Search

In a solar power tower plant, the role of the heliostat aiming strategy is to control the radiative flux distribution at the receiver surface to avoid thermally induced damage, while minimising spillage losses and maximising the receiver thermal efficiency. Flux limitations arise from factors including the heat transfer fluid stability limits, and thermo-mechanical stress limits in receiver pipes. Maximised flux, as close as possible to local flux limits, facilitates an overall smaller receiver with lower thermal losses. Methods exist to sequentially optimise aiming points of single heliostats, using fast convolution-based optical simulations to evaluate individual flux maps. However, to accurately determine receiver flux distributions, ray-tracing is preferred. Ray-tracing is computationally expensive and determination of the aim-points for every heliostat independently potentially leads to impractical simulation times. In this study, a new parameterisation of heliostat aim-point locations is introduced that significantly simplifies the aiming problem. The new aiming model, named Modified Deviation-Based Aiming (MDBA), enables efficient use of ray-tracing to optimise the aiming strategy and, together with receiver thermal and mechanical models, is able to closely match the flux distribution to local values of allowable flux on the receiver. This new parameterisation enables accurate aiming strategy interpolation, used to dynamically predict full field aim-points at different sun positions and values of direct normal irradiance (DNI). A reference case with a surround field and a cylindrical external receiver compatible with the Gen3 Liquid Pathway project is presented to test the capability of the method developed in this study.

Temperature distribution in nonaxisymmetrically concentrating solar thermal receivers tubes is calculated for the steady-state solution with a Gauss-Seidel iteration in cylindrical coordinates. The classical plane-biharmonic thermoelastic approach to nonaxisymmetrically heated tube stress is applied. Calculation of the dominant axial thermal stress component is included. Validation is obtained with the linear-elastic thermal stress OpenFOAM® solver. Thermoelastic stress in stainless steel 316 Schedule 5S DN25 (1″) tubes containing liquid sodium is found to be 35% lower than in tubes containing molten salt. The difference is due to the higher conductivity of liquid sodium which maintains a smaller temperature difference between the front and back tube sides. A simplified thermal stress formula is shown to be erroneous if not implemented as originally documented. The sensitivity of tube thermoelastic stress to tube material and flow properties is illustrated with parameter variation, and the impact of solar concentrated heat flux is explored with some typical and ideal tube circumference flux profiles.

The applicability of the cylindrical arrangement of vertical tube banks is evaluated for liquid sodium concentrating solar thermal receivers and compared with a molten salt reference case through a series of parametric studies. It is shown that sodium receivers experience less thermo-elastic stress load and can operate under higher flux which presents advantages in terms of size reduction and efficiency. While the cylindrical receiver configuration cannot reach the efficiency target of 91% in a high temperature configuration (480 °C to 640 °C), there is potential to improve efficiency by improving heliostat field optics. Flux limitations due to thermo-elastic stresses are less stringent due for sodium receivers due to the better heat transfer properties, and consequently better heliostat field optics would benefit sodium receiver concepts more than molten salts ones.

A new tubular cavity receiver for direct steam generation, 'SG4', has been built and tested on-sun based on integrated optical and thermal modelling. The new receiver achieved an average thermal efficiency of 97.1±2.1% across several hours of testing, and reduced the losses by more than half, compared to the modelled performance of the previous SG3 receiver and dish. Near-steady-state outlet steam temperatures up to 560°C were achieved during the tests.

For the prediction of convective heat loss from solar concentrating receiver cavities a number of empirical correlations exist. Geometry and the inclination angle determine the degree to which natural convection can infiltrate the cavity and remove stably stratified hot air out through the aperture. This makes the task of defining characteristic lengths for such Nusselt correlations difficult, neither does their use offer insight as to how one might reduce heat loss through the use of baffles, air curtains or small aperture-to-cavity-area ratios. Computational Fluid Dynamics (CFD) can assist in the design of better cavity receivers as long as the rules upon which it rests are respected. This paper is an exploration of the need for turbulence modelling in cavity receivers using some common linear eddy viscosity closure schemes. Good agreement was obtained with the CFD software OpenFOAMO® 3.0.1 for a deep cavity aperture but it under-predicted a shallow cavity. The experiments used for validation were in the Grashof region Gr ≈ 10 6 , well below the region for transition to turbulence between 10 8 < Gr < 10 9 .

Three high-temperature solar receiver design concepts are being evaluated as part of the Australian Solar Thermal Research Initiative (ASTRI): a flux-optimised sodium receiver, a falling particle receiver, and an expanding-vortex particle receiver. Preliminary results from performance modelling of each concept are presented. For the falling particle receiver, it is shown how particle size and flow rate have a significant influence on absorptance. For the vortex receiver, methods to reduce particle deposition on the window and increase particle residence time are discussed. For the sodium receiver, the methodology for geometry optimisation is discussed, as well as practical constraints relating to containment materials.

Although it also calculates the irradiation of a range of engineering and architectural surfaces, each edition of this Handbook is different from their simpler predecessors the so-called “Spencer Tables” (Spencer, 1974 and later) that use completely clear skies to calculate irradiation levels. The “Spencer Tables” provide equivalent information for architectural surfaces for the specific case of a cloudless sky. By contrast, the data used for this Handbook are all statistical values (means and frequencies of occurrence) that account for the reduction, particularly in beam radiation, caused by actual cloud conditions. Similarly, in the early years, solar position and shading design were tackled with no explicit detail on the irradiation energy involved, as in Spencer and Philips (1983).

Current implementation of Immersed Coil Heat Exchangers (IHX) into Solar Domestic Hot Water (SDHW) systems use simple guidelines for dimensioning their surface area based on an intended solar collector area or range thereof. The literature from whence these guidelines come (1970s and 80s) are based on experiments with different types of IHX (copper, flangeretrofitted) to those used today and are evaluated solely on the performance of heat transfer, thus a re-evaluation of these guidelines is considered necessary. The investigations presented attempt to combine an evaluation of the heat transfer performance and stratification together based on measurements from three IHX samples with varying material and geometrical arrangement. Preliminary analysis suggests a correlation between high convective heat transfer coefficients and low stratification efficiencies.