ASHRAE Journal - August 2010 - 66

VFDs for Residential Systems
By John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

Fifth in a Five-Part Series

ariable-speed operation of the compressor and indoor air blower is one way

to provide energy-efficient capacity modulation. This takes advantage of the

speed cubed power law, as discussed in previous columns in this series. Compressors and blowers are typically designed for worst-case operating scenarios. Residential cooling loads vary greatly. Therefore, by operating systems at levels that vary below design maximum, significant energy and cost savings can be achieved. Energy-efficient capacity modulation generally increases the Seasonal Energy Efficiency Ratio (SEER), determined by the DOE energy test procedure,1 by 30% to 40%, compared to a system with on-off capacity control and the same size coils, by eliminating on-off cycle losses, using condenser and evaporator heat transfer surface more effectively and reducing average blower power (once again, the speed cubed power law). A central air-conditioning system with a variable speed compressor and a variable speed blower operates with a compressor speed that matches the cooling capacity of the air-conditioning system to the cooling load of the residence. The speed of the indoor air blower and the resulting airflow rate are varied in tandem with the capacity, maintaining dehumidif ication capacity and taking advantage of the speed cubed power law. If the cooling load is less than the minimum speed cooling capacity, the system operates in on-off mode at the minimum compressor speed. Operating at reduced capacity reduces the heat rejection load on the condenser, lowering the condensing temperature and raising the EER of the compressor, a major factor in the increased SEER of variable capacity systems.
66 ASHRAE Journal

Recognizing the large energy saving potential of energy-efficient capacity modulation, the DOE energy test procedure1 for residential central air conditioners includes a variable speed test and SEER calculation procedure. The test procedure is illustrative of how capacity modulation leads to energy savings. The SEER is determined by a bin analysis (Table 1), where the individual bin cooling capacity and energy input for each ambient temperature bin is determined by interpolating between five test points. The bin cooling capacities and energy inputs, weighted by the fractional bin hours, are summed, with the SEER being the ratio of the summed capacity divided by the summed energy input.The fractional bin hours are based on average weather across the U.S. The three lowest outdoor temperature bins, below 79°F (26°C), account for 66% of total operating hours, where the compressor and indoor blower are operating at the lower end of the speed range. Five steady-state test points are the basis of interpolating to cover the range of fixed and variable cooling capacity possibilities. A series of formulas in the variable speed part of the test procedure specify how the interpolations are performed to arrive at the appropriate value for each of the eight temperature bins.
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Figure 1 illustrates the key aspects of the procedure graphically. A hypothetical design building cooling load is defined as 1/1.1 times the maximum speed capacity at the DOE A test condition (95°F [35°C] ambient, 80°F/67°F [27°C/19°C] DB/ WB indoor temperature, indicated by the dotted circle at 95°F [35°C], 100%). That is, the air-conditioning system is assumed to be 10% oversized if the DOE A condition is the design condition. The building load—the diagonal red line in Figure 1—is assumed to be 0 at 65°F (18°C) ambient temperature (accounting for solar load and internal heat gains) and is interpolated (or extrapolated) linearly at ambient temperatures above 65°F(18°C). The steady-state performance of the air conditioner is tested at the five points indicated on Figure 1. Testing is done at two outdoor temperatures (82°F [28°C] and 95°F [35°C]) at maximum speed (indicated by dotted circles), at two outdoor temperatures at minimum speed (67°F and 82°F [19°C and 28°C]), indicated by dotted circles, and at one specified intermediate speed at 87°F (31°C) outdoor temperature, indicated by an oval with a dotted line. The double-dashed line in Figure 1 traces the bin analysis over the 65°F to 104°F (18°C to 40°C) range of the outdoor temperature bins, through three district “cases”: • Case I (Figure 1) applies to temperature bins (Table 1) where the cooling capacity at minimum speed is more than the building load, resulting in cyclic operation at minimum speed. This is typically the lower two or three temperature bins. • Case II applies to the temperature bins where continuous operation at an intermediate speed meets the
August 2010

ASHRAE Journal - August 2010

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