Every day, the U.S. grid's 143 million commercial, industrial and residential customers flip on and off their light switches and TVs, turn up their air conditioning, or ramp up production from their assembly line. While the demand from any individual customer has discrete jumps as loads are powered on and off, the diverse timing of demands from millions of customers means aggregated load for an entire regional grid is relatively smooth.
On the other end of the power lines, utilities and grid operators have the task of providing safe and reliable power all day, every day. Because electricity cannot be easily and cheaply stored in bulk, a variable demand for electricity throughout the day means the utilities must produce a variable supply of electricity as well—relying on a portfolio of different generating resources that have different technical capabilities and costs.
The dynamic nature of variable 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). However, with good resource and demand forecasting and high availability of flexible demand and supply side resources, it is possible to operate an electricity system reliably with a high percentage of variable renewable energy. In this example of a renewables-heavy grid, natural gas is the only “conventional” dispatchable resource.
Energy efficiency measures can reverse peak demand growth and help smooth out the demand that must be met. While renewables like wind and solar are not always correlated with demand, electric auto charging, automated demand response, and storage (such as compressed-air energy storage (CAES), pumped hydro, or thermal energy storage) can reshape the demand profile to match renewable output better. Conventional (gas) or renewable peaking plants (like hydro) can be used to fill in the remaining gaps in demand and meet the system's physical needs. In a renewables-heavy electricity scenario, the increase in renewable capacity will decrease the need for gas generation, which means that more gas will be available as peaking reserves.
Stylized chart based on data from ERCOT dispatch simulations using projected 2050 Transform loadshapes and actual summer solar and wind data.
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