Integrative Investigation of Thermal Comfort and Energy Performance of Air Conditioning Systems


Student thesis: Doctoral Thesis

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Awarding Institution
Award date12 Aug 2019


Thermal comfort and energy efficiency are two critical issues for the design and operation of heating, ventilation and air conditioning (HVAC) systems due to high requirements of thermal comfort from occupants and heavy reliance on HVAC systems. Many observations and studies have identified that a trade-off always exists between these two issues, which signifies that the improvement of one may lead to the compromise of the other. Therefore, the simultaneous achievement of two goals is of great significance to the designers and managers of the building systems.

Since the issue is related to the balance between thermal comfort and energy efficiency of HVAC systems, this thesis attempts to use integrative approaches to address the trade-off from two perspectives: the static and dynamic investigation of the indoor thermal environment, and the conflation of thermal comfort simulation and building energy modelling (BEM). The comprehensive evaluation of the indoor environment is first carried out by integrating computational fluid dynamics (CFD) simulations and the wireless-sensor measurements. Based on the thorough assessment of the thermal environment, the coupling of CFD simulation and BEM is originally proposed and adopted in air conditioning (AC) systems to guarantee the thermal comfort of occupants with less building energy consumption. The dissertation mainly comprises three parts in terms of the integrated examination of thermal comfort and energy performance of AC systems.

Ⅰ. CFD simulations and wireless-sensor measurements of thermal comfort
By the definition in the international ASHRAE thermal standard, thermal comfort is that condition of mind that expresses satisfaction with the thermal environment and is assessed by subjects. The influence of the thermal environment on thermal comfort is clearly expressed within the definition. As the indoor thermal environment directly determines the thermal satisfactory of occupants, all of the environmental parameters are supposed to be considered for the evaluation of thermal comfort including air temperature, air speed, humidity and radiant temperature. Due to the limitations of CFD simulations and field measurements, the whole assessment of thermal comfort is conducted by integrating CFD simulations and wireless-sensor measurements. Based on the simulation results and sensor data, the temporal and spatial profiles of thermal comfort are assessed by PMV/PPD (predicted mean vote/predicted percentage of dissatisfied) thermal comfort model with consideration of environmental parameters and human factors.

Ⅱ. Dynamic investigations of the indoor environment
In terms of the enhancement of building energy efficiency, the intermittent operation mode is applied to an AC system, which aims to save energy consumptions as well as maintain thermal comfort of occupants. The optimal operation module is explored by means of transient CFD simulations with the dynamic supply conditions measured by wireless sensors. The changing characteristics of indoor air temperature at the occupied zone under ON/OFF mode of the AC system are estimated. According to the predicted functions of indoor air temperature, the operation periods for ON and OFF mode are determined respectively. A complete intermittent circulation involves 18 minutes of ON mode and 8 minutes of OFF mode to ensure the indoor air temperature ranging from 23.5ºC to 25ºC within the thermal comfort zone. More importantly, the energy performance of such an optimal intermittent mode presents to be superior to the traditional operation mode with an extra saving of 11% of cooling power energy.

Ⅲ. External coupling of building energy modelling and CFD simulations
The integration between building energy modelling (BEM) and CFD simulations has great potential to improve the prediction accuracy in terms of energy consumptions and thermal comfort. BEM programs can offer accurate boundary conditions for CFD programs, and in return, the airflow motion identified by CFD simulations can be output to BEM programs to optimize the energy simulation. In this study, the bidirectional external coupling of BEM and CFD simulation is performed through exchanging variables between the two programs. EnergyPlus, selected as a BEM program, sends the surface wall temperature, ambient temperature and supply air flow rates to the CFD program Fluent. While Fluent exports the mass exchange rates between interactive thermal zones to EnergyPlus. The co-simulation is implemented in a large-space office with an uneven indoor environment. The results show that the coupling of EnergyPlus and Fluent can achieve a uniform thermal environment. Moreover, the integrated simulation has a 3.5% of cooling energy saving potential compared to the independent EnergyPlus modelling.