Technical Papers and Presentations

In view of an energy system based on fluctuating renewable sources, the use of thermal energy storage (TES) systems plays a crucial role. In particular, large-scale underground water-based TES represent a promising solution as demonstrated by a number of cases currently being established in Denmark, Germany and China. To improve the energy performance of TES systems for a given limit on the specific TES costs, their design must take into account both the operational characteristics and the properties of TES components and envelope insulation and liner materials. The need for ease of installation and low cost limits the choice of insulation materials that can be considered. Moreover, the presence of high moisture contents and temperatures (water is stored at a temperature of 90/95 °C) requires an in-depth knowledge of the material behaviour. Possible materials that are considered suitable for this application are either mineral or stone wool or foam glass gravel, expanded glass and perlite. However, the direction of heat flow, upward in case of the cover and horizontal in case of the side wall, and the presence of open porosity can lead to the development of convective heat and mass transfer that increases the thermal losses. Moreover, there is a risk of water and vapour infiltrations, leading to the accumulation of moisture within the insulation layer. This situation should be avoided as it causes a further increase in heat transfer and degradation of the insulation. The influence of convective heat and mass transfer must therefore be quantified to implement suitable measures to tackle it. In order to assess the multiphysical aspects of convection, it is necessary to investigate the material’s behaviour under similar boundary conditions to that of the real application. In this analysis, experimental characterization and numerical simulations are complementary approaches required to define the properties of materials and predict their behaviour in real scale applications. A mock-up for the TES cover insulation is set up: in order to simulate the operation conditions, a heating plate is placed under the lower side of the case to ensure a constant set point temperature. The COMSOL Multiphysics® software has been selected as a suitable tool to perform the required simulation work. In the first part of the study, the insulation material is studied in dry conditions, where conductive, convective and radiative heat transfer play a major role. In this case, the heat transfer problem (“Heat transfer in Solid and Fluids” interface) is coupled with the fluid flow in the porous medium (“Brinkman Equations” interface). In a second phase of the test, the influence of moisture is investigated. A third equation is included to account for moisture transport in the model (“Moisture Transport in Porous Media” interface).The final model suitably validated by experimental tests allows studying the implementation of solutions to inhibit or at least reduce the effect of natural convection.