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Published: 2009

Total Pages: 15

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Katzenstein and Apt investigate the important question of pollution emission reduction benefits from variable generation resources such as wind and solar. Their methodology, which couples an individual variable generator to a dedicated gas plant to produce a flat block of power is, however, inappropriate. For CO2, the authors conclude that variable generators 'achieve (almost equal to) 80% of the emission reductions expected if the power fluctuations caused no additional emissions.' They find even lower NO(subscript x) emission reduction benefits with steam-injected gas turbines and a 2-4 times net increase in NO(subscript x) emissions for systems with dry NO(subscript x) control unless the ratio of energy from natural gas to variable plants is greater than 2:1. A more appropriate methodology, however, would find a significantly lower degradation of the emissions benefit than suggested by Katzenstein and Apt. As has been known for many years, models of large power system operations must take into account variable demand and the unit commitment and economic dispatch functions that are practiced every day by system operators. It is also well-known that every change in wind or solar power output does not need to be countered by an equal and opposite change in a dispatchable resource. The authors recognize that several of their assumptions to the contrary are incorrect and that their estimates therefore provide at best an upper bound to the emissions degradation caused by fluctuating output. Yet they still present the strong conclusion: 'Carbon dioxide emissions reductions are likely to be 75-80% of those presently assumed by policy makers. We have shown that the conventional method used to calculate emissions is inaccurate, particularly for NO(subscript x) emissions.' The inherently problematic methodology used by the authors makes such strong conclusions suspect. Specifically, assuming that each variable plant requires a dedicated natural gas backup plant to create a flat block of power ignores the benefits of diversity. In real power systems, operators are required to balance only the net variations of all loads and all generators, not the output of individual loads or generators; doing otherwise would ensure an enormous amount of unnecessary investment and operating costs. As a result, detailed studies that aggregate the variability of all loads and generators to the system level find that the amount of operating reserves required to reliably integrate variable resources into the grid are on the order of 10% of the nameplate capacity of the variable generators, even when upto25%of gross demand is being met by variable generation. The authors implicit assumption that incremental operating reserves must be 100% of the nameplate capacity of the variable generation, and be available at all times to directly counter that variability, excludes the option of decommitting conventional units when the load net of variable generation is low. In real power systems, generation response to wind variation can typically be met by a combination of committed units, each operating at a relatively efficient point of their fuel curves. In the Supporting Information, we conceptually demonstrate that the CO2 and NO(subscript x) efficiency penalty found by the authors can be significantly reduced by considering the unit commitment decision with just five plants. Real systems often have tens to hundreds of plants that can be committed and decommitted over various time frames. Ignoring the flexibility of the unit commitment decision therefore leads to unsupportable results. Anumber of analyses of the fuel savings and CO2 emission benefits of variable generation have considered realistic operating reserve requirements and unit commitment decisions in models that include the reduction in part load efficiency of conventional plants. The efficiency penalty due to the variability of wind in four studies considered by Gross et al. is negligible to 7%, for up to a 20% wind penetration level. In short, for moderate wind penetration levels, 'there is no evidence available to date to suggest that in aggregate efficiency reductions due to load following amount to more than a few percentage points'. As such, other studies using a more appropriate methodology have found a much smaller CO2 penalty from variability than found by Katzenstein and Apt. Less information is available on the NO(subscript x) emission penalty. Results from recent state-of-the-art integration studies in the United States indicate at the very least clear NO(subscript x) emission reduction benefits from variable generation. NO(subscript x) reductions estimated in these studies are discussed in more detail in the Supporting Information. Denny and O'Malley similarly find NO(subscript x) reduction benefits when forecasting is used in the unit commitment decision.