THERMAL EVALUATION OF A SOLAR POWER TOWER EXTERNAL RECEIVER WITH LIQUID METAL AS HEAT TRANSFER FLUID IN NORTHERN CHILE

Abstract

Molten salts have been the main heat transfer fluids used in solar power tower plants in recent times, but their limited operating temperatures limit the efficiency of current solar plants and their competitiveness. One option to improve the efficiency is to use liquid metals instead of molten salts, which in addition to withstanding temperatures even above 1000°C, have excellent thermophysical properties. This paper shows the comparative thermodynamics evaluation of a solar power tower external receiver using different liquid metals and molten salts as heat transfer fluid (HTF), located in northern Chile, a location with a high solar potential and where an important plant of this type recently started operation and others are projected. Tonatiuh and Engineering Equation Solver software were used to develop the work and implement the model. The results show that the operation with liquid metals produces lower energy efficiency than molten salts due to the higher operating temperatures, which significantly increase thermal losses, mainly radiative ones. However, the better thermophysical properties of some liquid metals, especially sodium, make the efficiency reduction not so relevant (only 2 percentage points in this analysis). It would be expected that this reduction in efficiency could be compensated by the increase in the thermal efficiency of the thermomechanical cycle in the case of electricity production due to the fact of operating at higher temperatures, but this work shows that the exergy efficiency of the process with liquid sodium is only 2 percentage points higher than with solar salt. Therefore, the reduction of losses, especially radiative losses, which can be as much as 400% greater than that ocurring with solar salt, becomes a necessity if we want to take advantage of the potential represented by being able to operate solar systems at temperatures higher than the current ones, with liquid metals or any other material.