What is the curtailment of renewables?
The electricity grid must always be balanced so that electricity generation nearly exactly equals electricity usage, this is for both safety and reliability reasons. As renewables are generally not dispatchable forms of energy, there are times of the year or even specific times of the day when they produce significantly more electricity than is needed on the grid. This excess electricity results in the curtailment of energy from renewable sources, which is a purposeful reduction in renewable electricity, so as not to overload or damage the grid.
For background on this increasingly prevalent issue, we must look at the increased use of renewables on both an EU and global scale. From a legislative perspective, the impetus to use renewables has increased as the EU Renewable Energy Directive set a target that 16% of a country’s total energy consumption should be from renewable energy sources by 2020 (including a 40% renewable electricity target). In 2020, both Ireland and Northern Ireland exceeded these targets, by integrating a significant level of renewable generation onto the grid, whilst also ensuring that the electricity system remained stable and balanced.
Ireland’s All Island Fuel Mix
In 2020, the share of electricity demand from renewable sources in Ireland and Northern Ireland was 42.5%. This is broken down as follows:
- 36.4% provided by wind
- 2.4% provided by hydro; and
- 3.6% provided by other renewable energy sources.
Total Wind Generation in Ireland & Northern Ireland
In 2020, the total wind energy generated in Ireland and Northern Ireland was 13,768 GWh.
- 1,909 GWh of this wind energy was dispatched down ( which is 12.1% of the total available wind energy in 2020
- Ireland accounted for 1,448 GWh, which is the equivalent to 11.4% of the total available wind energy in Ireland.
- Northern Ireland accounted for 461 GWh, which is equivalent to 14.8% of the total available wind energy in Northern Ireland.
The reduction in the output of renewable generators below their maximum available level is referred to as ‘dispatch-down’ (or curtailment). This occurs when not all energy produced by these generators can be accommodated safely on the grid in a safe and secure manner. A dispatch-down of renewables occurs when the set safety limits have been met.
Ireland must produce a report on why this renewable energy was dispatched down and how measures were being implemented to prevent unnecessary curtailment, this is in accordance with the EU Renewable Energy Directive. Mitigation measures are also taken in the form of a multi-stakeholder, multi-year programme of work designed specifically to securely and efficiently increase the capability of the power system and minimise curtailment.
Why is there a need to dispatch-down?
There are two reasons for the dispatch-down of wind and solar energy: constraint and curtailment.
· Constraint refers to the dispatch-down of wind and solar generation for localised network reasons, this is not a system wide approach.
· Curtailment on the contrary refers to the dispatch-down of wind for system-wide reasons. This occurs on a large scale across the system, when there is not enough demand for the electricity produced by renewables on the grid.
There are different types of system security limits that necessitate curtailment on a system-wide basis in Ireland. These are:
1. System stability requirements
2. Operating reserve requirements, including negative reserve,
3. Voltage control requirements,
4. System Non-Synchronous Penetration limit.
It is sometimes necessary to limit the maximum level of wind generation on the system for security or safety reasons. The impact of these limits on the level of dispatch-down will depend on two factors: the amount of wind generation installed on the system; and the capacity factor of the wind generation. The level of demand is another important factor which may affect the dispatch-down of renewable generation. Increased demand generally enables greater levels of wind and solar to be accommodated on the system. In 2020, 180 MW was added to the wind installed capacity on the island, which is lower than the 453 MW added in the previous year.
When do these curtailments occur?
There are more curtailments in the night hours (11pm to 7am). This is due to the lesser demand for electricity during these hours. The grid accumulates more electricity than its supply needs. In recent years significant capital works have been undertaken to upgrade the transmission system to allow more wind generation to be exported from wind farms on the system, particularly in the North West and South West regions of Ireland. These areas have previously experienced the greatest level of restrictions in relation to the export of wind.
Annual Constraints and Curtailment Levels Across Ireland
The total dispatch-down levels that occurred each year in Ireland can be divided into constraints and curtailments. There has been a clear steady rise in the dispatch-down levels, with the figure almost doubling from 2019 to 2020. These dispatch-down levels largely consist of curtailments, not constraints.
Future Wind Curtailment Levels
As more and more renewable capacity is installed in Ireland and Northern Ireland, there is an increasing level being added to the grid. The upward trend of dispatch-down suggests that these levels will only continue to rise, unless the grid system is updated to successfully integrate increased renewable generation.
Negative Impact of Curtailment
Wind curtailment contributes to a significant loss in economic value and energy efficiency. However, upgrading the grid system is a costly endeavour for small levels of curtailment. As this curtailment increases, so do the issues associated with it. There are several ways to help ease wind curtailment levels, including:
· Demand response,
· Electric vehicles (EVs).
Wind curtailment in Ireland, and other countries with high levels of wind generation, is only increasing, resulting in huge wasted potential. This issue needs to be tackled as more renewables capacity is built as part of countries switching from fossil fuels to renewable resources. It is imperative that to remain on the path to net-zero technical issues like this are addressed quickly and efficiently.