| Grid stability
be switched when ramping up to active power supply for power dispatch. The secondary fuel supply may be natural gas today, but it’s likely to shift to biogas or liquid renewable fuel in the long term. If the turbine must be able to supply power continuously for an extended period, storing liquid fuel can be more cost- effective and straightforward than storing hydrogen.
Liquified biogas (referred to as LBG or Bio- LNG = liquified methane) can be an option; however, it has the issue of boil-off due to heat ingress. The gas that boils off can be mixed with hydrogen during green idling. There are currently liquid renewable fuels available that are suitable for GT operation. For instance, Siemens Energy has recently sold an SGT-800 gas turbine designed for operation on 100% hydrogenated vegetable oil (HVO). Bio-methanol, e-methanol, and ethanol are also promising alternatives, with liquid ammonia potentially becoming a viable option in the future. If there’s a hydrogen supply for green idling mode in addition to another gaseous or liquid fuel for power dispatch, then increasing the size of the hydrogen storage to use hydrogen either partially or fully for active power dispatch is merely a cost optimisation strategy aimed at reducing fuel expenses. If hydrogen supply from a gas grid becomes available in the future, then all operations could be based on hydrogen.
Reducing maintenance costs When a gas turbine functions as a peaker, starting frequently (up to once daily) and operating for just a few hours at a time, the primary factor that determines the required maintenance activities and costs is cycle stress. Some OEMs claim that there’s no cycle penalty from frequent starting, and that only operated hours should count. This argument is often challenged by plant operators and
service providers. Maintaining the machine in an idle load condition between instances of active power supply, rather than stopping it entirely, can result in significant savings on start-cycle-related maintenance costs. When this is compared with supplying frequency stabilisation from a battery system, we find that charge and discharge cycles also take a toll in terms of battery wear and replacement costs. These can be avoided if a gas turbine with green idling operation is used.
Will green idling work for combined cycle too? By properly designing a downstream bottoming cycle to accommodate low and varied steam conditions, the operation of the steam turbine can be maintained at very low loads. Keeping the turbine warm and ready for load acceptance can enhance accumulated combined cycle power generation and efficiency during power dispatch. And with the turbine in operation, the steam turbine generator can also provide ancillary services to the grid.
Are there alternatives to green idling for this type of plant? By installing a clutch between the gas turbine and the generator, the generator can keep rotating even when the GT isn’t in operation. The generator then acts like a synchronous condenser, with somewhat less inertia available than when the mass of the GT rotor is connected. Operating only the generator may be feasible in situations where the supply of renewable power is significant but the electricity price isn´t low enough to incentivise the hydrogen production needed for green idling mode. Frequency stabilisation services or an active power supply can be provided once the gas turbine has started. Operators can switch between using only the generator and green idling mode, depending on the situation. I anticipate that market price
volatility will fluctuate between extremes of cheap and expensive, with few intermediate prices. This may suggest not investing in a clutch if the plant already has green idling capabilities – but having one does provide greater flexibility and adaptability.
Could green idling be a feasible concept for cogeneration? If electric power becomes more cost-effective than fuel (including carbon tax) for most hours of the year, it may be advantageous to supply industrial steam using electric boilers or heat pumps. If electricity is used for green idling of a gas turbine, the waste heat downstream from the turbine can be captured in a heat recovery steam generator (HRSG) to produce steam for the industrial site, either directly or via expansion in a steam turbine. The green idling GT then becomes just another way of supplying steam from electricity. The resulting energy losses, compared to supplying steam from an electric boiler, occur through the loss in the electrolyser and a minor stack loss after the HRSG. In an industrial cogeneration context, the energy expenses associated with green idling are lower compared to a standalone facility. The capital investment needed for electrolysers and small-scale hydrogen storage is also expected to be lower than equivalent battery systems combined with synchronous condensers. These alternatives would otherwise be required to maintain grid stability.
An industrial site owner could generate revenue by supplying grid services with a minor additional expenditure of power. Installing a cogeneration unit that runs in green idling mode could also be a strategy for electrifying the steam supply. For an industrial facility that hasn’t yet transitioned from fired boilers to electric boilers, implementing green idling can serve as an initial step towards decarbonisation. This approach also ensures grid resilience and provides an independent power supply for the facility. The backup
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