Integrating renewable energy sources into the grid

Carnegie Mellon University (CMU) Professor Jay Apt has developed an extensive body of work looking at how renewable energy sources can be better integrated into the existing electricity grid.

Writing in MRS Energy & Sustainability — A Review Journal, Professor Apt presents a summary of research carried out during the four-year RenewElec project by a team of technical and policy experts at CMU, the University of Vermont, Vermont Law School and environmental law firm Van Ness Feldman.

Focusing on the situation in the US, Professor Apt argues that increasing the market share of variable renewable electric power generation from the present 4% is both technically and economically feasible. He examines research results in several areas that could help to facilitate the large-scale integration of variable power sources into the electric power system, both in the US and around the globe.

“The amplitude of variability of wind and solar power is much less at high frequencies than at low frequencies, so that slow-ramping generators such as combined-cycle natural gas and coal can compensate for most of the variability,” said Professor Apt.

The research findings include the following:

• The seasonal variability of wind power is beginning to be understood, as are the sources of error in its day-ahead forecasts.
• Geographic aggregation of wind and solar power has long been proposed as a method to smooth their variability, but for wind power, it has been shown that there is little smoothing at timescales where the magnitude of variability is strongest.
• It has been shown that the point of diminishing returns is reached after a relatively few wind plants have been interconnected.

The costs of interconnecting renewable energy sources are likely to be higher than building new natural gas combined-cycle plants, the research shows. Large new investments in transmission systems designed to interconnect large areas of the country are neither required nor desirable to decrease the variability of electric power generated from wind, Professor Apt stated, since there are companies now shipping batteries with the aim of expanding utility-scale electric energy storage.

Professor Apt claims that while there are good prospects for lower cost electric storage for grid application, the profitability of storage for integration of renewable power is likely to remain a difficult issue. The good news is that new, efficient, low-pollution and fast-ramping natural gas plants have come on the market. So it is now possible to predict the amount of additional capacity of this sort that must be procured by system operators to cover the uncertainty in wind forecasts.