After hearing about its electricity provider offering a $50 per horsepower rebate for installing variable frequency drives (VFDs) on fan motors, a local cooperative was looking into VFDs for its ground pile fan system. The coop has a 1-million-bushel ground pile with eight 7.5-hp 480VAC motors for its fans and typically will hold the pile for 120 days (one piling season).
Based on the latest utility bill, the coop determined that its electrical cost is about $0.08 per kilowatt hour (kWh).
Also at the Opening Workshop Feb. 25 at GEAPS Exchange 2017 in Kansas City, MO, another topic related to energy efficiency dealt with reducing shrink losses in a temporary storage pile through the use of variable frequency drives (VFDs) on aeration fans. Mark Spindler, formerly with Lakeland Companies, Minneapolis, MN (763-544-1752), and now president of CIM Software, Minneapolis, MN (612-940-3169), provided this case study during the workshop to illustrate the cost justification.
For the purposes of this article, please use the following formulas for calculating shrink due to water loss:
Bushels lost = (total water shrink/100) x (total bushels).
Total water shrink = (moisture points removed) x (water shrink factor).
Water shrink factor = 100/(100 – % ending moisture).
Using this information, in conjunction with the speed-to-kW table on page 82, the coop manager used the following formulas to calculate the energy savings the VFD system would bring assuming that the average operating speed is 50%.
• Total hours in season = ________ hours.
• Difference in kW between 100% and 50% speed = ____kW.
• kWh (kilowatt hours) in a season for one fan = ______ kWh.
• Total energy savings for a season for one fan = $_________ .
• Total energy savings for eight fans = $_________.
The facility manager at the coop realized that with VFDs, they also would have better control of the fans and thus be able to control moisture loss more effectively. In a typical year, he will pile his corn at 15.5% moisture and will reclaim at 14.0%, and with the VFD system, he would be able to reclaim at 14.5%, saving 0.5%.
Using the formulas to calculate shrink on page 82, they calculated the water shrink savings that the VFD system would deliver, if the price per bushel is $3.50. Note: they needed to calculate the difference between the bushels lost going from 15.5% to 14% and the bushels lost going from 15.5% to 14.5%.
• Water shrink factor = ________.
• Total water shrink = ________%.
• Total bushels lost = ________ bushels.
• Difference in bushels lost = ________ bushels.
• Total shrink savings = $________
The manager calculated that the fan control system costs $30,000 to install. Using both the answers above and adding in the rebate, they determined the return on investment (ROI in piling seasons) with energy savings, utility rebate, and shrink savings.
• Total energy savings for eight fans for one season = $______.
• Total shrink savings for one season = $________.
• Total rebate savings for eight fans = $________ .
• Total savings for one season = $________ .
• ROI in number of seasons =_______ seasons.
Reprinted from GRAIN JOURNAL May/June 2017 Issue
