ABSTRACT
306Liquid metals have good thermodynamic properties for heat transfer processes (large thermal conductivity and large temperature range as liquids), making them efficient heat transfer fluids in a wide range of applications.
Liquid metals have been investigated from the early phase of nuclear fission reactors and have been extensively used as coolants in fast neutron reactors as their compact cores with high power density require good heat transfer coolants able to extract the power produced by the nuclear fission. Heavy liquid metals such as lead or lead–bismuth eutectic were investigated as coolants for fast reactors in the 1950s. Among all feasible liquid metals, sodium is the one that meets the requirements well; thus, it is the primary coolant in nearly all liquid metal–cooled fast neutron reactors constructed during the last 50 years. Sodium-cooled fast reactors have been widely applied, with over 400 reactor-years of operation worldwide.
Based on the operational experience gathered in the nuclear field and due to its good heat transfer properties, sodium was proposed as the heat transfer fluid (HTF) for power tower concentrating solar power (CSP) systems even in the 1980s. Power tower demonstrator facilities were operated with sodium in the United States and Spain and extensive operating knowledge was accumulated. However, the Almería, Spain, accident in 1986 has been probably the main factor that negatively affected the reputation of sodium as HTF in CSP plants and its potential was forgotten for about 25 years. Driven by the increasing demand on higher electrical output and higher plant efficiency, we are witnessing the renaissance of the liquid metal usage as HTF in CSP systems. This has been initiated by the appearance of several new CSP concepts using liquid metals, as well as by the start of comprehensive research projects in Germany (LIMTECH alliance on liquid metals) and in the United States (SunShot Initiative). In Australia, following the successful test of a sodium-based central receiver, the construction of the 6 MWth CSP pilot plant is finished and the commissioning tests are scheduled for early 2016.
This chapter discusses briefly the thermophysical properties of liquid metals and a comparison is made against other conventional HTFs used in energy systems. Numerical aspects specific to liquid metals, specific instrumentation required, and purification systems are also discussed. The chapter is focused on the latest applications of liquid metals in the nuclear field, in solar field on CSP systems, for thermal storage solutions, for thermoelectric converters, and heat pipe devices for parabolic dish systems.
