Numerical and experimental investigation of induced convective flow by high-temperature heat storage in water saturated sediments.

Seasonal or sub-seasonal large scale heat storage will be required for a switch of the heating market to renewable heat sources, due to the seasonality of the heating demand. Subsurface high-temperature heat storage (up to 90°C) is investigated here as a promising option for urban areas with strong land use pressure, as this technology provides the required high capacities. Surplus heat originating from solar thermal installations or industrial production can be stored and later on used when the heat demand is high. One technology option available is borehole thermal energy storage using borehole heat exchangers (BHE) to store the heat in the geological subsurface. However, storing heat at high temperatures in porous media can trigger convective density-driven flow. This interacting transport of heat and water may affect the storage efficiency of such storage systems. In this study, therefore, lab-scale experiments are numerically designed and experimentally conducted in order to identify, characterize and quantify the induced convective heat transport process at different storage temperatures.

A lab-scale analogon of a heat storage is constructed in a PP plastic barrel of 1.23 m height and 1.2 m diameter, consisting of water saturated homogeneous sand medium, with a hydraulic permeability of about 2.9x10^-10 m² and a thermal conductivity of 2.042 W/m/K. Coupled thermo-hydraulic process simulation applying OpenGeoSys was used to design and optimize the experimental set-up and the test cycles. Hot water is circulated in a coaxial BHE at 70°C for seven days to heat the storage medium, while tab water is used to recover the stored heat. The side of the barrel is cooled using ventilators while the top and bottom of the barrel are insulated.

The experimental results show that after four days of heat injection, a steady state temperature distribution is reached. The temperature distribution in the storage medium is vertically stratified with an average temperature approximately 39°C and 26°C in the upper and lower part, respectively. Thus the centre of the mass of stored heat is shifted to the top part of the storage medium, and a larger convection cell is formed, with water rising at the BHE in the middle and sinking at the barrel wall. The vertical temperature gradient decreases from the grout surface to the barrel wall with a rate of 0.153 K/m. The decreasing rate of the radial temperature gradient from the upper to the lower part of the sand medium is 0.174 K/m. The Rayleigh number, which characterizes the magnitude of the convective heat transfer, is about 44.15 for this experiment and thus greater than the critical value. Heat transfer process in the sand medium hence is influenced by density driven convective flow. Additional laboratory experiments at inlet temperatures of 30°C, 50°C, and 90°C show an increase of convective heat transfer with increasing temperature.

The numerical model qualitatively reproduces the convective heat transfer within the storage. An inverse model adaption is currently carried out to determine the effective heat transfer parameters for the storage components and to quantitatively fit the experimentally observed temperature distributions.