If you're still debating whether renewable energy will become a utility-scale asset and critical to this country's power needs, you're tilting at windmills.

The issue on the table is how to integrate variable resources with the power grid in the most efficient and reliable manner. What are the challenges, both technically and in terms of policy?

I'll touch on three developments in this area, one from the Massachusetts Institute of Technology (MIT), one from the Electric Power Research Institute (EPRI) and another from the Utility Variable Generation Integration Group (UVIG). (You may know the UVIG as the UWIG, but that's changed for the very reason we're discussing RE integration in today's column.)

Last year at this time, MIT held a symposium on large-scale integration of renewable energy and earlier this month it held a panel on the release of the symposium's report. (Click on the link to access Web video of the March 12 discussion of the report, and the report itself.)

In a nutshell, the MIT effort brought together subject matter experts in generation, transmission and regulation to discuss the impacts of large-scale renewables integration. Although many systems are handling the challenge, MIT noted that "there is limited system-wide analysis of how the deployment of large-scale renewables physically affects conventional thermal plants, the limits of their capabilities for such accommodation and the degree to which the integration of renewables is changing the physical and economic operations of power systems."

Yes, take a deep breath. The MIT report really sets up the issue well, however, providing some clarity to a potentially daunting subject. In 29 states and many countries, policies have encouraged renewable generation but "as renewable capacity has increased, the intermittent nature of wind and solar generation ... has led to operational difficulties and unintended consequences for emissions and economic efficiency."

The characteristics of intermittency and the need to balance supply and demand on the grid is creating a need for system flexibility to meet reliability goals and policy mandates such as renewable portfolio standards (RPS), the MIT group said in an "issues summary."

"In the absence of economically viable, large-scale storage, the burden of maintaining system reliability will fall mostly on the flexible operation of thermal generation units such as coal, natural gas and nuclear [generation]. However, the ability of these plants to operate flexibly is limited by both physical plant constraints and economic profitability considerations."

The impacts on regulation vary from the "economic dispatch in the short term to generation capacity investments in the long term," according to the issues summary.

"Ensuring the adequacy of the regulatory structure is an extremely complicated undertaking. In the U.S., the regulatory landscape for market rules and renewable policies is fractured and complex; planning and policy making for electric power systems occur at the state, regional and national level."

The MIT group framed five key topics:

  • Emissions: The integration of intermittent renewables forces the existing system to adapt and less efficient ramping and cycling of traditional generation can reduce the low-emissions contribution of those renewables.
  • Unintended consequences: RPSs can change market structures. Policies, markets and system requirements impact system planners and market participants, potentially raising the cost of the generation mix.
  • Future generation mix: What does a well-adapted generation mix look like? And what types of regulations would support units that contribute to reliability but may have low-utilization rates?
  • Electricity markets: Should the market treat renewables as any other generator, subject to scheduling penalties?
  • Regulation: The current regulatory system encourages cost reduction and reliability, not innovation. This may be inadequate to incentivize the development of new transmission and generation technologies required to fully enable large-scale renewable generation.


In practical terms, baseload generation—mostly coal and natural gas—must be cycled up or down to accommodate intermittent renewables, which leads to shorter lifetimes for components due to the physical stress of temperature changes, according to Howard Herzog, a senior research engineer at the MIT Energy Initiative. Cycling also reduces a plant's efficiency, which incurs higher fuel costs and, ultimately, to higher costs per kilowatt hour. Emissions equipment also doesn't run as efficiently at low loads, Herzog said.

One solution will be to find improved materials to withstand temperature fluxes. Another: better understanding how cycling can be done more efficiently. For instance, in France where 80 percent of generation comes from nuclear reactors, one-third the fleet has been modified for load-following of intermittent sources. However, cycling can introduce human error, an undesirable element in the case of nuclear power.

"Natural gas—in the absence of affordable storage—remains the technology of choice for integrating large-scale renewable energy resources," Herzog said. 

In related news, EPRI's just-released "Generic Models and Model Validation for Wind and Solar PV Generation: Technical Update" is an ongoing effort to answer "a need for generic, standard and publicly available models for variable generation technologies for the purpose of power system planning studies," according to the update's abstract. Frankly, generic models and model validation go beyond my current knowledge, but I get that modeling and system planning are among the nuts-and-bolts issues in RE integration, especially as utilities attempt to meet RPSs.

Lastly, UWIG has become UVIG. Yes, that took place last October, but I wasn't paying attention and even UWIG acknowledged that this change in scope will take time (i.e., most of 2012). The change reflected the growing need to accommodate increasing solar power on the grid and the potential for other variable energy resources such as wave and hydrokinetic power to assume greater importance. UVIG has worked for more than three decades to be a forum for solid engineering and operational practices in renewables integration at utility scale. 

Phil Carson
Intelligent Utility Daily