Energy harvesting: Creating useful power out of processor waste heat
December 01, 2014
Mitigating processor waste heat is a problem for embedded designers, but dealing with it could also help power the system if done right.
Embedded systems are often mobile or deployed in remote locations off the grid, and must run reliably for years. The small size of these embedded computing systems combined with performance demands creates small, localized processor hot spots. How can designers power mobile electronics and better address the concerns of hot spots? Harness the heat energy for power. Arizona State University School of Computing, Informatics, and Decision Systems Engineering Assistant Professor Carole-Jean Wu is investigating the use of energy harvesting capabilities of thermoelectric modules in processors. This technique harvests waste heat and converts it to electricity, which can be used to enhance system cooling or be stored for future use.
"The heat distribution of modern computing platforms offers an interesting opportunity for waste heat energy harvesting," Wu says. "In particular, the unique heat distribution enables the use of thermoelectric materials in embedded applications."
Thermoelectric coolers (TECs) are often used for active cooling of CPU hot spots, and thermoelectric generators (TEGs) can be used in other areas of the CPU to turn remaining heat waste into useful electricity.
"Thermoelectric modules operate based on the phenomenon where a difference in temperature creates an electric voltage difference and vice versa," Wu says. "When a voltage is applied to a thermoelectric material, the splitting and combination of electron hole pairs results in a temperature difference on the material, called the Peltier effect. Conversely, if the material is subjected to a difference in temperature, a voltage difference is created, called the Seebeck effect."
The energy harvesting technique used in Wu’s research exploits the spatial temperature difference between hot and cold components in a three-step process: perform system temperature and heat distribution characterization; identify thermal points and apply thermoelectric devices to generate electricity from temperature differences; and find native applications that exist in the system to use the harvested energy (Figure 1).
Wu and her team were able to recover 0.3 W to 1 W of power with an Intel Ivy Bridge processor running at 70 °C to 105 °C with a thermoelectric device on the CPU. The recovered energy when three TEG modules were used was at least enough to power a fan, and can be a significant amount of power for mobile and wearable applications.
Though preliminary studies show promise for thermoelectric modules, there are still challenges to overcome, Wu says. Material efficiency and additional thermal resistance introduced to embedded systems by the energy harvesting materials are two critical challenges that must be addressed for energy harvesting to become more widespread in embedded systems.
"We are currently investigating important applications using thermoelectric modules at the processor architecture granularity," Wu says. "Our preliminary results indicated that, if managed intelligently, the temperature increase caused by thermoelectric generators can be tolerated and will not increase the overall temperature of the processor. The harvested energy is then used to lower the operating temperature of processors, which will, in turn, improve the chip reliability and the total cooling cost of the chip. We have filed a provisional invention closure on this work and are working on the first prototype of the design."
Generating power for cooling from the waste heat that already exists is a very promising solution for embedded designers. Users’ battery complaints could be addressed and that bothersome excess heat from the processor could be mitigated and put to use. Designers should definitely keep an eye on where this research is going.