Terry Thomas

20th WEDC Conference Colombo, Sri Lanka, 1994

For Both Irrigation and domestic supply, gravity feed is not always possible: water often needs lifting. The power to lift a flow of water can conveniently be expressed as power = constant x mean flow rate x height lifted / duty x efficiency, where ‘duty’ is a time fraction (pumping hours per day) and ‘efficiency’ is a product of the efficiencies of the hydraulic circuit, the pump and the prime mover. Pipes are sized to give tolerable hydraulic efficiency and pumps are chosen to match the hydraulic conditions and the energy source available. Duty can also be varied to achieve better matching of the prime mover to the hy draulic circuit: high duties such as continuous 24-hour operation result in low power requirements and cheap piping.

Whilst in general the power for water-lifting can come from engines, electrical mains, animals, humans or re newable (climatic) sources, in the particular context of rural areas in poor countries the choice is more con strained. In many such countries there are virtually no rural electrical mains, engines pose problems of both fuelling and maintenance, draught animals may be una vailable or difficult to apply to water lifting, renewables are erratic, complex and import intensive. Therefore human-powered lifting and transporting of water is still constant x mean flowrate x height lifted duty x efficiency power =common, despite the very high cost of human energy (US$ 2 to 20 per kW hour).

Of the renewables, water power has the longest history, and under favourable conditions is the easiest to use. Several Asian and Latin American countries have devel oped the capability of building hydro-power systems. Although sites where power can be economically ex tracted from falling water are rather rare, they generally occur in the same terrain (mountainous) as the greatest water-lifting needs. The use of water power to pump water is therefore an interesting option. Figure 1 shows the main ways of doing this and illustrates the relative simplicity of the hydraulic ram-pump system. A typical such system is shown in Figure 2.

Ram-pumps (invented 200 years ago) are still manufac tured in over ten countries and were once commonplace in Europe, The Americas, Africa and some parts of Asia. They have however been largely displaced by motorised pumping in richer countries, whilst in developing coun tries their use is concentrated in China, Nepal and Colom bia. Ram-pump technology is not trivial: designing sys tems that are reliable, economic and durable (e.g. against flood, theft, silt ....) takes some experience. Generally, in rural areas of developing countries, this skill has been lost since about 1950, and the intermediaries that used to connect ram-pump manufacturers to pump users have disappeared. Old systems lie broken for lack of fairly simple maintenance: new systems are few.

For various reasons, discussed later, the potential for using ram-pumps seems to be increasing worldwide. Working, primarily in Africa, since 1985 the Develop ment Technology Unit of Warwick University has identi fied several obstacles to this potential being realised, and has been trying to remove them. This paper records that experience.

 

Full paper available from the conference site