Wind turbines on towers and rooftops can save energy costs by generating electricity for buildings from houses and farms to industrial plants and commercial facilities.
Small wind turbines suitable for supplying electric power to residential, commercial, industrial and institutional buildings can save electricity costs. They can also slow the rate at which new generating stations need to be constructed.
Small turbines have either of two basic constructions:
- Horizontal-axis wind turbines (HAWTs) are the more common of the two, having propeller-like blades.
- Vertical-axis wind turbines (VAWTs) have blades mounted on vertical shafts.
Both types can be mounted on towers or poles or on the roofs or walls of buildings.
The sizes range widely:
- Generators producing 100 watts (W), for charging batteries — a typical low-power device can generate 12V or 24V for charging lead-acid batteries and for powering energy-saving devices such as lamps, low-voltage refrigerators, televisions and consumer electronics. An inverter can convert the DC power to 110V or 230V AC for large appliances.
- Models for up to 50 kilowatts (kW) can be connected to a utility power grid and used singly or in banks for powering residences or facilities such as commercial buildings, industrial premises and farms.
Larger turbines capable of generating 110 or 230 V are for use where the power authority allows connection to the utility grid:
- When wind is sufficient to generate power, all electricity is generated by the turbine and none is drawn from the grid;
- When the wind generator produces more power than is consumed by the building or facility, the excess can be fed on to the utility grid to earn revenue from the utility;
- When wind is insufficient, power is drawn from the utility grid.
Horizontal-Axis Wind Turbines
Horizontal models generally have 3 or 5-blade rotors mounted on horizontal shafts, similar to aircraft. They are similar to the large wind turbines that are becoming familiar sights in some countries and are used to feed energy to utility grids.
Recent design advances have resulted in blade shapes that reduce noise and vibration. Although there is some disagreement among manufacturers, HAWTs generally tend to have a higher efficiency of power output for wind speed than VAWTs. The minimum wind speed for the HAWTs researched for this article is 3.5m/s (8mph) compared to 4.5m/s (10mph) for a VAWT.
Vertical Axis Wind Turbines
VAWTs use different shaped blades. Early types had bow-shaped blades attached top and bottom to a shaft.
Some recently-developed VAWTs use S-shaped blades. One manufacturer has shaped the blades into a spiral. It is for battery-charging applications.
Another has three bow-shaped blades formed into a triple helix.
Because the VAWT shaft is vertical the gearing can all be at ground level. There is no need for the gearing at the top of a mast of most HAWTs to convert horizontal shaft rotation to a vertical shaft driving a generator at ground level. However, at least one manufacturer produces HAWTs that have no gearing.
A VAWT can be quieter. One manufacturer claims noise is 2 deciBels (dB) only 2 metres distant from its generator. HAWTs produce 35 dB to 50 dB; 35 dB is the sound level in a library, according to the Noise Abatement Society; 50 dB is the noise level in an office.
VAWTs are unaffected by changes in wind direction, whereas HAWTs pivot with the wind. VAWTs are less affected by turbulence than HAWTs. The triple-helix VAWT in particular is claimed to be more suitable than HAWTs for use on urban buildings where it can perform more efficiently in turbulent wind conditions.
Wind Power Output
Manufacturers offer assistance through their websites for assessment of wind conditions at proposed sites.
One HAWT develops 1.5kW at 31 mph (14 m/s) and can deliver an annual power of 2000kWh (kilowatt-hours). Another delivers 5000 kWh per year with an average wind speed of 12.5 mph (5.6 m/s).
The triple-helix VAWT model develops a peak power of 7kW. It can deliver 7000 to 10,000kWh per annum, equivalent, says its UK manufacturer, of 10% of the energy required by a 600 square metre office building.
Payback Of Wind Power Costs
Both HAWTs and VAWTs can reduce the power drawn from the power utility to provide electricity for a building. Many can also generate excess power that can be sold to a utility.
Payback depends on the saving over the local power utility rate. A typical figure is 5 years.