The importance of flexibility
As the power system continues to evolve, with wind and solar generation playing an ever-greater role, we hear more and more about the importance of increasing its flexibility. The IEA considers flexibility a global priority, and addressed its May, 2019 report on the status of the power system on policy and technological advances that further enable a flexible system. Flexibility in this sense includes both being able to temporally shift generation – or, increasingly, demand – to when it is most valuable, and also to respond and change output (or consumption) levels quickly to respond to frequency deviations or short-term net load fluctuations.
New technologies and approaches to provide flexibility are proliferating as their value (or projected value) rises. The recent cost declines of lithium-ion batteries are of course well documented, but equally valuable may be advances in power electronics and communications protocols that allow the integration and control of building loads with behind-the-meter generation and storage. Likewise, generation technologies typically operated in a flat, baseload mode with few starts and stops, like geothermal and nuclear, are looking at new configurations or materials that allow them to turn on and off quickly or maximize efficiency at a wider range of load factors. Even wind and solar themselves are now able to provide a number of ancillary services, and if operated with headroom, considerable upward and downward ramping as well.
Hydropower as a flexible resource
Which brings us to hydropower. Like wind and solar, hydropower is fully renewable, with minimal life-cycle emissions and zero marginal cost. At the same time, however, many hydro facilities offer as much flexibility and responsiveness as any generation resource on the system. In a 2017 study of resources on their system, PJM ranked hydropower as the generation resource with the greatest range of flexibility, ahead of natural gas. Hydropower is used today to follow net load in all regions of the country. DOE’s own research has found that on a per MW basis, it is used to balance the system even more than gas. Add in pumped storage, which comprises 95% of grid-scale storage in the U.S. and is still the most cost effective solution available for large, long-duration storage needs, and it is clear that the current hydro fleet provides a backbone of flexibility.
But, if hydro is used as a default source of flexibility, it is likely that changes in the grid – and the specific flexibility requirements driven by changing system properties – will be reflected in the demands placed on the hydropower fleet. Some of this is being seen already in certain plants (for example, the AMP’s well-publicized Osage plant, which currently sees over 200 starts-stops a week). The Department’s Hydropower Value Study is currently undertaking fleet level analysis in multiple ISOs as well as specific utility case studies to track changes more systematically. The effect on pumped-storage is more widespread and consistent, with plants from California to Europe to Japan all moving away from a traditional day-night arbitrage model based on balancing inflexible baseload, to pumping as much if not more during the day, and varying output and consumption frequently.
Fig 1. Hydropower provides load following in every ISO market
What is the future of hydro – and what does that mean for the grid?
These changes raise important questions for both hydropower owners and operators, and for those responsible for the planning and operation of the grid. For those on the hydropower side, what effect may increased starts and stops, or running more often in the “rough zone,” have on O&M costs? Will predictive maintenance algorithms still hold up under new operating conditions? If costs increase, how will they be compensated – or further flexible operation incentivized? Some of these questions are specific to hydro, but others affect all technologies – so hydropower owners as well as others may have a particular stake in evolving market rules for capacity and ancillary services.
The future of hydropower brings up just as many questions for the grid itself. As net load patterns change, to what extent can hydropower continue to be counted on for load following and reliability services? Or perhaps with changes to technologies, planning and operations, or the rules and institutions governing how plants are run, the hydro fleet can provide even greater value? Answering these questions involves looking closely at how hydropower is being utilized now – as well as the extent of its technical capabilities, and how tradeoffs between different objectives (say, energy and water availability, or flow requirements) may change as the value to the power system becomes less about total generation, and the timing and flexibility of operations gain in importance.
Today, data on the hydropower fleet is relatively fragmented, and hydropower capabilities are not always well represented in power system models. DOE is looking to change that, with a new R&D initiative called HydroWIRES, focused on hydropower’s role in maintaining a reliable grid and integrating other renewables. By developing a deeper understanding of hydro’s current role and future value proposition, HydroWIRES will provide both hydropower community and grid planners and operators the data and tools to optimize their assets – and guide further DOE research into new technologies that are designed and optimized for the hydropower of tomorrow.
Director, Water Power Technologies Office
United States Department of Energy (DOE)