4.2. Inverter Control of the Convertible Frequency Air Conditioner
The convertible frequency air conditioner is the main air conditioning product for energy conservation. It is defined as the situation where the motor rotation speed could be adjusted continuously for regulating the refrigerant energy output by changing the input electric power frequency
Comparing with the fixed frequency control, the convertible frequency type could first rectify the 60 Hz electricity into the direct current type, and then modulate the frequency of output electricity continuously by pulse width modulation (PWM). According to the feedback temperature difference, the air conditioner could provide the air flow of stable temperature to control the temperature of indoor space by adjusting the refrigerator flow.
5. SMART SENSORS
Smart sensors, including mobile phones, wearable devices and other sensors, are introduced. They are the key elements of smart control for obtaining the human’s intention. Mobile phones would provide occupants’ information by connecting to the GPS and personal schedule for collecting the position and intentions. The air conditioner could cool down the indoor temperature rapidly before the occupants enter.
The wearable devices and their applications are predicted to have an exploding increase in the coming future. Wearable devices, such as watches or bracelets, may be adopted for detecting the human sleeping state as the feedback signals of the sleeping function. Figure 10a,b present bracelet with a digital accelerator (Kionix IC type) developed in our lab. The accelerator could detect the acceleration between 10 and 10?6 g, the velocity between 10 and 1.67 ×10?9 m/s, and the displacement between 10 and 2.78 ×10?13 m. It can collect human motion information and feedback to the smart air conditioner for further control.
5.1. Smart Control Based on Smart Sensors
Including the comparison of the fixed and convertible frequency control schemes, the quantitative analysis of the smart control based on the smart sensors is the main target in this paper. Smart control, based on the information collected by the use of mobile phones and wearable devices, intensifies the interaction with occupants and carries out the intention causing control. It may include the following aspects: (1) Mobile phones with GPS and personal schedules, for detecting the occupants’ position and intentions, could foresee the occupants’ intention of entering the enclosed space. t this moment, the compressor, which is off in the general situation, could turn on in the full power. Before entering, the circulating fan turns on at the highest speed, and the air deflector swings for 10 min to enhance the air circulation. Therefore, smart control may enable the enclosed space could be cooled down rapidly after the occupant enters. (2) The bracelet with the accelerator could detect the movement of occupants while the sleeping. After the occupant falls into a deep sleep, the air conditioner would lift the indoor temperature flexibly to avoid energy consumption. The smart air conditioner could adjust the compressor output actively according to the occupants’ active intention (going home) and passive one (falling into a deep sleep) for the goals of human comfort and energy conservation. The smart air conditioner includes the following devices:
(1) Air conditioner with adjustable refrigeration power;
(2) Novel sensors capable of interacting with occupants;
(3) Communicating units for mobile phone and network.
Figure shows the controlling structure of smart air conditioner, including air conditioner, temperature sensor, IR detector, mobile phone and wearable devices. The control structure of the smart air conditioner in Figure utilizes a multi-sensor system to achieve smart control. The indoor infrared sensor can detect humans’ position and carry out the air flow direction control. Mobile phones with GPS and personal schedules can be used to detect the occupants’ position and intention. Wearable devices can be the bracelet with the accelerator. It detects the movement of occupants while sleeping. After the occupant fallings into a deep sleep, the smart air conditioner would uplift the indoor temperature flexibly to avoid energy consumption.
Figure- The control scheme of a smart air conditioner with Inverter control: different from the fixed and convertible frequency air conditioner with difference causing control, it’s an intention causing control that adjusts the compressor output actively according to the occupants’ active intention (going home) and passive one (falling in a deep sleep) for the goals of human comfort and energy conservation.
5.2. Types of smart sensors integrated with the buildings automation system
Like other types of smart building technology, smart HVAC uses sensors that integrate with your building automation system. These sensors collect data about the conditions throughout your building. Other specialized HVAC equipment provides the ability to fine-tune temperature, humidity, and air flow in various zones (based on data from the sensors) to optimize comfort while reducing energy consumption.
Strategically-placed thermal sensors can detect the differences in conditions in each zone of your space. For example, a crowded conference room can get warm in a hurry, while an open office area with high ceilings can get chilly (since warm air rises and people are closer to the floor). A smart HVAC system uses that data to adjust to changing conditions throughout the day or week.
According to a recent study by Harvard School of Public Health, high CO2 levels in a building can have a direct negative impact on thinking and decision making. CO2 sensors can detect the levels of CO2 gas in a space, which can increase to undesirable levels as occupancy increases. When the threshold is reached, a smart HVAC system can increase levels of fresh air supplied to the space. This technology can have a significant impact on workforce wellbeing.
Occupancy sensors are useful for office environments (like most) that don’t have uniform usage all the time. Increasingly mobile workers are leaving desks and conference rooms empty as much as 50 to 60 percent of the time. Meanwhile, you’re heating and cooling space for people who are not there. Occupancy sensors detect the presence of people (typically by detecting motion) currently using individual spaces within an office. That data can be used to adjust temperatures based on real-time utilization, saving you money on energy consumption.
While your HVAC system consumes anywhere from 40 to 70 percent of your building’s energy usage, electricity for lighting is also a huge expense. That figure can be 25 percent or more. In addition to controlling a smart HVAC system, occupancy sensors also control lighting to further reduce lighting costs.
Today’s modern office spaces are being designed to let in more natural light. However, the variation in daylight from morning until evening, and from one part of the building to another, can wreak havoc on the operation of your HVAC system. As a result, sunny spaces wind up too hot while areas with less natural light can become too cold.
6. EXPECTED GOALS AND RESULTS
The expected goals of smart control for air conditioner include human comfort and energy conservation. They could be evaluated by:
(1) Human comfort represented by temperature response: the time to reach the indoor temperature of the setting value, oscillating amplitude of temperature after the steady state and the level of temperature shift.
(2) Energy conservation calculated by compressor energy: a smart socket is used for measuring compressor output during two experiment cases. For On-Off control of the fixed frequency air conditioner, the results of indoor temperature response and compressor output from the beginning to the steady state of 28 °C are shown in Figure 1.
Figure 1- The results of indoor temperature response and compressor output for the fixed frequency air conditioner with On-Off control.
In Figure 1, the solid line is the indoor temperature response, and the dotted line is the compressor output. When reaching the setting value of 28 °C, the compressor will be turned off/on to adjust the refrigerant. This will result in the compressor output ratio oscillating between 0 and 100%, and the indoor temperature also oscillating between ±0.6 °C. It is also noted that the indoor temperature increases from 27.4 °C to 29 °C gradually, due to the error of the temperature sensor feedback being within ±0.2 °C. In Figure 15, the accumulated errors of temperature feedback make the switching timing of the compressor occur later, and the indoor temperature also becomes higher subsequently. Therefore, the On-Off control obviously has problems resulting from feedback errors, especially the accumulated error.
For Inverter control of convertible frequency air conditioner, the results of the indoor temperature response and the compressor output from the beginning to the steady state of 28 °C are shown in Figure-2.
Figure 2- The results of indoor temperature response and compressor output for the convertible frequency air conditioner with Inverter control.
In Figure-2, the solid line is the indoor temperature response, and the dotted line is the compressor output. The compressor output decreases from 100% to a stable value of 12%. The indoor temperature keeps at 28 °C, with the error less than ±0.1 °C. Inverter control presents a more stable response of indoor temperature and compressor output than the On-Off one. However, the control output still depends on the error of temperature feedback. For the Inverter control, the PID algorithm in Equation (17) times the errors with a constant, differentiates the errors by time and accumulates the errors. All the complicated calculations take a longer time for the air conditioner to adjust the refrigerant, and the indoor temperature may reach the set value later, after 8 min.
From the theoretical analysis and experimental results, the design of a smart air conditioner with mobile phones and wearable devices could be carried out the intention causing control as the a significant improvement of air conditioner technology. There are some conclusions and recommendations for the smart air conditioners as follows:
(1) The total compressor output of a smart air conditioner is 48.4% less than the fixed frequency one. Indoor temperature can be controlled accurately with errors less than 0.1 °C. Rapid cool down can be achieved in 2 min to the optimized indoor capacity after occupants enter.
(2) The intention causing control of smart air conditioner could be practiced by combing with the GPS, personal schedule and setting information of the mobile phone for the optimized setting of compressor output.
(3) The smart air conditioner with wearable devices could detect the human temperature and action during sleep for determining the sleeping state and adjusting the sleeping function flexibly. The sleeping function optimized by the smart air conditioner with wearable devices could reduce the energy consumption up to 46.9% and maintain the human health. Based on these results, the smart air conditioner with mobile phones and wearable devices could carried out the intention causing control as a significant improvement of air conditioner technology, and be improved for human comfort and energy conservation in the coming future.
Acknowledgment the authors would like to acknowledge the cooperation of the project of ?Research and experience center of smart air conditioner? and related information provided by aiwan Hitachi Company. Author Contributions Chin-Chi Cheng coordinated the experimental work, analyzed the experimental data, and contributed to the writing and organization of the manuscript. Dasheng Lee proposed the research topic and the intension causing control approach, prepared the initial draft of the manuscript and supervised the study. Conflicts of Interest The authors declare no conflict of interest. Sensors 2014, 14 11203