Process steam required in industrial processes is conventionally produced by combustion of fossil fuels. Decarbonisation of this sector requires a switch into renewable energy sources, some of which are intermittent and weather-dependent. Integration with energy storage solutions is necessary to fully exploit the potential of renewable energy. In the presented case study, a photovoltaic (PV) farm is installed in a chemical factory. Electric energy from this farm is mainly used to cover own electric needs of the factory, but there are some periods when there is excess energy available. A thermal energy storage system (TES) has been designed to utilise the excess energy for production of process steam. Supply of process steam requires a certain temperature level in the storage tank. It can be achieved by utilising latent heat of a phase change material (PCM) whose melting temperature matches the required temperature of the steam. A zinc-aluminum alloy with melting temperature of 381°C has been selected as the PCM. This paper presents an analysis of the energy balance (electricity demand and production, steam demand) of the chemical factory in a hourly resolution. Steam is supplied by combusting hard coal in three grate boilers working in parallel. Surplus solar energy is used to heat up and melt the PCM; the stored heat is later used for steam production, reducing the fuel consumption in the boilers. A simulation model of the boilers + TES + steam demand system is used to calculate the consumption of electric energy and fuel in the variants with and without the use of PV farm and TES. Various sizing of the TES is analysed to find the economic optimum, considering the availability of excess solar energy - each subsequent tank will be utilised in a smaller degree.