In 2011, the U.S. Department of Energy’s Building Technology Office (DOE’s BTO), with help from the Better Buildings Alliance (BBA) members, developed a specification (RTU Challenge) for high performance rooftop air-conditioning units (RTUs) with capacity ranges between 10 and 20 tons (DOE 2013). Daikin’s Rebel RTU was recognized by DOE in May 2012 as first to meet the RTU Challenge specifications. This report documents the testing of a Rebel unit and a standard reference unit in the field and compares the seasonal efficiency of the two units.
The goal of the RTU Challenge demonstration was to estimate the seasonal performance of the RTU Challenge unit and the annual savings that could be achieved by installing the challenge unit instead of an alternate standard unit. The demonstration took place at two grocery stores located in New Smyrna Beach and Port Orange, Florida. The Rebel unit was installed as a replacement of an existing unit in July 2013 at the New Smyrna Beach store. The reference unit was an existing rooftop unit in the Port Orange store that is about 6 years old. The reference unit had two compressors for staged cooling and a constant-speed supply fan. Both units had the same rated cooling capacity of 7.5 tons and served each store’s office spaces with similar footprints.
A set of sensors were used to measure the dry-bulb temperature and the relative humidity for the outdoor-air, the return-air, the mixed-air, and the supply-air. RTU total power consumption was also measured using a power transducer. These sensor measurements, together with a number of control signals were monitored at 1-minute intervals from August 2013 to September 2014.
The average daily energy efficiency ratio (EER) was computed for each unit using the monitored data. The ratio of the average EER for the two units varied between 0.9 and 2.4. The Rebel unit had a higher daily EER than the reference unit for almost all days. The EER ratio increased as the daily average outdoor-air temperature decreased, as expected. This means that RTUs with variable-speed compressors and variable-speed fans, like Rebel, had better part-load efficiencies than units using constant speed supply fans and ON/OFF controls for compressors. The average of the daily EER ratio for all days was approximately 1.38, which means that on average, the daily EER of the Rebel unit was 38% higher than that of the reference unit.
In addition to daily EER, the seasonal cooling efficiency was also calculated over the entire monitoring period. Over the 12-month period, the reference unit and the Rebel unit had seasonal EERs of 8.3 and 10.9, respectively. The Rebel unit’s seasonal EER was about 31% higher than the reference unit. This result was slightly lower than the findings from our previous simulation work, which estimated that in hot and humid climates, Rebel would consume about 40% less electricity than a RTU with a constant-speed supply fan and a single-stage mechanical cooling. Possible reasons for this difference included: 1) the load that the two units in the field served were different, while the two units in the simulation served the same load; and 2) the reference unit had higher operating efficiency than the number used in the simulation runs.
The annual energy savings from the rooftop unit replacement with Rebel was about 16,000 kWh, which translated to roughly 3.8 years in simple payback.
It was a challenge to find two units running in two different spaces that had served similar cooling loads. Although two grocery stores with similar layouts were selected, the monitored data showed that they had noticeably different load profiles. Therefore, absolute energy savings between the two units could not be calculated. If the absolute savings measurement were desirable, then the existing RTU will have to be monitored for 1 year, followed by a year of monitoring of the Rebel unit after it replaces the existing RTU.
Other issues related to the installation of the Rebel unit included:
-The Rebel unit came with a different base footprint from the existing Lennox unit. Although a curb adapter was provided, it left the unit suspended over the front side of the base, and was ultimately supported by blocks.
-Although the new Rebel unit was considerably heavier than the unit it replaced, no roof reinforcement was needed.
The store that had the Rebel unit reported no comfort issues either positive or negative. The Rebel unit had a Micro Tech III controller, which was not compatible with the existing Emerson E2 BX controller, or the Emerson building automation system (BAS). Emerson had an application for the Micro Tech II controller but not for Micro Tech III. Therefore, the store had to install an output board with a set of dry contacts to control the RTU indoor fan. They also had to add an interface to monitor the indoor fan “On/Off” status and the supply/return temperatures, but they could not control any cooling/heating/speed control functions. All operations were controlled directly by the Micro Tech III controller in the unit with input from the zone temperature sensor.
The start-up and commissioning of the Rebel was challenging because the local Daikin distributor who installed the unit had very little experience in installing these new units. In addition, the controller had many features with a large instruction/operation manual, which made it difficult to properly configure. It took the distributor a couple of trips to configure the unit correctly, but after it was configured, the unit, as well as its metering and monitoring system worked as expected. Over the last 12-month period, maintenance requirements for this unit were similar to the other units.