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Top1. Introduction
The Heating, ventilation, and air conditioning (HVAC) system provides thermal comfort and acceptable indoor air quality. It is the single largest contributor to a home’s energy bills, accounting for 43% of residential energy consumption in the U.S., and 61% in Canada and the U.K., which have colder climates (E. P. B. S. Energy 1997; Rathouse & Young 2004). Studies have shown that around 25% of this energy could be saved by turning off HVAC system in residential/commercial buildings when nobody is inside (I. E. Center 2010). An 20%-30% reduction in HVAC energy translates to the savings of about $15 per month for the average households in the U.S. In order to improve HVAC systems and save energy at the same time, several communication protocols among HVAC units have been developed, such as Building Automation Control Network (BACnet), Modbus, Local Operating Network (Lon Works) and Lon Talk.
To observe the impacts of sensors-HVAC communications on the energy savings, we have used real hardware to emulate two scenarios in this work: energy saving control in commercial or residential buildings. Commercial building consists of multiple single offices and classrooms, while residential buildings refer to individual houses. Inside each room, different occupancy sensors are used to count people. The sensors include but not limited to: CO2 sensors, PIR sensors, image sensors, and accumulated sensors. The communications between sensors can be built via wired or wireless links. The wireless connections could use different types of communication protocols, such as Wi-Fi, Bluetooth, etc. In contrast, wired communication can be achieved by using Ethernet or USB connection, etc.
The interactions between occupancy sensors and HVAC controllers have the essential features of a typical Cyber-Physical System (CPS): (1) Physical-to-Cyber: The physical objects (occupants in the building) need to be counted in real time in order to better control the fan/temperature levels of the HVAV systems. Such physical parameters (i.e., the number of occupants in any region/room of a building) can be captured by using cyber units (i.e., computing hardware/software). Here we use wireless microsensors to serve as cyber units. The sensors can report the physical status to a HVAC server in real time. (2) Cyber-to-Physical: The cyber units can be used to change the physical world. In this case, the sensor data will be processed to find out the distributions of occupants in different regions of a large building. Thus, we can change the physical objects based on cyber data. For example, we can tune the fan/air circulation levels of HVAC units in these regions.
The architecture of such a building CPS is illustrated in Figure 1. There could be various uses of occupancy sensors. In particular, they can improve the security levels of homes and offices. They can also save energy by efficiently controlling the room temperature, lighting and appliances. Occupancy sensors generally use a human motion detector with a timer. Some others measure CO2 level, acoustic signals, or human images.
However, the CPS emulators for occupancy-sensor-based building energy control system are not available so far. Only some pure software-based simulators without hardware units have been built. In this work, we have built two hardware-based emulators. Both uses raspberry pi wireless board to serve as a communication platform between hardware units such as PIR sensors and thermostat. Our emulation platform shows the CPS interactions with HVAC/sensor communication control within residential and commercial buildings environment.
There are three features for our emulator designs.
First, the HVAC system can utilize different types of sensors (acoustic, CO2, PIR, etc.) based on different application requirements. Moreover, the communication can be constructed for both wired and wireless links. Regarding wired sensors, our platform allows the use of both analog and digital sensors. The sensors could send data back to the server through RF (radio frequency) waves or USB/Ethernet interfaces. The RF communication methods for wireless sensors could be Bluetooth, ZigBee and WiFi, etc. In our demos, both wired and wireless sensors can use TCP/IP protocols to communicate with other units. Compared with other solutions listed in Table 1, our hardware platform is more portable and has lower cost. Additionally, both wired and wireless communication methods can be used. It also allows the server to collect occupancy data from different sensors.
Figure 1.
Overview of Communication Between HVAC System and Sensors