Cold Energy Storage via Hydrates Production with Pure CO[sub.2] and CO[sub.2]/N[sub.2] Mixtures: Quantification and Comparison between Energy Stored and Energy Spent

Gas hydrates represent an attractive opportunity for gas storage. These ice-like structures can be produced both for the final disposal of greenhouse gases such as carbon dioxide in the solid form and for the storage of energy gases, such as methane, propane, and others, with the possibility of reaching energy densities comparable with those of pressurised vessels, but at lower pressures. In addition, gas hydrates can be directly produced for their capability to act as phase change materials at temperatures higher than 0 °C. This research deals with cold energy storage via the production of gas hydrate into a lab-scale apparatus. Hydrates were produced with pure carbon dioxide and with CO[sub.2]/N[sub.2] mixtures (70/30 and 50/50 vol%). For each mixture, the amount of energy spent for hydrates production and cold energy stored were calculated, and the results were compared among each other. The addition of nitrogen to the system allowed us to maximise the energy stored/energy spent ratio, which passed from 78.06% to 109.04%; however, due to its molecular size and the consequent impossibility to stabilise the occupied water cages, nitrogen caused a reduction in the total quantity of hydrates produced, which was obviously proportional to the energy stored. Therefore, the concentration of nitrogen in the mixtures need to be carefully determined in order to optimise the Estored/Espent ratio.