVTL. LEGAL

STUDIES, ENVIRONMENTAL COSTS OF

ELECTRICITY (New York, N.Y., 1990).

21. Note 1, supra.

April 1992 41

photovoltaics: the present presages the future

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