Direct grid-tie, batteryless wind systems are the fastest-growing segment of the U.S. small wind market. This growth is being fueled by demand for simpler, more efficient systems and incentive programs that compensate owners for the amount of energy their system produces. Wind turbines were previously used mostly by off-gridders, but the new market for residential-scale wind turbines is primarily for grid-tie applications.
The same arguments for batteryless grid-tied PV systems also apply to wind systems. Additionally, most direct grid-tie turbines are configured for higher voltages than their battery-charging counterparts (typically above 200 VDC, compared to 12 to 48 VDC nominal). These high-voltage turbines allow the use of smaller transmission wiring, which significantly reduces wire and conduit cost.
Batteryless wind systems offer an increase in operating efficiency that results in higher overall energy production compared to battery-based systems. Most grid-direct systems use a batteryless inverter with maximum power point tracking (MPPT) capability, which maximizes turbine output and can increase overall system production by 20% to 50% depending on turbine and wind conditions at the site.
There are several models of batteryless grid-tie inverters for residential-scale wind systems. The best known is SMA America’s Windy Boy, essentially a Sunny Boy PV inverter with firmware modifications that allow the MPPT function to work with the rapid voltage fluctuations unique to wind turbines. Several turbine manufacturers, including Abundant Renewable Energy (ARE) and Proven Wind Energy, have developed grid-direct systems using this inverter.
The Windy Boy requires an interface between the wind turbine and the inverter to protect it from overvoltage damage. The typical interface is a voltage clamp that consists of a rectifier to convert the turbine’s wild (unregulated) three-phase AC output to DC, and a control circuit to divert energy to a diversion load—typically an air- or water-heating element.
The potential for overvoltage exists when the turbine is producing in high winds and the inverter is not connected to the grid due to a utility outage, or during the turbine start-up phase. In the event of a utility outage, the voltage clamp sends all power produced to the diversion load, preventing turbine overspeed and protecting the inverter. During the start-up phase, a pulse-width modulation (PWM) circuit sends some energy to the diversion load to keep the voltage within the inverter’s DC input window.
The ARE110 and ARE442 turbines use this type of voltage clamp and diversion load assembly. The Proven WT2.5 uses mechanical turbine governing instead of a voltage clamp assembly to keep the voltage in range. Kestrel turbines, distributed by DC Power Systems, have a proprietary control package for grid-direct applications. Eoltec turbines use the Aurora inverter manufactured by Magnetek. The Aurora is a true MPPT inverter with wind interface functionality that acts as a voltage clamp. Additionally, the inverter can be programmed to optimize the power curve to match the output of a specific wind turbine. The Bergey Excel uses the Grid Tek 10 inverter, which loads the turbine to the maximum safe power point at any given rpm.
The S250 turbine by Endurance Wind Power does not use an inverter to interface with the grid. The S250 is an induction machine, similar in design to medium and large utility-scale wind turbines that use a speed increaser drivetrain to spin the generator above synchronous speed. This allows 60-hertz power to be delivered directly from the generator to the grid without the need for power conditioning or conversion. The overall efficiency between rotor power and grid-compatible electrical power is about 75% at 10% of rated output and 85% at rated output of 5 KW. An internal disc-brake system protects the turbine from overspeed if grid power is lost.
Southwest Windpower’s Skystream turbine has an inverter integrated into the body of the turbine itself. The turbine’s output is grid-synchronous 240 VAC that can be fed directly to a home’s main load center. The turbine uses electronic stall regulation that begins to slow the blades when the rotor speed exceeds 360 rpm.
Roy Butler is a technical editor for Home Power and owner of Four Winds Renewable Energy in Arkport, New York. Roy is a NABCEP-certified PV installer. He sits on the NABCEP technical review committee for PV and wind system installer certification, as well as the Small Wind Certification Council board of directors.
Wind Turbine Manufacturers:
Abundant Renewable Energy • www.abundantre.com
Bergey Windpower Co. • www.bergey.com
Endurance Wind Power • www.endurancewindpower.com
Kestrel • www.dcpower-systems.com
Proven • www.provenenergy.co.uk
Southwest Windpower • www.windenergy.com
Grid-Direct Inverters for Wind Systems:
SMA America • www.sma-america.com
Magnetek • www.magnetek.com