Natural reservoirs: Science and technology
How sunshine charges mountains with water and oceans with heat
What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.Nature stores energy for free: water held in lakes, reservoirs, snow and ice; heat in the oceans and rock; and pressure and temperature differences in the air. Humans cannot charge most of these stores, only manage how they are drawn down, so several battery-style dimensions apply only loosely.
Mechanism
FavourableWater lifted by evaporation and dropped as rain or snow on high ground carries gravitational energy: a kilogram falling 100 m holds about 1 kJ, so a cubic metre holds about 0.28 kWh (calculated). Reservoirs, snowpack and glaciers delay its release by months or years. The Earth's interior leaks about 47 TW of heat from a far larger stock in the rock, and ocean surface water is a store of solar heat usable where it is about 20 C warmer than deep water. Air pressure and temperature differences drive winds, a store that turbines tap but cannot fill.456
Energy in water at 100 m head about 1 kJ per kg, about 0.28 kWh per m3 (calculated)4 Heat flow from the Earth's interior 47 TW, plus or minus 25 Ocean temperature difference needed for OTEC about 20 C or more6Material scarcity and toxicity
MixedDoes not apply in the mineral sense: no materials are consumed to hold the energy. The scarce inputs are suitable sites, water and land. Dams use mostly concrete and earthworks. The toxicity concern is ecological: reservoirs emit methane and change river chemistry and sediment flows.3
Reservoir surface emissions, all reservoirs about 0.8 Gt CO2e a year (0.5-1.2)3Degradation and ageing
UnfavourableNatural stores degrade through sediment filling reservoirs, droughts and the retreat of glaciers and snowpack under climate change, which shifts when and how much water arrives. In 2025 severe droughts in parts of South America, South and Central Asia and Europe exposed this weakness. Geothermal fields can cool or lose pressure over decades if over-used. No global figures for snowpack or glacier energy storage loss were found in the sources reviewed.1
Variants compared156715
| Natural store | How it is used | Can humans charge it? | Scale indicator |
|---|---|---|---|
| Reservoirs and lakes | Hydropower dispatch | Only with pumped storage | 1,469 GW hydropower, 4,495 TWh a year (2025); Norway alone over 87 TWh of reservoir storage |
| Snowpack and glaciers | Seasonal river flow into hydro | No | no global energy figure found |
| Ground and geothermal heat | Heat pumps, aquifer stores, geothermal plants | Yes for shallow aquifer stores | over 2,800 aquifer stores; 47 TW natural heat flow |
| Ocean heat | Ocean thermal energy conversion, seawater heat pumps | No | needs about 20 C difference |
| Atmospheric gradients | Wind turbines | No | not a store that can be dispatched |
Sources
- Year of the water battery: global pumped storage capacity surpasses 200 GW (2026 World Hydropower Outlook) International Hydropower Association, 2026
- Greenhouse gas emissions from reservoir water surfaces: a new global synthesis (Deemer et al., BioScience 66) University of Amsterdam repository, 2016
- Lessons Learned: Developing Thermochemical Cycles for Solar Heat Storage Applications (Wong, General Atomics; conference slides whose energy density table includes a 100 m elevated water dam) US Department of Energy, 2013
- Earth's surface heat flux (Davies and Davies, Solid Earth 1, 5-24) Copernicus / EGU, 2010
- Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
- Innovation Outlook: Thermal Energy Storage IRENA, 2020
- Electricity production: Key facts about the Norwegian energy sector (data from NVE) Energy Facts Norway (Norwegian Ministry of Energy; data from NVE), 2025