Recent advances in thermophotovoltaic generator design make this technology increasingly attractive for generating electricity inside gas appliances, either to free the appliance from dependence on wall-plug power or to co-generate power for the home.

A TPV system generates electricity from radiant heat using photocells, and it has been around at least since the U.S. Army demonstrated an early TPV generator in 1956. Conceptually, TPV is just a collection of photocells arranged around a burner, as in the simple demonstration unit of Fig. 1. Almost any burner will work for TPV. Burners using natural gas, propane, diesel fuel, and wood chips have all been used in TPV generators. NASA and the U.S. Navy have even had nuclear-powered TPV programs.

TPV works on the same principle as solar panels. The sun is four times hotter than the TPV burner, but 10 trillion times farther away. With the burner very close to the photocells, TPV generators are much smaller than solar panels, and, therefore, small enough to fit inside appliances like furnaces, water heaters, and clothes dryers. An advanced TPV generator the size of a wine bottle can generate the same power as an outdoor solar panel the size of a car. But because the TPV burner is much cooler than the sun, the radiant heat is infrared. This means that special photocells, wavelength-selective optics, and other design innovations are necessary to achieve good efficiency. Developing all these elements in concert has taken time.

The challenge

Enlarge this picture Fig. 2. Burner Radiant Heat. Silicon solar cells can use only 1.3 percent of the burner radiant heat. GaSb cells use about 27 percent, but 70 percent or more of the radiant heat is unusable. A wavelength selective mirror reflects unusable, long wavelength radiant heat back to the burner.

The fundamental challenge in designing TPV generators is burner temperature. The surface of the sun is roughly 6,000 DegC, but temperatures much over 1,400 DegC are impractical for TPV burners, both from a materials standpoint and because very high-temperature combustion produces nitrogen-oxide pollution. Because the TPV burner is so much cooler than the sun, most of the radiant heat the burner gives off is infrared light. Traditional silicon solar cells cannot convert this infrared light to electricity, so infrared sensitive photocells made from gallium antimonide (GaSb) are typically used in TPV generators. But even GaSb cells use only part of the burner radiant heat.

Controlling the long wavelength radiant heat that even GaSb photocells cannot use is critical in TPV generators. If the unusable, long wavelength part of the burner radiant heat can be reflected back to the burner, the burner temperature is increased and the cooling requirement for the photocells is reduced. The result is a TPV generator that is smaller, more efficient and less costly. Such reflection requires a special type of mirror.

Fortunately, wavelength selective (dichroic) mirrors that reflect long-wavelength radiant heat while transmitting shorter wavelengths are widely used. These types of mirrors are used inside high-efficiency light bulbs to reflect unwanted infrared radiation back onto the filament while letting visible light out. For TPV, one needs wavelength selective mirrors set to a somewhat longer wavelength than those used in light bulbs.

Wavelength-selective mirrors are made by depositing thin, alternating layers of transparent materials with differing refractive indices. Titanium and silicon oxides are often used, but a wide variety of materials are available. To make the wavelength-selective mirror for TPV, the thickness of the layers is increased, compared to the process used for high-efficiency light bulbs, so these mirrors are easily made.

Photocell concerns

Fig. 3. Propane Heater with TPV. The Midnight Sun TPV stove was developed by JX Crystals of Issaquah, Wash. This propane-fired space heater incorporates a porous radiant burner and TPV generator with GaSb photocells and wavelength selective mirrors. The TPV generator powers the circulation fan and provides auxiliary 12 V power.

The next concern is the type of photocell. Semiconductors made from materials other than silicon are common today. Gallium arsenide (GaAs) is used in cell phone ICs and high-efficiency solar cells. Gallium phosphide (GaP) and gallium nitride (GaN) are used for light-emitting diodes and solid-state lasers. Gallium antimonide (GaSb) used for TPV photocells is similar to other gallium-based semiconductor materials, but it is easier and safer to make than GaAs because the ingredients are less toxic and processing temperatures are lower. While gallium is more costly than silicon, the process for making GaSb photocells is very similar to that for making low-cost silicon photocells used in solar panels. And, because the electrical output from a GaSb TPV photocell can be 100 to 200 times the output from a silicon solar cell the same size, GaSb photocell manufacturing is not a barrier to using TPV generators in appliances.

A few TPV appliances have already been built. Propane-fired TPV stoves using both GaSb photocells and wavelength-selective mirrors were built by JX Crystals, Issaquah, Wash., in 1999. Using a low temperature (1,200 DegC) burner and fan cooling of the cells, this stove delivered 2.5 percent efficiency and was not a commercial success. Greater efficiency, smaller size, and quieter operation were necessary.

Several things are needed beyond GaSb infrared-sensitive photocells and wavelength-selective mirrors to make TPV successful in appliances. Advanced burner designs that operate at temperatures above 1,200 DegC make TPV generators smaller and more efficient because their radiant heat is more intense and less of it is the long wavelength radiation that the infrared photocells can’t use. Direct, liquid cooling of the photocells, a technology used in high-end microprocessors and widely used in the 1980s for cooling supercomputers, can improve cell cooling, reduce cooling noise, and can help make TPV generators smaller than ever. But the greatest efficiency gains for TPV come from recuperation.

The exhaust gas leaving a very hot TPV burner is extremely hot. This heat in the exhaust carries with it a large part of the energy that was in the fuel. While this exhaust energy can be harnessed in a furnace, water heater or heating appliance, the heat energy in exhaust effectively bypasses the TPV photocells. Recuperation involves passing the exhaust gas through a heat exchanger where it heats in-coming combustion air. Recuperated, high-temperature burners like the one shown in Fig. 4, have been used with GaSb cells and wavelength-selective mirrors to achieve 16 percent conversion efficiency from fuel energy to electric output in laboratory tests.

In recuperated high-temperature burners, the in-coming combustion air is heated to 1,100 DegC or more before the fuel is introduced and combustion starts. It can be challenging to manage this high-temperature combustion air, along with the fuel, ignition system and flame-detection components that must operatein this environment. Controlling pollution from these burners is another challenge. As a result, sophisticated recuperator burners likethis may not be the solution for every TPV appliance application.

Sometimes compromises in efficiency can produce much simpler, smaller, and less costly TPV generator solutions that better fit a given application. For example, porous radiant burners are being used in more gas appliances to control pollution. These burners operate with simple, pre-mix fuel systems and offer efficient, low-pollution performance. Advanced versions offer a key to simple TPV generators well suited to gas appliances.

It can be generalized, based on years of design experience, about the efficiency obtainable with these two approaches. Pre-mix radiant burners limit TPV generator efficiency to about 7 percent because the energy in the exhaust effectively bypasses the photocell conversion process. More complex recuperated burners offer 15 percent or more conversion efficiency, but require relatively complex and expensive burner designs.

In gas appliances

Fig. 4. Recuperated Burner. TPV efficiency of 16 percent has been demonstrated with this recuperated burner for an Army TPV generator.

Consider how a TPV generator might work in a gas appliance. There are basically two application categories: self-powering and co-generating. For self-powering, TPV only needs to produce enough power for the electrical functions of the specific appliance. For a self-powered gas clothes dryer, that means enough power to run the drum motor and controls. For a self-powered furnace, that means just enought power to run the blower, draft inducer and controls. Many self-powering applications need less than 5 percent efficiency and, therefore, are a good fit for TPV generators with simple pre-mix radiant burners.

Co-generating appliances return power to the grid with the objective of reducing overall home power usage. While maximum generator efficiency and, hence, maximum power generation would seem ideal, this is not necessarily the case. For a simple, mass-market appliance that will connect to existing house wiring, there are limitations on how much power can be delivered, and this limitation sets an upper bound on how much conversion efficiency is actually useful.

Enlarge this picture Fig. 5. Efficiency. Sophisticated TPV generators with recuperated burners can achieve efficiencies of 15 percent or more while generators with pre-mix radiant burners are limited to about 7 percent efficiency or less. In spite of the lower efficiency, pre-mix burner based TPV generators may offer the right choice for gas appliances.

For example, furnaces in the U.S. are required by the National Electrical Code to be supplied by at least a dedicated 15 A/120 V electrical circuit. No more than 1.4 kW of power can be returned from a TPV furnace to the power grid using this circuit’s 12 A continuous rating. If the furnace uses 600 W internally, only 2 kW of total generator output can be utilized. For a 100,000 BTU/H input furnace, this equates to 6.8 percent generator efficiency. A more efficient generator used in this furnace would necessitate modifying house wiring to deliver the power. Again, a simple, pre-mix radiant burner TPV solution is the answer.

While focusing on TPV generator technology and some TPV history, it is important to remember that other small generator technologies are available for use in gas appliances. Appliance designers have fuel cells, Stirling engines, thermoelectric generators and other technologies to choose from when designing a self-powering or co-generating gas appliance. Where simplicity, on-off operation, low noise, modest cost, and small size are important, TPV may be a good fit.



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