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Variable renewable output (hourly)


The dynamic nature of variable renewable resources presents challenges to conventional electricity system operations. Production from wind and solar resources, in particular, is both variable (fluctuating throughout the day according to availability of the “fuel”) and uncertain (weather forecasting is required and by definition is not always accurate). The variability and uncertainty introduced by variable generation pose challenges to the traditional electricity grid at various levels—from grid connection to systems operations to resource planning.

Connection of variable generation to the grid often raises system stability questions. Does the generator have adequate control over the quality of injected power? Can the generator act to help moderate grid frequency and voltage? Will the generator respond safely in a grid outage?

Systems operations are often broken into three tasks with different timescales: regulation (seconds to minutes), load-following (minutes to hours), and unit commitment (hours to days). At the system regulation level, the system operator is concerned first and foremost with power quality and system health and stability. Typically, wind generators have very low variability on this timescale and do not pose problems for system stability. Solar photovoltaics, however, can have rapid changes in output as cloud cover shifts, and under certain conditions this may introduce a challenge to grid stability if the system is not properly designed. On a longer timescale, the system operator ramps the output of dispatchable generators to meet the changing electricity demand throughout the day. Variable generators such as wind and solar are not dispatchable; sometimes they may provide power in excess of demand, at other times they may not be generating at all. This often results in steeper ramps and lower turndown levels that other generation sources have to meet. As a result, a system with increased variable generation will value flexibility of both supply and demand side resources in order to balance the system. Over hours and days, the system operator plans ahead for the required quantity of generation and reserves needed to meet demand. Any uncertainty in the output of variable generators necessarily affects these unit commitment decisions, leading to either overcommitment of unneeded reserves (added cost), or undercommitment (possible grid outages).

The integration of variable generators into the electricity system presents challenges to resource and operational planning. As compared to a fossil-fueled plant with a firm capacity, it can be more difficult to accurately plan for the scale-up of transmission networks and needed reserve dispatchable generation capacity. Additionally, the future cost structure for renewable variable generators is highly uncertain: factors such as production tax credits, renewable portfolio standards, and utility integration fees are all undetermined.

Stylized chart based on data from ERCOT dispatch simulations using projected 2050 Transform loadshapes and actual summer solar and wind data.


RMI analysis based on:

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

GE Energy. 2010. Western Wind and Solar Integration Study. National Renewable Energy Laboratory. link

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