An energy performance study of several factors in air economizers with low-limit space humidity

• We develop an energy cost models of air-side economizers.
• We identify optimize outside air zones based on low-limit space humidity and energy prices.
• The simulation results show that energy savings can reach 15 Btu/h per CFM supply airflow.

Most air handling units in commercial buildings adopt an air-side economizer for free cooling when the outside air is cool. Since the outside air may be dry during the economizer operations, humidification is needed for special applications where stringent space humidity controls are required. Therefore, the economizer saves cooling energy but may lose space humidity control or require additional energy for humidification. A humidification energy penalty or loss of space humidity control may prevent or reduce air-side economizer usage. The barrier comes from the lack of systematic energy analysis on an economizer operation with a humidification process. In this paper, optimal economizer operation zones are simulated to minimize the total cost of mechanical cooling and humidification and an experiment is conducted to validate the energy savings. The results reveal that the significant energy benefits (up to 9.3 kW/(m3/s) or15 Btu/h-CFM) exist by using air economizers while the space low-limit space humidity is maintained. However, the energy benefits heavily depend on humidification cost and required low-limit space humidity. For the low-limit space relative humidity of 30% in a moderate weather, the energy performance of regular economizer excels non-economizer when the cooling price is twice of the humidification price or lower.