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.

A team of researchers at Stanford University led by Professor Nicholas Melosh recently published a paper describing tests conducted using PETE.

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.”

Justin Blows published the results of a recent study on his Clean IP blog regarding solar patent quality.  The study focuses on Australian patentees in particular but includes data from other countries as well.

Blows and his colleague George Mokdsi, both of the Griffith Hack law firm, attempted to measure the quality of patents relating to solar power technologies by using the number of times a patent was cited during prosecution of subsequent patent applications as a proxy for patent quality. 

The theory is that oft-cited patents are likely to involve foundational technologies that others in an industry may need access to.  These patents also provide strong protection to the patentee by forcing later patent applications to reduce their scope of protection:

A patent that discloses a technology with strong foundational characteristics is likely to be cited during the prosecution of a latter filed patent that discloses incremental advances on the same technology.  Often, the foundational patent will force down the scope of protection afforded by latter filed patents.  Thus, patents with high citation counts generally provide broad and robust protection.

The analysis was restricted to U.S. patents and the data was retrieved from the U.S. Patent and Trademark Office.  However, patentees from a number of countries were considered, including the U.S., Great Britain, Japan, Germany, France, Australia, Italy, China, Korea and Taiwan.

The study found that U.S. patentees have the highest quality solar power patents:

Patents from the US are on average cited the most, at 2.9 times per application.  By this metric, US solar patents have on average the highest quality. 

The study also notes trends in solar patent quality, with Japan’s quality increasing and the U.S.’s and Australia’s declining.

According to the study, the U.S. winner was U.S. Patent No. 6,297,539 (“’539 Patent”), which was cited 181 times.  The ’539 Patent, issued October 2, 2001, is entitled “Doped zirconia, or zirconia-like, dielectric film transistor structure and deposition method for same” and lists Sharp Laboratories of America as the assignee.

The Australian winner was U.S. Patent No. 6,429,037 (’037 Patent), cited 17 times.  The ’037 Patent issued August 6, 2002 and is entitled “Self aligning method for forming a selective emitter and metallization in a solar cell”.   The ’037 Patent lists Unisearch Limited, a Syndey company, as the assignee.

eSolar is a Pasadena, California solar thermal startup that makes solar power plants using flat mirrors, or heliostats, to concentrate sunlight onto a centrally located water tank suspended on a tower.  This type of structure is known as “power tower” architecture.

Founded in 2007, eSolar has seen very rapid success in business deals to implement and deploy its technology.  The latest, which is the biggest solar thermal deal ever, is a master licensing agreement with Chinese electrical power equipment manufacturer Penglai Electric (Penglai).

Penglai will develop at least 2 gigawatts of solar thermal power plants in China over the next 10 years using eSolar’s technology.

eSolar owns at least four published U.S. patent applications and at least half a dozen international applications relating to its solar thermal technology.  U.S. Patent Applications Pub. Nos. 2009/0241938 and 2009/0241939 are directed to solar receivers and U.S. Patent Application Pub. No. 2009/0107485 is directed to calibration and tracking control systems for heliostats. 

According to the technology description on eSolar’s web site, its cost-effective utility scale power plant is “based on mass-manufactured components and designed for rapid construction, uniform modularity, and unlimited scalability.”  The company’s heliostat is the “building block” of its power plants and is designed for deployment in pre-fabricated “heliostat sticks” for easy installation.

One factor that affects the efficiency of power tower plants is the positioning of the heliostats relative to the tower and to each other.  eSolar’s “modular field” of concentrating mirrors consists of thousands of systematically spaced heliostats arranged to optimize the layout and maximize efficiency. 

U.S. Patent Application Pub. No. 2009/0133685 (’685 Application) is directed to eSolar’s heliostat array layouts.  Some embodiments of the ’685 Application comprise three segments (128) with each segment divided into two variably spaced heliostat zones (128a, 128b).

Heliostats (328a) in the first zone (128a) are spaced closer together than heliostats (328b) in the second zone (128b).  The heliostat locations are also staggered with respect to heliostat locations in adjacent rows.  A thermal receiver (228) is mounted on a central power tower.

Last year, eSolar entered into a similar deal with an Indian partner, ACME Group (ACME), to build solar thermal plants generating up to 1 gigawatt of power in India.  ACME is the exclusive licensee of eSolar’s technology in India.

eSolar’s success in deploying its solar thermal technology in emerging markets such as India and China belies claims by those countries and other developing nations that IP rights are acting as a barrier to transfer of clean technologies (see China View article here and my previous post here). 

Various proposals to weaken or eliminate IP rights, including compulsory licensing and even excluding green technologies from patent protection, have been put forth by India, China and other emerging market and developing countries and were contained in the official UNFCCC negotiating text for the Copenhagen meetings in December.

However, eSolar’s success in finding willing partners in India and China may actually be driven, at least in part, by intellectual property protection:  it’s hard to imagine Penglai or ACME investing in such large scale projects without the exclusivity in their home markets guaranteed by the master license agreements.

eSolar’s China deal was announced in early January.  It’s interesting that while climate change treaty negotiators were discussing IP as a barrier to international clean tech transfer eSolar and Penglai were probably finalizing the terms of the master license agreement that would be part of the largest ever transfer and deployment of solar thermal technology to China.

Pyron Solar Inc. (Pyron) is a San Diego company that develops and makes solar concentrators.

Pyron and San Diego Gas & Electric (SDG&E) recently announced that SDG&E is building a demonstration project to test Pyron’s patented concentrated solar power system.  The system uses shallow pools of water as a passive cooling system for high efficiency solar cells.

Pyron’s U.S. Patent No. 7,299,632 (’632 Patent) is entitled “Solar electricity generator” and is directed to a solar electrical generator comprising a concentrator, a homogenizer and a photovoltaic (PV) cell.  The concentrator concentrates solar rays onto an entrance surface of the homogenizer, which is in turn attached to a PV cell.

The concentrators are positioned in troughs (1) that sit in bodies of water (5).  The water (5) acts as a passive coolant to disperse the heat generated by the PV cells.   

In addition, buoyancy torque created by pumping the water (5) between ballast compartment (8) and ballast compartment (9) and pressure differentials between the compartments pivots the troughs (1) to keep the lenses (2) aimed directly at the sun.

The lenses (2) concentrate solar rays (3) at focal spot (4).  According to the ’632 Patent, the highly concentrated “pencil” of solar rays (3) then enter homogenizer (43) and are evenly distributed onto PV cell (4′) by loss-free total internal reflection.

According to the ’632 Patent, this system makes better use of solar farm real estate by covering 87% of the set-aside land.  Pyron’s product description also touts the greater power production and reliability of its passive coolant design, noting that it protects the equipment from exposure to extreme wind. 

Pyron plans to stock the pools of water with fish to prevent mosquito infestation, leading Matter Network to speculate that “perhaps the fish farms of the future will double as solar energy collectors.”