Geodomes Camera Ready Inches Wide By Inches High

Using a DC Submersible Pump in a Domestic

Water System

Windy Dankoff

©1993 by Windy Dankoff

DC submersible pumps are a new technology, developed especially for solar power. In an alternative energy home, a low-powered DC pump can be more efficient and economical to use than a conventional 120 or 240 vac well pump.

Capabilities

DC submersible diaphragm pumps can lift water from wells as deep as 300 feet. They pump slowly, between 13 and 4 gallons per minute. Even such a low volume is enough water for a family's daily use if the pump runs a few hours per day and fills a storage tank.

Advantages

These low power pumps allow us to build a solar pumping system for a deep well at a modest cost. They are cheaper than windmills, and pump the most water during dry, sunny weather. They can be installed and pulled by hand. They work in wells of very low yield that conventional pumps may suck dry in minutes.

Pump Controllers

These solar pumps require a special controller if they are to be powered directly by PV modules (without batteries). The controller acts like an automatic transmission, allowing the pump to start and run in low light conditions. With a battery power source, the controller may not be required at all, or a special controller may convert 12 Volt battery power to 30 Volts to run the pump at top speed.

Installing a DC Submersible Pump

Carefully follow the instructions that come with your pump, and call your supplier if you have any questions. Also refer to "Installing a PV Powered Submersible Pump" in Home Power #31, and "Drilling a Well" in Home Power#33.

Drop Pipe

The pipe that drops from the well top down to the pump is called "drop pipe". We use flexible black polyethylene pipe. Get drinking water grade pipe, not utility grade pipe or irrigation tubing. It should have at least a 100 psi rating. This flexible pipe allows easy installation and removal by hand, without the need to disassemble joints every 20 feet.

In most cases, use 12 inch diameter pipe. If your pump is designed for 24 Volt use and has a 34 inch outlet, and you are using it at 12 Volts, adapt it to 12 inch pipe size. We use minimal diameter drop pipe for two reasons. First, water is heavy. Small pipe holds a small enough weight of water that the pump may be pulled by hand. Secondly, small pipe allows the water to flow upward at a higher velocity, so that sand or sediment can be exhausted from the pipe. If you use larger pipe, the water will rise so slowly that the sand may settle within the pipe. When sand accumulates, it causes abrasion and pump problems.

Water well professionals are accustomed to larger 120/240 vac pumps and use one inch pipe or larger, of a thick, rigid variety. This type of pipe is not appropriate for these pumps. The low power pump will not "kick" when it starts, so it does not require heavy-wall pipe (or a torque arrestor) for support.

Installing Polyethylene Pipe

When you buy your fittings, get extra connectors in case you break one or strip threads. Get plastic fittings, not plated-steel ones. Get extra hose clamps in case you strip one by overtightening. Get some extra couplers in case you kink the pipe and cause a restriction (cut out the kinked part and install a coupler). Use two clamps side-by-side on every poly pipe connection. Tighten each clamp with a wrench, until the "tail" just begins to turn sideways. Now you can trust your connections not to leak. Do not use any type of sealant on poly pipe connections. Take sample parts with you to the store to match sizes. Pipe sizing does not always match what you will measure with a ruler!

Personpower Required

One person can handle lowering the pump to its limit, if pipe, safety rope, and power cable are carefully laid out on the ground. Removing the pump is a much heavier job because of water held in the pipe. One person can usually handle a 100 foot pull. Two or three people are needed for greater pull. In addition, someone is needed to tend the pipe so it does not kink.

PIPE FROM WELLHEAD TO TANK Freeze Protection

Your pump's drop pipe must turn to horizontal where it exits the well casing. This can be done underground, below frost line, by using a clever device called a "pitless adapter". This fitting slides together, allowing you to install and pull your pump from above, without digging. Have your driller install one for you when your well is drilled. The smallest pitless adapter is for one inch pipe. Use a reducer bushing to adapt to your smaller drop pipe.

Supply Pipe

The horizontal pipe from the wellhead to your tank should be PVC, or whatever you prefer. Do not use polyethylene pipe underground, as it may develop joint leakage after many years. Use at least one inch pipe since who knows, maybe you'll put a bigger pump in someday. Also, you may be using the same pipe to let water out of your tank. If it flows down by gravity, you'll want big pipe for a good flow. It cannot be too big, only too small. Check a pipe sizing chart to be sure.

Check Valve

These diaphragm pumps have internal check valves, without which they would not function. When the pump stops, water does not readily flow back down the drop pipe. However, the valves aren't perfect, and may allow a slow downward trickle when the pump stops. If you want this to occur, in order to drain above-ground pipe for freeze protection, then do not install a check valve. Otherwise, place one or more check valves at the pump and/or in the line to the tank.

POWER SOURCE: DEDICATED OR INTEGRATED?

A dedicated power system is one which supplies power only to the pump. An integrated system is one in which the pump is wired to the home power system. Let's examine these two methods.

Dedicated System

Wire for low voltage power transmission must be relatively large and expensive to minimize power loss. If the distance from your home's power center to the well and down to the pump is more than 200 feet, the expense may be high. A dedicated system may be cheaper, particularly if batteries are eliminated. This is called PV array-direct. Price both systems and compare. The dedicated system gives the water system its own power supply divorced from the consumptive vagaries of the main home power system. This means that the energy used to supply water is not shared with other appliances like TVs, lights and what-not.

Integrated System

Connecting the pump to the home power system has advantages. Wired in this way, it is simply one of the home's appliances. During the summer, a home with photovoltaic power tends to produce excess energy. This energy can be put to work watering your land. A controller may be set up to do this automatically when your battery bank approaches full charge. The home's battery system and backup generator also provide an energy reserve that can be applied to pumping. An integrated system is more versatile and cheaper than adding a dedicated system, if your well is not too far from your power source. Powering the pump from the main system's batteries also allows use of the well pump to pressurize the water system if necessary. More on this below.

Pump Voltage

The pumps discussed here are primarily intended for solar-direct use at 24 Volts rather than 12 Volts. Larger home power systems are often based on 24 Volts, but smaller systems are 12 Volts. These pumps will operate at half-flow on a 12 Volt system. There is no problem using the pump this way.

Boosting the Voltage

A step-up controller will convert 12 Volts to 30 Volts. Solarjack makes a Pump Controller model PB10-28H that can be used with any 24 VDC pump (5 Amps maximum).

Use in Domestic Water Systems Because of the low flow capacity of these pumps, water must be accumulated in a tank so that it can be released on demand. There are three ways to do this. One, pumping directly to a pressure tank. Two, using storage tank with a booster pump and pressure tank. Three, using an elevated storage tank with gravity flow. The rest of this article deals with the first method.

Pumping Directly to a Pressure Tank

This is the simplest and least expensive setup. It is the same system used by most conventional AC

submersible pumps run on utility grid power. However, the low capacity of these DC pumps poses two limitations.

The pump is doing two jobs, lifting and pressurizing. Pressurizing 1 psi = lifting 2.31 feet. Pressurizing to 43 psi (a typical pressure) is equivalent to lifting 100 feet. So, a pump that can lift 230 feet maximum can lift only 130 feet if it is also pressurizing to 43 PSI. Remember however, vertical lift is measured from the depth of the pump down the well, not the level of the water in the well. These DC sub pumps are the positive displacement type and gain little pressure advantage from the water above them. Such is not the case with conventional submersible pumps (multi-stage centrifugal) pumps made for 120 or 240 vac.

The pump's volume is low. It may be as little as 12 gpm, which is like a pencil-size stream from a faucet. A pressure tank is used to accumulate water so that it can be released quickly when you open a faucet. An 80 gallon pressure tank can store about 30 gallons of water (the rest of the volume is air). The limitation to this system is that once you deplete that stored water, it will take as long as one hour to "recharge" the tank.

If people wait in line to take long showers, or you irrigate with a sprinkler, the pressure tank will be quickly depleted. But, small families get along well with this system, using common water-conserving measures, providing they are aware of the limitation. Drip irrigation is practical with this system.

As your water needs and/or budget expand, you can expand this system by adding a storage tank (large, non-pressurized) and a pressurizing "booster pump" to fill your pressure tank quickly. Meanwhile..

OPTIMIZING THE PERFORMANCE Pressure Tank

Get a captive air pressure tank, not a plain or galvanized tank. Get a large one, like the 80 gallon size. This can store over 30 gallons of water — enough water to fill a small bathtub before the pressure gives out. Go bigger if you have the space and the budget. It can't be too big. You can plumb more than one tank together to add volume, if it fits your space better, or if you wish to add to an existing tank. The tanks need not be equal in size. You can buy a horizontal or vertical tank (vertical tanks are cheaper).

Pressure Adjustment

Install a pressure switch and a pressure gauge on your system. Purchase a pressure switch of the type used with conventional AC pumps. You might buy a switch that says "cut-in 30 psi / cut-out 50 psi". This indicates the factory settings, but they are adjustable. The setting determines the pressures at which the pump turns on and off. The cut-out adjustment is also called "differential", since it sets the difference between cut-in and cut-out. It is desirable to use the lowest pressure that will satisfy your flow requirements. The lower you can set the cut-out, the less power your pump will require and the more water your pressure tank will store. Read the instruction card that comes with the switch.

Many homes are plumbed using the minimum required sizes of 12 inch and 34 inch pipe. In this case, use a 50 psi cut-out for good flow. If you have not yet plumbed your house, have it done with one size larger than minimum pipe sizing, all around. Your piping will have less resistance to flow, and you can use a lower cut-out pressure. Try 35 psi and see how it performs. You can try less. When you are satisfied with the flow you get, then go to the next step.

Cut-In Setting

Set this to a pressure that is not much lower than the cut-out. That is, set a low "differential". This way, the pump will switch back on before much water is drawn from the tank. A typical setting might now be 30 psi cutin, and 40 psi cut-out.

Pressure Tank Precharge

Inside your pressure tank is a big rubber balloon. It is filled, at the factory, with pressurized air from a valve on the tank that looks like a valve on your car's tire. It is pressurized at a higher pressure than you need. Check it with a tire pressure gauge. With this high setting, the water cannot compress the air balloon, so the tank is not yet effective.

Once you have set your pressure switch as described above, you need to let some air out of the tank. To do this, turn off the power to your pump. Open a water outlet to relieve the pressure in the tank, then close it again. Now let air out of the tank until the tire gauge indicates 2 or 3 psi lower than your cut-in pressure. This is also described on instructions that come with your pressure tank. If you have more than one pressure tank, adjust them equally.

Turn your pump on, and time how long it takes to charge the tank to cut-off. As soon as the pump starts, the pressure should quickly rise to the pre-charge pressure. Then it will rise very slowly as it compresses the air in the tank. Fix yourself a sandwich or something. When it finally reaches cut-out pressure and shuts off, note how long it took, and write down "cycle time." on the wall near the tank. Also record your cutin and cut-out pressure settings. If you have an ammeter measure the current (Amperes) that your pump draws at the beginning and at the end of the pumping cycle. If you have trouble in the future, changes in these readings will indicate where the problem lies.

Determining the Energy Requirement

These little pumps use less power than a 100 Watt light bulb. To estimate, look at the data sheet for the pump you intend to use. Calculate your total lift by adding your vertical lift plus the pressure (1 psi = 2.3 feet). A chart will indicate the current draw (Amps) and the flow rate. Calculate how many hours the pump will need to run to supply your daily needs.

Energy Required (Amp-Hours per Day) = Amps x Hours of pumping per day

You may need less than the output of one 50 Watt PV module to handle the energy requirement. Energy storage for one cloudy week may be less than the capacity of one battery. Or the water system could consume more. Energy consumption depends on the physical configuration of your water system and the volume of your water consumption.

Determining Optimum Pump Depth

Drillers and pump installers are in the habit of placing pumps down near the bottom of the well. Conventional pumps (centrifugal impeller mechanism) are not adversely affected by great submergence, so it doesn't hurt. Also, they cannot tolerate dry running if the water level should drop, so it is safer to place them low.

Diaphragm submersibles are fundamentally different. Diaphragm stress increases with pressure, so life expectancy decreases. They have good tolerance for running dry. Low voltage pumps require larger, more expensive wire, so length should be minimized to reduce cost. So, it is most advantageous to set the solar-powered pump high in the well, under just 5 or 10 feet of water, unless the water level is expected to vary. See manufacturer's ratings for maximum submergence. Do not approach the maximum unless you must.

The water level in your well may vary, and its long-term characteristics can only be speculated. In case of uncertainty, obtain the "Driller's Log" for your well. Most states require drillers to keep a log of their drilling results. The log will note locations of water-bearing strata, water yields, and possible variations in water quality. It will also indicate where the casing is perforated to allow ground water to enter. Collect any known information about neighbors' wells, including seasonal variations. In a mountain valley for instance, groundwater may rise with spring snowmelt and drop in winter. Or, it may vary from year to year according to rainfall. Large commercial irrigation can also lower the water table around nearby wells. You can have your well tested by a driller.

If the well yield is more than double the pumping rate, set the pump only 5-10 feet under the static water level.

If well yeld is less than double the pumping rate, anticipate the draw-down level of the well (take a guess or talk to the driller) and set the pump below that level.

If well yield is low, or water level is uncertain, purchase extra length of pump cable and pipe. Coil up the extra cable rather than cutting it. You can easily couple in the extra length of pipe if you need to drop the pump lower.

Measure the water level using a string with a weight. Run the pump a full day, and measure the level again. Also, listen. If the pump begins sucking air, you will hear it.

If your well yield is very low or uncertain, use a pump controller with level sensors. Place the sensor probes in the well to shut the pump off if water drops too low. Long-term dry running may damage the pump, especially if there is sand in it.

If Well Water Is Sandy

Ask your driller to bale or pump the well until it runs clear. Drillers don't always do this. Let him know that your pump is not only slow, but is not very tolerant of sand, which wears the rubber parts.

Keep the pump higher than casing perforations that may be introducing sand. If this is not possible, obtain a "sand shroud" from your supplier, or make one from a plastic soda bottle and a hose clamp. This fits over the pump like a skirt, so that if sand falls from above the pump, it will pass around the pump and continue to fall. If you have a four inch well casing, then you will not have enough room to fit a sand shroud.

Grounding And Lightning Protection

A long wire run, even buried, may act as an antenna receiving power surges from nearby lightning. Electrical grounding is essential for lightning protection. If you live in a dry climate, get a good earth contact for your grounding system. When you have a trench open for piping or wiring, lay in bare copper wire (#6 gauge, minimum). Connect it to the ground rods and/or to your grounding system. The wire buried and exposed to the earth will help drain off accumulated electrical charge during lightning conditions.

Access

Author: Windy Dankoff, Photovoltaic Systems Specialist, POB 548, Santa Cruz, NM 87567 • 505-3512100 Voice or FAX. Windy is a photovoltaic dealer and a manufacturer of solar pumps.

DIY Battery Repair

DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

Get My Free Ebook


Post a comment