Energy-intensive industrial processes, such as those occurring in steel billet reheating furnaces, usually release waste heat characterised by a fluctuating and/or intermittent nature, which limits the efficiency of current waste heat recovery systems. In fact, current waste heat recovery systems are usually designed for one single operating point, which forces them to operate at off-design conditions when subjected to power fluctuation of the waste heat. This paper investigates the impact of retrofitting a technology based on high temperature phase change materials (PCM-based technology) in an existing waste heat recovery system composed of an air preheater and an Organic Rankine Cycle (ORC) system - installed downstream of a steel billet reheating furnace. The analysis was carried out by using real data from a billet reheating furnace and ORC system to develop and validate a model in Modelica language; a comparison between the ORC system efficiency and capacity factor with and without the PCM-based technology was then performed. The results showed that the introduction of the PCM-based technology allows the capacity factor to increase from 38% to 52% and the average thermal efficiency to increase from 15.5% to 16.4%. The cost-benefit analysis shows that electric energy priced between 83 and 120 (sic)/MWh could guarantee a payback period between 3 and 5 years.