Few big city utility departments are more aggressive when it comes to renewable energy than Austin Energy. The Texas power provider plans to make renewable energy not just a supplement to conventional generation, but a replacement to natural gas, oil and coal sources of generation. There’s a catch, however, and it has to do with when the wind blows. Load demand is greatest during the daytime and falls off at night. But the strongest winds in West Texas and the Panhandle region—where much of the state’s wind resource is located—blow at night when the load lowest.

“The load and the wind supply are totally out of sync,” said Mark Kapner, senior strategy engineer, for Austin Energy. He spoke as part of a presentation on renewable energy and storage options given at Renewable Energy World Conference and Exhibition North America in Las Vegas in February. Synchronization is not limited to wind energy. Kapner said that even solar can be non-coincident with the peak load profile. Last August 13, the most recent peak energy demand day, demand peaked late in the afternoon. By that time, Kapner said, solar intensity was already falling.

For any other utility, such a load and energy supply mismatch could be met by switching to, say, a simple cycle natural gas-fired peaking plant. But for Austin Energy, whose goal is to replace conventional fossil generation, the problem of making wind and solar dispatchable is not so easily addressed.

Kapner outlined the system being considered by Austin Energy to solve the problem. Its essential subsystems include:

• Wind turbines directly coupled to air compressors or conventional wind turbines to power a central air compressor on the ground
• A high-pressure ;pipeline system to collect compressed air from several turbines
• High pressure, large-diameter pipelines to move the compressed air from the wind turbines to the solar site, which would also contain an expander-generator
• A possible compressed air storage cavern
• Solar thermal collectors with integral thermal storage for heating the compressed air to 200 C, and
• A compressed air expander-driven generator.

Kapner said some of the advantages of this concept are a single, dispatchable prime mover-generator; a need only to connect the wind farm to the expander-generator, simplified wind turbine gear box requirements; and the elimination of natural gas waste heat and cooling water.

Also, because West Texas wind energy production tends to be higher in the winter and spring and solar in the summer, seasonal energy supply fluctuations may be evened out by combining and integrating the two sources.

Utility officials also sought to calculate how much more dispatchable energy is worth than intermittent energy. They assumed that the incremental value of dispatchability would serve as a good indicator of how much more someone would be willing to pay for an integrated energy system. Calculations showed that dispatchability via daily storage and eight-hour uniform generation is worth 25 percent more than conventional wind plant generation. The annual incremental energy value of dispatchability was calculated at $3.3 million. The utility also determined that the firm capacity achieved for the proposed system was just over 100 MW (the average daily generation in August divided by eight hours). Finally, by assuming a capacity value equal to the installed cost of a new gas turbine (roughly $500/kW), Austin Energy officials found this 100 MW of capacity would be worth around $50 million.

Nadav Enbar, a research analyst with Energy Insights, also spoke during the energy storage session. He called energy storage the “holy grail” of renewable energy because it solves issues related to resource intermittency and reliability. He said two hurdles to more widespread adoption of energy storage are “sticker shock” and technology conflicts.

He said many forms of energy storage are still not economical and present protracted return on investment profiles. As for technology conflicts, he said that, for example, load leveling and spinning reserve strategies can compete with one another and can potentially cannibalize revenue streams.—David Wagman

The June issue of Power Engineering magazine will feature an in-depth look at one of the leading forms of energy storage, compressed air energy storage.