Electrical efficiency in grain facilities
September 3, 2015
Receiving, storing, aerating and transferring grains requires a significant amount of energy, which can impact already small profit margins.
Monitoring and managing energy consumption to reduce its usage can yield sizable energy cost savings, said Robert Fletcher with Lawrence Technological University in Southfield, Michigan, U.S., in his presentation at GEAPS Exchange 2015 in St. Louis, Missouri, U.S.
In the spring of 2012, the university was approached by the local utility company, DTE Energy, with funding to undertake an energy assessment of the Michigan Agricultural Commodities, Inc. (MAC) facility in Marlette, Michigan, U.S.
MAC is a private corporation that purchases, sells and stores agricultural commodity grains (primarily corn, soybeans, and wheat) throughout the U.S. and Canada. MAC is Michigan's largest grain handler with eight locations in Michigan. Rising utility costs and fixed commodities prices necessitated the need for a concerted effort by the MAC to reduce its energy costs, Fletcher said.
During his presentation, Fletcher outlined the process used to determine and implement opportunities for energy savings at the facility. He also discussed specific energy savings technologies, particularly variable frequency drives. The information from an energy assessment can be beneficial for grain facilities operations managers, facilities managers, facility engineers and key support staff who are looking for ways to reduce their grain handling operation costs through energy savings.
The project was completed in two phases. The first established power and energy usage data acquisition capability and included the testing of grain hopper aeration and drying with and without variable frequency drive power regulators, Fletcher said.
Phase 2 involved a full energy assessment of the Marlette facility including grain receiving, grain drying (which uses electric and natural gas), and outdoor temporary grain storage piles. Work on this project was conducted from the early summer of 2012 through the late fall of 2013.
The original Marlette facility dates to 1865 with some of the original structures still used for unique and small quantity grain storage. The primary-use storage facilities are much newer and are located toward the rear of the property. The facility has a rail spur that allows transfer of stored grain directly into rail cars that can then be transported by rail train for distribution around the U.S., or abroad.
It has 16 permanent storage bins in the primary-use storage area for a total storage capacity of 3.754 million bushels of grain. The facility can also accommodate an added 1.055 million bushels of temporary pile storage. Grain is primarily received from local farmers at the Marlette facility in trailers pulled by trucks or tractors. There are two receiving stations. The first is the front receiving station that allows grain dumps to the “front receiving pit” that transfers grain to the front receiving leg for routing to any of the permanent grain storage bins on the site.
The second is the back receiving station (located toward the rear of the facility) where grain can also be dumped into its own receiving pit that also allows grain to be distributed to all storage bins on site.
Before receival, the grain is weighed and a sample is evaluated for moisture content, grain size, quality, and the level of foreign materials present in the grain load. If the grain is dry (at or below 16% moisture by weight), the grain can then be received directly.
Grain is dumped directly from the trailer into the receiving pit. The grain is then transferred by the receiving pit drag to the receiving leg. The receiving leg lifts the grain to a high central shoot (a few hundred feet about the ground). The shoot can then direct and route the grain via a directional nozzle to specific gravity pipes that empty out to one or more in-series overhead drags located down-stream from the gravity pipe.
If the grain is not dry it can be routed to a bin for later drying, or it can be sent directly to the grain dryer. The grain dryer, which heats at 45 million BTU’s per hour, is a tall cylindrical structure that intakes outside air, heating it with a natural gas burner and blowing it into the dryer.
The grain enters the top of the dryer and flows downward while being exposed to the heated dry air. Once dried, the grain can be routed to longer-term storage bins, or temporary bins. Once the grain has been off-loaded to the receiving station from the trailer, the delivery truck then pulls the trailer back to the weigh station to determine the empty trailer weight. The difference in initial weight to final empty weight is used to then calculate the weight of grain received by the facility.
Once the grain has been routed to its initial storage destination it can be routed to three other on-site destinations: the dryer longer-term storage bin; or blended with similar grain.
Grain is moved from storage with an auger located at the base inside of the bin, were the grain drops down though shoots to a lower recovery drag that then routes the grain to a leg. The elevator dumps the grain to a shoot with a directing nozzle, which then dumps the grain onto an overhead drag to route to either a truck-loading hopper, or to a train-loading hopper.
It is important to note that all grain received at the Marlette facility is handled at least twice, once at receiving for storage, and once again for sale and shipment. Minimal handling and movement of grain at the facility is always desired, Fletcher said. Grain movement can damage the grain and also requires electric power use to energize motors for drags and legs.
Assessing electricity consumption
Most electric energy consumption at the Marlette facility is from electric motors for the operation of fans, legs and drags. All legs and drags motors are powered using 480 VAC delta connections. No motors in Marlette use a 240VAC wye-start that switches to a 480 VAC delta-run mode. The two 125-horsepower fans on the dryer each have a soft-start feature that ramps the power up. The soft-start feature limits the electric current per phase for each dryer fan motor to not exceed 400 amperes.
An energy assessment should start with a detailed review of energy usage, Fletcher said, through the review of historical utility billings. The Marlette facility has five power meters.
Natural gas consumption was also reviewed. The facility’s grain dryer uses natural gas to heat ambient air that is forced through the unit to dry moist grain. The dryer does have controls and monitoring systems, but proper maintenance and cleaning of vents and inlets are required. These require operator training to assure best operation of the unit.
One area of possible prediction of natural gas usage for the grain dryer at the facility could be the tracking of the local area’s relative humidity just before and through the harvest season. In studying the facility’s natural gas usage and grain moisture, Fletcher said they found a reasonably good correlation to average hourly relative humidity and the need for natural gas usage. Tracking this over time during the harvest season by Marlette personnel could be a useful predictor for the demand on the dryer, to anticipate personnel support and system maintenance, as well as the obvious natural gas demand.
On-site data collection was done with a portable, robust and reliable electric power and energy data acquisition (DAC) system. It was decided that the Fluke 435 II three-phase power quality and energy analyzer portable power meters were best suited for the project, Fletcher said.
The Marlette facility uses three-phase AC induction motors for its drags, legs and fans. Few of these motors need to be left on at full power, and could often run at reduced speeds at various process times without any detriment to product, process or productivity. Because of this it was natural to investigate possible motor control options for the facility, Fletcher said.
Wye-delta starting. One option would be wye-start to delta-run starting, which is used to reduce the significant current inrush encountered when starting AC motors, especially large motors. For large motors these uncontrolled inrush currents can be thousands of amps and as high as eight to 10 times steady-state operating currents. This current inrush induces significant stress on motor components and the resulting instantaneous loading of motor components and related connected hardware can eventually cause permanent failure.
In a wye-start to delta-run starting option, a motor is connected in an electrical “wye” configuration for initial starting and then with an electromechanical device is switched to a “delta” wiring configuration for run mode. This approach is sometimes still called “soft-start,” but should not be confused with the new solid state soft-start technology available that continually ramps up a motor from zero to full (or preset) revolutions per minute.
Wye-delta starting reduces high in-rush currents but doesn’t eliminate it. So while it works, it’s not the best option and newer systems are available that are superior solutions, Fletcher said.
Soft-start controller. Another option is a soft-start controller, which is an electronic solid-state device placed upstream from an electric motor. It ramps up the power to the motor using a programmable ramp rate so as to eliminate any significant current draw upon start-up of the motor, Fletcher said.
It operates by gradually ramping up the voltage to the motor over a programmed starting time. This permits smooth starting of the motor and eliminates high in-rush currents. Most soft-start devices also have a soft-stop option which does the same operation only in reverse.
Soft-start controllers are good for motors that undergo frequent starts and stops during the day, especially when the motor is under load when started. They are best for motors that do not need to come to full speed immediately and where the motor can tolerate ramping up to speed over several seconds without any detriment to the process, Fletcher said. This can help reduce significant wear and tear on the motor and other connected process equipment. The Marlette facility has soft-start controls on the two 125-horsepower motors in the grain dryer.
Variable frequency drives. There are numerous on-line references, as well as scholarly and technical literature documenting in great detail the operation and functionality of variable frequency drives (typically used for fans and pumps) and variable torque drives (typically used for conveyors and drive systems with fluctuating loads).
There are four basic types of variable frequency drives commercially available today. These include pulse width modulation (PWM), current source inverter (CSI), voltage source inverter (VSI), and the flux vector drive. The devices used in this study, and the most commonly used in industry, were PWM VFDs, and are the focus of the discussion here.
These devices operate using a standard 50 Hz or 60 Hz AC power input. Through power signal processing, they manipulate the frequency of their output by rectifying an incoming AC current into DC to provide a fixed-voltage pulse width modulated (PWM) output signal to a motor.
VFDs are able to accurately control the speed of standard AC motors, providing speed control with full torque ranging from 0 rpm through the maximum rated speed of the motor. If needed, VFDs can also deliver speeds above the rated speed, but at a reduced torque. Adjusting the frequency of the output signal to the motor under varying motor loads can result in significant energy savings, Fletcher said.