Solar Water Pumping: A Practical Introduction
February 1, 2008
Photovoltaic (PV) panels produce electricity from sunlight using silicon cells, with no moving parts. They have been mass-produced since 1979. They are so reliable that most manufacturers offer a 25-year warranty. They work well in cold or hot weather. Solar water pumps are specially designed to utilize DC electric power from photovoltaic panels. They must work during low-light conditions at reduced power, without stalling or overheating.
Low-volume pumps use positive displacement (volumetric) mechanisms, which seal water in cavities and force it upward. Lift capacity is maintained even while pumping slowly. These mechanisms include diaphragm, vane and piston pumps. These differ from a conventional centrifugal pump that needs to spin fast to work efficiently. Centrifugal pumps are used where higher volumes are required. A surface pump is one that is mounted at ground level. A submersible pump is one that is lowered into the water. Most deep wells use submersible pumps.
A pump controller (current booster) is an electronic device used with most solar pumps. It acts like an automatic transmission, helping the pump to start and not to stall in weak sunlight. A solar tracker may be used to tilt the PV array as the sun moves across the sky. This increases daily energy gain by as much as 55 percent. With more hours of peak sun, a smaller pump and power system may be used, thus reducing overall cost. Tracking works best in clear sunny weather. It is less effective in cloudy climates and on short winter days. Storage is important. Three to 10 days’ storage may be required, depending on climate and water usage. Most systems use water storage rather than batteries, for simplicity and economy.
A float switch can turn off the pump when the water tank fills to prevent overflow. Compared with windmills, solar pumps are less expensive, and much easier to install and maintain. They provide a more consistent supply of water. They can be installed in valleys and wooded areas where wind exposure is poor. A PV array may be placed some distance away from the pump itself, even several hundred feet away.
Some common uses:
- Livestock watering – Cattle ranchers in the Americas, Australia and Southern Africa are enthusiastic solar-pump users. Their water sources are scattered over vast rangeland where power lines are few, and costs of transport and maintenance are high. Some ranchers use solar pumps to distribute water through several miles of pipelines. Others use portable systems, moving them from one water source to another.
- Irrigation – Solar pumps are used on small farms, orchards, vineyards and gardens. It is most economical to pump PV array-direct (without battery), store water in a tank, and distribute it by gravity flow. Where pressurizing is required, storage batteries stabilize the voltage for consistent flow and distribution, and may eliminate the need for a storage tank.
- Domestic water – Solar pumps are used for private homes, villages and medical clinics. A water pump can be powered by its own PV array, or by a main system that powers lights and appliances. An elevated storage tank may be used, or a second pump called a booster pump can provide water pressure. Or, the main battery system can provide storage instead of a tank. Rain catchment can supplement solar pumping when sunshine is scarce. To design a system, it helps to view the whole picture and consider all the resources.
Thinking SmallThere are no limits to how large solar pumps can be built. But, they tend to be most competitive in small installations where combustion engines are least economical. The smallest solar pumps require less than 150 watts, and can lift water from depths exceeding 200 feet at 1.5 gallons per minute. You may be surprised by the performance of such a small system. In a 10-hour sunny day, it can lift 900 gallons. That’s enough to supply several families, or 30 head of cattle, or 40 fruit trees.
Slow solar pumping lets us utilize low-yield water sources. It also reduces the cost of long pipelines, because small-sized pipe may be used. The length of piping has little bearing on the energy required to pump, so water can be pushed over great distances at low cost. Small solar pumps may be installed without heavy equipment or special skills. The most effective way to minimize the cost of solar pumping is to minimize water demand through conservation. Drip irrigation, for example, may reduce consumption to less than half that of traditional methods. In homes, low-water toilets can reduce total domestic use by half. Water efficiency is a primary consideration in solar pumping economics.
Careful Design Approach UsedWhen a generator or utility mains are present, we use a relatively large pump and turn it on only as needed. With solar pumping, we don’t have this luxury. Photovoltaic panels are expensive, so we must size our systems carefully. It is like fitting a suit of clothes – you need all the measurements.
We will need to determine whether a submersible pump or a surface pump is best. This is based on the nature of the water source. Submersible pumps are suited both to deep well and to surface water sources. Surface pumps can only draw water from about 20 feet below ground level, but they can push it far uphill. Where a surface pump is feasible, it is less expensive than a submersible, and a greater variety is available. Now, we need to determine the flow rate required. Here is the equation, in the simplest terms:
Gallons per hour equal gallons per day divided by available peak sun hours per day.
Peak sun hours refer to the average equivalent hours of full-sun energy received per day. It varies with the location and the season. For example, the arid central-western United States averages 7 peak hours in summer, and dips to 4.5 peak hours in mid-winter.
Water StorageGenerally, your storage capacity should be equal to 3 days to 10 days of average water consumption, or more. This depends on your climate and your usage patterns. For domestic use in a cloudy climate, 10 days is minimal. In a sunny climate, this allows for a generous safety margin. For irrigation of deeply rooted crops or trees, 3 days’ storage may be adequate because the earth itself provides storage. For irrigating a garden, 5 days may be adequate. More always is better, unless evaporation loss is excessive.
Place your normal point of discharge higher than the bottom of your water tank, in order to hold a reserve so that the tank does not run completely dry. You can lose your water supply under any of these circumstances:
- Period of low sunshine and/or excessive water demand.
- Electrical or mechanical failure in the system.
- Leak in the tank or piping.
- Accidental discharge of stored water.
Pipe SizingThe pipe from the pump to the tank must be carefully sized, depending on the flow and the length of pipe. A single pipe may be used as both fill and discharge. In that case, size the pipe for the maximum discharge that you want to accomplish. Also, consider oversizing the pipe if there is a chance that you may want to install a larger pump in the future. Sizing the pipe larger than necessary will not influence the performance of the system.
If you will be using gravity flow to supply water from the storage tank at a relatively low pressure, then be sure that the discharge pipe is large enough to allow sufficient flow to meet the maximum water demand, with very little friction loss. Every 2.3 feet of vertical drop produces 1 pound per square inch pressure.
Float SwitchWe recommend the use of a float switch to prevent overflow of your tank. It will stop the pump when the tank fills, then will reset when the level drops. This conserves ground water, prevents overflow, and eliminates unnecessary pump wear. Solar pump controllers allow the use of small-sized cable to a remote float switch.
You may want to defeat your float switch to allow overflow for irrigation purposes, or to test or observe your system. To do this, install a switch to disconnect one of the two float switch wires. Use any size of on/off switch available from an electrical supply or hardware store.
Refresher ValveA refresher valve is an intentional slow leak. During times of low water demand, it reduces stagnation, and may help prevent freezing. It can be any small valve, installed just below the float switch reset level. When it is needed, adjust it for a very slow flow and pipe the flow to a place where it will drain away.
Monitoring ConsiderationsWill you be able to observe the output of your pump at the point of discharge? If not, you may not know if it malfunctions, especially in the case of a submersible pump. Consider installing a water meter, or additional valves so that the flow can be observed.
If your pump malfunctions, will you be able to observe the level of your tank dropping? If you cannot easily see into your storage tank, here are some methods of tank monitoring:
- Use a dipstick in the air vent.
- Employ a float with a visible rod that protrudes through the top of the tank.
- Use a float with a rope and pulley to a hanging indicator.
- Have a clear sight-tube alongside the tank.
- Employ a sensitive pressure gauge.
This article is provided through the courtesy of Conergy, a leading supplier of solar water pumps and photovoltaic, as well as solar thermal components. It is excerpted from the company’s training manual. The complete manual, along with a wealth of additional information, is available at www.conergy.us.