Photovoltaics - Solar Electricity

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PHOTOVOLTAIC MARKETS AND APPLICATIONS Robert Foster Southwest Technology Development Institute New Mexico State University Solar energy is the energy force that sustains life on the earth for all plants, animals, and people. The earth receives this radiant energy from the sun in the form of electromagnetic waves, which the sun continually emits into space. The earth is essentially a huge solar energy collector receiving large quantities of this energy which manifests itself in various forms, such as direct sunlight used through photosynthesis by plants, heated air masses causing wind, and evaporation of the oceans resulting as rain which can form rivers. This solar energy can be tapped directly as solar energy (thermal and photovoltaics), and indirectly as wind, biomass, and hydroelectric energy. Solar energy is a renewable resource that is inexhaustible and is locally available. It is a clean energy source that allows for local energy independence. The sun's power flow reaching the earth is typically about 1,000 Watts per square meter (W/m2), although availability varies with location and time of year. Capturing solar energy typically requires equipment with a relatively high initial capital cost. However, over the lifetime of the solar equipment, these systems can prove to be cost-competitive, as compared to conventional energy technologies. The key to successful solar energy installation is to use quality components that have long lifetimes and require minimal maintenance. ELECTRICITY FROM SUNLIGHT (PHOTOVOLTAICS) Electricity can be produced from sunlight through direct heating of fluids to generate steam for large scale centralized electrical generation (solar thermal electrical generation). Electricity can alternatively be produced from sunlight through a process called photovoltaics (PV), which can be applied, in either a centralized or decentralized fashion. "Photo" refers to light and "voltaic" to voltage. The term describes a solid-state electronic cell that produces direct current electrical energy from the radiant energy of the sun, as represented in Figure 1. Solar cells are made of semi-conducting material, most commonly silicon, coated with special additives. When light strikes the cell, electrons are knocked loose from the silicon atoms and flow in a built-in circuit, producing electricity. PV Terminology Solar Cell: The PV cell is the component responsible for converting light to electricity. Some materials (e.g., silicon is the most common) produce a photovoltaic effect, where sunlight frees electrons striking the silicon material. The freed electrons cannot return to the positively charged sites ("holes") without flowing through an external circuit, thus generating current. Solar cells are designed to absorb as much light as possible and are interconnected in series and parallel electrical connections to produce desired voltages and currents. PV Module: A PV module is composed of interconnected solar cells that are encapsulated between a glass cover and weatherproof backing. The modules are typically framed in aluminum frames suitable for mounting. PV Array: PV modules are connected in series and parallel to form an array of modules, thus increasing total available power output to the needed voltage and current for a particular application. Peak Watt (Wp): PV modules are rated by their total power output, or peak Watts. A peak Watt is the amount of power output a PV module produces at Standard Test Conditions (STC) of a module operating temperature of 25°C in full noontime sunshine (irradiance) of 1,000 Watts per square meter. Keep in mind that modules often operate much hotter than 25°C in all but cold climates, reducing operating voltage and power by about 0.5% for every 1°C hotter, thus a 100W module operating at 45°C (20° hotter than STC yielding a 10% power drop) would actually produce about 90 Watts. A thin silicon cell, four inches across, can produce more than one watt of direct current (DC) electrical power in full sun. Individual solar cells can be connected in series and parallel to obtain desired voltages and currents. These groups of cells are packaged into standard modules that protect the cells from the environment while providing useful voltages and currents. PV modules are extremely reliable since they are solid state and there are no moving parts. Silicon PV cells manufactured today can provide over thirty years of useful service life. Some manufacturers provide warranties of up to 25 years on their PV product (at 80 percent of original power rating). A 50 Wp PV module in direct sunlight operating at 25°C will generate 50 Watts per hour (referred to as a Watt-hour­[Wh]). This same module will produce less power at higher temperatures; at 55°C this same module can only produce about 42.5 W. Modules can be connected together in series and/or parallel in an array to provide required voltages and currents for a particular application. PV systems are made up of a variety of components, which aside from the modules, may include conductors, fuses, disconnects, controls, batteries, trackers, and inverters. Components will vary somewhat depending on the application. PV systems are modular by nature, thus systems can be readily expanded and components easily repaired or replaced if needed. PV systems are cost effective for many remote power applications, as well as for small stand-alone power applications in proximity to the existing electric grid. PV is a relatively new and unknown technology, which offers a new vision for consumers and business as to how power can be provided. PV technology is already proving to be a force for social change in rural areas in less developed countries. The unique aspect of PV is that it is a "radical" or "disruptive" type of technology as compared to conventional power generation technologies. PV is a technology that does not build from the old technology base, but rather replaces that base from the bottom up. PV allows people the opportunity to ignore traditional electrical power supply structures and meet their own power needs locally. In rural regions of the world today, where there are no power companies offering electricity, PV is often the technology of choice. The best performing renewable energy electrification systems are those that meet the expectations of the users. It is important to satisfy the basic needs of the user in order to ensure acceptance of renewable energy systems. Ownership and subsequent accountability is the key to system sustainability for PV. One 50 Wp PV module is enough to power four or five small fluorescent bulbs, a radio, and a 15-inch black-and-white television set for up to 5 hours a day. Obviously this is only a modest amount of energy, however this represents an important quality of life improvement for many rural people without electricity. A small PV solar home system can cost as little as $550 to 650. This represents a significant monetary investment for a rural campesino in Mexico and several months' worth of income. The need for financing and innovative financing instruments that can assist rural people to obtain better services through the private marketplace is obvious. GLOBAL PHOTOVOLTAIC MARKETS 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 percent per year over the decade with village power driving demand. Table 2 shows the total worldwide PV production in 1980 was only 6.5 megawatts (MW) and by 1997 this had increased to 126.7 MW.   Table 2. Worldwide PV production                         1980    1986    1989    1991    1993      1996     1997 TOTAL (MW) 6.5          26.0         40.2       55.4        60.1          88.6        126.7 Source: O'Meara, 1998. U.S. module production is leading world growth as well. In 1993 the U.S. 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 percent leap in worldwide production. The U.S. produced 46.4 MW with $175 million in sales and exported 33.8 MW (73 percent of production) overseas (EIA, 1998). There are over 500,000 homes using PV today in villages around the world for electricity (Flavin and O'Meara, 1998b). In Kenya, more rural households receive electricity from PV than from the conventional power grid (Kozloff and Shobowale, 1994). The single largest market sector for PV is village power at about 45 percent 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. PHOTOVOLTAIC COSTS 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 - $15 per installed Watt. PV modules on the market today are guaranteed by manufacturer's 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 percent efficient, and research laboratory cells demonstrate efficiencies above 34 percent. 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. 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 horsepower in size. Well installed quality PV water pumping systems can provide over 20 years of reliable and continuous service. The reason users like to use PV water pumping systems can be seen in example survey results obtained from Mexico for the SNL water pumping program (22 end-users surveyed) provided in Figure 7 (Foster, 1998). The increased leisure time, less site visits, and considerably less operation and maintenance.   Figure 7. Perceived end-user socio-economic benefits from photovoltaic water pumping systems use for ranchers. 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 feet wide and weighing up to 250 pounds. 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 PV power can be used to electrify fences for livestock and animals. Commercially available packaged units have maintenance free 6 or 12 Volt 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 Volt range. Commercial units are UL 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 De-Icers 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 feet 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 owner-installed system, including a PV module, compressor, and mounting pole is about $450 (Foster, 1994). 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 volts 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.   A great number of PV installations for homes have taken place in Mexico. The experience of PV electrification varies widely across Mexico and is demonstrative of the potential pitfalls of haphazard installations. Over 40,000 PV home lighting systems have been installed in Mexico, mostly through government programs (Foster, 1998). However, nearly half of these systems are not functioning today, mostly due to poor balance of systems hardware (i.e., the PV modules work fine), where improper batteries and poor quality charge controllers are used. It is important for any PV user to use quality equipment and install PV systems in accordance with local electric codes. This greatly reduces the potential for future problems. 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 PV systems where maintenance and power accessibility makes 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 PV technologies today. Solar powered watches, calculators, and cameras are all everyday applications for PV technologies. Typically, these applications use amorphous PV technologies that work well even in artificial light environments such as offices and classrooms. Electric Utility Applications A potential future growth area for photovoltaics is the utility market, which currently amounts to less than 10% of U.S. PV installations. While PV today is not very competitive in grid-tied applications, a number of utilities across the U.S. are evaluating PV for off-grid and grid-tied applications. The White House and Department of Energy are promoting the "Million Solar Roofs Initiative," whose goal is to put solar energy on these many roofs (both solar thermal and PV systems) by the year 2010. However, major power policy reforms and tax incentives are needed if this goal is going to be effectively realized. Utilities have numerous uses for small power sources, often at remote sites that are difficult to power conventionally, such as lighting for transmission towers, water pumping, cathodic protection for structures, sectionalizing switches, monitoring systems and microwave communication links. While this utility market is not large these applications are cost-effective today. The Photovoltaics for Utility Scale Applications (PVUSA) project initiated in 1986 is a joint effort between PG&E, DOE and other utilities across the country including the City of Austin Electric Utility. This project focuses on evaluation of PV within the U.S. utility sector. The City of Austin Electric Department is a PVUSA host site with an installed a 20kW experimental array in 1992. The City of Austin Electric Utility has also built a 300kW PV array by Decker Lake, as well as Several U.S. utilities are investigating using PV systems as a 2.7kW PV system located on a youth hostel in Austin to evaluate PV as a future demand-side management tool. Another Texas utility, Central and Southwest Services, has also built a pioneering renewable energy demonstration park near Fort Davis. They have installed over 300 kW of different PV technologies connected to the electric grid. Both the Austin Utility and Central and Southwest are members of the Utility Photovoltaic Group (UPVG) which is comprised of numerous utilities and trade associations across the U.S. CONCLUSIONS The future is bright for continued PV technology dissemination in the Southwest U.S. and around the world. PV technology fills a significant need in supplying electricity, creating local jobs and promoting economic development in rural areas, while also having the positive benefits of avoiding the external environmental costs associated with traditional electrical generation technologies. People who choose to pursue a renewable and sustainable energy future now, are the ones showing the way for the future. REFERENCES EIA, Energy Information Agency, "Solar Energy," ftp://ftp.eia.doe.gov/pub/solar.renewables/solar.txt, U.S. Department of Energy, Washington, D.C., October, 1998. Flavin, Christopher and Molly O'Meara, Karl Böer (editor), "Financing Solar Home Systems in Developing Countries: Examples of New Market Strategies," Advances in Solar Energy, Volume 12, American Solar Energy Society, Boulder, Colorado, 1998a. Flavin, Christopher and Molly O'Meara, "Solar Power Markets Boom," World Watch, Vol. 11, No. 5, Washington, D. C., September/October, 1998b. Foster, Robert, Photovoltaic Market Development and Barriers in Mexico, MBA Thesis, Graduate School of Business, New Mexico State University, Las Cruces, New Mexico, December, 1998, 206 pp. Foster, R. E., "Photovoltaic Energy for Agriculture," Energy Conservation and Management Division; Energy, Minerals and Natural Resources Department, Santa Fe, New Mexico, June, 1994, 6 pp. Kozloff, Keith and Olatokumbo Shobowale, "Rethinking Development Assistance for Renewable Electricity," World Resources Institute, Washington, D.C., November, 1994. O'Meara, Molly, "Solar Cells Shipments Hit New High," Vital Signs, 1998, WorldWatch Institute, Washington, D.C., 1998. System Design - Installation and More Here! What's the correct angle for your PV modules? Here's the answer!

 

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