Solar electric or PV systems convert some of the energy in sunlight directly into electricity. PV cells are made primarily of silicon, the second most abundant element in the earth’s crust and the same semiconductor material used for computers. When the silicon is combined with one or more other materials, it exhibits unique electrical properties in the presence of sunlight. Electrons are excited by the light and move through the silicon. This is known as the photovoltaic effect and results in dc electricity. PV modules have no moving parts, are virtually maintenance-free, and have a working life of 20 to 30 years.
There are three basic categories of PV systems with several types in each category. Crystalline silicon flat-plate collectors are the most developed and prevalent type in use today. These include single-crystal silicon and polycrystalline silicon, which are either grown or cast from molten silicon and later sliced into their cell size. They are then assembled onto a flat surface; no lenses are used. Thin-film systems are inherently cheaper to produce than crystalline silicon but are not as efficient.
They are produced by depositing a thin layer of PV material to a substrate such as glass or metal. This group includes amorphous silicon, like the kind found in calculators and watches. Concentrators use much less of a specialized PV material and employ a lens or reflectors to concentrate sunlight on the PV cell and increase its output. They can be produced more cheaply than either of the other type due to the reduced amount of expensive PV material. However, they can use only direct sun, so they must track the sun precisely and do not work when it is cloudy.
Photovoltaic System Terms
PV system terms progress from small to large as follows:
- PV cells, the smallest unit of a PV system, are wired together to form modules.
- Modules are usually a sealed or encapsulated unit of convenient size for handling.
- Modules are wired together to form panels.
- Groups of panels form arrays.
- A number of arrays form an array field.
- The total system includes the arrays and any other equipment, such as charge controllers, storage (batteries), tracking, and monitoring equipment, collectively called balance of system (BOS) components.
History of Photovoltaics
The history of photovoltaics dates back to 1839, and major developments evolved as follows:
- In 1839, Edmund Becquerel, a French physicist, observed the photovoltaic effect.
- In the 1880s, selenium PV cells were built that converted light in the visible spectrum into electricity and were 1% to 2% efficient. Light sensors for cameras are still made from selenium today.
- In the early 1950s, the Czochralski meter was developed for producing highly pure crystalline silicon.
- In 1954, Bell Telephone Laboratories produced a silicon PV cell with a 4 % efficiency and later achieved 11% efficiency.
- In 1958, the U.S. Vanguard space satellite used a small (less than 1-W) array to power its radio.
- The space program has played an important role in the development of photovoltaic ever since.
- During the 1973 to 1974 oil embargo, the U.S. Department of Energy funded the Federal Photovoltaic Utilization Program, resulting in the installation and testing of over 3,100 PV systems, many of which are still in operation today.
- The 1970s through the 1990s have seen a relative disinterest in solar power, with majority ownership of many U.S. PV manufacturers being transferred to German and Japanese interests.
- The Gulf War of 1990 again sparked America’s interest in non-fossil fuel energy alternatives.
The PV Power Market
PV systems traditionally have been economical in remote applications. Most common examples include wireless and cellular communications systems, off-grid homes, recreational vehicles and boats, power for offshore oil rigs, and highway sign lighting and call boxes. Water pumping, vaccine refrigeration, and water purification have all been important roles for photovoltaics in developing countries.
Market forces seem to have a hold of the PV market, since sales in 1995 rose 20%. Most U.S. manufacturers are increasing production significantly, and costs are expected to fall with the new volumes.
Current estimates of worldwide production of solar photovoltaic cells and modules for 1998 are about 120 MW, up steadily and dramatically from only 40 MW in 1990. Worldwide sales have been increasing at an average rate of about 15% every year during the last decade, although that growth rate has been slower in some markets and regions but faster in others. We believe that there is a realistic possibility for the market to continue to grow at about a 15% rate into the next decade. At this rate, the world production capacity would be 1,000 MW by 2010, and photovoltaics could be a $5 billion industry. These are realistic benchmarks, and show the solar business to be a very exciting market opportunity in the near term.
Developing countries today are the largest and fastest growing segment of the PV market. For the 2,000 million people in the developing world who currently have no access to basic electrical services, PV presents the opportunity for a giant leap forward and a much needed improvement in living standards. For the PV services industry, the developing world represents an enormous new business opportunity.
Photovoltaic prices are continuing a downward trend as manufacturers increase production. Cost decreases, combined with national and state incentive and subsidy programs, and renewable energy capacity mandates, have led to the emergence of a steadily growing market for bulk photovoltaic installations (greater than 100 kW capacity). Merchant PV plants are being built in places where
favorable feed-in tariffs make projects profitable. Bulk installations significantly decrease the installed cost of PV power at individual sites, while simultaneously driving market demand for PV equipment. The major players and PV technologies in each market are identified along with the mounting modes (roofing tiles, weather skins, carport shading, window walls, and so on) that are prominent. The potential impact of mass production of promising emerging PV technologies is examined. Market forecasts are provided for capacity, new projects, and annual revenue for the 2002 to 2008 time frame. The forecasts cover national, regional, and world markets for both single-site bulk PV installations and bulk purchases for national village power supplies and large residential projects. Project revenue is growing at over 40% per year, and will likely reach $1 billion annually by 2010.
International Activity : U.S. PV exports increased in capacity from 14.814 million ft2 in 1993 to 17.714 million ft2 in 1994, an increase of 19.6%. U.S. PV imports increased from 1.767 million ft2 in 1993 to 1.98 million ft2 in 1994, an increase of 12%. U.S. module production is leading world growth as well. In 1993, the United States produced 21 MW of PV, of which 14.8 MW was exported. By 1997, global demand led to a record breaking PV production year with a 42% leap in worldwide production.
The United States produced 46.4 MW with $175 million in sales and exported 33.8 MW (73% of production) overseas. India is boosting production and becoming a major world producer of PV modules. The Indian government plans to power 100,000 villages with renewable energy, primarily PV modules, and install solar-powered telephones in each of the nation’s 500,000 villages. Mexico planed to electrify 60,000 villages using photovoltaics by the year 2000. Hospital Bulape (serving 50,000 outpatients per year in Zaire) and several other major hospitals in Zaire depend totally on solar power for everything from x-ray equipment to air conditioning. In Morocco, solar panels are sold in bazaars and open markets, next to carpets and tinware.
In San Buenaventura, Guatemala, a local utility has installed PV panels
on 42 of the community’s 86 homes at one-third the cost of extending power lines into the village. Malta-Solar Power, Ltd. has begun construction of a new PV plant with a maximum production capacity of 3 MW per year. At full production, this plant will be able to produce 40% of all 1995’s module production capacity for all developing countries.
South African companies are building a PV manufacturing plant near Alexandra township that will serve to electrify 10,000 homes, 600 clinics, and 1,000 schools with solar power. Kenya has electrified 20,000 homes using photovoltaics in the last few years, compared with 17,000 new homes that were hooked up to the central power grid. Siemens Solar in 1995 sold just over 40% of its output in North and South America, nearly 40% to Europe and Africa, and “just under” 20% went to Asia.
5SI President Gernot Oswald expects the biggest growth in the next few years to come from Asia. There are over 500,000 homes using PV today in villages around the world for electricity. In Kenya, more rural households receive electricity from PV than from the conventional power grid. The single largest market sector for PV is village power at about 45% of worldwide sales. This is mostly comprised of small home lighting systems and water pumping. Remote industrial applications such as communications are the second largest market segment.
Global PV Market
The fast growing world market for PV greatly reflects the growing rural electrification demand of less developed countries around the world. The global PV market has grown at an average rate of 16% per year over the decade with village power driving demand. The total worldwide PV production in 1980 was only 6.5 MW, and by 1997 this had increased to 126.7 MW.
For many applications, especially remote site and small power applications, PV power is the most cost-effective option available, not to mention its environmental benefits. New PV modules generally retail for about $5 per peak watt, depending on quantities purchased. Batteries, inverters, and other balance of system components can raise the overall price of a PV system to over $10 to $15 per installed watt. PV modules on the market today are guaranteed by manufacturers from 10 to 20 years, while many of these should provide over 30 years of useful life.
It is important when designing PV systems to be realistic and flexible, and not to overdesign the system or overestimate energy requirements (e.g., overestimating water-pumping requirements) so as not to have to spend more money than needed. PV conversion efficiencies and manufacturing processes will continue to improve, causing prices to gradually decrease.
PV conversion efficiencies have increased with commercially available modules that are from 12% to 17% efficient, and research laboratory cells demonstrate efficiencies above 34%. A well designed PV system will operate unattended and requires minimum periodic maintenance, which can result in significant labor savings. PV modules on the market today are guaranteed by the manufacturer from 10 to 25 years and should last well over 30 years. PV conversion efficiencies and manufacturing processes will continue to improve, causing prices to gradually decrease; however, no dramatic overnight price breakthroughs are expected.
Common Photovoltaic Applications
PV is best suited for remote site applications that have small to moderate power requirements, or small power consuming applications even where the grid is in existence. A few power companies are also promoting limited grid-connected PV systems, but the large market for this technology is for stand-alone (off-grid) applications. Some common PV applications are as follows:
Water Pumping : Pumping water is one of the most competitive arenas for PV power since it is simple, reliable, and requires almost no maintenance. Agricultural watering needs are usually greatest during sunnier periods when more water can be pumped with a solar system. PV-powered pumping systems are excellent for small to medium scale pumping needs (e.g., livestock tanks) and rarely exceed applications requiring more than a 2 hp motor. There are thousands of agricultural PV water pumping systems in the field today throughout Texas. PV pumping systems’ main advantages are that no fuel is required and little maintenance is needed.
A PV-powered water pumping system is similar to any other pumping system, only the power source is solar energy; PV pumping systems have, as a minimum, a PV array, a motor, and a pump. PV water pumping arrays are fixed mounted or sometimes placed on passive trackers (which use no motors) to increase pumping time and volume. AC and dc motors with centrifugal or displacement pumps are used with PV pumping systems.
The most inexpensive PV pumpers cost less than $1,500, while the large systems can run over $20,000. Most PV water pumpers rarely exceed 2 hp in size. Well installed quality PV water pumping systems can provide over 20 years of reliable and continuous service.
Gate Openers : Commercially available PV-powered electric gate openers use wireless remote controls that start a motorized actuator that releases a gate latch, opens the gate, and closes the gate behind the vehicle. Gates are designed to stop if resistance is met as a safety mechanism. Units are available that can be used on gates up to 16 ft wide and weighing up to 250 lb. Batteries are charged by small PV modules of only a few watts. Digital keypads are available to allow access with an entry code for persons without a transmitter. Solar-powered gate-opening assemblies with a PV module and transmitter sell for about $700.
Electric Fences : P-power can be used to electrify fences for livestock and animals. Commercially available packaged units have maintenance free 6 or 12-V sealed gel cell batteries (never need to add water) for day and night operation. These units deliver safe (non-burning) power spikes (shocks) typically in the 8,000 to 12,000 V range. Commercial units are UL (Underwriters Laboratories) rated and can effectively electrify about 25 to 30 miles of fencing. Commercially packaged units are available from about $150 to $300, depending on voltage and other features.
Water Tank Deicers : For the north plains of Texas in the winter, PV power can be used to melt ice for livestock tanks, which frees a rancher from going out to the tank with an ax to break the surface ice so the cows can drink the water. The PV module provides power to a small compressor on the tank bottom that generates air bubbles underwater, which rise to the surface of the tank. This movement of the water with the air bubbles melts the tank’s surface ice. Commercially available units are recommended for tanks 10 ft in diameter or greater, and can also be used with ponds. Performance is best for tanks that are sheltered, bermed, or insulated. Installation is not recommended for small, unsheltered tanks in extremely cold and windy sites. Approximate cost for a complete ownerinstalled system, including a PV module, compressor, and mounting pole, is about $450.
Commercial Lighting : PV-powered lighting systems are reliable and a low-cost alternative widely used throughout the United States. Security, billboard sign, area, and outdoor lighting are all viable applications for PV. It’s often cheaper to put in a PV lighting system as opposed to installing a grid lighting system that requires a new transformer, trenching across parking lots, etc. Most stand alone PV lighting systems operate at 12 or 24 V dc. Efficient fluorescent or sodium lamps are recommended for their high efficiency of lumens per watt. Batteries are required for PV lighting systems.
Deep cycle batteries specifically designed for PV applications should be used for energy storage for lighting systems. Batteries should be located in protective enclosures, and manufacturer’s installation and maintenance instructions should be followed. Batteries should be regulated with a quality charge controller. Lighting system prices vary depending on the size; average systems cost from $600 to $1,500.
Residential Power : Over 500,000 homes worldwide use PV power as their only source of electricity. In Texas, a residence located more than a mile from the electric grid can install a PV system more inexpensively than extending the electric grid. A Texas residence opting to go solar requires about a 2 kW PV array to meet its energy needs, at a cost of about $15,000. The first rule with PV is always energy efficiency. A PV system can provide enough power for an energy-efficient refrigerator, lights, television, stereo, and other common household appliances.
Evaporative Cooling : PV-powered packaged evaporative cooling units are commercially available and take advantage of the natural relation that when maximum cooling is required is when maximum solar energy is available. These units are most appropriate for comfort cooling in the dry climate of West Texas where performance is best. Direct evaporative coolers save 70% of the energy over refrigerated units.
Battery storage is obviously required if cooler operation is desired at night. Array size would vary with the power requirements of the cooler motor. A linear current booster (LCB) is useful between the PV modules and the cooler’s dc motor if the cooler is coupled directly to the PV array. Packaged PV evaporative cooling systems for residences generally run from $500 to $1,500, depending on size.
Telecommunications : This was one of the early important markets for PV technologies, and continues to be an important market. Isolated mountaintops and other rural areas are ideal for stand-alone Photovoltaic systems where maintenance and power accessibility make PV the ideal technology.
These are often large systems, sometimes placed in hybrid applications with propane or other type of generators.
Consumer Electronics : Consumer electronics that have low power requirements are one of the most common uses for Photovoltaic technologies today. Solar-powered watches, calculators, and cameras are all everyday applications for PV technologies. Typically, these applications use amorphous Photovoltaic technologies that work well even in artificial light environments such as offices and classrooms.
Glossary of Solar and Photovoltaic Terms
Cell Efficiency : The ratio of the electrical energy produced by a PV cell (under full sun conditions or 1 kW/m²) to the energy from sunlight falling on the cell.
Charge Controller : A component that controls the flow of current to and from the battery subsystem to protect the batteries from overcharge and overdischarge. The charge controller also may monitor system performance and provide system protection.
Diffuse Radiation : Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other substances in the atmosphere.
Direct Radiation : Light that has traveled in a straight path from the sun (also referred to as beam radiation). An object in the path of direct radiation casts a shadow on a clear day.
Flat-Plate Array : A photovoltaic array in which the incident solar radiation strikes a flat surface and no concentration of sunlight is involved.
Fresnel Lens : A concentrating lens positioned above and concave to a Photovoltaic material to concentrate light on the material.
Grid-Connected : Referring to an energy-producing system connected to the utility transmission grid (also called utility interactive).
Hybrid System : A power system consisting of two or more power-generating subsystems (the combination of a wind turbine and a PV system).
Insolation : The amount of sunlight reaching an area usually expressed in watts per day.
Load : Electrical power being consumed at any given moment. The load that an electrical generating system supplies varies greatly with time of day and to some extend season of year. Also in an electrical circuit, the load is any device or appliance that uses power.
Parallel-Connected : Referring to a method of connection in which positive terminals are connected together and negative terminals are connected together. Current output adds and voltage remains the same. (See also series connected.)
Photovoltaic Cell : The semiconductor device that converts light into dc electricity. The building block of PV modules.
Series-Connected : Referring to a method of connection in which the positive terminal of one device is connected to the negative terminal of another. The voltages add and the current is limited to the least of any device in the string. (See also parallel-connected.)
Solar Constant : The rate at which energy is received from the sun just outside the earth’s atmosphere on a surface perpendicular to the sun’s rays. Approximately equal to 1.36 kW/m².
Thick Cells : Conventional cells, such as crystalline silicon cells, which are typically from 4 to 17 mil thick. In contrast thin-film cells are several micrometers thick.
Thin-Film Cells : Photovoltaic cells made from a number of layers of photosensitive materials. These layers are typically applied using a chemical vapor deposition process in the presence of an electric field.
Voltage Regulator : A device that controls the operating voltage of a PV array.