Free cooling leverages outdoor air or water sources to remove heat from buildings when external temperatures are lower than indoor conditions, eliminating or reducing mechanical cooling operation. This passive cooling strategy significantly reduces air conditioning energy consumption during mild weather periods. Free cooling can reduce annual cooling energy use by 30-60% depending on climate and building operating schedules.
Implementation Methods
Air-side economizers use outdoor dampers to introduce cool outdoor air into buildings when temperatures drop below setpoint (typically 55°F). Waterside economizers bypass chiller operation during cool weather and use cooling towers or ground water sources directly for building cooling. Hybrid systems combine both strategies to maximize free cooling hours while maintaining humidity control between 30-60% relative humidity.
Operational Constraints
Free cooling requires robust humidity control because introducing cool outdoor air without dehumidification can cause comfort problems and mold growth. Outdoor air quality must meet filtration standards to prevent contaminant introduction. Control systems must prevent short-cycling between cooling tower and chiller operation, which reduces efficiency. Buildings in climates with more than 2,500 annual cooling degree days realize maximum free cooling benefits.