Performance - Wind Energy: Renewable Energy and the Environment - page 157

The influence of wake effects on energy production [13] was estimated using data from met
towers at the northwest and east of the Horns Rev wind farm and from the SCADA database,
which contains all observed data for each wind turbine. For the majority of selected cases,
the wind turbines were operating at high wind speeds. An analytical model links the small-
scale and large-scale features of the flow in the wind farm with equidistant space between units
within a row and equidistant space between rows. For wind perpendicular to the row, there is a
large power drop between row 1 and 2 (around 30%), and then there is less power drop between
subsequent rows, which is almost linear. From row 2 to row 9 the power drop is around 10-15%.
For winds along the diagonal, the spacing is 9.3D; however, this gives only three lines with eight
turbines. At wind speeds of 9-10 m/s there is a large power drop (25-35%) from line 1 to line 2,
a slight drop in power from line 2 to line 5, and then essentially a constant drop from line 5
to line 8.
In the final analysis of performance, the issues are energy production, the return on investment,
and the value of that energy, which should include externalities. Capacity factors give an indication
about the wind regime and the relation between rotor area and generator size. However, the main
measures of performance should be annual energy production and average specific output, kWh/m 2 ,
per turbine type and model. Wind class should also be included as a check on comparison of per-
formance of wind turbines.

8.5 ENERTECH 44
A long-term performance test of an Enertech 44 [14] provided monthly values of energy produc-
tion, connect time, availability, and wind speed. The variation of power by month and year is shown
in Figure 8.7. Connect time, which is the time the unit is connected to the grid, is around 60%
( Table 8.3 ) , or over 5,000 h per year. From 1989 to 1996, when the unit was rated at 40 kW, it aver-
aged 78,000 kWh/year.
The prototype wind turbine (induction generator, constant rpm, stall control) was installed at
the USDA-ARS, Bushland, Texas, in May 1982. All three models had the same size rotor, 13.4 m
diameter. The original turbine was a 240 V, single-phase induction generator, rated capacity of
25 kW. The gearbox and generator were changed to a three-phase, 480 V, 40 kW induction genera-
tor ( Table 8.4 ) in 1984, and later that year, a gearbox and a three-phase, 480 V, 60 kW induction
generator were installed. In July 1988 the gearbox was replaced with the previous 40 kW gearbox,
making the rated power closer to 50 kW.

Jan
Feb
Mar
Apr
May
Jun
Jul

30

25

20

15

Aug

10

Sep
Oct
Nov
Dec

5

0

1985 1986 1987 1988 1989 1990 19911992 1993

FIGURE 8.7 Average power (kW, legend on right) by month for Enertech 44.