Leonid Minkin

Antireflection coated refractory metal matched emitters for use with GaSb thermophotovoltaic generators

GaSb thermophotovoltaic cells can be combined with infrared emitters to produce electric power. In this application, both power density and efficiency are important. High power density requires a practical target emitter temperature of 1600 K. In order to reach this temperature, spectral efficiency becomes extremely important. Radiation with wavelengths greater than 1.8 microns cannot be converted by the GaSb cells; instead, this long wavelength radiation overheats the cells, limiting power density and efficiency. A solution is to use refractory-metal coated emitters, because metals have low emittance at long wavelengths. Further, an antireflection (AR) coating on the metal can enhance the emittance in the cell convertible band. A spectral efficiency of 75% has been demonstrated for an AR coated tungsten emitter and a GaSb cell power density of 1.5 Watts/cm/sup 2/ has been measured with an AR coated tungsten emitter operating at 1555 K.

TPV History from 1990 to Present & Future Trends

Herein, the history of TPV at JX Crystals over the last 15 years is described. Our focus has been on commercial application for TPV components and systems. JX Crystals began as a spin off company when the inventors of the GaSb cell left Boeing with a license to make that cell at JX Crystals. We recognized that III‐V IR sensitive cells such as the GaSb cell were enabling for TPV systems. We began pilot production of GaSb cells in 1994. However, while these cells are necessary for TPV, they are not sufficient by themselves and the development of other system components has been necessary. The development of spectral control and burner and recuperator subsystems will be described here. Unfortunately, a complete TPV system still needs to be fully developed. Various R&D efforts around the world are underway targeting this objective. JX Crystals continues to supply small quantities of cells for these development efforts. Given such a TPV system, cell prices can fall dramatically with volume production.

TPV Power Source Using Infrared‐Sensitive Cells with Commercially Available Radiant Tube Burner

Over the last several years, JX Crystals has invented and systematically developed the key components for thermophotovoltaic systems. These key components include GaSb infrared sensitive cells, high power density shingle circuits, dielectric filters, and hydrocarbon‐fueled radiant tube burners. Most recently, we invented and demonstrated an antireflection (AR)‐coated tungsten IR emitter which when integrated with the other key components should make TPV systems with efficiencies over 10% practical. However, the use of the AR tungsten emitter requires an oxygen‐free hermetic seal enclosure. During a 2003 Small Business Innovative Research (SBIR) Phase I contract, we integrated a tungsten emitter foil and a commercial SiC radiant tube burner within an emitter thermos and successfully demonstrated its operation at high temperature. We also designed a complete stand alone 500 W TPV generator. During the upcoming SBIR Phase II, we plan to implement this design in hardware.

Soda-can sized thermophotovoltaic battery replacement

A portable cylindrical thermophotovoltaic (TPV) battery replacement is described. This small 8 cm diameter unit is designed to consume fuel at a rate of approximately 200 W and to produce 20 W of DC power. It consists of a cylindrical TPV receiver assembly using simple diffused junction GaSb IR cells along with a novel omega recuperator and a novel doped ceramic matched IR emitter. With a TPV conversion efficiency of 10%, this lightweight TPV cylinder along with a fuel cylinder can have a very high specific energy of approximately 1000 Wh/kg. This specific energy is 6.5 times higher than the specific energy for a Li-ion battery with a similar power rating making this TPV unit very lightweight. The key is the 10% system efficiency. An earlier design for a TPV battery replacement is also reviewed with the key differences in the recuperator, IR emitter, and TPV converter circuit enumerated. The highlighted improvements then lead to the 10% overall system efficiency.

Portable concentrating solar power supplies

JX Crystals Inc is developing a class of compact portable solar generators using commercially available 32% efficient multijunction solar cells. Three different sized solar generators with nominal power ratings of 12 W, 36 W and 75 W are described. The high solar cell efficiency will allow all three units to be small, portable, and light weight. (For example, the 12 W generator will fit in a pocket.)

Performance of 3-Sun Mirror Modules on Sun Tracking Carousels on Flat Roof Buildings

Commercial buildings represent a near term market for cost competitive solar electric power provided installation costs and solar photovoltaic module costs can be reduced. JX Crystals has developed a carousel sun tracker that is prefabricated and can easily be deployed on building flat roof tops without roof penetration. JX Crystals is also developing 3-sun PV mirror modules where less expensive mirrors are substituted for two-thirds of the expensive single crystal silicon solar cell surface area. Carousels each with four 3-sun modules have been set up at two sites, specifically at Oak Ridge National Lab and at the University of Nevada in Las Vegas. The test results for these systems are presented.

Long Term Outdoor Testing of Low Concentration Solar Modules

A 1‐axis carousel tracker equipped with four 3‐sun low‐concentration mirror modules has now been under test outdoors at the University of Nevada in Las Vegas (UNLV) for three years. There are three unique features associated with this unit. First, simple linear mirrors are used to reduce the amount of expensive single crystal silicon in order to potentially lower the module cost while potentially maintaining cell efficiencies over 20% and high module efficiency. Simple linear mirrors also allow the use of a single axis tracker. Second, the azimuth carousel tracker is also unique allowing trackers to be used on commercial building rooftops. Third, an experiment is underway comparing aluminum based mirrors with novel 3M Company multilayer polymeric mirrors which are potentially very low cost. Comparing the data from March of 2008 through March of 2011 shows that the aluminum mirror degradation to date is negligible and that the carousel tracker has been operating continuously and reliable. Also, no degradat...

Spectral control development for thermophotovoltaics

Gallium Antimonide (GaSb) infrared photovoltaic cells are light weight whereas batteries are heavy. The specific energy in a hydrocarbon fuel is high at 12,900 W-hr per kg but a lithium ion rechargeable battery has a specific energy of only 145 W-hr per kg. A 10% efficient portable lightweight thermophotovoltaic (TPV) cylindrical generator when compared with a battery can have 6.5 times higher specific energy, operates 7 times longer, and be quickly refueled. The key is the 10% efficiency goal. This efficiency goal depends on matching the spectrum from a combustion heated ceramic IR emitter to the response band of the IR PV cell. Herein, spectral measurements are presented for a propane burner and ceramic IR emitter assembly surrounded by a fused silica glass envelope. The cylindrical IR ceramic emitter consists of an array of NiO doped MgO rods or slats. The Ni ion emission line is centered at 1.6 microns and it matches the GaSb IR response band extending out to 1.8 microns. The ceramic IR emitter is combustion heated to approximately 1200 C. The fuel and air combustion efficiency and hot gas coupling efficiency to the IR emitter are also important parameters.