Unconstrained Optimisation of Combined Gas Turbine & Supercritical Carbon Dioxide Cycles for Off-Shore Applications

Supercritical CO2 (sCO2) cycles have attracted interest due to their high energy conversion efficiency, small footprint and flexibility for integration into different energy sources (solar, waste heat recovery, nuclear...). These are very attractive features that could help decarbonise the operations of off-shore platforms for oil extraction, where the adoption of a bottoming sCO2 power cycle downstream of existing gas turbines could boost the efficiency of power generation without constraining operational flexibility. This could be seen as a more feasible alternative to conventional steam turbines used in Combined Cycles, not only in terms of performance but also cost and volume/weight. The aforedescribed combined power plant has already been explored by other authors in the past. Nevertheless, these past studies have relied on gas turbines existing in the market, into which tailored bottoming sCO2 cycles have been integrated, not really assessing if modifications of the gas turbines could yield benefits in terms of combined cycle performance. This paper explores if new design parameters should be adopted in gas turbines used in combined sCO2 cycles to achieve the highest efficiency possible. With this in mind, a global optimization of topping and bottoming cycles has been conducted using detailed performance models. The gas turbine model incorporates a secondary air system for blade cooling, supported by a mean-line design code to supply input data for the developed blade cooling model. The study focuses on a 25 MWe gas turbine paired with a preheating sCO2 cycle, known for its effective waste heat recovery capabilities and suitability for this application. In a thermodynamic study, the impact of different pressure ratios and combustion temperatures of the gas turbine on the bottoming cycle performance is evaluated, aiming to determine the highest achievable combined cycle efficiency. The outcomes of this study show the potential of a combined optimisation in terms of cycle efficiency in comparison to commercial gas turbines. It is interesting to confirm that the designs obtained for the gas turbines that would yield highest efficiency are in a range of 18–22 bars and therefore not far from those already employed by engines in the market; this is excellent news for the industry, as it means that only slight modifications are needed. With a combined optimization of topping and bottoming cycle, peak efficiencies of close to 54% and a combined power output of 40 MWe with a gas turbine TiT of 1400°C can be achieved. Moreover, certain configurations of the bottoming system seem to be very resistant (performance-wise) to changes in the specs of the gas turbine, what is again good for standardisation. Overall, this suggests that the reduction of greenhouse gas emissions coming from the operations of off-shore platforms is within reach for these systems.