Design and optimisation of a heliostat field and a sodium receiver for next-generation CSP plants

Abstract

The heliostat field and receiver subsystems are crucial in a concentrated solar power (CSP) plant, where solar irradiance is converted into thermal power. It is essential to optimise the subsystems at the design stage to maximise the energy yield and to minimise the cost. This thesis presents different modelling methodologies and case studies for the optimal design of the heliostat field and tubular receiver subsystems for next-generation CSP plants.

Firstly, heliostat aiming strategy affects the thermo-mechanical performance of the receiver and is a key factor for receiver reliability and interactions between the field and receiver. For the external cylindrical receiver, the modified deviation-based aiming (MDBA) method is proposed as a fast and accurate heliostat aiming strategy based on ray-tracing. The new aiming model enables efficient use of ray-tracing together with receiver thermal and mechanical models to closely match the flux distribution to local values of allowable flux on the receiver. The MDBA method maximises the thermal output while respecting thermal stress limits on the receiver, and is then coupled to a new co-optimisation technique to design the heliostat field and receiver together.

In the co-optimisation method, instantaneous optical, thermal and mechanical models are integrated in an annual system-level model to capture the highly transient behaviour of the subsystem, and the design is optimised using a genetic algorithm. Several techniques are implemented to make this complex and computationally expensive problem tractable. The co-optimisation method can be used to maximise the annual solar-to-thermal efficiency or to minimise the levelised cost of energy (LCOE).

It is found that the receiver flow configuration, including the flow path pattern and pipe diameter, affects receiver performance. Hence, the proposed integrated design methodology is used to explore the optimal flow configuration for a receiver with an oversized field at both design-point and annual conditions. The results show that the optimal receiver flow configuration achieves a low fraction of heliostat defocusing with a 20% oversized field, although the benefits on the annual energy yield are weakened by capacity limits of other system components.

Therefore, a system-level optimisation is implemented with relative sizing of the field, receiver, sodium-salt heat exchanger, storage and power block to achieve the lowest LCOE. An iterative surrogate-based optimisation (SBO) technique is proposed to accelerate the optimisation process. The best achieved LCOE is below 60.0 USD/MWh, within the range targeted by the DoE Gen3 program. A high capacity factor of 83.2% is achieved in the optimal design.

A further topic in this PhD thesis is the option of spillage skirts and secondary reflectors for performance enhancement of a cavity receiver. Show more