Micro-hydro power is bringing electricity and prospects for a better future to remote communities across the world. As well as replacing polluting and dangerous kerosene for lighting, it can also power radios, TVs and machinery, providing new education, leisure and livelihood opportunities.
- Micro-hydro schemes usually supply a mini-grid and provide electricity to a whole community.
- Installation typically costs US$4,000 per kW, but varies greatly depending on the site.
- Newer schemes are designed to connect to the mains grid if it becomes available.
How small-scale hydro power works
The power available in a river or stream depends on the rate at which the water is flowing, and the height (head) which it falls down. Hydro schemes are sometimes classified into four groups, although there’s no universal agreement on the boundaries between them, and the basic principles of operation are the same for all.
- Large scale: outpower about 2 MW and above
- Mini:100 kW to 2 MW
- Micro: 5 kW to 100 kW
- Pico: less than 5 kW
The core of a small hydro scheme is the turbine, which is rotated by the moving water. Different types are used, depending on the head and flow at the site (see box). For example, Ashden Award-winner Practical Action Peru uses Pelton, Francis and Cross-flow turbines (see box for details). The turbine rotates a shaft, which is often used to drive an electrical generator.
Pelton turbine (for high head, low flow) consists of a set of small buckets arranged around a wheel onto which one or more jets of water are arranged to impact.
Francis turbine (lower head and higher flow) has a spiral casing that directs the water flow through vanes on a rotor.
Cross-flow or Banki turbines (even lower head and higher flow) are made as a series of curved blades fixed between the perimeters of two disks to make a cylinder. The water flows in at one side of the cylinder and out of the other, driving the blades around. They are much easier to make than most other designs.
Propeller turbine (very low head and large flow) has fixed blades, like a boat propeller. A more complex version, the Kaplan turbine, has blades that can be adjusted in pitch relative to the flow.
River current turbine, which is like a wind-turbine immersed in water, can be used to extract power from with a large flow in a river, where there is virtually no head.
Most small hydro systems are ‘run-of-river’ which means that they don’t need large dams to store water. However, they do need some water-management systems.
A small dam in the river bed directs the water to a settling tank. This allows silt to settle out of the water, and the clean water to flow into a canal or a pipe to a second settling tank called the ‘forebay’, which is sited above the power house. The canal or pipe can be fairly long, 1 km or more, if a suitable stream is distant from where the power is required. The outlet from the forebay has a screen to trap silt and floating debris. Water flows out into a pipe called the ‘penstock’, which is made as steep as possible to transfer water to the turbine. Water leaving the turbine is led back to the stream through the outlet pipe or ‘tail-race’.
How small-scale hydro systems are used
How small-scale hydro systems are used Many micro-hydro schemes are remote from the mains grid, and a local grid is then constructed to distribute the electrical power. In the local grids installed by Practical Action Peru, the voltage output from the generator is stepped up to about 10,000 volts using a transformer, for transmission to the village, because this reduces electrical losses. At the village the voltage is stepped down to 220 V for distribution to individual customers.
The output from the generator must match the demand for electric power on the local grid, otherwise the voltage and frequency can vary suddenly, which can damage some electrical equipment. The demand for power in an off-grid system is often very variable, because people switch lights and machines on and off, so the supply from the micro-hydro system must be varied to keep close control. This can be done by varying the water flow, or by using an electronic load controller.
Small hydro schemes can be connected to a mains grid if available. CRERAL and CRELUZ are electricity supply cooperatives in southern Brazil, which between them have installed eight grid-connected mini-hydro schemes with a total capacity of 5.9 MW to reduce the amount of electricity which they have to purchase from the national electricity supplier, and also improve the reliability of the supply to their customers. The not-for-profit IBEKA was set up to provide community-managed micro-hydro schemes for off-grid communities in Indonesia. And if the mains grid is extended to the region, IBEKA helps the community to get the micro-hydro connected to the grid, so that they can earn income from electricity sales. It also helps communities to set up grid-connected micro- and mini-hydro schemes from scratch.
In some micro-hydro schemes, the rotating shaft directly drives machinery.CRT-N upgrades water mills in Nepal, so that they can grind more flour and in some cases run oil expellers and other machines as well.
What are the benefits of using micro-hydro?
In remote areas, small-scalehydro schemes can bring electricity for the first time to whole communities. This provides lighting, TV and communications for homes, schools, clinics and community buildings. The electrical power generated can be enough to run machinery and refrigerators, thus supporting small businesses as well as homes. This is a major benefit of using micro-hydro to provide electricity compared to individual solar home systems which – although simpler to manage – provide less power.
A prime aim of the micro-hydro schemes developed by GIZ-Integration in Afghanistan was to provide enough electricity for workshops and other small businesses, thus offering alternatives to opium production for earning income. Practical Action Peru also found that young people were more likely to stay in villages with micro-hydro, and that business activities grew.
The main environmental benefit of micro-hydro is reducing greenhouse gas emissions and local pollution from fossil fuels. This includes kerosene for lighting, diesel for running machinery, and fossil fuels for generating electricity.
There are concerns about the environmental impact of large-scale hydro schemes, because they require substantial areas to be flooded to provide reservoirs, and can have a serious impact on water management. Carefully designed small-scalehydro schemes take only a limited amount of water from a river or stream, have a small storage volume, and return the water a short distance downstream, and thus have very little environmental impact. Several small hydro systems have less environmental impact than a single large hydro scheme supplying the same power.
The cost of small-scalehydro varies significantly with location. For example, GIZ-Integration schemes in Afghanistan cost between US$3,700 and US$5,300 per kW installed. This cost covered the micro-hydro plant, distribution and metering, and was provided by grants. The plants supply about 600 homes and 50 businesses per 100 kW, and users pay per kWh of electricity used, at a tariff which covers the management and maintenance of the scheme. Schemes installed by Practical Action Peru average US$3,400 per kW.
The installed costs of CRELUZ schemes are lower because they feed an existing grid and there is no additional cost for distribution. This means it is cheaper for CRELUZ to generate its own electricity than to buy from the national supplier.
Globally, hydro-power is the largest source of non-fossil-fuel electricity. The 1,000 GW of installed hydro capacity provided about 16% of the world’s electricity in 2013 (3,800 TWh – compared to just 2,500 TWh from nuclear power). However, most of this is from large-scale systems. In 2009, the global capacity of small-scale hydro was estimated at about 60 GW.
The main micro-hydro programmes are in mountainous countries, such as Nepal (around 2,000 schemes, including both mechanical and electrical power generation) and other countries in the Himalayas. There are also many schemes in South America, particularly in countries along the Andes such as Peru and Bolivia. China has seen the main growth in use of small-scale hydro in recent years.
There is substantial potential for supplying off-grid communities with electricity from small hydro schemes. The technology can be made compatible with the national grid, so that an off-grid hydro scheme could subsequently be connected to the grid: all the GIZ-Integation schemes in Afghanistan have been designed in this way.
Micro-hydro systems were widely used in the Europe and the USA from the end of the 19th century, but most were abandoned when national grids arrived. Their potential for supplying power to the grid is now recognised, in particular where there are preferential tariffs for low-carbon electricity, and new schemes are being installed by companies such as Dulas in the UK.
Lead author: Dr Anne Wheldon