ASHRAE Journal - February 2011 - 70

emerging technologies	

Ac capacity modulation
By John Dieckmann, Member	ASHRAE; James Brodrick, Ph.D., Member	ASHRAE

A

ir-conditioning	systems	are	usually	specified	with	enough	cooling	

capacity	to	meet	a	maximum,	or	near	maximum,	design	space	

cooling	load.	It	is	common	for	systems	to	be	specified	with	excess	capacity	to	allow	for	discrepancies	between	as-built	and	as-designed.	 This	 ensures	 that	 the	 cooling	 load	 will	 be	 met	 under	 the	 worst	 case	 conditions.	In	most	applications,	over	a	full	air-conditioning	season,	the	

actual	cooling	load	reaches	the	design	load	only	a	fraction	of	the	time.
For example, Table 1 (Page 72) summarizes the distribution of cooling hours in the cooling season outdoor temperature bins that are used in the U.S. Department of Energy (DOE) energy test procedure for residential central air conditioners.1 This distribution is intended to be broadly representative of the U.S. climate. The associated cooling load in each temperature bin as a fraction of the rated cooling capacity is tabulated in the fourth column of the table. While this is based on the oversimplif ied assumption that the cooling load varies linearly with the difference between the outdoor dry-bulb temperature and 65°F (18°C) (neglecting the effect of variations in solar loading, outdoor humidity, and other factors), the cooling load is 50% or less of the rated cooling capacity for 80% of the cooling season. The air-conditioning system cannot be allowed to run continuously at full capacity when the load is less than design. If it did run continuously at full capacity, the conditioned space would be overcooled and a significant amount of energy would be wasted.
70	 ASHRAE	Journal	

The simplest way to reconcile the difference between cooling capacity and the instantaneous cooling load is on-off control, the method used in a majority of the residential air conditioners used in the U.S. A thermostat in the space cycles the air-conditioning system on and off as the space temperature cycles above and below the setpoint over a small dead band. An alternative approach is to modulate the cooling capacity so it matches the instantaneous cooling load, running the air-conditioning system continuously at the modulated capacity. In larger capacity air-conditioning systems, some form of capacity modulation is commonplace. Benefits of operating with capacity modulation include improved energy efficiency, steady control of space temperature, better control of relative humidity, and elimination of the large in-rush currents associated with frequent starting of large compressor drive motors. The June 2010 “Emerging Technologies” column discusses some methods for compressor capacity modulation in large systems.2 Many of the benefits of capacity modulation apply to residential air-conditioning systems as well:
ashrae.org	

• Improved energy efficiency (assuming that the compressor efficiency does not fall significantly as the capacity is reduced); • Steady temperature control; and • Better humidity control. A variety of capacity modulation methods are commercially available for residential capacity air-conditioning compressors. Two categories are continuously variable capacity modulation and step-wise (or staged) capacity modulation. With continuously variable capacity modulation, the compressor can deliver any capacity from the design capacity down to a minimum capacity. With stepwise modulation, the compressor capacity is reduced in one or more discrete steps from the maximum capacity. Two metrics are relevant to the effectiveness of a given approach to capacity modulation: the efficiency fall-off as the capacity is reduced and the ratio of maximum to minimum capacity (turndown).

continuous capacity modulation
Two approaches to continuously variable capacity modulation are commercially available in residential capacity refrigerant compressors: variable speed operation of several common types of refrigerant compressors, and rapid (on a 10 to 30 second time scale) unloading and loading of a constant speed scroll compressor. Variable speed compressors are driven by electronically synthesized variable frequency electric power, freeing the speed range from the 60 Hz or 50 Hz speeds of an induction motor. The drive motor can be a three-phase induction motor or a permanent magnet rotor type motor. Variable speed models of reciprocating, scroll and rotary compressors are all commercially available. They have been
	 February	 2011



ASHRAE Journal - February 2011

Table of Contents for the Digital Edition of ASHRAE Journal - February 2011

ASHRAE Journal - February 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Thermal Coupling of Cooling and Heating Systems
10 Common Problems in Energy Audits
Hall of Fame Feature: History of the Changing Concepts in Ventilation Requirements
A Guide to Wireless Technologies
Building Sciences
Solar NZEB Project
Emerging Technologies
People
Special Section
InfoCenter
Commissioning
Products
Washington Report
Classified Advertising
Advertisers Index
ASHRAE Journal - February 2011 - ASHRAE Journal - February 2011
ASHRAE Journal - February 2011 - Cover2
ASHRAE Journal - February 2011 - 1
ASHRAE Journal - February 2011 - 2
ASHRAE Journal - February 2011 - Contents
ASHRAE Journal - February 2011 - Commentary
ASHRAE Journal - February 2011 - 5
ASHRAE Journal - February 2011 - Industry News
ASHRAE Journal - February 2011 - 7
ASHRAE Journal - February 2011 - Letters
ASHRAE Journal - February 2011 - 9
ASHRAE Journal - February 2011 - 10
ASHRAE Journal - February 2011 - 11
ASHRAE Journal - February 2011 - 12
ASHRAE Journal - February 2011 - 13
ASHRAE Journal - February 2011 - 14
ASHRAE Journal - February 2011 - 15
ASHRAE Journal - February 2011 - Meetings and Shows
ASHRAE Journal - February 2011 - 17
ASHRAE Journal - February 2011 - Thermal Coupling of Cooling and Heating Systems
ASHRAE Journal - February 2011 - 19
ASHRAE Journal - February 2011 - 20
ASHRAE Journal - February 2011 - 21
ASHRAE Journal - February 2011 - 22
ASHRAE Journal - February 2011 - 23
ASHRAE Journal - February 2011 - 24
ASHRAE Journal - February 2011 - 25
ASHRAE Journal - February 2011 - 10 Common Problems in Energy Audits
ASHRAE Journal - February 2011 - 27
ASHRAE Journal - February 2011 - 28
ASHRAE Journal - February 2011 - 29
ASHRAE Journal - February 2011 - 30
ASHRAE Journal - February 2011 - 31
ASHRAE Journal - February 2011 - 32
ASHRAE Journal - February 2011 - 33
ASHRAE Journal - February 2011 - Hall of Fame Feature: History of the Changing Concepts in Ventilation Requirements
ASHRAE Journal - February 2011 - 35
ASHRAE Journal - February 2011 - 36
ASHRAE Journal - February 2011 - 37
ASHRAE Journal - February 2011 - 38
ASHRAE Journal - February 2011 - 39
ASHRAE Journal - February 2011 - 40
ASHRAE Journal - February 2011 - 41
ASHRAE Journal - February 2011 - 42
ASHRAE Journal - February 2011 - 43
ASHRAE Journal - February 2011 - A Guide to Wireless Technologies
ASHRAE Journal - February 2011 - 45
ASHRAE Journal - February 2011 - 46
ASHRAE Journal - February 2011 - 47
ASHRAE Journal - February 2011 - 48
ASHRAE Journal - February 2011 - 49
ASHRAE Journal - February 2011 - Building Sciences
ASHRAE Journal - February 2011 - 51
ASHRAE Journal - February 2011 - 52
ASHRAE Journal - February 2011 - 53
ASHRAE Journal - February 2011 - 54
ASHRAE Journal - February 2011 - 55
ASHRAE Journal - February 2011 - 56
ASHRAE Journal - February 2011 - 57
ASHRAE Journal - February 2011 - 58
ASHRAE Journal - February 2011 - 59
ASHRAE Journal - February 2011 - 60
ASHRAE Journal - February 2011 - 61
ASHRAE Journal - February 2011 - Solar NZEB Project
ASHRAE Journal - February 2011 - 63
ASHRAE Journal - February 2011 - 64
ASHRAE Journal - February 2011 - 65
ASHRAE Journal - February 2011 - 66
ASHRAE Journal - February 2011 - 67
ASHRAE Journal - February 2011 - 68
ASHRAE Journal - February 2011 - 69
ASHRAE Journal - February 2011 - Emerging Technologies
ASHRAE Journal - February 2011 - 71
ASHRAE Journal - February 2011 - 72
ASHRAE Journal - February 2011 - 73
ASHRAE Journal - February 2011 - 74
ASHRAE Journal - February 2011 - 75
ASHRAE Journal - February 2011 - People
ASHRAE Journal - February 2011 - 77
ASHRAE Journal - February 2011 - InfoCenter
ASHRAE Journal - February 2011 - 79
ASHRAE Journal - February 2011 - 80
ASHRAE Journal - February 2011 - 81
ASHRAE Journal - February 2011 - 82
ASHRAE Journal - February 2011 - 83
ASHRAE Journal - February 2011 - 84
ASHRAE Journal - February 2011 - 85
ASHRAE Journal - February 2011 - Commissioning
ASHRAE Journal - February 2011 - 87
ASHRAE Journal - February 2011 - 88
ASHRAE Journal - February 2011 - 89
ASHRAE Journal - February 2011 - 90
ASHRAE Journal - February 2011 - Products
ASHRAE Journal - February 2011 - Washington Report
ASHRAE Journal - February 2011 - Classified Advertising
ASHRAE Journal - February 2011 - 94
ASHRAE Journal - February 2011 - 95
ASHRAE Journal - February 2011 - Advertisers Index
ASHRAE Journal - February 2011 - Cover3
ASHRAE Journal - February 2011 - Cover4
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