As the proportion of wind and solar photovoltaics (PV) in an electrical grid extends into the 50-100% range a combination of additional long-distance high voltage transmission, demand management and local storage is required for stability [1, 2]. Pumped Hydro Energy Storage (PHES) constitutes 97% of electricity storage worldwide because of its low cost.

We found about 616,000 potentially feasible PHES sites with storage potential of about 23 million Gigawatt-hours (GWh) by using geographic information system (GIS) analysis. This is about one hundred times greater than required to support a 100% global renewable electricity system. Brownfield sites (existing reservoirs, old mining sites) will be included in a future analysis. //

An approximate guide to storage requirements for 100% renewable electricity, based on analysis for Australia, is 1 Gigawatt (GW) of power per million people with 20 hours of storage, which amounts to 20 GWh per million people [2]. This is for a strongly-connected large-area grid (1 million km2) with good wind and solar resources in a high-energy-use country. Local analysis is required for an individual country. For example, Australia needs about 500 GWh (and has storage potential that is 300 times larger) and the USA needs about 7000 GWh (and has storage potential that is 200 times larger).

Finding PHES sites
Potential sites for off-river PHES are identified using GIS algorithms [4] with defined search criteria. The surveyed latitude range is up to 60 degrees north and 56 degrees south [5]. For each reservoir the following attributes are identified:

• Latitude, longitude, and elevation of the reservoir
• Area of the reservoir (in hectares)
• Water volume of the reservoir (in Gigalitres)
• Length of the dam (in meters)
• Dam wall height (in meters): the maximum height of earth and rock wall; different wall heights will produce different dam and reservoir shapes and volumes
• Volume of rock in the dam wall (in Gigalitres) based on a 3:1 upstream and downstream slopes
• Water-to-rock (W/R) ratio: ratio between volume of the stored water and volume of rock in the dam wall; reservoirs with higher water-to-rock ratio are economically more competitive.