Energy storage has become an everyday element of grid planning and energy network management – driven by technology advances, proven benefits, and steadily falling prices. As storage goes mainstream, it’s no longer unusual to see deployments in the tens of MWh. Although about 95 percent of operational storage in the U.S. is in the form of pumped hydro, which can store massive amounts of energy cheaply for days, virtually all of the remaining storage resources are lithium-ion batteries.
According industry analysts, energy storage deployments in the U.S. will triple in 2018 compared to 2017. As applications for energy storage have expanded with systems on both sides of the meter, there is growing interest in technology that can provide the best of both worlds: the long-duration, long-life benefits of pumped hydro (but without the lengthy siting process) — at a levelized cost of storage (LCOS) at or below that of li-ion batteries.
While lithium-ion battery deployments have ramped up steeply in recent years as costs have dropped, project developers are reluctant to place long-duration (four hours or more) demands on the technology. Lithium-ion batteries have shorter cycle lives than alternative long-duration solutions, making them uneconomical over the long-term when exercised deeply and frequently. The LCOS of flow batteries, by comparison, is markedly better for long-duration use cases, thanks to virtually unlimited cycle life without storage capacity degradation.
As with any technology choice, developers and project owners are also trying to figure out which long-duration storage applications will provide the greatest return on investment, best meet their energy needs, and/or help to build an integrated application stack to boost revenues.
Long-duration Storage Use Cases
Long-duration storage can deliver significant value in some widespread and increasingly important grid services areas, including:
- Support for areas of the grid with a high renewables penetration; i.e. avoiding the “duck curve”;
- Grid-tied community or commercial and industrial (C&I) microgrids, where renewables plus storage can offer grid resilience and enable renewables self-consumption;
- Demand management – using long-duration storage to reduce electricity charges at peak times, and multi-hour time shifting to avoid new transmission buildout;
- Islands or remote microgrids, where the need exists for savings, stability, security and sustainability.
The value of long-duration storage is also recognized by regulators, utilities, and industry experts for its flexibility in addressing multiple use cases with a single storage asset.
Current and Emerging Long-duration Storage Technologies
Pumped hydropower — One of the most widely used forms of energy storage currently is pumped hydropower. Pumped hydro offers the lowest cost per MWh; the longest cycle life (40-50 years); and field-proven, unlimited storage capacity. But its drawback is geographical: it requires access to two reservoirs at different altitudes, the building of which can impact the environment and require years of permitting and construction.
A variation on pumped hydro but with a modern spin (and few geographical limits) is a new technology that uses excess electricity to pump water into the underground shale rock in abandoned oil and gas wells, creating pressure that can be released to run turbines and re-generate electricity. In theory, this type of system shows promise and is currently being piloted at small-scale.
Thermal — There are several different types of thermal energy storage. So-called “sensible storage” is the most common form, and is often paired with large-scale solar plants. A sensible heat system involves heating or cooling material with no phase change present to store either heating or cooling potential. This can be achieved using water as a storage medium, although other materials including glycol, concrete, rock, sand and molten salt are also used.
Liquid air — Liquid Air Energy Storage (LAES) super-cools ambient air to a frozen liquid state, stores it in a tank and turns it back into a gas that spins a turbine when power is needed. LAES uses off-the-shelf components with long lifetimes (30+ years), resulting in relatively low technology risk. The systems share performance characteristics with pumped hydro and can utilize industrial low-grade waste heat/waste cold from co-located processes. Storage capacity extends from around 5 MWh to hundreds of MWh; with capacity and energy decoupled, the systems are very well suited to long-duration applications.
Compressed air — Compressed air energy storage (CAES) plants compress and store ambient air under pressure in underground caverns. When that energy is required, the pressurized air is heated and expanded in a turbine, thus driving a generator for power production. This technology offers an attractive LCOS, long operating life and no cyclic fatigue, but challenges include geological conditions, environmental permitting and a long construction time.
Flow batteries — Many types of flow-battery chemistries have been developed and deployed, including those based on vanadium, iron-chromium, zinc-bromine, and all-iron. They provide wide flexibility to independently tailor power and energy ratings for a given application, versus other electrochemical means and are a good solution for long-duration grid-scale storage. Flow batteries are a safe, low-cost way to store energy at grid scale, with power ratings from tens of kilowatts to many megawatts for periods of 4 or more hours. They offer reduced system complexity and maintenance; lower material and operational costs (i.e. low LCOS); greater than a 20-year cycle life with no capacity fade; and chemistries such as all-iron flow battery are cleaner, safer and non-corrosive.
The choice of a long-duration storage solution will depend on the grid application(s) involved, as well as performance attributes, life span, LCOS, and, in some cases geological conditions and availability of real estate.
For long-duration use cases, the solution should be able to shift four or more hours of energy capacity, with frequent cycling during each day. It should also operate at high efficiency over an unlimited number of deep charge and discharge cycles with little degradation. With this in mind, local electricity costs and new types of tariffs, such as time-of-use rates, will also be an important factor in the decision.
The world’s energy markets are moving inexorably toward a distributed and decarbonized model, and it’s becoming clear that long-duration energy storage will be needed in massive amounts, both to smooth the intermittencies of wind and solar, as well as to enable deferral (or avoidance) of higher-cost transmission and distribution assets. Forward-looking states like California are taking the lead with regulatory changes that boost long-duration storage as compared to fossil-fueled peaker plants.
An ideal long-duration storage technology is one that is cheap, safe and lasts a “utility lifetime” (20+ years) without degradation. In that regard, flow batteries represent a commercially viable solution. As production volumes increase, multi-hour use cases become more prevalent, and costs continue to fall, their advantages should become even more compelling.