Solar Cells

Types of Solar Cells

There are several ways of producing solar cells and photovoltaic modules.

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 advent of large scale photovoltaic production over the last few years (2010 to 2016) has led to development of solar grade silicon with a price tag of $12 to $20 per kilogram. his is expected to decline even further as some very large production plants are completed.

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  Then, over the course of many hours, the liquid silicon is cooled while the seed is slowly rotated and withdrawn.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 wafer-slicing techniques, and to improve the overall design of modules.

There are two or three major steps in producing solar cells from silicon wafers.  The photovoltaic effect is produced by forming a P-N junction <need link here> on the surface.  Some of the higher efficiency solar cells add a third layer of semiconductor, and then a system of contacts are need to provide electrical connections while allowing sunlight to pass through, generally called a grid.

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 15 to 22 percent of the sunlight that strikes them into electricity.  In the laboratory, module efficiencies of more than 30 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, and space between the solar cells.  Consequently, modules have 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 12 to 18 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 20 to 25 percent with efficiencies as high as 34 percent for a single cell.

Although they use less of the costly photovoltaic material, other elements increase their cost.  Concentrator cells use more expensive materials and processes since they are a small proportion of the system cost. Because of their lenses and mirrors, for example, concentrator cells must aim 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. Separate cells are not generally produced, the process forms photovoltaic modules with the solar cells interconnected.

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, non-crystalline 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 25 percent in the laboratory and theorist predict efficiencies as 35 to 40 percent.

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, but some are substantially larger.

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, 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 becoming common in Arizona, and the biggest reason is lower costs.  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, Arizona utility companies are already preparing for that possibility.  Since about 2015 have been changing their policies concerning solar, making the solar systems less effective in reducing costs. See our article on Arizona Electric Utility Information.

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 reached 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. Low cost, high quality inverters are now available that avoid this problem.

Central Power Plants

There are now some enormous fields of photovoltaic arrays 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 (now decommissioned. 

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 out of business) 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 early 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.

For many years (established in 1998) APS operated a Solar Test and Research Center to test the effectiveness of different photovoltaic equipment in the Arizona climate.  It featured many 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 provided 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. With the basic research completed, APS closed the facility.

Arizona’s also has a small photovoltaic central power plant that 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.

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.

Fundamentals of Photovoltaic Systems

The basic element of a photovoltaic system is the solar cell. Modern solar cells are made in sizes up to about 6 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. 

A single cell produces only about 0.45 volts. It takes 36 cells connected in series to generate the voltage required for charging a 12-volt battery. The power produced in full sunlight is a function of the size of the solar cell and the efficiency (always getting higher as the technology improves).  These cells are usually mounted (laminated with a clear plactic) behind a tempered glass sheet with the cells wired together to form a photovoltaic module (also called a” panel” or “flat-plate collector”). For instance, the most popular design in 2016 is to use 60 cells, each about 6" square, to produce 230 to 310 watts (depending on efficiency) from modules that are about 39" x 66".  The rear of the module is sealed with plastic or glass for protection, long life requires a moisture tight assembly. The modules connected together for one system form an “array.”  Large systems will have these photovoltaic modules connected physically and electrically in "sub-arrays". 

Single cells produce little power and are not often used individually.  They can be found, however, in some items such as small yard 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, but the tracking equipment costs more than a simple fixed mounting.

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.  It is even possible to have 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.  However, it is generally more cost effective operate larger photovoltaic systems inter-actively with the local electric utility.

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.

If a system uses batteries, charge controllers are also important components.  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, and last a shorter amount of time.

See also: The Fundamentals of Photovoltaic Systems

Photovoltaics (PV) - Introduction

Photovoltaics (PV) - Introduction

Photovoltaic (PV) cells convert sunlight directly into electricity.  PV cells are usually assembled into weather resistant modules to protect the cells and provide a means of mounting.

They have successfully powered space satellites for more than 60 years and now furnish electricity for a wide variety of applications on earth.

Tens of thousands of Arizona homes, schools, and businesses now have photovoltaic power systems that are connected to the utility grid, reducing the need for energy from other power sources, improving the environment.  Arizona electric utilities have embraced photovoltaic power and now have many square miles of large PV arrays.

PV-generated electricity is also used to power water pumps, weather monitoring stations, fire watchtowers, billboard lights, irrigation system, streetlights, boat battery chargers, and numerous other devices in Arizona and throughout the world.  Glistening photovoltaic panels can easily be seen at bus stops, atop many roadside emergency telephones, and as canopies in many parking lots where they provide power and shade.

Nevertheless, photovoltaic systems have established a record of reliability and have proven cost effective for many uses.  They produce no pollution in creating electricity and require no water to operate.  As environmental problems escalate and solar cells costs are reduced, these systems will almost certainly play an important role in our energy future.

For more information, follow the topics below.

• Photovoltaics Birth

• The Fundamentals of Photovoltaic Systems

• From Calculators to Power Plants: PV Systems in Action

• Solar Cells

• Electrical Generation - PV & Thermal Solar

• Sizing a photovoltaic system seems missing, needs a lot of work

• Glossary of Terms

• Economics of Photovoltaics Needs updating

• Economics of Residential Rooftop Solar Needs updating

• Price of Installed PV

Links about larger scale photovoltaic systems:

• Frequently asked questions about SRP's commercial solar electric program
• Parking lots provide solar power- SRP
• SRP Launches Major New Renewable Energy Project with Apple