As the penetration of variable renewable energy increases, the curtailment of solar PV generation will only increase. Because curtailment will almost always be cheaper than investing in new transmission capacity or new grid-scale storage, curtailed energy must 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 studies by IRENA, IEA, GEM, WNA and GWEC, the world’s total installed solar capacity exceeded nuclear capacity in 2017, wind capacity in 2022 and hydropower capacity 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 by 2025 and become the energy source with the largest installed electricity generation capacity in the world . At this rate, by the turn of the century, more solar PV capacity will be installed on Earth than all other electricity generation technologies combined.
This exponential growth poses significant challenges for the integration of solar energy into the electricity grid. One of these challenges is the phenomenon known as the “duck curve” or variations such as the “canyon curve”, which shows the impact of solar energy generation on the load curve of the electricity grid. As solar energy penetration increases, it becomes increasingly important and necessary to control solar energy generation to maintain the stability and reliability of the electrical system. In this context, the concept of ‘constrained-off’ emerges, which is often used interchangeably in the international literature (and in national news) with the term ‘curtailment’, and these can be considered synonyms.
Curtailment involves reducing the production of a renewable resource from what it could otherwise produce. It may apply to large-scale centralized PV power plants and to distributed and dispersed generation from residential rooftop solar PV systems, where the power system operator can remotely shut down large-scale or rooftop solar PV systems when there is a there is a risk of overloading the electricity grid. Limiting PV production during times of high solar radiation and moderate-low electricity demand will increase as solar PV penetration grows.
At higher volumes, curtailment has the potential to undermine the economics of new solar PV projects by reducing revenue security for PV installations that sell electricity on the wholesale market. However, modeling prior to project development could predict this outcome and is likely taken into account. In any case, prices are low during such periods in a solar-dominated electricity grid, making lost revenue relatively small.
Continued curtailment and negative prices are fueling new markets, including utility battery charging, hydropower plants and factory thermal storage. Frequent negative prices during the day also strongly encourage coal-fired power stations to switch to low or zero levels often during the day in places like Australia. For domestic systems, using rooftop solar to charge electric cars, home batteries and hot water storage systems can absorb excess solar energy.
Countries with greater penetration of photovoltaic generation into the electricity grid have been working to redefine the perception of containment and deal with this new reality. Looking at the glass from a “glass half full” perspective, curtailment becomes a valuable tool for integrating more renewable energy into the grid. This change in perspective is critical for understanding the role of curtailment in the evolution of electricity systems with high penetration of intermittent sources such as wind and solar energy.
Increase in solar energy production
The short-term variability of solar energy availability can lead to steep slopes in solar PV generation. At the grid level, weather-related diurnal slopes are greatly mitigated by distributing large-scale PV power plants over large areas to facilitate solar energy supply. Moreover, high-quality solar forecast is available. The steep slopes at sunrise and sunset are predictable, which greatly helps with grid management.
At a local level, a single cloud can move over thousands of rooftop solar systems in a few minutes, causing supply problems. There are many solutions that are gradually being implemented in leading countries. These include utility-controlled interruptible loads such as air conditioning and home battery charging, EV batteries and hot water storage. Temporary curtailment of rooftop solar can also be used to reduce rates of increase by taking advantage of high-resolution solar forecasts in cities. Stronger transmission interconnection within and between cities also significantly reduces problems. Very often, such measures lag behind the use of solar energy, which causes temporary problems.
The increasing penetration of both large-scale and rooftop PV is leading to more and more containment events. In the recent lecture ‘Global Patterns of Solar Resource Short-Term Variability Based on Solargis Time Series Data’ by Solargis during the EU PVSEC in Viennathe figure below was presented, which shows the average number of solar energy disasters with a power of 400 W. As the figure shows, the steepest slopes occur in the sun belt, and that is where countries with high PV penetration are rapidly limiting solar power generation. Although the distribution of PV power plants over large areas can potentially reduce power output variability due to a spatial smoothing effect, PV power plants are obviously often concentrated in regions with the best solar energy availability. With the cost of photovoltaic energy continuing to decline, solar power plants and rooftop photovoltaic systems will become more widespread, and this problem will obviously be minimized.
Because PV has become so cheap, overbuilding is an option. The concept of overbuilding in solar energy systems is similar to the power of the cars we use every day. We buy vehicles with engines that can reach speeds well above legal limits, even without having an Autobahn – a German highway without a speed limit – where we can exploit all this potential. This additional power capacity provides flexibility, consistent performance and reliability in situations that require more power, such as steep hills or overtaking. Likewise, overbuilding solar power plants allows for more constant and reliable energy generation, even if not all capacity is used constantly. This excess photovoltaic capacity acts as a virtual form of storage, resulting in more predictable and controllable generation, and allowing storage systems to be sized in an optimized manner. In addition, implicit storage provides further operational flexibility, allowing system operators to adjust solar energy production in real time to meet grid demand, improving the stability and reliability of the energy system.
Containment, 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 photovoltaics and wind energy, these strategies are becoming 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 alignment and new business models. With the continued decline in the cost of solar energy and its increasing share of the energy mix, curtailment (and implicit storage) are not only options, but necessary. Currently, the curtailment does not guarantee compensation for producers who cannot fulfill their contracts using their own generation, even if the outage is due to limitations in the transmission network. This situation must be resolved with appropriate compensation for the curtailed energy, so that investing in PV remains an attractive option.
Authors: Prof. Ricardo Rüther (UFSC), Prof. Andrew Blakers (ANU)
Andrew.blakers@anu.edu.au
rruther@gmail.com
ISESthe International Solar Energy Association is a UN accredited member NGO founded in 1954 working towards a world with 100% renewable energy for all, used efficiently and wisely.
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