Battery energy storage systems: enhancing the operational performance of flexible combined cycle industrial gas turbines
There is no doubt that both the grid and power market structure are changing. Energy independence, sustainability, carbon targets and all the mechanisms designed to help achieve low carbon goals, have meant that globally, investments in green or renewable energy now exceed investments in conventional generation.
Authors: John Charlton; Uwe Fuchs; Bridgit Hartland-Johnson; Mike Welch
Variable generation from renewable energy resources, distributed generation, retiring conventional generation fleet and the threat of cyber attack mean that system operators require additional and alternative services for balancing and system security, to ensure the stability and reliability of the grid. Whilst modern combined cycle gas turbines (CCGTs) based on an industrial gas turbine are designed for multiple starts, modern system operation, as well as demand requirements often necessitate much faster response times that cannot easily be met by conventional resources. Combining fast response battery electrical energy storage systems with CCGTs provide solutions for economic and reliable supply of balancing services such as frequency response, system security services such as black start and fast ramping support to address spurious peaks.
There is no doubt that both the grid and power market structure are changing. Energy independence, sustainability, carbon targets and all the mechanisms designed to help achieve low carbon goals, have meant that globally, investments in green or renewable energy now exceed investments in conventional generation. As a consequence, faced with price and policy uncertainty, thermal fleet are being retired. Demonstration of this change could be seen this year when Germany, Portugal and the UK were all able (at times) to completely meet demand with renewable energy [ 1 ]. Although conventional generators still had to be run to manage volatility and ensure system stability.
Whilst this is great news for achieving EU2020 climate targets [ 2 ], it brings additional challenges to system operation. More variable generation, which is less dispatch-able vis-à-vis demand requirements, means that more ancillary services, namely balancing services such as frequency and voltage regulation are required. A bigger mix of inverter connected generation opposed to rotating machines also means a loss of rotational system inertia. This creates a need for new super-fast-to-respond alternatives to maintain grid stability. Further, uncertainties regarding future investment in critical base-load power plants, peaking plants and interconnectors, and threats of cyber attacks as a result of global unrest make our grids ever more vulnerable.
Co-locating battery electrical energy storage systems (BESS) with combined cycle gas turbines (CCGTs) can provide solutions to such challenges. First to enhance the start up of CCGTs thus enabling them to provide much faster services such as enhanced frequency regulation (EFR) and firm frequency response (FFR), whilst still being available and running to provide longer duration services such as short term operating reserve. Second as a solution to replace the diesel generator (DG) normally used to provide power for black start to a CCGT used to rebuild the grid in the event of a black-out, thus providing a much faster restoration service. Finally as a resource to support fast ramp-up to meet peak demand requirements.
Thankfully, as a result of the accelerated downward price trends (seen between July 2015 and July 2016) for the highest performing lithium-ion 1C or 2C (ratio between power and energy) cells it means that it is now economical to install a high-capacity solution that enables start up and peak support for both the gas turbines (GTs) and steam turbine (ST) of a CCGT plant.
National Grid in the UK procures balancing services as tools to help support grid stability. Frequency is one of the measures of grid stability. As part of its licence conditions, National Grid is obliged to maintain grid frequency within a very narrow bandwidth around the nominal frequency of 50 Hz.
There are two types of frequency response: (i) dynamic frequency response is on line and provides second-by-second support; (ii) non-dynamic or static frequency response is triggered according to specific frequency deviations. National Grid offers different products for provision of these services such as FFR, which requires delivery within 10 s and covers both primary and secondary reserves, and EFR, which is a pre-fault service; designed to support system inertia and requires delivery within 1 s.
In the event that there should be a shutdown of the entire transmission system, National Grid also requires system security tools such as black start, whereby the grid is rebuilt gradually by bringing on generation and load simultaneously. Under normal conditions, generators require an electrical power supply provided by the grid to start up, but in the event of a grid collapse this power supply is not available. Therefore, black-start power supplies are required. In addition to this, merchant peak power plants may need to start very quickly from cold, in order to meet changing demand requirements and contractual obligations.
In the UK, electrical energy is traded on the wholesale market and price is a reflection of current supply and demand. During September 2016, several incidents due to outages, resulted in extremely high market prices (>£1000/MWh). Fig. 1 is an extract from Cornwall Energy showing recent price volatility and extreme price peaks [ 3 ].
Fig 1: Power prices – Cornwall Chart of the Week commencing 12 September 2016 (source: Cornwall Energy Market Bulletins)
Combining BESS and CCGTs mean that conventional generators can continue to be used to support grid services whilst maintaining operational efficiency. The event of an outage BESS can provide sufficient power to the CCGT to enable it to start and provide black-start functionality to rebuild the grid. In addition to this service, the BESS can provide faster-start capability to CCGTs that normally start up in minutes rather than seconds. This enables access to fast response services such as EFR and FFR and therefore provides improvement in operational flexibility and access to secondary revenue streams. Further, the BESS can be used to provide fast ramp-up requirements to meet spurious peaks; again providing access to additional revenue streams, but without compromising the life cycle of the plant.
Fig. 2 shows the typical operation of a GT, an example of the FFR and EFR balancing service requirements and how the BESS could provide support until the GT can start up under normal operation. Further diagrams in the results section will show how the BESS can interface with the GT and ST of a CCGT. This configuration can provide full cycle support for the applications mentioned previously, i.e. black start, fast start and fast ramping.
Fig 2: Industrial Trent 60WLE (gas fuel) – 10 min start sequence
The above approach sounds straightforward; however there are some challenges technically, economically and as a result of current market design that may prevent access to markets for certain classes of assets.
As mentioned previously, an increasing penetration of variable renewable generation requires more flexible plant to compensate volatility and maintain grid stability. In this instance, flexible means resistant to multiple start-ups and shut-downs per day. However, start times and power supply requirements for starting GTs vary greatly. For example, aero-derivative industrial GTs require low starting power (350 kW for the 50 MW Industrial Trent 60), whereas it is closer to 2200 kW for a single shaft industrial GT [ 4 ] of similar output and this would have a significant impact on the sizing and cost of the battery storage system (between £300/kW and £500/kW) [ 5 ]. Therefore, it is important to ensure the most economical and technically feasible combination.
A BESS+DG may be considered a more flexible option due to concern that too many GT start attempts (due to GT combustion chamber burning problems, etc.) could deplete the battery storage. However, this paper is based on an industrial aero-derivative GT that have >99% reliability [ 6 ] and negligible starting load versus the large battery capacity (Fig. 3 ).
Fig 3: Example of varying start-up times for GTs compared with the requirements of UK grid services
Economics are another challenge when comparing the investment cost of a BESS to a DG used for black start. However in order to meet carbon budgets or targets it is expected that globally costs for DGs will escalate as pressure increases to reduce noise and pollution and ultimately strive towards lower carbon solutions. It is also anticipated that in the near future cost parity between BESS and DGs can be achieved with BESS playing a superior role due to the additional services that can be provided.
In 2014, Siemens delivered a turnkey BESS project to Vulkan Energiewirtschaft Oderbrücke GmbH (VEO) to ensure the black-start capability of a 40 MW GT at the Eisenhüttenstadt power plant that supplies electricity and heat to the ArcelorMittal Eisenhüttenstadt GmbH (AMEH) steel mill.
VEO is responsible for the operation and maintenance of all electrical networks at the plant. The gas-fired power plant has an installed electrical capacity of 153 MW. AMEH operates the largest integrated steel and rolling mill in eastern Germany at this location, with a closed metallurgical cycle comprising iron works, converter steel mill, hot rolling mill and cold rolling mill as well as various galvanising and finishing plants.
In the case of failure or a severe black-out of the local 110 kV distribution network, VEO's power plant switches over to island mode to continue supply to the site. This island network keeps the critical production processes at the AMEH steel mill operating, and thus prevents consequential damage, which could cost millions of Euros (Fig. 4 ).
Fig 4: Example of VEO plant interaction with the BESS
Switching a running GT plant from grid connection to island mode is not an easy task and because testing is impossible during normal operation there remains a risk of failure. To mitigate this risk, the black-start functionality provides assurance against extended outages that would result in large consequential losses.
To start the stand-by GT from a de-energised state, a starting motor is used to bring the turbine up to speed. Under normal conditions, this starting motor requires power from the grid. However, this power would not be available in the event of a grid outage. Therefore, the BESS is now used to provide the power supply, thus ensuring reliability. The BESS at VEO uses lithium-ion batteries and has a power rating of 2.8 MVA/1.2 MW and a capacity of 1080 kWh.
This solution guarantees the black-start capability of the gas-fired power plant at any time and ensures the power supply in the event of failure of the upstream 110 kV distribution network.
Having the storage facility located at the power plant not only enables the plant operator to have secure and reliable black-start capability but also means that the plant can now participate in the primary reserve market and attract additional revenues.
Fig. 5 shows the operation of a typical CCGT configuration as used in peaking/cycling applications, combined with BESS to provide ultra-fast response and ultra-flexible operation.
Fig 5: Process flowchart of flexible CCGT
Standard CCGT operation can be enhanced in the following ways to make the plant more flexible:
- Addition of inlet spray inter-cooling to boost power output (optional).
- Flexible, fast-start CCGT using single shaft concept with double end drive alternator to boost power during peak periods, and to enable higher fuel efficiency for longer periods of operation, with full combined cycle power output available within 30 min.
- Addition of BESS to enable black-start capability of the gas GT, allow instantaneous response to grid demand for balancing; providing 100% station output power until GT or CCGT starts up, with option for additional power during peak periods.
- BESS recharged by power plant, during run down at end of operation period or by grid at times of low power prices.
- Optional addition of duct burner and larger ST to boost power output of ST during peak periods.
- Optional addition of clutch between GT and alternator to enable plant to operate as synchronous condenser to provide voltage regulation.
The timing diagram in Fig. 6 shows the optimal starting sequence for a CCGT plant based on an industrial GT and how the BESS could support almost instantaneous start-up; allowing the plant to participate in higher-paying/faster grid services.
Fig 6: Example of start-up sequence for GT and ST and ramp-up to meet peak demand
The BESS capacity requirement considering the combined start-up needs of the CCGT is shown in Fig. 7 . The hot-start sequence represents a 2 + 1 configuration using an industrial TRENT 60 GT with a once-through steam generator and Dresser-Rand ST with a total rated capacity of 150 MW.
Fig 7: Example of energy storage capacity requirements
Note: For this example, hot-start means running up again, e.g. after midday downturn due to photo-voltaic, warm-start means running up after an overnight downturn and cold-start means start-up after a weekend downturn. Warm-start moves the knee-point to the right by 10 min and cold-start by 40 min.
For the foreseeable future CCGTs will be required to support the integration of renewable energy and the transition towards a low carbon economy. However, many current regulatory and market environments do not provide investors enough confidence to build or modernise plants. Combining CCGTs with BESS provides the flexibility that enables plant operators to access additional revenue streams whilst optimising performance and minimising environmental impact.
- https://www.theguardian.com/environment/2016/may/18/portugal-runs-for-four-days-straight-on-renewable-energy-alone .
- http://ec.europa.eu/europe2020/pdf/targets_en.pdf .
- http://www.cornwallenergy.com/Publications/Energy-Market-Bulletin .
- Rolls-Royce Trent 60 GT Power Generation Packages Application Handbook .
- Recent price trends as a result of the National Grid EFR Tender .
- Measured results 99.6% start reliability achieved in 2016 at a US installation of 8 Trent GTs and daily or twice daily starts, with the longest ‘failed start’ delay being 15 minutes .