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 MJ/m3, 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.
Conclusion:
The sensible thermal-energy storage technology using subcritical water as the TES material is discussed for the first time. Experiments to investigate the TES characteristics at different initial temperatures were performed. The temperature distribution and TES characteristics were studied. Subcritical water has greater TES capacity than normal water. The energy storage amount of subcritical water at 150°C is 4.12 GJ, which is more than two times that of the normal water at 80°C; the exergy storage amount of subcritical water at 150°C is 682.7 MJ, which is 4.3 times that of normal water at 80°C. The thermal energy is primarily stored in the cylinder part. 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, dES/dx 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 characteristics are similar to its section energy density characteristics.
In conclusion, the temperature distribution of subcritical water is decisive to the TES capacity. Future work will focus on the physics mechanism of the thermal stratification of subcritical water in the heat preservation process and establish a model to simulate the temperature distribution.
The results have been published on Journal of Energy Engineering, 2017, 143(6): 04017061.
