Rural part of Nepal has places still devoid of the national grids. A water pump designed for these regions to lift water needs high wattage power (5kW to 20kW for a single pump, and from water demand can be multiple) as the elevation difference varies minimum of 500m from the source to the project region. In this case, solar pumps are the most efficient choice over the generator system. Due to the lack of efficient transportation system, the choice fuel powered generator system is not used.
Water pumping is one of the simplest and most appropriate use of energy output from solar panel. New systems have the advantage of storing water for use when the sun Is not shining, eliminating the need for batteries, enhancing simplicity and reducing overall system costs. Initial cost of these systems seems to be high, but the cost spread over the design period, it comes in affordable region and if designed efficiently, upon comparing by installation cost (including labor), fuel/electricity costs over 10 years, this solar system will be cheaper than its 11kVA counterpart system.
Solar-Powered Water Pumping System Configurations
Solar-Powered Water Pumping System Configurations There are two basic types of solar-powered water pumping systems, battery-coupled and direct-coupled. A variety of factors must be considered in determining the optimum system for a particular application.
Battery-coupled water pumping systems consist of photovoltaic (PV) panels, charge control regulator, batteries, pump controller, pressure switch and tank and DC water pump (Figure 1). The electric current produced by PV panels during daylight hours charges the batteries, and the batteries in turn supply power to the pump anytime water is needed.
The use of batteries spreads the pumping over a longer period of time by providing a steady operating voltage to the DC motor of the pump. Thus, during the night and low light periods, the system can still deliver a constant source of water for livestock. The use of batteries has its drawbacks. First, batteries can reduce the efficiency of the overall system because the operating voltage is dictated by the batteries and not the PV panels. Depending on their temperature and how well the batteries are charged, the voltage supplied by the batteries can be one to four volts lower than the voltage produced by the panels during maximum sunlight conditions. This reduced efficiency can be minimized with the use of an appropriate pump controller that boosts the battery voltage supplied to the pump. In direct-coupled pumping systems, electricity from the PV modules is sent directly to the pump, which in turn pumps water through a pipe to where it is needed
Figure 2 Direct coupled solar pumping system
(Figure 2). 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 intensity of the sun and the angle at which it strikes the PV panel changes throughout the day, the amount of water pumped by this system also changes throughout the day. For instance, during optimum sunlight periods (late morning to late afternoon on bright sunny days) the pump operates at or near 100 percent efficiency with maximum water flow. However, during early morning and late afternoon, pump efficiency may drop by as much as 25 percent or more under these low-light conditions. During cloudy days, pump efficiency will drop off even more. To compensate for these variable flow rates, a good match between the pump and PV module(s) is necessary to achieve efficient operation of the system.
Direct-coupled pumping systems are sized to store extra water on sunny days so it is available on cloudy days and at night. Water can be stored in a larger-than-needed watering tank or in a separate storage tank and then gravity-fed to smaller watering tanks. Water-storage capacity is important in this pumping system. Two to five days’ storage may be required, depending on climate and pattern of water usage. Storing water in tanks has its drawbacks. Considerable evaporation losses can occur if the water is stored in open tanks, while closed tanks big enough to store several days water supply can be expensive. Also, water in the storage tank may freeze during cold weather.
Main solar powered stock watering system components
A typical solar-powered stock watering system includes a solar array, pump, storage tank and controller shown in Figure 3,
Figure 3 A typical solar-powered stock watering system
Solar electric systems are sometimes called photovoltaic systems. The word “photovoltaic” is often abbreviated PV. Most solar panels, or modules, generate direct current (DC) electricity. A group of modules is called an array.
There are two ways to mount solar modules: either on a fixed structure or on a tracking structure. Fixed mounts are less expensive and tolerate higher wind loading but have to be carefully oriented, so they face true south (not magnetic south). An array can easily be mounted on a trailer to make it portable. A tracking array follows the sun across the sky. A tracker will add at least 40,000 to 80,000 NRs to the cost of a system but can increase water volume by 25 percent or more in the summertime, compared to a fixed array.
DC water pumps in general use one-third to one-half the energy of conventional AC (alternating current) pumps. DC pumps are classed as either displacement or centrifugal and can be either submersible or surface types. Displacement pumps use diaphragms, vanes or pistons to seal water in a chamber and force it through a discharge outlet. Centrifugal pumps use a spinning impeller that adds energy to the water and pushes into the system, similar to a water wheel. Submersible pumps, placed down a well or sump, are highly reliable because they are not exposed to freezing temperatures, do not need special protection from the elements, and do not require priming. Surface pumps, located at or near the water surface, are used primarily for moving water through a pipeline. Some surface pumps can develop high heads and are suitable for moving water long distances or to high elevations.
Batteries are usually not recommended for solar-powered livestock watering systems because they reduce the overall efficiency of the system and add to the maintenance and cost. Instead of storing electricity in batteries, it is generally simpler and more economical to install 3 to 10 days’ worth of water storage. Controller or Inverter The pump controller protects the pump from high- or low-voltage conditions and maximizes the amount of water pumped in less than ideal light conditions. An AC pump requires an inverter, an electronic component that converts DC electricity from the solar panels into AC electricity to operate the pump.
A float switch turns a pump on and off when filling the stock tank. It’s similar to the float in a toilet tank but is wired to the pump controller. Low water cut-off electrodes protect the pump from low water conditions in the well. Designing and Installing Systems Every pumping and stock-watering situation is unique. The average consumer is likely to be intimidated by the prospect of sizing and designing a solar pumping system, and most people need the assistance of a qualified solar dealer. In general dealers are eager to help. Many will provide a no-cost proposal based on a few simple questions that can be asked over the phone. If the price seems too high, you can easily get bids from other dealers. In order to size and design a system correctly, the dealer will want to know:
• how much water you need;
• when you need the water;
• whether your water source is a stream, pond, spring, or well;
• water available in liters per second(lps);
• well depth;
• how far the water needs to be pumped, and with what elevation gain;
• water quality problems (e.g., silt or high mineral content) that may damage the pump;
• how much volume is available in storage tanks and how the tanks are arranged.
Installing a solar pump is a complex task, combining elements of electrical work, plumbing, and heavy construction (often including earthmoving, pouring concrete, and welding). Written instructions are not always as complete as they should be. A backhoe or tractor with a front-end loader is almost a necessity for some larger projects.