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
The IEA’s phases of VRE integration framework outlines six phases of increasing solar PV and wind impacts on the power system. Each phase presents new challenges requiring targeted measures to enable the secure and cost-effective uptake of VRE. Phases 1 to 3, considered low phases of VRE integration, experience relatively low impacts, with most challenges addressable through straightforward modifications to existing assets or operational improvements. Phases 4 to 6 are considered high phases and mark increasing influence of VRE in shaping system operations, requiring a fundamental transformation of the power system.
The IEA’s phases of VRE integration framework outlines six phases of increasing solar PV and wind impacts on the power system. Each phase presents new challenges requiring targeted measures to enable the secure and cost-effective uptake of VRE. Phases 1 to 3, considered low phases of VRE integration, experience relatively low impacts, with most challenges addressable through straightforward modifications to existing assets or operational improvements. Phases 4 to 6 are considered high phases and mark increasing influence of VRE in shaping system operations, requiring a fundamental transformation of the power system.
Phase 1
VRE has no significant impact at the system level
The first set of VRE plants are deployed, but their impact is largely insignificant at the system level and the typical operating parameters of the system remain unchanged. Any effects are very localised, for example at the grid connection point of plants.
Load versus net load
The difference between load and net load is minimal
Phase 2
VRE has a minor to moderate impact on the system
As more VRE plants are added, changes between load and net load become more noticeable with a minor to moderate impact on the system such as faster and more frequent ramping of generators. Upgrades to operating practices such as integrating forecasting into dispatch and making better use of existing system resources are usually sufficient to achieve system integration.
Load versus net load
The difference between load and net load is noticeable
Phase 3
VRE determines the operation pattern of the power system
VRE determines the operation pattern of the power system and increases the uncertainty and variability of net load. Greater swings in the supply-demand balance prompt the need for a systematic increase in flexible operation of the power system that often goes beyond what can be readily supplied by existing assets and operational practice.
Load versus net load
The "duck" curve starts emerging, suggesting that more pronounced and longer ramps are required
Phase 4
VRE meets almost all demand at times
VRE output is sufficient to meet a large majority of electricity during certain periods, which may impact power system stability. A key operational challenge is related to the way the power system responds to maintain stability immediately following disruptions in supply or demand, which may involve advanced operational solutions and changes in regulatory approaches.
Percentage of hours covered by VRE
During a few hours of the year, almost all demand is covered by VRE
Phase 5
Significant volumes of surplus VRE across the year
Rising shares of VRE mean that without additional measures VRE availability will exceed demand during many hours and be in overall surplus for periods of a day or more. Achieving such shares under decarbonisation goals in an economic and secure manner requires increased measures to support VRE utilisation, such as large deployment of demand response, energy storage and grids, and more extensive solutions to ensure stability at low levels of conventional supply.
Percentage of hours covered by VRE
VRE generation can be higher than 100% of the local demand: surplus energy must be managed
Phase 6
Secure electricity supply almost exclusively from VRE
Phase 6 applies to regions looking to meet extremely high shares of annual electricity demand with VRE. The main challenges in this phase include operating a system largely dependent on converter-connected resources and meeting demand during extended periods of low wind and sun availability. Addressing flexibility needs can involve long-duration energy storage or extensive electricity trade with other regions.
Electricity generation and load profiles
Prolonged periods of low VRE availability need to be compensated for by storage and dispatchable generation
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
Open
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.
Integrating Solar and Wind
This report underscores the urgent need for timely integration of solar PV and wind capacity to achieve global decarbonisation goals, as these technologies are projected to contribute significantly to meet growing demands for electricity by 2030.
