Run-of-river and small hydro: Efficiency

Why a tiny turbine still converts over 90% of falling water

What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.

Small hydro uses the same physics as large dams, and good turbines still convert over 90% of the energy of falling water. Water wheels and hydraulic screws reach about 75-85% at best, hydraulic rams 50-80%, and in-stream turbines, which sit in an open current with no head, are bound by the same 59.3% ceiling as wind turbines.

Chart: Efficiency ranges

Commercial Record Theoretical limit

Efficiency by measure

Measure	Range	Type	What is measured
Turbines for run-of-river and small hydro (Pelton, Francis, Kaplan)	90 to 92%¹²	Commercial	Energy of falling water to shaft power at the best operating point; crossflow turbines are lower
Gravity water wheels	75 to 85%⁴	Record	Maximum hydraulic-to-shaft efficiency: about 85% for overshot and undershot wheels, 75-80% for breastshot wheels
Archimedes screw turbines	80 to 82%¹⁴	Record	Hydraulic power to shaft power: about 80% reported in earlier studies, 82% simulated for a real-scale run-of-river screw
Hydraulic ram pumps	50 to 80%⁵¹⁵	Commercial	Energy delivered as lifted water over energy of the drive water; more than 50%, typically about 60% for commercial units, up to 80%
Open-flow limit for in-stream turbines	59.3%⁷	Theoretical limit	Betz limit for a rotor in a free stream (same actuator-disc physics as wind)

Where the energy is lost

Step	Loss
Turbine hydraulic losses	about 8-10% at the best operating point (calculated from 90-92%), more at part load¹
Drive water spilled through a ram's waste valve	most of the inflow; only a fraction is lifted¹⁵

The difference between small and large hydro is less in conversion efficiency than in control. With little or no reservoir, a run-of-river plant can only use the water that arrives, so output follows rainfall and snowmelt. Simpler designs such as crossflow turbines trade some peak efficiency for steadier performance when flow varies.

Sources

Hydropower: Technology Brief E06 (Table 2, turbine efficiency) IRENA and IEA-ETSAP, 2015
Renewable Energy Technologies: Cost Analysis Series, Hydropower (Volume 1, Issue 3/5) IRENA, 2012
Gravity water wheels as a micro hydropower energy source: a review based on historic data, design methods, efficiencies and modern optimizations (Quaranta and Revelli, Renewable and Sustainable Energy Reviews 97, 414-427) Elsevier, 2018
Performance characteristics of a small water-hammer head pump (Manohar, Adeyanju and Vialva, Drinking Water Engineering and Science 12, 59-64) Copernicus Publications, 2019
Physical limits of wind energy within the atmosphere and its use as renewable energy: from the theoretical basis to practical implications (Kleidon, Meteorologische Zeitschrift 30(3)) arXiv (author manuscript of peer-reviewed paper), 2021
Computational flow analysis on a real scale run-of-river Archimedes screw turbine with a high incline angle (Edirisinghe et al., Energies 14(11), 3307) MDPI, 2021
Hydraulic ram pumps (technical brief) Practical Action