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Solar pump

The Solar Pump is a Volanta pump, driven by a thermal solar engine. It has the potential to be considerably cheaper than solar PV pumps and the solar collector is less prone to theft. The main applications of this pump are for deep well drinking water and shallow well small-scale irrigation.

The context

Where water tables are deep or where large volumes of water are needed, mechanized pumping is often required. Where there is sun, a solar pump would be the preferred option as it is a clean and renewable energy source. The large scale application of solar pumps is hampered by their high initial cost, sensitivity of the electronic equipment, high cost of replacement parts and the risk of theft of the solar panels.

The thermal Solar Pump will be cheaper to buy, cheaper to run and its solar collector has no value for thieves. The introduction of this technology may open the way to the wide spread use of solar pumps, especially in developing countries.

The technology

The Solar Pump works on the Rankine principle i.e. a fluid is evaporated through heat from the sun; the pressurized vapor drives an engine; the vapor is then condensed in a cooler and returned to the evaporator through a condenser pump. The evaporation takes place at 130C and the condensation at 60-100C.

With these temperatures a maximum theoretical efficiency can be obtained of 10-15%. With 60% collector efficiency and 50% expander efficiency, an ioverall efficiency of 3% may be obtained. Efforts in the past usually obtained no more than 1% overall efficiency. Extensive research by PRACTICA has discovered that “entrance condensation” is the main reason for these low efficiencies.

With this knowledge, the expander (engine) has been designed to minimize entrance condensation losses and make otherwise optimum use of the vapor energy. On the test bench, overall efficiencies of 3%, sunlight to shaft power have been measured.

The application

The power output of the Solar Pump depends on the installed surface area of solar collector. In practice, it will vary between 50W and 800W. For deep wells, the maximum capacity will be 20cubm per day when the water is at 50m of depth. For lifting irrigation water from a shallow source, the maximum capacity will be 400cubm per day.

The maximum depth from which water can be lifted will be 100m. If water needs to be lifted from a surface source to a higher elevation (as may be required in mountainous areas), the maximum lift will be 200m.

The facts

NB. The given investment costs may be much reduced if the Solar Pump will be produced in China or India.

Status of development

One Solar Pump is installed in Burkina Faso for field testing, since early 2006. Problems encountered were; loss of pentane through leakage, high back pressure in the condenser because of high ground water temperature (31C) and throttling of the vapour inlet and outlet valves. The leakages have been fixed by using better connectors and changing the valve design, throttling has been remedied by fitting larger valves.

In 2007, the solar pump has been modified so as to work on steam (water). The advantages are: no problem with availability of the working fluid, no sensitivity to small leakages and no more problem with back pressure in the condenser. In order to reach the higher temperatures necessary for steam, the flat plate collectors have been replaced by vacuum tube collectors.

The complete system was tested in the Netherlands and worked well. The equipment has been shipped to Burkina Faso and field testing on steam is expected to begin early 2008.