INTRODUCTIONAdvanced packaged rooftop units (RTUs) are emerging with higher efficiencies and additional features designed to save energy off-peak and improve reliability. Features include improved fans and economizers; better controls for the fan, refrigerant cycle, and economizer; and advanced monitoring and diagnostics. Projected package costs are relatively high, but should come down with greater acceptance of the feature set.
BACKGROUND AND DESCRIPTIONRooftop packaged air conditioners are commodity air cooling and handling equipment, often including gas fired or electric air heating equipment. They account for about 0.74 quads of energy annually, or around 54 percent of commercial building cooling primary energy consumption, and are used to cool about half of all commercial space. These products are often selected based on their initial cost, as most buyers are builders and developers who are more interested in minimum purchase price, rather than the occupants who have to pay the operating costs.
EER, or energy efficiency ratio, is a measure of the cooling efficiency of the equipment at full load (measured at 95°F dry bulb). As the EER only measures the full load efficiency of the equipment in the lab, it may not be a very accurate reflection of the equipment’s energy efficiency when it is run at part load in the field. Also, EER does not fully reflect the potential energy efficiency benefits that can be achieved by advanced rooftop packaged air conditioners through part-load measures such as effective economizers and dampers, demand controlled ventilation, variable speed fans, ventilation lockout during the startup, and evaporative pre-cooling of the condenser unit.
CURRENT STATUSOver the years, the efficiency of rooftop packaged air conditioners has steadily improved. Manufacturers are continuing to develop advanced RTUs with improved energy efficiency and incorporating new features like economizers and advanced sensors. As part of the California Energy Commission’s Public Interest Energy Research (PIER) program, Architectural Energy Corporation (AEC) prepared a document describing the features that should be incorporated into an advanced rooftop packaged air conditioner. These features include:
• Economizer improvements;
• Fan improvements;
• Unit efficiency;
• Refrigeration cycle;
• Fan control;
• Refrigerant control;
• Thermostat capability;
• Installation and check-out capabilities;
• Advanced monitoring; and
• Advanced diagnostics.
The Consortium for Energy Efficiency is currently exploring the possible development of a program framework for a voluntary initiative to promote awareness of advanced rooftop packaged air conditioner features, based on the specifications in the PIER program. This would be accomplished through utility-run consumer education and incentive programs.
SAVINGS POTENTIAL AND COST-EFFECTIVENESSA cost-benefit assessment was conducted by AEC as part of the PIER project. AEC estimated that a 5-ton advanced rooftop unit incorporating 36 features to improve operational performance, maintenance/serviceability, and reliability, and providing monitoring and diagnostics, would have an installed cost of about $9,800, about 72 percent more than the estimated installed cost of a baseline unit ($5,700). The report also estimates that the features would result in annual energy savings of between $300 and $530, based on conditions in several California cities, as well as non-energy benefits of around $300 yearly. This results in a simple payback period of five to seven years for the advanced rooftop unit features.
MARKET BARRIERSThe cost premium for an advanced rooftop air conditioner is relatively high. In the 5-ton example, the installed cost was 72 percent higher for the advanced unit. This can be overcome with incentives, and may be attractive to energy service companies (ESCOs) and similar programs in areas with high utility costs, including high demand charges.
However, widespread adoption of the advanced RTU is most likely to be seen where owners, contractors, and other service vendors are receptive to new business models. Currently, RTU service is based on a scheduled check-up basis (change filters and check units twice yearly, for example). Advanced units with on-board diagnostics will allow an as-needed service model, in which technicians are dispatched when the unit sends notice that it requires service, whether more or less frequently than would have been scheduled. Adopting such a model will involve some risk.
NEXT STEPSNow that a coherent and realistic specification is available, the next steps include raising visibility, which is being undertaken by organizations like the Western Cooling Energy Center. When national account-scale enterprises understand the benefits of the advanced RTU, they will be ready for trials. Success will open opportunities for coordinated rebates for these products, such as a Consortium for Energy Efficiency program.