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Solar irrigation designed for performance

Robert Fears for Progressive Forage Published on 30 April 2020
Extra water being stored

In many cases, a good option for powering irrigation systems is solar energy. This is especially true when the site is distant from the electrical grid.

Another advantage of using solar systems is immunity from variable costs of electric power or fossil fuels. Once system installation is complete, solar energy is free.

“Solar energy is used to generate electricity through a photovoltaic (PV) system,” said Dr. Kynda Curtis from Utah State University. “Solar PV systems are an efficient energy source for rural areas, since they are often more cost effective than installing new electrical lines and transformers. Additionally, they do not have moving parts or require fuel, making them more convenient to operate and maintain than traditional fuel-based generators.”

“A solar-powered water pumping system is made of two basic components – photovoltaic panels and a pump,” says Dr. Michael Buschermohle from the University of Tennessee. “The PV panels are the power supply, with a solar cell being their smallest element. Each solar cell has two or more specially prepared layers of semiconductor material that produce direct current [DC] electricity when exposed to light. This DC current is collected by wiring in the panel. It is then supplied either to a DC pump or occasionally stored in batteries for future use by the pump.”

“The heart of any successful rural water program is an efficient, reliable and sustainable pumping system driven by efficient whole-system design,” emphasizes Eric Macias, chief operating officer of Lorentz. “The design should include use of products with a long life, affordable maintenance and the ability to monitor and measure system performance.”

“In planning an irrigation system, it is important to determine the water requirements of the crops. Geographic location, soil type, time of the season and a crop’s response to water all affect the amount of needed irrigation,” says Dr. Juan Enciso with Texas A&M AgriLife Research. “Irrigation companies and extension service crop specialists can usually assist in determining water needs.”

Location of panels

“Solar energy is very predictable, but location of the PV panels or modules is important,” Macias continues. “Solar insolation is the amount of electromagnetic energy [solar radiation] that falls on the earth’s surface. By knowing the insolation levels of a particular region, we can determine the required size of the solar array [set of PV panels] and energy output. An area with poor insolation levels will need a larger array than an area with high insolation levels.”

Insolation values are generally expressed in kilowatt hours per square meter per day (kWh/m2/day). This is the amount of solar energy (measured in kilowatt hours) that strikes a square meter of the earth’s surface in a single day. Values are averaged to account for differences in day lengths.

“Irradiation level [process of receiving energy] on the earth varies dramatically depending on the atmospheric conditions, particularly the amount of cloud cover,” Macias says. “This variation becomes important when calculating solar generator size, so accuracy is critical.”

If the solar pump system is expected to perform during winter months, the array should be oversized to account for low sunlight. Peak irradiation is 1,000 watts per square meter. If we have a 740-watt solar array and our motor requires 600 watts to achieve the desired flow rate, double the array size to 1,480 watts. This will ensure maximum flow rate from the pump on days with irradiation of 500 watts per square meter.

“Consider the changing arc of the sun at different times of the year in the planning process,” Macias says. “To maximize capture of sunlight modules, PV panels must be mounted tilting toward the sun, which means solar modules must be installed facing south in the northern hemisphere and facing north in the southern hemisphere. Optimum tilt angle is determined by the location’s latitude. In summer, tilt should be the latitude minus 15 degrees, and in winter, it should be latitude plus 15 degrees. As a year-round compromise for pumping applications, tilt can equal the latitude.”

“Consider shading when planning a solar-powered water system,” says Macias. “Shading from near objects such as structures, hills or trees will cause severe drops in power and can easily stop the complete output from a module. Production levels of any modules connected in series to a shaded module is reduced to that of the shaded modules. Shading can reduce output from a module or a series of modules to zero. Shading caused by distant objects does not have a significant effect on output because diffused light, or light bouncing off of other surfaces, will contribute to module outputs.”

Providing enough water

Solar systems are only productive when the sun is shining, which is also the period when plants can become water stressed. Plan irrigation for periods of sunlight when solar power is generated and plants need water. Modern solar pumping systems use high-efficiency motors that are direct current (DC) and brushless. This allows them to operate effectively as the available energy from the sun changes. Current systems have an option of remote monitoring and management, which allows operation without being on site.

Larger systems have variable-speed drives, which provides an opportunity for variable pumping volumes to match plant needs. Hybrid systems are available that blend solar and generator power when sunlight is not sufficient to achieve desired flow, pressure or level.

“Storing water in tanks or surface ponds requires almost no maintenance and is much more cost effective than storing power in batteries or hauling a generator. The only reason a generator or batteries should be used in an irrigation-water supply system is when well production is low or water demand cannot be met by daytime pumping,” says Macias. “If sized right, a solar pumping system should work independently of any other power source to meet water demand.”

Safety

“PV panels generate DC electricity, which requires respect,” says Macias. “Develop and follow a safety program based on local safety rules and practice codes. Three 185-watt modules in a series generate 150 volts, whereas any amount over 42 volts of either DC or AC [alternating current] electricity is considered unsafe. Handle PV modules with care and respect.”

“A switch to disconnect solar panels from the pump is a very important safety measure and is part of every professional installation. Although a disconnect switch is used to cut off power to the pump, PV modules produce power when they are in daylight and cannot be switched off,” Macias adds. “Before working on the equipment, cover PV modules with a sheet or cover of opaque material to stop power generation.”

Solar systems offer an economical and efficient method of irrigating, but do not erase their advantages by not practicing good safety.  end mark

PHOTO: Extra water can be stored in tanks or surface ponds for irrigation on cloudy days.  Photo provided by Eric Macias, Lorentz US Corp.

Robert Fears is a freelance writer based in Georgetown, Texas.

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