Anantha Chandrakasan, MIT's Joseph F. and Nancy P. Keithley Professor of Electrical Engineering and director of the MIT Microsystems Technology Laboratories, and Yogesh Ramadass (PhD '09) believe they have figured out how to avoid doctors getting beneath a patient's skin to replace batteries for implanted biomedical monitoring or treatment systems. They have been working to get as close as possible to the theoretical limits of efficiency in thermoelectrically tapping this heat energy. Most thermoelectric devices are designed to exploit differences of tens to hundreds of degrees C but the new MIT-developed devices use only one or two degrees, producing a tiny but usable 100 microwatts or so. The key to the new technology is a control circuit that optimizes the match between the energy output from the thermoelectric material and the storage system connected to it, in this case a storage capacitor. The findings were presented in February 2010 at the International Solid State Circuits Conference in San Francisco.
The experimental versions of the device require a metal heat-sink worn on an arm or leg, exposed to the ambient air. "There's work to be done on miniaturizing the whole system," Ramadass says. This might be accomplished by combining and simplifying the electronics and by improving airflow over the heat sink. He notes that, that as a result of research over the last decade, the power consumption of various electronic sensors, processors and communications devices has been greatly reduced, making it possible to power such devices from very low-power energy harvesting systems such as this wearable thermoelectric system. Such a system might enable 24-hour-a-day monitoring of heart rate, blood sugar or other biomedical data, through a simple device worn on a patient's arm or a leg and powered by the body's temperature. A similar system could monitor the warm exhaust gases in the flues of a chemical plant, be embedded with sensors in engines and machinery or monitor air quality in the ducts of a heating and ventilation system, where replacing batteries is difficult or impossible.
David Lamb, chief operating officer of Camgian Microsystems, a company that produces a variety of low-power, lightweight semiconductor chips, says that "we believe the wireless sensor products we are developing will all migrate to energy harvesting, as we push their size, weight and power down." He adds that the research of Chandrakasan and Ramadass "is in the critical path of technologies required by companies such as Camgian that are developing next-generation microsystems."
Devices to use this power would in most cases still need some energy storage system, so that the constant slow trickle of energy could be accumulated and used in short bursts, for example to operate a transmitter to send data readings back to a receiver. Different ways of storing the energy are possible, such as the use of supercapacitors, Ramadass says. "These will play a critical role, in order to build a complete energy harvesting system," he says, noting that, "after years of work on these highly efficient energy-scavenging devices, currently funded by a seed grant from the MIT Energy Initiative," Chandrakasan says, "the time has come to find the real applications and realize the vision."
Top image of Anantha Chandrakasan source MIT
For more read : Energy Harvesting and Storage for Electronic Devices 2009-2019
