Monument, Colo. has blossomed from a Rio Grande Railroad stop into a bustling bedroom community for Colorado Springs and Denver. Lewis-Palmer School District #38 has grown, too, from a patchwork of log-cabin schoolhouses to one of Coloradoís top-performing school districts. With upwards of 6,000 students, the district has experienced growing pains.
In 2008, the District Building Maintenance Supervisor saw state funding evaporate. He knew that PLCs and his 40 years of electrical experience could help keep dollars, and teachers, in classrooms. Labor was among Lewis-Palmerís highest costs. Layoffs are often the first wave of cost cutting. If he could save $50,000, it might save a job.
The supervisor zeroed in on 70 diesel-powered buses shuttling 67% of the student body to nine schools daily. Upon returning to a central bus lot, 25-ft. extension cords hitch the electric engine block heater in each bus to receptacles within two 120 V junction boxes. (The engines run from 500 W to 1,000 W.) This daily routine ensured reliable startups for the diesel engines, despite ever-changing weather and average winter temperatures of 12°F. Unless physically shut off, each 120 V circuit would needlessly heat engines on holidays, weekends, and full snow days.
On smaller buses, a 500 W block heater starts warming an engine block after the daily drop-off run at 3:00 p.m., and will operate until the next dayís 6:00 a.m. pickup run. When multiplied by 70 buses, the true costs for the fleet were surprising. In 2008, the fleetís block heating cost $19,500 and in 2010 $22,000.
PLC programming could prevent charging, especially if the heaters were plugged in on warm days or when the fleet is idled. By automating engine block heating, Lewis-Palmerís transportation costs could be cut significantly. Specifically, a PLC-based system could enable heating to run as needed, not year-round. The necessary investment could be recouped in just five years.
The entire bus lot is a half-mile long by a quarter-mile wide. Potential suppliers proposed full-fledged industrial SCADA systems, requiring a substantial hardware investment, including HMIs and PCs. One potential solution, the Wago-I/O-System, was compatible with the districtís existing computers, which minimized hardware costs. Given the bus lotís size, a Bluetooth transceiver I/O module with a one-kilometer line-of-sight range could provide an infrastructure savings. This was important because one junction box is a quarter-mile from the transportation building.
Without a wireless solution, the lot would have to be cut up, a $15,000 endeavor. In addition, itís likely that any asphalt patching used to cover the cabling wouldnít withstand rapidly changing weather and daily bus traffic.
Deft PLC programming and simplicity cut engine block heating costs by nearly 73 percent, from an annual average of $20,000 to just $5,500 for the 2012-2013 school year. Similarly, the systemís ROI has fallen to just three years.
Existing junction boxes were transformed into full-fledged control panels within NEMA 3 enclosures. Each contains a Wago PLC, a Bluetooth transceiver module, a 16-channel digital output module, a terminal block, a power supply, and a 20 A relay. Bluetooth links the panels to a supervisory PLC in the districtís transportation office. There, a master PC directs the enclosuresí PLCs to activate/deactivate relays routing to breakers. This energizes each receptacle for engine block heating.
With the bulk of the savings coming from eliminating seven to fifteen hours of wasted energy to warm 70 engine blocks daily, a simple program was required with provisions for rapidly changing weather. Flexibility was also critical, since 75% of the fleet operates daily. A two- to three-hour heating time was chosen for the blocks. Avoiding simultaneous activation of the heaters would allay inrush current concerns and mitigate demand metering.
The programming and Web-based management provide options as temperatures (and school opening times) change. The supervisor can log into the system from any location and activate/monitor activity.
Based on Lewis-Palmerís success, and the potential to help other districts cope with escalating energy costs and dwindling funds, plans are underway to market the diesel engine block heating system to other districts.
Amanda Bell, of Wago Corp., holds a BA degree in communications from the University of Wisconsin-Madison.