How It Works: Water Well Pump
If you live in a town or city, you probably don't give much thought to how the water you use each day gets to your house. Even small villages often provide a network of supply pipes that transport water to each home in the neighborhood. All you need to know is how to open the tap at the sink.
Move a few miles out of town and the picture can change. While the inner workings are still—thankfully—invisible, your water supply is independent from the neighbor's down the road. Each home has its own well from which to draw water. More than that, each home has its own electromechanical system for getting the water from the well to the house. At the heart of each system is a pump, and the most common types are jet pumps and submersible pumps.
In many areas of the country, finding potable water is as easy as getting out a shovel and digging a hole in the ground. Okay, maybe "easy" isn't the right word, but wherever the water table is only several feet below the surface of the ground, part of the battle may already be over. In such a shallow-well situation, lifting the water up to the house is going to be a little easier, if only because the distance you have to move it is modest.
If your area doesn't have a high water table, or if it lacks a stable supply of potable water near the surface, you must dig deeper to achieve the same result. And because a deep well means that the water has to be lifted farther, the strategies for moving it change.
These days, the most common pump for a shallow well is a jet pump. Jet pumps are mounted above the well, either in the home or in a well house, and draw the water up from the well through suction (see Single-Drop Jet-Pump System diagram on next page). Because suction is involved, atmospheric pressure is what's really doing the work. Think of the system as a long straw. As you suck on the straw, you create a vacuum in the straw above the water. Once the vacuum is there, the weight of the air, or atmospheric pressure, pushes the water up the straw. Consequently, the height that you can lift the water with a shallow-well jet pump relates to the weight of the air. While air pressure varies with elevation, it's common to limit the depth of a jet-pump-operated shallow well to about 25 ft. There is also deep well pump.
Jet pumps create suction in a rather novel way. The pump is powered by an electric motor that drives an impeller, or centrifugal pump. The impeller moves water, called drive water, from the well through a narrow orifice, or jet, mounted in the housing in front of the impeller. This constriction at the jet causes the speed of the moving water to increase, much like the nozzle on a garden hose. As the water leaves the jet, a partial vacuum is created that sucks additional water from the well. Directly behind the jet is a Venturi tube that increases in diameter. Its function is to slow down the water and increase the pressure. The pumped water–new water that's drawn from the well by the suction at the jet–then combines with the drive water to discharge into the plumbing system at high pressure.
Because shallow-well jet pumps and deep well water pump use water to draw water, they generally need to be primed–filled with water–before they'll work. To keep water in the pump and plumbing system from flowing back down into the well, a 1-way check valve is installed in the feed line to the pump.
Solar Powered Water Pumping
Any renewable energy source can make the electricity needed to power various appliances, including pumps. Solar electric power in particular is a reliable and economic choice for powering remote water pumping. Solar water pumping systems are in common use for garden fountains, livestock watering, and large-scale watering needs for commercial installations.
Cattle ranchers all over the world are enthusiastic solar pump users because their water sources may be spread over large areas of rangeland that lack utility power and where generator use would be expensive and impractical. Photovoltaic (PV) panels are therefore in widespread use for reliably producing electricity directly from sunlight to power livestock and irrigation watering systems. When properly designed, PV-powered pumping systems can result in significant long-term cost savings and a smaller environmental footprint compared to conventional power systems.
System Design Considerations
A typical solar-powered water pump system, which includes a solar array, controller, pump, and storage tank. (Source: "The Montana Agsolar Project – Expanding the Agricultural Uses of Solar Energy in Montana.")
A solar-powered water pumping system consists of four parts: the actual pump which moves the water, the controller which adjusts the pump speed and output power as the solar panel input varies, the engine, and the solar panels. The specifics of the system design are determined by the following considerations:
The site-specific available solar energy (or insolation).
The volume of water required in a given period of time for the application at hand. This may include additional water to be stored for periods when the PV is not operating or has diminished output.
The total dynamic head (TDH) for the pump (the equivalent height that water must be raised, taking friction losses in the pipes into account.)
The quantity and quality of available water.
The system's proposed layout and hydraulic criteria.
Pumps for Solar Pumping Systems
There are two major types of solar pumps, direct current (DC) and alternating current (AC). DC solar pumps are generally suitable for small applications (garden fountains, landscaping, etc.) and are relatively low-priced, particularly because they do not require inverters to produce AC power from the solar panels. These pumps are generally designed to operate with minimum electrical power, so they have rather low flow rates. Such pumps often find use in submersible deep wells where a slow but steady pump rate is acceptable.
AC solar pumps are driven by inverters producing AC power from PV panels. They are suitable for all kinds of applications from landscaping to irrigation, particularly large-scale applications such as farmland irrigation, desert control, and so forth. AC solar pumps are available in power output ranges from 150W to 55kW.
Solar-powered pumps are characterized as either positive displacement pumps (e.g. diaphragm, piston, or helical rotor) or centrifugal pumps. Positive displacement pumps are typically used when the TDH is high and the flow rate (measured in gallons per minute) required is low. Conversely, centrifugal pumps are typically used for low TDH and high flow rates.
There are many resources available that provide specific information and advice for implementing solar water pumps in a variety of situations. These should be consulted to meet the needs of specific applications.
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