We address two critical environmental and technical problems for the integration of subsurface salt cavern hydrogen storage with 100% renewable electricity. First, the storage/production rate of hydrogen must match the unpredictable pattern of renewable electricity supply and (more predictable) demand for electricity by creating and strategically locating enough salt caverns. Secondly, creating and maintaining so many salt caverns requires large volumes of fresh/seawater. We develop two static and dynamic models for Denmark as a successful case of wind power development, considering the surplus energy and demand forecasts. The static model predicts the minimum amount of hydrogen needed for balancing the average annual supply and demand of electricity and fresh water necessary for the construction of required salt caverns. The model considers all the round-trip exergy losses of electricity-H2-electricity in the electrolyzers, fuel cells, compressors, and pipelines. The dynamic model considers the variable supply from wind farms and user demand over time; We also include the effect of the inertia of the electrolyzers, fuel cells, and compressors, and technical constraints, e.g. salt cavern pressure and pipeline flow capacity, to design sufficient storage sites that can dynamically balance the fluctuating supply of renewables and variable user demand. The static model predicts a realistic volume of salt caverns for storing the surplus green hydrogen; however, in the absence of small-scale storage solutions (batteries), we show that the number of required caverns and injection/production wells become unrealistically high, with high energy demand and cost for maintenance water treatment.