Kenetech is proposing replacing its entire fleet of 3410, 56-100 turbines in the Altamont Pass with 1705 of its larger KVS-33 models. This redevelopment, according to evidence in the two studies, could reduce bird collisions in the area by as much as 80%, while greater output from the larger machines would also increase income. However, the US Fish and Wildlife Service (USFWS) has opposed the repowering proposal, saying the new technology would increase mortality of protected bird species, including golden eagles. USFWS contends the KVS-33 machine, with a blade diameter of 33 metres instead of the 18.5 meters of the older machines, would increase collisions by 33% given that the rotor swept area of each turbine is more than three times larger than its older turbines.
The study that most directly addresses USFWS' concerns was performed by Sausalito-based Judd Howell & Associates. Howell compared avian mortality at adjacent strings of 20 and 16 KVS-33 turbines in the Altamont Pass and another 17 KVS-33 turbines in Solano County. In both cases, Howell identified nearby strings of the older, KCS-56 turbines, whose combined rotor swept area equalled that of the larger machines. The study started in December 1993 in the Altamont Pass and November 1994 at the Solano site. Both studies were concluded in September of this year.
During the study period, which included biweekly visits to the turbine strings, a total of 104 dead birds and one dead bat were found. Field surveyors identified a total of 72 deaths in the Altamont Pass during an 18 month period when both types of turbines were operating. Of these deaths, 44 were raptors. During a ten month period in Solano County, 13 confirmed casualties were discovered; ten were raptors. Ten birds recovered at the KCS-56 turbines in the Altamont Pass were excluded from the study because the KVS-33 turbines were not operating for three months. Another seven deaths were attributed to other causes, such as predator attack, and two injured birds were rehabilitated and released back into the wild.
All told, 20 dead birds were found by the KVS-33 units, while 66 were found near the KCS-56 units in both study areas, or 75% of all kills. Collision rates were 2.85 strikes per 100 turbines in the Altamont Pass and 2.84 per 100 turbines at Solano. Focusing on raptors, the KVS-33 turbines had one third the mortality ratio associated with the KCS-56. Howell concludes that Kenetech's proposal to retrofit the Altamont Pass with its larger turbines "would reduce mortality by two-thirds." In addition, the KVS-33 had relatively low numbers of non-raptor species recovered during mortality searches.
Fixed speed more deadly
The other study that offers a ray of hope for Kenetech was released by Vance Tucker, a scientist from Duke University's Department of Zoology. Tucker is a member of Kenetech's task force dedicated to solving the American wind industry's public relations challenge in relation bird kills. His study indicates that dramatic reductions in avian mortality may be achieved if larger, variable speed machines featuring slow-moving blades are utilised.
Based on several rather esoteric concepts such as "collision theory," Tucker examines how factors such as rotation speed, the point of penetration of the wind swept area and blade length all contribute to the probability of a bird colliding with a wind turbine. Tucker also distinguishes between variable and fixed speed rotors, factors which also distinguish the KVS-33 from the KCS-56.
Tucker developed what he calls "a safety index " to judge impacts of different attributes of wind turbines on probabilities of bird collisions. A fixed speed turbine scores low marks on this safety index since, if evaluated over the course of an annual wind season, "It is inefficient and produces little power at low wind speeds, yet it rotates at the same rate as it does at the rated speed. Although its power output is low, its probability of colliding with birds is undiminished." This contrasts with a variable speed turbine of the same size, which scores 50% higher on Tucker's index "because at low wind speeds, it has higher efficiency, higher power output and lower rotation rates."
Unsurprisingly, Tucker concludes that the KVS-45, which has a rotor radius of 22.5 metres, is six times safer than the 56-100, which features a nine metre rotor. The KVS-33, with 16.5 mere blades, is three times safer, states Tucker. He concludes: "The larger rotors are much safer for birds. For example, one KVS-45 could theoretically replace six KCS-56 turbines with no decrease in the amount of electrical energy generated in a year, but an 83 percent reduction in the number of bird collisions."
Even some of Kenetech's past critics on the bird issue seem impressed. Hans Peters, a local biologist who has helped focus attention on the raptor kill problem , has recently praised the company. "I'm no fan of wind turbines, but I'll say this of Kenetech: they're trying." Peters went on to note that the firm has already installed "perch guards" on a number of the wind turbines to reduce roosting, which Peters believes is a major cause of raptor mortality. Peters claims these guards have reduced roosting by almost 50%.