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In the work of Ref. [8], a survey was conducted on thermal comfort of various buildings with conventional fixed-reference temperature types of air-conditioning systems from different regions of Thailand. The appealing analysis reported that the reference temperatures in most buildings were set at lower temperatures, which were indicated by sensation votes of thermal comfort from slightly cool to cool. In hot and humid climates, controlling the room temperature to be lower than the indoor comfort temperature causes not only less satisfaction of thermal comfort but also air-conditioning systems consume much more energy. It was also estimated that the percentage of energy saving is about 6% with 1°C increase of the reference temperature. A similar trend of reduction in energy consumption was obtained via analysis of the adaptive comfort temperature to air-conditioned building in Hong Kong [9]. In applied work, the monthly thermostat setting scheme was recommended by the thermal comfort chart under the climate conditions of Riyadh [10]. It was conveyed that the emphasis on manual adjustments of reference temperature encouraged occupants to be aware of the energy-saving measures in buildings with acceptability of the thermal comfort. However, the monthly manual setting of the reference might not be convenient and effective in real-time control implementation of adaptive comfort temperature.
Recently, there have been many analytical attempts in research to develop an adjustable reference temperature for an air-conditioning system according to thermally environmental changes, so that the thermal comfort condition can be satisfied by most of the occupants all the time [11-15]. Those proposed adaptive thermal comfort models based on field studies allow feasibility of real adaptable behavior of occupants on usage of clothing, activity and even available climate control in altering their thermal environment. In this work, both theoretical analysis and particularly experimental study are used to demonstrate a practical real-time determination and computerized implementation of such adaptive indoor comfort temperature, via an embedded system for a typical air-conditioning control system under dynamic environment.
2. Experimental setup and field studies
In implementing a prototype of an embedded system for an adaptive indoor comfort temperature, many high-efficient microcontrollers available in the market can be deployed to perform real-time executables in fast and accurate computations for digital I/O control of an air-conditioning unit. Fig. 2 shows a schematic diagram of an embedded system for controlling indoor comfort temperature via an air-conditioning unit.
As depicted in Fig. 3, two temperature transducers of Shinko Technos Co., Ltd. are connected to thermocouples to nmeasure the indoor temperature and the outdoor temperature.
Two temperature signals are fed to an analog-to-digital converter in I/O module in order to determine the indoor comfort temperature and the corresponding status of a control signal for the relay output of the compressor according to the programmed algorithm being executed in the central processing unit. After that, the control signal is sent to an air-conditioning unit in order to alter the indoor air temperature within an air-conditioned space accordingly. In this work, the embedded FreescaleTM MPC555 is installed and interfaced to a 10.55-kW (36000-BTU) split-type air-conditioning unit of Sanjo Denki, Co.,Ltd for the purpose. For field studies, well-organized questions are asked of persons and students using services in a small 35-m2 library room of a primary school in Chachoengsao province located at latitude of 13.78° and longitude of 101.34°. An example of the questionnaire is in the Appendix. It is filled out with necessary information on thermal comfort, together with date and time from a time-recording machine. Two data loggers 175-T1 of Testo. Co. Ltd., are used for data acquisition on the temperature of the indoor air, the outdoor temperature, and the corresponding date, as well as time.