Concentrating Solar Thermal Energy

5Heat Transfer Fluids for Solar Power Plants

Gilles FLAMANT

PROMES-CNRS, Font-Romeu, France

5.1. Introduction

In a solar power plant, the heat transfer fluid (HTF) flows through the solar receiver and transfers heat to the heat storage system or for the conversion into the electricity system (power block). The heat transfer fluid differs from the working fluid. The latter is employed in a thermodynamic system that generates work, which is most often a steam turbine. In general, the thermal energy contained in the heat transfer fluid is transferred to the working fluid by a set of exchangers. Solar power plants with direct steam generation are a specific case in which the heat transfer fluid and the working fluid are one and the same. Steam flows through the solar receiver and is directly sent to the turbine. A steam storage system (more precisely pressurized water) can be integrated in the loop (see Chapter 8). The heat transfer fluid is most often a single phase (liquid or gas), but it can also be a two phase (water–steam or solid–gas).

This chapter begins by reviewing the macroscopic physics of fluid–wall heat transfers in section 5.2 and then presents the properties and domains of thermal stability of the most common heat transfer fluids in section 5.3. A detailed presentation of fluid–wall heat transfer coefficients is provided in section 5.4. Finally, current research on the new heat transfer fluids at high temperature is summarized in section 5.5.

5.2. Review of thermal ...