byRalph Masiello, Rick Fioravanti and Jillis RaadscheldersPublished: 2010-06-04
Energy storage offers huge potential for helping renewables meet periods of peak power demand, but the financial and policy cases must keep up with the technology, explain Ralph Masiello, Rick Fioravanti and Jillis Raadschelders.
Our power system is embarking on significant changes that may require storage to be as necessary in the electricity industry as it is in every other type of industry operating today. One area where energy storage may become a necessity is with the integration of renewables into the electricity grid. As more systems look to adopt renewable generation into the electricity system, the network can become more difficult to balance. Before the advent of renewables, variability was evident only in the load. Now, as increasing amounts of renewable generation are added to the grid, variability is introduced to the supply side as well, making the system more unpredictable.
In the long term, the implications of widespread, mass deployment of electricity storage across the power system are profound. Integrating storage into the electric grid can add the flexibility required for the system to adjust to future increases in renewable energy generation penetration by acting as a bridge, a buffer and a reliability component. Harnessing today’s rapidly advancing energy storage technology for the sustainable and distributed grid of tomorrow requires keen insight into storage applications, the financial implications, and the public policies needed to encourage its use.
Renewable resources, especially wind generation, are often characterized as intermittent or variable in nature. This variability can impact the electricity grid in multiple areas. For operators, as renewable deployments hit penetration rates of 20% or more, sudden drops in output can create scenarios where the grid may not be able to respond in the available time. For renewable operators, this intermittency can create problems in trying to meet their forecast output. While it is well within the state-of-the-art to forecast that tomorrow will be a windy day, it is difficult to forecast the actual wind speed accurately hour-by-hour a day ahead. Wind power production forecast-versus-actual typically show significant variations, especially in the timing of wind fall-off and ramp-up.
Though this second-by-second variation in renewables is not generally an operational problem, the variability in how the diurnal cycle occurs certainly can be. This becomes apparent when examining systems that have a high wind and solar penetration. Highly predictable renewable patterns, such as the production from a concentrating thermal solar plant, may exhibit ramp rates that are very high compared to normal load ramping or conventional generation ramping. The potential of diurnal wind production falling off in the morning, while shortly afterwards the concentrating thermal solar plant is ramping up, can create major difficulties. Trying to balance this with conventional generation resources is impractical if not infeasible.
Storage can assist with renewable integration in the ways outlined below:
Evening out Fluctuations, Moderating Ramping
Levelizing these kinds of fluctuations as well as moderating ramping – up or down – is a perfect application for large-scale storage. Today, however, there are no financial incentives for renewable developers to install storage and there is no mechanism to allocate the costs elsewhere. Instead, the prospect is that the market will socialize these costs via increased procurement of regulation and balancing energy services. As regulation is paid for whether or not it is used, the implication is that the market operator must procure enough regulation services to cover the worst case, even if that only occurs on one day in every 10.
Transmission Curtailment
Another important aspect of renewable balancing has to do with transmission curtailments that may be imposed on remote wind farm production. In fact, if transmission is built on sound economic principles, this concept is almost a given. The capacity factor of a given land-based wind farm is typically 25%–30%, meaning that the average output of the wind farm is 25%–30% of its peak capacity. It may not be economic at all to build transmission with that kind of capacity factor in mind. But if the transmission is built to some level less than peak capacity, there will be occasions when the wind farm has to be curtailed. This is another perfect use for storage. When wind production is at peak and transmission curtailments are in effect, you can store the energy, then discharge it when wind production falls off and transmission capacity is available. The difficulty with realizing such a solution today has both economic and policy roots. The cost of storage needs to align with the benefit of the opportunity. Storage technologies with lower costs, such as compressed air storage, may work – provided a convenient cavern is located nearby – but the cycle efficiencies are an issue. However, this market requires that storage comes down in cost before it can be realized.
Diurnal and Longer Time Shifting
Because we don’t have direct control over when renewable systems produce power, production shifting is a key area of interest for storage applications. For wind, in some areas production peaks tend to be at night, when prices are low, while production drops during the hottest days, when prices are high and the energy is needed most. Though large solar does align with mid-day peaks, utilities are seeing peaks extend into early evenings, when people return from work and turn on appliances, after the sun has set. For renewables, production shifting with storage has tremendous potential value.
Adjusting to Forecasting Error
Because our ability to forecast output of renewable applications such as wind and solar is limited by our ability to predict the weather (or better stated, the exact timing of expected weather changes), storage can act as a buffer to allow the renewable generation output to more accurately match predicted outputs.
Pumped hydro storage has long been used to levelize energy demand versus production over daily and weekly cycles. As such, it is a great benefit to generation fleets that have a preponderance of lower cost baseload (nuclear, coal, run of river hydro) units that are less flexible in varying their output. Unfortunately, pumped hydro is inherently limited by siting issues. A convenient valley that can be dammed without too much adverse environmental and public consequence is needed, and the reservoir needs to be near adequate water supply – meaning a river, typically.
While many large reservoirs behind major dams also serve as an economic engine for recreational purposes, the typical pumped hydro facility does not because the level fluctuates severely during daily and weekly cycles. This is also a siting issue on the downstream side – the river, lake, or other body of water on the low side has to be able to tolerate widely fluctuating inflow/demand for water as well.
Below: Storage allows wind production to be used when transmission is available
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