Installing a PV-powered Submersible Pump Bob-O Schultze Ä KG6MM It's hot and dry. Small creeks and springs that have been flowing since anyone can remember are drying up. You've saved up your money, drilled a well, and hit water. Now what? Here's how to get your water from down there to up here using the solar power. Submersible pumps Submersible or "Sub" pumps have been around for a long time. In a typical grid-connected home or farm, these are fairly high volume pumps usually powered by 240 or 120 vac. A pressure sensitive switch is employed to turn the pump on when the house water pressure falls below a certain point and turn it back off when the pressure rises to the high set point of the switch. In an inverter-based renewable energy system, this is known as a priority load because it may turn on or off at any time regardless of whatever other loads are being operated. That means that the inverter (and battery bank and charging system) must be sized to handle the sub pump and all other priority loads, like ac refrigeration, lighting, etc., CONCURRENTLY. Not only can this get expensive and inefficient, but if any part of the system goes fails, you're out of water! DC direct Sub pumps A more cost-effective and reliable system is to use a dedicated PV or wind system directly coupled to a sub pump. A water storage tank holds the water until it is actually needed. There are some definite advantages to this type of system. The PVs can be located directly at or nearby the wellhead, saving long wire runs from the inverter, generator, or battery bank. This can be cost-effective even for utility grid-connected folks who want to develop a well in the "back 40". At some sites the water storage tank can be located above the point of use to provide a gravity flow system. Even if tank's head is insufficient to provide adequate water pressure in the house, a pressure boosting pump with positive pressure at its intake will use far less energy than one which has to suck the water up to the pump intake. A pump/storage system sized to provide a number of days or weeks of storage will do double duty as a fire protection water source. Here a lot of water is needed quickly to prevent a disaster. System Sizing and Pump Selection Efficient DC direct sub pumps are available which will pump from 250 feet total head. Total head is the vertical distance from the pump to outflow at the storage or stock tank. For example, if a pump is placed 150 feet down a well and the tank is located 50 feet vertically up a slope to provide gravity feed, the total head would be 200 feet. If the tank is a long way from the wellhead, any pipe friction losses incurred will add to the total head. Choosing the right pump means striking a balance between your water needs and the capabilities of your water source. No pump is a bargain if it doesn't meet your needs or if it overtaxes your water source. Consult the manufacturer's literature or your solar professional to help you make the right choice. Here's a chart showing typical DC sub pump dynamic head in feet versus gallons per minute output. This chart was done for a Solarjack SDS-D-228. INSERT CHART To figure out the best depth placement for your sub pump, you'll need to know the diameter of the well casing, the total depth of the well, the static (standing) water level, and, if possible, the recharge rate of the water coming into the well. All the manufacturers of currently available sub pumps make pumps which will fit into 4 inch diameter well casings. Some of the higher output pumps, however, require a 6 inch casing. As a rule, the deeper the pump is located, the less volume of water will be pumped. If the recharge rate of the well is greater than the pump's capacity, locate the pump 10-20 feet below the static level of the water for greatest output. If the recharge rate is less or unknown, locate the pump deeper in the well and use water level sensors in the well to protect the pump from dry running. NEVER rest the pump on the bottom of the well or cistern as this can cause the pump to fill with mud and sand or reduce the water flow past the pump screens causing overheating. If possible, suspend the pump at least 5 feet above the bottom of the well to reduce water turbidity. Set-up After determining the proper pump depth, cut your safety rope and drop-pipe to length. Add (and keep track of) an extra foot or two of safety rope to allow for knots and tie off to your wellhead anchor point. Next cut your waterproof pump wire to length allowing plenty to get from the wellhead to the pump controller or LCB. If the possibility exists that you may want to locate the pump deeper in the well at some later time and if the wire is sized so that voltage loss is not a major problem, it's a good idea to cut the wire long and store the extra down the well. If you are using water level sensors, cut the sensor drop wire to the same length as the pump wire. Splicing the Pump Wire One of the greatest causes of pump failure is water intrusion into the pump via the wire splice. This is a place that pays big dividends later on if you pay attention to detail now. While some pump manufacturers use a waterproof cable connector to insure against water intrusion into the pump motor, most rely on double heat shrink tubing to prevent water intrusion. With any scheme, a well-connected, watertight joint is imperative. I prefer to use crimped and soldered compression sleeves to connect the short pump cable to the main pump wires. Each wire joint is then covered with glue-filled shrink tubing and all three connections covered with a larger piece of glue-filled shrink tube. See photos for detail. Water Level Sensors Most pump controllers use three water level sensors or electrodes, although at least one LCB uses a single sensor to turn the pump off or on depending upon whether the sensor is immersed. In the three sensor system, the "ground" electrode is the lowest and must be in the water at all times. It is placed within a few inches of the pump. The low-water electrode is placed at the desired "turn-off" point, usually about 6 inches above the ground electrode and the high-water electrode is placed at the desired "turn back on" point. This needs to be well below the static level and no more than 20 feet or so from the low water electrode. For low volume wells, it's usually within 5 feet of the low water electrode. In operation, the controller turns the pump off when the water level falls below the low-water electrode and keeps it off until the water level rises to above the high water electrode. This prevents unnecessary on-off cycling of the pump. Small gauge waterproof sensor wire is kinda hard to find. At a tip from Wes Edwards of Alternative Energy Engineering, I tried some 3 wire, direct burial sprinkler control cable from a local drip irrigation supply house. It worked well enough, but it's a little fragile. Try rolling a little electrical tape around the wire insulation to "fatten" it up where it goes into the sensor seal. Drop-Pipe All of the smaller sub pumps are designed to pump into 1/2 inch poly pipe. The larger pumps use 3/4 inch poly pipe. Even though the small diameter pipe can cause some friction losses in long runs, the velocity of the water in a bigger pipe is not enough to carry sediments to the surface at low flow rates. They tend to settle back into the pump at night and can cause problems. Even though I've never seen it mentioned in any of the manufacturer's literature, I strongly recommend installing a check valve in the pipe at or just above the wellhead if you are pumping into the bottom of a storage tank. Here's why: all the sub pumps use a diaphragm-type pump with 2 push-pull check valves. Normally, these valves will seal when the pump stops and hold the water in the drop-pipe. Frequently, however, a grain of sand or small bit of ick will temporarily lodge under a valve and hold it open. The in-line check valve will prevent draining the tank back into the well. Installation Bundle the drop-pipe, wires, and safety rope together using plastic wire ties or electrical tape every 5-10 feet. If you use tape, use a high quality vinyl PVC tape such as Scotchtm Super 33+ or better to prevent unraveling. Lose tape is a mess in the bottom of the well. It can even clog the pump's intake! After making sure that the safety rope is well anchored to the wellhead, GENTLY lower the pump down taking care to keep the bundle centered in the well so as not to nick any of the wires or the drop-pipe. Run the wires through the well seal and connect them to the pump controller. Connect the controller to the PV array through a properly fused disconnect. Pass the drop-pipe out of the well seal or make sure the in-well connection is secure if using a pitless type adapter. Switch the disconnect switch on and you should be pumping water! Fine Tuning the Pump Controller or LCB On most controllers, the "factory" settings will provide an adequate flow rate. If you need or want to get the most water out of your sun powered pump, here's how: If you're using an adjustable LCB or Controller, make the adjustments at mid-day with the PV at their normal operating temperature, if possible. With the system pumping, turn the array away from the sun or shade the panels until the pump flow rate is reduced about 50%. Connect a DC voltmeter to the pump or "load" side of the controller or LCB. Slowly turn the adjusting screw until the maximum voltage is obtained. Uncover or return the PVs to their normal position. The pump should now be operating at its maximum flow rate. Note: this "peaked" LCB setting will change from summer to winter sun conditions. In most installations, a controller peaked for summer conditions will provide adequate winter flow rates. If you're looking for maximum flow on a yearly basis, however, you'll need to readjust the controller at least twice yearly. Access Author: Bob-O Schultze, Electron Connection, POB 203, Hornbrook, CA 96044 ù 1-800-945-7587 ù FAX 916-475-3401 Special Thanks to Jim Allen at SolarJack, Wes Edwards at Alternative Energy Engineering, and Bob Maynard at Energy Outfitters for helpful hints and kinks.