Svenskt Stål AB (SSAB) is a global steel company who aim to eliminate all of their CO2emissionsby 2045. Today, SSAB Borlänge releases 20 GWh of waste heat in the form of steam into theatmosphere annually. This steam could instead be stored in a thermal storage unit, and then usedagain to reduce fossil fuel burning that is currently running the steam boilers. This report willinvestigate if SSAB Borl ̈ange’s old oil tank of 1200m3can be redesigned into a thermal storageunit to store the steam that is being wasted.Results show that the tank can be designed as a 95◦C hot water storage with a two cm thick foampolyurethane insulation, and a full tank will be able to hold 107,2 MWh. This will lead to annualoil savings of 2,14 GWh, which will save the company around 800 000 SEK per year in oil costs.A theoretical analysis prove that the SSAB Borl ̈ange previous oil tank can be designed as a hotwater storage and be profitable both environmentally and economically.