I read an interesting article in The Economist’s most recent Technology Quarterly about new ways to recycle waste heat from power plants and other types of machinery such as computers. 

In the power generation context this typically is done using a heat recovery unit to capture heat from a combustion unit’s exhaust stream.

Another way to way to recycle waste heat is to capture infrared radiation emitted by hot objects using photovoltaic cells. 

The difficulty with this approach is that only photons that travel at a near perfect right angle to the surface of the hot material can escape and be picked up by PV cells.  Photons traveling at any other angle are reflected back inside the material.

One of the companies profiled in the Economist article is Boston, Massachusetts startup MTPV Corporation (MTPV), which takes its name from an acronym for a technology called micron-gap thermal photovoltaics. 

MTPV discovered that by placing PV cells just a few hundred nanometers from a hot surface of silicon carbide alloy - so the gap is smaller than the wavelength of the infrared radiation - the photons are not reflected inward but instead continue to travel into the PV cells.

According to MTPV’s web site, U.S. Patent No. 6,084,173 (‘173 Patent), entitled “Method and apparatus for the generation of charged carriers in semiconductor devices,” is its “fundamental” patent on the micron-gap technology. 

The ‘173 Patent is directed to methods of enhancing electrical current generation in a conductive surface by adjusting the gap between a hot surface and the conductive surface to the order of microns or submicrons.

The MTPV web site describes U.S. Patent No. 6,232,546 (‘546 Patent) as a version 1 “implementation” patent.  The ‘546 Patent is entitled “Microcavity apparatus and systems for maintaining a microcavity over a macroscale area” and is directed to a microscale generator (10) having two elements (14, 16) within a vacuum (12).  

The first element (14) acts as a thermal source for transferring energy and a second facing element (16) receives the energy transferred.

 

Moveable panels (18) are disposed on one of the elements.  The panels (18) are thermally coupled to element (14) and spaced from facing element (16) a predetermined, sub-micron distance to efficiently couple the energy between the elements so it can be converted to electricity.

To maintain and control the requisite sub-micron distance between the elements, each individual panel includes spring-like actuating flexures (20).  These flexures (20) urge each panel (18) towards facing element (16) to maintain the predetermined sub-micron spacing between the elements.

 

Each panel (18) has its own flexures (20) so the panels act independently of each other to conform to and compensate for surface variations in element (16).

In addition to the ‘173 and ‘546 Patents, MTPV owns U.S. Patent Applications Pub. Nos. 2008/0060694, 2009/0188549 and 2009/0277488.  According to the company’s web site MTPV continues to innovate and grow its patent portfolio:

The intellectual property continues to extend with six pending patent applications and over fifty disclosures in addition to ongoing research and development efforts.

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