Client in Sioux City with a large heat pump system is getting killed by demand charges during defrost cycles. MidAmerican's Schedule G-41 demand rate is brutal and these defrost spikes are adding $800-1200 per month in winter. System has 6 heat pumps that all seem to defrost around the same time, creating 180 kW spikes that last 8-10 minutes. The controls guy says they're programmed to defrost independently but it's not working. Anyone know how to better stagger heat pump defrost cycles?
MidAmerican Energy demand spikes from heat pump defrost cycles
Dana, I've dealt with this exact issue on Duke Energy accounts. The problem is usually that all the heat pumps see the same outdoor conditions and have similar runtime hours, so their defrost algorithms trigger simultaneously. You need to manually offset the defrost initiation temperatures by 2-3 degrees between units. So if one defrosts at 35°F, set the next at 32°F, then 29°F, etc. Also stagger the time-based backup triggers by 15-20 minutes. Takes some trial and error but can eliminate simultaneous defrost cycles.
Sam's approach works but you have to be careful not to compromise efficiency. In Michigan with DTE Energy, I've found that installing defrost demand limiting controllers is more reliable. These monitor total building demand and delay defrost cycles if demand is already high. Had a manufacturing plant reduce defrost-related demand spikes by 70% this way. The units defrost when needed but not all at once. Cost about $3,500 for the controller but saves $15K annually in demand charges.
Good suggestions from Sam and Tina. Here at MLGW we also see this with large heat pump installations. Another approach is installing auxiliary heat staging to reduce the defrost demand spike. Instead of using full electric heat during defrost, stage it in smaller increments. A 40 kW heat pump might use 60 kW of auxiliary heat during defrost, but if you stage that auxiliary heat in 15 kW increments with 2-minute delays, the total demand spike is much smaller. The defrost takes slightly longer but demand savings are significant.
Randy's point about auxiliary heat staging is crucial. Up here in Fargo with Xcel Energy, I learned this lesson after a client got hit with a $4,800 demand spike during a cold snap. Six heat pumps all went into defrost mode simultaneously and the auxiliary heat came online at full capacity. 220 kW spike that lasted 12 minutes. Now I spec all heat pump installs with staged auxiliary heat and defrost coordination controllers. The upfront cost is higher but the demand savings pay for it in one winter.
Thanks for all the suggestions. I'm meeting with the controls contractor next week to discuss implementing defrost coordination and auxiliary heat staging. The client is willing to invest in upgrades if it saves money long-term. At $800-1200 per month in extra demand charges, even a $10K control upgrade would pay for itself in less than a year. Will report back on what we implement and how it works out.
Dana, definitely look into the Belimo defrost coordination systems if your contractor isn't familiar with them. We've installed several in Greensboro with Duke Energy accounts and they work well. The system monitors all heat pumps and prevents more than one from defrosting at a time. Also integrates with BAS systems for manual override during maintenance. Cost runs about $1,200 per heat pump but the demand savings usually justify it within 8-10 months in our territory.
One more thing Dana - make sure to get baseline interval meter data before making any changes. You'll want to document the current defrost patterns and demand spikes so you can quantify the improvement after implementing controls. MidAmerican should provide 15-minute interval data for the past 12 months. Having solid before/after data really helps justify the investment to your client and proves the system is working as designed.