An edited version of this article appeared in WindStats Vol 8 No 4, (Autumn 1995).
Data from the California Energy Commission’s Wind Project Performance report annual summaries for 1985 through 1994 confirm a trend that good turbines on good sites with competent management will deliver outstanding yields. Though this may seem obvious today, it wasn’t always so, and data from the CEC’s performance reports have been the only means for convincing skeptics in the U.S. of what has become an industry truism. The results from California mirror that of data collected from wind turbines in Denmark by Risoe National Laboratory showing a gradual increase in annual specific yields within each size class during the past decade. As with the Risoe data, reports of performance from California show a marked improvement at certain sites with the introduction of new, more productive turbines.
Specific Yields of Operators Statewide
WindStats has examined the annual specific yields of the top six California wind plant operators. Altogether these operators account for 80 percent of California’s 1,609 MW of total installed capacity. During the past nine years the specific yield of nearly all major operators in California has risen steadily and seemingly converges at about 800 kWh/m2/yr. The exceptions are FloWind, which has been plagued by poor performance of their Darrieus turbines for most of the decade, and the stellar performance of San Gorgonio Farms. The similarity in performance among Cannon, Kenetech, SeaWest, and Zond is surprising considering the differences between them in equipment, terrain, wind resources, and management styles.
Chart: Specific Yield in California by Selected Operator. See WindStats Volume 8, No. 4, Autumn 1995
Since 1989 the specific yield of turbines operated by San Gorgonio Farms atop Whitewater Hill have hovered around 1,200 kWh/m2/yr. Whitewater Hill is widely regarded as one of the best sites in California and illustrates the performance that can be expected from large arrays of well performing turbines on good sites.
Table: California Installed Capacity 1994. See WindStats Volume 8, No. 4, Autumn 1995
Yields of Selected Turbines in Southern California
With the exception of the WEG turbines in the Altamont Pass, few arrays have produced yields in excess of 1,100 kWh/m2/yr. The lower wind speeds of Northern California have brought in yields of 800-900 kWh/m2/yr at best, that is during good years.
The generally higher winds of Southern California sites coupled with their generally newer, more reliable turbines consistently produce yields of 900 kWh/m2/yr to in excess of 1,100 kWh/m2/yr during above average wind years.
Zond’s Sky River site is reputedly the windiest site in the Tehachapi-Mojave wind resource area. The site is windier than Zond’s Victory Garden in the Busee Hills of the Tehachapi Pass proper. The difference is apparent in annual yields. The 342 Vestas’ V27 turbines at Sky River deliver specific yields 10-15% higher than the 99 Vestas V27 turbines at Victory Garden.
Chart: Specific Yield of Selected Turbines in Southern California. See WindStats Volume 8, No. 4, Autumn 1995
The performance of the Mitsubishi turbines at SeaWest’s Mojave site clearly match that of the Vestas turbines at Victory Garden. Analysts have long marveled at the surprisingly good winds at the SeaWest site which lies downwind of the steep escarpment of Cameron Ridge. In 1994 the yield of the 660 Mitsubishi 28-meter machines was nearly 9% greater than the Vestas turbines at Zond’s Victory Garden.
The 35 Vestas DWT turbines on Whitewater Hill in the San Gorgonio Pass are among the most productive in California and probably represent an upper boundary on yields for large arrays on good sites in California of about 1,500-1,600 kWh/m2/yr. Though individual turbines and small arrays elsewhere in the world have performed as well as those on Whitewater Hill, there are no arrays the size of San Gorgonio Farms’ DWT project.
Selected turbines on the North Sea coast in Denmark and selected turbines in Spain have produced 1,500 kWh/m2/yr. A Vestas V27 in New Zealand produced 1,700 kWh/m2/yr during its first year of operation. One of the few arrays anywhere to rival San Gorgonio Farms operation of the DWTs on Whitewater Hill are the Nedwind 25 meter turbines at Tera Kora on Curacao in the Netherlands Antilles. The 12 turbine array generated 1,525 kWh/m2/yr during its first year of operation.
Yields of Selected Turbines in Northern California
The data from Northern California is much more erratic than that from Southern California. First, some of the turbines selected for examination experienced extensive repairs and modifications, notably the Howden turbines. Second, 1993 was an extremely poor wind year and this affected all wind turbines in the Altamont Pass and Solano County.
Of the five turbines examined, Kenetech turbines exhibited the most consistent performance. This is especially significant considering the number of Kenetech turbines operating in Northern California: 4,500 by 1994. Kenetech performance, mostly that of its workhorse 56-100 model, climbed steadily during the late 1980s, improving about one-third from 1986 to 1988. Performance has held steady with the exception of 1993, a poor wind year.
The Wind Energy Group’s 20 turbines operated by Kenetech outstripped all others in Northern California during good years. But yields of WEG’s MS2 turbines have been inconsistent, reflecting either disputes with Kenetech or problems with the turbines.
The Howden turbines demonstrate that bigger isn’t always better. The 33 meter Howden design represented the largest wind turbines operating in California from 1985 until installation of the Vestas DWT turbines in the San Gorgonio Pass in 1990. Reliability is a key factor in overall performance and, despite their size, the Howden turbines have not performed any better than Kenetech’s 18-meter 56-100 model, the 20-23 meter Bonus turbines, and the 25-meter WEG MS2. Only in 1990 and 1994 did the Howden turbines produce yields comparable to other designs.
The lesson from Howden may be that it’s difficult to maintain reliability if designers push the technology too far too fast. In the mid 1980s when the Howden turbine was introduced, most European manufacturers were building turbines in the 19 meter size range. Howden, with no previous wind turbine manufacturing experience, jumped into the market with its 31 meter design after scattered testing of a somewhat smaller prototype. The original Howden 330/31 machine swept 2.5 times more area than Danish machines of the era. After extensive–and expensive–modifications in the field, the rebuilt Howden 330/33 swept 3 times more area than other machines of the day. The performance of the Howden turbines still lagged the industry even after the repairs. It wasn’t until 1990, half a decade later that turbines of the Howden size were installed in California in any number. This was a costly lesson for Howden, forcing it from the market. As Kenetech starts testing its 46 meter prototype in Texas and Zond begins assembling its new 46 meter model in Tehachapi, the Howden lesson should not be forgotten.