HomeWorld RegionsEuropeCOMBINED CYCLE SYSTEMS: An Irish Incentive


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The Huntstown Power Company Limited (HPCL) is building a 343 MW combined cycle gas turbine (CCGT) generating station near Dublin airport to meet the rapidly growing demand for power in Ireland.

In autumn 2000, the Huntstown project was granted an allocation of gas network capacity under a competition administered by the Commission for Electricity Regulation (CER). The competition was essential because of the limited gas transmission capacity within the Irish network and the need to allocate network capacity to the most developed independent power producers (IPPs).

Market development

The Irish electricity market entered its first stage of liberalization on 19 February 2000. The CER was created under the Electricity Regulation Act (1999). The Commissioner reports to the Minister for Public Enterprise, who is responsible for the national energy policy.

The Republic of Ireland’s electricity sector has a peak demand of over 4000 MW and an average demand of approximately 2600 MW. Approximately 28 per cent of this market is now open to competition thanks to the IME Directive. However, recent draft directives have indicated that 60 per cent of the market could be open by 2003, and the Irish government has announced its intention to open the entire market by 2005.

Despite the apparent size of the open market, and a significant shortfall in national generation capacity, there are very few prospective new market entrants. This is because the initial market system has been designed to provide the incumbent utility with a significant level of transitional protection, and has done so in a fashion that presents significant market trading risks to new entrants. The initial market system will be reviewed and replaced in 2005.

Viridian Group

Energia, a wholly owned subsidiary of the Viridian Group, is rapidly becoming established as a major force in the independent competitive electricity supply sector within Ireland. Energia was the first independent company to engage in the new competitive market in the Republic of Ireland, and is presently supplying customers across the entire country.

Due to the developing and emergent nature of the Irish market, decisions need to be made swiftly and decisively. Therefore, Viridian Group has chosen to fund the Huntstown project from its own resources for the time being. However, it is intended that project finance will be sought at the earliest practicable opportunity.

The engineering, procurement and construction (EPC) turnkey contract for the Huntstown CCGT project is between HPCL and Siemens Ltd. who are doing the engineering and project management.

The construction programme under the contract has a duration of 26 months, with completion in the fourth quarter of 2002. Open cycle operation has been requested for the second quarter of 2002 to match Irish load growth.


The Siemens V94.3A gas turbine which is being used in the plant is a single shaft machine of single casing design. The turbine can burn gas or liquid fuel in both diffusion and low emission premix modes. The basic design follows previous models and includes:

  • One disc type rotor with central tie bolt and radial serrations
  • Two outboard bearings
  • Generator drive at the cold compressor intake end
  • Axial exhaust diffuser to the boiler.

The rotor is supported by two bearings. A combined journal and thrust bearing is located at the inlet side of the compressor and a journal bearing at the exhaust side of the turbine.

The rotor is an assembly of discs, each carrying one row of blades, and hollow shaft sections all held together by a pre-stressed central tie bolt. Hirth serration provides the alignment of discs and hollow shaft sections to allow free radial expansion and contraction, to transmit the generated torque. The turbine rotor is internally air cooled.

A Siemens ring type annular combustion chamber is connected to the turbine common outer casing. The combustor has 24 hybrid burners designated hybrid burner rings (HBR). These form a continuous ring flame with reasonably uniform temperature profile. The hybrid burners are of proven design and provide low NOx combustion without injection of steam or water.

Figure 1. Huntstown has a large potential output volume within the expanding eligible market sector
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The gas turbine uses a 17 stage Siemens designed compressor. The turbine section comprises four stages of turbine with complex labyrinth air-cooled blades.

The heat recovery steam generator (HRSG), supplied by Hitachi Babcock, is of a standard three pressure reheat design using natural circulation. The unit has horizontal gas flow and vertical tubes with steam drums above.

There is a diverter damper and exhaust bypass stack before the HRSG which allows open cycle operation of the gas turbine. This is an important feature given the commitment made by HPCL to the CER to provide generation during the summer of 2002 before the completion of the steam turbine cycle.

The steam turbine is a single flow high pressure (HP) turbine connected directly to a combined single flow intermediate pressure (IP)/low pressure (LP) turbine. The turbine is of a tandem compound design.

Generating power

The steam turbine rotors are designed with a single bearing system having only one bearing between the two turbine casings. Journal bearings are arranged at both ends of the turbine. The individual turbine shafts are rigidly coupled together with integral flanges. Each turbine is equipped with a combined stop and control valve for HP, IP (and hot reheat) and LP steam admission. Each valve has an electro-hydraulic actuator. The jacking oil and lube oil is supplied by electrically driven lift and main oil pumps during normal operation.

Separate generators for the gas turbine and steam turbine are of two pole design with closed air cooling. Indirect cooling is provided for the stator winding and direct radial cooling for the rotor winding. The cooling air at the generator outlet is cooled in a secondary glycol water circuit with outdoor mounted radiator coolers.

An excitation transformer is used to take the excitation current from the auxiliary power system. The digital automatic voltage regulator is of two channel design.

The gas turbine generator is used with a static frequency converter as motive power to start the gas turbine up to self-sustaining speed.

Figure 2. Huntstown, located close to Dublin, will help Ireland meet rapidly growing demand for power
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An air cooled condenser is used to condense the steam exhausted by the steam turbine since no direct cooling system is available at the site. This solution provides a completely plume free arrangement. The condenser will be provided by GEA and the single tube design is well proven and of modular configuration.

The condenser comprises four streets of bundles mounted on ‘A’ frames, each of the four streets have four large diameter fans below. The condenser comprises parallel flow steam condensing modules and counter flow dephlegmator modules where non-condensable gases are removed by water ring pumps.

Both gas and steam turbine generators are equipped with generator circuit breakers but are then connected by isolated phase busducts to a single three winding step-up transformer. The total generation is exported to the Finglas 220 kV substation via a single cable circuit.

The station has a comprehensive auxiliary electrical system using 6.6 kV and 400 V equipment. In general the equipment is single failure tolerant and is provided by two auxiliary transformers from the generator terminals. An emergency diesel generator is supplied for safe and secure shutdown of the station in the event of loss of grid connection.


Figure 3. Huntstown will use a Siemens V94.3A gas turbine coupled to a steam turbine and a Hitachi Babcock HRSG. The output of the plant will be 343 MW in combined cycle operation
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The CCGT power station is being connected to the ESB 220 kV grid by a cable from the generator transformer high voltage terminals to an air insulated switchyard within the site boundary owned and operated by ESB. From there ESB is laying its own 220 kV cable to the Finglas substation.

A gas connection agreement has been arranged between Bord Gais Eireann and HPCL for a connection from the high pressure Ballough to Brownsbarn No 72 gas pipeline with a spur pipeline from Kilshane AGI, and an on-site above ground installation comprising filters, heaters, pressure reduction and metering equipment with an on-site terminal point connection to the Siemens power station equipment.

The project also includes back-up distillate fuel oil capability with on-site storage of liquid fuel.

A dual train reverse osmosis plant was selected to produce pure water for the water-steam-cycle from on site well water. Each train is sized for filling the boilers and normal blow down requirements. However the plant is relatively small since there is no need for steam or water injection for emission control. The plant has an innovative boiler blowdown recycling scheme using cartridge filters and anion exchangers to minimize the waste water discharge from the site.

Auxiliary steam from the auxiliary boiler is provided to prewarm the HRSG feed water and provides seal steam for the steam turbine glands to allow vacuum to be maintained during overnight shutdowns.

The plant is controlled using a Siemens Teleperm XP distributed control system (DCS) to perform control functions, plant status acquisition and data collection and storage. This is structured on a redundant arrangement to ensure maximum availability and reliability. Secondary systems are controlled by dedicated PLC controllers which are interfaced with summary information to the main DCS.

Site location

Figure 4. Ireland is facing a shortfall in generating capacity.
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The gas turbine and steam turbine generators are located in a single turbine building with a common overhead gantry crane. These buildings are steel framed with insulated cladding walls and roof. The HRSG is an outdoor design with frost and weather protected systems. Electrical switchgear and controls are contained in modular power control centres with weatherproof design.

Boiler feed pumps, condensate polishing and the water treatment plant are enclosed within steel clad buildings. The project also includes a workshop store facility and administration building which includes the central control room for the plant. All foundations, roads, fences and landscaping are included in the EPC contract.

The site covers approximately 15 ha of agricultural land adjacent to the working Huntstown Quarry, so as a precaution in view of the scheduled blasting activities, the power station is designed for seismic peak particle velocity of 12 mm per second in the frequency range 4-40 Hz. Air over-pressure of 129 dB has also been allowed in the design.

The main exhaust stack from the HRSG and the bypass stack are limited in height to 112 m AOD Malin Head which is 45 m level surface above the aerodrome level. The bypass stack will be used for open cycle gas turbine operation prior to completion of the steam turbine cycle or in the event of steam turbine cycle outage.


The plant is rated at 343 MW and offers a net LHV efficiency of 54.8 per cent. Plant capability on distillate fuel in premix low emissions mode is rated at 316 MW.

Atmospheric emissions from the plant will be minimized by using a premix hybrid burner combustion system. The main emissions will include nitrogen oxide (NOx), carbon monoxide and carbon dioxide (CO2). Sulphur dioxide will only be emitted when emergency distillate fuel oil is used and the sulphur content of the distillate fuel will be limited to minimize this.

The proposed power station will have a high thermal efficiency and, together with the benefits of burning gas with low carbon to hydrogen ratio, this will result in significantly reduced emissions of CO2 per unit of electricity produced in comparison with existing thermal power plants. The contribution of the project to global warming will therefore be significantly lower than for alternative thermal power stations.

In order to meet the requirements of the Irish Grid, the facility is designed to be entirely suitable for base load, two-shift, full load and part load operation.

Operational team

In line with international practice, HPCL intends to employ a core team of maintenance and operations staff to provide day-to-day operations, preventative and first line maintenance services. The bulk of the services required to operate and maintain the station will be contracted out to approved suppliers.

Technical support will be provided through long-term arrangements with OEMs and key specialist service providers among others. Typical among these will be the use of long-term service agreements to provide for scheduled outage work on the main power components such as the gas turbine.

The key role of maintaining and ensuring a safe working environment, which minimizes the overall environmental impact of the production and maintenance process, will reside with the core operations and maintenance team.

ISO 9000 and ISO 14001 accreditation will be key business objectives to be pursued and achieved by the plant by the end of the first year of commercial operation.