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Arizona Solar Center Blog

Commentary from Arizona Solar Center Board Members and invited contributors.

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AZ Dept. of Commerce Report - Sizing a Photovoltaic System

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Sizing a Photovoltaic System

(What follows is a quick and easy method for sizing photovoltaic systems. However, the formula is not intended

to be a sizing procedure for design purposes, but is offered only as an aid to readers seeking preliminary sizing information.  For accuracy in system design, the guidance of an experienced photovoltaic system designer is highly recommended.)

The size, and subsequently the cost, of a photovoltaic system depend upon two factors: the electrical requirements of the devices (loads) relying on the system and the amount of sunshine available to power the system.  Both factors determine the quantity and size of panels, batteries, and other components.

  • The Load

The load is defined as the amount of electric power being consumed at any given moment.  To determine the load, it is necessary to select the units that will rely on the system for power.  If the system is used to power a home, the load will consist of appliances, lights and other common home items.

The next step is to determine the wattage of each item.  The wattage of a device is usually stamped or printed on a nameplate or identification plate on the rear of the unit.  If the unit lists VA (volts x amps), that is the wattage.  If only amps are listed, multiply the amps by the volts listed to find the wattage.

Finally, decide how many hours per day (average) each item is to be used.  The load estimate must be as precise as possible to avoid oversizing or undersizing the system. If design is oversized, money is wasted on excess capacity. If it is undersized, power shortages during operation may result.

The average daily load then, is found by the following formula:

Daily load = wattage X time in use (DL = W x T) DL = 100 watts x 2 hours DL = 200 watt-hours

The load profile, together with the amount of sunshine, can be used to determine the size of the array.

  • Available Sunshine

Sunshine is rated in peak hours, the hours of the day at which you can expect the maximum rated performance from a solar panel.  On average, Arizona has six peak hours of sun daily.

Panels are rated in peak watts, the amount of electricity they can produce at peak sun. Consequently, the number of watt-hours available from a panel is found by this formula:

Panel                 Number hours peak sun
Watt-hours = x rated panel output
(Wh = Sp x P)
Wh = 6 hours x 40 watts
Wh = 240 watt-hours per panel

Size of the Array

Because batteries and inverters consume a certain amount of the power generated by the solar cells, it is wise to allow for at least a 20 percent safety factor over and above the exact calculated load needs.

The number of panels is thus calculated as follows:

number of panels =

 

(Daily load x 1.2)

needed

Watt-hours

Number of Batteries

To determine the number of batteries required for the daily load, the owner must decide how many days of reserve he or she desires.  Storage batteries must be capable of operating the load during periods of little or no sun, without any electricity generated by the photovoltaic array.  (A PV system that requires one to five days’ storage capacity should be outfitted with special, deep-cycle batteries.)

To determine the number of batteries required, the designer must know how much energy the batteries can store (energy capacity) and compare that to the daily load and desired reserve.  Batteries are normally rated in amp-hours instead of watt-hours.  To convert to watt-hours, use the following formula. (Battery capacity and discharge average voltage are usually stamped on the battery)

Battery Daily load (watt-hours) x Capacity =  discharge average voltage in watt hours

Once the battery capacity is knows, the number of batteries required can be calculated from the figures previously determined as follows:

 

Number

Batteries=

Watt-hours       Days

required     x   reserve

by load

 

needed

Energy capacity

(watt-hours)

per battery

 

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AZ Dept. of Commerce Report - Solar Cells

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Single – Crystalline Cells

The oldest and most efficient type of photovoltaic cell is made from single-crystalline Silicon.  It is called single-crystalline because the atoms form a nearly perfect, regular lattice – if you could see into the cell, it would look exactly the same in almost every spot.  In these cells, electrons released during the photovoltaic effect have clear, unobstructed paths on which to travel.

Most silicon comes from ordinary sand and several steps are required to turn it into a crystalline solar cell.  The silicon must first be separated from the oxygen with which it is chemically bound.  Then it must be purified to a point where the material includes less than one non-silicon atom per billion.  The resulting semiconductor grade silicon is one of the world’s purest commercial materials and has a price tag of $40 to $50 per kilogram.

The process of growing crystalline silicon begins with a vat of extremely hot, liquid silicon.  A “seed” of single-crystal silicon on a long wire is placed inside the vat.  Then, over the course of many hours, the liquid silicon is cooled while the seed is slowly rotated and withdrawn.  As they cool, silicon atoms inside the vat bond with silicon atoms of the seed.  The slower and smoother the process, the more likely the atoms are to bond in the perfect lattice structure.

When the wire in fully removed, it holds a crystal about 8 inches in diameter and 3 feet long – the size of long salami.  It is cut into wafers, 8/1000 to 10/1000 of an inch thick with a diamond-edge blade and much of the silicon crystal, now worth hundreds of dollars per kilogram, is turned into dust in the process.  The wafers are polished, processed into cells, and mounted in modules.

More than a hundred industry and university research teams have worked to upgrade and automate the manufacture of crystalline silicon solar cells.  They try to further reduce the cost of purified silicon, to develop high-speed crystal pullers and water-slicing techniques, and to improve the overall design of modules.

One of the main objectives of PV research, however, has been to increase the efficiency with which photovoltaic modules convert sunlight into electricity.  Commercial solar modules typically turn 10 to 14 percent of the sunlight that strikes them into electricity.  In the laboratory, module efficiencies of more than 20 percent have been achieved.

NOTE: Photovoltaic conversion efficiency is generally based on module output rather than cell output. Modules include many connections and tiny wires in which electricity is lost.  Consequently, they give lower efficiencies than individual cells.

Polycrystalline Silicon Cells

Polycrystalline photovoltaic cells are exactly what the name implies – a patchwork quilt of single-crystalline silicon molecules.  Connections between these molecules are random and do not form a perfect lattice structure. Polycrystalline cells are less efficient than single-crystalline cells because released electrons cannot follow clear paths.

These cells are produced by pouring hot, liquid silicon into square molds or casts.  The silicon is cooled to form solid blocks, which are sliced like single-crystalline silicon.

These cells are less expensive to produce than single-crystalline cells because their manufacturing process does not require many careful hours of cooling and rotating silicon material.

The main challenge of polycrystalline cells is attaining a sufficiently high efficiency.  Typically, the boundaries between crystals impede the flow of electrons, resulting in module efficiencies of only 7 to 10 percent.

Concentrator cells

Concentrator cells employ lenses and mirrors to focus the sun’s light onto a high-efficiency, single-crystalline cell. Concentrators help gather sunlight so that a smaller-than-normal cell can produce the same amount of electricity as a standard module.  Efficiencies range from 15 to 20 percent with efficiencies as high as 26 percent for a single cell.

Although they use less of the costly photovoltaic material, other elements increase their cost.  Because of their lenses and mirrors, for example, concentrator cells must air directly at the sun.  A tracking system is crucial for effective operation.

Thin-film technologies

In the past decade, much progress has been made in developing and refining thin-film photographic cells.  These cells are created by depositing hot, liquid silicon or other semi-conductor materials onto glass, metal or plastic.

One thin-film technology, which is already employed in many PV modules, is called “amorphous silicon”. It is composed or randomly arranged atoms, forming a dense, noncrystalline material resembling glass.  The silicon layer is less than a millionth of a meter (a micron) thick requiring considerably less pure silicon then other cell types.

Researchers are working to obtain higher efficiency from this material, which lacks the ordered structure and inherent photovoltaic properties of crystalline silicon.  Today’s commercial efficiency average 5 to 6 percent but efficiencies as high as 14.5 percent have been exhibited in laboratories.

Tandem Cells

These cells are still in the developmental stage but offer great potential for the future of photovoltaics.  Tandem, or multiple-junction cells, are actually several cells stacked on top of each other.  Each cell layer is able to convert a different wavelength, or color, of the light spectrum into electricity.

Tandem cells have displayed efficiencies higher than 14 percent in the laboratory and theorist predict efficiencies as 35 to 40 percent.

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AZ Dept. of Commerce Report - From Calculators to Power Plants

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From Calculators to Power Plants: PV Systems in Action

Photovoltaics systems are quite different from traditional methods of generating electricity. Their power production is directly affected by the weather and the time of day – I. e. they can’t produce electricity without sunshine. Ironically, photovoltaic cells are also affected by the sun’s heat becoming less efficient at high temperatures.

One important quality of photovoltaics is their flexibility. Unlike nuclear plants, for example, PV systems can be made small enough to power a hand-held calculator, or large enough to power an entire community. When the demand for electricity increases, a PV system can simply be enlarged, provided the owner can afford it.

Photovoltaic is a young technology and important questions remain as to how it should best be used. Among these questions are:  What kind of backup should be provided for nighttime or cloudy days?  should solar systems be installed at homes or at special generating stations?  and should utilities or individuals own and operate solar electric systems?

Despite these questions, photovoltaics are already used in hundreds of different ways.  Those applications fall into four broad categories: stand-alone systems, grid-connected units, central utility stations, and consumer products.  Each category is discussed below.

  • Stand-alone systems

As the name implies, stand-alone photovoltaic systems are virtually self-sufficient.  They provide all the electricity for a particular application, operating without a utility-line backup.  They are typically small systems, generating less than 10 kilowatts of electricity.

Stand-alone systems are most commonly found in areas far from power lines.  Ranchers, for example, often install solar-powered water pumps to replenish livestock watering holes in distant grazing areas.  These solar systems do not require constant refueling like diesel generators and often cost half as much as power line extensions.

More than 1200 Arizona homes or cabins rely on stand-alone photovoltaics as their main source of electricity.  In particular, PV powered homes are becoming a common sight on Arizona’s Native American reservations.  More than 140 homes on the Navajo Reservation get their electricity from the sun, and the Hopis are working toward 350 such homes.  The Hopis do not allow power lines to enter their villages and have relied for years on diesel generators or batteries, or have simply lived without electricity.

“Four villages have (utility) power lines running 1/8 to 1/4 miles from the village”, said Doran Dalton, sales manager for the Hopi Solar Electric Enterprise.  “We don’t hook up to the power lines because of our long-standing tradition of self-sufficiency.  We don’t have an objection to electricity, we simply want to own the source by which we get it.

Most Hopis live with very little electricity, and rely on small PV systems that power only a few lights and perhaps a television set.  Families that consume more energy install larger PV systems that provide electricity for all the conveniences of a modern home – microwave ovens computers, stereo systems, washing machines, evaporative coolers and lights.  Home PV systems can also power energy-consuming air conditioners and clothes dryers but these sizeable PV systems can cost more than $40,000.

Despite their vast electricity resources, utility companies also use stand-alone photovoltaic systems instead of extending costly power lines.  Many warning sirens at Palo Verde Nuclear Generating Station draw their power from solar panels.  Photovoltaics also furnish electricity for mountaintop microwave repeater stations owned by Salt River Project.

Stand-alone photovoltaics are used for numerous other applications in Arizona, including:  nearly 100 monitoring stations owned by the U.S. Geological Survey, some fire watch towers, a commercial radio station transmitter near Prescott, numerous emergency roadside telephones, billboards, lights and fans at the Picacho Peak Rest Area, and many irrigation or watering system controls.  At a recreation area near Roosevelt Lake, photovoltaics furnish all the electricity for indoor and outdoor lights and toilet fans.

  • Grid-connected systems

Homes or devices connected to photovoltaic systems as well as utility power lines are called “grid-connected systems.”  The utility can supply power at night, when electricity is cheapest, or simply serve as a backup. Federal law mandates that utility companies must purchase any excess power produced by the PV system, although at a reduced rate.

Homes or devices that employ both types of power are not common in Arizona, and the biggest reason is probably money.  Photovoltaics can be extremely cost-effective when compared to the price of extending power lines.  However, for individuals who live in cities or already have grid electricity, the choice to install PV is more difficult.  Homeowners must weigh the cost of utility bills – a few hundred dollars a month at the most – against the price of a photovoltaic system that can cost thousands of dollars for a typical house and a typical energy lifestyle.

As the price of PV decreases, and the cost of utility power increases, PV systems will compare more favorably for grid-connected applications.  In fact, utility companies like Salt River Project are already preparing for that possibility.  SRP has been researching a house in Chandler that employs electricity from both the grid and a small PV system.  “Our experience is that (the PV system) is reliable.  It produces power day after day with very little maintenance,” said Biff Hoffman, manager of SRP’s Research and Development division.

Individuals who are not connected to the utility grid sometimes install utility lines to power only one or two home items.  One Scottsdale couple lived without utility power for seven years, relying entirely on a PV system and propane generator backup.  This off-grid system was sufficient to power an evaporative cooler and the other electrical devices in their home.  When in 1990, temperatures reach 122 degrees Fahrenheit, they decided to install an air conditioner.  To meet the unit’s electrical demand, the couple had to choose between doubling the size of the solar system (another $17,000), or paying for a power line at less than $8,000.  The rest, as they say, is history.

The same couple discovered an interesting problem that sometimes occurs with photovoltaic systems. Electricity provided by their PV system did not follow perfect sine-wave form, as does utility power.  That change in the quality of electricity ruined their computer printer two times before the couple discovered the reason—a faulty inverter.

Because of the occasional problem in power quality, some utility companies require “power conditioning systems” between the home PV system and the grid.  These systems serve two functions: they ensure that electricity entering the grid is of the same quality as power produced by the utility company, and, they can serve as an off switch so that line workers are not hurt by electricity traveling from the photovoltaic system.  SRP determined that power-conditioning systems were necessary from its research on the Chandler house.

Some photovoltaic systems are connected to the grid for reasons not related to power requirements.  Bus stops in downtown Phoenix, for example, have a utility line back up to satisfy the city’s insurance requirements.

  • Central Power Plants

One day, enormous fields of photovoltaic arrays may stand among the Saguaro and Ocotillo of Arizona’s deserts. Connected to the utility grid, their combined power may equal production at large coal plants such as the Navajo Generating Station near Page.

Today, however, the world’s largest central photovoltaic power plant generates a maximum of only six megawatts of electricity – 125 times less than each of three units at the Navajo station.  This plant, called the Carrisa Plant project, was built in 1984 by ARCO Solar Corporation (now called “Siemens Solar”) and is connected to the grid owned by Pacific Gas & Electric Company.

The Carissa Plain plant covers dozens of acres of land with photovoltaic arrays mounted on two-axis trackers. Mirrors, placed next to the arrays, help reflect light to increase the potential power output.  Unfortunately, the intense reflected light has partially destroyed the protective module coatings and has actually decreased production of electricity.  Further development of better coatings should solve this problem.

For those reasons, the Carissa Plain plant is slated to be dismantled. Since, 1984, its only revenue has come from electricity sales to Pacific Gas and Electric Company.  The electricity is purchased at PG&E’s “avoided cost” of producing electricity – a price even lower than wholesale.  In the meantime, worldwide demand for photovoltaic panels has dramatically increased and their value has risen.  Consequently, it is more economical for the owners of Carissa Plain to sell the individual modules than to sell the electricity they produce.

Near Sacramento, California is another photovoltaic central power plant that has operated since 1984. The plant was built in two stages, called SMUDPV-1 and SMUD PV-2 and together they generate up to two megawatts of electricity at maximum production – enough to power 400 to 500 homes.  The total plant employs more than 58,000 photovoltaic modules and occupies more than 20 acres of land.

So far, three photovoltaic central power plants have been built in Arizona.  One completed in 1982 at Phoenix’s Sky Harbor Airport, was once the world’s largest grid-connected photovoltaic power plant.  It was designed to produce 225 kilowatts of power using concentrator solar modules.  The plant was dismantled in 1987, when the lease was not renewed.

APS has donated many of the panels from the plant to Arizona high schools and others are still being researched at the Solar Test and Research Center (STAR Center) in Tempe.

The Solar Test and Research Center was established in 1998 to test the effectiveness of different photovoltaic equipment in the Arizona climate.  It features five photovoltaic arrays, each producing 2 kilowatts of power used in the APS grid.  Many different types of photovoltaic cells are represented in the arrays.

Data obtained from the STAR Center provides valuable information about photovoltaic systems and how they operate.  For example APS has found that single-axis sun tracking systems improve electrical output by about 20 percent and double-axis trackers improve output another 20 percent.  They have also found that output decreases about 10 percent in midsummer, when the weather is hottest.

Arizona’s other photovoltaic central power plant is not utility-operated.  It belongs to a 24-home subdivision in Glendale, Arizona called “Solar One.”  The first-of-its-kind subdivision was constructed by John F Long Homes, with a photovoltaic field along its south side.  The 2600-panel system provides 192 kilowatts of electricity at peak output and provides much of the electricity used by homeowners during daylight hours.  The utility company provides nighttime power and purchases any excess produced by the PV system.  Until rates changed in 1991, some Solar One homeowners actually received refund checks from the utility company.  Photovoltaic central power plants have also received attention abroad.  They have been constructed in Denmark, Greece, Spain, Germany, Saudi Arabia and Japan.

Number cells or modules needed to power various applications

Number cells or modules

Item powered by photovoltaics

Electricity (in watts) produced at peak output

1 Small Cell

(1” X 2”

Calculator

.1 Watt

1 Standard Cell

(4” X 4”)

Small Yard Light

.5 Watt

Module  4’ x 1.5”

 

Color TV for 3 hours

60 Watts

110 Modules

(47 watts each)

A 1500-1800 square foot house with an evaporative cooler, not air conditioning

5.2 kilowatts

169 Modules

Same house with an air conditioner

8 kilowatts

2,600 Modules

Solar One, 24-home subdivision in Glendale, Arizona

192 kilowatts (PV system provides only part of total power used here)

58,000 Modules

SMUD PV-1 and PV-2 photovoltaic central power plant in California

2 megawatts

  • Consumer Products

From toys to security systems, ever-growing arrays of consumer products operate on electricity supplied by photovoltaic cells.  These products are available through catalogs, at many Arizona photovoltaic companies, and even in some department stores.

An estimated 200 million people already own PV-powered calculators and wristwatches. Other solar-powered devices include: portable camping lights, Frisbee-sized pool cleaners, small fans that roll up in car windows, and hats with tiny fans for extra cooling.

Car manufacturers such as Mazda and Audi now offer a “solar sunroof” option in some new car models.  These sunroofs, incorporating see-through photovoltaic cells, power ventilation systems that help cool a parked car as much as 20 degrees.  PV cells have also been used to power entire electric cars.

Landscape lights and security systems are practical, photovoltaic-powered products that are growing in popularity.  Depending upon the size and complexity of these systems, owners can avoid hundreds of dollars in expenses for digging power line trenches or hooking-up to a power line.

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AZ Dept. of Commerce Report - Fundamentals of Photovoltaics

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The Fundamentals of Photovoltaic Systems

The basic element of a photovoltaic system is the solar cell. Modern solar cells are approximately 4 inches square and are most often made from silicon, a semiconductor.

The photovoltaic effect occurs when sunlight shines on the silicon, freeing electrons and generating an electric current. The electricity is collected and transported by metal contacts on the top and bottom of the cell. The current flows through a wire to provide electricity.

Groups of cells are mounted on a rigid, rectangular plate and wired together to form a module often called a” panel” or “flat-plate collector.”  The module is sealed with plastic or glass for protection. Two or more modules connected together form an “array.”

Single cells produce little power and are not often used individually.  They can be found, however, in some items such as small yard lights. Individual panels, which usually produce 40 to 60 watts of electricity, power larger devices such as downtown Phoenix bus stop lights.  Arrays, depending on the number of panels used, can provide all the electricity for a home or even create a huge generating station.

Modules or arrays are sometimes mounted on tracking systems, which follow the sun across the sky.  These devices help maximize electricity production because sunlight shines directly on the PV modules throughout the day.  Single-axis trackers move as the sun changes position from the east to west.  Two-axis trackers not only follow the suns east to west movement, but also allow for its apparent change in attitude with different seasons. Trackers can increase the energy production of a photovoltaic system by nearly 40 percent.

Electricity storage is a critical component of many PV systems. If power is needed at night or on cloudy days, solar-generated electricity can be stored in batteries.  In large PV-powered homes, for example, it is not uncommon to find banks of 50 or more batteries.  These battery banks usually store sufficient electricity to power the home through one or two cloudy days and nights.

All photovoltaic, cells produce direct current (dc) electricity.  That electricity can be used immediately if the PV cell is connected to a device designed for dc power – many refrigerators in recreational vehicles, for example. However most homes and appliances are designed to operate on alternating current (ac) electricity provided by utility companies.  For applications, an inverter, which changes dc electricity to ac must be added to the PV system.

Charge controllers are also important components of many PV systems.  These devices protect batteries from excessive charge when the modules produce more electricity than the batteries can store.  They also keep batteries from releasing electricity if their charge is too low.  Without charge controllers, batteries suffer extreme wear-and-tear and become less effective, last a shorter amount of time and possibly even short circuit.

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AZ Dept. of Commerce Report - Birth of a Technology

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The Birth of a Technology

French scientist Edmund Becquerel in 1839 discovered the photovoltaic effect – that light falling on certain materials can produce electricity.  Later physicist, including Albert Einstein, found that tiny photons, or particles of light, could interact with electrons surrounding the nucleus of an atom.  That interaction causes a free stream of electrons - the basis of electricity.

Scientists in the 1930s developed a theory for the electrical properties of silicon and other crystalline materials, substances that came to be known as “semi-conductors.”  Primitive photovoltaic cells were developed using selenium, but they were very expensive, only one percent efficient, and little more than a scientific curiosity.

Early in 1954, a small team of scientists at Bell Laboratories tried to find a practical way to generate electricity for telephone systems in rural areas not connected to power lines.  Using crystalline silicon, they fashioned an enormous solar cell capable of turning six percent of the sunlight that struck it into electricity.  Soon the efficiency was raised to eleven percent and scientists realized that the new devices could have practical applications. They had another reason for optimism; the material they were using, silicon, is the world’s second most abundant element, comprising 28 percent of the earth’s crust.

These achievements were greeted with much fanfare amid the technological developments of the 1950s.  Solar cells seemed to promise an unlimited supply of electricity and roused considerable excitement.  A 1957 article in “Business Week” envisioned an automatically controlled solar car in which “all riders could sit comfortably in the back seat and perhaps watch solar-powered TV.”

It was an unfavorable time, however, to develop a new energy technology.  Oil was priced at less than $2 per barrel, and large fossil-fuel power plants were built at a record pace.  Moreover, in 1954 construction began on the world’s first commercial nuclear reactor.  Nuclear power was envisioned as a source of electricity “too cheap to meter,” and most government energy funds were devoted to that technology.

Photovoltaic researchers also faced an unsettling economic reality. Silicon cells developed in the 1950s were extremely expensive, with costs as high as $600 per watt (compared to $5 or $6 today).  Funding for research to reduce the cost was not available in an era with falling electricity prices and minimal concerns about the environment.

The space program rescued photovoltaics from the technological scrap heap.  American scientists in the late 1950s went searching for a lightweight, long-lasting power source for satellites.  Photovoltaic cells, which could take advantage of the continuous sunlight of space, were their choice. In 1958 just four years after the Bell laboratory breakthrough, silicon solar cells were boosted into orbit aboard Vangard I, the second U.S. Satellite.

With the help of large contracts from the National Aeronautics (NASA) four U.S. commercial companies enter the photovoltaics business and by the late 1960’s were producing hundreds of thousands of solar cells a year.  Amid the heady competition of the post-sputnik space race, the Soviet Union began equipping its satellites with photovoltaics as well.  Today, solar cells power virtually all satellites.

Achievements in photovoltaic cell research during the peak years of the space program included a major increase in efficiency and reduction in cost of more than 400 percent.  However, the space-related PV market leveled-off and photovoltaic cell production declined.

In 1973, America was jolted by its first oil crisis – gasoline prices soared and interest in alternative fuels was awakened.  Beginning in 1975, the U.S. government funded a steadily growing research and development program aimed at making photovoltaics economical for terrestrial use.  Japan and European countries followed suit.  Perhaps a dozen private companies entered the solar cell research or production business.

Since the 1970s, the overall market for photovoltaics has increased more than ten-fold.  In the same period, the cost of PV modules has dropped from about $50 per watt to $5 or $6 per watt.  Production has increased to more than 250 times the highest level during the peak years of the space program, and can barely keep up with demand.

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