Abstract
The increase of CO2 emissions is causing critical consequences such as glacier melting and sea level rising; to reduce these effects, DeSolination project proposes an innovative power cycle for a desalination system where supercritical CO2 blends are targeted as the next generation of working fluids. This paper aims to provide an overview of the benefits of the whole project and to list the technical challenges faced and overcome in the design of turbomachinery.
Multiple aspects have been considered in the definition of the thermal cycle and of the working fluid: hazards, environmental impact and thermal stability of the fluid, compatibility with the materials of the equipment and cycle efficiency.
The needed turbomachinery for pure CO2 Rankine power cycle applications consists of a pump and an expander, coupled with a motor and a generator respectively: the cycle objective is to recover the heat of the primary cycle which exploits solar energy. The equipment will provide a controllable heat production and it can simulate various solar field heat productions, reproducing transient conditions.
The preliminary design phase of the thermal cycle and of the equipment evidenced conflicting requirements in the selection of the working fluid: after the study of the dopant effect on fluid properties, the most critical part of the analysis was to reach the desired hydraulic performance. CO2 blends allow an optimized integration of the power cycle to the desalination plant: the use of a dopant inside the blend makes it possible to operate with a pump instead of a compressor by increasing the critical temperature of CO2; therefore, by maintaining a minimum temperature of the liquid phase cycle compatible with the temperature of the hot source, the heat exchange can take place. The goal of making the cycle as efficient as possible led to the study of several CO2 dopants.
The material compatibility of the possible working fluids was discovered particularly critical for exposures at the maximum temperature of the cycle (550 °C), which is representative of the current CSP technology: the selection of the working fluid, therefore, has implied a trade-off between the material compatibility constraints and the maximization of the cycle efficiency. The dopant that showed the least overall corrosive interactions with tested materials was therefore selected for the DeSolination project.
The challenge of this project, whose first pilot plant will be launched in 2025, was to ensure competitive solutions on the market that develop high Concentrating Solar-thermal Power (CSP) energy efficiency (>42% at 550 °C) and high conversion efficiency from heat to fresh water, with a reduction in CO2 emissions per cubic meter of desalinated water up to 70% compared to existing desalination systems.
