Mixed-mode office buildings: energy savings and illuminance levels in a high-altitude tropical climate

Mixed-mode buildings (MM), which use both natural ventilation and mechanical cooling to provide thermal comfort and energy savings, have a high potential in climates where it is favorable to use natural ventilation (e.g., Brazil). In these buildings, operable windows are the source of natural ventilation, lighting and heat gains, and their characteristics can influence energy consumption. This work aims to evaluate the energy and luminous performance of mixed-mode office buildings with operable windows in a high-altitude tropical climate in Brazil. The performance is evaluated concerning energy savings, air-conditioning operation hours, the period of use of natural ventilation and balance between energy demand and illuminance levels. It is considered variations in envelope parameters (window-to-wall ratio and solar orientation). The method is composed of computer simulations on Daysim and EnergyPlus programs. The results showed that the mixed-mode strategy has great potential to reduce air-conditioning energy consumption. The most significant reductions occurred with larger WWRs, reaching 39.4% of energy savings (north facade, WWR = 80%). Nevertheless, while the larger WWR provided more natural ventilation availability, it increased the discomfort by high illuminance, which reached about 90% of the office’s area. Therefore, when the mixed-mode system was used, the cases with a better balance between adequate illuminance levels and low energy consumption were the south facade with all WWRs and the east and west facades with WWR of 30%. It is important to emphasize that the adoption of these solutions depends on the building's design requirements