These days the world is switching to wind and solar generation instead of conventional generation because of their relatively reduced carbon emissions; Also there are many favorable policies, rebates and incentives that have been established that make people prefer wind and solar over fossil fuel generation. Add to it the fact that wind and solar have almost negligible marginal costs, hence wind and solar tend to displace conventional generation. However, increasing wind and solar penetration levels can lead to transmission and operational problems, hence only some amount of wind or solar can be absorbed into the grid system.
In the process of Curtailment the generation from a power plant (solar/wind) is curbed beyond what it is capable of producing. High levels of wind and solar power can be hard to integrate into grid because of their intermittency and unpredictability. When high levels of RE are planned, infrastructural, operational changes to the grid is necessitated which is a long term process, hence RE is curtailed to better manage the demand-response behavior on the grid and in the most economical way that can give profitable returns on the incumbent grid infrastructure.
Renewable curtailments can be compared to traditional load shedding events and are conducted for roughly the same reason which is that: Electricity consumption and generation have to be kept in balance to maintain the grid system reliability.
What exactly is curtailment?
As per a research report on curtailment by the National Renewable Energy Lab (NREL), curtailment is defined as: “A reduction in the output of a generator from what it could otherwise produce given available resources, typically on an involuntary basis”. NREL’s report titled Wind and Solar Energy Curtailment: Experience and Practices in the United States notes that, “Curtailment of wind and solar resources typically occurs because of transmission congestion or lack of transmission access, but it can also occur for reasons such as excess generation during low load periods that could cause base load plants to reach minimum generation thresholds because of voltage or interconnection issues or to maintain frequency.”
To explain this better, curtailment of solar and wind on a grid is a planned choice by the system operators because “ there’s a limit to how much one can turn down traditional fossil fired generation plants like coal plants which account for major percent to a nation’s electricity generation and have an average life of 38 years. These old base load plants have the reliability requirements as noted in a research that keep the grid in balance, including:
Load following – The ability to vary output with electricity demand
Inertia – The stored energy that comes from rotating mass in synchronous and induction generation turbines
Frequency response – The ability to sense and automatically correct for frequency excursions
Volt/VAR control – Production and absorption of reactive power to maintain voltages and minimize real-power losses
These traditional fossil fuel-based generation sources don’t have the ability to shut down and ramp up quickly. So, they necessitate a ‘must-run generation’ scenario that clashes with overproduction as greater percent of renewables increasingly come online. Small percentages of RE can be integrated into the system but accommodating more of RE will require major changes in how the current grids are governed and regulated all over the world.
Renewable energy can be accommodated in the grid when demand and renewable supply are matched i.e. when both rise and fall together. But when demand and renewable supply don’t fall and rise together, the cost of accommodation of RE in the grid can become a costly affair. For example, it is a known fact that renewable energy like wind blows strongly during night times when the power demand is low. This wind can be utilized in the grid only if base load power plant like coal or nuclear is curtailed. This is an expensive decision to make that may cause reliability issues in the grid.
Take another case where on a particular day no wind is blowing hence negligible wind power is produced. The peak demand times must be now met entirely by conventional generation sources. How these two scenarios (too much wind and negligible/zero wind) are to be dealt with is a major challenge in RE integration in the grid, hence to save the profitability of conventional sources of power, RE curtailment is resorted to by system operators.