Enhanced closed-loop underground thermal energy storage systems, using experimental materials via numerical modelling and laboratory analysis

Abstract

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time for heating and cooling applications and power generation.  TES systems can be divided into three main categories: sensible heat storage (SHS), thermochemical storage (TCS) and latent heat storage (LHS) associated with phase-change materials (PCMs). PCMs are able to release or absorb thermal energy changing their physical state at a quite constant temperature.This work aims to analyse the properties of different PCMs and supporting materials to be used coupled in closed-loop underground thermal energy storage (UTES) system. The physical and thermal properties of different PCMs (from lower to medium – high temperature) and their supporting materials, in particular diatoms, are analysed separately through laboratory analysis and numerical modelling, in order to find out the best material providing high thermal conductivity and capacity, efficiency, durability and low cost. However, research is also open to other types of materials such as graphene, zeolites, carbon nanotube and biochar.Then, the best performances PCM is tested to define the best mix between PCMs (the capacitive part) and diatoms (part that will prevent the leakage during the phase change) under different boundary conditions. With the best mix, combined with cement, cylindrical samples 30 cm in diameter and 1 m in length will be constructed with the patented mixtures with a metallic finned tube inserted within it, to represent a thermal storage mass with a tube inside.This sample will be tested, for low-temperature experiments (below 100 °C), using water as carrier fluid, in a physical model hosted at the UNIPD laboratory constituted by a box (1 m3 volume) that can be filled with dry or saturated loose materials with a metallic tube inserted in the middle to represent the heat exchange between the water flowing in the tube at a certain temperature, and the surrounding material, under different boundary conditions. The experimental device, allows to assess, under controlled conditions, the evolution in time and space of the energetic processes that occur between an heat exchanger and the surrounding ground. A second sample with similar characteristics, on the other hand, will be tested with high temperatures (between 200 °C and 300 °C) using a diathermic oil as heat transfer fluid and will be tested in a different device with a high-temperature heating system. The experimental data obtained will be also used in the construction and calibration process of numerical models by using commercial software (FEFLOW).Finally, the simulation results are expected to identify the best conditions to apply the new conceived mix in a real test site.