Question of the Week
Answer: While solar water-pumping systems are not very complicated, there are a few steps to take and some figuring that goes into choosing the right size pump and solar panels. The type of water source, the distance it is from the crops being irrigated, and the type of irrigation system all greatly influence the type and size pump you will need, which, in turn, determines the size and number of solar panels you need to do the job.
1) You need to determine how much water you will need or the flow rate. This is usually determined in gallons per minute (gpm) or gallons per hour. If you are using drip irrigation, which would use the water resource most efficiently and decrease the size of the pumping system you will need, then look at the emitter spacing and flow rate per hour per emitter to determine how much water you will need. The manufacturer of your irrigation equipment would be helpful to you in determining this number.
2) Next, determine the pressure you will need to provide the flow rate determined in step one. This number is expressed in pounds per square inch (psi), and is determined by adding up the total head (the elevation from the water source surface to the outlet), friction losses from the pipe and filters and elbows in the supply pipe, and the pressure needed to run the drip tape or sprinklers or other irrigation equipment. The friction loss is based on the size pipe being used and the flow rate in gpm.
3) With information about flow rate and the pressure, or psi, needed for the system, you can now choose a pump that will do the job. Centrifugal pumps are used a lot for irrigating. They are good at moving large amounts of water from one place to another, but not under a lot of pressure. They are not good at pulling water from a source that is lower than the pump. Positive displacement pumps can lift water much higher and under more pressure. You have to look at a pump curve for different solar pumps, which shows how many gpm of water a pump can supply at different pressures and different amounts of power. Pump manufacturers can supply you with pump curves for each pump they sell.
4) Finally, once you choose a pump that can supply the amount of water you need at the pressure you need, based on the pump curve, you can choose the size and number of solar panels you will need to supply the power that the pump requires. The number of sun hours in your area and equipment efficiency need to be considered in panel selection, so consulting the pump and panel manufacturers for sizing recommendations would be helpful. Also consider consulting a solar installer in your area. If you want to install the system yourself, you could pay for a design consultation to make sure the system will work.
For more information, consult the ATTRA publication Solar-Powered Livestock Watering Systems.
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