Airdolphin Technical Notes
Here are some technical notes based on the information gained by Zephyr's engineers through field tests.
In the first issue, we will explain through Airdolphin's two week's worth of data from the field test at "Cape Erimo Kaze-no-Yakata".
Cape Erimo is one of the windiest areas in Japan, and this field test was held at the befitting "Kaze-no-Yakata (Wind Museum)" in Erimo Quasi-National Park on the Hidaka Mountain Range. At Cape Erimo, strong winds of over 10m/s blow on more than 290 days a year. The data for this test shows how Airdolphin operated at a maximum wind gust speed of 35m/s and at a 10-minute average wind speed of maximum 25m/s.
Airdolphin Installation Details
【[Installation Pole】Retractable 9m pole
【Installation Site】Kaze-no-Yakata in Erimo Quasi-National Park on the Hidaka Mountain Range at latitude 43 north/longitude 143 east
【Battery】GPL-4D x 2 units (rating capacity: 210Ah)
【Load】Heaters within Kaze-no-Yakata (power consumption: 700W)
Specifications for Wind Condition Observation
【Anemometer】DAVIS (U.S.) cup anemometer (arm type), measurement resolution 1m/s
【Installation Site】Attached to pole 1.4m directly below the wind turbine hub
What Is Stall Operation?
Many propeller type wind turbines have blades with airplane-like cross sections (Fig. 1).This form allows airplanes to gain lifting power and keep flying. However, the wind needs to blow against the cross section at a certain angle -- inflow range -- to gain the lift. When the wind blows outside of this range, the lifting power is not be created and causes a stall condition -- unable to stay afloat.
During strong winds, Airdolphin's blade slows the rotation, changes the wind-inflow angle, and shifts to stall mode.
Why is stall mode necessary? Because lifting power-type propellers usually rotate faster as the wind speed increases. When the speed is too fast, the propeller may eventually break from its own centrifugal force or wind pressure. As the rotation speed approaches sonic speed, shock waves may occur.
With conventional wind power generation, damage from strong wind is prevented by changing the pitch (attachment angle) of the propeller, or deflecting the wind turbine (furling). On the other hand, Zephyr has adopted the stall operation based on the following development concept.
1) Blades featuring carbon fiber technology has become "ultra light", "highly rigid", and "longer lasting", capable of enduring stress from strong winds.
2) Less parts improves the machine's reliability. Compared with methods such as pitch angle change and furling structure, Airdolphin apparently uses fewer parts, thus less chance of failure.
3)The simple structure with less parts reduces costs.
4)Steady and continuous power generation in high wind conditions.
If within the stall range, Airdolphin can suppress the rapid acceleration of the rotation speed and maintain a relatively stable rotation.
However, the trade off for stall mode's stable operation with restrained rotation is less readiness to the wind and a drop in power generation efficiency. So with Airdolphin, we have set the following conditions when switching to stall mode:
1)High winds with the average wind speed of over 12m/s for several tens of seconds.
2)Battery-overcharge detection.
Fig. 2 shows how the 10 minute average generated output changes with the ten-minute average wind speed transition (operation stopped on 12th and 13th for maintenance).
The graph shows that Airdolphin generates power non-stop in stall mode, even in 20m/s strong winds. The data on 3rd/8th (avg. 26m/s, 440W), 14th (avg. 17m/s, 400W), 19th (avg. 21m/s, 360W), and 20th (avg. 28m/s, 460W) especially stand out.
Fig. 3 shows the 10-minute average generated power curve with the 10-minute average wind speed. You can see how Airdolphin switches between normal mode and stall mode. During stall mode, Airdolphin continues to operate at a slower rotation of 500 to 600 rpm. At a wind speed of 10 to 15 m/s, it generates an average of around 300W, at 20m/s wind speed, it generates an average of 400W power.
This means that Airdolphin can continuously generate power at 40% of its rated generated output during high wind conditions. With conventional wind turbines, operation needs to be stopped for safety during storms with an average wind speed of over 20m/s. Airdolphin has come up with a solution for the paradox that wind turbines can not generate power in strong winds, enabling non-stop power generation even in storm conditions. Zephyr focused on this continuous power generation, and succeeded to improve the capacity utilization in strong wind areas.
