Electric power generator including a thermophotovoltaic cell assembly, a composite ceramic emitter and a flame detection system

A thermophotovoltaic generator apparatus includes a thermophotovoltaic converter assembly and a cooling fan positioned beneath the assembly for generating an updraft around the assembly. A fuel source is connected to the converter assembly by a fuel line. A new control system, which may include a non-metallic electrode for flame sensing and, regulates flow of fuel from the fuel source to the converter assembly. A housing encloses the cooling fan and the converter assembly. The converter assembly includes a fuel injector cup having a fuel inlet connected to the fuel source and a fuel outlet. A combustion chamber is positioned above the cup for receiving fuel from the fuel outlet and for allowing hydrocarbon combustion. A combustion fan is positioned between the cup and the cooling fan for generating an updraft into the combustion chamber. An infrared emitter is positioned around the combustion chamber for emitting infrared radiation when heated by combustion gases resulting from the hydrocarbon combustion. The emitter includes infrared emitter further comprises a first refractory, infrared transparent ceramic material layer, a second refractory, infrared transparent ceramic material layer, a reinforcing material layer sandwiched between the first layer and the second layer, and a doped, refractory, infrared transparent ceramic matrix material layer positioned over the second layer.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
The present invention is a TPV generator that produces heat, light and 
electricity safely, efficiently and quietly. 
The new thermophotovoltaic generator includes an infrared emitter 
positioned above a flame-generating assembly. The flame-generating 
assembly includes a fuel injector cup having a fuel inlet. A venturi 
chamber is positioned above an upper end of the cup. A slit is provided 
along the upper end of the cup for radially injecting fuel into the 
venturi chamber. A combustion air fan is provided beneath the injector cup 
for moving air upward and into the venturi chamber. Hydrocarbon combustion 
occurs in a combustion chamber above the venturi chamber. That combustion 
heats an infrared emitter positioned above the venturi chamber. A circuit 
of low bandgap cells surrounds the emitter and collects infrared energy 
radiating from the heated emitter. Fins are provided on the photovoltaic 
cell circuit for cooling the cells. An exhaust chimney extends upward from 
the combustion chamber. Exhaust gases from the emitter flow upward between 
the inner walls of the chimney and exit through a top of the chimney. A 
visible light-emitting mantle is positioned in the chimney above the 
emitter. A line delivers fuel and air to a region of the chimney proximate 
the mantle. The air burns the fuel, thereby heating the mantle, which in 
turn emits visible light. Heat exchangers, such as fins, are provided 
along the chimney above the mantle for transferring heat from the exhaust 
gases to the updraft air flow. A housing surrounds all components of the 
generator, with air flow passages created between the chimney and the 
inner wall of the housing. A cooling fan for blowing air past the cooling 
fins of the photovoltaic cell circuit is provided at a bottom of the 
housing beneath the combustion fan. The fan creates an updraft that flows 
past the fins of the photovoltaic cells and between the chimney and the 
housing and exits through ports provided in the housing. A control box for 
housing the safety and ignition controls and gas plumbing connection is 
provided on an outer side wall of the housing. 
The present invention further includes a robust, bandgap matched emitter 
and a method for making the same. The new emitter includes advanced 
ceramic matrix composites that provide the desired combination of high 
fracture toughness and refractory temperature operation and that are able 
to withstand severe thermal stress. The method for making the new ceramic 
matrix composite emitter includes a wet cloth lamination process. With the 
judicious selection of materials, that lamination process produces 
emitters that maintain excellent spectral properties and that possess the 
mechanical stability to resist severe thermal stress. The present method 
provides for wall thicknesses substantially less than those achieved using 
conventional methods, including extrusion, slip casting and isopressing 
techniques. 
The present invention further includes new control systems for TPV 
generators that eliminate the problems associated with metal electrodes. 
One embodiment involves minimizing the metal electrode area within the 
combustion chamber and using a photovoltaic cell component to produce the 
flame sense detection signal. A second embodiment involves replacing the 
metal electrode with a higher temperature non-metallic material such as 
silicon carbide. 
The present TPV generator is a tri-energy source capable of producing 6,000 
BTUs per hour of heat and 40 Watts of electricity. That generated 
electricity is circulated for powering the cooling fan and the combustion 
fan and for energizing battery packs. Combustion gases are exhausted to 
the outdoors through a chimney. Heat is transferred from the combustion 
gases and exhaust flow into an air stream that is used to heat the 
surrounding room. The present invention has numerous potential 
applications and is particularly beneficial for use in off-grid markets 
such as sailboats and other marine vessels, recreational vehicles and 
cabins. 
A thermophotovoltaic generator apparatus includes a thermophotovoltaic 
converter assembly and a cooling fan positioned for generating an updraft 
from beneath the assembly. A fuel source is connected to the converter 
assembly by a fuel line. Controls regulate the flow of fuel from the fuel 
source to the converter assembly. A housing encloses the cooling fan and 
the converter assembly. The converter assembly preferably includes a fuel 
injector cup having a fuel inlet connected to the fuel source and a fuel 
outlet. A combustion chamber is positioned for receiving fuel from the 
fuel outlet and for allowing hydrocarbon combustion. A combustion fan 
generates air flow into the combustion chamber. An infrared emitter 
positioned around the combustion chamber emits infrared radiation when 
heated by combustion gases resulting from the hydrocarbon combustion. A 
photovoltaic cell receiver positioned around the infrared emitter receives 
the infrared radiation and converts that radiation to electric power. Heat 
fins extend outward from the receiver for cooling the photovoltaic cells 
of the receiver. An exhaust chimney extends from a top of the combustion 
chamber for exhausting combustion gases. 
The burner assembly preferably includes a venturi section positioned around 
the fuel outlet where air and the fuel mix. The emitter is positioned 
above the venturi section. 
A light-emitting mantle may be positioned in the chimney. A mantle fuel and 
air line extends between the fuel source and the chimney proximate the 
mantle and delivers fuel and air to the chimney. That fuel mixes with the 
air, combusts and heats the mantle, causing the mantle to emit visible 
light. 
The housing preferably encloses a portion of the length of the chimney. 
Updraft air stream channels are formed between the converter assembly and 
the housing. Openings are provided in an upper end of the housing for 
venting heated air. Heat exchangers, such as fins, are provided along the 
chimney above the mantle and the emitter for transferring heat from 
exhaust gases to the updraft air stream channels. 
The present generator preferably includes a control system. In one 
embodiment, the control system includes a condition sensor positioned in 
the converter assembly for generating signals relating to hydrocarbon 
combustion status, a valve positioned the fuel line leading from the fuel 
source to the converter assembly and the mantle, and a safety and ignition 
controller for receiving the signals and for controlling a position of the 
valve in response to the signals. A control box is preferably connected to 
the housing. The control box houses the controller and connections between 
the fuel source and the converter assembly. 
The fuel source for the present generator is preferably a hydrocarbon fuel 
source such as a natural gas fuel source or a propane gas fuel source. 
At least one electric conduit preferably extends between the receiver and 
the fans for carrying wires routing at least a portion of the generated 
electric power from the receiver to the fan. Additional electric conduits 
for routing at least a portion of the generated electric power to a device 
for recharging batteries may also be provided. 
The emitter is a host refractory compound doped with substitutional ions to 
create a desired emissivity band. The emissivity band of the emitter is 
matched to the response band of the photovoltaic cells of the receiver. In 
preferred embodiments, the infrared emitter includes a first refractory, 
infrared transparent ceramic material layer, a second refractory, infrared 
transparent ceramic material layer, a reinforcing material layer 
sandwiched between the first layer and the second layer, and a doped, 
refractory, infrared transparent ceramic matrix material layer positioned 
over the second layer. The emitter can be perforated. The first and second 
layers include substrates impregnated with primarily alumina. The 
reinforcing layer is a mesh of continuous fibers. The doped layer includes 
a substrate impregnated with primarily magnesium oxide doped with cobalt 
oxide. In preferred embodiments, the reinforcing layer constitutes 
primarily continuous filaments made of refractory materials selected from 
the group consisting of alumina, silicon carbide, silicon nitride, boron 
nitride and refractory metals, and the first, second and doped layers are 
cloth layers impregnated primarily with materials selected from the group 
consisting of alumina, magnesia, spinel, garnet, thoria, yttria, titanium 
dioxide, zirconia, silica, sapphire, hafnia, erbium oxide and ytterbium 
oxide. A variety of emitters are possible with the two important elements 
being continuous fiber reinforcement for durability and doping for special 
control. 
In preferred embodiments, the control of the present generator includes a 
high temperature, non-metallic electrode made of material that is stable 
at temperatures of about 1500.degree. C. or greater. That electrode is 
positioned in the combustion chamber for detecting the presence of 
combustion. An electronic circuit associated with the electrode generates 
flame sense detection signals. A voltage comparator receives the flame 
sense detection signals and converts those signals to logical signals. A 
logic controller receives the logic signals and controls fuel delivery to 
the converter assembly in response to the logical signals. The 
non-metallic electrode is preferably made primarily of a material such as 
silicon carbide. 
In other preferred embodiments, the control includes a voltage comparator 
electrically connected to the photovoltaic cell receiver for receiving 
flame sense detection signals from the receiver and for converting the 
signals to logical signals. A logic controller receives the logic signals 
and controls fuel delivery to the converter assembly in response to the 
logical signals. 
A method for making an infrared emitter includes the step of providing a 
first suspension of highly concentrated ceramic powders. A first cloth is 
soaked in the suspension and is then applied over a forming structure. A 
second cloth is soaked in the suspension. A continuous fiber reinforcing 
material layer is placed over the first cloth and the second cloth is 
positioned over the reinforcing material layer, thereby sandwiching the 
reinforcing layer between the first and second cloths. A second suspension 
of highly concentrated ceramic powders is then provided. A third cloth is 
soaked in the second suspension and is positioned over the second cloth to 
form a four layer assembly. The assembly is then sintered to form a 
ceramically stable, thermal stress fracture resistant emitter. 
The first suspension preferably constitutes primarily alumina. The second 
suspension primarily includes magnesium oxide doped with cobalt oxide. The 
first cloth, the second cloth and the third cloth are made of highly 
absorbent material such as hydroentangled cellulose incorporating a 
polyester binder and are generally thin. The first suspension may further 
include a defloculation agent. In one preferred embodiment, the first 
suspension includes about 220 parts by weight of 2.9 micron alumina 
particles, about 50 ml of deionized water and multiple drops of 
dispersant. 
The step of providing a second suspension preferably includes jar-milling 
electrically fused magnesia powder in deionized water. 
The forming structure is preferably a mandrel or a mold. 
The sintering step preferably includes ramping the assembly to a sintering 
temperature up to about 1500.degree. C. at a rate of about 3 degrees per 
minute, ramping the sintering temperature down at a rate of about 10 
degrees per minutes. 
These and further and other objects and features of the invention are 
apparent in the disclosure, which includes the above and ongoing written 
specification, with the claims and the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the new thermophotovoltaic generator 1 includes a 
thermophotovoltaic converter assembly 3. The assembly 3 includes a 
combustion chamber 5, an infrared emitter 7 positioned around the 
combustion chamber 5, and a photovoltaic cell receiver 9 positioned around 
the emitter 7. In preferred embodiments, a fuel injector cup 11 is 
provided beneath the combustion chamber 5. The cup 11 has a fuel inlet 13 
connected to the fuel source 15 and a fuel outlet 17. Preferably, the 
outlet 17 has dimensions such that fuel is radially dispersed from the 
outlet 17 into a venturi section 19 below the combustion chamber 5. An 
updraft mechanism 21, such as a combustion fan, is provided for generating 
air flow into the venturi section 19 below the combustion chamber 5. That 
air flow mixes with the fuel from the cup 11 in the venturi section 19 and 
combusts. That combustion generates intense heat in the combustion chamber 
5. The emitter 7, which is positioned in the combustion chamber 5 above 
the venturi section 19, increases in temperature and radiates energy. The 
radiated energy is collected by the photovoltaic cells 23 of the receiver 
9. Filters 22 may be positioned between the photovoltaic cells 23 and the 
emitter 7 that allow only selected bands of radiation to pass to the 
photovoltaic cells 23. In preferred embodiments, the emitter 7 includes a 
host refractory compound doped with substitutional ions to create a 
desired emissivity band which matches the response band of the 
photovoltaic cells 23. An electronic circuit 25 bonded to the photovoltaic 
cells 23 convert the collected energy to electric power. The photovoltaic 
cells 23 and electronic circuit 25 are cooled by radial fins 27 extending 
from a back surface of the receiver 9. 
A housing 29, which is preferably cylindrical in shape, surrounds the 
converter assembly 3. A chimney 31 extends from the top of the combustion 
chamber 5 for venting exhaust gases. Preferably, the chimney 31 is 
continuous with the combustion chamber 5 to insure that substantially all 
combustion gases exit through the chimney 31. A cooling fan 33 is 
positioned at the base of the housing 29 for generating an air stream. 
That air stream is heated as it travels over the cooling fins 27 of the 
receiver 9 and continues upward in channels 35 defined by the inner walls 
of the housing 29 and the outer walls of the chimney 31. As shown in FIG. 
1, the chimney 31 preferably extends beyond the top 37 of the housing 29. 
Openings 39 are provided for venting the heated air stream to the 
surrounding room. The openings 39 may be provided at the top 37 of the 
housing 29, as shown in FIG. 1, or along sides 41 of the housing 29. For 
increased efficiency, additional heat exchangers 43, such as heat fins, 
are provided. Those heat exchangers 43 preferably extend from the chimney 
31 and transfer heat from the exhaust gases to the air stream in the 
channels 35. 
A fuel source 15 is connected to the converter assembly 3. The fuel source 
15 is preferably a hydrocarbon fuel source such as a natural gas fuel 
source or a propane gas fuel source. 
In preferred embodiments, a light-emitting mantle 45 is positioned in the 
chimney 31. A fuel line 47 extends between the fuel source 15 and the 
chimney 31 proximate the mantle 45. Fuel and air enter the chimney 31 via 
the fuel line 47 and mix with the exhaust gases. Hydrocarbon combustion 
results, generating heat. The mantle 45, as it increases in temperature, 
radiates visible light. The mantle 45 may be positioned at any point along 
the length of the chimney 31. 
A control box 49 is provided on the wall side of the housing 31. The 
control box 49 houses the safety and ignition controls as well as the fuel 
plumbing connections. As shown in FIG. 1, a first fuel inlet 13 extends 
from the fuel connection in the control box 49 to the fuel injector cup 
11. A second fuel inlet 47 extends between the fuel connection in the 
control box 49 to the chimney 31. The valve for interrupting fuel flow to 
the fuel inlets 13, 47 may be provided in the control box 49, along with a 
microprocessor 51 for controlling the valve; 
The present generator includes a control system 53 for regulating flow of 
fuel from the fuel source 15 to the converter assembly 3 and the chimney 
31. The control system 53 preferably includes a condition sensor 55 
positioned in the converter assembly 3 for generating signals relating to 
hydrocarbon combustion status. That sensor 55 may be suspended in the 
combustion chamber 5 or positioned along the receiver 9. The sensor 55, 
which may be an electrode, generates signals when combustion is occurring 
in the combustion chamber 5. Those signals are sent to a controller 51 
which, in turn, instructs the fuel valves to remain open. When no 
combustion occurs, no signals are generated by the sensor 55. The 
controller 51, in the absence of signals, instructs the valves to close, 
thereby preventing further fuel flow to the converter assembly 3 and 
chimney 31. Manual controls, such as knobs, may be provided for 
interrupting fuel flow. All parts of the control system 53 except the 
sensor 55 are preferably housed in the control box 49. 
Electric conduits preferably extend between the receiver and the fans. 
Those conduits carry wires that rout at least a portion of the generated 
electric power from the receiver to the fans. Additional electric conduits 
are provided for routing at least a portion of the generated electric 
power to a device, such as a battery pack. 
As shown in FIGS. 2 and 3, the present invention preferably includes a 
ceramic matrix composite emitter 7 that is constructed in four layers. The 
first layer 61 and the third layer 63 are made of a refractory, infrared 
transparent ceramic material such as alumina. The second layer 65, which 
is sandwiched between the first layer 61 and the third layer 63, 
constitutes an open mesh of fiber-reinforcing material. In preferred 
embodiments, the material of the second layer 65 is NEXTEL 610 high purity 
alumina fiber. The fourth layer 67 is made of a refractory infrared 
transparent ceramic matrix material, preferably magnesium oxide, that is 
doped with a small amount of cobalt oxide or other suitable compound. 
Layers of material are applied by soaking thin, highly absorbent cloths, 
such as hydroentagled cellulose, in a traditional slip, or a suspension of 
colloidal ceramic powders that are highly concentrated using a 
defloculation agent. The hydroentangled cellulose cloth incorporates a 
polyester binder that gives the cloth excellent wet strength and allows 
easy application of the cloth to a mandrel or a mold without tearing. A 
preferred slip includes about 220 parts by weight of 2.9 micron alumina 
particles (Alcoa A17), about 50 ml deionized water and several drops of a 
dispersant (DARVAN C). The capillary action of the cellulose cloth and the 
dense slip should be present to create a strong non-clay ceramic body that 
exhibits very low shrinkage after sintering. The minimal wall thickness 
that may be achieved using the present method while maintaining very low 
shrinkage after sintering is of great practical importance because a 
continuous fiber mesh is sandwiched between layers without deformation or 
rupture due to the different shrinkage ratios of the fiber and the ceramic 
impregnated cloth laminate. A minimal wall thickness is also desired for 
facilitating heat transfer during combustion. It is possible to achieve a 
total wall thickness far smaller than 50 mils using the present method. 
Magnesia is difficult to stabilize for the purpose of slip casting because 
of hydrogen bonding reactions that take place in aqueous solutions. Using 
the new method, excellent results are achieved by using a dense 
electrically fused magnesia powder, as opposed to commercially available 
calcined magnesia powder. The electrically fused magnesia powder is 
jar-milled in deionized water for a period of time that is dependent on 
the size of the starting powder and the hydration reaction taking place 
during jar-milling. The resulting jar-milled product is an aqueous 
magnesia slurry having optimal particle distribution for impregnating a 
cloth laminate. Once the cloth laminate is impregnated with the product, 
the impregnated cloth is dried and sintered, with the shrinkage of the 
magnesia layer being well matched to the fiber reinforced alumina 
substrate layers. The magnesia layer also shows excellent adhesion. 
A paddle-wheel mandrel having thin, radial alumina felt supports is 
preferably used for maintaining the shape of the unsintered parts and for 
preventing warping during the sintering process. The sintering process is 
preferably a two step process wherein the temperature is first ramped up 
to about 1500.degree. C. at a rate of about three degrees per minute and 
then ramped down at a rate of about ten degrees per minute until the 
paddle-wheel mandrel can be removed. The sintered product is then heated 
for about two hours at a temperature of about 1650.degree. C. to provide a 
chemically stable, thermal stress fracture resistant emitter. 
The new ceramic emitters are preferably made of alumina and magnesia. Other 
possible refractory materials for the continuous fiber layer include, but 
are not limited to, silicon carbide, silicon nitride, boron nitride and 
refractory metals, such as platinum or boron coated tungsten. Preferably, 
the infrared emitting materials constitute only a small fraction of the 
total mass of the emitter. In addition to alumina and magnesia, possible 
ceramic matrix materials include, but are not limited to, spinel, garnet, 
thoria, yttria, titanium dioxide, zirconia, silica, sapphire, hafnia, 
erbium oxide and ytterbium oxide. 
The present invention further includes a control system compatible for use 
with high temperature TPV generators. Standard control systems include 
electronic circuitry and electrodes for flame sense detection. An analog 
signal produced by the flame sense circuit is converted to a logical 
signal using a voltage comparator. That logical signal is routed to a 
logic sequencer, such as a microprocessor. In one embodiment of the new 
control system, a photovoltaic cell of the generator produces the flame 
sense detection signal. In that embodiment, no electrode or complex flame 
sense circuit is needed, as the photovoltaic cell directly generates a 
signal that is delivered to the voltage comparator, converted to a logical 
signal and directed to the microprocessor. Since the remainder of standard 
control systems may be used for all other functions, the new embodiment 
has minimal impact on current industry standard design. When the new 
system is on and working as expected, the photovoltaic cell generates a 
signal indicating that the gas is combusting properly as follows. The hot 
combustion gases heat the emitter. The emitter radiates energy to the 
photovoltaic cell. The cell converts the energy to electric power and 
generates an analog voltage signal. That analog voltage signal is 
delivered to a voltage comparator, where the input is converted to a 
logical signal and directed to the microprocessor. In the absence of 
combustion, the emitter is not heated and therefore does not emit radiant 
energy. The photovoltaic cell, in turn, does not collect radiation or 
generate voltage signals. In the absence of the signals, the controller 
logic of the microprocessor instructs the gas valve to close or shut off. 
By using the photovoltaic cell as the flame sense detector, existing 
requirements for complex flame rectification circuitry and for acceptable 
metal or conductive electrodes within the high temperature TPV combustion 
chamber are eliminated. 
In another preferred embodiment of the new control system, metal electrodes 
are replaced with electrodes made of material that is stable at high 
temperatures. In this embodiment, the industry standard controller is used 
directly. The electrode material of the new control system is stable at 
high temperatures, is formable into suitable electrode geometry, is 
supportive of the flame rectification process and has an electrical 
conductivity that is compatible with existing flame sense electronic 
circuitry. In preferred embodiments, the electrode is a non-metallic 
electrode made primarily of silicon carbide. In one embodiment, the 
electrode takes the form of a thin piece of NOTOX. In another preferred 
embodiment, the electrode is a SC-6 silicon carbide filament. Using the 
new electrodes, flame sense signal levels are enhanced by increasing the 
surface area of the ground electrode. With silicon carbide based 
electrodes, acceptable signals for satisfying the controller are obtained 
using reasonable surface areas inside the combustion chamber. The new 
non-metallic, high temperature electrodes remain stable and do not cause 
undesired deposits in the TPV generator. 
While the invention has been described with reference to specific 
embodiments, modifications and variations of the invention may be 
constructed without departing from the scope of the invention, which is 
defined in the following claims.