Thin films enhance thermoelectric energy harvesting

G. Jeffrey Snyder of Materials Science at California Institute of Technology has provided an excellent exposition of the design principles of thermoelectric energy harvesting in the book Energy Harvesting Technologies (Springer 2008). He summarises the available design space, with the approximate limits of air and water cooling, as follows:

 

He notes that traditional bulk devices are most appropriate for low heat flux, passive, air-cooled exchangers whereas high performance air cooled heat exchangers require higher heat flux than is generally available with thin bulk modules. Consequently, to utilise the very high heat fluxes at the limit of water cooling, thin film devices are needed. Of course, highest power is obtained from both large heat flux and temperature difference at the top right corner of the figure.

 

Experimentally, Snyder pioneered use of thermoelectric films down to five microns in 2003 but noted in 2008 that, "So far, these devices have lower efficiency due to the larger fraction of electrical and thermal contact resistance losses." He noted that the inherent disadvantage of in-plane thermoelectric is that the substrate used to deposit the thermoelectric acts as a thermal short, reducing the efficiency - but work continues.

Nextreme has recently developed a thin-film thermoelectric generator (eTEG™) that converts heat directly into electricity delivering power generation densities greater than 3W/cm2. Nextreme claims a unique advantage over other thermoelectric device manufacturers in that its devices use very thin (5-15 microns) thermoelectric material. This allows exceptionally high heat fluxes and low thermal resistances. As a result, Nextreme thin film TECs can support much higher power densities than conventional thermoelectric modules.

Nextreme points out that, inherently, the technology can also be used to generate low levels of electricity from waste heat, by converting temperature differences of as little as 120°C into electrical energy. It says,

 

 

 

 

Nextreme's thin film thermoelectric materials are claimed to deliver the smallest ever eTEG size as well as unique output power density. Their thermoelectric generator (eTEG) enables convenient recycling of heat to electricity. The eTEG's advantages include high power density in a very thin, lightweight form factor; solid-state construction with no moving parts; and cost efficiencies/scalability as a result of semiconductor fabrication techniques. Nextreme's DBAM™ assembly process is highly flexible and enables a vast range of manufacturable designs to meet specific customer needs.

 

Applications include:

 

 

Nextreme's eTEG is optimized to provide power for high heat fluxes (>20 W/cm2) with a very small form factor. Conventional bulk thermoelectrics are limited to heat fluxes <20 W/cm2 with 10x the footprint. In a thermal environment where heat rejection was well matched to the heat flux specification of the TEG design, fabricated device performance was consistent with analytical and FEA model predictions. Output power levels >100 MW at ΔT =70 K and >300 mW at ΔT=120 K were achieved with modules that measured 3.5 mm x 3.5 mm in size, corresponding to output power densities of ~1-3 W/cm2."

 

For more attend: Energy Harvesting and Storage Europe 2009 or Printed Electronics Europe 2009.

 

Also read: Energy Harvesting and Storage for Electronic Devices 2009-2019