Experimental Investigation on the Thermal-Energy Storage Characteristics of the Subcritical Water

This paper first proposes the concept of using subcritical water as a thermal energy storage (TES) material. Subcritical water has a maximum TES temperature of 330°C and an internal energy storage density as high as $977.75\mathrm{MJ}/{\mathrm{m}}^3$, which is very suitable for industrial waste heat recovery. This paper investigates the TES characteristics of subcritical water in the heat preservation process. An experimental apparatus was set up, and experiments at different initial temperatures were performed. The experiments show that subcritical water has greater TES capacity than normal water because of its high section energy density. The water tank can be divided into three regions according to the section energy density. The section energy density rapidly increases in the lower head region, slowly increases in the transition region and remains stable in the isothermal region. The section energy density is distributed according to the tank shape and temperature distribution. When the initial temperature increases, the changing rate of the section energy density along the axial direction rapidly increases in the lower head region, slowly increases in the transition region and does not change in the isothermal region. The section exergy density has similar characteristics to the section energy density.