There are many situations when you need to pump water, perhaps in a DIY solar water heating or rainwater harvesting system, or a larger application such as a water supply from a well or river, or for irrigation. If there is no mains electricity nearby to run the pump, then a renewable energy technology could be ideal, as a grid connection may be expensive and a diesel generator noisy and polluting.
One of the simplest renewable energy pumping systems is a solar fountain, which powers a low-voltage pump using a small photovoltaic (PV) panel. A complete solar fountain will cost upwards of £100, and the beauty of it is that it will work best on sunny days when you most want a water feature.
Sizing a pump
The two main factors to consider when seeking a suitable pump are the flow rate - the amount of water that the pump will deliver, and the head - the height through which it will raise the water. These are related - increasing the head will decrease the delivered flow. It's important to minimise bends and other friction losses in pipework, as navigating these will require greater pressure, and as pressure and head are directly related, this effectively means a greater head.
Manufacturers' technical data sheets will give the performance range of each pump, with graphs showing optimum combinations of flow and head. A pump sized properly to your needs will operate most efficiently. Suction pumps are limited to a depth of a few metres, so to draw water from a well or borehole, you'll almost certainly need to lower in a submersible pump. Pumping wastewater or sewage necessitates one designed to handle drainage or effluent.
Small electric pumps for moving water around a domestic system will cost tens of pounds, whilst those for a well or borehole supply are likely to be a few hundred pounds. The main cost is in providing power to the pump, particularly when off-grid. Therefore, do first take all possible water-saving measures (i.e. installing spray-head taps, a low-flush device, mulches on plants to minimise water loss etc) as these easily pay for themselves in the energy saved by reduced demand.
Off-grid Electric pumps
Meeting a year-round water demand with a renewably-powered pump may require a combination of PV panels and a wind turbine, as this will balance energy production over the year. Sunshine and wind are naturally intermittent, so you may need some form of storage. Pumping water up to a tank (with demand then fed by gravity) during sunny or windy periods is more efficient than transferring the energy to batteries. If storing lots of water, you'll need to balance the costs of a large tank (and supporting structure) against the costs of batteries (and their environmental impact and toxicity). An inexpensive control system can pump when needed, and otherwise divert power to batteries, giving extra backup facility.
The price of a small-scale renewable energy system will depend on the power and the maximum capacity needed. A very rough estimate is around £5 to £10 per installed watt. Siting generating equipment close to the pump minimises the cost and power loss incurred by cabling. As small turbines and PV panels usually produce power at 12 or 24 volts, a low-voltage pump would enable you to do without a costly inverter (for stepping up to 240 volts). The CAT publication Off the Grid contains advice on sizing wind turbines or solar panels to meet a specific off-grid energy demand.
For larger-scale pumping applications, you can avoid the losses in electrical systems by using mechanical power directly. See the related questions on hydraulic ram pumps and wind pumps for more on these options.