Decarbonisation Technology - August 2023 Issue

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

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.

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

Figure 1 NREL plans to harness depleted oil and gas wells as a reservoir for the storage of compressed natural gas Courtesy: NREL