As the penetration of variable renewable energy increases, curtailment of solar photovoltaic (PV) generation will only increase. Since curtailment will almost always be cheaper than investing in new transmission capacity or new grid-scale storage, curtailed energy should be rewarded, so that PV investment decisions can include curtailment as one of the flexibility options for grid operators.
Solar PV is experiencing unprecedented growth on a global scale. According to surveys by IRENA, IEA, GEM, WNA and GWEC, the total installed capacity of solar power in the world surpassed nuclear capacity in 2017, wind in 2022 and hydropower last year. It is expected to surpass natural gas before the end of this year and, maintaining current growth rates of 20% per year, it will surpass coal in 2025, becoming the energy source with the largest installed capacity for generating electricity in the world. At this pace, by the turn of the decade, there will be more solar PV capacity installed on Earth than all the other electricity generation technologies combined.
This exponential growth brings with it significant challenges for the integration of solar energy into the electrical grid. One of these challenges is the phenomenon known as the “duck curve” or variations such as the “canyon curve”, which represents the impact of solar generation on the electrical grid load curve. As solar penetration increases, it becomes increasingly important and necessary to control solar generation to maintain the stability and reliability of the electrical system. In this context, the concept of constrained-off emerges, often used interchangeably with the term curtailment in the international literature (and in national news), and they can be considered as synonyms.
Curtailment entails reducing the output of a renewable resource from what it could have otherwise produced. It can apply to large-scale centralized PV power plants, and to distributed and dispersed generation residential rooftop solar PV systems, where the electrical system operator can remotely shut down large-scale or rooftop solar PV when there is a risk of grid overload. Curtailing PV output at times of high solar irradiance and moderate-low electricity demand will increase as the penetration of solar PV grows.
At larger volumes, curtailment has the potential to undermine the economics of new solar PV projects by reducing revenue certainty for PV plants that sell electricity on the wholesale market. However, modeling in advance of project development would predict this outcome and is presumably taken into account. In any event, prices are low during such periods in a solar-dominated grid and so lost revenue is relatively small.
Persistent curtailment and negative prices stimulate new markets including charging utility batteries, pumped hydro storage and in-factory thermal storage. Frequent daytime negative prices also strongly encourage coal power stations to frequently cycle to low or zero levels of output during daytime in places like Australia. For domestic systems, the use of rooftop solar electricity to charge electric cars, home batteries and hot water storage systems can soak up excess solar electricity.
Countries with greater penetration of photovoltaic generation in the electrical grid have worked to redefine the perception of curtailment and learn to deal with this new reality. Looking at the glass from a “half-full” perspective, curtailment becomes a valuable tool for integrating more renewable energy into the grid. This change in perspective is critical to understanding the role of curtailment in the evolution of electricity systems with high penetrations of intermittent sources such as wind and solar.
Solar output ramping
Short-term variability of the solar resource availability can lead to steep ramps in solar PV generation. At a grid level, weather-related daytime ramps are greatly diluted by distributing large-scale PV power plants over large areas to smooth out solar supply. Additionally, high-quality solar forecasting is available. The steep ramps at sunrise and sunset are predictable, which helps greatly with grid management.
At a local level, a single cloud can move across thousands of rooftop solar systems in a few minutes, which can lead to supply problems. There are many solutions, which are gradually being implemented in leading countries. These include utility-controlled interruptible loads such as air conditioning and the charging of home batteries, EV batteries, and hot water storage. Temporary curtailment of rooftop solar can also be used to reduce ramp rates by taking advantage of high-resolution solar forecasting in cities. Stronger transmission interconnection within and between cities also greatly reduces problems. Quite often, such measures lag behind solar deployment rates, causing temporary problems.
Increasing penetration of both large-scale and rooftop PV is leading to increasing curtailment events. In the recent “Global Patterns of Solar Resource Short-Term Variability Based on Solargis Time Series Data” talk by Solargis during the EU PVSEC in Vienna, the figure below was presented, showing the average number of solar ramps with 400W magnitude. As the figure shows, the steepest ramps are happening in the sunbelt, and there, countries with high PV penetration are curtailing solar generation at a fast pace. Although the distribution of PV power plants over large areas can potentially decrease power output variability due to a spatial smoothing effect, PV power plants naturally tend to be concentrated in regions with the best solar resource availability. With the costs of photovoltaics still declining, solar power plants and rooftop photovoltaic systems will become increasingly widespread, and this problem will naturally be minimized.
Because PV has become so cheap, overbuilding is an option. The concept of overbuilding in solar systems is similar to the power of the cars we use every day. We buy vehicles with engines capable of reaching speeds far above the legal limits, even without having an Autobahn – a German highway with no speed limit – where we can exploit all this potential. This extra power capacity offers flexibility, consistent performance and reliability in situations that require more power, such as steep hills or overtaking. Similarly, overbuilding in solar plants provides a more constant and reliable power generation, even if not all the capacity is constantly used. This excess photovoltaic capacity acts as a virtual form of storage, resulting in a more predictable and controllable generation, and allowing storage systems to be sized in an optimized manner. Additionally, implicit storage provides further operational flexibility, allowing system operators to adjust solar output in real time to meet grid demands, thereby improving the stability and reliability of the power system.
Curtailment, combined with the concept of implicit storage represents a paradigm shift in the integration of large-scale solar energy. As the world moves towards an energy mix increasingly dominated by solar photovoltaic and wind energy, these strategies become essential tools to ensure the stability, reliability, and affordability of the electrical system. The successful implementation of these approaches requires a combination of technological innovation, regulatory adaptation, and new business models. With the continuous decline in the costs of solar energy and its increasing share in the energy mix, curtailment (and implicit storage) are not only options, but also necessities. Currently, curtailment does not guarantee compensation to generators, who are unable to fulfill their contracts using their own generation, even when the outage is due to limitations in the transmission network. This situation must be solved with appropriate compensation to curtailed energy, so that investing in PV remains an attractive option.
Authors: Prof. Ricardo Rüther (UFSC), Prof. Andrew Blakers (ANU)
ISES, the International Solar Energy Society is a UN-accredited membership NGO founded in 1954 working towards a world with 100% renewable energy for all, used efficiently and wisely.
International Solar Energy Society (ISES)
