Solar-electric (photovoltaic) pumping systems provide a welcome alternative to fuel burning generators, cumbersome windmills and hand pumps. A solar pump is alot like a windmill, which fills a tank when the energy is available. The BIG difference is that solar pumps don't slow down in summer, when winds are low. They provide the most water precisely when it is needed the most -- when the sun shines the brightest! Solar pumps are simple to install and maintain. The smallest systems can be installed by one person in a couple hours, with no experience or special equipment required.
While multi-national corporations commit massive resources to developing photovoltaics, small companies and private inventors are putting PV power to use in small-scale, appropriate applications. Special water pumps required for solar use are among these innovations. Solar power differs fundamentally from conventional electric or engine-powered systems, so solar pumps often depart from the conventional. PV arrays produce DC power, rather than the AC from conventional sources. And, the power available varies with the sun's intensity. Since it costs less to store water (in tanks) than energy (in batteries) solar pumps tend to be low in power, pumping slowly through the duration of the solar day.
Simple, efficient systems are the key to economical solar pumping. Special, low-power DC pumps are used without batteries or Ac conversion. Modern DC motors work well at varying voltage and speed. They are more efficient than small AC motors, too. The better DC motors require maintenance (brush replacement) only after periods of 5 years or more.
Most solar pumps used for small scale application (homes, small irrigation, livestock) are "positive displacement" pumps which seal water in cavities and FORCE it upward. This differs from faster, conventional CENTRIFUGAL type pumps (including jet and submersible pumps) which spin and "blow" the water up. Positive displacement pumps include piston, diaphragm, rotary vane, and pump jacks. They work best for low volumes, particularly where variable running speeds occur. Centrifugal, jet and turbine pumps are used for higher volume systems.
Electronic matching devices known as Power Trackers and Linear Current Boosters allow solar pumps to start and run under low-light conditions. This permits direct use of the sun's power without bothersome storage batteries. Solar trackers may be used to aim the panels at the sun from morning to sunset, extending the useable period of sunlight. Storage tanks hold a 3-10 day supply of water, to meet demands during cloudy periods.
Solar pumps use surprisingly little power. They utilize high efficiency design and the long duration of the solar day, rather than power and speed, to lift the gallons required. Solar pumps are available in the power range from 1/30 to 1 1/2 horsepower. System costs range under $1000 to the tens of thousands, depending on water requirements, lift and climate.
TO DESIGN A SOLAR PUMPING SYSTEM, WE ASK FOR THE FOLLOWING DATA:
• WELL DEPTH (or description of water source)
• DEPTH TO WATER SURFACE -- Does it vary? Describe
• YIELD OF WELL, estimate in gallons per minute
• TOTAL VERTICAL LIFT from WATER SURFACE to storage tank/pipe outlet
• SIZE OF CASING (inside diameter)
• QUALITY OF WATER (silty, mineralized)
• WATER REQUIREMENTS in gallons PER DAY, ACCORDING TO SEASON
• APPLICATION for water: Home? Livestock? Irrigation (what kind of system)?
• Is PRESSURE required (home, sprinkling)?
• Can STORAGE TANK be located higher than point of use (easily)?
• Is system to be located near a home/battery? Distance?
• Elevation above sea level (determines suction limitations)
• Complex terrain? Include map or diagram
• DESCRIBE EXISTING EQUIPMENT for pumping, distribution, storage etc.
You should provide ALL this information to your supplier when requesting a solar pump design!
If your well is near your home, and a battery system is present at the home, then it is usually most economical to run the pump FROM THE HOME SYSTEM. That way in summer, when you use less energy for lights, you have more to spare for pumping and need not add much to your generating capacity. A battery system allows you the options of pumping on demand (any time) to charge a pressure tank, and also the option of using a conventional AC submersible pump powered by an inverter -- a good option in some circumstances. And, storage tank requirements may be reduced or eliminated.
STORAGE AND DISTRIBUTION: Homes generally require some pressure for proper water delivery. When considering using an elevated tank to provide pressure, be aware that 2.3 feet of elevation is required for every 1 PSI pressure. Conventional "Town Pressure" is 30 to 60 PSI -- over 100 feet! In most cases, it is FAR CHEAPER to obtain pressure using a DC PRESSURE (BOOSTER) PUMP charging a conventional pressure tank. The booster pump runs from the home battery system, as usage demands. Energy requirement is generally less than that supplied by one PV panel, for typical family requirements.
An exposed storage tank will be subject to freezing in most parts of North America. We favor BURIED storage tanks made of food-grade polyethylene (like a giant milk jug) combined with a booster pump system. The buried tank keeps water cool in summer, and liquid in winter! The pump may be placed inside the house, even if it is a little higher than the buried tank. A plastic tank also costs much less than a steel one, and will last longer.
IRRIGATION should be satisfied with gravity flow whenever possible, so energy is not spent pressurizing. Drip systems can usually operate from low pressure by gravity. Irrigation water may also be supplemented by RAIN WATER CATCHMENT and storage, to reduce pumping requirements. The author catches water from 1200 square feet of roof, stored in a 3000 gallon steel tank elevated 4 feet above the ground. This has provided 95% of the requirements for 25 new trees, for the past 3 years -- in NEW MEXICO! Rain storage is also an alternative for domestic water, using a purifier, if ground water development is too costly.
For our home, we use a prototype "Hydra-Jack", a new kind of deep well DC pump. Ours draws about 3/4 GPM from a 145 foot deep well (the well only produces 1 gpm). It fills a 1200 gallon polyethylene tank that is so clean even my wife can't taste the plastic (she's very sensitive to it). A Flowlight Booster Pump provides pressure. We use about 100 gallons per day for our household of (average) 2 1/2 people. We have a clothes washer and dishwasher and an "Info" 1-gallon flush toilet.
This article is vague, of necessity -- the subject matter could easily fill an entire book AND a good sized catalog. Solar pumps are utilizing many new, innovative designs and specifications are ever-changing. For more information, refer to the latest catalogs from PV suppliers, and consult a competent solar power specialist. Watch for more articles on this vital subject in future issues of Home Power.
Windy Dankoff is owner/manager of Flowlight Solar Power, manufacturer of "Slowpump", "Flowlight Booster Pump" and "Flowlight Micro-Submersible" DC well pumps. You may reach Windy at PO Box 548, Santa Cruz, NM 87567 or call (505) 753-9699.
FLOWLIGHT SOLAR PUMPS
DC SOLAR WELL & BOOSTER PUMPS
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