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Office buildings are expensive to operate and maintain. Heating, ventilation, and air conditioning (HVAC), as well as lighting, make up 59% of commercial building energy costs. Newer buildings are typically more efficient, though—they use more modern and efficient equipment and incorporate myriad sensors that provide better feedback for energy control.
Energy usage can be cut by 40% by using the latest, more advanced HVAC and lighting controls. Thus, operating costs for older buildings can be lowered by retrofitting equipment and controls. However, the cost of rewiring is often prohibitive. That’s where wireless sensor networks (WSNs) can help; they eliminate wiring costs and provide sensors that are able to deliver significant energy savings and other benefits.
Benefits of a WSN Retrofit
Adding a WSN to an existing building can lead to a double-digit percentage decrease in operating costs over a period of years. Not only do energy costs decline significantly, but wiring costs and hassles become a thing of the past. Even without new HVAC equipment, the WSN will improve monitoring and control of environmental conditions that, in turn, leads to energy savings since equipment is only operated when and where needed. Essentially, WSNs will significantly reduce waste.
Safety and security are other benefits of a WSN. A security system is readily implemented with sensors that can detect unauthorized entry, thereby ensuring the safety of employees and their possessions. A WSN also deploys fire alarms and air-quality monitors to boost safety.
Employee comfort is another benefit of a WSN. By more closely monitoring temperature, humidity, and ventilation, environmental control helps improve comfort level depending on the number of people involved. One study indicated a 3% increase in employee productivity when optimizing the comfort level.
The Key to WSN Success
Good sensors are the secret to building an outstanding WSN monitoring and control system. Temperature and humidity monitors are essential, as is ambient light sensing. However, other factors like time of day and occupancy are important, too. Time of day can be programmed depending on building usage. Occupancy can be determined by using people-sensing and people-counting systems.
People-sensing systems and motion detectors determine whether or not an area is occupied. This enables control of lighting and ventilation so that energy can be saved if no one is present. Passive-infrared (PIR) sensors on a wireless sensor node can be used for this purpose. PIR sensors only indicate the presence of a person or persons in a given area.
People-counting systems are more sophisticated in that they can accurately determine the actual number of people occupying a room. This permits the implementation of demand controlled ventilation (DCV) that can optimize the ventilation based on occupancy.
One such system is Texas Instruments’ 3D Time-of-Flight (ToF) design. It uses four infrared lasers to illuminate an area while a stereographic camera captures the three-dimensional image of the scene. Analysis software offers a way to determine human shapes and count the number of bodies present in an area with an accuracy of better than 90%. The system can also track movement, which provides an accurate measure of occupancy in an area.
TI’s 3D time-of-flight solutions are ideal for occupancy-management and people-counting applications such as more intelligent security monitoring, improved control of HVAC in offices and elevators, and traffic and queue analysis. But that’s not all. The 3D time-of-flight sensors should open up new applications that enable consumer and industrial robots to autonomously navigate while avoiding collision.
Today’s buildings are adding intelligence for energy and system efficiency through wireless sensor nodes, which eliminate wiring issues. The main requirement is that these sensor nodes must maintain a long battery life of up to 10 years while constantly monitoring key parameters. Battery requirements and energy harvesting for wireless sensor nodes in building-automation systems are a major design factor.
A WSN for building-automation retrofit can be built with several different types of wireless technologies, such as Wi-Fi, Bluetooth, ZigBee, or sub-1-GHz ISM band radios. Most of these technologies use the 2.4-GHz ISM band, though, which comes with various constraints. For instance, it can limit range. In addition interference from other sources using this band is a potential hazard. On top of that, these higher-frequency radios also typically consume more power, meaning that the battery life of a node will be short. Replacing batteries in many sensor nodes is expensive and a nuisance in terms of maintenance. That’s why low power consumption and long battery life (many years) are essential design criteria.
One ideal solution is to use ultra-low-power, sub-1-GHz ISM band transceivers. These devices not only minimize power consumption, but also operate on the lower ISM frequencies such as 315, 433, 868, and 915 MHz, among others. These lower operating frequencies provide much longer range and improved wall-penetration capability critical to creating a reliable building network.
An example of a typical WSN node is one that senses both temperature and humidity (see figure). It uses a combination digital temperature and humidity sensor such as the TI HDC1010. That device sends 14-bit digital readings over an I2C interface to a low-power ARM Cortex M3 microcontroller embedded in the wireless transceiver. The transceiver is a CC1310 sub-1-GHz radio that operates as a node in a WSN built using TI’s SimpliciTI star network protocol (other protocols can also be used).
Also part of the design is the TPS61291 dc-dc boost converter to provide a stable dc-regulated 3.3 V to the system even as battery voltage declines over a long life. A TPL5111 nanopower timer controls when the system transmits.
In this application, the timer allows one measurement per minute with transmit on time of only 30 ms. The TPS22860 ultra-low-leakage load switch keeps power cut off until the sensor and transceiver are to be enabled. This short duty cycle and the very low power consumption of all devices have shown that the life of the CR2032 lithium-ion battery can exceed 11 years.