An energy service company agreed to convert all 12 CV AHUs within a commercial building into VAV systems by installing variable frequency drives (VFDs) and VAV terminal boxes. To verify fan energy savings, it was agreed to meter only one AHU. Metering all 12 AHUs was determined to be economically infeasible. The metering of one AHU revealed the following:

(a) the supply fan is rated at kW_e,fan = 40 kW and is operated for 6,000 hr/year, and

(b) the VFD operates according to the following load profile:

It was agreed that the energy use of one motor is proportional to its speed squared. (The reader is referred to Chapter 7 for more details on fan performance.) Determine the fan energy and cost savings for the CV to VAV conversion for all the AHUs as stipulated in the agreement, which states that the annual fan energy savings for all the AHUs is simply 12 times the fan energy savings estimated from the metered AHU. Assume that electricity costs $0.07/kWh.

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At 100 percent of its speed, 500 hr/year (i.e., 8.33 percent of total operat- ing hours) At 80 percent of its capacity, 3,500 hr/year (i.e., 58.33 percent of total operating hours) At 60 percent of its capacity, 1,500 hr/year (i.e., 25.0 percent of total operating hours) At 40 percent of its capacity, 500 hr/year (i.e., 8.33 percent of total oper- ating hours)