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class="style6">HELPLINE: </span>(610) 362-1320<br> <span class="style6">WEBSITE: </span><a href="http://www.empf.org">www.empf.org</a><br> <a href="http://www.aciusa.org">www.aciusa.org</a></div></td> </tr> </table> <p>&nbsp;</p> <p><br> <br> </p> <div align="center"> <p align="center" class="style7">Michael D. Frederickson<br> <strong><em>EMPF Director </em></strong><br> <br> Barry Thaler, PhD.<br> bthaler@aciusa.org <br> <strong><em>Empfasis Technical Editor<br> </em></strong><br> Paul Bratt<br> pbratt@aciusa.org <br> <strong><em>Empfasis Editor</em></strong><br> </p> <p align="center" class="style7"><img src="http://www.empf.org/empfasis/archive/images/iab_sm.gif" alt="IAB" width="100" height="52"><br> <span class="style9">Industrial Advisory Board</span><br> <span class="style14"><em>Gerald R. Aschoff</em>, The Boeing Company<br> <em>Dennis M. Kox</em>, Raytheon<br> <em>Gregory X. Krieger</em>, BAE Systems<br> <em>Edward A. Morris</em>, Lockheed Martin<br> <em>Jane Krueger </em>, Rockwell Collins<br> <em>Gary Kirchner</em>, Honeywell<br> <em>Andrew Paradise</em>, Northrop Grumman<br> <em>Richard Kidwell </em>, ITT Industries</span></p> </div> <p align="center" class="style7"><br> </p> </div> </td> </tr> </table> </div></td> <!-- InstanceBeginEditable name="Body1" --><td width="70%" rowspan="2" valign="top"><div class="body_style" id="body_content"> <p align="center"><img src="images/advanced.jpg" alt="title"></p> <p class="body_text"><span class="style20">T</span>hermophotovoltaic (TPV) devices convert thermal energy in the form of black-body radiation into electricity in the same way photovoltaic (PV) devices convert solar energy into electricity. They can be packaged to fit easily into a variety of locations to capture waste heat from engines, motors, or other sources of heat and transfer that energy to a storage device where it can be made available for future use. A potential application for the U.S. Navy is to develop a manufacturing process to incorporate TPV systems in turbine generators used shipboard to generate power. The 550°C to over 800°C temperatures in these engines can generate enough waste heat to recycle using TPV technology on DDG 1000, LCS, and other platforms that use turbine engines. Potential commercial applications could include lining the engine compartment of an automobile where the excess energy would be held in storage until needed to run some other device such as a power steering pump. This concept can also be extended to any sufficient heat source providing a continuous supply of electricity<sup>1</sup>.</p> <p>Thermophotovoltaic energy conversion is a direct conversion process from heat to electricity via photons. The device converts secondary thermal radiation, re-emitted by an absorber or heat source, into electricity. The device is designed for maximum efficiency at the wavelength of the secondary radiation. Currently, TPV devices are more sensitive to infrared radiation (»>1500 nm) from surfaces that are at temperatures from 950°C to about 1350°C. TPV devices are based on diodes with band gaps lower than 0.75 eV. They can contain a single type of semiconductor or several different types to cover a broad range of temperatures. The typical TPV device consists of four main parts; emitter or source, radiator or filter, collector, and reflector (Figure 1-1). The radiator or filter control component acts to modify the emitter radiation to match the characteristics for the TPV diode. Proper matching between the emitter radiation and diode is essential for optimum efficiency of the system. The filter also reflects long wavelength photons back to the emitter to be reabsorbed. The radiator can be of two general types; broad or narrow band. Tungsten has been the choice in the industrial application because it is a refractory metal and it has good emissivity in the visible and near-IR range. Silicon carbide has been used for the broad band type because its emissivity is close to unity, it can withstand high temperatures, and it is inexpensive to manufacture. Several other materials such as ytterbium oxide, erbium oxide, and photonic crystals have been examined for use in narrow band radiators. While these rare earth materials have met with some success, more often a filter control of the source is used to select the wavelength for the diode. Photonic crystals are useful but right now are too expensive to manufacture and are not cost effective.</p> <p align="center"><img src="images/fig1-1.gif"></p> <p>Since thermophotovoltaic systems have few, if any, moving parts they are small, quiet, and require low maintenance. They also have the potential for high energy densities, versatility in the choice of energy source, and will provide a continuous source of electricity if they are in contact with a steady source of heat. These properties make thermophotovoltaic systems suitable for use in the recovery of waste heat from consumer and industrial processes. The source of heat for a TPV device can theoretically be any source of heat, a flame, a concentrated solar array, or waste heat from a catalytic converter. However, since the most efficient devices operate at temperatures between 950°C to about 1350°C, there are limited sources of heat that can be utilized. Developing devices or systems that operate efficiently at lower temperatures will expand the uses of these devices.</p> <p>The collector is typically manufactured from a low band gap semiconductor that produces an electrical current when exposed to infrared photons with energies that are above the band gap. Currently, the collectors with the lowest bad gaps are those made from indium phosphide arsenide antimonide (InPAsSb). InPAsSb have been manufactured by liquid phase epitaxy (LPE) and organometallic vapor phase epitaxy (OMVPE) but research into its use at lower temperatures is still being explored<sup>2</sup>. Early work on TPV diodes focused on silicon germanium diodes, however the conversion efficiencies were low. Recent work with gallium antimonide (GaSb) and quaternary gallium indium arsenide antimonide (GaInAsSb) have produced devices with band gaps from 0.4 eV to 0.72 eV. Currently GaSb diodes are the basis for most devices and are about 20% efficient. Indium gallium arsenide (InGaAs), another III-V semiconductor, has a band gap of 0.74eV when lattice-matched to an InP substrate. While not an improvement over GaSb, the band gap may be engineered by changing the ratio of In to Ga in order to absorb higher wavelength photons. This lattice mismatched InGaAs material with a bandgap of 0.55 eV provided the best experimental data for single junction cells. A sizeable efficiency improvement could be made in an InGaAs based TPV cell by monolithically combining two or more lattice mismatched InGaAs subcells on InP<sup>3</sup>.</p> <p>The large band gap lowering seen in low nitrogen fraction GaAs1-xNx has led to the development of long-wavelength devices for IR lasers and PV cells. The lattice matched quaternary material Ga1-yInyNxAs1-x<sup>4</sup> has a band gap ranging from 1.42 eV to below 1.0 eV on GaAs and as low as 0.6 eV when strained to InP<sup>5</sup>.</p> <p>There are other lower band gap materials that are currently being developed are those using BiTe and quantum well technology<sup>6</sup>. Work with thin film, super-lattice, and quantum well materials currently holds the most promise for low band gap collectors. This would allow for harvesting at ambient temperatures in almost any application. The development of lower band gap semiconductors is essential for lowering the temperature range where these devices can be effective. </p> <p>The filter/radiator serves to reflect the sub-bandgap photons back to the emitter and a mirror surface on the back of the diode reflects longer wave radiation back to the source, thus improving the system efficiency. It has been shown that TPV efficiencies of 27-28% can be achieved with the addition of back surface mirrors. The radiator/filter must be tailored to the collector in order for the system to operate efficiently. Work has been done using Yb2O3, photonic crystals, and rare earth materials but these must also be tailored to the specific collector. Work with the reflectors made from Zn diffusion process has improved the efficiency of the GaSb cells by recycling the energy in the long wavelength photons which occur in a higher percentage in low temperature applications.</p> <p>The TPV system is a relatively simple one that has few components. Since the efficiency of the system depends on the efficiency of each part (radiator/filter, collector, and reflector), steps are taken to optimize each component. The technology in the semiconductor industry is close to making a device that can operate with some efficiency at lower temperatures, however, the materials and manufacturing costs are prohibitive at this point. Photovoltaic (PV) power has faced similar challenges in the past and still faces some of those challenges today. The start up costs of PV is high and the return of investment can take a long time. However, over the years advancements in manufacturing have driven down costs significantly while increasing the system efficiency. Materials such as hybrid solar cells and manufacturing techniques like roll-to-roll manufacturing will help PV become more and more competitive with other energy sources. The steps to making TPV efficient are similar to the ones facing PV today. Building on that technology and focusing on manufacturing an inexpensive TPV system holds the best hope for making this a viable technology.</p> <p>References</p> <ul style="list-style-type: decimal"> <li>Gamota, Daniel. "Near Term Opportunities for Large Area Flexible Electronics." <i>Circuits Assembly</i> 20.4 (2009): 26-29. Print.</li> <li>Wang, C. A. "Antimony-Based III-V Thermophotovoltaic Materials and Devices." <i>Sixth Conference on Thermophotovoltaic Generation of Electricity: TPV6.</i> Proc. of THERMOPHOTOVOLTAIC GENERATION OF ELECTRICITY:. Vol. 738. 2004. 255-66. AIP Conference Proceedings.</li> <li>Buyanova, I. A., W. M. Chen, and B. A. Monemar. "Electronic Properties of Ga(In)NAs Alloys." <i>MRS Internet Journal of Nitride Semiconductor Research</i> 6.2 (2001): 1. Materials Research Society. <http://www.mrs.org/s_mrs/doc.asp?CID=26415&DID=321178>.</li> <li>Bhusal, L., A. Alemu, and A. Freundlich. "Short-period Strain-balanced GaAs_{1-x}N_{x}/InAs_{1-x}N_{x} Superlattices Lattice Matched to InP(001): A Material for 0.4-0.6eV Midinfrared Applications." <i>Physical Review</i> B 72.7 (2005).</li> <li>Köhler, K., J. Wagner, P. Ganser, D. Serries, T. Geppert, M. Maier, and L. Kirste. "The Realization of Long-wavelength (l <_ 2.3 µm) GaInAsN Quantum Wells on InP by Molecular-beam Epitaxy." <i>Journal of Physics</i>: Condensed Matter 16.31 (2004): S2995-3008.</li> <li>Anderson, David J. <i>NASA Radioisotope Power Conversion Technology NRA Overview.</i> Rep. NASA/TM-2005-213981. National Aeronautics and Space Administration, Nov. 2005. 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