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Micro-hydro

Micro-hydro schemes produce power from streams and small rivers. The power can be used to generate electricity, or to drive machinery. Micro-hydro can bring electricity to remote communities for the first time, replacing kerosene for lighting, providing TV and communications to homes and community buildings, and enabling small businesses to start.

Micro-hydro schemes are already benefiting many remote communities in the Himalayas and Andes, as well as in hilly parts of China, Sri Lanka and the Philippines. In the developed world, micro-hydro schemes supply power to existing mains electric grids.

Read on for more information about micro-hydro, or go to our database for films and case studies of Ashden Award winners who use micro-hydro.

How micro-hydro 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 classified by the output power which they produce as approximately:

  • Large scale: 2 MW and above
  • Mini: 100 kW to 2 MW
  • Micro: 5 kW to 100 kW
  • Pico: less than 5 kW

but the basic approach is similar for all.

The core of a micro-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. 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 micro-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.

Penstock and turbine house for micro-hydro scheme installed by SITMo at Bokiawan, the Philippines

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’.

Penstock and turbine house for micro-hydro scheme installed by SITMo at Bokiawan, the Philippines


How micro-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 kV 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, 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.

Micro-hydro schemes can be connected to a mains grid if available. CRERAL is an electricity supply cooperative in southern Brazil, which has installed two grid-connected mini-hydro schemes with total capacity 1.9 MW to reduce the amount of electricity which it has to purchase from the national grid, and also improve the reliability of the supply to its customers

In some micro-hydro schemes, the rotating shaft directly drives machinery such as a grain mill. SITMO in the Philippines has installed micro-hydro mills for rice and coffee. 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.

Grinding maize using an improved water mill in Nepal


What are the benefits of using micro-hydro?

In remote areas, micro-hydro 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 from micro-hydro also is sufficient to run machinery and refrigerators, thus supporting small businesses as well as homes. For SITMo, a major reason for installing micro-hydro was to provide a more pleasant life with increased employment opportunities in rural areas, and discourage young people from drifting to the cities. Practical Action Peru also found that young people were more likely to stay in villages with micro-hydro, and that business activities grew

Two benefits from micro-hydro in the village of Conchan, Peru: running a sewing business and watching TV


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 driving machinery, and diesel and other fossil fuels for generating electricity,

There are concerns about the environmental impact of large-scale hydro schemes, because they require large areas to be flooded to provide reservoirs, and can have serious impact on water management. Carefully-designed micro-hydro 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, thus have very little environmental impact. Several small hydro systems have less environmental impact than a single large hydro scheme supplying the same power.

Cost

The cost of micro-hydro varies significantly with location. Schemes installed by Practical Action Peru provide an average of 33 kW for about 100 households, and cost about US$3,400 per kW (equivalent to US$1,000 per household) to install. About 55% of the cost is subsidised, and the remainder provided through labour and as a loan. Users pay per kWh of electricity used, at a rate which covers the management and maintenance of the micro-hydro scheme, and pays back the loan.

CRERAL costs are lower because the schemes feed an existing grid and there is no additional cost for distribution and management. CRERAL earns carbon finance for displacing electricity generated by fossil fuels with its micro-hydro schemes.

Numbers

Globally, hydro-power is the largest source of renewable electricity, providing about 16% of the world’s electricity (3,100 TWh in 2008), but most of this is from large-scale systems. In 1995, the micro-hydro capacity in the world was estimated at 28 GW, supplying about 115 TWh of electricity. About 60% of this capacity was in the developed world, with 40% in developing areas.

The main micro-hydro programmes in the developing world are in mountainous countries, such as Nepal (around 2,000 schemes, including both mechanical and electrical power generation) and other countries in the Himalayas. In South America, there are micro-hydro programmes in the countries along the Andes, such as Peru and Bolivia. Smaller programmes have been set up in hilly areas of Sri Lanka, the Philippines, China and elsewhere.

The future

There is substantial potential for supplying off-grid communities with electricity from micro-hydro schemes. A study in Peru suggested that less than 5% of the economic hydro potential in the Eastern Andes had been used.

Micro-hydro systems were used in the Europe and the USA from the end of the 19th century, but most were abandoned as grid power became available. Their potential for supplying power to the grid is now recognised, in particular where there is carbon finance or preferential tariffs for low-carbon electricity. New schemes are being installed and some mill owners like the South Somerset Hydropower Group in the UK are refurbishing old water mills to generate electricity.

 

Lead author: Dr Anne Wheldon