History of Danish Wind Power - Wind Power for the World: The Rise of Modern Wind Energy - page 54

the tower became close to the natural frequency of the tower. This
caused heavy tower vibrations and cracks in the concrete, but
the problems were eliminated by stifening the towers with three
outside “pilasters” and increasing the natural frequency above the
critical level. Another weak point showed to be the durability of
the blades. They had a main beam made from laminated wood and
wooden ribs. On the prototype the covering was canvas, as used
on aeroplanes, but later a changeover to water-resistant plywood
was made. But even that had to be treated often with a special
lacquer to withstand the rather harsh Danish climate. The two
longest-living Aeromotors, in service for 16 and 19 years, both
had to have their blades changed.

Figure 3.18 Vital parts from the two longest-living Aeromotors have been

preserved. The Danish Wind Historical Collection has the
complete gear and generator unit from a two-bladed turbine—
illustrated here—and the two new metal-covered blades from
1954. At the Nordic Folkecenter for Renewable Energy, the
gear and blade shaft unit from a three-wind Aeromotor has
been preserved (Photo: Benny Christensen, DVS).

Though the Aeromotor was an important step in the deve-
lopment towards modern wind turbines, it was still producing
direct current (DC). And the performance was not only dependent
on local wind resources, but also on the capacity of the local
grid. Some of the 2-bladed Aeromotors had a yearly production
of 80 000 kWh or more, while others were producing less than
30 000 kWh/year. The 3-bladed version, mostly placed on locations
with good wind resources, produced from 90 000 kWh/year to
more than 130 000 kWh/year.