As part of the second phase of the OptiEnR research project, the present work deals with improving multi-energy district boilers operation, by adding optimally sized and controlled thermal storage tanks to the plants. Previous studies focused on both a sizing approach based on a parametric analysis and an operation strategy. The aim of the present work is to improve the design of the tanks, in particular by evaluating the thermal losses and finding the adequate insulating materials. We focused on the stratification phenomenon and highlighted that the effectiveness of a thermal storage tank is related to the time during which the stored energy can be kept without considerable degradation. A case study allows the impact of thermal energy storage to be quantified.