Xiangguo Xu

Steady-state operating performance modelling and prediction for a direct expansion air conditioning system using artificial neural network:

A two-in two-out steady-state artificial neural network (ANN)-based model for an experimental variable speed direct expansion (DX) air conditioning (A/C) system has been developed for simulating its total output cooling capacity and equipment sensible heat ratio under different combinations of compressor and supply fan speeds. Experiments were carried out, and totally 169 sets of experimental data were obtained for ANN training and testing. An ANN-based model having the configuration of 2 neurons in the input layer, 2 neurons in the output layer and 6 neurons in each of the 2 hidden layers, i.e. 2-6-6-2 configuration, was thus developed. The ANN-based model developed can be used to predict the operating performance of the DX A/C system with a higher accuracy. It is expected that the model developed can help design a multivariable-input multivariable-output strategy to simultaneously control indoor air temperature and humidity. Practical applications: The work reported in this paper demonstrates that the operating performance of a DX A/C system under different compressor and supply fan speeds can well be represented using ANN model technique, as an alternative to traditional physical-based modelling techniques, with a higher predicting accuracy and less computational effort. The ANN-based model for the DX A/C system is expected to be very useful in developing an efficient control algorithm for simultaneous indoor temperature and humidity using the ANN technique.

Thermal comfort in an office with intermittent air-conditioning operation

An effective way to reduce A/C energy use is to switch off air conditioning during unoccupied periods. The main concern is, however, whether occupants’ thermal comfort is sacrificed. This paper reports a study of the impacts of switching off A/C during unoccupied hours in an office on thermal comfort of occupant after returning to office and resuming the operation of A/C system. Site measurements for both indoor thermal environmental parameters and energy consumption of A/C were carried out and the thermal comfort of occupants was assessed. Measurement and assessment results indicated that up to 11% of annual energy consumption of A/C for the office could be saved, but the level of thermal comfort of occupants was not greatly degraded. Practical applications: Intermittent operation of A/C is often ignored when people talk about energy saving. It requires no additional cost and equipment, but the saving is significant. Moreover, no advanced technology or knowledge is required. The most important result is that thermal comfort level is not scarified during occupied hours.

A New Viewpoint to Studying the Condensate Retention and Drainage Process on Air Cooling Coil

Normally simultaneous air cooling and dehumidification takes place in an air cooling coil. The condensate accumulated on the cooling coil surfaces could significantly affect its thermal and hydraulic performance. The condensate retention and drainage has bee usually regarded as a static process. The total mass of condensate retained was viewed as being influenced by the balance among gravitational, air flow drag forces and surface tension retaining force only. With fixed geometric and surface characteristics and under a constant air Reynolds Number, a cooling coil was therefore considered to have a fixed amount of condensate retained. However, some researches pointed out that the total mass of condensate retained could be influenced by system operating parameters such as temperature and moisture content of inlet air. A previously related study by authors developed a mathematic model and considered this process as being a dynamic but without giving a detailed discussion. In this paper, through a further analysis of data obtained in the previously study, a new viewpoint on condensate retention and drainage process has been suggested in order to better explain the experimental results and to provide a base for proposing some future related research work.Copyright

Residential indoor humidity control in tropics and sub-tropics:

The use of direct expansion (DX) air conditioning (A/C) systems is widely seen in residential buildings in tropics and sub-tropics. However, most DX A/C systems are equipped with single-speed compressor and supply fan, relying on On—Off cycling as a low-cost approach to maintain only indoor dry-bulb temperature, whereas the indoor air humidity is not controlled directly. This also leads to potential inefficient energy use due to space over-cooling. This paper suggests a novel control strategy (termed Hi—Lo strategy) of using two-speed compressors in DX A/C systems for better indoor thermal environment control and higher energy efficiency. Experimental work has been carried out to test the Hi—Lo control strategy. Experimental results suggested that the novel control strategy can help improve the indoor humidity control without sacrificing energy efficiency. Practical application: The use of ON/OFF control at part load conditions in conventional DX A/C systems will lead to undesirable relative humidity levels. The problem is more prominent when the latent load is dominant. In order to have a better dehumidification effect, a lower indoor dry bulb temperature has been often set. On the other hand, the use of variable speed compressors in DX A/C systems can be a solution to the problem of humidity control, but its cost is comparatively high. The suggested novel control strategy is a low cost approach, which can save energy and provide a better indoor humidity control using DX A/C systems.