If you were building an electrical grid from scratch (with no regard to regulations or finance), then long-duration energy storage would be a requisite. It just makes sense — store energy when it’s cheap and/or abundant, and discharge when the price is high, or the energy is needed by the grid. Use it to load-shift, peak-shift and smooth; to replace fossil-fuel-fired peaker plants; and to integrate intermittent renewable resources onto the grid.

Long-duration storage fits in with what utilities, independent system operators (ISOs), and regional transmission operators (RTOs) understand. “Most utilities seem to want much longer-duration storage systems, with 6 to 12 hours discharge, to do serious load-shaping over the day,” suggests an analyst at a U.S. energy think tank. Some of these expectations are being driven by the performance of pumped hydro, once the only source of grid-connected storage.

Surely, balancing a photovoltaic- and wind-heavy U.S. grid, while retiring fossil-fueled generators, is going to require long-duration energy storage to deliver power when it’s needed and to absorb curtailed power when it’s not.

Storage booming, lithium-ion dominates

The U.S. energy storage market was booming until the Covid-19 virus hit — and it continues to boom, despite, or because of the pandemic. The U.S. energy storage market is forecast to grow from 523 MW in 2019 to 7.3 GW in 2025, according to energy analyst Wood Mackenzie.

Residential solar installers such as Sunrun have revealed solar-plus-storage attachment rates of up to 15% nationwide and up to 60% in the San Francisco Bay area. Utility-scale developers such as 8minute Solar Energy are coupling energy storage with PV on most, if not all, of their large solar projects.

Today’s energy storage market is absolutely dominated by lithium-ion batteries, but a host of new energy storage technologies are being brought to market — offering longer durations and potential improvements in project economics and functionality.

New technologies — thermal storage

There is an enormous set of new energy storage technologies with greater than four-hour discharge times — ranging from flow batteries to stacked concrete blocks. Here’s a brief survey of technologies and vendors.

Thermal storage uses excess or curtailed power to charge a thermal “battery” made of materials such as molten salt or cryogenic liquids.

California startup, Pintail Power uses molten salt in its “liquid salt combined-cycle hybrid” to integrate thermal energy storage with existing turbomachinery and heat transfer equipment than can repurpose existing peaking and combined cycle facilities as energy storage assets. Stored heat is used to generate steam when needed — lowering a natural gas plant’s fuel demand and increasing its flexibility. The company claims that its fast charging capability combined with 24-hours of more of compact storage aligns with the PV production profile to decarbonize overnight generation.

London-based Highview Power uses liquid air to store energy and plans to develop a 50-MW/8-hour energy storage project in northern Vermont. Highview uses off-peak or excess electricity to chill and liquefy air at -320°F, storing the liquid air in insulated, low-pressure tanks. Upon exposure to ambient temperatures, the liquid air rapidly returns to a gas, expands by 700 times its liquid volume and powers turbines to generate electricity.

Malta’s storage system operates as a heat pump in charge mode — storing electricity as heat in high-temperature molten salt. In discharge mode, the system operates as a heat engine, using the stored heat to produce electricity.

Brayton Energy is developing what it calls a “key component” to integrate turbomachinery into a cost-competitive thermal energy storage system. Brayton is looking to create a system in which each turbomachinery stage can act as both a compressor and turbine, alternating between charging and discharging cycles to increase efficiency and reduce capital costs.

Echogen is developing an energy storage system that uses a CO2 heat pump cycle to convert electrical energy to thermal energy by heating a reservoir of low-cost materials such as sand or concrete. To generate power, liquid CO2 is pumped through the high-temperature reservoir to a supercritical state, after which it expands through a turbine to generate electricity.

Swedish start-up Azelio stores thermal energy in 600°C molten aluminum. Its storage system has been installed alongside the 580-MW Ouarzazate solar complex in Morocco. When power is required, the stored thermal energy is transferred to a Stirling engine via a heat-transfer fluid.

Australia’s 1414 Degrees stores molten silicon at, yes, 1414 degrees. The startup acquired the Aurora Solar Energy Project in South Australia (a combination of 70 MW of PV and 150 MW of CSP) that 1414 Degrees is looking to expand and pilot its thermal storage technology.

Alumina is a California-based startup that uses a high-temperature ceramic material with high thermal conductivity, heat capacity, and low cost to store and recover thermal energy up to 1,500 °C.

Antora Energy is an early-stage startup, incubated at Cyclotron Road, that is developing a thermal battery that combines high-temperature thermal storage media with high-efficiency thermophotovoltaic energy conversion.

New electrochemistries

Electrochemical energy storage approaches such as flow batteries and some non-lithium battery chemistries boast long-duration capabilities.

Flow batteries circulate a liquid electrolyte through stacks of electrochemical cells and have long held the promise of 10-hour durations, tens of thousands of cycles, minimal degradation, and no limitations on depth of discharge.

This performance promise has lured venture capital investment and R&D — but so far, the investments have yielded few commercial, competitive flow battery products.

Flow battery firms such as Primus, Invinity, Sumitomo, UET, ESS and ViZn use a variety of different electrolytes and materials ranging from vanadium to zinc to iron. Installations are increasing but megawatts deployed are tiny compared to lithium ion.

Form Energy, a secretive startup thought to be developing a flow-battery variant and backed by Bill Gates’ Breakthrough Energy Ventures, has raised over $50 million in funding. Form’s first commercial project is a 1-MW, grid-connected storage system with Minnesota-based utility Great River Energy capable of delivering its rated power continuously for 150 hours, a real achievement if it can be commercialized.

Gravity-, air- and mechanical-based approaches

Despite some prominent startup failures, compressed air energy storage (CAES) remains a contender. Los Angeles Department of Water and Power selected Range Energy and Mitsubishi Power Systems to develop enormous underground salt caverns to store high pressure air (and eventually hydrogen) to replace some of the energy from the 1,900 MW coal-fired Intermountain Power Plant in Delta, Utah.

Startups Gravity Power, Ares Power and Energy Vault respectively raise a water column, a rail car and a weight with low-cost or curtailed energy — and generate power when the mass is lowered.

Quidnet Energy recently closed a $10 million financing round and won a contract with the New York State Energy Development Authority for a 2 MW/20 MWh (ten-hour duration) demonstration project of its geomechanical pumped storage. Quidnet looks to use excess renewable energy to store pressurized water underground at dry oil and gas wells — an alternative to traditional pumped hydro. CEO Joe Zhou said, in an interview with pv magazine, “Today, the duration is ten hours, but we can get to tens of hours, maybe hundreds of hours, dependent on the volume of the cavern.”

Regulation drives deployment

Grid-scale energy storage was rarely deployed by utilities until it was required by regulators – now it’s rampant in utility RFIs, RFPs and project awards. Recently, a federal appeals court in Washington D.C. upheld the Federal Energy Regulatory Commission’s Order 841 — maintaining that FERC has control over energy storage in its regulated interstate markets and energy storage connected at the distribution level must have the option to address wholesale markets.

Storage and long-duration storage technologies are here today – but regulators and utility commissions at the federal and state level are still adjusting.

William Conlon, president of thermal storage startup, Pintail Power offers an example: Long-duration storage technology in California is locked out because of the nature of California’s resource adequacy (RA) requirements. “Four hours is what California wants for RA. If you provide eight hours you only get paid for four hours. We’re at four hours today because that’s what you get paid for.”

Economically viable long-duration energy storage could accelerate solar and wind penetration, grid resiliency, and serve to stabilize volatile energy prices. But, long-duration energy storage will not become pervasive until regulators adapt to the capabilities of the technology.

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Tim Sylvia contributed to this article