May 31, 2011Nextreme Thermal Solutions, a Durham, N.C., thermoelectric products company, and Arkansas Power Electronics International (APEI) are developing a wireless sensor system to be installed on military aircraft engines, that will capture engine heat to generate electricity and power the transmission of sensor data regarding the health of that engine's turbine bearings. The solution, known as the Thermal Energy Harvester for Turbine Health Sensor System, includes sensors, a wireless transmitter and an energy-harvesting device. Nextreme is providing the energy-harvesting technology to APEI, which is building the sensors.
APEI has contracted with a division within the U.S. military to provide the wireless sensors for monitoring bearing health in the F-35, a single-seat, single-engine jet airplane developed for the Joint Strike Fighter program by Lockheed Martin. The agency has asked to remain unnamed. The sensor system is expected to be ready for installation by the end of this year.
Both military and commercial aircraft have engine bearings that must periodically be replaced to ensure healthy engine operation. A failed bearing in flight could have catastrophic results. To ensure that doesn't happen, most airlines and military agencies replace bearings after a specific number of operational hours; however, this means the bearings are often changed out long before approaching the end of their lives, which costs money not only for the replacement bearings, but also for the labor required to make those unnecessary replacements. The solution that aims to provide aircraft maintenance workers with greater knowledge about the bearings' condition is a sensor system that can help determine a bearing's condition, based on the amount of vibration coming from it, as well as its temperature.
APEI and Nextreme are building a sensor solution to provide that information without wires. Every additional item placed within an airplane can result in the need for more fuel to operate that plane over time. For that reason, says Karl von Gunten, Nextreme's director of marketing, the absence of wires means a reduction in weight. The energy harvester weighs approximately 5 milligrams (0.0002 ounce), says Dave Koester, the company's VP of engineering, while the weight of the entire system (including sensors, transmitters and receivers) is not yet known.
Nextreme's Thermobility thermal energy-harvesting platform includes an interface to pull heat from the engine and convert that energy into electrical power. The energy harvesters generate electricity via the Seebeck Effect, in which voltage is produced from the temperature differential created by heat flowing through the device. The energy harvester can come with a rechargeable battery to store the electricity, in situations in which the heat source is intermittent. For the turbine health system, however, the battery will not be required, since the heat source used to generate the electrical power will be constant, von Gunten explains.
The readers attach to the bearing's raceway and could be affixed with bolts, though Nextreme is still determining the mounting methodology. The devices will be designed to transmit signals in an environment containing a lot of metal. A prototype—already built to send data wirelessly (not necessarily for the aircraft turbines industry)—can transmit to a distance of about 30 feet, using a Texas Instruments transmitter (the Nextreme Thermobility Wireless power generator is designed to work with Texas Instruments' EZ430 platform, which includes the MSP430 ultra-low-power microcontroller and the CC2500 2.4 GHz transceiver). That prototype is part of an evaluation kit that Nextreme designed to demonstrate how thermal energy could be used to power a wireless sensor.
Once the sensor data and a unique ID number are received by a reader on the aircraft, that information could then be stored on a computer onboard, or sent via radio to a server on the ground, where data regarding various in-flight aircraft could be stored and monitored.
Nextreme is now also marketing the thermal energy-harvesting solution in the form of the wireless power generator (WPG-1) evaluation kit for other RFID and wireless sensor applications, because the harvesting is able to save users money that would otherwise be spent on battery replacements or wiring. Because thermal-energy technology is long-lived, the firm estimates, the harvesters would not need to be replaced for decades.
Thermal energy-harvesting technology is typically used to generate power for a variety of electronic devices, such as wireless sensors for monitoring and surveillance, process control, and gas detection. Thermoelectrics can also be used for opto-electronics (electronics that control light), or to cool laser diodes. Because the wireless sensors that Nextreme and APEI are developing do not draw power from batteries, they can be very small, require no battery replacement, and be relatively inexpensive. Von Gunten estimates that the sensor devices to be used for bearing-health detection would cost $1 or less apiece in large volume. He foresees the systems being used in locations such as utility companies, or at other industrial locations in which pipes become hot, as steam or some other gas or liquid passes through them. That heat, he says, can be used to generate electricity to power a sensor that could detect other information, such as temperature, humidity or vibration.
