Austin Energy’s GreenChoice® Program
Austin Energy's GreenChoice is the nation's most successful utility-sponsored
green energy program. Most of Austin Energy’s green power comes from wind
turbines in McCamey and Sweetwater, Texas, which have been operating since
summer 2001 and December 2005, respectively. More turbines should be online
soon. Austin Energy also receives electricity from several solar installations and
three landfill gas projects in Austin and San Antonio.
With 753 million k Wh in subscriptions, GreenChoice is currently fully subscribed,
and Austin Energy is not taking new applications at this time. However, the
company plans to make more GreenChoice energy available at long-term fixed
rates in January 2009.
When customers subscribe to GreenChoice, Austin Energy contracts for green
power to meet their needs. The green power reaches Austin over the statewide
transmission system. Once it enters Austin Energy’s system, it mixes with power
from the generating plants. This means the electricity generated from ‘green’
sources is not being directed to a specific home or business. Rather, as more
customers subscribe to GreenChoice, the proportion of green power in that
mix grows larger and larger.
Source: Austin Energy
would mean that 6 percent of the campus’s yearly electrical needs would be
supplied by wind energy.
The basic assumption is that this
6 percent is clean, renewable, green energy
and that, once established, is energy generated without emissions. Disregarding
life-cycle costs of the wind turbines, and
when fully integrated into a large-enough
power grid, this assumption holds true.
When evaluated over smaller grids such
as the Austin Energy grid, and especially
the university grid, there are two drawbacks to this assumption:
Variations in wind energy generation
require the use of low-efficiency ‘peak-
ing units’ to offset sudden fluctuations.
The purchased energy would cause the
UT CHP plant to operate at times below
the designed baseload generation, caus-
ing overall plant efficiency to suffer.
These drops in efficiency in no way
fully counteract the environmental benefits
of wind energy. However, when paying the
higher premium for renewable energy, it is
important for the university to fully consider the environmental impact of wind
energy and not to simply assume it is 100
percent clean energy.
It is important for the university
to fully consider the environ-
mental impact of wind energy
and not to simply assume it is
100 percent clean energy.
Large-scale wind farms are able to predict
wind levels a day in advance with up to
94 percent accuracy. The remaining 6
percent represents ‘noise’ in the energy
capability of the wind farm and must be
offset by some other energy source in the
grid. On a large-enough grid, the distribution of large-scale combined-cycle power
plants can handle a majority of these
variations. However, in a smaller grid such
as that maintained by Austin Energy,
these abrupt swings require faster, near-immediate offsets.
If there are sudden drops in wind-generated power, Austin Energy fills in the
gap with a variety of reserve sources –
ramping up coal and natural gas plants,
purchasing electricity off the Texas grid
as regulated by the Electric Reliability
Council of Texas (ERCOT) and bringing
natural-gas peaking units online. Short-term purchases of ERCOT electricity are
typically sold at a high premium and
avoided if possible. During the summer,
the coal and natural gas plants are baseloaded and unable to account for large
variations in wind power, leaving peaking
units as the most cost-effective option.
During the winter, coal plants account for
part of the variations in wind.
Each of these options is referred to as
electricity ‘reserves’: power sources that
can handle unexpected fluctuations on the
grid. As the total capacity of wind energy
increases on a grid, so does the utilization
of these reserves, both in frequency and
duration. Figure 1, derived from a National
Renewable Energy Laboratory case study
on the grid impact of wind power, demonstrates the increase in reserves usage
relative to the increase in total wind
capacity on the grid.
Austin Energy utilizes single-cycle
aero-derivative gas turbines for peaking
units, which can come fully online in 15
minutes. Compared to a combined-cycle
unit, these units have a much lower effi-
Figure 1. Grid Capacity Impact on Reserves.
Percentage Increase in
Use of Reserves
Offsetting Wind Variations
The inherent instability of wind results
in rapid fluctuations of power output.
Source: National Renewable Energy Laboratory case study