The Wind for Schools program uses the Skystream 3.7 wind turbine manufactured by Southwest Windpower in Flagstaff, Arizona. The WfS program uses the Skystream turbine for several reasons:
- Southwest Windpower offers Skystreams at a 30% discount to WfS Host Schools
- The Skystream is one of the few (if not the only) small, grid-connected turbine that is IEEE 1547.1 and UL 1741 certified
- The data acquisition system for the Skystream is configured to allow connection to the Center for Advanced Energy Studies at Idaho National Laboratory. Data from WfS Host School turbines is available for public use from the CAES site.
- The turbine comes with a five-year limited warranty. The expected life of the turbine is 20 years.
The Skystream is a popular residential wind turbine. It was designed in partnership with the U.S. Department of Energy’s National Renewable Energy Laboratory. The turbine generates electricity that feeds back into the electrical system of the building on the customer's side of the electric service, so it offsets the electricity used in the building.
The Skystream 3.7 has a rotor diameter of 3.7 m (12 feet). At the rated wind speed of 29 mph, the turbine generates 2.4 kW of electric power.
Wiring for the turbine is run up through the core of the tower. For the Colorado Wind for Schools program, the default tower is a 45 foot monopole tower that tilts up with the use of a gin pole.
The turbine is a downwind design so that the blades of the turbine are downstream from the nacelle, which is quieter and inherently better at finding the wind direction than upwind designs.
The inverter constantly monitors the turbine and the electrical connection to assure that the electric energy generated by the turbine synchs with the frequency and voltage of the building's electrical system. The inverter actually draws about 5-7 watts to operate the monitoring system. Because of this, the turbine will not generate electricity when the electrical grid to the building is down. The inverter is UL 1741 certified which means that it meets UL standards for islanding protection, frequency synchronization, flicker, and the ability to withstand surges.
The alternator includes of 42 sets of powerful rare earth magnets. The motion of the rotor within the stator generates electricity, but there is no transmission or mechanical braking required. The only moving part is the stator hub. The inverter senses the need for braking and uses the repelling action of the magnets to slow the turbine, much like the regenerative brakes on a hybrid vehicle (but in reverse). Once the winds are strong enough to get the turbine up to speed, the inverter keeps the speed of the turbine between 300 and 340 rpm.
The blades are constructed from two halves of compression molded fiberglass. The curve of the blade helps to more efficiently capture the energy in the wind and to reduce the sound of the the blades as they move through the air.
Because of the many features incorporated in the turbine and blade design, the Skystream emits about 45-55 db at 50 feet from the base of the tower. Background sound level is usually between
30 and 45 dB. Because sound perception at a receptor is measured on a logarithmic scale,
a perceived doubling of sound is represented by a
difference of 10 dB. Much of the sound generated by the turbine is masked by the wind itself. Sound decreases significantly with distance from the source. Doubling the distance from the turbine decreases the sound level by a factor of four. For example, sound level readings at 50 feet from the turbine hub drop by a factor of 4 at 100 feet.
It's important to understand that this turbine is built to supply power to homes and buildings directly. As long as there is at least 4 kW of electricity being used by the school (very likely), a Skystream connected to the school's electrical system won't generate enough electric power to go onto the utility grid.
In a Class 2 or 3 wind and proper siting, the Skystream is expected to generate an average of 200-400 kWh per month.
The Skystream has a built in 2.4 Ghz wireless radio that sends performance data to a desktop computer with a wireless receiver. A software package called Skyview is included with the turbine. With the software, schools can track the generation of the turbine and download data for use in the classroom. This data can also be sent to a site at the Center for Advanced Energy Studies at Idaho National Laboratory to compare with other schools.
The typical installation used for the Colorado Wind for Schools program is a 45 foot segmented monopole tower.
After the turbine location is sited and permitted, a trencher or backhoe is used to build a 2 foot deep trench from the tower foundation back to the building and the electrical connection.
The preferred foundation design is a round, shallow pill-shaped design. The anchor bolts are arranged in a pattern and supported on a stanchion. Concrete is then poured around the bolt assembly, finished, and allowed to harden for 28 days to allow the concrete to reach its full strength.
The tower is assembled on the ground and the turbine is attached to the end of the tower. The blades are assembled onto a hub and attached to the turbine.