A few reasons why the rope pump really is a remarkable invention!

I thought I might talk about why the rope pump is such a uniquely appropriate design for use with a solar panel.

Firstly, I should point out that there is a theoretical limit imposed by the maximum depth a pure vacuum can suck water up from atmospheric pressure. In theory this limit is about 10 metres. However, in practice, the maximum depth water can be sucked from is more like 7-8 metres. This means that all pumps located at ground level are limited to drawing water from this depth.

Some designs locate the pump below ground level, connecting it to the power supply at ground level via a rotating shaft (or with electric cables) but this always adds complexity, and cost!

The rope pump's unique design means that the weight of the water column inside the pump's pipe is spread evenly across all the pistons, and the water is 'pushed' rather than 'sucked'. The pump can therefore draw water from a theoretically unlimited depth (in practice, water has been drawn from up to 90 metres!) whilst keeping the bulk of the mechanism above ground. The fact that the water's weight is spread across all the pistons also means that the pressure remains relatively low throughout.

Therefore, cheap PVC piping can be used as the rising main material- the pistons can even be cast by the user from melted plastic bags! Alternatively, the user can plat the rope from sisal (a native plant in many parts of Africa) and the pulley can be made from a recycled car tyre! This not only means that the pump mechanism can be made for just £20-30 but also that it is light enough to be removed by just one man (this is important as most pumps require specialist lifting equipment to remove them from the well). Combining this, with the simplicity of the design, means that the rope pump can be easily maintained by the user.

The rope pump is also unusual in its property of requiring the most 'torque' when it's only running normally, once the pipe is full. Torque is the measure of how much a force acting on an object causes that particular object to rotate by. This is unlike other pumps which have friction at start up, making them 'sticky'. This is important as most motors only produce their highest torque once up and running. The rope pump is also unique in being the only rotary hand pump. These two facts mean that the rope pump is particularly suited to being powered both by hand and by a motor.

Did you know that there are over 100,000 rope pumps are in operation worldwide? Most of these are hand powered! However, there is little to no data available for either the torque and power requirements of the pump or for the delivered flow-rates for a given head (or 'drawn depth') and rotational speed.

Over the last year, whilst at Bristol University, I've developed a theoretical flow model of the rope pump and tested it against a real model rope pump. I now am reasonably confident in predicting exactly what type and size of motor and solar panel I will need for a range of well depths.

SolarAid have been put me in touch with a very useful academic at the Department of Energy, at the University of Mzuzu in Malawi. His name is Maxon Chitawo. He has offered to let me use some of the facilities at the university which should be invaluable. I hope to set up a working solar rope pump on a range of well depths. I will record the methods and materials needed and collate enough concise information so that others will be able to install solar pumps on any well once I've left Malawi.

My other main aim while I am in Malawi is to establish a substantial number of suppliers of the solar rope pump by demonstrating its low cost, ease of maintenance, versatility and output flow rates. I believe that with a little more work the solar rope pump could be commercially supplied, especially to those who already have the solar panel or who have budget horizons of more than a couple of years. These might include schools, small businesses, hospitals and other community centres or groups.

Bobby Lambert received a lot of interest when he visited Tanzania last year, and I hope to follow up on these as well as find new potential suppliers.

I hope you've found it useful to read a little more about why the solar rope pump is such a simple yet highly effective invention, and how it works well with solar power. If you would like to support our work and help us with further research, please donate to this project!

Thank you,
Cai