Most analyses and reviews of utility-scale, highly intermittent new renewables, especially wind power which will be the focus here, are lacking in perspective. This makes marginal aspects appear to have significance out of proportion to the very little value they represent.
A few examples are:
· A focus on the energy contribution (MWh) from wind power leads to error in assessing the contribution to electricity costs, reliability, impact on fossil fuel consumption and CO2 emissions, transmission needs and the operation of an electricity system.
· The possibility of some improvements in wind forecasting. Given the current state of weather forecasting in general, it seems difficult to believe that wind can be forecast for short time intervals, say 24 hours in advance. In any event, even if such forecasting was possible, it does not change the need for balancing generation plants to be ramped frequently to mirror wind conditions.
· The use of statistical averaging over long time periods, which obscures the more important real time effects, to show low impacts of various aspects of introducing utility-scale wind plants.
Similarly in media coverage,
· The Economist Technology Quarterly review in the March 6, 2010, issue highlighted the use of a technology that will allow individual wind turbines to sense the upwind conditions and adjust blades accordingly. It concludes with, “The result is a system that can already improve electricity production by 5%.” What if increased electricity production from wind actually increases fossil fuel use and CO2 emissions, stresses the grid and other generation plants and leaves users more at risk when such production falls rapidly shortly thereafter?
· A Canadian national newspaper column proclaiming the virtues of new renewables over traditional electricity sources included a picture of a mass of electricity transmission towers to illustrate a disadvantage of existing technologies. What was not considered was that substantially increased grid deployment would be required to transport the output from new renewables in necessarily wide-spread locations to demand centers.
The result of these marginal, and often invalid, representations is to artificially make unfeasible new intermittent renewables, especially wind plants, somehow appear attractive.
To assist in providing a basis for the effective evaluation of these renewable sources, my recent article in the USAEE Dialogue proposes a broad framework to assess how the available electricity generation sources, both new renewables and those that are more traditional, meet public policy objectives. These objectives are suggested to be:
The article discusses how public policy in support of an emphasis on undependable renewable energy sources for electricity generation is in conflict with these societal goals. A more effective alternative is provided, which is a re-commitment to existing generation technologies, and is shown to be substantially more consistent with these goals, including combating one of the main public concerns, climate change. The funding required for undependable renewables would be better directed to research and development of advanced generations of existing, traditional technologies and of newer technologies that better incorporate the desired characteristics.
This is one of the topics discussed in the article and is re-emphasized here because of its importance. Capacity value, as defined below, is a rarely used, but vital, measure of wind plant performance. Some do use this term interchangeably with capacity credit. I have come to realize that it is necessary to make a distinction, and this article defines them separately. The following definitions are not universally accepted, but they serve here to make an important point. So, in discussions with those who use the terms differently, or interchangeably, the intended underlying meaning should be clarified.
Capacity credit is the statistical expectation of availability over time, which is useful primarily for capacity planning for a set level of system reliability. Most types of generation have capacity credits approaching 100%, offset only by scheduled and unscheduled maintenance. It is measured at peak demand. Because of its random nature and low general production over time, wind’s capacity credit is very low. To ensure high overall electricity system reliability it is less than 10% where wind provides in excess of about 1-2 percent of the total production (MWh) as reported by the German Energy Agency (dena).
Capacity value is significantly different. It is determined by the ability of an electricity generation means to be dispatchable when required, that is capacity available on a real time demand basis. We live, work and play in “real time”, and in many ways vital to our welfare, depend upon electricity to be available on this basis. It is therefore an important measure of the utility of any generation means. As for capacity credit, most types of generation have capacity values approaching 100%, a necessary condition. Wind plants have no capacity value because of their complete dependence upon instantaneous wind conditions. As a result, their electricity output can change rapidly and randomly over the range of 0-100% of capacity. This is the much misunderstood aspect of wind power that renders it ineffective as a utility-scale source of electricity.
Further exacerbating this is the fact that electricity output from a wind turbine varies as the cube of the wind speed. As a result, for a wind speed change of a factor of 2, the electricity output changes by a factor of 8. For wind speed changes of 3 times, the electricity output changes by 27 times.
Until such time as highly-intermittent, new renewables can meet the societal goals described above, care must be taken in otherwise promoting or reviewing their value in providing utility-scale electricity. Analyses and reviews that focus on (1) marginal issues like those described above, and (2) applications, such as electric car battery charging and extensive demand management schemes, neither of which will not be broadly applicable for decades, and in a form not necessarily predictable, do not serve as useful contributions to the discussion.
The full USAEE Dialogue article can be seen here.