Variable renewables in flexible electricity systems

A recent report from IEA explores how variable renewables can be integrated on a large-scale into the electricity system.

The report proposes to replace the term 'intermittency' with 'variability'. Calling renewables intermittent is misleading, since wind or solar power do not drop from full power to zero and vice versa - they are available at some level much of the time, and ramp up or down following gradients dictated by weather.

The main theme of the report is that hosting capacity of the electricity system for variable renewables can be much larger than previously reported, provided that a number of measures are taken to increase system flexibility. The approach recognises the problem, but offers solutions.

A number of strategies are presented to smooth the effect of variability or to increase flexibility of the electricity system to absorb it.

Smoothing factors:

• system-wide aggregation: aggregating wind or solar power over a wider area reduces variability
• Aggregation of the output of different generation technologies
• Better forecasting

Flexibility sources:

• Fast-response capacity in the generation portfolio
• Availability of storage
• Stronger interconnections of transmission systems
• Demand-side management and response

While there is no intrinsic ceiling to hosting variable renewables, current networks may be inadequate for large-scale incorporation. Substantial changes to networks and system operation will be needed. And whatever the mix of above solutions adopted, there remains large uncertainty on the cost and reliability of the resulting system.