EValuation For Solar Water Pumping Guide

Tech notes: Evaluating the well
One of the most important phases in designing a solar water pumping system is evaluating the well. If the rancher is fortunate enough to have unused wells on his property, the expense of drilling a new well may be avoided by evaluating the well. If the well is found to be usable, installing the PV pumping system on-site can be a simple matter. Using compressed air, the well driller can determine water yield and draw-down level and remove some accumulated trash from an old well. The water will almost certainly be dirty at first but can clear up after prolonged pumping if the original screen and casing are intact. An experienced well driller can have a good idea of the well depth needed for a good water supply in his working area. In many cases artisan pressure pushes water close to the ground level. If this is the case and the well yield is adequate, a centrifugal pump can be used. Otherwise, submersible positive displacement type pumps must be used to push the water up and out of the well (Table 1). The theoretical pumping limit for a suction pump is approximately thirty-four feet at sea level, and less at higher altitudes (about 1 foot less per 1000 feet). That is the limit regardless of the motor size connected to the pump because suction is limited to one atmosphere of negative pressure. Over 34 feet you would be pulling a perfect vacuum. The practical limit for a centrifugal pump is about twenty-feet because of pipe friction, non-perfect seals, etc. Other types of pumps often have much less suction capacity. When evaluating an existing well for conversion to a solar pumping system, the following questions must be answered:
• What is the size of the well casing?
• Is the casing in good condition with no pin hole leaks?
• How deep is the well?
• What is the depth to the water surface?
• How much water will the well produce - the replenishment rate?
• What is the draw-down level during water flow?
• Is the water free of silt and sand?
• How many gallons of water will be needed each day?