Wind turbines
Swept area
The Airforce has a blade diameter of 4.09 metres. This means it has a swept area, or capture area of 13.14 square metres. This is all important when comparing wind turbines, since it directly relates to the amount of passing wind energy
the turbine blades can intercept.
Airforce power output curve
The graph below shows the power produced (in watts) for the Airforce at different wind speeds, based on
data supplied by the manufacturer.
The green line shows the actual output that the wind turbine achieves at the given wind speed,
according to the manufacturers.The dashed line in the graph shows the theoretical maximum energy that this turbine could extract from the wind,
according to Betz's Law (59.3%). At a typical wind speed of
5 m/s, the Airforce is extracting
400 Watts of power from the wind, which is an efficiency of
39.6%.
How efficient is the Airforce?
Wind turbines can only convert some of the energy in the wind into electricity. The graph here shows how efficient the
turbine is across the range of wind speeds you might get.
The orange line in the above graph shows the efficiency ranging from 0 to 1. The grey line is the previously mentioned maximum efficiency - The Betz Limit. If this limit is exceeded, the manufacturer's claims are not to be trusted!
While these graphs give an indication of the instantaneous power and efficiencies you might expect from the
Airforce turbine, working out annual power production is more complex.
Read more about the
Power Predictor
Power output
The Airforce 4.1 will begin rotating in very low wind speeds (1.8m/s), but starts generating energy at around 3m/s. It has a rated wind speed of 5kW at 12m/s and cuts out at 60m/s.
Cost
Listed price of $19,800 (£12,301) with an approximate total installed price of £17,000.
Our view
The Airforce 4.1 has slightly smaller blades than other 5kW wind turbines, which Hopeful suggest means it may be less obtrusive and may lead to easier planning permission. However, smaller blades mean less swept area for capturing wind energy.
Blade diameter
4.09 metres
Rated output (W)
5000
UK grants available?
No
Warranty
Hopefull tell us they offer a 5 year warranty with this product.
Voltages
240 Volt AC
Manufacturer
Hopeful Energy
Noise levels
The manufacturer claims the turbine generates only "low noise levels" but doesn't quantify this by stating a decibel value.
More information
Hopeful Energy claims that they have an installation and maintenance network worldwide, including agents or offices in the US, Europe and Africa.
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