GHG reductions from intermittents in actual use

November 8, 2009, at 5:50 AM, on a Sunday, Spain’s windmills supplied 53% of its electricity, and over half between 3 and 8:30 AM. OK, not the biggest demand portion of the week, but certainly an indication of Spanish trust in wind to reduce greenhouse gas emissions. Wind produced 11% of Spain’s electricity in 2008, and this year is expected to provide even more. According to an article in MIT’s Technology Review, “If Spain meets its goal of generating 30 percent of its electricity needs from renewable power by 2010, with half of that amount coming from wind power, it will reduce carbon dioxide emissions by 77 million tons”.

How much actual emissions reductions results when intermittent sources of electricity, solar and wind, use fossil fuel backup? The operating assumption has been that if wind supplies 10% of the power, greenhouse gas emissions from natural gas decline 10%.

An analysis in Environ. Sci. Technology, Air Emissions Due To Wind And Solar Power, examines this assumption.

Utilities match demand with supply. Demand varies during the day with work hours, temperature, etc. Power plants are turned on and off, ramped up and down, to accommodate shifting demand. Using intermittents complicates things, as now utilities must also follow sun or wind, sometimes quickly. Note: this study assumes natural gas backup, but some utilities use hydro as backup, and others use coal.

If a generator produces 2 tons of CO2 per MWh, averaging 10% solar plus wind over the year has been expected to cut emissions by 0.2 ton. If the reduction is only 0.1 ton, this is only 50% of expected reductions.

The use of solar and wind with natural gas backup was found to achieve 76-79% of expected GHG reductions, and 20-45% of expected NOx reductions at best (day time only, obviously for the solar). In some instances, NOx production of intermittent plus natural gas backup exceeded that of natural gas alone, because the natural gas was more often run at less than optimal power levels. The poor results for NOx indicate that relying on wind to help meet clean air requirements may be unsuccessful.

Running natural gas generators at suboptimal levels increases maintenance costs as well.

natural gas generator
This natural gas generator is used to provide peak power, rather than to run all the time. The report examines the effect of backing up intermittents on both efficient and peak natural gas generators.

How complicated is it to integrate intermittents into the grid?

A report from North American Electric Reliability Corporation (NERC), Accommodating High Levels of Variable Generation (pdf), discusses this issue at some length. One problem is that wind declines during heat waves, down to 5 – 10% of nameplate capacity during a recent California heat wave (compared to capacity factors averaging over 30% in the US).

While error in forecasting demand is normally 3% and unlikely to be more than 10%, wind forecast could readily be 20%, or as much as 100%. Experience in Texas shows that wind output can decline dramatically in 1 – 2 hours.

Because wind is stronger during hours of low demand, the use of wind requires a more rapid ramping of non-wind sources. In one example, without wind, conventional sources need to ramp from a low of 9,600 MW to a high of 14,100 MW, or 4,500 MW. Wind lowers nighttime use of conventional sources to 7,000 MW. To increase to 13,600 MW (plus some wind power) requires 6,600 MW ramping capability.

While solar power is greater when wind tends to be low, under some weather conditions, photovoltaics (solar panels) can change output by ± 70% in 2 – 10 minutes, several times each day.

Future analysis, or technology change, may alter these results. For now, wind and solar perhaps should be credited with only 75-80% of expected greenhouse gas reductions based on capacity factor.

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