a b s t r a c t Solar power tower (SPT) systems are viewed as one of the most promising technologies for producing solar electricity, in which direct solar radiation is reflected and concentrated by a field of giant mirrors (heliostats) onto a receiver placed at the top of a tower. However, the optimized design of a heliostat field is a rather complex problem because the annual performance of a heliostat is a function of not only the instants of time considered and its own position, but also the relative location of neighbouring heliostats, which cause shadows and blockings. A variety of procedures may be found in the open literature, although there is great lack of information on the details of an optimized layout. This review shows that these complex problems have partially led to the expansion of parabolic trough technologies in USA and Spain in spite of their lower thermodynamic efficiencies compared with solar tower power. As a modest support of SPT systems, the authors have presented elsewhere the abilities of a new code called campo for fast and accurate calculations of the shadowing and blocking factor for each and every heliostat. This work explores a review of the optimized heliostat field layouts yielded by campo. Campo commences the optimization search based on the densest layout, with the worst shadowing and blocking factor, but with good values for the other optical factors, and then progresses towards gradually expanded distributions. The search for maximum annual energy through campo results in a clear, steady and reproducible procedure. Finally, as an example of this new procedure, some options of optimized heliostat field layouts are reviewed using as input parameters the scarce open literature data on Gemasolar, the first solar power tower commercial plant with molten salt storage in the world.

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Recently, Collado (Proc, IMECE 2001, Symposium on Fluid Physics and Heat Transfer for Macro- and Micro-Scale Gas-Liquid and Phase Change Flows) suggested calculating void fraction, an essential element in thermal-hydraulics, working with the “thermodynamic” quality instead of the usual “flow” quality. The “thermodynamic” quality is a state variable, which has a direct relation with the actual vapor volumetric fraction, or void fraction, through phase densities. This approach provides a procedure for predicting void fraction, if values of “thermodynamic” quality are available. However, the standard heat balance is usually stated as a function of the “flow” quality. Therefore, we should search for a new heat balance between the mixture enthalpy, based on “thermodynamic” quality, and the absorbed heat. This paper presents the results of such analysis based on the accurate measurements of the outlet void fraction measured during the Cambridge project by Knights (1960, “A Study of Two-Phase Pressure Drop and Density Determination in a High-Pressure Steam-Water Circuit,” Ph.D. thesis, Cambridge University Engineering Lab, Cambridge, UK) in the 1960s for saturated flow boiling. In the 286 tests analyzed, the pressure and mass fluxes range from 1.72 MPa to 14.48 MPa and from 561.4 to 1833.33 kgm−2s−1, respectively. As the main result, we find that the slip ratio would close this new thermodynamic heat balance. This has allowed the accurate calculation of void fraction from this balance, provided we can predict the slip ratio. Finally, the strong connection of this new thermodynamic heat balance with the standard one through the slip ratio is highlighted.