The gas turbines/battery hybrid: Ideal partner for wind and solar

June 26th, 2019, Published in Articles: Energize

The increasing amount of renewable energy in networks has led to challenging situations which require flexible balancing generation plant that can respond rapidly to changes in generation and demand. Open-cycle gas turbines (OCGTs) have been used successfully in this function, but the growth of renewable energy (RE) in some networks has outstripped the ability of OCGTs to respond quickly to changes, so additional flexible resource capabilities are required to stabilise networks.

Because of their rapid response to demand changes, gas turbines play a crucial role in electricity supply systems by matching supply to demand. The growth of variable, renewable-generating resources, particularly wind and solar power, has changed the operating and economic constraints on gas-fired generation. Many systems are adding high-flexibility, open-cycle gas turbines to provide a wide range of necessary functions for successful grid operation. Technologically advanced, open-cycle gas turbines provide fast-starting and rapid power-ramping functions necessary to accommodate high penetrations of power from wind and solar sources.

Gas turbines also provide essential reliability services, including adding system inertia and accepting secondary frequency control. However, delivery of these essential services requires a continuous, committed gas turbine for power generation. When the cost of the power produced by the unit exceeds the marginal value of electricity, there is a significant economic penalty to operation, even at minimum load [1].

Fig. 1: Operation of a HOB system [3].

Challenges and changes to the way the system operates, make the need for greater flexibility and adaptability paramount. Large amounts of wind and solar generation will ultimately prove more cost effective, and will lessen the need for fossil power  generation, and renewables will become become the preferred power supply. With less power demand from fossil fuels during the intervals of wind and solar production, synchronous power generation will be displaced. This will take two forms: units will either be:

  • Decommitted (stopped), or
  • Dispatched down, often to the minimum allowable level

Many of the economic challenges facing the industry derive from these operational constraints in conjunction with other competitive forces. Operators are faced with an environment of [1]:

  • Flat systemic load energy growth
  • Requirements for faster ramps and higher peaks
  • Multiple starts per day
  • Frequent dispatch to minimum
  • Close scrutiny to primary frequency response, and frequency response obligation
  • Increased need for spinning and other short-term reserves
  • System synchronous inertia and fast-frequency response requirements
  • Tighter emissions requirements

It would be ideal if it were possible to have all benefits of having the gas turbine committed without burning any fuel and producing unwanted electricity, but there are limits to the capability of the turbine on its own to meet this requirement.  Some of these limitations are described in the following:

Turbine start-up capability

This is the time taken to reach full operational speed and power from a cold or hot start. Current models available can achieve start-up times of ten minutes from cold start and five minutes from hot start. This is still not fast enough to respond to sudden changes in load, and for this reason gas turbines are kept at a low power spinning reserve state.

Turbine ramping capability

This is the rate at which the gas turbine (GT) can increase or decrease generation. The rate for a GE LM6000 50 MW turbine is 50 MW/min. Ramping rate governs the speed with which the GT can follow load variations.

Spinning reserve requirements

Spinning reserve can be defined generally as additional or unused generating capacity that is on line, synchronised and available to meet demand when activated.  GTs used as partners to RE must be running and synchronised to be used as spinning reserve. Running the GT during times of high RE production means that some of the RE or other generating capacity needs to be curtailed, with an associated increase in cost. The GT needs to be run at the lowest level possible in spinning reserve mode, and this means operating at low efficiency. GTs have minimum dispatch levels as dictated by firing stability, emissions control and other practical limitations. Common GT can run down to power levels of 25% but operating at a level below this has problems, and GTs simply cannot be run at zero power.

Hybrid systems

The drop in the cost of storage and the development of smart battery energy storage systems (BESS) makes the OCGT/BESS hybrid a possible solution to the problem. BESS are used extensively at various places in the grid to stabilise load, load demand response, peak shaving and to smooth out the output of solar PV and wind generators. They have recently been used to complement and improve the operation of auxiliary service generators such as open and closed cycle gas turbines.

The hybrid OCGT/BESS (HOB) system consists of an open cycle gas turbine operated in parallel with a BESS system. The BESS can be either a custom built unit or a multipurpose unit such as the Siemens Siesstart [4] which is designed for use in such a hybrid system. The unit will consist of the battery, power converters, one or more control units and switchgear and transformers for interconnection with the grid and the GT. The heart of the system is the smart controller, which regulates the operation of both the BESS and the GT.

To meet operating requirements the battery must retain sufficient charge to meet immediate demands (both to supply energy and to absorb energy) under all situations .This means that the battery must be recharged in a controlled manner. This function is carried out by the battery controller or storage management system.

Operation of the HOB

The operation of the system is illustrated Fig. 1.

Black start

This is the ability of a generating unit to start without an outside electrical supply. In the case of the hybrid unit, black start is the capability of the battery to supply the necessary electrical power and possibly the frequency reference to enable the GT to start up. The battery needs to have the capacity to power the turbine start-up motor as well as the various pumps,valves, ignitors and controls associated with the GT.  When the GT starts up, the BESS absorbs the energy produced until customers can be re-connected to their supply from the grid.

Grid support

The battery needs to be able to supply the additional demand from the grid instantaneously. The Bess can ramp up to full capacity within milliseconds and continue to do so until the GT has reached full capacity. The battery capacity may equal that of the GT or be set at a lower figure as shown in Fig. 1.

Frequency response (primary and secondary)

The bi-directional nature of BESS means that they can both absorb energy from the grid as well as feed energy back into the grid. Grid frequency is an indicator of grid stability and, under ideal conditions, will be 50 Hz. Primary reserves are the fastest services and are first in line to stabilise frequency deviations. The fast response times of BESS technology can provide both upward and downward regulation, and can allow the GT to maintain a stable output while the hybrid provides frequency regulation to the grid.

The BESS/GT hybrid  is capable of providing PFS and SFS while the turbine is running, by either injecting energy into the grid in addition to the GT output,  or absorbing energy from the GT, thus reducing its contribution to the grid. The battery assumes the task of fast response (both positive and negative), while the gas turbine only supplies the energy to deliver extended service requirements and balance the battery charge level. Such an optimized operating mode significantly reduces the stress on the gas turbine. The BESS is also capable of providing frequency response services when the GT is not operational.

Ramp up and ramp down support

The battery provides ramp up/down support by either injecting additional  power into the grid or absorbing power from the GT during ramp down. The BESS stabilises the GT during sudden drops in load such as disconnection by absorbing power and providing a soft shutdown.

Spinning reserve

The BESS is capable of providing additional spinning reserve power while the GT is operational, depending on the state of charge.

Minimum environmental load management

The technical minimum environmental load (MEL) is defined as the minimum condition at which the GT is able to operate, while still meeting the environmental limits, in particular NOX and CO emissions. When the grid demand drops below the MEL, the BESS can absorb energy from the GT, allowing continued operation. Without the BESS, the GT would have to shut down.

Stabilisation of turbine during grid outages

In the event of a grid outage, rotating power-generating machines tend to speed up very quickly because the kinetic rotational energy is no longer balanced by a sufficiently high generator load. The most important objective in stabilising the  GT on grid outages is to limit the speed increase as well as to maintain turbine operation at the auxiliary power requirement or in island mode. On disconnection from the grid, the excess energy in the rotating system is removed through the generator as electrical energy and input to and stored in the BESS. The residual energy is removed from the process through controlled instantaneous charge of the BESS for a specific time. This significantly reduces the acceleration and thus the speed increase. Battery charge power is gradually reduced again after the speed has stabilised.

Hybrid configurations

In a network hybrid the generator can be decommited while the BESS remains synchronised and provides the equivalent of spinning reserve.  The system can operate without the turbine being operational and without the generation of power.

Gas turbine primary

In this configuration the GT  is the main source of power and the battery is used to assist with start up, ramp rates and low level output . A prime example is the Stanton energy reliability centre reserve power plant [2]. The constituents of the plant are an aero-derivative gas turbine (GE LM60001 – nominal 50 MW rating), and a 10 MW/4,3 MWh packaged battery energy storage system. The gas turbine features state-of-the-art controls with 5 min fast start, fast synchronisation, and improved NOx controls at ultra-low output. The battery system is fully integrated controls, with 4-quadrant self-commutating inverters, and complementary balance-of-plant. The annual number of starts is expected to decrease by approximately 50% and run hours by 60% [2].

Battery primary

In this configuration the battery is the primary source of power and the GT is used to provide additional power when the capacity of the battery is exceeded. This configuration is claimed to be useful for extending the useful life of older gas turbines. An example the would be combined gas and battery grid services power plant by Technische Werke Ludwigshafen and Younicos [7]. The system consists of a 4 MW gas turbine with a 6,5 MWh battery.

References

[1] NW Millar, et al: “Hybridizing Gas Turbine with Battery Energy Storage: Performance and Economics” Cigre, Paris session 2018, Paper A1-101.
[2] GE: “Gas turbines and batteries : A perfect pairing”, Power engineering international, March 2018.
[3] U Fuchs: “Battery energy storage solution: Enhancing the operational flexibility of combined cycle gas turbines”, Third International Hybrid Power Systems workshop, May 2018.
[4] C Miekowski: “Battery gas turbine combination provides power plant flexibility”, Siemens Power, February 2018.
[5] U Fuchs: “Energy storage on the grid: Supporting energy transition”, SA Energy Storage conference, 2017.
[6] S Hoffman: “Investigating battery storage in combination with gas turbine generation for frequency regulation”, University of Sheffield.
[7] M Fuhs: “Combined gas and battery grid services power plant by Technische Werke Ludwigshafen and Younicos”, PV magazine, March 2018.

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