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Since dumping will almost always be cheaper than investing in new transmission capacity or new storage, dumping should be rewarded so that investment decisions can include dumping as one of the flexibility options for network operators. Solar photovoltaics are experiencing unprecedented growth worldwide. According to studies by the International Renewable Energy Agency ( IRENA), the US Energy Information Administration (EIA), GEM, the World Nuclear Association (WNA) and the Global Wind Energy Council (GWEC), the total installed solar power capacity in the world surpassed nuclear 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 to generate electricity in the world. At this rate, by the end of the decade there will be more solar photovoltaic capacity installed on Earth than all other electricity generating technologies combined. This exponential growth brings with it significant challenges for the integration of solar energy into the 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 load curve of the grid. 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 arises , often used interchangeably with the term curtailment , and they can be considered synonyms. Curtailment involves reducing the output of a renewable resource relative to what it could otherwise have produced. It can be applied to large-scale centralised PV plants and to distributed and dispersed generation residential rooftop PV systems, where the power system operator can remotely switch off large-scale or rooftop PV power when there is a risk of grid overload. The reduction in PV output during times of high solar irradiance and moderate-low electricity demand will increase as solar PV penetration grows. At higher volumes, spills can undermine the profitability of new solar PV projects by reducing the revenue certainty of PV plants selling electricity on the wholesale market. However, modelling prior to project development would predict this outcome and is presumably taken into account. In any case, prices are low during such periods on a solar-dominated grid, so the revenue loss is relatively small. Persistent curbs and negative prices stimulate new markets, such as battery charging, pumped hydro storage, and factory thermal storage. In places like Australia, frequent negative daytime prices also encourage coal-fired power plants to reduce or eliminate their daytime output. For home systems, using rooftop solar electricity to charge electric cars, home batteries, and hot water storage systems can absorb excess solar electricity. Countries with the highest penetration of PV generation in the electricity 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 to integrate more renewable energy into the grid. This change in perspective is fundamental to understanding the role of curtailment in the evolution of electricity systems with high penetrations of intermittent sources such as wind and solar. Solar production ramp Short-term variability in solar resource availability can lead to steep ramps in PV generation. At the grid level, weather-related diurnal ramps are largely diluted by distributing large-scale PV plants over wide areas to smooth out solar supply. In addition, high-quality solar forecasts are available. Strong ramps at sunrise and sunset are predictable, which greatly aids grid management. At a local level, a single cloud can move across thousands of rooftop solar systems in a few minutes, potentially causing supply problems. There are many solutions, which are slowly being implemented in major countries. These include utility-controlled interruptible loads such as air conditioning and home battery charging, electric vehicle 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. Greater transmission interconnection within and between cities also greatly reduces problems. Often, these measures lag behind solar deployment rates, causing temporary problems. The increasing penetration of large-scale and rooftop PV is leading to an increase in discharges. At the recent Solargis talk “Global Patterns of Solar Resource Short-Term Variability Based on Solargis Time Series Data” during the EU International Photovoltaic Science and Engineering Conference (PVSEC) in Vienna, it was shown that the steepest ramps are occurring in the Sunbelt, and there, countries with high PV penetration are increasing PV discharges at a rapid pace. Although distributing PV plants over large areas can potentially reduce the variability of electricity output due to a spatial smoothing effect, PV plants naturally tend to be concentrated in regions with higher solar resource availability. As the costs of photovoltaic energy continue to decrease, solar power plants and rooftop photovoltaic systems will become more and more widespread, and this problem will naturally be minimized. Since PV power 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 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, excess capacity in solar plants provides more consistent and reliable power generation, even if not all capacity is constantly used. This excess PV capacity acts as a virtual form of storage, leading to more predictable and controllable generation, and allowing storage systems to be optimally sized. In addition, implicit storage provides greater 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. Discharges, combined with the concept of implicit storage, represent a paradigm shift in the integration of large-scale solar energy. As the world moves towards an energy mix increasingly dominated by solar PV and wind, these strategies become essential tools to ensure the stability, reliability and affordability of the electricity system. Successful implementation of these approaches requires a combination of technological innovation, regulatory adaptation and new business models. With solar energy costs continuing to decline and its growing share in the energy mix, curtailment (and implicit storage) are not just options, but also necessities. Currently, discharges do not guarantee compensation to generators, who are unable to fulfil their contracts using their own generation, even when due to constraints in the transmission grid. This situation must be resolved with adequate compensation for the interrupted energy, so that investment in PV remains an attractive option. |
