Harvesting CPU Waste Heat Through Pyroelectric Materials

Abstract

Modern CPUs generate considerable wasted heat due to increased power dissipation from high-performance computation. Lots of research effort has extensively focused on using thermoelectric generators (TEGs) to harvest CPU waste heat to increase overall system energy efficiency. To harvest waste heat using TEGs requires a significant temperature differential between the processor as a heat source and the heat spreader/heat sink, as well as a high heat flow. However, the heat-to-electricity conversion efficiency is typically limited to 15 to 20 percent, due to large heat conductivity, low Seebeck coefficient, and low figure of merit of TEGs. In addition, TEGs on a CPU could significantly increase CPU junction temperature compared to the baseline CPU temperature due to its high thermal resistance. Contrary to using TEGs to harvest waste heat from a fixed, spatial temperature differential, this paper presents an approach to harvest CPU waste heat using pyroelectric (PE) materials from the time-varying, temporal temperature differential that is common in current processors. PE materials can generate electricity when subjected to a temporal temperature gradient. The operation of PE materials is distinctly different from TEGs and they have the following advantages. First, the theoretical efficiency is up to 50% using thin films. Second, the overall optimization of PE material is easier than thermoelectric material, since the conversion ratio, the ratio of net harvested energy divided by the heat taken from the hot reservoir, of PE material is independent of the material properties, whereas that of TEG is highly dependent on material properties. Although PE material is also a long-researched energy harvesting material, it is less explored by researchers compared to TEG in the application domain of processor waste heat management. In this paper, we review current PE materials in terms of pyroelectric coefficient and thermal conductivity, and also investigate the harvested power generation from CPU waste heat in a modern computing system.