I've got a retail client here in Grand Rapids getting hammered by demand charges and I think the culprit is heat pump defrost cycles. They have six Carrier heat pumps on a strip mall, each about 15 tons. During defrost, the backup electric heat kicks in and we're seeing 20-minute demand spikes of 180kW. This is happening 3-4 times per day in winter weather. Consumers Energy's GS-2 rate is $11.50/kW for demand. Anyone dealt with heat pump defrost demand issues before?
Heat Pump Defrost Cycles Killing My Client
Tina, that's a classic problem with electric backup heat. 180kW for 20 minutes is setting your monthly peak every single day. Here in Memphis with MLGW we see this constantly. Two options - either install demand limiting controls that can temporarily shut down non-critical loads during defrost, or look into variable capacity heat pumps that defrost more efficiently. The payback on either solution should be under 18 months with those demand charges.
We had this exact issue in Spokane with Avista Utilities. The problem is that all six units are probably defrosting at random times, so you never know when the spike is coming. We installed a simple demand controller that sheds lighting loads during defrost cycles. Saved the client about $2,400 per month during heating season. The controller monitors total facility demand and starts shedding non-essential loads when it hits preset thresholds.
Larry, that's a great idea about load shedding. What kind of loads did you shed? This is a retail strip mall so we can't really touch much without affecting customers. Maybe some of the parking lot lighting or back-office equipment? Randy, the variable capacity heat pumps would be ideal but that's a $80,000+ replacement project. The demand limiting might be more realistic short-term.
For retail you're pretty limited on what you can shed. Maybe water heaters if they're electric, some interior lighting zones, non-refrigerated display lighting. The key is finding 30-40kW of loads that can be off for 15-20 minutes without impacting sales. Even if you can only shed 25% of the defrost spike, that's still significant savings. Have you logged the actual defrost patterns? Might be able to stagger the defrost timing across units.
Tina, another option is upgrading the defrost controls themselves. Some of the newer smart defrost systems only run when actually needed instead of on fixed time intervals. We had good results here in Spokane with Honeywell adaptive defrost controls. They monitor coil temperature and only defrost when ice buildup actually occurs. Cut defrost frequency by about 40% which proportionally reduced the demand spikes.