There are several different types of water turbine. Each type operates over a limited range of site conditions in terms of head and flow. Given a typical efficiency, a 1 kW scheme could acquire its energy from 200 m head with a flow rate of 1 litre/s or 2 m head with a flow rate of 100 l/s.
What is Pico Hydro? (Home Page)
Pico hydro is water power up to 5 kW. It was given the name "pico" by Nigel Smith because it needs some different ways of thinking to micro, mini and larger hydropower. There are thousands of sites where people have a source of falling water but do not have electricity. For these rural communities, pico hydro is the lowest-cost technology for generating electricity. Lighting from this source is cheaper than using kerosene lamps, and safer, too. For more facts and figures on pico hydro for lighting, there is an article in Boiling Point (May 2007). On the News page is an interesting cost comparison, taken from a report prepared for the World Bank. It shows that Pico hydro is potentially the lowest cost technology for off-grid electrification.
This web site includes information on ways to design, manufacture and install the various parts of pico hydro projects. It includes documents that you can download to assist in the process.
The main parts of the system, as shown in the picture, are: intake from stream or river, pipe (known as the penstock), water turbine, electrical generator, electronic controller, electrical distribution system.
For higher head schemes, a Pelton turbine is the ideal choice. Pelton turbines use a jet of water which drives a runner with "buckets" which are specially shaped to capture as much energy as possible. This site has details of a particular layout of Pelton turbine, known as the Pico Power Pack, where the turbine is mounted on a shaft extension fitted to a standard induction motor used as a generator. The layout of the turbine, with a runner attached to a shaft extension from the horizontally-mounted generator, was developed through a synthesis of design ideas from two experienced manufacturers of micro and pico Pelton turbines Akkal Man Nakarmi from Nepal and Mauricio Gnecco from Colombia. Bucket patterns for the Pico Power Pack are available for three sizes of runner.
Pumps as turbines are one option for sites with medium head, which have the advantage of being widely available and usually supplied with an induction motor that can be used as a generator. More information on these can be found on a dedicated Pumps as Turbines website.
Turgo turbines are a another good option for medium to high head schemes, with efficiencies of over 70% possible even for pico turbines. Crossflow (Mitchell-Banki) turbines are commonly used and are relatively easy to manufacture locally, e.g. in Indonesia (add link). However, for pico hydro they tend to be large in size and run at relatively slow speeds. Even for turbines with power of 10 kW, efficiencies of above 70% are difficult to obtain in the field although higher efficiencies have been reported from optimized laboratory turbines (add link ).
There are a few different options for low head sites, including traditional water wheels (European type horizonal axis and Asian/South American type vertical axis). These tend to be large and to run slowly, but they have the advantage of allowing leaves and other matter to flow through without blockage.
There are also at least three types of water turbine - tube turbines (with axial guide vanes), open flume turbines and closed volute turbines. A joint project between Nottingham Trent University and Practical Action in Peru (funded by the Leverhulme Trust - 2004-07) has developed a closed volute design, which uses a scroll casing. The aim of this project was to produce a general design for pico propeller turbines and the results, which are described here, have been very encouraging. In particular, it was found that over 70% turbine efficiency can be achieved without the need for expensive materials or manufacture. However, care must be taken with the design of the turbine components and matching of the design parameters. A design guide is available to download from this website.