Instead
of designing electronic devices to wastefully dissipate their excess heat, a
new quantum-dot based thermo-electric effect aims to harvest that heat to
generate enough electricity to double battery lifetimes.
Every
electronic device wastes energy dissipating heat that it could be using to
extend battery life, according to the Massachusetts Institute of Technology
(MIT). By combining micro-gap thermal effects with nanoscale quantum dots, MIT
researchers recently demonstrated thermophotovoltaic materials that recover
lost heat by generating electricity with it.
Thermophotovoltaics
is traditionally a low-efficiency process, but MIT researchers have found two
ways to boost the efficiency and current carrying capability simultaneously.
Currently a commercial version of MIT's thermophotovoltaic material that boosts
its performance by 10-times is due out next year from MTPV Corp. (Micron-gap
Thermal Photo-Voltaics Corp., Boston). MTPV
was founded by Bob DiMatteo to commercialize the micro-gap innovation invented
at MIT and subsequently developed by DiMatteo at Draper Laboratories (Cambridge). Now a
second round of improvements wrought by quantum dots has been reported by MIT
professor Peter Hagelstein, promising a second 10-fold boost in performance,
Together, the two improvements boost thermophotovoltaic materials to 100-times
better performance than today.
Thermophotovoltaic
polymers generate electricity by reabsorbing emitted heat photons in
photovoltaic layers that MTPV has boosted in performance with microgap
dimensions. Quantum dots were recently demonstrated by Hagelstein, and his
graduate student Dennis Wu, to further increase that performance by harnessing
the dipole quantum effects of near-surface electric fields.
Quantum
effects only kick in when materials--here electrons--are confined into spaces
sized so small that overall bulk properties of the material are swamped out. In
Hagelsteins' current formulation, two quantum dots with a thin barrier between
them, allows conversion of thermal energy into electricity at an efficiency
levels near 90 percent of the theoretical maximum.
The
layered material works by separating its hot side from its cold side with a
nanoscale air gap that couples dipoles on either side through Coulomb
interactions. On the cold side, electrons in a quantum dot are boosted in
energy enough to tunnel through an oxide barrier to the a nearby quantum dot,
where they generate an electrical current.
The
technique works because of near-field coupling by evanescent waves that only
propagate short distances, thereby making their contributions negligible in
conventional thermophotovoltaic materials. By making the thermophotovoltaic
microgap ultra-thin, DiMatteo and colleagues at MTPV have shown that throughput
power per unit area can be greatly increased. In addition, Hagelstein's recent
demonstration shows that performance can be further improved by harnessing
thermal fluctuations in the near-surface electric fields, thereby coupling dipoles on the hot side to the cold side by
promoting carriers. The smaller the quantum dots can be made, the higher the
material's efficiency becomes, hinging commercial success on perfecting
practical fabrication methods for ultra-small quantum dots.
Funding was provided by Draper Laboratory and MTPV Corp.
