Topic last reviewed: 10 April 2013
Category: Power and heat generation
A combined-cycle power system typically uses a gas turbine to drive an electrical generator, and recovers waste heat from the turbine exhaust to generate steam. The steam from waste heat is run through a steam turbine to provide supplemental electricity. The overall electrical efficiency of a combined-cycle power system is typically in the range of 50–60% — a substantial improvement over the efficiency of a simple, open-cycle application of around 33%.
A combined-cycle power system is the traditional technology of choice for most large onshore power generation plants, and is therefore well established. The technology have also been used on a few offshore installations for over 10 years. Most offshore installations are designed to generate power from open-cycle gas turbines which offer reduced capital costs, size and weight (per MW installed), but with compromised energy efficiency and fuel costs per unit output. Combined-cycle system operation is suitable for stable load applications, but less suitable for offshore applications with variable or declining load profiles. In a new ‘greenfield’ development incorporating a combined-cycle system design, the size of the gas turbine can be optimized and is likely to be smaller than an equivalent open-cycle configuration. Additionally, the waste heat recovery unit (WHRU) can replace the gas turbine silencer, thereby mitigating some of the space and weight constraints. Residual heat may be used instead of fired heaters, thereby improving the overall system efficiency. As such, the use of combined-cycle power technology is dependent on the power and heat demand of the installation. Combined-cycle technology is most cost-effective for larger plants. On an installation where the heat demand is large, the waste heat from the WHRU will normally be used for other heating applications, and hence there will be little residual heat left for power generation.
Retrofitting gas turbine generator technology to convert from simple, open-cycle systems to combined-cycle operation is complex and costly; hence this is not common in offshore installations. The additional topside weight and space necessary to incorporate a steam turbine, as well as the need for additional personnel on the platform to manage the steam system operations, makes a combined-cycle retrofit a challenging project.
A combined-cycle power system typically consists of the following equipment: gas turbines (GTs); waste heat recovery units for steam generation (WHRU-SG); steam turbines (STs); condensers; and other auxiliary equipment. The figure below illustrates a combined-cycle power system using a gas turbine generator with waste heat recovery and steam turbine generator.
Figure 1: Combined cycle power system using a gas turbine generator with waste heat recovery and steam turbine generator
For a more detailed description of this technology in typical onshore applications, please refer to:
- BREF on Large Combustion Plants (see Reference 3)
- BREF on Energy Efficiency (general info under the chapter for cogeneration, see Reference 4)
- Offshore Gas Turbines (and Major Driven Equipment) Integrity and Inspection Guidance Notes (see Reference 6)
|Years experience in the industry:||5-10|
|Range of application:||10 ~ 20 MW (not including the gas turbine) power units already installed in the industry; potentially up to 50 MW (Wall, et al.)|
|Efficiency:||50–60% (overall power generation efficiency), a significant improvement over simple cycle efficiencies of around 33%|
|Guideline capital costs:||Offshore brownfield: No known cases|
|Guideline operational costs:||Less fuel and energy is used, saving operational costs|
|Typical scope of work description:||For new offshore installations, it is important to analyse the need for power and heat, the available space, and weight restrictions to design an optimal solution to balance capital costs, logistical constraints and energy costs.
For existing offshore platforms with open-cycle gas turbine generators, the space and weight constraints to install a waste heat recovery unit and steam turbine generator must be considered. Such modifications may be costly or technically infeasible for some offshore installations, and the capital cost for modification, operational cost savings from using less energy / fuel, and reduced greenhouse gas (GHG) emissions must then be evaluated before the decision to retrofit the power system can be taken.
|Technical:||Footprint: size, weight, plot area required. Current design places WHRU-SG on top of the GT, alleviating plot area issues. Weight is an issue and needs to be optimized. Could potentially lead to higher overpressures due to more equipment congestion. Load profile of installation needs to be relatively stable. Brownfield integration—waste heat capture and transport, tie-ins.|
|Operational:||Operators need to be trained in steam systems; Operational complexity|
|Commercial:||Driven by fuel gas price and potential gas savings and/or value of CO2 reduction versus incremental capital costs|
|Environmental:||Improved energy efficiency over simple cycle. Combined cycle’s improved efficiencies lead to reductions in GHG, nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), and particulate matter (PM).|
The following are technologies that provide similar benefits (high efficiency power generation) and may be considered as alternatives to a combined-cycle system:
- Organic Rankine Cycle (refer to Reference 5)
- Aeroderivative Gas Turbines (see Reference 6)
- Offshore electrification (bringing power from shore)
Figure 2 illustrates high-level applicability of technologies based on the demand for power and heat. For low to moderate heat demand applications, combined-cycle technology may be appropriate for stable load applications; however, the trade-off between capital costs and fuel / emissions savings must be evaluated.
Figure 2: The applicability of technologies based on the demand for power and heat
Issues and risks are few and known. Combined-cycle technology has been used for many years for onshore applications. The technology has also been used for more than 10 years for offshore applications, including both floating (Snorre B) and fixed (Oseberg D) installations.
- Steam turbine power potentially replaces an additional GT generator
- Design may be optimized, especially for greenfield applications
- Design allows for heat extraction, eliminating the need for fired heaters
- Peak saving duties (under additional firing)
- Integration with nearby platforms, central power generation unit.
- Svalheim, Stig M. (Norwegian Petroleum Directorate, 2002). ‘Environmental Regulations and Measures on the Norwegian Continental Shelf’. SPE paper 73982.
- Kloster, P. (ABB Miljø AS, Norway, 1999). ‘Energy Optimization on Offshore Installations with Emphasis on Offshore Combined Cycle Plants’. SPE Paper 56964.
- European Commission (2006). ‘Large Combustion Plants’. Best Available Techniques Reference Document (BREF).
- European Commission (2009). ‘Energy Efficiency’ (see general information in the sections on cogeneration). Best Available Techniques Reference Document (BREF).
- Leslie, Neil P. et al. (2009). ‘Recovered Energy Generation Using an Organic Rankine Cycle System’. ASHRAE paper CH-09-024; in ASHRAE Transactions, vol. 115, pt. 1, Chicago 2009.
- Wall, Martin et al. (2006). ’Offshore gas turbines (and major driven equipment) integrity and inspection guidance notes’. Research Report 430, UK Health and Safety Executive.
- Offshore Magazine (2000) (website). ‘Combined cycle plant to power Snorre production platforms.’