Solar Water Pumping Systems
Need a direct-coupled or a battery-coupled solar water pumping system? Learn the pros and cons here.
Solar water pumping systems are used to pump small amounts of water using solar energy. They can be practical solutions in rural areas not connected to an electrical grid. They an also be used by families or farms in smaller or larger scale to save electricity costs.
Below we'll explain the differencfe between a direct-coupled and a battery-coupled solar pumping system, to help you make the right choice for your needs. For a general overview over solar powered water pumps, click here.
Solar Water Pumping Systems
Direct-coupled solar pumping systems
In direct-coupled solar water pumping systems the electricity from the PV modules is sent directly to the pump, which pumps water through a pipe to where it is needed. This system is designed to pump water only during the day. The amount of water pumped is totally dependent on the amount of sunlight hitting the PV panels and the type of pump.
Because the pump runs exclusively on sunlight the amount of water pumped by this system varies throughout the day in direct correlation to the intensity and amount of sun is striking the PV panel at any given moment. This means that during periods of sunlight (late morning to late afternoon on bright sunny days) when the sun is in the optimum position and the sky is clear the pump operates at or near 100 percent efficiency with maximum water flow. However, under these low-light conditions, such as during early morning and late afternoon when the sun is not an an optimal angle or there is cloud cover, pump efficiency may drop by as much as 25 percent or more During cloudy days, pump efficiency will drop off even more. To compensate for variations in available sunlight, you must calculate a good match between the pump and PV module(s) to achieve efficient operation of the system. You need to make sure you have enough PV panels to provide average power at all times based on the sunlight estimations for your area.
Direct-coupled pumping systems are able to store extra water on sunny days so it is available on cloudy days and at night. Water can be stored in a larger watering tank or in a separate storage tank. Water can then be fed to smaller watering tanks by a gravity system. Water-storage capacity is important for a solar pumping system. You may need to store between two and five days worth of water depending on your climate and your water usage patterns. There are some drawbacks to storing water in tanks. If you use an open storage tank you can lose a lot of water through evaporation. Closed tanks work well but large closed tanks can be expensive. Depending on your climate and winter cold conditions, water in your storage tank may freeze. You need to carefully consider how you will use your solar water pump to avoid these problems.
A DC solar water pump also requires a pump controller which trades voltage for current. This allows the pump to start and run at reduced output during periods of weak sunlight. If you carefully match the pump motor performance to the available sunlight in your area you can increase the amount of water pumped in a day by 10 to 15% with a properly sized controller.
Battery-coupled solar pumping systems
Battery-coupled solar water pumping systems consist of photovoltaic (PV) panels, a charge control regulator, batteries, pump controller, pressure switch, tank and a DC water pump. The electric current produced by the solar panels during periods of high sun charges the batteries. The batteries then supply power to the pump anytime water is needed. By using batteries the pumping is spread over a longer period of time by providing a steady operating voltage to the DC motor of the pump.
The use of batteries does have drawbacks. Batteries can reduce the efficiency of the overall system because the operating voltage is controlled by the batteries and not the PV panels. Battery supplied voltage can run 1 to 4 times lower than voltage produced by the solar panels themselves under maximum sunlight. The voltage produced depends on the battery temperature and how well the batteries are charged. Any reduction in efficiency however, can be minimized by using a pump controller which boosts the battery voltage supplied to the pump. This reduced efficiency can be minimized with the use of an appropriate pump controller that boosts the battery voltage supplied to the pump.
System componentsBattery-coupled solar water pumping systems contain more system components than direct-coupled systems. The main components are the following:
In a battery-coupled pumping system the primary function of a pump controller is to boost the voltage of the battery to match the input voltage of the pump. Without a pump controller, the PV panel operating voltage is controlled by the battery bank and is less efficient than voltage achieved by the solar panels themselves.
Charge Control Regulators:
Solar panels that are wired directly to a set of batteries can produce voltage levels high enough to overcharge the batteries. You should install a charge control regulator between the PV panels and the batteries to prevent overcharging the batteries. Charge controllers allow the full current produced by the PV panels to flow into the batteries until they are nearly fully charged. At that point the charge controller lowers the current and the basttery trickle charges until it is fully charged. The regulator you install should be rated at the appropriate system voltage (i.e., 12-volt, 24-volt, etc.) and the maximum number of amperes the solar panels can produce. Install the regulator near the batteries or as recommended in the manufacturer's instructions. Normally the installation only requires four connections: The "POS" and "NEG" terminals on the solar panel and on the battery.
You need a low-voltage or battery state-of-charge control to prevent deep-discharge damage to the batteries. The low-voltage relay acts as an automatic switch to disconnect the pump before the battery voltage gets too low. The relay is activated and switches when battery voltage drops to "low-voltage" threshold, and de-activates and switches back when the battery voltage rises to the "reconnect" threshold. Most suppliers of PV equipment offer a charge control regulator that combines both overcharge protection and low-voltage disconnect to protect the batteries.
Batteries: One type of battery used in a stand-alone PV system is the lead-acid battery. A new type of lead-acid battery referred to as a "gel cell" uses an additive that turns the electolyte into a non-spillable gel. These are sealed batteries and they can be mounted sideways or even upside down. Another type of battery using nickel cadmium (NiCd) plates can be used in PV systems. Their initial cost is much higher than lead-acid batteries, but in many PV applications the cost over the lifetime of the battery may be cheaper. Some advantages of NiCd batteries include their long-life expectancy, low maintenance requirements and their ability to withstand extreme conditions. The NiCd battery is also more tolerant to complete discharge. It is important to choose a quality battery rated that is rated at a minimum of 100 amp-hour storage capacity.
Car batteries (shallow cycle batteries) should never be used for PV applications. These batteries are lighter, cheaper and designed to produce a high-current, cold-cranking amps for a short period. The battery is then quickly recharged. Generally, shallow-cycle batteries should not be discharged more than 25 percent of the rated battery capacity.
Battery banks are often used in PV systems. These banks are set up by connecting individual batteries in series or parallel to generate the desired operating voltage or current. The voltage achieved in a series connection is the sum of all the voltages of all the batteries. The current (amps) created in series-connected batteries is equal to The current of the smallest battery. For example, two 12-volt batteries connected in series produce the equivalent voltage of a 24-volt battery with the same amount of current (amps) output as a single battery. When wiring batteries in parallel, the current (amps) is the sum of the currents (amps) from all the batteries and the voltage remains the same as that of a single battery.
Batteries must be protected from weather. Batteries should be buried below the frost line in a watertight enclosure or placed in a building where the temperature will remain above freezing. If the batteries are buried, Make sure the are is a well-drained location. Batteries should never be set directly on a concrete surface because self-discharge will increase, especially if the concrete surface gets damp.
I hope this will help you choose between different solar water pumping systems. For more solar home products, check out the articles below.
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