Scientists at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) found that super-efficient rooftop units reduce energy costs an average of about 41 per cent compared to units in operation today. The newly published report (including a link for builders and others to simulate tests of their own buildings) analyzes the operation of the commercial rooftop HVAC unit known as the Daikin Rebel, which was one of two units to meet DOE’s Rooftop Challenge, a competition for manufacturers to create a rooftop unit that significantly exceeds existing DOE manufacturing standards. Daikin Applied’s unit was studied in depth by PNNL researchers; Carrier Corp. also met the challenge. The PNNL team estimates that if current rooftop units were replaced with devices similar to the Rebel over a 10-year period, the benefits in terms of energy saved and reduced pollution would be about equal to taking 700,000 cars off the road each year. If all rooftop units with a cooling capacity of 10 to 20 tons were replaced immediately, DOE officials estimate the cost savings at around $1 billion annually.
Scientists Srinivas Katipamula and Weimin Wang ran simulations for a typical 75,000-square-foot big-box store in Chicago, Houston, and Los Angeles. They compared performance of the Rebel to three types of units: those in use today, those that meet current federal regulations for new units, and those that meet more stringent requirements, known as ASHRAE 90.1-2010 standards. DOE designed the Rooftop Challenge to exceed the ASHRAE standards.
• Compared to units in operation today that are ready for replacement, energy costs were 33 per cent less in Chicago, 44 per cent less in Houston, and 45 per cent less in Los Angeles. The Rebel slashed energy demand by 15 per cent, 37 per cent, and 36 per cent, respectively.
• Compared to new units that meet current federal regulations, costs were cut 29 per cent, 37 per cent, and 40 per cent, in Chicago, Houston, and Los Angeles, respectively. Likewise, energy demand was reduced 12 per cent, 30 per cent, and 32 per cent in those three cities.
• As expected, savings were a bit less when compared with new units that meet today’s strictest ASHRAE standards. Costs to run the Rebel system were 15 per cent lower in Chicago, 27 per cent lower in Houston, and 18 per cent lower in Los Angeles. Energy demand was eight per cent, 23 per cent, and 15 per cent lower, respectively.
While the cost of the unit was not part of the team’s analysis, Katipamula estimates it would take at least a few years for the latest technology to pay back the increased investment in the newer units. The team’s analysis did not include a look at some of the unit’s additional features, such as its potential to save energy used for heating.
“The savings depend very much on the particular conditions — the climate, the size of the store, the materials used in the construction, and so on,” said Katipamula. “We’ve developed Energy Plus software models that allow building designers or owners to calculate for themselves the cost-effectiveness of installing a newer unit that meets the DOE rooftop challenge.”
