Determine the reduction in the annual energy costs due to staggered duty cycling of three identical fan motors (each rated at 30 kW [40 hp]). The motor manufacturer specifies 20 minutes on and 10 minutes off as the minimum duty cycle. The utility monthly demand charge is $10/kW.

Question

Determine the reduction in the annual energy costs due to staggered duty cycling of three identical fan motors (each rated at 30 kW [40 hp]). The motor manufacturer specifies 20 minutes on and 10 minutes off as the minimum duty cycle. The utility monthly demand charge is $10/kW. First, determine the recommended NEMA duty period assuming 2-pole motors.

Accepted Answer

(a) Using Table 10.2, the allowable number of starts per hour for 40 hp, 2-pole motors is 3.7 starts/hr. Thus each start should last:

Start period = (60 min/hr)/(3.7 starts/hr) = 14.4 min/start

The duty period is thus about 15 minutes. Because the duty period is the sum of off-time and on-time, and because the minimum off-time is about 2 minutes (130 seconds based on Table 10.2), the maximum on-time allowable by the NEMA standard is 13 minutes. Thus, the manufacturer on-time is longer than that recommended by NEMA.

(b) The reduction in the electrical demand peak due to the staggered duty cycling approach is 1/3 of the total demand of all three motors (or 1/3 * 90 kW = 30 kW). Indeed, at any given time, only two out of the three motors are operating. Thus, the annual savings in electrical demands charges is given as follows:

ΔkW =12*30kW = 360kW/yr

Therefore, the staggered duty cycling control provides an annual energy cost savings of $3,600.

Table 10.2 Summary of Comparative Results for the Control and Measurement Techniques Tested by Krarti et al. (2000).

Outside Air Intake Rate Measurements
Electronic Thermal Anemometry			Averaging Pitot-Tube Array
RMS (cfm)	stdev (cfm)	Mean (cfm)	RMS (cfm)	ev (cfm)	Mean (cfm)	Validity	Set-Point (cfm)	Case	Measurement Control1	System Description
1048	564	682	1,150	658	656	14%	1,600	1A	NA	Fixed Damper Position
1,199	680	1,407	1,199	678	1,410	23%	2,400	1B	NA	Fixed Damper Position
1,513	870	2,124	1,309	819	2,178	26%	3,200	1C	NA	Fixed Damper Position
89	70	1,544	75	69	1,630	100%	1,600	2A	NA	Plenum Pressure Control
118	88	3,279	129	94	3,288	100%	3,200	2C	NA	Plenum Pressure Control
71	46	1,546	52	38	1,635	100%	1,600	3A	P	Direct Control with Economizer Duct
53	47	3,225	51	50	3,192	100%	3,200	3C	P	Direct Control with Economizer Duct
67	55	1,637	110	55	1,695	94%	1,600	4A	E	Direct Control with Economizer Duct
80	50	3,263	57	49	3,228	100%	3,200	4C	E	Direct Control with Economizer Duct
953	458	2,436	936	439	2,427	0%	1,600	5A	E	Volume Tracking
58	47	1,634	55	39	1,639	100%	1,600	6A	P	Direct Control with Dedicated Duct
77	51	2,457	51	41	2,430	100%	2,400	6B	P	Direct Control with Dedicated Duct
58	43	1,640	56	36	1,643	100%	1,600	7A	E	Direct Control with Dedicated Duct
50	42	2,428	40	39	2,404	100%	2,400	7B	E	Direct Control with Dedicated Duct
42	36	1,622	37	31	1,621	100%	1,600	8A	P	Injection Fan
54	36	2,440	40	28	2,429	100%	2,400	8B	P	Injection Fan
40	36	1,617	44	38	1,622	100%	1,600	9A	E	Injection Fan
42	32	2,427	37	33	2,418	100%	2,400	9B	E	Injection Fan
11[latex]9^{3}[/latex]	11[latex]9^{3}[/latex]	1,60[latex]5^{3}[/latex]	141	137	1,632	75%	1,600	[latex]NA^{2}[/latex]	C	Direct Control

1: P = Averaging Pitot-Tube Array, E = Electronic Thermal Anemometer, [latex]C = CO_{2}[/latex] Concentration Balance.
2: A different system setup was used for testing the concentration balance measurement technique.
3: Value is for [latex]CO_{2}[/latex] concentration balance measurement technique, not electronic thermal anemometry.

Question 10.2: Determine the reduction in the annual energy costs due to st...

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