Aug-2023

Solutions for long-duration energy storage

A renewable grid requires storage that lasts more than a few hours, but as the need increases, technology is evolving at a rapid pace.

Jim Berry
Just In-Time Energy Company

Article Summary

As the penetration of renewable energy on the grid increases beyond about 30%, energy storage becomes an essential requirement. While batteries are a good approach for short-term storage, mechanical and fuel-burning systems will be needed when solar and wind cannot meet peak demand – both to supply natural gas for peaking power generation and to provide cost-effective, long-duration storage of excess renewable energy to be reclaimed at peak periods.

But there is a problem. The growing presence of renewables on the grid means that natural gas usage is likely to decline steadily. There will be a collision of peak gas demand at the same time as the need for peak backup power ramps up. This presents a real challenge.

The good news is that the need for affordable long-duration energy storage is driving innovation. Several solutions have been proposed, including a combined gas and electric storage system. These systems store both natural gas and excess electric energy in various forms and repositories, including within the pipeline network, using renewable energy sources. In this way, both natural gas and green energy are available whenever the direct supply of wind or solar energy falters.

New approaches to energy storage
Despite the enormous expansion of renewable energy over the last two decades, 40% of the electric power in the US continues to be produced from natural gas-fired generation  (EIA, 2023). As the penetration of renewable power generation rises even further, the number of operating hours for fossil-fuel units will drop. Thus, natural gas consumption will gradually fall along with the average amount of gas in storage reservoirs. There will be no incentive to store gas that will not be used.

That said, demand for gas-based generation during peak demand periods is likely to remain. When renewable generation is hindered by cloud cover or lack of wind, reserves of stored energy will be needed to help meet grid demand. In some cases, this can come from the storage of excess renewable energy in battery energy storage facilities. But that will not nearly be enough. A mix of short-, mid-, and long-duration storage options will be required to maintain a stable grid.

Traditional approaches to longer-term storage include compressed air energy storage (CAES) and pumped hydro. CAES uses air that is compressed using excess renewable energy and stored at high pressure underground. When needed, electricity is produced by feeding the high-pressure air to a turbine to produce power. Pumped-hydro plants use excess energy to pump water to a higher elevation. When the power is needed, water is released to power a turbine in the same way that hydroelectric power is generated.

But there are plenty of advances on the horizon. One concept funded by the Department of Energy (DOE) is based on what is known as Compressed Natural Gas Energy Storage (CNGES) technology. It is currently being studied at the Abbott Power Plant in Illinois. The main difference to CAES is that natural gas is stored rather than air and in a pipeline rather than an underground cavern.

How does it work? Gas pressure is raised during off-peak periods and discharged at lower pressure during peak demand periods through a turboexpander coupled to an electrical generator. This concept has the advantage over CAES of lower capital costs, as it takes advantage of the existing pipeline, compressor, and natural gas storage network. However, it is not large-scale technology. It may work well for mid-duration storage but not for long durations.

The National Renewable Energy Laboratory (NREL) is evaluating a somewhat similar way to store pressurised natural gas but in depleted hydraulically fractured oil and gas wells. The hope is that this approach can be co-located with solar and wind farms situated on or near old wells so natural gas generation can take over seamlessly when needed by using the stored gas as fuel. It recovers some of the heat of compression of the stored gas. One challenge to overcome is flow resistance while compressing the gas into the fractured well and when withdrawing it (see Figure 1). 

Another promising concept is combined electric and gas storage (CEGS) developed by Just in-Time Energy Company (Just In-Time Energy Company, 2022). CEGS is a hybrid, long-duration energy storage system storing both excess renewable electric energy and natural gas in the form of liquefied natural gas (LNG) at off-peak times. The LNG is converted back to the gaseous form at the required pipeline pressure during times of peak demand, along with the production of electric energy to the grid. This is accomplished at a fuel rate equal to a modern combined cycle power plant while increasing the deliverability of natural gas storage and minimising the amount of time peaking gas turbines are required.

The CEGS system is one of two applications of the patented Re-condensing Power Cycle for Regasification (the other use is for liquid air energy storage) (see Figure 2). CEGS returns the stored gas to the pipeline at peak times at a flow rate 3-4 times the off-peak flow rate into the LNG plant. As it is a hybrid system that uses stored electric energy (as LNG), and a small amount of fuel, it is possible to return more power to the grid than is consumed in making the LNG.

Energy can be stored as heat as part of the system if desired. Hot molten salt heated with excess renewable energy is stored in insulated tanks, with this stored heat added to the CEGS system to lower fuel consumption. The electrical resistance heaters used to heat the molten salt can start almost instantly and can be used to store excess electric energy that often is available for short periods on sunny, windy afternoons, periods of time too short to start the LNG plant. SSS over-running clutches are used between each of the turbines and their generators. A third clutch is positioned between the gas turbine and its generator. These clutches allow the generators to remain spinning and connected to the grid when the system is not producing power and turbines are shut down. These spinning generators add value as they can be used to provide ancillary grid services, including voltage support, spinning reserve, inertia for grid stability, and power factor correction to allow the transmission lines to transmit more power without overheating. In many regions, grid operators pay for these services. Such services are becoming especially important as more wind and solar come online: renewable power sources lack the system inertia or the ability to correct the power factor traditionally supplied by the fossil-fuel generators they are gradually replacing.