Renewable Integration
What is it? / What is variable renewable energy (VRE)?
Sources of renewable energy (usually electricity) where the maximum output of an installation at a given time depends on the availability of fluctuating environmental inputs. Includes wind energy, solar energy, run-of-river hydro and ocean energy. VRE is a preferable term as it does not convey an inaccurate impression that the output is always subject to sharp or sudden outages or changes. For example, while wind energy is variable, it may operate for long periods without output dropping to zero.
Why is this important?
Increasingly, power system planning exercises are incorporating assessments of flexibility requirements and integrating across power market segments and economic sectors. Such integrated approaches can help to uncover smart solutions, but policy makers may need to intervene to encourage these kinds of approaches in an unbundled system.
What are the challenges?
High shares of VRE can create operational challenges, particularly short-term flexibility related to power system stability on the sub-second timescale. System inertia, a property derived from synchronous generators, acts to mitigate the rate of change of frequency following a contingency event in the power system. VRE generators do not have a direct, electro-mechanical coupling to the grid, which makes them different to traditional, synchronous generators.
Power system transformation
Power system flexibility refers to the capability of a power system to maintain continuous service in the face of rapid and large swings in supply or demand, whatever the cause. Flexibility has always been an important requirement for power systems due to the need to plan for unexpected contingencies such as plant and transmission outages. However system flexibility has become increasingly important for policy makers as the share of VRE generation increases and needs to be addressed in all time domains from real-time operations to long-term system planning.
Power system flexibility refers to the capability of a power system to maintain continuous service in the face of rapid and large swings in supply or demand, whatever the cause. Flexibility has always been an important requirement for power systems due to the need to plan for unexpected contingencies such as plant and transmission outages. However system flexibility has become increasingly important for policy makers as the share of VRE generation increases and needs to be addressed in all time domains from real-time operations to long-term system planning.
Key characteristics and challenges in the different phases of system integration
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Phase 1 captures very early stages where VRE deployment (often no more than a few percent of annual energy demand) has no immediate impact on power system operation. Phase 2 flexibility issues emerge but the system is able to cope with them through minor operational modifications. Phases 3 through 6 respectively indicate greater influence of VRE in determining system operations; starting from the need for additional investments in flexibility; structural surpluses of VRE generation leading to curtailment; and structural imbalances in energy supply at seasonal and inter-year periods requiring sector coupling.
Power system flexibility is required across a range of timescales, and different flexibility hardware solutions and operational practices solutions offer timescale-specific capabilities. Shorter flexibility timescales help to provide services such as system stability and balancing, whereas longer timescales help to address seasonal imbalances in supply and demand, often driven by weather and the availability of renewable energy resources.
The first issues that become apparent are at short to medium timescales, followed by stability concerns at ultra-short timescales. As VRE becomes a dominant source of supply in the system, long- to very long-term issues are encountered in the highest phases.
The first issues that become apparent are at short to medium timescales, followed by stability concerns at ultra-short timescales. As VRE becomes a dominant source of supply in the system, long- to very long-term issues are encountered in the highest phases.
Issues seen at different flexibility timescales
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Depending on the institutional aspects of the system and markets, there are four key categories of infrastructure assets that feed flexibility into the system; these include: (a) power plants (both conventional and VRE); (b) electricity network interconnections; (c) energy storage; and (d) distributed energy resources.
Conventional power plants, electricity networks and pumped storage hydropower have historically been the primary sources of flexibility. However, operational improvements in VRE power plants, electricity networks and the advent of affordable distributed energy resources and battery energy storage systems, are enabling a wider set of flexibility options for consideration.
As power systems transition towards higher phases of system integration, these flexibility resources can work together to enhance system flexibility in a cost-effective, reliable and environmental sound manner. Modifications to policy, market and regulatory frameworks ensure that battery energy storage systems and distributed energy resources can participate in the power system to provide flexibility services.