WATER SUPPLY FOR THE INDEPENDENT HOME: RUNNING SUBMERSIBLE WELL PUMPS ON INVERTER POWER Windy Dankoff The submersible well pump is one of the great inventions of the 20th century. From domestic use to remote livestock watering, the "SUB" has replaced all sorts of hand pumps, jack and piston pumps, chains, buckets, windmills, etc. It is inexpensive, reliable and reasonably efficient. Millions are in use worldwide. Homes beyond the power lines often have alternative energy systems, usually photovoltaics with storage batteries. There are perhaps 50,000 such homes already in the U.S. and a growing industry to serve their special needs -- low voltage DC lights, appliances, pumps and electronic inverters. An inverter converts the stored DC power to household AC. Most independent homes use a combination of DC and AC appliances. A variety of special DC pumps are available. See articles in Home Power #5 and #11. DC "Solar Pumps" are typically more energy-efficient than AC submersibles powered by inverters, but they are often more costly. Even if less efficient, there are times when AC pumping makes sense in alternative energy systems. The lower cost of the AC pump must be weighed against the cost of additional PV modules, batteries, and the inverter required to power it. AC pumping may be economical if one or more of these factors apply: (1) Water requirements are low and/or energy system is relatively large so energy usage is not a critical factor. (2) The appropriate DC pump for your particular needs is not available at a reasonable price (compared to inverter/AC sub). (3) Your well is hundreds of feet from the power system. (Inverter's high voltage output greatly reduces line loss and therefore the need for large-size, expensive wire.) (4) You already have a good AC sub in your well. (5) You need an inverter for other tasks, and it will have enough available capacity to power an AC sub. HOW DOES A SUBMERSIBLE PUMP WORK? THE PUMP: All conventional AC subs work by centrifugal force. Water is drawn into a spinning disc called an IMPELLER, and forced outward at high speed. It is then funneled upward to another impeller, which adds more pressure, and another and another. The more impellers the pump has, the higher it will push and the larger the motor must be. The impeller stack is a single moving part, without sliding surfaces to wear. THE MOTOR: A submersible AC motor is sealed and filled with water or oil. It is exceptionally slender, fitting in well casings as small as 4" in diameter. It is an "induction" motor with only one moving part. Home-size pumps range from 1/3 to several horsepower. The power required depends on vertical lift, pressure required at the house, capacity of the well, and water demands of the home. ADVANTAGES OF AC SUBS Given proper selection, proper power, no dry running and fairly clean water, AC Subs are very reliable. Many have lasted 10-20 years with little attention required. They are common, easily available and competitively priced. DISADVANTAGES OF AC SUBS (1) ENERGY LOSSES: Small AC subs are consumer products that are not designed with efficiency as a primary factor. Their energy losses are most severe at low flow rates (under 6 GPM) in deep well situations. Inverter and battery losses compound to bring overall efficiency down to the poor-to-fair (15- 45%) range. (2) STARTING PROBLEMS: Induction motors require a high STARTING SURGE of current. The sub's surge requirement is higher than other motors of similar HP, due to high speed design and constricted motor diameter. Modern inverters are specially designed with induction motors in mind, but a large 2000 watt inverter may exceed its surge limit starting even a small (1/2 HP) AC sub. NOTE: Why can't you put a DC motor on a submersible pump? A true DC motor (with brushes) cannot be liquid-filled, so DC subs use either unique sealing methods, or use a combination of inverter electronics and a specialized AC motor (called a "brushless DC motor"). This is a new and growing field. DC subs are used for solar-direct power where there is no battery system nearby. They are less mass- produced, and are more expensive. HOW TO FIND THE MOST EFFICIENT AC SUB: (1) Ask your driller or pump dealer. Specify ALL your pumping requirements AND the characteristics of your well. Good pump distributors have engineers on staff who can understand your needs. Get a second opinion from a distributor who carries different brands. (2) Higher flow pumps tend to use energy more efficiently. Shop for the highest flow rate you can get for the HP, without exceeding your well's capacity or the capacity of your inverter. If you will be in danger of overpumping your well (running pump dry) consider the Franklin "Pump Tech" dry run controller. NOTE: Vertical lift or "head" on an AC sub is measured from the water surface in the well. (Submergence does not effect the work the pump will do, since the water seeks its own level in the pipe.) In many wells the water level draws down during pumping. It is this pumping level that is important to consider. Your driller (or written records) can give you an idea of your well's "recovery rate" and anticipated draw-down. HOW TO MINIMIZE STARTING PROBLEMS: (1) Get a "Three-Wire" pump rather than a "Two-Wire". It employs an above-ground control box that reduces surge requirement and eases maintenance. (2) Avoid pumps with "Solid-State Starter". They are not tolerant of extreme dips in voltage during starting surge. However, if your pump of choice has one, you can either get a relay kit to convert it to a conventional starter, or use another brand of control box. MORE TIPS: (1) Don't skimp on wire size. The wire is sized according to the power requirement and the length -- the TOTAL length from the power source to the motor. Check the pump manufacturer's recommendations. WARNING: A 115V pump requires larger wire than the more common 230V pump. Be sure your installer uses the proper wire for the lower voltage. (2) Get one or two spare "start capacitors" from your supplier, right away. Obtain ones with SLIGHTLY higher microfarad (MFD) rating than the original. They tend to fail if sluggish starting occurs. HOW TO SELECT AN INVERTER FOR DEEP WELL PUMPING: (1) Remember that AC subs are the hardest motors to start. Be sure the inverter you select can handle its power needs, including the surge requirement. (2) If you are using a 230 Volt pump, try to get 230V inverter output without buying an accessory transformer. Note however, that this may limit the 115V power that the inverter will deliver. If you use a transformer, wire the pressure switch in its primary circuit, or it will draw power when the pump is off. (3) If you expect your inverter to run other appliances at the same time as the pump, be sure it is large enough. OR -- (a) Design for more DC utilization, to relieve loads from inverter (b) Use more than one inverter (c) Pump several days' supply of water into a storage tank, then use a DC booster pump for your pressurizing. This way the AC pump can stay off for days at a time. (4) If your inverter will be dedicated to NOTHING but pumping water, consider a specialized motor- starting inverter that may be more economical. NOTE ON GENERATOR POWER: Small generators have the same potential problems starting AC subs as inverters do. A 1500 watt generator could theoretically run a 3/4 HP sub with ease, but it will never start it. The guidelines above apply to generator power too. WARNING: Inadequate generator power may run your pump, but low running voltage will overheat and ruin the motor. Be sure to obtain professional advice in all power system design. HOW TO MINIMIZE ENERGY USE FOR PUMPING WATER: (1) MINIMIZE WATER USE! The less water pumped, the less energy consumed. Low-flush toilets can cut domestic water use in half. (The Eljer Ultra-One one-gallon model is available nationwide.) Keep hot water lines short. Consider waste-water recycling. Consider drip irrigation to make extra-efficient use of water. Use water timers so that irrigation is not left on by mistake. Catch and store rain water for irrigation. Plant drought-tolerant species and use mulch to conserve water in your soil. (2) USE A LARGE PRESSURE TANK. A typical home pressure tank is 40 gallons in size and will store/release about 12 gallons of water between pump cycles. This is called "draw-down between cycles". A larger tank is better, to reduce start/stop cycles and energy-robbing surges. This also reduces wear on the pump. If you have a minimal sized tank now, you may add a second tank for more capacity. Use the modern "captive air" pre-charged tank, rather than the old "galvanized" or "plain tank" which needs re-charging periodically. (3) PLUMB FOR EFFICIENCY. If you haven't yet plumbed your house, use one size larger piping than usual throughout (such as 3/4" instead of 1/2"). This will reduce the pressure required to provide satisfying flow at your faucets. It is FLOW that you perceive, NOT PRESSURE. Typical house pressure is 30-50 PSI (lbs. per sq. inch). To produce this pressure, your pump does the equivalent work of lifting water an additional 100 feet high! Generous pipe sizing allows you to obtain the same satisfying water delivery at 1/3 less pressure. The result is increased pump flow and efficiency. (4) SET YOUR PRESSURE TO THE MINIMUM amount that will satisfy your flow requirements. This is done by adjusting the pressure switch. The lower the pressure, the higher the pump's flow rate and efficiency. A lower pressure range will also allow your pressure tank to deliver a longer cycle. After you have determined a good pressure setting, readjust the pre-charge pressure in your tank to maximize its capacity (see tank manufacturer's directions). (5) CONSIDER A STORAGE TANK and a separate DC booster pump. This way, you can pump enough water in an hour to last for several days. This will also keep your inverter free for other tasks nearly all the time. If you will be dependent on a generator for much of your pumping power, this is definitely the best way. Stored water can also be held as reserve for fire protection. See Solar Pumping article in HP #11. HOW TO DETERMINE ENERGY REQUIREMENTS: In order to design the energy system that will run your pump, calculate its energy requirement in WATT- HOURS PER DAY. (1) CALCULATE YOUR DAILY WATER REQUIREMENT: Typical domestic use requires 50 gallons per person per day (using 1-1.5 gal. toilets -- double that for 4-5 gal. toilets). A young fruit tree in dry weather needs 15 gal/day. A typical lawn sprinkler uses 360 GPH. Cattle average 10 gallons per head per day in summer. Estimate average GALLONS PER DAY requirement as best you can. Remember, if you overestimate here, it can cost you a lot of money. (2) SELECT THE PUMP YOU PROPOSE TO USE. Consult the pump's specification sheet (or ask your driller/dealer) to determine the optimum flow rate and the required horsepower. (3) ESTIMATE ELECTRIC POWER REQUIRED using this table: INSERT TABLE NOTE: For induction motors, multiplying voltage by amp rating will give you a higher than true watts figure. The current draw is out of phase (not synchronized) with the voltage. ALSO NOTE: Many brands of pumps use "Franklin" motors, as described in chart above. Some other motor manufacturers have a so-called "1/3 HP" model that is really a 1/2 HP motor and will surge accordingly. If you want a true 1/3 HP pump, be sure its current rating is not over 9 amps. (4) CALCULATE AC ENERGY REQUIREMENT: AC WATT-HOURS/DAY = PUMP WATTS X GALLONS PER DAY PUMP FLOW RATE (GPM) X 60 (5) CALCULATE DC ENERGY REQUIREMENT: Inverters have conversion losses in converting DC to AC power. Inverter manufacturers advertise peak efficiencies exceeding 90%. HOWEVER, losses are higher for "highly inductive loads" including our beloved AC sub. Efficiency depends on many factors but it is safe to assume an average inverter conversion efficiency of 82%. DC WATT-HOURS/DAY = AC WATT-HOURS/DAY 0.82 If you use a step-up transformer to obtain 230 Volts, change the 0.82 figure to 0.75 to accommodate transformer loss. (6) DETERMINE THE SIZE AND COST OF THE ENERGY SYSTEM REQUIRED: Use a guide book or your dealer's help to determine how many watts of PV modules and kilowatt-hours of battery storage you will need (relative to your climate) to supply the water pumping portion of your energy budget. Don't forget to figure in the battery loss (15-20%). Include costs for wiring, controls, mounting racks, installation etc. (7) OPTIONAL -- CALCULATE OVERALL SYSTEM EFFICIENCY: SYSTEM EFFICIENCY = TOTAL DYNAMIC HEAD (FEET) X PUMP GPM PUMP WATTS X 5.31 Total Dynamic Head (Ft.) = Vertical Lift + Piping Friction Loss (Ft.) + Service Pressure in Feet (PSI X 2.31) NOTE: THIS FORMULA MAY BE APPLIED TO ANY ELECTRIC PUMPING SYSTEM. An inexpensive, low-efficiency system MAY be economically viable if your water needs are minimal -- "It's a gas hog, but it was cheap and I only drive it on Sundays". BUT, if you need all the water you can get from a modest sized energy system (especially photovoltaic power in a cloudy climate) consider a high-efficiency DC pump. The higher cost of the pump system may be more than offset by savings in energy system cost. OVERALL SYSTEM cost is the BOTTOM LINE. REQUEST FOR FEEDBACK: We are gathering performance data on various pump/inverter combinations. If you have experience in this field, we would appreciate knowing -- PUMP: make, model and voltage, 2 or 3-wire, any modifications. INVERTER: make, model, voltage SYSTEM SET-UP: pressure-demand or storage tank, how long in use? PERFORMANCE: flow rate, amp draw AC and DC (if known) LIMITATIONS: problems encountered, particularly with multiple loads on inverter. ACCESS WINDY DANKOFF is the owner of FLOWLIGHT SOLAR POWER. He has been selling and living with wind and PV home power since 1977 and has specialized in solar pumping since 1982. Flowlight manufactures Solar Slowpump, Flowlight Booster Pump and Solaram solar pumps. Contact Windy at Flowlight Solar Power, PO Box 548, Santa Cruz, NM 87567 (505) 753-9699.