Thermophotovoltaics

Three new developments have now occurred making economical TPV systems possible. The first development is the diffused junction GaSb cell that responds out to 1.8 microns producing over 1 W/cm2 electric given an IR emitter temperature of 1200 C. This high power density along with a simple diffused junction cell makes an array cost of $0.5 per Watt possible. The second development is new IR emitters and filters that put 75% of the radiant energy in the cell convertible band. The third development is a set of commercially available ceramic radiant tube burners that operate at up to 1250 C. Herein, we describe a 1.5 kW TPV generator / furnace incorporating these new features. This TPV generator / furnace is designed to replace the residential furnace for combined heat and power (CHP) for the home.

burning a fuel. If very low cost solar cells can be made, a homeowner can generate his own electricity at rates below the electric utility rate. However, while this can be true during summer months, there is a problem in winter months when the sun doesn’t shine. Low Bandgap photovoltaic or “Solar” cells can solve this problem. A homeowner can put solar cells on his roof for electric power in the summer months and “solar” cells in his furnace for electric power during the winter months. The idea of “solar” cells in a heating furnace is called ThermoPhotoVoltaics or ThermoPV or TPV. The idea is that a ceramic element is placed in the flame in the furnace and this element then glows like the coals in a fireplace. “Solar” cells near by then convert the glow into electricity. Using TPV, the homeowner generates electricity whenever he needs heat. Therefore, it is not necessary to burn additional fuel.

While this TPV concept is simple, the problem has been that the two types of solar cells (or more accurately photovoltaic cells) are not the same. While the solar cells on the roof convert visible light into electricity, the TPV cells in the furnace need to convert infrared radiation into electricity. JX Crystals has invented and developed the required GaSb TPV cells.

After inventing the required IR sensitive cells, JX Crystals began to develop complete TPV systems. This effort then required us to invent and integrate several key components into a practical generator that can be manufactured economically. While the TPV idea was first proposed in 1960, three new developments have taken place in the last 10 years that now make economical TPV systems possible. The first new development is the diffused junction GaSb cell [1,2,3,4] that responds out to 1.8 microns producing over 1 W/cm2 electric given an IR emitter temperature of 1500 K (1225 C). Two TPV circuits incorporating these new cells are shown in figure 1a along with a power curve in figure 1b showing 2 Watts per cell. The power density is approximately 100 times more than the traditional planar
solar cell making cost of $0.5 per Watt possible.

TPV cells and solar cells are natural allies. They both produce electricity without the need for additional fuel. The bar graph shown in figure 6 is for the electricity use in a typical residence in NY. It illustrates how TPV and solar can work together to produce home electric power. This bar graph is typical for Mid Atlantic & New England states as well as states around the Great Lakes, Alaska and Canada. The economics for photovoltaics is now very exciting as the following calculation shows. Given investment for manufacturing and marketing, we expect our Residential TPV Furnaces (figure 3) to be selling for $1500 on top of the standard heating equipment cost. At what price will a Residential TPV Furnace begin to be cost effective? Referring to the bar graph in figure 5, the annual savings in a home-owner’s electric bill will be $376 per year at 10 cents per kWh. At this savings rate, the payback time will be 4 years.