Design of a Secondary Concentrator for a Small Particle Heat Exchange Receiver

Abstract

The design of a secondary concentrator for the Small Particle Heat Exchange Receiver (SPHER) using a Monte Carlo Ray Tracing (MCRT) method is discussed in this paper. Applying basic MCRT rules, a modular solver logic for secondary concentrators is established. The logic is coded into FORTRAN subroutines to be compatible with MIRVAL, a ray trace code for heliostat fields created by Sandia National Laboratories. Based on a 3D Compound Parabolic Concentrator (3D-CPC) example the power of the simulation based on the Sandia heliostat field calculations is demonstrated. The results of the simulations are used to calculate the solar flux distributions in the ideal 3D CPC inlet and outlet planes as well as the incident ray distribution hitting the secondary concentrator. Code modifications to account for surface irregularities and spectral reflectivity are implemented in the appropriate FORTRAN subroutine. Using the automated simulation routines first the optimal receiver tilt angle and secondly the secondary concentrator acceptance angle are determined. These parameters combined with the fixed secondary concentrator outlet radius — which is determined by the SPHER window diameter — fully constrain the 3D CPC geometry. The flux maps generated using MATLAB post processing on the derived concentrator results clearly indicate the strengths and weaknesses of the specific concentrator and heliostat field combination. The influence of the secondary concentrator on the window incident flux distribution and window transmission, absorption and reflection properties is evaluated. Early findings using the code suggest higher yearly average power entering the receiver when compared to a non-secondary case. The reason for this effect is found in increased heliostat efficiency towards the edges of the heliostat field. At the same time the peak power hitting the window is found to increase very slightly only. This means the maximum window design specifications do not need to be adjusted dramatically to be able to accommodate the average power increase. First indications using the MCRT output in preliminary receiver simulations suggest increased receiver efficiency and a boost in receiver outlet gas temperature. The combined effect of these improvements is expected to raise overall power generation efficiency by improving the gas- / steam turbine combined cycle efficiency.