Converting with Rectifiers and Inverters
Direct current to alternating current and back again
Often lost amid the fanfare of improved efficiencies and design breakthroughs for alternative energy sources is the need for converting that power into usable form. This is especially true for renewable and solar-based power sources, which generate electricity in the form of direct current. Photovoltaic (PV) solar cells and fuel cells produce only direct current. Wind power is highly variable and needs leveling and balancing prior to use. These and other energy sources must be converted for use by appliances, lighting, etc., by means of an inverter. Recent advances in inverter design, manufacturing, and operation are the keys to further development and acceptance of alternative energy systems.
Direct Current and Alternating Current Basics
Direct current (DC) is a flow of electric charge through an electrical conducting wire that always flows in the same direction. DC is often used in low-voltage situations, such as currents generated by batteries, and in most electronic circuits. Solar power is another example of DC power since PV cells can only produce DC. Direct current is represented graphically over time by either a constant line—representing electrical energy generated by a battery or solar cell—or by a series of either completely positive or completely negative, half sine waves—representing alternating current that has been converted into DC by a rectifier. DC can be generated from AC (alternating current) by means of a rectifier.
AC is a flow of electric charge through an electrical conducting wire that repeatedly alternates the direction of its flow. AC is often used in high-voltage situations, such as power-transmission lines. Portable power generators are another example of alternating current. AC is represented graphically over time by a series of alternating positive and negative, full sine waves. AC is delivered in two forms: single-phase and three-phase. AC can be generated from DC by means of an inverter. This device is the primary problem in most renewable and solar energy applications—and the necessity of converting what is typically DC into AC for home or office use at the highest possible efficiency.
The most common and efficient means of delivering AC power to significant users (large buildings and neighborhoods, for example) is three-phase generation. Three-phase AC is generated quite simply by combining three separate coils in the generator’s stator. The coil windings are offset at 120 degrees around the circumference of the stator, with the three coils together covering all 360 degrees of the rotating stator. Together, the three coils produce three AC sine waves of equal loads that are 120 degrees out of sync with each other.
Not to be confused with three-phase AC distribution is the three-wire, single-phase AC system. This system branches out from the main three-phase delivery power line from a center-tapped transformer, resulting in two live wires. This distribution scheme allows for the delivery of single-phase AC to smaller customers (such as individual homes, small businesses, or construction sites where power tools are used). Only the two conducting wires and the neutral wire are connected to the property.
Converters: Diodes, Rectifiers, and Inverters
Converters transform AC into DC and vice versa. There are two types of converters—rectifiers and inverters. Rectifiers use diodes in various configurations to perform the conversion. The more complicated inverters rely on microprocessor circuits and transistors.
AC is converted into DC by means of a rectifier. A rectifier is an electrical device consisting of one or more diodes. Diodes are used to rectify AC by blocking the negative or positive portion of the alternating current’s sine wave form. Almost all rectifiers consist of a series of diodes for efficient conversion of AC to DC power. Rectifiers are rated as either half-wave or full-wave. Half-wave rectifiers consist of a single diode when converting single-phase AC.
DC is converted to AC by means of an inverter. The output waveform (voltage over time) varies with the quality and cost of the inverter from rectangular (poorest quality and least cost) or trapezoidal (better quality and more cost) to a true sine wave identical to that directly produced by an AC generator (best quality and most expensive). Inverters can be connected either in parallel for higher power or in series for higher voltage. The operating power of an inverter varies with voltage; typically a 100-W inverter will operate at 12–48 V.
Figure 1. The circuit diagram of a typical inverter with the source of DC shown at left.
Figure 2. A diagram of the operation of a simple IGBT; the pathways are for hole and electron flow.
Inverters are designed with microprocessor circuits and transistors. In the more advanced models reduced instruction set computer (RISC) microprocessors and insulated gate bipolar transistors (IGBT) are utilized (see Figure 1).
RISC microprocessors are based on simple design philosophy combined with simple instruction sets. IGBT transistors expand the high-current and low-voltage characteristics of standard bipolar transistors with the simple polysilicon gate of a standard metal-oxide semiconductor field-effect transistor (MOSFET). By combining these two sets of characteristics, an IGBT has fast switching capability without the danger of voltage breakdowns (see Figure 2.)
Most inverters generate single-phase AC. For very large systems, three-phase inverters are used. However, the use of three-phase inverters is limited since most DC generating systems that need to be converted to AC are small-scale, relatively low-power units designed for individual (single-dwelling or -building) use. Three-phase inverters find their widest use in the conversion for DC generated by PV cell arrays that generate electricity for sale and distribution back into the local power grid.
Inverters are rated by several parameters: power, voltage range, maximum current and voltage, and rated current and voltage. Each parameter is used to measure incoming DC and outgoing AC. The overall efficiency of each parameter is measured by the ratio of the AC component of the parameter to the DC component.
Corporate Researchers and Technological Advances
Some of the most exciting research is being done in the field of converting direct current generated by PV cells into alternating current fit for home or small-business use. Whenever a technical specification quotes a given efficiency for a solar cell’s conversion of sunlight into electricity, that is only half the story. This DC inevitably loses more efficiency as it is inverted into AC. It does a solar electrical generating system no good to have 90% efficient solar cells and only a 10% efficient DC-to-AC conversion. It’s whole system efficiency that matters.
Massachusetts-based Heliotronics Inc. designs, sells, markets, and promotes products that increase the usability of grid-connected PV energy systems. These include data-acquisition devices (hardware, software, power supplies, weather sensors, and transducers) to monitor energy production and concurrent weather conditions, grid-connection inverters, Web-enacted data monitoring for remote data collection, and transducers to measure current and voltage on either the AC or DC side of a PV energy system. Heliotronics’ research department is currently developing a grid-tied 2.4-kW inverter for the educational PV energy market (museums, schools, demonstrations, etc.). The performance data gained from this research were commissioned in 1999 under a contract with the New York State Energy Research and Development Authority.
California-based SatCon manufactures a line of utility-grade inverters for commercial-scale solar power applications (PowerGate model PV inverters). The units are grid-tied and line-interactive, enabling them to convert direct-current power from solar arrays to alternating-current power that is compatible with the utility voltage for export to the grid. SatCon manufactures commercial solar inverters ranging from 30 kW to 500 kW. The company’s financial incentives shorten the payback cycle for the solar-power installation. The California Energy Commission required all providers of PV inverters to resubmit their products for a standardized testing protocol based on a UL testing regime. This new testing protocol resulted in weighing DC-to-AC conversion efficiency at values that more closely reflect realistic inverter operating parameters rather than peak efficiency as measured in a controlled laboratory environment. The test results showed that SatCon’s PowerGate PV inverters had efficiencies on average 1% higher and as much as 2% higher than comparable products.
SMA Technologies AG of Germany has a distribution office in California that supplies the North American and South American renewable-energy markets with highly efficient, reliable inverters. SMA provides a wide variety of inverters that can be combined to create power ranges from 500 W up to more than 1 million W. SMA’s product line also includes central inverters for the large commercial or industrial solar plants, and a battery inverter for the off-grid and backup power markets. SMA’s inverter model series, the Sunny Boy (SB) is the company’s main inverter product line. At the high end of this series is the SB6000U, which is designed for use with PV cells, fuel cells, wind turbines, hydro-turbines, and micro-turbines. The SB6000U follows SMA’s modular design philosophy for utility, commercial, and residential PV installations of 6 kW and up. Automatic sensing of the site utility voltage makes installation on almost any utility system trouble-free. The wide DC input voltage range allows connection to almost any type or model of PV module. SMA offers a variety of hardware and software solutions from low-cost wireless system monitoring to complex data-acquisition systems that integrate a large number of inverters with external sensors to networked PCs and the Internet.
Xantrex Technology Inc. develops, manufactures, and markets products and systems for the alternative energy markets. These include decentralized and mobile power sources as well as programmable power systems. Xantrex specializes in converting any power source (central grid, distributed alternative-energy source, backup generators, etc.) into power for use by electronic equipment. The company’s main product line is a series of solar inverters (Xantrex GT 3.8) and grid-tie solar inverters (GT 2.5 and GT 3.8) for residential and commercial markets. These grid-tie inverters are based on the basic 3-kW Xantrex GT 3.0 Grid Tie Solar Inverter platform. Xantrex performs verification testing to ensure long life and reliability. The inverters are tested using highly accelerated life test (HALT) methods, which expose the inverters to extreme thermal and mechanical conditions. HALT involves the repeated application of vibration and thermal stresses to determine potential failure modes.
Sputnik Engineering manufactures its SolarMax line of PV energy inverters. The SolarMax line includes both single-phase and central PV inverters. SolarMax’s single-phase inverters combine high efficiency (97%) and low weight (12 kg), and are manufactured with a weatherproof aluminum casing that allows for either outdoor or indoor installation. The company’s central inverters are three-phase and range in power from 20 kW to 300 kW, suitable for medium to large electrical power plants.
OutBack Power Systems of Washington state is an engineer-owned manufacturer of renewable energy and backup power system equipment. Outback manufactures inverter/chargers that serve as DC to AC sine-wave inverters, battery chargers, and AC transfer switches. The products produce true sine-wave AC energy with a high-surge power capability. The AC transfer switch reacts in less than 16 milliseconds. Flexibility is provided with the ability to be connected at any time in either parallel, series, or three-phase power configurations.
The Ingeteam Group comprises 26 companies. Ingeteam SA possesses a wide range of products and services suited to wind energy, solar PV, hydroelectric, and biomass requirements. Ingecon Sun inverters are single-phase inverters for PV plants connected to electrical mains. Sun inverters are designed for connection to the electrical mains of PV plants. This equipment has rated power levels of 2.5 kW, 3.3 kW, and 5 kW. The Sun TL is used for PV plants connected to electrical mains without transformers. The Ingecon Hybrid inverter is used for isolated areas without access to main power grids.
Government Research Options and Trade-Offs
The US Department of Energy (DOE) is the single largest inverter researcher in the world. Its Solar Energy Technologies Program emphasizes a systems-driven approach to inverter research and development. The culmination of this effort is its “High-Tech Inverter R&D 5-Year Strategies” document. The stated goal of this program is to achieve marketable PV energy at a cost of $0.06 per kilowatt-hour. Of key importance is the role inverters will play in reducing the overall costs of PV energy through cost improvements in inverter manufacturing and efficiency improvements in inverter performance. Reaching this goal is the objective of the DOE’s five-year plan. The plan seeks improvements in seven key technologies: semiconductors, magnetic materials, control algorithms, automated manufacturing, thermal management, energy-storing capacitors, and surge-protection devices.
New and advanced semiconductor devices are needed. There are three areas of ongoing research. First is the integration of silicon-carbon diodes and wide-band switching devices. Second are smart (and smarter) switching devices. Lastly are higher-temperature printed wiring boards.
Research in magnetic materials focuses on the development of higher-performing, lower-cost materials for use in inductors and transformers. One promising area is nano-crystal magnetic materials.
Control algorithms are to be improved to allow feed-forward controls and to reduce the need for energy storage. This can be accomplished by reducing the number of parts (resulting in higher reliability) and “plug and play” network support. Reduction and integration of various functions will achieve in effect an inverter on a chip.
Improvements in automated manufacturing of inverters will require process standardization to allow for standard inverter packages to be used in multiple applications.
Advanced thermal management technologies seek to incorporate advanced coatings and insulated metal substrates to eliminate the need for cooling fans and heat sinks.
Advanced, long-life capacitors based on high-temperature metal film, multilayer ceramic capacitors, and thick-film capacitors will provide superior energy-storage capacity. Research is also being done on a wear-out mechanism made from aluminum.
Surge-protection devices serve to reduce “wear and tear” on the inverters’ other electronic components. Though not devices in and of themselves, advanced circuitry layout design can greatly reduce surge potential. The goal is a rugged, long-lasting, and—most importantly—maintenance-free surge protector.
The DOE’s effort isn’t aimed at hitting a technological “home run.” The aim rather is a series of “base hits”—and if we are lucky, the occasional double or triple—that incrementally improve the performance and reduce the cost of the overall inverter system. There may not be an equivalent of “Moore’s Law” for power systems, but there doesn’t need to be a doubling of inverter performance efficiencies every year to make DC-generating alternative-energy sources cost-competitive.
So where is all this research and development taking us? What kind of market will there be for these improved and more efficient inverters? The inverter market remains strong and is likely to get stronger as alternative energy sources become more common. The market itself is segmented according to the end user (energy conversion, telecommunications, etc.). Reliance on only one market segment can make an inverter manufacturer extremely vulnerable to market fluctuations. Inverter vendors can avoid this by diversifying which allows them to take advantage of opportunities in other market segments while counteracting downturns or sluggish growth in other segments. However, for the near future at least, strong growth is expected in all market segments.
Inverters represent 20% of the value of commercial-scale PV systems. They represent a slightly lower value percentage of small-scale and decentralized solar energy and fuel cell power conversion. The commercial scale segment of the US solar energy market is projected to be $45 million in 2005 with a 30% growth rate for the foreseeable future. Today’s commercial-scale solar inverter market of $9 million in annual sales can therefore expect to grow to more than $33 million by 2010.
Converting DC from alternative energy sources to AC for home use isn’t the only growth market for inverters. Cars, trucks, and boats will continue to be equipped with auxiliary and integral infotainment electronic applications. Expansion in the special-vehicle segments in North America and Europe provides a strong impetus for the growth of the inverter markets. Continuous technology development has also fed increasing demand in the form of replacements and upgrades.
However, the renewable energy–based market is likely to remain the biggest consumer of inverters. Though driven by increased demand for reliable, decentralized energy sources, tax incentives and subsidies to the solar energy market are expected to increase the demand for the inverters used in these systems. Great assistance is given to the inverter market by tax incentives promoting the use of alternative energy sources. Governments are driven by the need to find alternatives to an increasingly volatile fossil fuel market and the issue of environmental protection. The State of California leads the nation in these incentives. Financial incentives for qualifying commercial PV power systems in California are calculated for 2005 at $3.50 per watt. For a 100-kW system, each point of higher efficiency would yield an additional $3,500 to the system purchaser. Renewable Portfolio Standards goals, mandated by states such as California, New Jersey, and New York determine the levels of these subsidies.
Energy conversion isn’t the only growth market for inverters. Inverters allow for variable speed controls and greater energy efficiency in home appliances. The number of major home appliances (mostly refrigeration appliances, room air conditioners, and washing machines) using inverters is projected to increase by 400% by 2008, from 25 million units in 2003 to nearly 100 million units in 2008. Though Japan is the current world leader in this field of inverter application, strong growth is expected for all major markets (Western Europe, North America, and China).
Ever-increasing demands for energy and electrical power will require the development of a wide variety of energy sources, not just the standard mix of fossil fuels, nuclear, and hydroelectric. Renewables will provide a diversified energy base while increasing the amount of energy available for future needs. Inverters are integral to the development and utilization of alternative energy sources; their utilization will increase with the wider acceptance of non-traditional energy systems.
Author's Bio: Daniel P. Duffy, PE, writes frequently on the topics of landfills and the environment.