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-