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CAMBRIDGE, Mass. -- State regulators are required to develop formal plans describing the monitoring programs, emissions standards and other measures they will undertake to ensure that their regions meet federal clean air laws. Now an MIT study can help them obtain reasonable estimates of the emissions reductions they could achieve by including solar-generated electricity in their plans.
Researchers at MIT's Laboratory for Energy and the Environment's Analysis Group for Regional Electricity Alternatives (AGREA) used information from numerous databases, including that of the U.S. Environmental Protection Agency, to look at how emissions reductions from solar photovoltaic systems vary across the contiguous 48 states.
The researchers examined hourly electricity generation from photovoltaic systems and matched it with the per-kilowatt-hour emissions from fossil-fuel-fired units in the same power grid, in the same hour. To make their "avoided emissions" estimates more accurate, they used EPA's generation and emissions data from 1998 to 2002 to calculate emissions rates from those fossil units that respond to changes in electricity demand, including changes due to the operation of small sources of generation such as photovoltaic systems and small and moderately sized wind farms.
Their results confirm that emissions reductions from "nondispatchable" resources such as solar and wind generation and electricity conservation are highly dependent on exactly where and when they are used.
For example, in Texas, photovoltaic systems displace more pollutant emissions in winter than they do in summer, even though the photovoltaic systems generate more electricity in summer. The explanation: in summer, Texas brings on more expensive, cleaner natural-gas-fired methods of generating electricity to meet air-conditioning needs, so the first units to respond to the solar-generated electricity are the cleaner ones.
Another comparison shows that a photovoltaic system used in the Southwest will produce 30 percent more kilowatt-hours than similar systems in the Ohio Valley will. However, the resulting reduction in annual sulfur dioxide emissions will be greater in the Ohio Valley because coal-fired power plants in that region use coal with higher sulfur levels. Both observations demonstrate the importance of putting future renewable systems such as solar and wind not only where it's sunny or windy but also where electricity production is dirty.
The study was conducted by graduate students Michael Adams and Katherine Martin of MIT's Engineering Systems Division, assisted by AGREA director Stephen Connors and LFEE research engineer Edward Kern.
The MIT group is looking to extend this work to emissions reductions from wind power and electricity conservation. With estimates of where and when wind power is being generated or where energy savings from conservation programs occur, the researchers can easily calculate the emissions savings there.
Other AGREA activities include continuing to help the Mexico City Program design integrated emissions-reduction strategies and undertaking a new project with universities in Norway and Sweden to identify viable ways of moving to a sustainable energy future.