ASHRAE Journal - August 2008 - 77

eral, wireless communications can realize the greatest benefit chosen to sell products with wireless radios that only operate in retrofit applications with the following characteristics: in the 2.4 GHz band because they have designed their products • High-churn applications. Wireless sensors can be readily to serve a global market.8,15,* relocated as uses change. Examples include data centers and On the other hand, stairwells and elevators are, for radio some office spaces.4,8 waves, effectively, large holes that facilitate transmission • Occupied spaces. Contractors can usually install wireless between floors.4 solutions in less time than wired solutions, decreasing or elimiIt appears that the interval between battery replacements is nating business interruptions and occupant disturbances. increasing rapidly, driven by improvements in device power • Longer cable runs. Longer runs increase the cost of wired electronics and controls. For example, in 2005, Zebrick17 solutions. noted that a battery would power a wireless temperature sensor • Spaces with special wall or ceiling treatments that cannot that communicates at one-minute intervals every two years. be disturbed. Examples include historical buildings, marble Two years later, Huseth8 estimated that a temperature sensor walls, asbestos-containing areas, firewall penetrations and with the same duty cycle would have about a five-year life. cleanroom alterations.8,14 Earlier this year, measurements of the residual battery life of • Spaces with problematic access.1 wireless temperature sensors deployed at a university campus • Exterior installations that require trenching.14 indicated that the batteries will have an average lifetime of • More sensors. This reflects a tradeoff between higher just over seven years, albeit with a two-minute update rate. component costs for wireless systems and higher labor (instal- Moreover, the contractor noted that a radio recently released lation) costs for wired systems. One receiver can serve many by the vendor is five times more efficient, in which case the transmitters, so wireless sensors also are highly scalable, i.e., functional lifetime of the battery should exceed its 10-year adding additional sensors only incurs the incremental cost of the shelf life.4 sensor and the cost to install and configure that sensor. In the short term, electric utilities’ demand response All fully wireless transmitters must strike a balance between battery life, device en- (DR) programs may drive wider deployment of wireergy consumption, and radio transmission power. These factors, in turn, impact the less controls in buildings. cost-effectiveness of a wireless system in a potential application. As a result, the following attributes, Recently, sensors integrated with devices that harvest ambiwhich tend to increase battery life and reduce maintenance ent energy sources, such as light, temperature differences, or costs, enhance the attractiveness of wireless solutions: vibrations, have begun to appear in the market. Under design • Control applications requiring infrequent communication. conditions, the harvesting devices provide sufficient power The radio turns off when not broadcasting to reduce energy to power the sensor and its radio for the effective lifetime of consumption. the downsized onboard battery.11 • Sensors with low power draw levels. This excludes sensors that draw more power, such as pressure and CO2 sensors.1,4 Reliability • Higher density deployments of wireless sensors. The more Functional reliability is a key attribute for wireless networks devices that are in relatively close proximity to each other, the in commercial building control applications to avoid dropped greater the number of potential transmission paths for data; data. Although the redundant communication paths of mesh this tends to improve system robustness and reduce transmit networks enhance their reliability, interference can limit the efdistances, decreasing power draw and maintenance costs.15 fective range of wireless systems in some applications. This, in Device energy consumption also depends on the radio signal turn, increases the required transmit-signal strength (decreasing strength needed to achieve the necessary in-building range. battery life) or spatial density of wireless system components Several materials commonly used in building construction (increasing wireless system costs).2 can significantly attenuate radio transmissions, including Controls vendors indicate that wireless solutions are relaconcrete, structural steel, office furniture and electronic noise tively robust in most applications, i.e., they often are plugsources.4,8,10 Radio wave attenuation increases with frequency. and-play and do not require RF surveys prior to deployment Accordingly, an evaluation of 900 MHz and 2.4 GHz radio to establish feasibility.15 In some applications, however, insources in a seven-story institutional laboratory building found terference can cause dropped messages and devices dropping that the 900 MHz source achieved superior coverage to the 2.4 GHz source operating at the same transmit power.13 Despite * The 900 MHz band is unlicensed—and, therefore, usable for wireless radios— this advantage, at least two major controls manufacturers have only in the U.S., Australia, and Israel.16 August 2008 ASHRAE Journal 77
ASHRAE Journal - August 2008

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