Efficient, User-Friendly Solar Systems Extend Wireless Sensor Network Capabilities
Ty Fayfield, Director of Wireless Products
Banner Engineering

Advancements in power management technology have now yielded physically smaller and more efficient solar systems to power wireless sensor network applications using a range of commercially available sensors and radios. These radios are equipped with industrial I/O and high output power. Today, a small solar panel, power optimized radios, and duty-cycled sensors replace the traditional 80 watt solar panel and large batteries to provide a more convenient and longer-lasting solution.

With compact solar systems powering wireless sensor devices, gathering control application parameters such as operational status, level, temperature and other crucial information can now be accomplished remotely, even in locations not wired for power and data and with a minimum amount of sunlight.

These applications require a remote power supply capable of powering both the radio and sensing device, making solar energy a natural extension of wireless sensor technology, and allowing system integrators to install a wireless sensor network independent of wired power availability.

Using Solar Power
To extend the longevity and application potential of a wireless installation, wireless devices optimized for the low power consumption of a battery-based system can be combined with a solar panel used to recharge those batteries.

Though it seems there is always enough sunlight available, in many parts of the world the lack of adequate sunlight available during winter months, when days are shortest, can make implementing a solar solution challenging. Today’s smaller solar systems coupled with optimized electronics can reliably power a sensor network and most commercially available sensors even in locations with as little as one hour of sunlight a day.

For example, a relatively small 10 to 15 watt solar system can supply 25 to 40 mA at 12 V or 50 to 80 mA at 5 V per hour of sunlight per day. Based on these power output specifications, an optimized 1 watt radio and multiple 4-20 mA loop-powered sensors can be installed in areas not wired for power, even in locations with limited solar availability.

By manipulating the duty cycle, most commercially available 4-20 mA loop-powered sensors can be optimized for low power operation. The radio is powered continually, but it cycles power to the sensor only when the sensor is programmed to take a reading and only long enough to ensure a stable measurement. This arrangement minimizes the overall power requirements of the application while maintaining reliable network communication.

These small, efficient solar systems are less expensive to implement and can be easily relocated and reinstalled as the application parameters change. An added advantage of using smaller solar systems is that they are more environmentally friendly. The smaller systems employ nickel and lithium based battery chemistries as an alternative to standard lead acid options.

Rechargeable battery packs power wireless radios and sensors autonomously when sunlight is not available, then recharge when sunlight returns. Sunlight varies with geography and latitude, making the length of time the battery pack can autonomously power an application important to determine prior to installation. For most applications, solar power supplies can be designed using average sunlight values for December. Critical monitoring and control applications should use a solar system designed to withstand worst-case weather and sun conditions.

Good Applications for Solar Power
Applying solar energy to power wireless network devices and sensors is particularly advantageous in outdoor applications spread over large geographical areas. Landfills or wastewater treatment facilities often span many miles and require multihop networks to meet range demands.
Wireless Instrumentation

Many process variables must be collected and analyzed in a typical landfill or wastewater treatment facility, such as in tank and groundwell level monitoring, pump control, or pump cycle counting. Because these applications may be located far from power and data cabling, they are well suited for solar power and wireless sensor networks.

Typically these sensors are not optimized and generally have high power requirements, making operation from a single-use battery pack difficult. However, because these sensors can be duty-cycled, the total power demand is low enough to make solar powered rechargeable batteries a viable option.

Wireless sensor networks also prove advantageous in retrofitting an application using existing instrumentation. One radio can be connected to one or more sensors monitoring tank or well levels for easy data collection and logging.

Solar power additionally extends the capability of a wireless sensor network in outdoor monitoring applications, such as controlling pond levels. An industrial wireless radio and submersible pressure sensor can be easily powered by a solar panel and rechargeable battery system. Because most of these pond or well monitoring applications are usually not time-sensitive, as data is transmitted back to a central control location infrequently, the solar system powers the sensors and radio for many years. What was once a time-consuming manual process to collect a single data point is now simplified into an automatic process, complete with status alarms and electronic notifications sent in real time to update operators about process and error states.

Wireless Sensor Network Infrastructure
Large-scale applications spread over miles of open land are good examples of wireless sensor networks that require repeater radios. High output repeater radios using frequency hopping spread spectrum are smart enough to form self-healing, auto-routing networks. These power-optimized radios increase network range and offer reasonable data throughput with latencies of less than one second.
Because these repeaters have been optimized for efficient operation, they are easily powered by small solar panels and only limited sunlight. Their small size also allows these radio repeaters, and their accompanying solar panels, to be easily added or removed from a wireless sensor network to accommodate range or topography changes.

Easy-to-relocate wireless sensor devices are additionally beneficial in large landfill monitoring applications where heavy machinery or earthmoving equipment is frequently moving around the site. The ability to move a wireless sensor device reduces the likelihood of the machinery damaging expensive equipment. Also, an application independent of power constraints eliminates power lines susceptible to damage in these active environments, minimizing downtime and reducing maintenance costs.

New Wireless Opportunities
Other applications for solar power involve wireless networks using multihop radios to send sensor data around large obstacles. As long as the solar panel’s access to the sun is not obstructed, installing multihop radios powered by solar panels and rechargeable battery packs is an easy way to send sensor data around mountains, buildings, or other large obstructions that block the radio signals.
As well as making existing installations easier to implement and more mobile, the combination of power-optimized radios, sensors and solar panels also enables a new class of wireless sensor network applications.

One example of an application that leverages the efficiency of a small solar panel and the availability of power-optimized sensors is a wireless perimeter guard for a large outdoor storage area. The size of the perimeter is limited to the range of the sensor, but many sensors have ranges well into the hundreds of feet. Using solar panels and rechargeable battery packs to power the radios and energy-efficient photoelectric sensors, such as Banner’s SM30 emitter/receiver pair, facilitates the creation of a reliable guard system.

With the receiver outputs wired to the radio’s inputs, breaking the “beam” between any emitter/receiver pair changes the input status. If the beam surrounding the storage area is broken, indicating someone has entered the perimeter, alarms sound and e-mail alerts can be sent from the host system to notify designated personnel of the burglary or vandalism in progress.

This application uses a 10 to 15 watt solar system to power the radio and a pair of photoelectric sensors. Because the location of this perimeter guard system is not bound by available power lines, the operator can move or resize the perimeter guard system as production levels increase and higher storage volumes are required.

The Future of Wireless Sensor Networks
The full potential of wireless sensor technology can only be exploited when wireless devices are no longer limited by power constraints. New power-optimized industrial radios require less power to transmit data and activate or read sensors. Efficient power management technology offers the ability to use solar panels as a cost-effective power source for every device in a wireless sensor network.

Incorporating solar power with wireless sensors enables the creation of larger networks that can operate independent of power constraints and without the high costs of running conduit and wiring. Because the individual components are small, the solution can be expanded or relocated as application needs change.

Adding to the convenience of solar power, with efficient duty-cycled sensors and power-optimized radios, smaller solar panels can accomplish what once took an expensive and cumbersome solar system. As energy technology improves, wireless sensor networks will become more and more efficient, using less energy, generating less waste and having a reduced effect on the environment.

About the Author
Ty Fayfield, director of wireless products for Banner Engineering, has worked in the industrial automation and sensor industries for over 14 years. In his present role at Banner, he is responsible for wireless product development and market direction. Before working in industrial automation, he developed wafer processing equipment for the semiconductor industry. For more information please visit www.bannerengineering.com.

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