Environmental issues for windpower developers gel the most press. But equally persistent is the question of intermittency-the fact that wind is the least predictable energy fuel for electricity production. The question is challenging on many levels for both generation and delivery.
Maintaining the Margin
A thermal plant may be able to change and maintain output levels very precisely, but wind probably won’t blow at, say, a required 25 miles per hour for a particular length of time. In fact, on a minute- by-minute basis, electric generation from a wind facility can fluctuate enormously.
To smooth out those fluctuations, there must be additional instantaneous generation-that is, regulation service. Only powerplants that can precisely follow these fluctuations will do. For instance, if you expect to have 50 megawatts of wind capacity in the next hour but the wind is not blowing, you need to have 50 megawatts of something else instantaneously available. As wind generation increases as a percentage of total electric generation, this need will also increase. That means having several large powerplants available with the capabilities to provide regulation service.
Plus, on a broader basis, wind speed and duration often do not complement system peak demands. For example, the wind may not blow on very hot summer days. Or the windiest time of day may be at night when low-cost energy from thermal power plants is widely available. In many regions, wind generation is "richest" during the blustery days of spring and fall, when heating and cooling loads ebb.
But to ensure economic use of generation assets, owners generally conduct plant overhauls and other major maintenance programs during spring and fall. With these plants unavailable, system operators have two choices to ensure reliability and account for wind’s intermittency-buy energy or reserves from elsewhere on the grid, or alter their plant maintenance schedules, thus affecting the ability to meet peak demands in other seasons. Again, as wind energy increases in proportion of total production, the increasing need to smooth out these fluctuations-minute-by-minute and over time-may significantly affect maintenance schedules for thermal plants.
Still, it is possible to redispatch other generation resources. But utilities and other system operators dispatch powerplants to ensure that customers receive the benefit of least-cost use of the generation fleet and delivery networks. Redispatch increases total production costs, thus raising traditional cost-allocation and – recovery issues to a much more prominent level, especially where wind generation becomes a large proportion of overall electric generation. In some regions, wind penetration is likely to accelerate needs to address long-term resource adequacy issues.
lntermittency is also an issue for wholesale transmission service, where wind energy is usually not a network resource. Transmission customers must reserve and schedule service in advance, under the Federal Energy Regulatory Commission’s open access transmission tariff. So, if actual wind generation does not match schedules, transmission providers may impose an energy imbalance charge on the transmission customer. This charge serves as a proxy for costs that other generators incur when they must pick up the slack and meet those real-time load requirements. And this charge can be as high as $100 per megawatt-hour. Since hour-to-hour wind generation is difficult to predict on a day-ahead basis (the usual scheduling requirement), such penalties can sap a wind project’s economic viability. In addition, various types of generation reserves-replacement, contingency, spinning, nonspinning-must be available to respond over longer periods of time.
From a generation and transmission planning perspective, system planners still can’t accurately estimate the extent to which wind resources can provide capacity to comply with reliability requirements. It’s true that in some regions and some specific sites planners can rely on wind during periods of system peak demand. Plus, wind and weather forecasting models have advanced rapidly and can measure more precisely the profiles of actual performance. Nevertheless, planners must meet reliability planning and operating requirements, state-based capacity reserve margins, and other resource adequacy requirements. The consequences of a reliability failure provide strong incentive for conservative planning approaches.
A regional transmission organization (RTO) can mitigate those problems in various ways. The PJM RTO, for example, already has specific provisions for wind farms in its generator interconnection and market rules. Moreover, where states have renewable portfolio standards (RPS), many utilities "cover" intermittency-related RPS and reliability compliance costs in state-approved bundled retail service tariffs for purchased-power agreements and resource planning approval processes.
But the main point is that intermittency has costs. Policymakers will need to balance two basic goals:
* ensuring cost recovery for actions taken by utilities to comply with renewable energy goals and reliability requirements; and
* determining how industry participants will share in the costs of integrating wind energy into the nation’s supply infrastructure.
Windpower is coming on strong. Generation and transmission planners anticipate several thousand megawatts over the next few years, so it is critically important to reach solutions soon.
Dave Dworzak, manager for transmission policy Edison Electric Institute