Coupled Optimization of Non-uniform Air Distributions and Air Conditioning System

Description

Buildings consume over 40% of end-use energy worldwide and this percentage is much higher in Hong Kong. Air conditioning systems account for around half of the energy use in Hong Kong buildings. Non-uniform collective air distributions (e.g., displacement ventilation, stratum ventilation, etc.) have recently been developed for energy saving. These collective air distributions focus on the occupied zone in rooms in order to minimize the energy used to condition the unoccupied zone. However, the non-uniformity of collective air distributions is challenging because building management systems usually only monitor the conditions of supply and exit air, with little information on the non-uniformity in rooms. On the other hand, elevated room temperatures are recommended by governments and standards for energy saving. Room air velocities have to be elevated simultaneously for thermal comfort, but building management systems do not provide information on room air velocity. These deficiencies could result in poor indoor air quality, thermal discomfort and energy wastage in the operation of collective air distributions. Physics-based models and mathematical/statistical models can predict the non-uniformity in rooms. Nevertheless, the physics-based models require high computational power. All these models need inputs of wall temperatures/heat fluxes and thereby more sensors which increase the complexity of the sensor network and cost. Furthermore, the locations of sensors could significantly affect room modulation quality due to non-uniformity. The indoor environment is determined by the supply air condition and the internal and ambient conditions. Similar to the wall temperatures/heat fluxes that are presently used as inputs, the exit air condition also results from the aforementioned conditions. Both supply and exit air conditions are readily monitored and/or controlled by the building management systems. Therefore, this proposal originates the idea of modeling the non-uniform indoor environments with conditions of supply and exit air as inputs, for modulation of collective air distributions. This approach is feasible because the necessary data are readily available in building management systems. Mathematical/statistical modelling techniques will be employed with the merits of a low computational load for operation control. Because energy consumed by a desired indoor environment can vary over a wide range, the energy performance of the air conditioning system will be optimized coupled with the desired indoor environment. With this coupled operation optimization, the indoor environment of the collective air distributions could be modulated explicitly and reliably by the current building management systems, without retrofitting the sensor network. Because the correlations depend on the positions in a room, they could satisfy individual occupants’ preferences on the environment. As a result, the desired indoor air quality and thermal comfort with substantial energy saving could be expected for practical operations of air conditioning systems with collective air distributions. Explorative tests have found this new approach promising. The proposed study will be carried out in our unique environmental chamber which conveniently allows for configurations of offices and classrooms with different ventilation strategies to be tested. Stratum ventilation provides typical non-uniform indoor environments with elevated room temperatures, velocities, and energy efficiency. Hence, it will be tested first. Objective measurements, subjective surveys and validated CFD simulations will be conducted to collect data for the development and validation of the mathematical/statistical models. With these models, a dynamic control algorithm will be developed for operation control. The effectiveness of the coupled operation optimization for real-time operation will be demonstrated. Finally, a set of guidelines will be compiled for the application of this optimization for the desired indoor environment under collective air distributions, and for optimal energy efficiency of the air conditioning system.Note: Only the essential information of the project is presented in this proposal due to the limit in length.