Handling Wind Variability

Issue 4 and Volume 115.

By Dr. Michael Brower, Chief Technical Officer, AWS Truepower LLC

Often the first question people ask when they learn I work in the wind business is, “Is wind energy reliable?” This is usually followed by, “Don’t we need storage, and isn’t that very expensive?”

These are important questions, ones that power engineers are playing a key part in answering. While the answers depend on characteristics of the electric power system and the local wind resource, the overall conclusion is that wind energy can be used without storage and at moderate cost – typically 0.15 to 0.45 cents/kWh – up to levels of deployment far beyond the 1 percent of annual generation the United States consumes today.

Wind variability is a familiar part of our lives, from mild gusts that blow leaves around to storms that knock down power lines. The variations span time scales from seconds and less (the domain of turbulence) to years (the scale of coupled atmospheric-ocean oscillations such as El Niño).

One key question for power system planners and operators is, how do such fluctuations translate into variations in wind power feeding into the grid? Another is, how will the power system respond to them?

These two questions are the focus of power system analyses called wind integration studies. A number of such studies performed in North America and Europe over the past decade have begun to provide rigorous and largely consistent answers.

Three important elements in most wind integration studies include: the development of wind power expansion scenarios and data sets, the analysis of transmission needs and the modeling of wind impacts on grid operations.

The wind power scenarios typically envision wind penetrations of 10 to 40 percent of the total generating capacity on the power system. To determine the demand that must be met by non-wind power plants, time-synchronized load data and wind power outputs (based on weather simulation models and/or historical measurements) are necessary. The data must account for correlations between wind variations on different temporal and spatial scales, as these correlations determine the overall variability of the aggregated wind plant output.

Since some of the country’s best wind resources are located far from load centers, new transmission lines will be required for much of the future wind power. Because it takes longer to build new transmission than to build wind plants, it is imperative to plan ahead for this transmission.

Finally, the grid operation analysis reveals the costs, benefits and risks to reliability of the various scenarios. System reliability and stability may be analyzed for a wide range of time scales of interest to grid operators, from seconds to days.

The Oahu Wind Integration and Transmission Study (OWITS) examined a particularly challenging case. This study was sponsored jointly by the Hawaiian Electric Company, the state of Hawaii Department of Business, Economic Development and Tourism, and the U.S. Department of Energy. It was carried out by a project team that included General Electric, AWS Truepower, LLC, Electric Power Systems Inc., and Electranix.

A principle goal of the study was to assess the feasibility of bringing up to 400 MW of wind energy from the islands of Lanai and Molokai to Oahu — the state’s main load center — by means of undersea high-voltage direct current cables. This is to help satisfy the Hawaii Clean Energy Initiative (HCEI) target of generating 40 percent of the state’s energy from renewable resources by 2030. The study also looked at ancillary services (such as spinning reserves) and potential mitigation measures (such as storage and real-time wind forecasts) to offset the variable nature of the planned wind generation, as well as potential modifications to the utilities’ existing conventional generating units and operational practices and procedures.

The results of the study indicated that the proposed plan, although challenging from engineering and environmental permitting aspects, is technically feasible with the present system. Moreover, identified changes in operational strategies would both increase the amount of wind generation that could be absorbed by the system (without curtailing, or spilling, the wind generation) and reduce the annual variable operating cost.

The variability of wind energy is an important concern, but today’s power system studies can provide clear insight and direction to meet the technical challenges of fulfilling 20 percent or more of our power needs with wind energy.

Author: Dr. Michael Brower has conducted research and analysis on energy and environmental issues for federal and state governments, private business, and nonprofit organizations. He has authored or co-authored two books, “Cool Energy: Renewable Solutions to Environmental Problems” and “The Consumer’s Guide to Effective Environmental Choices.” As Chief Technical Officer of AWS Truepower, Dr. Brower is responsible for company-wide research and development and technical standards.

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