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What we'll be looking at in this article:
Wind-electric systems fit into three categories: (1) grid-connected, (2) grid-connected with battery backup, and (3) off-grid. In this section, we'll examine each system and discuss the pros and cons of each. We'll also examine hybrid systems, consisting of a wind turbine plus another form of renewable energy. This information will help you decide which system suits your needs and lifestyle. To begin, let's take a look at two of the main components of wind systems, wind turbines and towers. Subsequent sections contain more detailed discussions of these and other components. Wind Turbines Most wind turbines in use today are horizontal axis units, or HAWTs, (explained shortly) with three blades attached to a central hub. Together, the blades and the hub form the rotor. In many wind turbines, the rotor is connected to a shaft that runs horizontal to the ground, hence the name. It is connected to an electrical generator. When the winds blow, the rotor turns and the generator produces alternating current (AC) electricity. (See the accompanying article for an explanation of AC electricity.) One of the key components of a successful wind generator is the blades. They capture the wind's kinetic energy and convert it into mechanical energy (rotation). It is then converted into electrical energy by the generator. == AC vs. DC Electricity Electricity comes in two basic forms: direct current and alternating current. Direct current (DC) electricity consists of electrons that flow in one direction through the electrical circuit. DC electricity is the kind produced by flashlight batteries or the batteries in cell phones, laptop computers, or portable devices such as iPods. Most wind turbines produce alternating current electricity. In alternating current, the electrons flow back and forth. That is, they switch, or alternate, direction in very rapid succession, hence the name. Each change in the direction of flow (from left to right and back again) is called a cycle. In North America, electric utilities produce electricity that cycles back and forth 60 times per second. It's referred to as 60-cycle-per-second - or 60 hertz (Hz) - AC. The hertz unit commemorates Heinrich Hertz, the German physicist whose research on electromagnetic radiation served as a foundation for radio, television and wireless transmission. In Europe and Asia, the utilities produce 50-cycle-per-second AC. AC electricity is also produced by electrical generators in hydroelectric and power plants that run on fossil fuels or nuclear fuels. No matter what form of energy is used to turn a generator, all of them operate on the principle of magnetic induction - they move coils of copper wire through a magnetic field (or vice versa). This causes electrons to flow through the coils, producing electricity. == The generators of wind turbines are often protected from the elements by a durable housing made from fiberglass or aluminum ( Ill. 1a). However, in many modern small wind turbines, the generators are exposed to the elements ( Ill. 1b). Most wind turbines in use today have tails that keep them pointed into the wind to ensure maximum production. However, some very successful turbines like those made by the Scottish company Proven (pronounced PRO-vin) are designed to orient themselves to the wind without tails. Ill. 1a and 1b: Wind Turbine Design. (a) The generators in many small wind turbines are housed in a protective case made from aluminum or fiber glass. (b) Others, like this one, are not. Towers Another vital component of all wind systems is the tower, discussed in more detail in Section 6. Residential wind generator towers come in three varieties: (1) freestanding, (2) fixed guyed, and (3) tilt-up (Ill. 2). Freestanding towers may be either monopoles or lattice structures. Freestanding monopole towers consist of high-strength hollow tubular steel like that used for streetlight poles. Lattice towers consist of tubular steel pipe or flat-metal steel bolted or welded together to form a lattice structure like the Eiffel Tower or trans mission towers used for high-voltage transmission lines. Freestanding towers must be strong enough to support the weight of the wind turbine, but more importantly, they need to be strong enough to withstand the forces of the wind acting on the turbine and the tower itself. Towers must have a large foundation to counteract the tremendous forces applied by the wind, forces that could easily topple a weak or poorly anchored tower. Large amounts of steel and concrete are required to accomplish this task. This makes freestanding towers the most expensive tower option. The second tower type is the fixed guyed tower ( Ill. 2b). Guyed towers may be made of high-strength steel pipe or may be lattice structures supported by high-strength steel cables, known as guy cables or guy wires. Guy cables extend from anchors in the ground to tower attachment points. They stiffen towers so they don't buckle and hold them in place so they don't lean or fall over. Because guy cables strengthen and stabilize the tower, less steel is required for them than in freestanding towers. Guy cables are attached to the tower every 20 to 30 feet, although wider spacing is also possible. A 120-foot-tall tower may require four sets of cables. The third type of tower is the tilt-up tower, so named because it can be raised and lowered - tilted up and down. This makes it possible to lower a turbine so it can be inspected, maintained and repaired at ground level. Tilt-up towers typically consist of high-strength steel pipe or a lattice structure. Both are supported by guy cables. Ill. 2: Tower Types. Three types of towers in use today: (a) freestanding, (b) fixed guyed, and (c) tilt-up. Towers are as important as the wind turbine itself. Unfortunately, many wind turbines are mounted on towers that are much too short. This occurs out of ignorance and misguided frugality. Installing a turbine on a short tower is a common and costly mistake. Properly sized towers place wind turbines in the path of more powerful winds and raise turbines above turbulence created by ground clutter, which severely diminishes the quality and quantity of wind. Don't let anyone talk you into a short tower! Electricity produced by a wind turbine runs down wires attached to the tower. The wires typically run externally - for example, alongside a tower leg of a lattice tower - to a junction box at the base of the tower. From there, they typically run underground in conduit to the point of use. As you shall soon see, wind-electric systems involve a number of additional components. We'll describe them as we explore the three main types of wind energy system. Wind Energy System Options As noted in the introduction, wind systems fall into three main categories: (1) grid-connected, (2) grid-connected with batteries, and (3) off-grid. We'll begin with the simplest, the grid-connected system. Grid-Connected Systems Grid-connected systems are so named because they connect directly to the electrical grid. They are also referred to as battery less grid-connected or batteryless utility-tied systems because they do not employ batteries to store surplus electricity [ Because the term "grid" refers to the high-voltage transmission system, not local electrical distribution systems, customers are connected to the grid through their local utility and indirectly to its distribution system, the grid. Because of this, the terms "grid-connected" and "grid-tied," which are used by the renewable energy community, are not entirely accurate. "Utility-connected" or "utility tied" would be better terms, but that is not the common usage in the renewable energy world.]. In batteryless grid-tie systems, the electrical grid accepts surplus electricity - electricity produced by the turbine in excess of demand. When a wind system is inactive, the grid supplies electricity to the home or business. The grid therefore serves as the storage medium. Ill. 3: Grid-Connected Wind System. The grid-connected wind system is the simplest of all systems. Wild AC electricity produced by the turbine is first fed into the controller. The inverter produces grid-compatible AC electricity to power household loads. Surpluses are back-fed onto the electrical grid. 1. Electric wire from turbine carries wild AC electricity to house 2. DC control panel 3. DC disconnect 4. Inverter 5. AC disconnect 6. Breaker box 7. Electrical outlets 8. Electrical meter 9. Electric wire carries AC electricity to and from house As shown in Ill. 3, a batteryless grid-connected system consists of six main components: (a) a wind generator specifically designed for grid connection, (b) a tower, (c) an inverter/power conditioner, (d) a main service panel (e) meters, and (f ) safety disconnects. In most batteryless grid-connected systems, the wind generator produces "wild AC" electricity. Wild AC is alternating current electricity the frequency and voltage of which vary with wind speed. Frequency is the number of times electrons switch direction every second and , as noted earlier, is measured in hertz, or cycles per second. The flow of electrons through an electrical wire is created by an electromotive force that scientists call voltage. Voltage is electrical pressure, the driving force that causes electrons to move. The unit of measurement for voltage is volts. In most small wind turbines, the faster the blades spin, the higher the voltage and the greater the frequency. Wild AC, produced by wind turbines, is not directly usable. Appliances and electronic devices require a tamer version of electricity - alternating current with a fairly constant frequency and voltage, like that available from the grid. In a grid-connected system, the wild voltage must first be "tamed." That is, its frequency and voltage must be converted to standard values. This occurs in two additional components, the controller and inverter (Ill. 3). The inverter converts the electricity to grid-compatible AC - 60 cycle per second, 120-volt (or 240-volt) electricity. Because the inverter produces electricity in sync with the grid, it's often referred to as a synchronous inverter. While grid-compatible wind generators typically produce wild AC, another type of wind generator is also found in the small wind market. It is an induction generator. As explained more fully in Section 5, an induction generator produces grid-compatible AC electricity without a controller or inverter. The 120-volt or 240-volt AC produced by the inverter (or directly by an induction generator) flows to the breaker box, which is where the circuit breakers are found. From here, the electricity flows along wires in a house or business to electrical devices drawing power. If the wind machine is producing more electricity than is needed, the excess is fed onto the grid. Surplus electricity backfed onto the grid travels from the main service panel through the utility's electric meter, typically mounted on the outside of the building. It then flows through the wires that connect to the grid. The surplus electricity then travels along the power lines where it flows into neighboring homes or businesses. A utility electric meter monitors the electricity fed onto the grid so the utility can credit the producer for its contribution. The meter also keeps track of electricity the power company supplies to homes and businesses when their wind systems are not generating. To learn how the electric company measures what you are putting onto the grid and how they "pay" for it, check out the accompanying box, "Net Metering in Grid-Connected Systems." In addition to the electric meter - or meters (some utilities require two or more meters) - that monitor the flow of electricity onto and off the local utility grid, grid-connected wind energy systems often contain safety disconnects. These are manually operated switches that enable service personnel to disconnect at a couple of key points in the system to prevent electrical shock if service is required. As shown in Ill. 3, an AC disconnect is located between the inverter and breaker box. When switched off, it disconnects and isolates the wind energy system from the household circuits and the grid. The AC disconnect must be mounted out side so that it is accessible to utility company personnel so they can isolate the wind system from the grid when working on the electric lines in your area without fear of shock - for instance, if a line goes down in an ice storm. The AC disconnect must also contain a switch that can be locked in the off position by the utility worker so that the homeowner or a family member doesn't accidentally turn the system back on prior to the completion of repairs. Lockable AC disconnects are required by most utilities. However, many experienced electric companies like the large California utilities and Colorado's Exel, which collectively have thousands of solar- and wind-electric systems connected to their systems, have dropped the requirement for utility company-accessible, lockable disconnects. The California utilities have found that they can't use them because there are too many to disconnect at once. More importantly, these companies have come to realize that they're simply not needed. That's because grid-compatible inverters automatically shut off when the utility power goes down. As a result, no electricity can flow onto the grid. A properly installed grid-connected wind-electric system will not backfeed a dead grid. Period.
The Pros and Cons of Grid-Connected Systems Batteryless grid-connected systems represent the majority of all new wind systems in the United States. Their pluses and minuses are summarized in Tbl. 1. == Utilities keep track of the two-way flow of electricity from grid-connected renewable energy systems in one of two ways. In many cases, they install a single dial-type electric meter. This meter measures kilowatt-hours of energy and can run forward or backward. It runs forward (counting up) when your home or business uses more energy than your wind turbine is producing. It runs backward (counting down) when the wind turbine produces more electricity than is being used. In many new installations, utilities install digital meters that tally electricity delivered to and supplied by a home or business. They don't "run backward," as older, dial-type meters do. They simply keep track of electricity coming from and going to the grid so the utility can determine whether you've produced as much, more, or less than you've consumed. Still other utilities install two meters, one to tally the electricity delivered to the grid and another to keep track of electricity supplied by the grid. (They typically charge to install the second meter and may charge a separate monthly fee to read it.) Most companies employ a billing system known as net metering. Net metering is a system in which the electric bill is based on net consumption - consumption minus production. That is to say, a customer's electric bill is based on the amount of utility energy consumed minus the amount of energy provided to the grid from a renewable energy system. The "net" in net metering refers to net kilowatt hours. In a net metering arrangement with two meters, one meter tracks consumption while the other tallies a customer's contribution to the grid. The customer is charged for the electricity he or she consumes and is credited for the electricity he or she feeds onto the grid at the same rate (retail rate) up to the point where there's a surplus backfed onto the grid. The differences in net metering programs in different states stem from differences in ways utilities treat surpluses (net excess generation or NEG). Net excess generation can be reconciled at the end of the month (monthly net metering) or at the end of the year (annual net metering). If you're thinking that wind could be a profitbl. venture, don't get your hopes up. While a few utilities pay retail rates, most reimburse for net excess generation at their wholesale rate - that is, what they pay to generate electricity. Many utilities don't write checks at all. They simply "take" the surplus without payment to the customer, like cell phone companies do. It all depends on state law. As a rule, monthly net metering is generally the least desirable option, especially if surpluses are "donated" to the utility company or reimbursed at wholesale rates (a.k.a. avoided cost). Annual reconciliation is a much better deal. It permits wintertime surpluses to be "banked" to offset summertime shortfalls, if any. The ideal arrangement from a customer's standpoint is to negotiate a continual month-to-month roll-over, so there's no concern about losing credit for your wind-powered kilowatt-hours. We're only aware of one state in which credits carry forward indefinitely, and that's Kentucky. Net metering is mandatory in many states. At this writing (May, 2009), 41 states and the District of Columbia have instituted net metering, although there are substantial differences among them. The programs vary with respect to which utilities are required to participate in the program, the size and types of systems that qualify, payment or lack of payment for net excess generation (wholesale, retail or no payment), and so on. Some states only require "investor-owned" utilities to offer net metering. These companies typically serve urban regions where wind energy systems are not well suited. In many states, municipal power companies and rural cooperatives are exempt from net metering laws. The exemption of rural co-ops from net metering is unfortunate, because rural areas are more likely to have the best wind resources, less ground clutter, fewer zoning restrictions, and homes on larger acreage, which are ideal for installing wind turbines. Utilities that don't offer net metering may use another system called buy-sell or net billing. In these arrangements, utilities typically install two meters, one to track electricity the utility sells to the customer, and another to track electricity the customer feeds onto the grid. At first blush, this arrangement may seem very similar to net metering, how ever, it is quite different. Unlike net metering, utilities that engage in buy-sell arrangements typically charge their customers retail rates for all the electricity they draw from the grid but pay customers wholesale rates for all the electricity fed onto the grid by renewable energy systems. For example, a utility may charge 10 to 15 cents per kilowatt hour for electricity they supply to the customers, but pay wholesale rates of 2 to 3 cents per kilowatt-hour for all the electricity that customers deliver to the grid. How does this work out financially for the small-scale producer? As you might suspect, not very well. Suppose that a customer consumed 500 kilowatt-hours of electricity from the grid but fed 1,000 kilowatt-hours of electricity onto the grid in December, a typically windy month. Let's suppose that the retail rate for electricity was 10 cents per kilowatt-hour and the generation and delivery costs (that is, the wholesale cost) came to 3 cents per kilowatt-hour. In a buy-sell agreement, the utility would charge the customer 10 cents per kilowatt-hour for all the electricity delivered to them, or $50 for the 500 kilo watt-hours. The utility would then credit the customer 3 cents per kilowatt-hour for the 1,000 kilowatt-hours of electricity delivered to the grid. That is, the customer would be paid or credited $30.00 for the 1,000 kilowatt hours of electricity sold to the utility. As a result, the customer would end up owing the utility $20 plus a meter reading fee of $10 to $20. Although (at this writing) nine states do not have net metering laws for wind, progress in this area is moving quickly. More and more utilities have come to realize that it is often cheaper - and less hassle - for them to net meter than to install two meters. For information about net metering rules for individual states, go to the Database for State Incentives on Renewables and Efficiency (dsireusa.org). Click on your state and scroll down to net metering. == Net Metering in Grid-Connected Systems On the positive side, batteryless grid-connected systems are relatively simple and typically the least expensive option - often 25% cheaper than battery-based systems. They also require less maintenance than battery-based systems. Another substantial advantage of these systems is that they can store an unlimited amount of electricity on the grid (so long as the grid is operational). Although grid-connected systems don't physically store excess electricity on site like a battery-based system for later use, they "store" surplus electricity on the grid in the form of a credit on your utility bill. When winds fail to blow - or a wind turbine isn't producing enough electricity to meet demand - electricity is drawn from the grid, using up the credit. Unlike a battery bank, you can never "fill up" the grid. It will accept as much electricity as you can feed it. By crediting a producer for electricity fed onto the grid, a utility says, "You've supplied us with x kilowatt-hours of electricity. When you need electricity, we'll supply you with an equal amount at no cost. If at the end of the month you've supplied more than you consume, we'll either pay you for it or carry the surplus over to the next month." Another advantage of grid storage is utility storage of electricity is not subject to losses that occur when electricity is stored in a battery. As described in Section 7, when electricity is stored in a battery, it is converted to chemical energy. When electricity is needed, the chemical energy is converted back to electrical energy. As much as 20 to 30 percent of the electrical energy fed into a battery bank is lost due to conversion inefficiencies and other factors. In sharp contrast, electricity stored on the grid comes back in full. If you deliver 100 kilowatt-hours of electricity, you can draw off 100 kilowatt-hours. (The grid has losses too, however, net metered customers get 100 percent return on their stored electricity.) Another advantage of grid-tie systems is that they are greener than battery-based systems. Although utilities aren't the greenest entities in the world, they are arguably greener than battery-based systems. Battery production requires an enormous amount of energy and raw materials. Batteries also contain highly toxic sulfuric acid. Although old lead-acid batteries are recycled, they're often recycled under abysmally poor conditions in less developed countries, exposing employees (often young children) and the environment to toxic chemicals. Grid-tie systems, when net metered, can provide some income. In windy sites, they may produce surpluses month after month. If the local utility pays for surpluses at retail rates, the surpluses can generate income that helps reduce the cost of the system and the annual cost of producing electricity. On the downside, grid-connected systems may require extensive negotiations with local utilities. This, though, may become a thing of the past. Although some utilities may throw up roadblocks, more and more are becoming cooperative as they become more comfort able with these systems. Another downside of these systems is that when the grid goes down, so does a batteryless grid-connected wind system. Even when winds are blowing, batteryless grid-tied wind energy systems shut down if an electric line comes crashing down in an ice storm or lightning strikes a nearby transformer, both of which result in a power outage. Even though the winds are blowing, you'll get no power from your system. If power outages are a recurring problem and outages occur for long periods, you may want to consider installing a standby gas or diesel generator that switches on automatically when the grid goes down. Because a backup generator takes many seconds to start up and come on line, you may want to consider installing an uninterruptible power supply (UPS) on critical equipment such as computers. A UPS contains a battery and a small inverter. If the utility power goes out, it supplies power instantly until its battery runs low. Another alternative is to install a grid-connected system with battery backup, discussed next. In these systems batteries provide backup power to a home or business when the grid goes down. == Tbl. 1 Pros and Cons of Batteryless Grid-Tie Systems - Pros - Simpler than other systems Less expensive Less maintenance More efficient than battery-based systems Unlimited storage of surplus electricity Greener than battery-based systems - Cons - Vulnerable to grid failure unless an uninterruptible power supply and /or generator is installed == Grid-Connected Systems with Battery Backup Grid-connected systems with battery backup are also known as battery-based utility-tied systems. These systems ensure a continuous supply of electricity, even when freezing rain wipes out the electrical supply to your home or business. Ill. 4 shows the components of these systems: (1) a wind turbine on a tower, (2) a charge controller, (3) an inverter, (4) safety disconnects, (5) breaker box or main service panel, and (6) meters to keep track of electricity delivered to and drawn from the grid. Ill. 4 : Grid-Connected Wind System with Battery Backup. 1. Carries wild AC from turbine 2. Controller 3. Battery bank 4. Transfer switch 5. Inverter 6. Subpanel (for critical loads) 7. Main breaker box (all household loads) 8. Utility meter 9. Service line Although grid-connected systems with battery backup are similar to batteryless grid-connected systems, they differ in several ways. The most obvious difference is that battery-based grid-connected systems contain a bank of batteries. They also require a different type of inverter. These systems also contain a meter that monitors the flow of electricity into and out of the battery bank and a device known as a charge controller. Batteries for grid-connected systems with battery backup are either flooded lead-acid batteries or sealed lead-acid batteries. Battery banks in grid-connected systems are typically smaller than those in off-grid systems because they are usually sized to provide sufficient storage to run a handful of critical loads for a day or two until the utility company restores electrical service. Critical loads might include a refrigerator and freezer, a few lights, a well pump, and the blower of a furnace or the boiler and pump in a radiant heating system. Keeping batteries fully charged is a high priority in these sys tems. Battery banks are maintained at full charge day in and day out to ensure a ready supply of electricity should the grid go down. It's only when the batteries are topped off and a household's demands are being met that excess electricity is backfed onto the grid. Batteries are called into duty only when the grid goes down. They're a backup power source. They're not there to supply additional power to run loads that exceed the wind system's output. When demand exceeds supply, electricity is supplied by the electrical grid, not the batteries. When the winds are dead, the grid, not the battery bank, becomes the power source. Maintaining a fully charged battery bank requires a fair amount of electricity. That's because batteries self-discharge when sitting idly by. Thus, a good portion of the surplus electricity a wind sys tem generates may be devoted to keeping batteries full. Keeping batteries topped off consumes 5 to 10 percent of a system's daily output. (In systems with a low-efficiency and technologically unsophisticated inverter and a large or older battery bank, consumption can be as high as 25 to 50 percent.) Battery banks in grid-connected systems don't require careful monitoring like those in off-grid systems, but it is a very good idea to keep a close eye on them - just to be sure they'll be functional when the grid goes down. Owners can monitor batteries through a meter that indicates the total amount of electricity stored in the battery bank at any one time. These meters give readings in amp hours or kilowatt-hours. What do these terms mean? As most readers know, electricity is the flow of electrons through a wire. Like water flowing through a hose, electricity flows through conductors at varying rates. The rate of flow depends on the voltage. The flow of electrons through a conductor is measured in amperes or amps for short. An amp is 6.24 x 1018 electrons passing by a point on a conductor per second. The greater the amperage, the faster the electrons are flowing. One amp of electricity flowing through a wire for an hour is one amp-hour. This term is also frequently used to define a battery's storage capacity. A flooded lead-acid battery, for example, might store 420 amp-hours of electricity. Amp-hours can also be converted to kilowatt-hours, as explained shortly. Meters also typically display battery voltage. Battery voltage can provide a very general approximation of the amount of energy in a battery - if you know how to interpret this parameter. We'll discuss this topic in Section 7. Another component found in wind energy systems with battery backup is the charge controller, shown in Ill. 4. Charge controllers contain a component known as a rectifier. It converts AC from the wind generator to DC electricity. It is then fed into the batteries. Charge controllers also monitor battery voltage. They use this information to protect batteries from being overcharged - having too much electricity driven into them. Overcharging can permanently damage the lead plates in batteries, dramatically reducing battery life. When the charge controller sees that the batteries are fully charged, it terminates the flow of electricity to them. Surplus electricity is then fed onto the grid, or if the grid is not operational, to a diversion or dump load. Diversion loads are typically resistance-type devices that convert surplus electricity into heat. They are installed in water heaters or as separate space heaters in the basement or a nearby utility room and help put to use the surplus electricity (Ill. 5). - Pros and Cons of Grid-Connected Systems with Battery Backup - Ill. 5: Dump Load. Resistive heaters like this one are used as dumps for surplus electricity from off-grid wind energy systems. Grid-connected systems with backup power protect against utility failures, although typically only a handful of critical loads can be run from a battery bank. These systems allow homeowners to heat their homes, keep food cold, power a radio, and run emergency medical equipment. In businesses, they protect computers and other vital equipment required to continue operations. Grid-connected systems with battery backup do have some drawbacks. They cost more to install and operate than batteryless grid-connect systems. Flooded lead-acid batteries used in these systems require periodic maintenance and replacement every five to ten years, whether they're used or not. Keeping batteries topped off can also consume a fair amount of a system's daily electrical output. When contemplating a battery-based grid-tie system, ask your self three questions: (1) How frequently does the grid fail in your utility's service area? (2) What are your critical loads and how important is it to keep them running? (3) How do you react when the grid fails? If the local grid is extremely reliable, you don't have medical support equipment to run or need computers for critical financial transactions, and you don't mind using candles on the rare occasions when the grid goes down, why buy, maintain, and replace costly batteries? See Tbl. 2 for a quick summary of the pros and cons of battery-based grid-connected systems.
- Off-Grid (Stand-Alone) Systems - Those who want to or must supply all of their needs through wind energy or a combination of wind and solar and don't want to be connected to the grid install off-grid systems. As shown in Ill. 6, this system bears a remarkable resemblance to a grid-connected system with battery backup. - Tbl. 2 Pros and Cons of a Battery-Based Grid-Tie System - Pros: Provides a reliable source of electricity Cons: More costly than batteryless grid-connected systems; Less efficient than batteryless grid-connected systems; Less environmentally friendly than batteryless systems; Requires more maintenance than batteryless grid-connected systems. The main source of electricity in an off-grid system is a battery charging wind turbine. These turbines produce wild AC electricity that is converted (rectified) to DC electricity by rectifiers located in the charge controller. The controller delivers DC electricity to the battery bank. When electricity is needed, it is drawn from the battery bank via the inverter. The inverter converts the DC electricity from the battery bank, typically 24 or 48 volts, to higher-voltage AC, either 120 or 240 volts, required by households and businesses. The AC then flows to active circuits in the house via the breaker box. Although off-grid systems resemble grid-connected systems with battery banks, there are some noticeable differences. The first and most obvious is that there are no power lines running from the house or business to the grid. In these systems, then, the wind turbine produces all of the electricity required to meet the owner's needs. Surplus generated during windy periods is stored in batteries for use during low- or no-wind periods. If the batteries are full, the surplus is typically sent to the diversion load. Off-grid systems are also typically equipped with another source of electricity, often a PV array or a gasoline or diesel generator (gen-set). They help make up for shortfalls. Off-grid systems also require safety disconnects to permit servicing. A DC disconnect is located between the charge controller and inverter. These systems also contain charge controllers to protect the batteries from overcharging and a low-voltage disconnect to prevent deep discharge of the battery bank. Off-grid wind energy systems are the most complex of all options. Some systems contain DC circuits. These circuits are fed directly from the battery bank, bypassing the inverter, to power DC lights or refrigerators. Bypassing the inverter saves energy, because inverters are not 100 percent efficient. It takes a little energy to convert DC to AC - usually about 5 to 10 percent. DC appliances are generally small, difficult to find, expensive, and not always that reliable or as fully equipped as AC appliances. DC refrigerators, for example, do not come with the features that many individuals expect, such as automatic defrost or ice makers. DC circuits also require larger, more costly wires and special receptacles. Moreover, the energy lost as low-voltage DC electricity flows through wires is about the same as the losses in an inverter. Ill. 6: Most wind turbines in off-grid wind systems produce AC electricity that's converted to DC electricity by the controller. The inverter draws electricity from the batteries, converting it into AC electricity for household use. 1. Electric wire carries wild AC to controller 2. Controller 3. Battery bank 4. DC disconnect 5. Inverter 6. AC disconnect 7. Main breaker 8. AC circuits To simplify installation of battery-based systems, you may want to consider installing a power center (Ill. 7). Power centers contain many of the essential components of a renewable energy system, including one or more inverters, the meters needed to monitor system performance, safety disconnects, and the charge controller. Power centers provide connection points to which the wires to the battery bank, the inverter and the wind generator connect. Although power centers may cost a bit more than buying all the components separately, they are easier and cheaper to install. Ill. 7: Power Center. Power centers like this one contain all of the components needed for a successful installation, all mounted on one panel. They're easy to wire and pass inspection with ease. - Pros and Cons of Off-Grid Systems - Off-grid systems provide freedom from power outages, energy independence, and total emancipation from the electric utility (Tbl. 3). If designed and operated correctly, they will provide sufficient energy to meet your needs for many years. Although, they do free you from utilities, you will still very likely need to buy a generator (gen-set) and fuel to power it. Gen-sets produce pollution and cost money to maintain and operate. Off grid systems are also the most expensive of all systems because of the need for batteries and backup power (via PV systems and /or gen-sets), which add substantially to the cost. They also require more wiring and additional space to house battery banks and generators. They require more maintenance, too, thanks to the batteries and generators. Batteries require replacement every five to ten years, depending on the quality of batteries you buy and how well you maintain them. Battery production and recycling also exact a toll on the environment. Although cost is a major downside, there are times when off grid systems cost the same or less than grid-connected systems - for example, if a home or business is located more than a few tenths of a mile from the utility lines. Under such circumstances, it can cost more to run electric lines to a home than to install an off-grid wind system. Tbl. 3 Pros and Cons of Off-Grid Systems Pros: Provide a reliable source of electricity; Provide freedom from the utility grid; Can be cheaper to install than grid-connected systems if located more than 0.2 miles from grid; Cons: Generally the most costly wind energy system; Less efficient than batteryless grid-connected systems; Require more maintenance than batteryless grid-connected systems (you take on all of the utility's operation and maintenance jobs and costs) -- Hybrid Systems -- Many homeowners and business owners, especially in rural areas, install hybrid systems to meet their needs, as discussed earlier (Ill. 8). In fact, most residential off-grid wind systems in use in North America are hybrids that combine solar electricity with wind. In some locations (like the North American Great Plains) you can count on consistent winds for power production. But for most of the world, wind turbines and PVs are a marriage made in heaven because winds vary throughout the year. They tend to be strongest in the fall, winter and spring - from October or November through March or April. At Dan's educational center in central Missouri, The Evergreen Institute's Center for Renewable Energy and Green Building, for instance, the highest average wind speeds occur from October through May (Ill. 9). During these months, a properly sized wind generator can meet most of a family or business's needs. Winds continue to blow, but less frequently and less forcefully through the rest of the year. Fortunately, though, sunshine is more abundant during this less-windy period. In such instances, a solar electric system can supplement a wind energy system, providing the bulk of the electricity while the wind turbine plays a backup role. Because solar and wind resources are often complementary, hybrid systems provide a more consistent year-round output than either wind-only or PV-only systems. Sized correctly, in areas with a sufficient solar and wind resource, hybrid wind/PV systems can provide 100 percent of one's electricity This complementary relationship is shown graphically in Ill. 9. Ill. 8: Hybrid System. This system consists of a PV array and wind turbine, two renewable energy technologies that complement each other very nicely. Ill. 9: The Complementary Nature of Wind and Solar. Wind and solar energy often complement each other, creating a reliable, year-round source of electricity. Note how well solar makes up for the reduction in wind during the summer months at Dan's education center, The Evergreen Institute, in east-central Missouri. A hybrid wind/PV system may even eliminate the need for a backup generator. Moreover, hybrid systems often require smaller solar electric arrays and smaller wind generators than if either were the sole source of electricity. However, if the combined output of a solar and wind system is not sufficient throughout the year, you will either need to cut back on consumption or run a backup generator. A generator is useful because it is used to maintain batteries in peak condition (discussed in Section 7), and it may allow installation of a smaller battery bank. -- Choosing a Wind Energy System - By far the cheapest and simplest wind energy option is a battery less grid-connected system. The occasional power outage will shut your system down, but in most places these are rare and short lived events. Grid-connected systems are usually cheaper than going off-grid. Moreover, if you are installing a wind system on an existing home that is already connected to the grid, it's best to stay connected. Use the grid as your batteries. Grid-connected systems with battery banks are suitbl. for those who want to stay connected to the grid, but want protection against occasional blackouts or brownouts. They cost more, but provide peace of mind and security. However, if you are building a new home and you are more than a few tenths of a mile from existing power lines, connecting to the grid can be expensive. While some utility companies foot the bill for line extension, others charge to run an electrical line to your home. Utility company policies vary considerably when it comes to line extension costs. Hook-up fees can run up to $50,000 - or more - if you live more than half a mile from the closest electric lines. Dan has a client who spent $65,000 to run a utility line two miles to her property, which could have purchased a huge renewable energy sys tem for her cabins. So be sure to check with your local utility when considering which system you should install. In some locations, a quarter-mile grid connection is costly enough to justify an off-grid system. Next: Prev.: |