Water. Because it's so often hard to come by without the application of a considerable amount of electric power, those of us living off the power grid seem to have a preoccupation with it. Generally, we either bring water home from town in a big tank lashed into the bed of a one-ton pickup, use a fossil-fuel-guzzling generator to run a power-hungry AC (alternating current) well pump, or we simply beef up our solar arrays and upgrade our inverters to handle the load imposed by the ravenous well pump.
Of the three, I prefer the option that doesn't require me to fire up an internal combustion engine-either the one in my temperamental '72 Chevy 4 x 4, or the one that runs my quiet and compliant 6.5 kW Honda generator. Pumping water from a deep (540-foot, in our case) well with no other input than sunlight and wind is quiet, automatic, and esthetically pleasing.
But the system is not without drawbacks. On very rare occasions-like when LaVonne is simultaneously running the dishwasher and clothes washer, and the 11-amp, 240-volt well pump kicks on at the same time I'm cutting wood with a 15-amp miter saw-the sum of the loads can overburden the system. This creates an instant blackout that persists until I, with a mini-mag flashlight clenched firmly between my teeth, restart the inverter. But this seldom happens. The biggest annoyance is simply the extra power the pump requires; always in large chunks of wattage and often when the batteries can least afford it.
What would I replace our system with? Ideally, my dream solar water-pumping system would:
- Run independently from the house system.
- Work without batteries, since batteries, convenient though they may be for storing power, are expensive, require maintenance, and have a short lifespan compared to the solar panels that keep them charged. Most importantly, batteries introduce an extra degree of inefficiency into a system.
- Operate on DC power, and would thus not require an expensive inverter and an additional loss in efficiency.
- Be completely automatic-I want to install it and forget about it. And always have all the water I need.
Though this may sound overly wishful, it really isn't. In fact, once all the variables are taken into account, my hypothetical system is the cheapest and most efficient water pumping system you could buy for your off-grid home. Not only does it spare you the expense of extra batteries and a beefier inverter, all the energy from the separate solar array goes directly into pumping water. This means that less array wattage will be required overall.
The components of a stand-alone system
Most stand-alone solar water pumping systems have just a few basic components. They include a solar array (and/or a wind turbine, both separate from the house system), a pump controller, a well pump, a storage tank, a float valve, a pressure tank, and a booster pump.
The solar array-and, with some types of well pumps, a wind turbine-will be sized for your application and can be anywhere from 80 watts to several kilowatts of capacity. For better performance in summer, the array can be mounted on a solar tracker that automatically follows the sun as it moves across the sky. Bear in mind, however, that the extra yield from a tracker drops off considerably in winter when the sun is confined to the southern horizon. In addition, solar trackers are notorious for breaking down, or simply not being able to figure out where the sun is in low-light conditions.
The pump controller will be specific for each type of pump. The purpose of the controller is to monitor the power coming in from the array or wind turbine and to adjust the voltage and amperage for optimal pump performance. The controller, therefore, protects the pump and ensures that there is enough power to run it safely. When power begins to wane, the controller will shut the pump down.
A storage tank is used to store the water needed at night and during cloudy spells. You will want it large enough to store several day's worth of water. It should be fitted with a float valve that tells the pump to shut down when the tank is full. If pressurized water is needed for domestic use, a small, inexpensive booster pump, powered by the main house PV (photovoltaic) system, can be plumbed in tandem with a separate pressure tank. Since water cannot easily be compressed, the air inside the pressure tank acts as a cushion for the booster pump between the kick-on and kick-off pressure settings.
This leaves us with the heart of the system, the solar-powered well pump.
Types of well pumps
There are dozens of makes and models of solar-powered well pumps, but all are either surface pumps, which are mounted above the water line and draw water from not much more than 20 feet (the exception being jet pumps, which pump water from deeper down by using extra power to force water through a venturi-like ejector set inside the well); or submersible pumps, which are lowered into the well and can pump water from 700 feet or more.
These can be further divided into two basic types: positive displacement pumps that trap water in cavities and force it upwards, and centrifugal pumps that force water outward with an impeller turning at a very high rpm. In general, positive displacement pumps are used where high pressure is needed directly from the well, and/or in situations not requiring very high volumes of water. In addition, positive displacement pumps can run in low-light conditions, owing to the fact that they are able to pump water effectively at low rpm. Centrifugal pumps, on the other hand, are noted for their ability to pump large volumes of water quickly, but they begin to slip (and thus to waste energy) in high-pressure applications, and they often cannot develop the power they need to operate in sub-optimal sunlight.
The pump you choose will depend entirely on your application. The idea is to buy the thriftiest pump, in terms of energy usage, that will adequately fill your needs. If, for instance, you are in a situation where you can pump water slowly throughout the day and store it for later usage, you might want to look at a Lorentz PS Series helical rotor positive displacement submersible pump. These pumps operate on 24 to 48 volts and can pump several gallons per minute using energy from a solar array as small as 80 watts.
Where more volume is required, as for livestock operations, check out the Grundfos line of SQFlex submersible pumps. They have both helical rotor and centrifugal models, capable of pumping up to 4 gallons per minute (gpm) at 525 feet, or as much as 50 gpm at 20 feet. These highly versatile pumps run on 30 to 300 volts of DC power, or 90 to 240 volts of AC, and can operate on as little as 130 watts of solar input.
Or, if you should happen to have a spring or well under 20 feet in depth, the Conergy SlowPump is a rotary-vane (positive-displacement) surface pump that can push water uphill as high as 440 feet, and is designed to run optimally on 12, 24 or 48 volts.
For more information on these and other pumps, visit: www.grundfos.com, and www.conergy.com.
Before you lay down good money for a pump, or begin gathering any other components for your install-it-and-forget-about-it solar water-pumping system, you owe it to yourself to talk to a dealer who sells several types of pumps. Every application comes with its own peculiarities, and by asking you questions you never even thought to ask yourself, the dealer will be able to design the perfect solar and wind pumping system for your needs. The best part is, the system design is free. All you have to do is buy the components. And then you get to put the whole thing together all by yourself, just like you always wanted to.
Then you can call me and gloat.
Rex Ewing is the author of several renewable energy books. He lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind in the foothills of Colorado.
