Material Insights · Understanding the Material World

Natural reservoirs: Engineering and performance

Why reservoirs are the only natural store you can dispatch

What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.

Reservoir hydro is the one natural store that behaves like dispatchable storage. The others are flows that can be tapped but not timed.

Round-trip efficiency

Mixed

Mostly not applicable, because nature does the charging. What matters is conversion: modern hydro turbines turn up to about 92% of the water's energy into shaft power at their best point. Ocean thermal plants are limited to about 7% by the small temperature gap. Aquifer stores that are charged run as heat pump systems with very high cooling efficiency, a COP (heat moved per unit of electricity) of 30 to 40 in Dutch systems.6813

Hydro turbine efficiency, maximum up to 92%13 OTEC maximum (Carnot) efficiency about 7%6 Dutch ATES COP, cooling / heating 30-40 / 4-58

Power and energy decoupling

Favourable

For reservoir hydro, turbine capacity (MW) and reservoir volume (MWh) are set independently, and operators often oversize turbines to serve peaks: plants commissioned in 2024 averaged a 48% capacity factor (output as a share of the maximum possible). Extra energy means a bigger reservoir, which is costly in land and people rather than money per kWh. Norway shows the scale possible: about 1,100 reservoirs hold over 87 TWh, about half of Europe's reservoir storage, behind 33,947 MW of hydro capacity, enough to store well over half a year of its 137.6 TWh normal annual hydro output (calculated).215

Hydro capacity factor, new plants 2024 48%2 Norway reservoir storage over 87 TWh in about 1,100 reservoirs; about half of Europe's15 Norway hydro capacity and normal output 33,947 MW; 137.6 TWh a year15

Footprint and siting flexibility

Unfavourable

The least flexible of all stores: they exist only where geography provides them. Because water holds so little energy per m3, storing 1 GWh at 100 m head needs about 3.6 million m3 (calculated). Reservoirs flood valleys, and only 37% of rivers over 1,000 km long still flow freely.410

Water needed per GWh at 100 m about 3.6 million m3 (calculated)4 Long rivers still free-flowing 37%10

Safety and thermal runaway risk

Mixed

No thermal runaway, but dam failure is a rare, catastrophic risk: including the Banqiao dam failure in China, hydropower has caused about 1.3 deaths per TWh since 1965, against 0.04 without it. Dam safety codes and reservoir inspection regimes apply.11

Deaths per TWh, 1965-2021, including / excluding Banqiao 1.3 / 0.0411

Sources

  1. Renewable Power Generation Costs in 2024: Summary (Table S1) IRENA, 2025
  2. 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
  3. Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
  4. 35 years of Dutch experience on ATES (Godschalk, SpeicherCity GeoTHERM workshop) University of Halle / SpeicherCity, 2024
  5. Mapping the world's free-flowing rivers (Grill et al., Nature 569) Michigan Technological University repository, 2019
  6. What are the safest and cleanest sources of energy? Our World in Data, 2024
  7. Hydropower: Technology Brief E06 (Table 2, turbine efficiency) IRENA and IEA-ETSAP, 2015
  8. Electricity production: Key facts about the Norwegian energy sector (data from NVE) Energy Facts Norway (Norwegian Ministry of Energy; data from NVE), 2025