Outside air economizers and fan speed control comprise significant energy saving strategies. Adding multiple cooling stages and multi-speed fans improves savings. These strategies can be applied to unitary air conditioners or rooftop units (RTU) that serve more than 40% of commercial floor area. The leading energy simulation programs do not accurately model economizer operation and fan control in RTUs. Typically, they treat economizer benefits for RTUs with direct expansion coils the same as for hydronic coils that have modulating output. Current modeling software simulates only one economizer mode during each time step, when in fact RTUs have two modes during integrated economizer cooling: (1) a mode with direct expansion cooling combined with economizer operation, and (2) a mode with only economizer operation. Single-stage direct expansion coils require that economizer integration be reduced in some conditions to avoid comfort problems or coil freezing. The needed reduction reduces the economizer benefit below what is simulated.
Fan control options in energy simulation programs are generally limited to a variable speed drive with airflow proportional to load or intermittent fan operation. Advanced fan control strategies for RTUs are typically multi-speed rather than variable speed and speed is selected based on stages of cooling or heating operating or ventilation-only mode. This control fan strategy saves significant energy. Proper modeling needs to account for multimode fan operation in each time step.
A method to adapt the building simulation program EnergyPlus(TM) using the Energy Management System feature to more accurately simulate multi-speed fans and outside air economizer operation with single- and two-stage direct expansion cooling is presented. The impact on energy saving predictions is shown for a stand-alone retail building in four climate zones. Until simulation programs are upgraded to better reflect actual RTU operation, the proposed method can simulate subtle improvements in economizer and fan controls.
Product Details
- Published:
- 2013
- Number of Pages:
- 8
- File Size:
- 1 file , 1.7 MB
- Product Code(s):
- D-DE-13-C058
- Note:
- This product is unavailable in Russia, Belarus
