Solar Water Pumping
The sun is the natural source of energy for an independent water supply. Solar pumps operate anywhere the sun shines. While energy production from solar pumps is impacted by cloudy weather, having adequate water storage and decreasing water needs during cool or rainy weather mitigates these impacts.
Solar water pumping systems operate on direct current (DC). The output of the solar power system varies throughout the day and with changes in sunlight intensity and weather conditions, requiring specialized pumps and controls that operate within a wider range of voltage and current compared to most AC pumps.
Conventional AC pumps are usually centrifugal pumps that spin at a high speed to pump as many gallons per minute as possible. They also consume a large amount of power and their efficiency suffers at low speeds and when pumping against high pressure. If you run a centrifugal pump at half speed, it pumps one quarter the volume.
To minimize the size of the solar PV system required, solar pumps generally use more efficient motors and pumping mechanisms. The most efficient pumps are “positive displacement” pumps, which pump a fixed amount of water with each rotation. If it is cloudy or early morning, the pump will receive less energy and run more slowly, but with no loss of efficiency—so at half speed, it simply pumps half the amount of water at the same pressure.
To use solar energy economically, solar pumping systems typically pump more slowly than conventional well pumps (many solar pumps are designed to produce less than 6 gallons per minute) and they don’t run at all between sunset and sunrise, so an adequately sized storage tank is usually required. Solar powered water pumps can provide an equal volume of water per day without the high and inefficient energy demands of a large capacity AC pump. Instead of pumping a large volume of water in a short time and then turning off, the solar water pump works slowly and efficiently all day. Often a solar pump can be used in a well with a recovery rate too slow for a conventional AC pump.
If your water sources are remote from power lines, compare the cost of a low-maintenance solar pumping system to what you would spend on a generator, with continual fuel and maintenance costs, or on a utility power-line extension. In most cases, a good solar pumping system is far more economical, which is why many non-profits and NGOs use solar pumping to provide clean water to remote villages around the world.
If you are pumping from a well, we have solar pumps that can deliver from 1 gallon per minute to over 75 gpm. The SHURflo 9300 can be powered by two 50- to 100-watt solar modules, depending on the “head” (vertical distance or elevation change) they are pumping. They can pump 500 to 1,000 gallons per day and lift water 200 feet. These pumps require service every 2 to 4 years.
If you have a higher lift, need more water, or want a pump that does not require service for 15 to 20 years, the Grundfos SQFlex pump is a good choice. The SQFlex can lift water over 800 feet and can pump over 20,000 gallons per day at lower lifts. The SQFlex pump can be powered by solar modules, a wind generator, a fuel powered generator, an inverter, the utility grid, or a combination of several of these.
Surface pumps are typically less expensive than submersible pumps and can draw water from a spring, pond, river, or tank, and push it far uphill and through long pipes to fill a storage tank or to pressurize it for home use or for irrigation, livestock, etc. The pump may be placed at ground level, or suspended in a well in some cases.
All pumps are better at pushing than pulling, since the vacuum a pump can draw is limited to atmospheric pressure (about 14 psi). At sea level, a pump can be placed no higher than 10 or 20 feet above the surface of the water source (subtract one foot per 1,000 feet elevation). Most wells are much deeper than this and therefore require a submersible pump, which can push the water up to the surface.
Suction piping for surface-type pumps must be oversized a bit and not allow air entrapment (much like a drain line) and should be as short as possible.
Pumps can push water very long distances through a pipe. The vertical lift and flow rates are the primary factors that determine power requirements.
Water Storage and Pressurization
Many conventional AC powered water systems pump from a well or other water source into a pressure tank that stores water and stabilizes the pressure for household use. When you turn on water in the house, an air-filled bladder in the tank forces the water into the pipes. When the pressure drops, a pressure switch turns on the pump, refilling and re-pressurizing the tank. This works because an AC pump delivers high volume and pressure on demand; however, this will not work with pumps operating directly from PV modules because the sun may not be shining when you want to take a long hot shower.
For pumps operating directly from PV modules, a non-pressurized water tank or cistern is used to store water for usage during times when the sun is not shining. If the tank can be located above the house on a hill or on a tower, gravity can supply the water pressure.
Gravity water pressure can be calculated in two ways:
Pressure in psi = head (in feet) x 0.433, or
Head (in feet) x 2.31 = psi.
For reasonable pressure, the tank needs to be at least 40′ above the house, although to obtain a pressure of 30 psi will require about 70′ of elevation. Alternatively, a DC or AC pressure booster pump, powered from a battery or inverter respectively, can be used to maintain a pressure tank as needed from a storage tank that is filled by a solar pump during the day. You must use a pressure pump that can deliver
the maximum flow rate required by the house, or have a pressure tank that is large enough to make up the difference between what the pressure pump can deliver and what is required for as long as it may be required. This is called the “draw down volume” of the pressure tank.
Calculation of Solar Power Needs
If you are using a pump driven directly by PV modules, the array’s nameplate output should be at least 20% higher than the power required by the pump to achieve the desired head and flow rate. A larger array or a tracking system can maximize the power available to the pump, providing more gallons per day.
Since the pump will only draw the power it needs, it will not be damaged by oversizing the array. A larger array will produce the needed power in less light, extending the pumping time and volume delivered in the morning, afternoon, and on cloudy days. For instance, a 1 kW array will produce 200 W in 1/5 the amount of sunlight that you would get on a sunny day at noon.
Designing a Solar Pumping System
We carry many types of pumps that can be used in a variety of applications. Which pump and related equipment are needed for a solar pumping system depends on many factors, including what the water source is, how much water is needed, when the water is needed, how far the water source is from another power source, etc.
If the well or other water source is close to an existing source of power, such as the utility grid or the power system of an off-grid house, it’s usually better to power the pump from that existing source rather than set up a dedicated PV array.
If grid power is available, it can be used to power a water pump, and if desired, a grid-tied PV system can be installed to offset the cost of the grid power.
In off-grid situations, if the well or other water source is close to the house’s off-grid power system, it’s usually easier to power the pump using the house’s power system, either directly from the battery bank with DC, or with AC from the inverter. Additional PV modules may be needed to accommodate the pump’s power requirement, but they can be added to the house’s PV system and used to help charge the batteries when the pump isn’t running.