Inherent operational characteristics aided fuzzy logic controller for a variable speed direct expansion air conditioning system for simultaneous indoor air temperature and humidity control
Introduction
Variation in indoor air humidity has always been regarded as one of the factors that influence indoor thermal comfort, which is particularly true in hot and humidity regions. Although ASHARE Standard [1] recommends a wide comfort range of relative humidity (RH) from 40% to 60%, at RH level above 60% adversely affects indoor thermal comfort and indoor air quality significantly. However, when a DX A/C system is used for indoor thermal environment control, the complex dynamic characteristics of heat and mass transfer taking place in a DX evaporator and the coupling of air temperature and humidity make it very difficult to simultaneously control both indoor air temperature and humidity using a DX A/C system.
It has been however demonstrated that when a DX A/C system is used, the simultaneous control of indoor air temperature and RH could be achieved by simultaneously varying its compressor speed (Cs) and fan speed (Fs) [2], taking the advantage of variable speed (VS) technology. Hence, a number of multi-variable controllers (MVCs) to simultaneously control indoor air temperature and humidity using VS DX A/C systems have been developed [3], [4], [5], [6]. However, the development of MVCs are usually complicated and mathematical modeling supports are usually required. The cost of development is thus higher. Alternate simpler and low-cost controllers should hence be sought. Compared to MVCs, a simpler alternative is to use a fuzzy logic controller (FLC), which has become popular for controlling the operation of building HVAC systems [7], [8], because of its characteristics of capturing the approximate and inexact nature of a controlled process, such as human thermal comfort [7], [9], [10], [11], [12], [13], [14]. FLC may be combined with PID control strategy [15], [16], [17] and neuro network [18] for HVAC applications. However, indoor air humidity was usually assumed unchanged or left uncontrolled in previously developed FLCs [7], [8], [9], [12], [13], [15], [16], [19], [20], [21], [22], [23], [24].
On the other hand, the inherent operational characteristics of a VS DX A/C system, relating its output total cooling capacity (Qt,E) to its output sensible heat ratio (SHRE) at different Cs and Fs combinations were extensively studied [25], [26], [27], [28], and may be used to help develop such a simple and low-cost controller.
The purpose of this paper is to develop a FLC for a VS DX A/C system to simultaneously control indoor air temperature and humidity, based on the known inherent operational characteristics of the VS DX A/C system [26], [28]. The organization of the paper is as follows. Firstly, the development of the FLC is presented in Section 2. An experimental VS DX A/C system to be controlled by the FLC is reported in Section 3. This is followed by presenting the results of controllability tests for the FLC in Section 4. Finally, conclusions are given.
Section snippets
FLC for a VS DX A/C system to simultaneously control indoor air temperature and humidity
In this section, the development of a simpler and low-cost FLC as compared to MVCs for a VS DX A/C system to simultaneously control indoor air dry-bulb temperature (Tdb) and wet-bulb temperature (Twb), based on its inherent operational characteristics, is presented.
Experimental DX A/C system
The proposed FLC was tested using an experimental VS DX A/C system, whose inherent operational characteristics at 25 °C and 50% RH are shown in Fig. 2.
As shown schematically in Fig. 5, the experimental DX A/C system was composed of two parts, i.e., a DX refrigeration plant (refrigerant side) and an air distribution sub-system (air side). The major components in the DX refrigeration plant included a VS rotor compressor, a PI controlled electronic expansion valve, a high-efficiency
Experimental controllability test results and discussions
Using the experimental DX A/C system and following the experimental conditions listed in Table 3, seven tests to examine the controllability of the FLC developed were carried out. In this section, only the test results for Tests 0-1, 1-1, 2-1 and 3-1 are presented, but the results for all the seven tests are summarized in Table 4. During the operation of the DX A/C system, the developed FLC was activated every 100 s to avoid frequent changes in both Cs and Fs.
In Fig. 6, the variations profiles
Conclusion
In this paper, the development of a FLC for a VS DX A/C system based on its inherent operational characteristics for simultaneous indoor air temperature and humidity control is reported. The control algorithm for the FLC are detailed and its controllability test results presented. The controllability test results demonstrated that the FLC was capable of realizing the simultaneous control of indoor air temperature and humidity, with a reasonable control accuracy and sensitivity. The FLC reported
Acknowledgements
The authors would like to acknowledge the financial supports from The Hong Kong Polytechnic University (PolyU G-YBDB) and Zhejiang Provincial Natural Science Foundation of China (Grant No. Y17E060002).
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