Adaptive HVAC Comfort Concept for a Hot and Humid Climate

Worldwide, today’s buildings require massive energy input for air conditioning in terms of heating, cooling and de-/humidification, including auxiliary energy demand for the mechanical system itself. The way we define “comfortable” thermal conditions plays a significant role in this environmental impact. Adaptive comfort models, developed on the basis of field studies in tropical and subtropical climates, give evidence that extended operative temperature and humidity ranges are acceptable for indoor climate especially when combined with elevated air speed controlled by the occupants. Adaptive comfort can typically be achieved with fewer mechanical systems. In this context, the present work was embarked upon with the overall objective of developing a comfort calculation methodology that evaluates thermal comfort on the basis of dynamical thermal building simulation, considering the effect of air movement under hot and humid thermal conditions. Theoretical analysis and comparison study of thermal comfort indices are presented. The Standard Effective Temperature (SET) and the Predicted Mean Vote for elevated air speed (PMVeas) according to the ASHRAE Standard 55-2013 are proposed as preferred indices for evaluating the effect of air movement in a tropical climate. A calculation method is developed, implementing SET and PMVeas, and variably adapting their input parameters for thermal comfort regulation. The dynamical building simulation program TRNSYS 17 3D in combination with the developed calculation method is used for elaborating comfort design strategies for a generic classroom model in the hot and humid summer climate of Dhaka, Bangladesh. The designs are compared with regard to achieved comfort, required systems and energy demand. Following this, an adaptive Heating, Ventilation and Air Conditioning (adaptive HVAC) design is proposed as an alternative strategy to deterministic HVAC control systems with a static “comfort zone”. In this context, ‘adaptive’ does not imply changing people’s comfort expectations. On the contrary it means calculating environmental parameters, especially the air speed, by a new evaluation method to create comfort. The adaptive HVAC comfort design provides fresh air quality and good thermal comfort: A simple decentralized mechanical system supplies the room with 20°C air temperature, while dehumidifying the air to 20°C dew point temperature. In addition, ceiling fans create an average air movement at 0.7 m/s. Compared to the conventional concept of returned air with full heat recovery, aiming at 26°C operative room temperature, the energy demand is only about 75%. Both approaches provide the same comfort according to PMV for elevated air speed. This means that 25% energy savings can be achieved without compromising the thermal comfort of occupants.