Choosing a Battery-Based PV Inverter

The inverter is the heart of a battery-based PV system, converting DC from the batteries into AC for lights and appliances. High-power options, better surge capacity, lower cost per watt, and more bells and whistles are now available.

Matching the Inverter to the System

There are basically two different system configurations that utilize battery-based inverters: “off-grid” (also referred to as “stand-alone”) and those that have utility power available. Within the two system types are numerous variations. Determining which inverter is appropriate for your system requires answering several questions:

• Is there is access to a supplemental power source, such as the grid or a generator?
• What are the goals for your system? If you’re planning an off-grid system, do you want to minimize generator size? If you are on the grid, do you want to maximize the solar power that’s exported to the grid? Or do you want to maximize your on-site consumption of energy produced by your system?

There are myriad possibilities. Some inverters are built to serve only one or two system configurations, while others can accommodate several different system types­—and selecting how the system functions can be as simple as a quick programming change. The basic battery-based system configurations are discussed below. However, selecting the best inverter for your system requires spending some focused time with inverter cut sheets and manuals, and/or working with an installer who has solid experience with battery-based systems.

Off-grid. As the name suggests, these systems do not have access to utility power. Off-grid homes commonly use a generator for supplemental power for large AC loads or during times of little sun. These systems require an inverter/charger that can operate in off-grid mode and can use outside AC input from the generator for charging the battery bank. Several battery-charging inverters have expanded programming options, optimizing the working relationship between the generator and the inverter. As a result, the generator capacity needed can be reduced (see “Generator Support”).

Grid available. If there is utility power available, you can design a grid-tied system where excess energy is sold back to the grid, but a battery bank is available for backup (aka “grid-tied with battery backup”). These systems require an inverter that has a grid-interactive mode, but can be configured several different ways. The most common method is to have the inverter operate in parallel with the grid when it is available, and to provide backup power to specific AC loads when the grid goes down. This minimizes battery use, since it only draws from them if the grid is down.

There are also newer options for systems—“grid support” and “grid zero.” These are programming modes for some grid-interactive inverters that allow you to fine-tune how your system interacts with the utility. These options can be useful in areas where rules and incentives for grid-interactive systems have changed, such as not allowing exporting of PV energy to the grid or not allowing net metering, making consuming energy from the on-site solar and battery bank more desirable. Some inverters can also accommodate a second AC power source, such as a generator, to provide another level of backup power.

Alternatively, an inverter/battery system can function as a backup system to the grid (i.e., a UPS system) or can use the grid as a backup power source to a solar/inverter/battery system—without exporting any energy to the grid. These systems require inverters that can accept AC power from the grid for battery charging, but do not have to be listed as “grid-interactive.”

Surge Capacity

Inverter surge is a measure of how much power the inverter can put out to start motor loads that may draw much higher than normal power upon startup. Depending on the particular motor, this may take from less than a second to tens of seconds, and may be from 1.5 times to 7 times the motor’s normal load. There is no standard in rating inverter surge capacity, so what one inverter reports as “surge” may not directly compare to another one. A “surge duration” is more useful information than a generic “surge” rating with no specs on duration. One way to determine how an inverter handles surge is reflected in its weight—heavier transformer-based inverters can sustain a good surge for much longer (minutes versus seconds) than a lighter-weight high-frequency inverter. This is one large difference between the inverters designed for whole-house use included in this article compared to many RV and consumer-electronics inverters.

Generator Support

Many off-grid inverters can operate in parallel with a generator, instead of just switching the loads to generator power when the generator comes on. This allows an inverter to “assist” a small generator with large loads. Historically, generators were sized to simultaneously power the largest loads and charge the batteries. Now, with greater inverter capacity, the inverter may be sized to serve the largest loads, with a small generator sized to handle only battery charging. The inverters that can operate in parallel with a generator (often called “generator support”) can help a smaller generator start a large load like a well pump or table saw by briefly drawing power from the batteries.

AC Output Needs

Some inverters provide only 120 VAC output; if your off-grid house (or the critical loads subpanel in your battery backup system) requires 240 volts, a second inverter is added to provide the other phase. Instead of adding a second inverter, an external step-up transformer can be used to get 240 VAC from a 120 VAC inverter. The efficiency is reduced, but the cost may be quite a bit lower than adding a second inverter. Some inverters come with 120/240 VAC split-phase output. Which is best depends on your situation.

If you have no 240 VAC loads, you can use a single 120 VAC inverter to energize both 120 VAC legs of your load panel (see the “Beware: Multiwire Branch Circuits” sidebar). If you have an appliance that requires 240 VAC, such as an existing well pump, you have a couple of choices:

• If you need 240 VAC, but don’t need the combined power of two inverters, then it can make sense to get a single 120/240 VAC split-phase inverter. For example, if you have a 1 hp deep well pump that draws 2,000 W with a 7,000 W surge at 240 VAC, you could save money by using a single 4,000 W 120/240 VAC inverter to power it, rather than two 3,600 W 120 VAC inverters. On the flipside, these split-phase inverters won’t put out full power on a single leg—they are usually limited to about 67% or 75% of full power on a single leg. So if you have a very large 120 VAC load, a single 120 VAC inverter may be better than a split-phase inverter of the same rating. For example, a 4,000 W, 120 VAC load could not be powered by a 4,000 W, split-phase inverter.
• If you need 240 VAC and the combined power of two inverters, there are two options. One is to use two 120 VAC inverters stacked in series, and the other is to use two 120/240 VAC inverters stacked in parallel. Using two 120/240 VAC inverters gives redundancy—if one fails, you can still get 240 VAC from the other inverter. This method can also be more efficient, because, for small wattage loads, only one inverter needs to be on. Sometimes the choice depends on the model. Some, notably SMA America’s Sunny Island series and OutBack Power’s line of FX inverters, only come in 120 VAC, so you will be selecting one inverter per phase when using multiple inverters.

A 120/240 VAC inverter is often selected for a battery-backup grid-tied system because it’s cheaper and easier to install. The amperage of the tie-in is half as much at 240 V compared to the tie-in at 120 V. This means you can fit twice as much PV power on a given service size following NEC 705.12(D), which commonly limits the size of the solar input to 20% of the busbar amperage.

Balance of System

Remember that an inverter is only one part of the system—many people focus on selecting and buying the inverter, and then face the challenge of integrating it with rest of the equipment. Magnum Energy, OutBack Power, and Schneider Electric offer wiring solutions (aka “power centers” or “power panels”) for use with their inverters, simplify the wiring considerably. There are also third-party options, such as MidNite Solar’s E-Panel, which provide complete Code-compliant wiring systems to simplify an inverter’s installation. Most inverters from Magnum Energy, OutBack Power, and Schneider Electric require a separate system control panel for programming and monitoring. There are no controls or displays on the inverter itself. This can be good when the inverter is located in a utility room, but, for example, you also want an inverter control/monitor in the living room. This functionality comes at an extra cost—between $150 and $400 depending on the model.

Many of the advanced functions, such as automatic generator-start, are part of the system control panel, not the inverter firmware—without the control panel, you may be limited to just turning the inverter on and off, and not be able to adjust the settings.

Many battery-based inverters can connect to a computer for remote monitoring, control, and data logging. Some allow users to remotely monitor the inverter’s operation via the Web. This usually requires an extra communications box (which may or may not be the same as the remote system control panel).

MPPT Charge Controllers

In a battery-based PV system, a charge controller is used between the PV array and the battery bank to monitor battery voltage, optimize charging, and keep the array from overcharging the batteries.

There are a few common types of charge controllers: single or two-stage (shunt or relay type); pulse-width modulated (PWM); and maximum power-point tracking (MPPT). While non-MPPT charge controllers are less expensive and still have their place in the battery-based PV market—especially for lighting and small developing-world systems—just about all modern home- and cabin-scale PV systems include an MPPT charge controller, as they offer several advantages.

MPPT Advantages

More watts. Recall the power equation—volts × amps = watts. The more voltage captured from an array, the more power (watts) can be sent to the battery bank. An MPPT charge controller keeps the array operating at the peak of the current-voltage curve, and converts array voltage above battery voltage into extra amperage, thus absorbing more watts from the array. A non-MPPT charge controller chains the array’s voltage to the battery’s voltage, effectively limiting the array’s power output.

Array voltage varies with cell temperature. For example, when the cells are cold during winter, yet receiving full sun, the array voltage is higher. Higher array voltage translates into greater wattage. Here’s an example: Considering average winter and summer temperatures in Boulder, Colorado, there would be about a 12% difference between average winter versus summer array power output, and up to a 25% difference on a cold winter day versus a hot summer day. For off-grid systems that have higher loads in the winter, the extra energy input offered by MPPT-based systems can be a big benefit. At higher temperatures, which usually occur in the summertime or year-round in mild climates, array voltage drops, and an MPPT controller may be less advantageous.

Step-down. Voltage conversion is another benefit that is built into MPPT charge controllers. An MPPT charge controller is a DC-DC converter—with computerized controls. It can take a higher voltage and lower amperage, and convert those to a lower output voltage at higher amperage. For example, instead of an array producing a nominal 24 V and charging a 24 V battery, an MPPT controller can step-down an array producing 60 V to charge that battery. This frees the array from having to be matched to the battery voltage, and mitigates some wire-sizing (and cost) issues.

In that example, pushing 30 A at 24 V a distance of 40 feet would require large-gauge (expensive) cable—2 AWG—to keep voltage drop under 2%. For the same amount of power, pushing 12 A at 60 V that same 40 feet with 10 AWG will keep voltage drop under 2%, with the MPPT charge controller stepping the output voltage down to 24 V for the batteries. THHN #2 wire retails for about $1.24 per foot, and #10 sells for about $0.19 per foot, saving $84.00 on that two-way wire run, even without considering conduit size and the physical difficulties of pulling large wire.

Higher Input Voltages

Until recently, most charge controllers could accept a maximum input voltage of only 150 V. Today, one manufacturer has models that accept 200 or 250 V input, and two have models that accept up to 600 V input. Having these options provides more flexibility in designing module strings for battery-based systems. For example, instead of designing strings of three modules in series, strings of six modules in series are possible. This reduces the number of strings needed by half. At half the amperage and twice the voltage, the same size wire can be used, but at four times the distance—without losing power. A 600 V charge controller may be able to accommodate a single series string of 12 modules, negating combiner boxes completely. This translates into less equipment, wire expense, and labor.The 600 V charge controllers may be used for transforming batteryless grid-tied PV arrays to grid-tied with battery backup. In many cases, rewiring the array is unnecessary.

A disadvantage to using a controller with a higher input voltage is that the disconnects and combiner boxes (if required) are typically more expensive and harder to find. Note that one of the 600 V input charge controllers (Morningstar’s TS-MPPT-60-600) has an optional integrated DC disconnect, which can help mitigate sourcing and finding space on the wall for an external 600 V DC disconnect, though the controller’s additional cost is similar to the cost of a separate DC disconnect.

Single-Module PV Systems

Most module manufacturers have switched to a 60-cell design, resulting in modules in the 200 W to 300 W range with a maximum power point of 25 to 35 V. Nominal 12 V and 24 V modules (having 36 and 72 cells, respectively) are harder to find and more expensive per watt. Several manufacturers have introduced MPPT charge controllers to accommodate a single 60-cell module on a 12 V battery system (which might power, for example, remote lighting or communications, or an off-grid cabin). Blue Sky Energy offers several products for 12 V systems, and MidNite Solar and Morningstar have introduced smaller (30 A) MPPT controllers, which will work for a single module on a 12 V system.

These charge controllers cost more than a simple PWM charge controller that you might use on a system with 36-cell (12 V nominal) modules. However, when you take into account the total system cost—PV module(s) plus charge controller—it can be 10% to 20% less expensive to use the 60-cell module with the MPPT charge controller. Plus, you get the advantage of MPPT. In addition, the wiring of the system often is simpler, since it involves one large module and no combiner boxes.

Matching Controllers to Inverters

For off-grid systems, matching the brand of charge controller to the inverter isn’t usually important, since there is very little coordination between these two. The charge controller routes energy into the battery, and the inverter takes it out—neither of them really cares what the other is doing. However, for a grid-tied system, synchronizing them can matter. While there are thousands of battery-based grid-tied systems that operate without communications between the charge controller and inverter, system programming can be simplified and efficiency can be improved if they are matched. Compatible communications systems enable the inverter to tell the charge controller that the grid is available. At this point, the charge controller’s job is not to regulate battery charge but to track the array’s MPP and get the most energy out of the array that it can. (The inverter will regulate the battery voltage by selling excess energy to the grid.)

Monitoring & Data Logging

All but the most basic charge controllers come with some system monitoring. All of the charge controllers included offer remote display options, enabling you to monitor the system’s operation in the house, for example, rather than at the controller’s location. Most of the MPPT charge controllers include a digital display on the controller as well. If your system has multiple charge controllers (from the same manufacturer), they can communicate with each other to coordinate charging, and can all send data to a single remote monitor.

MidNite Solar offers an amp-hour-counting state-of-charge meter with their Classic charge controllers, and as an option on its smaller KID controllers. Battery state-of-charge (SOC) metering, which shows battery SOC as a percentage, is an important tool that enables users to easily see how full (or empty) their batteries are. But it is often left out of systems because it comes at an extra cost.

Data logging can be another important feature, especially with systems that are not monitored daily. The larger  MidNite Solar, Morningstar, OutBack Power, and Schneider Electric charge controllers include data logging, so you can see how many kWh the system produced over a period of time. Having access to this data can be useful for installers when troubleshooting a system.

MidNite Solar, OutBack Power, and Schneider Electric’s charge controllers can be connected to a computer or smartphone (directly for MidNite Solar, and through an extra communications device for OutBack Power and Schneider Electric charge controllers) for monitoring, programming, and accessing historical data.

GEAR: SolarEdge Rapid Shutdown Function

SolarEdge (solaredge.com) inverters now have the rapid shutdown functionality that the 2014 NEC Section 690.12 requires. The inverters’ provider has completed Intertek Laboratories (ETL) testing to verify that its Safe DC technology meets the requirements.

Turning the inverter’s safety switch to the off position de-energizes PV source circuits to less than 30 VDC in less than the NEC’s required 10 seconds. For equipment manufactured prior to built-in rapid shutdown functionality, SolarEdge offers kits for field-retrofitting inverters to meet the requirements.

PV Circuit Sizing & Current Calculations

Section 690.8 of the National Electrical Code (NEC)  deals with PV circuit sizing and current calculations, and defines how to calculate four maximum circuit current values. These maximum circuit currents are used in additional calculations in sections 690.8(B). But before jumping into calculations, a few NEC definitions will be helpful, since the rules for correction factors and overcurrent requirements can change based on the specific circuit. Working from the array to the inverter, we have:

PV source circuits are conductors between the modules, and from modules to a common point of connection, typically a junction box or combiner box. In industry terms, these are often called the “home runs” from the individual strings.

PV output circuits are conductors between the PV source circuits and the inverter or DC utilization equipment. These are the circuit conductors after a combiner box to the inverter or charge controller. 

Inverter input circuits, in a battery-based system, are the conductors between the inverter and the battery bank. In a grid-tied system, they are the conductors between the inverter and PV output circuits. Typically, these are the conductors between the inverter’s integrated DC disconnect and the inverter’s DC input connection.

Inverter output circuits are the AC conductors from the inverter to the ultimate connection to the AC distribution system for either stand-alone or utility-interactive systems.

Calculations

The first calculation, from 690.8(A)(1), results in the maximum PV source-circuit current. The rated short-circuit current (Isc) is multiplied by 125%. For example, if a PV module has an Isc of 8.8 amps, this calculation is: 8.8 A × 1.25 = 11 A.

Section 690.8(A)(2) covers the maximum current for PV output circuits. For output circuits, multiply the Isc by the number of circuits in parallel, and then by 125%. A common installation method is to keep the source circuits separate until they reach the inverter’s integrated DC combiner and disconnect. In that case, there are no output circuits to consider because the source circuits are not placed in parallel outside of the inverter.

Section 690.8(A)(3) defines the maximum current for the inverter’s output circuit. For utility-interactive inverters, there isn’t a calculation required, since the maximum current is defined as the inverter’s continuous output rating.

Section 690.8(A)(4) shows the calculation for the highest input current of a stand-alone inverter. This value helps determine the conductor size and overcurrent protection device (OCPD) rating between the batteries and the inverter. Divide the inverter’s continuous power output rating by its lowest DC operating voltage, and then multiply by the inverter’s rated efficiency under those conditions.

Part 5 of 690.8(A), added to the 2014 Code, defines the maximum output current of DC-to-DC converters as the rated output per the manufacturer’s specifications. No additional calculations are required.

In the 2014 NEC, 690.8(B), which outlines the rules for calculating minimum conductor sizes in PV circuits, is titled “Conductor Ampacity.” The OCPD section has been relocated to 690.9. The method for conductor sizing has not changed, although the 2014 sections incorporate some clarifications.

In 690.8(B)—690.8(B)(2) in the 2011 edition—two calculations must be run; the circuit conductor size must be based on the larger of the two values calculated. The first calculation is in 690.8(B)(1)—690.8(B)(2)(a) in the 2011 edition. Because PV system currents are considered continuous, the maximum currents calculated in 690.8(A) must be multiplied by 125% to calculate the minimum conductor size. This calculation ensures that the conductors do not carry more than 80% of the continuous current value (0.8 is the inverse of 1.25), a standard procedure in earlier Code articles. In the PV industry, the result of this calculation is commonly referred to as the “156% factor.” When this rule is applied, the module’s rated Isc has been multiplied by 156% (125% × 125% = 156%). However, don’t just multiply everything by 156%. Inverter output circuits were not multiplied by 125% originally, so the 156% factor doesn’t apply to them.  This calculation is done before applying any adjustment and correction factors, commonly referred to as “conditions of use,” which include corrections for conductors exposed to temperatures in excess of 30°C or more than three current-carrying conductors within a conduit. The ampacity of the conductor, at a minimum, then, needs to be greater than or equal to the maximum current in 690.8(A) × 1.25.

Solar Energy

Solar energy is radiant light and heat from the sun harnessed using a range of ever-evolving technologies such as solar heating, solar photovoltaics, solar thermal electricity, solar architecture and artificial photosynthesis.

Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

In 2011, the International Energy Agency said that "the development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries’ energy security through reliance on an indigenous, inexhaustible and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuelprices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across thevisible and near-infrared ranges with a small part in the near-ultraviolet.

Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and thebiomass from which fossil fuels are derived.

Yearly Solar fluxes & Human Energy Consumption
Solar	3,850,000 EJ	[8]
Wind	2,250 EJ	[9]
Biomass potential	~200 EJ	[10]
Primary energy use (2010)	539 EJ	[11]
Electricity (2010)	~67 EJ	[12]
1 Exajoule (EJ) is 1018 Joules or 278 billion kilowatt-hours (kW·h).

The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass. The technical potential available from biomass is from 100–300 EJ/year.The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and mined uranium combined,

Solar energy can be harnessed at different levels around the world, mostly depending on distance from the equator

In 1897, Frank Shuman, a U.S. inventor, engineer and solar energy pioneer built a small demonstration solar engine that worked by reflecting solar energy onto square boxes filled with ether, which has a lower boiling point than water, and were fitted internally with black pipes which in turn powered a steam engine. In 1908 Shuman formed the Sun Power Company with the intent of building larger solar power plants. He, along with his technical advisor A.S.E. Ackermann and British physicist Sir Charles Vernon Boys, developed an improved system using mirrors to reflect solar energy upon collector boxes,

increasing heating capacity to the extent that water could now be used instead of ether. Shuman then constructed a full-scale steam engine powered by low-pressure water, enabling him to patent the entire solar engine system by 1912.

Shuman built the world’s first solar thermal power station in Maadi, Egypt between 1912 and 1913. Shuman’s plant used parabolic troughs to power a 45-52 kilowatt (60-70 H.P.) engine that pumped more than 22,000 litres of water per minute from the Nile River to adjacent cotton fields. Although the outbreak of World War I and the discovery of cheap oil in the 1930s discouraged the advancement of solar energy, Shuman’s vision and basic design were resurrected in the 1970s with a new wave of interest in solar thermal energy.In 1916 Shuman was quoted in the media advocating solar energy's utilization, saying:

We have proved the commercial profit of sun power in the tropics and have more particularly proved that after our stores of oil and coal are exhausted the human race can receive unlimited power from the rays of the sun.

Wind Turbine

A wind turbine is a device that converts kinetic energy from the wind into electrical power. A wind turbine used for charging batteries may be referred to as a wind charger.

The result of over a millennium of windmill development and modern engineering, today's wind turbines are manufactured in a wide range of vertical and horizontal axis types. The smallest turbines are used for applications such as battery charging for auxiliary power for boats or caravans or to power traffic warning signs. Slightly larger turbines can be used for making small contributions to a domestic power supply while selling unused power back to the utility supplier via the electrical grid. Arrays of large turbines, known as wind farms, are renewable energy and are used by many countries as part of a strategy to reduce their reliance on fossil fuels.
becoming an increasingly important source of
History 

Windmills were used in Persia (present-day Iran) as early as 200 B.C.^[1] The wind wheel of Hero of Alexandria marks one of the first known instances of wind powering a machine in history.^[2 However, the first known practical windmills were built in Sistan, an Eastern province of Iran, from the 7th century. These "Panemone" were vertical axle windmills, which had long vertical drive shafts with rectangular blades. Made of six to twelve sails covered in reed matting or cloth material, these windmills were used to grind grain or draw up water, and were used in the grist milling and sugarcane industries.

Windmills first appeared in Europe during the Middle Ages. The first historical records of their use in England date to the 11th or 12th centuries and there are reports of German crusaders taking their windmill-making skills to Syria around 1190. By the 14th century, Dutch windmills were in use to drain areas of the Rhine delta.

The first electricity-generating wind turbine was a battery charging machine installed in July 1887 by Scottish academic James Blyth to light his holiday home in Mary kirk, Scotland. Some months later Charles F. Brush built the first automatically operated wind turbine for electricity production in Cleveland, Ohio Although Blyth's turbine was considered uneconomical in the United Kingdom electricity generation by wind turbines was more cost effective in countries with widely scattered populations.
American inventor 

In Denmark by 1900, there were about 2500 windmills for mechanical loads such as pumps and mills, producing an estimated combined peak power of about 30 MW. The largest machines were on 24-meter (79 ft) towers with four-bladed 23-meter (75 ft) diameter rotors. By 1908 there were 72 wind-driven electric generators operating in the United States from 5 kW to 25 kW. Around the time of World War I, American windmill makers were producing 100,000 farm windmills each year, mostly for water-pumping.

By the 1930s, wind generators for electricity were common on farms, mostly in the United States where distribution systems had not yet been installed. In this period, high-tensile steel was cheap, and the generators were placed atop prefabricated open steel lattice towers.

A forerunner of modern horizontal-axis wind generators was in service at Yalta, USSR in 1931. This was a 100 kW generator on a 30-meter (98 ft) tower, connected to the local 6.3 kV distribution system. It was reported to have an annual capacity factor of 32 percent, not much different from current wind machines.

In the autumn of 1941, the first megawatt-class wind turbine was synchronized to a utility grid in Vermont. The Smith-Putnam wind turbine only ran for 1,100 hours before suffering a critical failure. The unit was not repaired, because of shortage of materials during the war.

The first utility grid-connected wind turbine to operate in the UK was built by John Brown & Company in 1951 in the Orkney Islands.

 Resource 

A quantitative measure of the wind energy available at any location is called the Wind Power Density (WPD) It is a calculation of the mean annual power available per square meter of swept area of a turbine, and is tabulated for different heights above ground. Calculation of wind power density includes the effect of wind velocity and air density. Color-coded maps are prepared for a particular area described, for example, as "Mean Annual Power Density at 50 Metres ".  In the United States, the results of the above calculation are included in an index developed by the National Renewable Energy Laboratory and referred to as "NREL CLASS". The larger the WPD calculation, the higher it is rated by class. Classes range from Class 1 (200 watts per square meter or less at 50 m altitude) to Class 7 (800 to 2000 watts per square m). Commercial wind farms generally are sited in Class 3 or higher areas, although isolated points in an otherwise Class 1 area may be practical to exploit.

Wind turbines are classified by the wind speed they are designed for, from class I to class IV, with A or B referring to the turbulence.

Class	Avg Wind Speed (m/s)	Turbulence
IA	10	18%
IB	10	16%
IIA	8.5	18%
IIB	8.5	16%
IIIA	7.5	18%
IIIB	7.5	16%
IVA	6	18%
IVB	6	16%

Despite these diverse developments, developments in fossil fuel systems almost entirely eliminated any wind turbine systems larger than super micro size. In the early 1970s, however, anti-nuclear protests in Denmark spurred artisan mechanics to develop micro turbines of 22 kW. Organizing owners into associations and co-operatives lead to the lobbying of the government and utilities and provided Vestas is the world's biggest wind-turbine manufacturer.

incentives for larger turbines throughout the 1980's and later. Local activists in Germany, nascent turbine manufacturers in Spain, and large investors in the United States in the early 1990's then lobbied for policies that stimulated the industry in those countries. Later companies formed in India and China. As of 2012, Danish company 

Horizontal axis

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox,

which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted forward into the wind a small amount.

Downwind machines have been built, despite the problem of turbulence (mast wake), because they fatigue failures, most HAWTs are of upwind design.

don't need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclical (that is repetitive) turbulence may lead to

Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. These have high tip speeds of over 320 km/h (200 mph), high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored white for daytime visibility by aircraft and range in length from 20 to 40 meters (66 to 131 ft) or more. The tubular steel towers range from 60 to 90 meters (200 to 300 ft) tall. The blades rotate at 10 to 22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 90 meters per second (300 ft/s).A gear box is commonly used for stepping up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades into the wind which ceases their rotation, supplemented by brakes.

Vertical axis design

Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. One advantage of this arrangement is that the turbine does not need to be pointed into the wind to be effective, which is an advantage on a site where the wind direction is highly variable. It is also an advantage when the turbine is integrated into a building because it is inherently less steerable. Also, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, improving accessibility for maintenance.

The key disadvantages include the relatively low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.

When a turbine is mounted on a rooftop the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of a rooftop mounted turbine tower is approximately 50% of the building height it is near the optimum for maximum wind energy and minimum wind turbulence. Wind speeds within the built environment are generally much lower than at exposed rural sites, noise may be a concern and an existing structure may not adequately resist the additional stress.

Darrieus wind turbine

"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus.They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing

Giromill

A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used

Savonius wind turbine

These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops.

Twisted Savonius

Twisted Savonius is a modified savonius, with long helical scoops to provide smooth torque. This is often used as a rooftop windturbine and has even been adapted for ships.

Another type of vertical axis is the Parallel turbine, which is similar to the crossflow fan or centrifugal fan. It uses the ground effect. Vertical axis turbines of this type have been tried for many years: a unit producing 10 kW was built by Israeli wind pioneer Bruce Brill in the 1980s

Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modelling is used to determine the optimum tower height, control systems, number of blades and blade shape.

Wind turbines convert wind energy to electricity for distribution. Conventional horizontal axis turbines can be divided into three components:

• The rotor component, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy.
• The generator component, which is approximately 34% of the wind turbine cost, includes
  
  the electrical generator,the control electronics, and most likely a gearbox (e.g. planetary gearbox),^[31] adjustable-speed drive or continuously variable transmission component for converting the low speed incoming rotation to high speed rotation suitable for generating electricity.
• The structural support component, which is approximately 15% of the wind turbine cost, includes the tower and rotor yaw mechanism.

A 1.5 MW wind turbine of a type frequently seen in the United States has a tower 80 meters (260 ft) high. The rotor assembly (blades and hub) weighs 22,000 kilograms (48,000 lb). The nacelle, which contains the generator component, weighs 52,000 kilograms (115,000 lb). The concrete base for the tower is constructed using 26,000 kilograms (58,000 lb) of reinforcing steel and contains 190 cubic meters (250 cu yd) of concrete. The base is 15 meters (50 ft) in diameter and 2.4 meters (8 ft) thick near the center.

Among all renewable energy systems wind turbines have the highest effective intensity of power-harvesting surface^[36] because turbine blades not only harvest wind power, but also concentrate it