scope:

Although often garnering scant attention, commercial unitary HVAC systems, such as rooftop air conditioners, are estimated to consume 0.88 quads of energy annually, or about 46% of commercial building cooling site energy consumption, and are used to cool over 60% of all commercial space in the U.S. The as-installed energy efficiency of unitary systems can be half that of central systems, and the efficiency gap widens as systems age due to maintainability issues. When tuning systems, energy engineers and service technicians use indirect indicators of equipment performance and make adjustments according to manufacturer guidelines and standard field practice, which varies with their level of experience. Growing numbers of unitary systems combined with shrinking budgets result in deferred maintenance, and long-term operation of equipment at degraded levels. Energy efficiency is a metric that must be measured to be optimized. This paper reports on field testing of continuous sensing of operating energy efficiency to control unitary equipment operating parameters, provide remote fault detection diagnostics, and support maintainability. Optimization systems were installed on package units at three sites in diverse climate locations: Cape Canaveral, FL; Mojave Desert, CA; and Beaufort, SC. The systems utilize a relational control strategy to continuously maximize the ratio of cooling delivered versus power consumed as operating conditions vary over a day and across seasons, and as components degrade over time. Condenser fan speed, supply airflow, evaporator temperature, outside airflow, and refrigerant charge were continuously adjusted by the system to maintain a state of optimized operation. The systems successfully detected and attempted to compensate for faults such as low refrigerant charge or condenser coil fouling, and reported operating EER, pressures, temperatures, and efficiency degradation to service technicians in an actionable way. Analysis of resulting data from the field tests shows considerable unitary energy efficiency gains and maintenance improvements can be obtained cost effectively.