A Central Nebraska farmer was faced with enormous electrical bills and found the answer to reducing energy costs in variable frequency drives. The environmentally responsible solution saves money while conserving the natural resources of energy and water.
One of the more common types of irrigation systems used in the agricultural industry today is the center pivot sprinkler system, so named because the irrigation sprinklers pivot about a central point and water is applied from a continuously moving lateral pipe. The center pivot is fixed at the pivot point and rotates to irrigate a large circle with a radius of up to 3000 feet. The longer the pivot, the larger the area covered, and typically the lower the cost per acre.
In most installations, the control panel, fertilizer injection equipment, and the pump are located at the pivot point or center of the irrigated circle. The pump is typically driven by a fossil fuel type engine or by an electric motor. The lateral irrigation pipe is supported about 10 feet above the ground by a truss network, supported by wheeled towers located every 100 to 150 feet. Each tower is an A-frame design with a small electric motor, usually 1 HP to 1.5 HP, providing the drive mechanism the power to move the lateral pipe around the axis. The lateral pipes are typically 6”, 8” or 10” in diameter. Pivots are fitted with a variety of impact sprinklers, spray heads, and drop pipes to accommodate crop, wind, and field conditions. Depending on the system, they will operate at between 25 and 80 psi at the center pivot.
The farmer in Nebraska contacted Danfoss Water & Wastewater to help determine if there was any potential to use a variable frequency drive on the main 125 HP pump in order to reduce energy costs. Annual electrical costs averaged approximately $15,000 dollars per year, depending on weather and soil conditions.
The center pivot system consisted of the 125 HP well-pump feeding into the irrigation lateral which extended approximately 1000 feet with an end gun designed to irrigate the corner sections of the 100 acres. He was operating his system with an across-the-line starter system and would typically start the system several times per year. The system was designed to operate at approximately 80 psi and pumped approximately 1000 GPM of water with the end gun operating.
System design review showed the pump size was selected to accommodate both the end gun requirements at each corner of the field and the higher elevation points in the field, meaning that the only time the pump operated at the designed discharge head was when the end guns were in use and when the lateral was at the design elevation. When the system was operating at the lower elevations, excess water was being applied, resulting in runoff or pooling, wasting energy and water. Pressure regulation valves had been installed to help remedy this; however, the pump still ran continuously at full speed, maximizing energy consumption. (See chart 1) Controlling the system pressure using a variable frequency drive would allow the pump to slow down at lower elevations, reducing the water flow and lowering the energy demand. Use of a variable frequency drive in combination with a pressure transmitter located at the end of the lateral near the end gun would control the speed of the pump based on pressure changes in the system, resulting in constant pressure in the lateral and more uniform distribution. A Danfoss VLT 8125 AQUA Series variable frequency drive was installed, in combination with a Danfoss pressure transmitter, which was placed approximately three feet from the end of the lateral. The transmitter was cabled back to the AQUA drive, which provided it with power and used the 0-100 psi feedback signal as a reference to control the setpoint at 80 psi. The pressure regulating valve was removed from the system.
In determining what type of payback could be expected, it was necessary to look at several aspects of the system:
- Starting Currents—with the old installation, each time the pump was started, the result was a peak current demand of approximately 7 times the 177 FLA (full load amps) of the motor, or approximately 1120 A (See Chart 2). The power company does not presently bill based on demand charges supplying this farm, but many utilities have begun doing so, resulting in increased energy costs. With the Danfoss drive installed, the motor would never demand more than the 177 FLA rating of the motor.
- Affinity Laws—The Affinity Laws indicate that a reduction of the speed of a motor by 10 percent results in a 27 percent reduction in energy costs. This is primarily of most benefit when changing head conditions are present. In reviewing the center pivot installation, there were wide swings in system head pressure and the system was only required to operate at full speed 30 percent of the time—when the end guns were open and when the lateral was at the maximum field elevation.
- Weather and soil conditions and peak crop water requirements—An irrigation system is typically designed to satisfy the peak crop water requirements or net irrigation requirements. Since these change seasonally, usually peaking in July, there is significant opportunity to further reduce energy usage during lesser demand periods.
Upon reviewing these factors, it was determined the potential annual energy savings in this application would allow for a payback of less than two years based on the average conditions. Additionally, there were several other benefits to operating the system using variable speed drives with constant pressure control:
- More uniform distribution of water on the crops.
- Reduced maintenance on the irrigation equipment. The “soft starting” of the pump prevented water hammer and eliminated sprinkler head damage as well as flexible coupling damage.
- A secondary feature, which is becoming more necessary, was the addition of a submersible pressure transmitter to monitor well water depth. This allowed control of the speed of the pump to prevent over pumping of the well. This can also be data logged to provide reporting information as required.
- Ability to adapt to changing conditions. It was easier to adjust the setpoints on the VFD to compensate for changes in soil conditions and application depths than to change the speed of the center pivot.
- Use of the VFD is a major step towards increased water conservation. As these resources become increasingly scarce, technologies such as VFD control are a vital part in helping to insure that we use these resources wisely.
- Motor protection during periods of voltage swings in the power distribution line. With across-the-line starting of many pump motors in the surrounding area, the power grid suffered wide fluctuations in voltages which caused motor failure in many instances. The variable frequency drive can maintain full output voltage at full load with voltage sags as low as 15 percent, helping to protect the motor.
In conclusion, there is a tremendous potential for energy savings by retrofitting an existing center pivot irrigation system with variable frequency drives. The installation in most cases has a very short payback period based solely on energy conservation. The additional benefits derived from the use of VFDs on center pivots also provide significant return on investment. And they help farmers to be good stewards of our natural resources. Increased energy savings can be further realized by incorporating additional automatic controls through the use of devices designed to measure the moisture contact or crop temperature in conjunction with drives.