Since the earliest days of the power industry, energy storage has been a key factor in maintaining reliable electricity. Whether in the form of a coal pile, natural gas in a pipeline or water stored behind a dam, supplies of latent energy available for power production have been a key focus of resource planning.
Now as the decarbonization movement gains momentum, energy storage is gaining renewed attention, both as a means to offset intermittent production from renewable energy sources and to maintain grid stability. Here are five energy storage options that are attracting much current attention.
Several viable lithium-ion battery chemistries are commercially available, and together they have become the leading technology in the marketplace. Because of the massive scale of manufacturing production, costs of lithium-ion technology continue to trend lower. The primary downside of lithium-ion battery storage is the fact that, with current technology, only a fixed number of charge and discharge cycles can be expected over the battery’s useful life. In today’s market, the maximum duration of discharge is relatively short — generally four to six hours (although eight hours has been reported in some cases). Moreover, installations with a high frequency of charge/discharge cycles — such as batteries installed to maintain power quality on distribution feeders — can accelerate battery degradation, with replacement needed in as little as two to five years depending on cycling frequency.
Redox Flow Batteries
Although there have been interesting recent developments with redox flow batteries, the primary obstacle continues to be cost. Unlike lithium-ion batteries, flow batteries incorporate liquid electrolyte stored in separate tanks and are activated by pumps that circulate the fluid into a stack with electrodes separated by a membrane. The batteries can produce electricity with very little degradation of materials over long durations of usage. However, the high capital cost of equipment, plus the cost of electrolyte, typically makes it difficult for flow battery installations to compete with lithium-ion on an installed-cost basis.
There are dozens of different chemistries being tested for flow battery applications, but vanadium-based chemistries are currently among the most prevalent offerings. Costs of raw vanadium are starting to decline, which is an encouraging development.
Another encouraging sign is the amount of venture capital being attracted to the space. A 2021 initial public stock offering for a manufacturer with a proprietary iron-based chemistry technology attracted an astonishing $3 billion in net proceeds. Another emerging technology featuring a chemistry combining iron and air is an interesting development. All these trends indicate a high level of optimism in future developments of redox flow batteries as an energy storage technology option.
Liquefied Natural Gas
Even with the large amount of renewable capacity being installed, natural gas-fueled power plants continue to provide the highest percentage share of capacity on the grid. Supply disruptions such as the one caused by unexpectedly bitter cold weather in Texas and other parts of the South in February 2021 have prompted much discussion about adding liquefied natural gas (LNG) as a backup energy supply.
LNG is a very flexible energy resource that can allow plants to keep running during supply interruptions or when spot market prices begin to spike, simply by vaporizing supplies stored on-site or trucked in from centralized facilities. Plants that install enough stored LNG to run up to 72 hours can meet their own demand and may even see arbitrage opportunities to bid into the wholesale markets during peak demand. A reciprocating engine, simple-cycle or combined-cycle power plant can install storage and vaporization equipment for LNG as needed. The overall cost can be optimized by liquefying gas for storage during periods when spot market prices are lower and then vaporizing it on demand.
Though conventional hydrogen production processes can emit significant amounts of carbon dioxide, newer technologies, such as electrolysis production of hydrogen powered by excess renewable power, are gaining attention as a carbon-free energy storage resource. Much like LNG, hydrogen can be produced and stored on-site, either in a gaseous or liquefied state, and blended with natural gas during periods of peak demand. Hydrogen storage presents certain technical challenges, however.
Hydrogen has approximately one-third the volumetric energy density of natural gas. This means that to store the same amount of energy within the same volume, hydrogen must be compressed and stored in specialized tanks designed for ultra-high pressures of up to 900 bar. Alternatively, it can be stored in a liquefied state where the hydrogen is chilled to minus 423 degrees Fahrenheit, a temperature that is significantly lower than that needed to liquefy natural gas.
Despite the challenges, hydrogen is attracting much attention around the world, and leading reciprocating engine and combustion turbine manufacturers are developing units that will soon be able to burn 100% hydrogen fuel. Demonstration projects could be operational by 2023 and commercial-scale plants may be coming as soon as 2025 — far sooner than recent forecasts would suggest.
Thermal energy storage projects are also being considered for geographic regions where underground storage like depleted salt caverns or other voids are available. The process involves heating up an inexpensive raw material such as salt, crushed rock or concrete through a heat exchanger that would be powered by renewable energy resources during periods when that capacity would be otherwise curtailed. By storing molten salt or heated rock in highly insulated caverns, the heat energy is available to be extracted as hot air that can propel wind turbines during calm periods. Most thermal energy projects are currently in demonstration phases, but they represent another interesting alternative to utilize renewable power as a primary energy source to heat very inexpensive materials that do not degrade.
As the power industry continues to evolve and meet expectations for reducing carbon, development of storage technologies will be among the most interesting to watch. Options being explored by Burns & McDonnell for utility and development clients will always revolve around evaluating technical feasibility, in relation to overall risk posed by capital investment. It’s a position that will be exciting and intriguing with many possibilities on the horizon.
In the race to develop long-duration battery energy storage systems, various flow battery chemistries are showing great potential.