Variable Speed Compressor and Supply Fan Controller

It is well understood that matching the cooling capacity of an air conditioner (A/C) with the varying cooling load in a given space can potentially reduce electrical power demand and energy use. The objective of this project is to quantify the energy efficiency benefits of a variable frequency drive (VFD) that modifies both the compressor and indoor fan speed to match the A/C cooling capacity to the cooling load of the space being conditioned. To achieve this objective, the project team compared the performance of a 5-ton packaged rooftop unit (RTU) with and without the VFD technology under controlled laboratory conditions at Southern California Edison’s Technology Test Centers. The baseline-air cooled RTU under test was equipped with a constant-speed compressor and indoor fan. The evaluation tested the unit under 32 different test scenarios that reflected varied cooling loads, as well as different outdoor and indoor conditions. The technology evaluated in this project was an add-on VFD controller suitable for a commercial A/C with the several main components: the controller, programmable thermostat, supply and outside air temperature probe, return air carbon dioxide sensor, and the VFD. The study focused on the technology’s ability to vary the frequency, and hence the speed, of the compressor and indoor fan to maintain the target thermostatic setpoint. For this project, the manufacturer’s certified contractor installed and set up the controller. The controller was set to vary the frequency from 40 Hz to 60 Hz in cooling mode. When the compressor cycled off, the indoor fan’s frequency could drop to as low as 20 Hz. In initial testing of the technology under low cooling load or part-load condition of 37%, an improper VFD setting caused frequent short cycling of the A/C, which resulted in compressor failure. The manufacturer replaced the compressor and properly reprogrammed the controller prior to the testing presented here. The first set of tests were run with an indoor dry-bulb temperature (DBT) of 75°F and indoor wet-bulb temperature (WBT) of 63°F. These tests were done at outdoor DBTs of 85°F, 95°F, and 105°F at full A/C load, as well as at 90%, 80%, and 70% of the full load. Note that the test condition of 95°F outdoor DBT at full load is the standard AirConditioning, Heating, and Refrigeration Institute (AHRI) rating condition for the unitary A/Cs. The second set of test were run with an indoor DBT of 80°F and WBT of 67°F. These tests were done at the outdoor DBTs of 95°F, 81.5°F, 68°F, and 65°F under full A/C load as well as at 75%, 50%, and 25% of the full load. Results of the successful tests showed that under full-load conditions, the VFD had no or little impact on the overall power demand and energy usage of the RTU. As anticipated, the impact of the VFD was more evident under the part-load conditions. Reductions in the total power demand of the unit varied as a function of the cooling load and the outdoor DBT and ranged between less than 10% (6,021 kW without VFD vs. 5,547 kW with VFD) and up to 40% (4,927 kW without VFD vs. 2,959 kW with VFD). The reductions in the power demand resulted in decreased energy usage, even though the compressor rang longer with the VFD than without the VFD. This longer run time was expected, as it is an inherent characteristic of A/Cs with variable speed driven compressor. Overall, varying the speed of the indoor fan and compressor is more beneficial for the milder climates, where the nominal capacity of the A/C is typically lower than the building cooling load. That is, the potential for demand reductions and energy savings are more feasible for the A/Cs that are expected to operate largely under low part-load conditions. Although not studied in this project, varying the condenser fan speed when the A/C is at part-load conditions, would likely offer further energy savings benefits. Also recommended is proper installation and commissioning of the VFD to eliminate the possibility of the compressor failure—and the associated replacement costs—due to frequent short cycling under low part-load conditions. In next steps, the data obtained here can be used with the energy simulation modeling to establish the annual energy savings for the rebate programs. Specifically, annual energy savings can be determined by using the energy savings potential as a function of part-load ratios and the A/C’s energy usage at different part-load ratios, which can be found for different climate zones by performing energy simulation modeling.