Photon enhanced thermionic emission, or PETE, is a relatively new concept in solar energy production. It is considered a major breakthrough because it utilizes both energy from sunlight (solar photons) and from the heat of the sun.
By harvesting both thermal and photon energy, the PETE process significantly improves the efficiency of solar energy conversion.
Melosh is also a named inventor on a patent application directed to PETE, the eponymously titled U.S. Patent Application Publication No. 2010/0139771 (’771 Application), owned by Stanford University. The ‘771 Application describes the PETE principle and claims a PETE apparatus.
The process works by thermalizing electrons (110), thereby boosting electron distribution (112). In general terms, when cathode (104) is illuminated, photons (106) are absorbed to generate electron-hole pairs, and emitted electrons are received by anode (102).
The important thing, according to Professor Melosh, is that instead of electrons separating into their electron-hole pairs inside the semiconductor material, the electrons leave the material, are forced into a small vacuum gap and then collected by another electrode (102). The ‘771 Application states:
PETE is based in part on thermionic emission, where a fraction of Boltzmann-distributed electrons have sufficient thermal energy to overcome the material’s work function and emit into a vacuum.
One key advantage of the process is that the heat of the sun, which goes unused in standard PV, can be harvested and used. The ‘771 Application describes a PETE device set up so that waste heat can go to a thermal engine.
A PETE/solar thermal architecture is shown in Figure 6a of the ‘771 Application (shown below). In this embodiment, radiant energy (610) from the sun (608) is incident on cathode (602). Electron current (614) emitted from cathode (602) is received at anode (604).
As a result, the PETE device (620) can provide electrical power to an external load (606), and no separate heating is needed for the cathode (602). Electrons and photons emitted from the cathode deliver heat energy to the anode, and a thermal engine (618) can remove this excess heat (616) from the anode and use it to generate additional electrical power.
According to a Stanford Report article, the PETE device hits peak efficiency at over 200 degrees Celsius, while most silicon solar cells are rendered inert by the time the temperature reaches 100 degrees. This is borne out by independent claim 1 of the ‘771 Application, which specifically recites a photocathode temperature of greater than 200 degrees Celsius.
The article also says that Melosh calculates the efficiency of the PETE process at 50%, and combined with a thermal conversion cycle, the PETE process could reach 55 or even 60 percent efficiency, almost triple the efficiency of existing systems. In the words of Professor Melosh, PETE seems to be a “conceptual breakthrough” and a “real win.”
Eric Lane is a patent attorney at Luce, Forward, Hamilton & Scripps in San Diego and the author of Green Patent Blog. Mr. Lane can be reached at firstname.lastname@example.org.