The present invention relates to thermophotovoltaic (TPV) power generators which convert infrared radiant energy to electric power using low bandgap photovoltaic cells.
Existing TPV generators include an emitter inserted in a flame of a burner and surrounded by a circuit of low bandgap TPV cells. In those units, the heated emitter emits infrared energy that is received by the TPV cells and converted to electric power. Excess heat is removed from the TPV cells by convective air cooling through fins attached to the outside surface of the cell circuit. One existing generator, as described in an earlier filed application, takes the form of a wall mounted TPV lantern. That lantern includes an infrared emitter, low bandgap photovoltaic cells for converting infrared energy to electric power, fins for cooling the cells and a cooling fan for blowing air past the fins. A cylindrical housing surrounds the emitter, the photovoltaic cells, the fins and the cooling fan. While existing TPV generators have proven effective, needs exist for economically viable, durable compact TPV generators having added features for improving efficiency and that are adaptable for use in a variety of applications.
Several high temperature emitter materials had been discovered that appeared promising for use in TPV generators. Those materials have the ability to emit radiation at wavelengths that are nearly ideal for efficient conversion to electricity using infrared responding GaSb photovoltaic cells. In particular, it was found, using spectral measurements in the infrared range of about one to five microns, that magnesium oxide doped with cobalt oxide and aluminum oxide doped with cobalt oxide evidence optimum spectral selectivity. Thin disk emitters including those materials have been made by dry pressing suitably sized powders and sintering the disks at about 1650.degree. C. Both magnesia and alumina, when highly pure and sintered to a reasonable density, provided for a low emissivity matrix. The addition of small amounts of cobalt to those low emissivity matrices created an energy band between about 1 and 1.9 microns with very high emissivity, attributable to the outer d orbitals of the cobalt ions. Unfortunately, when those dry powder pressed ceramic disk emitters are heated to temperatures in the 1100.degree. C. to 1400.degree. C. range, a range in which emissive power reaches useful levels, the emitters tend to fracture and shatter when thermal stress in the form of spatial temperature gradients become too high. In a similar fashion, cylindrical tubes manufactured using conventional means such as slip casting and extrusion are also unsuitable for TPV applications, as the tubes tend to fracture even when conventional anti-failure measures, such as the implementation of stress relief slots, are taken. Needs exist for bandgap matched emitters that overcome the problems of fracturing and thermal stress.
Thermophotovoltaic generators having combustion heat sources require a fuel flow control system, which minimally includes an on/off valve and a combustion initiation means. Products which are similar in form to existing gas heaters are currently being developed for use in recreational vehicle, marine and other off-grid markets. Those products include ceramic emitters and combustion chambers capable of producing and withstanding emitter temperatures exceeding 1000.degree. C. Arrays of gallium antimonide infrared sensitive solar cells are provided for collecting energy radiated by the emitter and for converting that energy into electric power. The requirements for commercial TPV product control systems include safety, timing, gas valving and logical sequencing. Needs exist for control systems for TPV generators meeting those requirements.
Several control systems for gas appliances exist. The industry standard technique for flame sensing typically provides that the primary safety interlock cuts off fuel flow in the case of unexpected flame out. Metal electrodes in the combustion chamber, together with an associated electronic circuit, use the physical principle of flame rectification to detect the presence of combustion. Whenever an unexpected flame sense signal is not properly detected, the control system shuts off the fuel flow. In existing control systems, the metal electrodes are generally made of the metal Kanthal. That metal cannot be used in applications where the temperature of the electrode exceeds about 1200.degree. C. and thus cannot be used in TPV products currently being developed, where temperatures generally exceed 1500.degree. C. While the use of high temperature platinum metal electrodes has been suggested, evidence exists which strongly suggests that high temperature metals would vaporize in the combustion chamber, resulting in unacceptable deposits within the TPV generator. Needs exist for flame sense detection methods and apparatus for use in TPV generator control systems.