Performance - Wind Energy: Renewable Energy and the Environment - page 150

Generator,
Pump, Other

Transducer Load

FIGURE 8.1 Schematic diagram of wind energy conversion system.

Connect time: The connect time, or energized hours, is the amount of time or percent of time
the unit was actually generating power. In the Texas Panhandle, a typical unit should be generating
power around 60% of the time. This is a large number and can be put into perspective by compar-
ing wind turbines to automobiles (a mature industry). Suppose your car went 160,000 km (100,000
miles) with no maintenance. At an average speed of 50 km/h, that is only 3,200 h of operation,
which is equivalent to just over a half a year operation for a wind turbine.
Lifetime: Wind turbines are designed for lifetimes of 20-30 years. This can be done within the statis-
tical lifetime for the components [1] and with preventive maintenance. Some components, such as bear-
ings in gearboxes, will have to be replaced within that time period. As noted above, 25 years of operation
for a wind turbine would be equivalent to 8,000,000 km for a car, so there will be some major repairs.
Jamie Chapman, who was with U.S. Windpower at the time, made the following statement,
“Estimated minimum standards for nonroutine maintenance are one down tower per 5 years and one
up tower per year.” Down tower means that the nacelle or rotor had to be removed, a major problem.
Some first-generation wind turbines had quite a few problems, and those units were replaced within
5 years or dismantled. Others had major retrofits. Some of the early wind farms in California began
replacing the 50-100 kW wind turbines with megawatt-size wind turbines, starting in 1998, which
is called repowering. The smaller wind turbines were then refurbished for the distributed market.
Design of generators and gear trains is well known. Loads produced by the rotor are the major
unknown factor, especially loads due to the turbulent character of the wind, stochastic loads. As the
industry matured, engineers designed blades, gearboxes, and generators specifically for wind tur-
bines. Airfoils have been designed for horizontal-axis wind turbines with characteristics to improve
overall performance for increased energy production.
Reliability: Most of the first-generation wind turbines [2] suffered from a lack of reliability and
quality control. Prototypes generally have failures within the first few months. Lack of reliability
means larger maintenance and operation costs after installation. Manufacturers and dealers were
caught in a bind, as retrofit programs in the field cost a lot of money. The most successful wind
farms are those that have reliable wind turbines and a good operation and maintenance program.
If a dealer has to service a small wind system more than one time during the first year of war-
ranty, he has probably lost money. Typical service charges are $60/h or greater, and a large service
area means the dealer is spending most of his time on the road. As the cost of gasoline increases,
service charges will increase.
Specific output: The most important factors for determining the annual energy production
are the wind regime and the rotor swept area. One way to compare wind turbines is by annual
specific output, kWh/m 2 . Stoddard [3] tabulated some data for wind turbines in California,
where the best values were 1,000 kWh/m 2 . This still does not take out the factor of the wind
regime; however, if the average of a large number of units is compared for similar locations, it
will give a good estimate of performance. The annual kWh/(weight of rotor or weight on top
of the tower) gives an idea of the goal for cost comparisons, as for a mature industry, the costs
would be based primarily on $/(weight of material). Another specific output is kWh/kW; how-
ever, it is not as useful.