Design methodology for a prototype helical receiver adopted in the MOSAIC solar bowl system

This paper presents the design methodology of the receiver developed within the MOSAIC H2020 project, which proposes and is now commissioning an innovative solar bowl system, also known as SRTA (Spherical Reflector Tracking Absorber), aiming at reducing the Levelized Cost Of Electricity (LCOE) with respect to the most popular Concentrated Solar Power (CSP) technologies. The solar bowl technology consists of a fixed (non-tracking) spherical mirror that concentrates the solar radiation on a receiver, which moves tracking the sun. Past investigations of this technology were limited so far, although it can potentially reduce both capital and operational costs because of the adoption of a non-tracking mirror, which imposes special attention and care in the design of the receiver. The design methodology for the MOSAIC receiver presented in the paper consists of three main steps. First, a preliminary study is performed, which results in the choice of the helical receiver configuration among other alternatives. Second, the “macroscopic” features of the chosen configuration, which define the external size of the receiver, are determined, based on an ad-hoc developed optical model, adopting the Monte-Carlo ray-tracing technique implemented in the Tonatiuh software to compute the heat flux distribution over the receiver surface. Third, and last, the “microscopic” design parameters, i.e. the absorber tubes diameter and the number of parallel threads in the helix, are optimized by means of an ad-hoc developed quasi-3D thermal-fluid-dynamic model. This model solves the 1D mass, momentum and energy balance of the coolant and the lumped heat conduction problem in the solid wall, calculating the heat losses and, consequently, the thermal performance of the receiver. Finally, an example of application of the model to the analysis of transient operation is presented: the resulting rate of change of the tube wall temperature in the case of a fast start-up determines the minimum transient duration required to avoid excessive thermal stresses on the receiver.