The ESTMAP project considers subsurface and above-ground energy storage technologies. The subsurface storage options include:
› natural gas
› hydrogen
› compressed air
› thermal
› underground pumped hydro

These technologies are associated with reservoirs defined in geological formations (porous aquifers and hydrocarbon reservoirs or mined cavernous spaces in salt, mineral or hard-rock formations).

Above-ground storage options encompass a wide range of technologies including:
› various types of battery systems
› super capacitators
› thermal storage
› fly wheels
› pumped hydro systems

Except for pumped hydro systems, the above ground storage options rely on man-made storage devices, which in theory are independent of location. Pumped hydro relies on the presence of two or more lakes at different elevation levels.

The ESTMAP database describes the various natural subsurface and above-ground reservoirs that are potentially suitable for energy storage development. These include:
› depleted hydrocarbon reservoirs
› aquifers
› salt formations and caverns
› rock formations and excavated caverns
› mines
› lakes

Each reservoir type is characterized in terms of the geological and technical properties that are relevant for defining feasibility, capacity and performance of storage. Besides the reservoirs, the database also contains existing, planned and future potential energy storage facilities including known technical and economic parameters.