This paper proposes a methodology for the economic optimisation of the sizing of Energy Storage Systems (ESSs) whilst enhancing the participation of Wind Power Plants (WPP) in network primary frequency control support. A generalised approach was taken for the design of the methodology, so it can be applied to different energy markets and concerning different ESSs. The methodology includes the formulation and solving of a Linear Programming (LP) problem.The methodology was applied to the particular case of a 50 MW WPP, equipped with Vanadium Redox Flow battery (VRB) in the UK energy market. Analysis is performed considering real data on the regular and frequency response markets of UK. Data for wind power generation and energy storage costs are estimated from literature.Results suggest that, under certain assumptions, ESSs can be profitable for the operator of a WPP that is providing frequency response. The ESS provides power reserves such that the WPP can generate close to the maximum energy available. The solution of the optimisation problem establishes that an ESS with a power rating of 5.3 MW and energy capacity of about 3 MWh would be enough to provide such service whilst maximizing the incomes for the WPP operator considering the regular and frequency regulation UK markets.Methodology for the economic optimisation of energy storage systems for frequency support in wind power plants Abstract This paper proposes a methodology for the economic optimisation of the sizing of Energy Storage Systems (ESSs) whilst enhancing the participation of Wind Power Plants (WPP) in network primary frequency control support. A generalised approach was taken for the design of the methodology, so it can be applied to different energy markets and concerning different ESSs. The methodology includes the formulation and solving of a Linear Programming (LP) problem.The methodology was applied to the particular case of a 50 MW WPP, equipped with Vanadium Redox Flow battery (VRB) in the UK energy market. Analysis is performed considering real data on the regular and frequency response markets of UK. Data for wind power generation and energy storage costs are estimated from literature.Results suggest that, under certain assumptions, ESSs can be profitable for the operator of a WPP that is providing frequency response. The ESS provides power reserves such that the WPP can generate close to the maximum energy available. The solution of the optimisation problem establishes that an ESS with a power rating of 5.3 MW and energy capacity of about 3 MWh would be enough to provide such service whilst maximizing the incomes for the WPP operator considering the regular and frequency regulation UK markets.