Quick question for the group. I've got a small office building in Rapid City with two 25-ton rooftop units. Black Hills Energy hits us with $9.25/kW demand charges on their Large General Service rate. Building owner is asking about variable frequency drives to reduce startup demand. Each unit pulls about 35kW at startup for 2-3 minutes, then settles to 22kW running. Is the VFD investment worth it just for demand control, or should we look at simpler solutions?
Variable Speed Drives - Worth It for Demand Control?
Joanne, VFDs are great for energy savings but they're probably overkill if demand control is your only goal. For two 25-ton units, you're looking at $15,000-20,000 for proper VFD installation. A simple soft starter would give you most of the demand reduction for maybe $3,000-4,000 total. The soft starter limits inrush current during startup but doesn't give you the ongoing energy savings of variable speed operation.
Randy's right about the cost difference. We see this decision a lot here in Oklahoma with OG&E. VFDs make sense if you have significant part-load operation - the energy savings can justify the higher cost. But if the units are mostly running at full capacity, soft starters give you 80% of the demand reduction at 20% of the cost. What's the typical load profile on those units?
Susan, the units run pretty much full capacity during peak summer and winter months, maybe 60-70% capacity during shoulder seasons. Sounds like soft starters are the way to go for this application. The building owner was sold on the "energy savings" pitch from a VFD vendor but the demand charge reduction is really what we need to focus on. Thanks for the reality check on costs!