Session: 30-02 Cycles

Paper Number: 153074

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

Supercritical 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 decarbonize the operations of offshore 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 technically-feasible and cost-effective alternative to conventional steam turbines used in Combined Gas & Steam Cycles, not only in terms of performance but also, and very importantly, 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. This might nevertheless not be optimal in terms of thermal performance. Indeed, same as open cycle gas turbines (OCGT) are typically designed with higher pressure ratios to minimize waste heat in the exhaust, and standard combined cycle gas turbines (CCGT) have lower pressure ratios to favor the performance of the bottoming steam turbine, this paper explores if new design parameters should be adopted in gas turbines used in combination with bottoming supercritical Carbon Dioxide power cycles in order to achieve the highest efficiency possible.

With this idea in mind, a global optimisation of the topping and bottoming power cycles has been carried out, using performance models of increasing fidelity. First, simple gas turbine and sCO₂ models have been used to identify trends for increasing firing temperature. Then, the performance model of the gas turbine has been extended to account for the impact of turbine cooling on performance, as turbine inlet temperature increases. Finally, preliminary designs of turbomachinery and heat exchangers have been produced to understand the performance of these individual major components that can be expected.

The analysis is based on a 25 MWe gas turbine and a bottoming preheating sCO₂ cycle. The preheating cycle offers particularly good waste heat recovery capabilities and is proven by other authors to be most suitable for this application. As said, realistic boundary conditions for components as well as an analytical cooling air model for the gas turbine are integrated to address the impact on cycle performance with blade cooling. 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. By considering a wide range of parameters, the entitlement of the technology as well as the margin for cost reduction and standardization of components are examined.

The outcomes of this study show the potential of a combined optimisation of the gas turbine and sCO₂ bottoming cycle for cycle efficiency, power output and footprint in comparison to commercially available solutions. It is very interesting to confirm that the designs (pressure ratio, in particular) obtained for the gas turbines that would yield highest efficiency are not far from those already employed by engines in the marketplace; this is excellent news for the industry, as it means that only slight modifications are needed. 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 very good for standardization. Overall, this suggests that the reduction of greenhouse gas emissions coming from the operations of offshore platforms is within reach for these systems. Its feasibility will have to be confirmed by a future economic analysis.

Presenting Author: Leonard Muke University of Seville

Presenting Author Biography: Leonard Muke is a Ph.D. Student at the University of Seville under the ISOP project. Mr. Muke started his research activity in 2023 after completing his bachelor’s and master’s degree at the University of Duisburg-Essen. He is especially interested in turbomachinery, thermodynamics, power cycles and numerical simulation. His research activity at the moment is focused on the transient simulation of supercritical CO2 cycles.

Authors:

Leonard Muke University of Seville
Amgad Khamis University of Seville
Adonis Constantinidis University of Seville
Francesco Crespi University of Seville
David Sánchez University of Seville