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Thermoeconomic Analysis and Optimisation of Air-Based Bottoming Cycles for Water-Free Hybrid Solar Gas-Turbine Power Plants (Mechanical Project)

The growing worldwide energy demand and the impacts of climate change due to anthropogenic greenhouse gases emissions are among the major issues facing humanity. The global energy system, responsible for most of the greenhouse gases emissions, is therefore at the heart of global concerns. In particular, the search for a reliable, sustainable and environmentally friendly means of generating electricity is a crucial matter, with growing worries about the scarcity of fossil resources, air pollution and water acidification.

For these reasons, alternatives for the sustainable production of electricity are to be found.Among the plethora of alternatives available, concentrated solar power (CSP) appears as one of the most favourable options. The stability and dispatchability of production achievable by the integration of storage and fuel-solar hybridisation are amidst the major advantages of this technology.

Nevertheless, conventional CSP plants are based on stream-turbine cycles which consume large amounts of water. In addition to the low thermodynamic efficiency of this type of cycle, the installation of such plants in water-scarce areas is complicated by their reliance on water resources. Thus, the study of new concepts that overcome these drawbacks is necessary for the future of this technology. The availability of high temperature solar receivers for solar tower systems opens the way for the use of gas-turbines in hybrid solar-natural gas configurations. In order to increase the efficiency of the cycle while keeping the water consumption as low as possible, a promising alternative to the recovery of the waste heat in steam-turbines is to use a low-temperature intercooled-recuperated gas-turbine cycle.

This work focuses on the analysis and optimisation of the performance of an innovative hybrid solar gas-turbine power plant with an air-based bottoming cycle (ABHSGT). The evaluation considers thermodynamic performance, economic viability and environmental impact as interrelated concerns. With this in mind, detailed steady-state and dynamic models of the power plant have been developed and validated by comparison with existing components. A second model without bottoming cycle has been built for comparison. A multi-objective optimisation using an evolutionary algorithm has then been performed, optimising both capital cost and specific CO2 emissions and resulting in a Pareto-optimal set of possible designs.

The analysis of the trade-off curves resulting from the optimisation reveals promising outcomes. The global minimum for the levelised cost of electricity, found at relatively high solar shares, proves the economic potential of the technology. The integration of the bottoming cycle decreases significantly the levelised cost of electricity and the CO2 emissions of the system compared to the reference plant, and higher efficiencies are achieved.

The optimal design selected for an in-depth thermoeconomic and environmental analysis exhibits a levelised cost of electricity of 109 [USD/MWhe] for a solar share of 20% and an overall exergetic efficiency 38.5%. The specific CO2 emissions are reduced by 33% compared to simple gas-fired power plant. The water consumption is kept at very low levels compared to other CSP plants, making the system suitable for the deployment in water-scarce areas.

In addition, the environmental impact induced by the land use requirements is considerably lower than that of other renewable energy technologies. The sensitivity analysis performed to assess the consequence of changes in varying financial conditions on the levelised cost of electricity and the net present value reveals that the system studied represents a profitable investment in the presence of feed-in tariffs.In the light of the performance obtained in the three aspects considered (thermodynamic, economic and environmental), it can be concluded that the ABHSGT represents a promising alternative to other renewable energy technologies, especially in water-scarce areas.
Source: KTH
Author: Sandoz, Raphael

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