ASHRAE Journal - February 2011 - 70

emerging technologies

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

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:
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• 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

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