When renewable power meets most of the demand and the residual load (ie, load not met by renewables) is near zero, grid stabilisation services become crucial. Stopping conventional generators typically reduces inertia and increases the need for frequency stabilisation. Traditionally, keeping power plants running at low load addresses this, but that leads to inefficiency and high emissions.
An emerging solution that’s gaining attention is the installation of battery systems in conjunction with synchronous condensers. The battery system stabilises frequency, while the synchronous condenser offers inertia, short-circuit capacity, and voltage control. A synchronous condenser operates as a generator synchronised with the grid, but it doesn’t have driving machinery. Instead, it relies on the rotating mass of the generator rotor, which may be combined with a flywheel. A generator connected to a gas turbine (GT) can function as a synchronous condenser when the turbine isn’t operating provided that a clutch is installed between the gas turbine and the generator, with an optional flywheel attached to the generator shaft. This concept requires a battery system for frequency stabilisation.
Another potential method involves operating a gas turbine at full speed and zero power generation (idle), but, instead of using conventional fuel, renewable hydrogen is employed, produced via electrolysis from electricity sourced from the grid at the same time as it’s combusted. As a result, the GT operates on renewable electricity, resulting in zero carbon emissions. The additional equipment required includes an electrolyser system with a capacity sized to meet the GT’s fuel consumption at zero load. When the GT ramps up power for active power dispatch, natural gas, biogas, or other fuels may be used if the hydrogen supply isn’t sufficient for full load operation. In addition to providing essential grid ancillary services at a competitive cost compared to alternatives, this operational mode offers the advantage of avoiding gas turbine maintenance costs associated with frequent start-ups and shutdowns, as the machine remains hot and spinning. This option, called “green idling”, is explored below.
Renewable power supply: variable and lacking critical grid forming capabilities
To maintain grid stability during periods of high renewable energy supply, inertia, reactive power, short-circuit current capability, and quickly deployable active power supply are required. A balanced use of battery systems and synchronous condensers can manage this.
In periods of limited renewable power, residual load must be met, and this can be handled by adding combinations of battery systems (or other storage solutions) and peaking/backup gas turbine plants.
Replacing traditional thermal baseload power plants with renewable wind and solar energy requires substantial investments in equipment to provide both intra-day and long duration residual load capacity as well as grid stabilisation services. The effort necessary to produce renewable fuel for the peaking gas turbine plants should also be considered.
The total investment in renewable power generation equipment and all the other necessary assets accumulates, and the levelised cost of renewable electricity must account for these additional grid-stabilising and balancing assets.
Although the anticipated total investment cost is high, the average cost of electricity is expected to be reasonable in the future, because wind, solar, and hydroelectric power sources don’t consume fuel.
Can a gas turbine be used instead of synchronous condensers and batteries?
Yes, definitely. A gas turbine provides grid stabilisation services when operated to meet residual load, and its effectiveness is even enhanced during idle (ie, non power generating) operation. When a GT is connected to the grid and operating at full speed, the combined rotating masses of the turbine rotor, gearbox, and generator contribute significant inertia.
Particularly significant is the inertia that can be provided by single-shaft industrial gas turbines. When the GT is in idle operating mode, the generator isn’t heated by supplying active power, and so, just like a synchronous condenser, it can provide a significant amount of reactive power. In the event of a short circuit in the grid, it can temporarily supply up to seven times its rated current, also similar to a synchronous condenser.
This capability ensures that fuses operate correctly. In contrast, power electronics like those used in battery systems can only deliver their maximum rated current. When the gas turbine is kept spinning, it’s also ready to instantly provide active load. It’s already hot and synchronised to the grid, so the fuel supply is only ramped up to increase power at the rate requested, up to the allowable ramp rate limits of the machine. Certain machines have the capability to accept a limited amount of reversed power, which means frequency stabilisation in both directions of power flow, even under zero load conditions.
The disadvantages of maintaining an idle operating mode typically include continuous fuel consumption and potentially high emissions from the machine. Therefore, under normal fossil-fuel operation, idling is considered to be a temporary measure when other grid resources are under significant stress.
How does hydrogen operation enhance idle mode?
When operating in idle mode during periods of high renewable power availability and using hydrogen as fuel, the operational cost of hydrogen should be low. Producing hydrogen from cost-effective renewable energy sources and supplying it to the gas turbine effectively means utilising inexpensive renewable grid power to maintain grid operations, with hydrogen only serving as an internal intermediate energy carrier. In the future, the operating cost should be compensated by revenues generated from supplying these grid services, strongly needed for the management of the grid. And because hydrogen is burned at the same time as it’s produced, no gas storage is needed except for minute scale damping. In addition, a hydrogen gas pipeline wouldn’t be needed.
Operating the gas turbine on hydrogen doesn’t produce carbon monoxide, because there’s no carbon present. The usual problems with poor combustion performance, high carbon monoxide, and unburned hydrocarbon emissions at low load are magically eliminated. Thanks to the high reactivity of hydrogen, the proportion of gas supplied to pilot burners can be reduced compared to natural gas operation. As a result, NOx formation may be lower during idle conditions when burning hydrogen, as opposed to operating in the same mode with natural gas. NOx emissions at idle may thus be kept at a level that can be sufficiently reduced by a downstream catalyst. By operating continuously, just alternating between green idle mode and active power dispatch, the downstream catalyst remains hot and prepared and experiences minimal thermal cycling stress because it rarely cools down.
Additional grid services through flexible operation of the electrolyser
The required storage volume for hydrogen between the electrolyser and the gas turbine is relatively small when fuel is consumed at the same rate and time as it’s produced. A small amount of storage (minute scale) for the purpose of system control may be needed, because the GT control needs to be stable and able to respond to frequency disturbancies. When specifying the storage, the designer should consider adding a little extra volume to allow for operating the electrolyser for short-term demand response: in other words, an even greater provision of grid support. But the total storage volume required is still very small compared to when storage needs to be scaled to bridge hours of full load operation.
How should the fuel supply for power dispatch be provided?
When the gas turbine needs to supply residual power, it may ramp up instantly by adding another more easily stored fuel. Even if the machine can operate entirely on hydrogen in the future, it may be more practical to use a combination of fuels if there isn’t a nearby hydrogen pipeline. This approach would allow each fuel to serve its specific purpose, even at full load. This means that the fuel supply can 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 power supply could help in obtaining permits to increase electricity use for electrifying the rest of the steam production, because it can help eliminate grid congestion issues.
When will the technology be ready?
Constructing this type of plant is already technically possible, and H2-capable burners are in development. Siemens Energy’s DLE burner gas turbines are commercially available for a 30 to 75% hydrogen-to-natural-gas ratio, depending on the turbine frame. However, that range applies to high load operation; low load hydrogen combustion is easier to manage, and therefore full hydrogen capability for low-load operation is expected soon. Some of the fuels mentioned above require the creation of dual-fuel hydrogen burners and modifications to fuel supply systems. These developments are underway and are relatively straightforward. Once there’s market demand for a new fuel capability, as OEMs we’ll develop and engineer the appropriate systems to meet that need.
Steam turbine green idling and use of high-temperature storage
Steam can be produced using an electric boiler to run a steam turbine, not at zero load, but at least at a very low load. The benefit of providing grid services in this mode is similar to that for GT green idling, except that the frequency stabilisation capability is rather limited because providing active power depends on starting up another boiler. If the steam turbine is situated in a CHP plant and the outlet steam downstream of the steam turbine is used, then the energy losses are negligible.
One of the benefits of steam turbine green idling operation occurs in a combined cycle that is intended for cycling operation. At start of the gas turbine, the steam cycle is already up and running, and bottoming cycle startup time and related stress are significantly reduced.
An interesting concept is the option of integrating a high-temperature thermal storage system with the bottoming steam cycle of a combined cycle, especially in CHP applications. The storage system doesn’t require a separate re-electrification system, because the steam cycle is shared. And again: when the storage is emptied and it’s time for the GT to start, the steam turbine is already hot and running.
When this kind of storage system is combined with GT green idling, electricity produced internally from the storage system in certain periods may be cheaper than external grid power for keeping the green idle GT operation running. These periods may occur when other storage systems dispatch power, but the grid price is still too low for GT dispatch.
Heat from storage or an electric boiler may also be used in periods of GT standstill for pre-warming or maintaining the HRSG temperature.
Green idling: an option to consider
In summary, keeping generators spinning ensures grid stability, and the concept of green idling makes this possible without the drawbacks of emissions and fuel costs associated with low-load fossil fuel operation.
Of course, its feasibility depends on the availability, and thus the economic viability, of green hydrogen. If dedicated electrolysers are installed it’s not enough that the price of electricity to feed them is low and compensation for grid services is high, but it must also occur for a sufficient number of hours per year to pay for the electrolyser investment.
Nevertheless, it makes sense to consider this approach as a possible option when designing new power generation, or as an upgrade to an existing plant, to improve revenues, reduce cycle stress and become hydrogen ready at the same time.
