All About Solar Water Pumps and Performance
Solar Water Pumping Guide
This guide is primarily about DC powered pumps, as used in typical solar electric systems. Information is also provided on using AC powered pumps on systems that have an inverter available. DC powered pumps are used for deep and shallow well pumping, stock tanks, irrigation, water pressure systems, and many other areas. This guide is recommended reading for installers, users, and well drillers - especially those that are new to solar electric pumping systems. DC pumps are different in many ways from the AC pumps that many people are used to.
Capabilities & Limitations
DC pumps come in a variety of types. One of the most common is the small pressure booster pumps (Shurflo) commonly used in RV’s to supply water from the on board water tank. Others include diaphragm and piston positive displacement pumps for wells, booster (pressurizing) pumps, circulating pumps, ground water sampling pumps etc.
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, when it is need most. They can be installed and pulled by hand. They work in wells of very low yield that conventional pumps may suck dry in minutes. Pumps are available that pump as low as 1/2 gallon per minute.
Many of these solar pumps require a special controller if they are to be powered directly by PV modules (without batteries). The controller, or linear current booster (maximum power point tracker) acts like an automatic transmission, allowing the pump to start and run in low light conditions, such as overcast or early morning & evening. 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.
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 1/2 inch diameter pipe. If your pump is designed for 24 Volt use and has a 3/4 inch outlet, and you are using it at 12 Volts, adapt it down to 1/2 inch pipe size.
We use minimal diameter drop pipe for two reasons:
(1) Water is heavy. Small pipe holds a low enough weight of water that the pump may be pulled by hand.
(2) 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 AC pumps and use 1 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 over tightening. 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. If you are unfamiliar with plumbing, take sample parts with you to the store to match sizes. Pipe sizing does not always match what you will measure with a ruler!
Person power 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 at least 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
This is generally standard pipe, in most cases flexible or rigid PVC or poly pipe can be used.
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 1 inch pipe-size. Use a reducer bushing to adapt to your smaller drop 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 1 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.
These diaphragm pumps have internal check valves, without which they would not function. So 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.
Pump 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. Lets examine these two methods.
Wire for low voltage power transmission must be relatively large (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. Price it out both ways 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. Using a dedicated system also allows installation of a solar water pump that is totally independent of utility power, allowing water pumping even if grid power is down.
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 provides an energy reserve that can be applied to pumping. In 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.
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. Pumps are available for voltages up to 180 volts. Higher voltages are an advantage with larger pumps because they reduce voltage drop and allow the use of smaller wire.
Boosting the Efficiency
A wide variety of pump controllers are available. Although commonly called "controllers", these are actually specialized DC to DC converters, often called "LCB"’s, or linear current boosters. The purpose of these is to maximize the daily water delivery. These work by boosting the current, especially under low light conditions, cloudy days, and early morning or late evening. The voltage output of the PV panels is often too low to run a pump under these conditions, so the controller boosts the voltage enough to run the pump. In effect, these act like a perfect "gearbox", and match the output of the panels to the pump. These typically increase water flow by 25% to 50% over the day. Most controllers have extra inputs for remote control and/or low or high water shutoff, using water level sensors. We sell controllers for all types of pumps from a variety of manufacturers, including Dankoff, Shurflo, Solarjack, and Power Tracker inc.
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: (1) pumping directly to a pressure tank, (2) using storage tank with a booster pump and pressure tank, or (3) using an elevated storage tank with gravity flow. The rest of this article deals with method 1. Methods 2 and 3 will be discussed in upcoming articles.
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 most 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 for most submersible pumps is measured from the depth of the pump down the well, not the level of the water in the well. These DC sub jumps are of the positive displacement type and gain no pressure advantage from the water above them. Such is not the case with rotary pumps like the multiple stage rotaries made for 120 or 240 vac. The pump's volume is low. It may be as little as 1/2 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..
Get a "CAPTIVE AIR" pressure tank, not a "plain" or "galvanized" tank. Get a large one, like the 80 gallon size suggested above. 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). We sell the complete line of Challenger pressure tanks.
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 1/2 inch and 3/4 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.
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 cut-in, 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 measure 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 cut-in 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.
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 as by adding your vertical lift + 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.
(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 the Optimum Depth to Set a Submersible Diaphragm Pump
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 effected 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 yield 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 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.
Tech notes: Evaluating the well
One of the most important phases in designing a solar water pumping system is evaluating the well. If the rancher is fortunate enough to have unused wells on his property, the expense of drilling a new well may be avoided by evaluating the well. If the well is found to be usable, installing the PV pumping system on-site can be a simple matter.
Using compressed air, the well driller can determine water yield and draw-down level and remove some accumulated trash from an old well. The water will almost certainly be dirty at first but can clear up after prolonged pumping if the original screen and casing are intact.
An experienced well driller can have a good idea of the well depth needed for a good water supply in his working area. In many cases artisan pressure pushes water close to the ground level. If this is the case and the well yield is adequate, a centrifugal pump can be used. Otherwise, submersible positive displacement type pumps must be used to push the water up and out of the well (Table 1).
The theoretical pumping limit for a suction pump is approximately thirty-four feet at sea level, and less at higher altitudes (about 1 foot less per 1000 feet). That is the limit regardless of the motor size connected to the pump because suction is limited to one atmosphere of negative pressure. Over 34 feet you would be pulling a perfect vacuum. The practical limit for a centrifugal pump is about twenty-feet because of pipe friction, non-perfect seals, etc. Other types of pumps often have much less suction capacity.
When evaluating an existing well for conversion to a solar pumping system, the following questions must be answered:
- What is the size of the well casing?
- Is the casing in good condition with no pin hole leaks?
- How deep is the well?
- What is the depth to the water surface?
- How much water will the well produce - the replenishment rate?
- What is the draw-down level during water flow?
- Is the water free of silt and sand?
- How many gallons of water will be needed each day?