Run-of-river and small hydro: Techniques

Pelton, crossflow, screw or ram: matching machine to river

What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.

Ways of using rivers with little or no storage, from run-of-river plants and village micro-hydro to current turbines and direct water power.

Comparison

Method	Efficiency	Cost	Life-cycle emissions	Maturity	Scale
Run-of-river plants¹²⁸	Over 90% at the turbine	Large schemes: USD 1,050-7,650/kW (IRENA 2012, 2010 USD)	Hydropower median 24 g CO2e/kWh (IPCC AR5); no separate run-of-river figure	mature	Not reported separately
Small, mini and micro hydro¹²³⁹¹⁶	Over 90% for good turbines; crossflow lower but tolerant of varying flow	USD 1,300-8,000/kW; LCOE USD 20-100/MWh in developing countries (IRENA 2012, 2010 USD; no newer global benchmark found)	6-147 g CO2e/kWh across hydropower sites (UNECE 2022)	mature	About 79 GW up to 10 MW (6% of hydropower in countries covered); 64% of 221.7 GW potential untapped (UNIDO 2022)
In-stream (hydrokinetic) turbines⁷⁶	At most 59.3% of the current's kinetic energy (Betz limit)	No benchmark found	No life-cycle studies found	pilot	RivGen, Igiugig, Alaska: 70 kW in two units (2019, 2023)
Waterwheels and watermills⁴	Up to about 85% (overshot, undershot); 75-80% (breastshot)	No benchmark found	Not studied	mature	Heritage and small new installations
Hydraulic rams⁵¹⁵	Typically about 60%, up to 80%	No benchmark found	None in operation	mature	Village and farm water supply

Turbine-based small hydro can be cheap where head and flow are good (from USD 20/MWh), but costs per kW climb as schemes shrink, with electro-mechanical equipment 18-50% of total cost. Run-of-river avoids most reservoir flooding but cannot store water. Wheels, screws and rams are simple and robust, suited to low heads or pumping without power. In-stream turbines are at pilot stage.

How each method works

Run-of-river plants

A low weir diverts part of a river through a channel or pipe to a powerhouse downstream, then returns it. With little storage, output follows river flow.¹²⁸

Small, mini and micro hydro

Small 1-20 MW, mini 100 kW-1 MW, micro 5-100 kW, pico up to 5 kW. Typical heads: Pelton 50-1,300 m, crossflow 5-200 m, Kaplan 2-40 m.¹²³⁹¹⁶

In-stream (hydrokinetic) turbines

A turbine anchored in a river current captures kinetic energy only, with no dam.⁷⁶

Waterwheels and watermills

Water pours onto (overshot) or pushes under (undershot) a wheel of buckets or paddles, driving millstones or, today, generators.⁴

Hydraulic rams

Water speeds down a drive pipe until it slams a waste valve shut; the pressure surge pushes some water up a delivery pipe. Two valves are the only moving parts.⁵¹⁵

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
World Small Hydropower Development Report 2022: English summary UNIDO, 2022
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
RivGen Power System, Igiugig, Alaska (project site and monitoring summary) Tethys, Pacific Northwest National Laboratory, 2024
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
Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters, Table A.III.2 (life-cycle emissions, min/median/max) IPCC AR5 Working Group III, 2014
Carbon neutrality in the UNECE region: integrated life-cycle assessment of electricity sources (ECE/ENERGY/141) United Nations Economic Commission for Europe, 2022
Hydraulic ram pumps (technical brief) Practical Action
Design models for small run-of-river hydropower plants: a review (Tsuanyo et al., Sustainable Energy Research 10, 3), Table 2 head ranges Springer Nature, 2023