Water is the lifeblood of agriculture, and the lack of it can limit the productivity of an otherwise economically attractive piece of property. Therefore, getting water to the crops is a critical part of a grower’s job and often represents a major cost. When pumping is necessary, the cost of getting power to the pump and the cost of electricity for operation can be significant.
Foster Family Vineyards in Lewisburg, North Carolina, is the largest grape supplier to the largest wine-making operation in the southern U.S. The vineyard was faced with the challenge of irrigating with efficiency in mind.
The company was pumping deep-well water from two 400-foot wells at 30 gallons per minute.
But this rate was insufficient to adequately serve the 70 acres of vines, and the power consumption was expensive.
To ease the cost of pumping well water, vineyard management wanted to take advantage of a creek on the property. But estimates showed the costs at $8,000 to $10,000 per pole to run electrical wire from the nearest access point to the creek, which was located in a low-lying spot on the property approximately a half-mile away. From this creek, a pump would send the water to a holding pond on higher ground, approximately 75 feet above the vineyard and 2,500 feet away.
Vineyard management looked for a solution that would reduce installation costs and the vineyard’s dependence on the local power grid. Open to using new technologies, the team thought that this application may be a good candidate for a solar-powered pump system.
The company found a provider who had experience with solar power. At the same time, they learned about an available university grant that would help fund the project and submitted an application.
A pump manufacturer introduced vineyard employees to a possible technology solution (see Figure 1). Key to the system’s capability is its variable frequency drive (VFD) control that runs the pump at varying speeds to achieve the most possible flow at all times of the day, regardless of the solar array’s momentary electrical power output fluctuations.
A key advantage of VFD systems is that they allow the pump systems’ operational speed and output to be adjusted on an as-needed or as-available basis. This technology works well with solar-powered pump systems. Because available sunlight energy is always changing, the controller continuously adjusts system performance to ensure that the end users are receiving the maximum water output possible.
The solar pumping system also works with standard water well pump equipment. End users can apply their existing knowledge of system installation, troubleshooting, sizing and maintenance the same way they would apply it to any other grid-tied application. This technology could supply the pressurized water needs for a small cabin or a drip irrigation system for a strawberry patch, for example, or pump water over dozens of miles of plumbing to a series of remote watering stations. Solar systems can be employed in virtually all the same situations as any other well pumping system.
Another alternating current (AC) solution would be to use a separate inverter to convert the array’s direct current (DC) to AC power for the pump, but this process would add a significant cost to the system. Some systems’ built-in VFDs for pump motor control make using a solar inverter unnecessary.
The solar solution employed at the vineyard powers a 200-volt (V), 3-phase, 60 hertz, 4-inch submersible motor. The system runs between 190 V and 330 V DC, which is typically supplied by four to 10 standard solar modules. The pumps have flow rates between 5 and 90 gallons per minute (gpm) and can pump from 1,000 feet in some lower-flow applications. At about 100 gpm the systems are still able to produce 30 pounds per square inch (psi) of operating pressure. Thanks to the VFD-based control unit, the pump systems often run from sunrise to sunset with few interruptions.
To provide flexibility for multiple applications, the system can be switched on and off by different start-stop mechanisms. These could include system timers, float controls or pressure switches. The system can also be allowed to run. When the sun is up, the pump is on; when the sun is down, it is off.
The Foster Vineyard system pumps 26 gallons per minute when the weather is cloudy and 46 gallons when it is sunny. Every eight minutes the control system sets a power point according to how much power is coming from the arrays and runs the pump at the new set point.
As a standard feature, each system also supports continuous data reporting. This allows for simple connectivity to aftermarket remote communication systems, using standard interfaces such as Modbus. This data is available through an RS-485 serial connection data port. With the appropriate connections, remote operators can view system status online at any time.
The control system couples with the improving cost-efficiency of the latest solar arrays to make solar pumping far more economically attractive than before. As the cost of installing solar arrays approaches $1 per watt, it is coming much closer to the cost of connecting to the utility grid. The company only needs six to 10 solar panels to do the work that older DC-only systems required many more panels to do.
By not requiring electrical wires to be run to the pump, the company saved about 75 percent of the cost of the typical rural installation. With a pumping station running on a locally placed solar array, water can be moved economically to and from various points in the vineyard, eliminating the need to drill extra wells. With the water from the creek moved by the solar system, the company has not had to use their deep-well pumps.