Residential Pre-Cooling: Mechanical Cooling and Air-Side Economizers
This study used an advanced airflow, energy and humidity modeling tool to evaluate residential air-side economizers and mechanical pre-cooling strategies using the air conditioner, in all US DOE Climate Zones for a typical new home with ASHRAE Standard 62.2 compliant ventilation. A residential air-side economizer is a large supply fan used for night ventilation. Mechanical pre-cooling used the building air conditioner operating at lower than usual set before the peak demand period. The simulations were performed for a full year using one-minute time steps to allow for scheduling of ventilation systems and to account for interactions between ventilation and HVAC systems. The short time steps also allow for more precise evaluation of HVAC system cycling operation. The results showed that a residential economizer can save large quantities (more than 2000 kWh/year) of cooling energy – in some cases the energy savings offset all of the mechanical ventilation related energy use. Using a high performance Brushless Permanent Magnet (BPM) air handler in the HVAC system saved an additional 12% of cooling energy compared to more typical Permanent Split Capacitor (PSC) air handlers. However, economizers may cause problems with excess humidity in climate zones 2A and 3A (Houston and Memphis) due to increases in indoor humidity. The economizer was most effective in the hot and dry Climate Zones 2B, 3B (Phoenix, El Paso) and the marine Climate Zone 3C (San Francisco). There were less significant, but still desirable, energy savings in mixed temperature Climate Zones 4A, B & C (Baltimore, Albuquerque and Salem).
The effectiveness of pre-cooling is highly dependent on climate zone and the selected pre-cooling strategy. In this study the pre-cooling strategies included two levels of temperature depression: 22.2° and 23.3°C and pre-cooling times of 8, 5 and 3 hours. The results showed the expected tradeoff between peak savings and increased energy use. In looking to maximize peak energy savings and minimize the extra cooling energy used, our results showed that the high cooling climates (zones 1A-3B) gave the best results for the shortest time and least temperature depression. Climates with less cooling showed better results for pre-cooling with the least temperature depression. Climate Zones 3C and 8 showed no advantage with pre-cooling strategies due to near-zero air conditioning loads. One caveat with the pre-cooling recommendations is that they were for a lightweight wood frame construction and may change for heavier brick/block structures not included in this study.