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Hourly operability on a microgrid

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The dynamic nature of variable resources presents challenges to conventional electricity systems operations, especially on a small grid. But if a microgrid has sufficient dispatchable supply- and demand-side resources and storage capacity, these can enable the integration of a large percentage of local variable renewable energy. Grid power can play a supporting role in balancing the microgrid, complementing the onsite resources, and allowing demand to be met throughout the year.

In this example, RMI used an hourly dispatch model to model the supply from variable renewables and to dispatch generation and storage resources to meet demand in all hours of the year. With approximately 17 MW of peak demand in August, this Texas-based microgrid has typical diurnal demand swings, largely driven by afternoon space cooling loads. After applying savings from energy efficiency measures, however, the “load to meet” is reduced to around 11 MW at peak, and is somewhat smoothed. The microgrid has some local capacity of rooftop solar photovoltaics and distributed wind power, as well as some battery storage and a small fleet of electric vehicles. The primary dispatchable generation on this microgrid is from a combined heat and power (CHP) plant, which typically generates only during the day when the power and process heat are most needed. All of these generation and storage resources interact to match supply and demand throughout the day, and to continue to provide power even in the case of grid or local outages. These outages are modeled as lasting a full day—much longer than the typical grid power disturbance—and there is still sufficient storage and generating capacity to meet all loads at these times. (Danish grid operators, who use a similar “cellular” pattern of grid organization, routinely stress-test their microgrids by isolating them from the surrounding grid for a full day to ensure that critical loads are still met.)

This microgrid illustrates the need for a full suite of resources—dispatchable generation, demand response (both load-shifting and load-reduction), energy storage, and a viable grid connection—to balance the output from distributed variable renewables. Balancing a microgrid's resources is nearly identical to that of the full grid’s, except for two main differences. First, combining variable renewables and demands over a larger balancing area reduces fluctuations, and second, the unspecified “grid power” resource modeled here would be replaced with specific dispatchable generation capacities.

Sources

RMI analysis using data from:

Electric Reliability Council of Texas. 2004. “FERC Form No. 714-ERCOT.” link

GE Energy. 2010. Western Wind and Solar Integration Study. Report prepared for the National Renewable Energy Laboratory. link

National Renewable Energy Laboratory. 2011a. National Solar Radiation Data Base 1991–2005 Update. National Renewable Energy Laboratory. link