# Run-of-river and small hydro: Techniques

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

Source page: https://www.materialinsights.org/energy/e_small_hydro/techniques/

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 plants**^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2015/IRENA-ETSAP_Tech_Brief_E06_Hydropower.pdf)^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/RE_Technologies_Cost_Analysis-HYDROPOWER.pdf)^[8](https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7) | 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 hydro**^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2015/IRENA-ETSAP_Tech_Brief_E06_Hydropower.pdf)^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/RE_Technologies_Cost_Analysis-HYDROPOWER.pdf)^[3](https://www.unido.org/sites/default/files/files/2023-05/ENGLISH%20SUMMARY-2022.pdf)^[9](https://digitallibrary.un.org/record/4020227)^[16](https://doi.org/10.1186/s40807-023-00072-1) | 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) turbines**^[7](https://arxiv.org/abs/2010.00982)^[6](https://tethys.pnnl.gov/project-sites/rivgenr-power-system) | 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 watermills**^[4](https://doi.org/10.1016/j.rser.2018.08.033) | Up to about 85% (overshot, undershot); 75-80% (breastshot) | No benchmark found | Not studied | mature | Heritage and small new installations |
| **Hydraulic rams**^[5](https://doi.org/10.5194/dwes-12-59-2019)^[15](https://Sswm.info/sites/default/files/reference_attachments/PRACTICAL%20ACTION%204000%20Hydraulic%20Ram%20Pumps.pdf) | 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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2015/IRENA-ETSAP_Tech_Brief_E06_Hydropower.pdf)^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/RE_Technologies_Cost_Analysis-HYDROPOWER.pdf)^[8](https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7)
**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.^[1](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2015/IRENA-ETSAP_Tech_Brief_E06_Hydropower.pdf)^[2](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/RE_Technologies_Cost_Analysis-HYDROPOWER.pdf)^[3](https://www.unido.org/sites/default/files/files/2023-05/ENGLISH%20SUMMARY-2022.pdf)^[9](https://digitallibrary.un.org/record/4020227)^[16](https://doi.org/10.1186/s40807-023-00072-1)
**In-stream (hydrokinetic) turbines**

A turbine anchored in a river current captures kinetic energy only, with no dam.^[7](https://arxiv.org/abs/2010.00982)^[6](https://tethys.pnnl.gov/project-sites/rivgenr-power-system)
**Waterwheels and watermills**

Water pours onto (overshot) or pushes under (undershot) a wheel of buckets or paddles, driving millstones or, today, generators.^[4](https://doi.org/10.1016/j.rser.2018.08.033)
**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.^[5](https://doi.org/10.5194/dwes-12-59-2019)^[15](https://Sswm.info/sites/default/files/reference_attachments/PRACTICAL%20ACTION%204000%20Hydraulic%20Ram%20Pumps.pdf)

#### Sources

1. [Hydropower: Technology Brief E06 (Table 2, turbine efficiency)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2015/IRENA-ETSAP_Tech_Brief_E06_Hydropower.pdf) IRENA and IEA-ETSAP, 2015
2. [Renewable Energy Technologies: Cost Analysis Series, Hydropower (Volume 1, Issue 3/5)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2012/RE_Technologies_Cost_Analysis-HYDROPOWER.pdf) IRENA, 2012
3. [World Small Hydropower Development Report 2022: English summary](https://www.unido.org/sites/default/files/files/2023-05/ENGLISH%20SUMMARY-2022.pdf) UNIDO, 2022
4. [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)](https://doi.org/10.1016/j.rser.2018.08.033) Elsevier, 2018
5. [Performance characteristics of a small water-hammer head pump (Manohar, Adeyanju and Vialva, Drinking Water Engineering and Science 12, 59-64)](https://doi.org/10.5194/dwes-12-59-2019) Copernicus Publications, 2019
6. [RivGen Power System, Igiugig, Alaska (project site and monitoring summary)](https://tethys.pnnl.gov/project-sites/rivgenr-power-system) Tethys, Pacific Northwest National Laboratory, 2024
7. [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))](https://arxiv.org/abs/2010.00982) arXiv (author manuscript of peer-reviewed paper), 2021
8. [Climate Change 2014: Mitigation, Annex III Technology-specific cost and performance parameters, Table A.III.2 (life-cycle emissions, min/median/max)](https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf#page=7) IPCC AR5 Working Group III, 2014
9. [Carbon neutrality in the UNECE region: integrated life-cycle assessment of electricity sources (ECE/ENERGY/141)](https://digitallibrary.un.org/record/4020227) United Nations Economic Commission for Europe, 2022
10. [Hydraulic ram pumps (technical brief)](https://Sswm.info/sites/default/files/reference_attachments/PRACTICAL%20ACTION%204000%20Hydraulic%20Ram%20Pumps.pdf) Practical Action
11. [Design models for small run-of-river hydropower plants: a review (Tsuanyo et al., Sustainable Energy Research 10, 3), Table 2 head ranges](https://doi.org/10.1186/s40807-023-00072-1) Springer Nature, 2023

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_small_hydro/index.md)
- [Overview](https://www.materialinsights.org/energy/e_small_hydro/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_small_hydro/correlation/index.md)
- [Efficiency](https://www.materialinsights.org/energy/e_small_hydro/efficiency/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_small_hydro/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_small_hydro/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_small_hydro/links/index.md)