Different renewable portfolio compositions place differing demands on the generation and storage resources of the grid. In hours when variable renewable supply is not enough to meet the full load, the remaining demand must be met with dispatchable generators. When variable renewable supply exceeds the full load, the excess renewable supply must be stored or curtailed (spilled). The frequency of over or under-supply is highly dependent on the amount and mix of variable renewables on a given system.
In this example, RMI used an hourly dispatch model to compare six different portfolios of wind and solar power on the Electric Reliability Council of Texas (ERCOT) grid. Each portfolio meets 40% of the annual electricity demand from wind and solar, though in different proportions. At any time during the year, the combined wind and solar renewable supply either 1) is not enough to meet the full load, or 2) exceeds the full load at that hour. By defining the net load as the total load minus the wind + solar renewable supply, we can refer to these two situations as either 1) positive net load, or 2) negative net load. In the case of positive net load, there is additional load that must be met with other resources; in the case of negative net load, the excess renewable generation must be stored or curtailed. Different renewable portfolio compositions result in differing frequency and duration of positive and negative net load events, and thus have important implications for grid operations and control.
For easiest load-following, a grid should have frequent and long positive net load events. Practically, this means that the variable renewable supply only exceeds total electricity demand infrequently and with long breaks in between. Long positive net load events allow the system operator to dispatch resources more steadily, rather than requiring them to ramp up and down constantly. And having more frequent positive net load events means that the dispatchable resources will be used more consistently throughout the year. Both long and frequent positive net load events reduce the cost and operational difficulty of the electricity system. In this analysis, the generation portfolio of 25% solar and 15% wind yields the most frequent and longest periods of continuous positive net load when compared to other portfolios of more wind or more solar.
Equally important to positive net load events are the frequency and duration of negative net load events. These will place a burden on storage and demand-side resources to absorb surplus energy, and in contrast to positive net load events, it is best for negative net load events to be infrequent and short. In practice, this means that variable renewable supply does not exceed demand very often, and when it does it is only for a brief period. Short and infrequent negative net load events will require the least amount of storage capacity and also tie up storage resources the least with required charging from excess energy. The rest of the year, these storage resources will be available to shift loads around during the day, flattening out positive net loads to make power dispatching easier. In this example on the ERCOT grid, portfolios with high wind adoption can have negative net load events longer than 48 consecutive hours. With less wind and more solar energy, both the frequency and duration of negative net load events are reduced.
RMI analysis using data from:
Electric Reliability Council of Texas. 2004. “FERC Form No. 714-ERCOT.” link