**ABSTRACT** - A steel mill extrudes steel billets at temperatures above 1400 °C continuously 24 hours a day. The steel billets then go to cooling beds where they slowly cool down to below 1100 °C. JX Crystals Inc makes Gallium Antimonide (GaSb) thermophotoyoltaic (TPV) cells that can generate over 1 W/cm when exposed to infrared radiant energy from glowing steel at temperatures above 1100 °C. There is a great opportunity to integrate GaSb TPV receivers into steel mill operations to generate electricity economically utilizing this now wasted radiant energy.

In a recent visit to a steel mill in China producing 10 million Metric-Tons (MT) of steel a year, the operators of the mill told us that they have 2000 in' of glowing steel in process continuously. At 1 W per cm', this equates to the potential to produce 20 MW of electricity for that steel mill. In 2012, the world steel production was 1,548 million MT. So, the world wide potential for TPV electricity production could be 3.1 GW.

At what cost will TPV be affordable? This application has two advantages over solar PV. The first is the high power density, a factor of 100 over solar PV modules, translating to a potential cost advantage. The second distinct advantage over solar PV is the 24 hours of operation since the sun is only available on average for 8 hours per day. One can estimate the potential value of a TPV plant from the potential annual revenues. Assuming electricity at 8 cents per kWh, 1 kW of TPV electric power capacity will produce 8765x0.08 = S700 dollars per year. This implies a 3 year payback at S2.1 per W. Cost is a function of volume but should come down to below SI.5 per W at volumes above 1 MW.

Index Terms — Co-generation, GaSb, Photovoltaic Cell, Steel-Mill, Thermophotovoltaics, TPV.

I. Introduction

The melting temperature of steel depends on the type of steel. Carbon steel has a melting point of 1425 degrees C to 1540 degrees C while stainless steel has a melting point of 1510 degrees C. A black body at 1400 K (1127 C) emits 3.4 W/cm2 of infrared (IR) radiant energy at wavelengths equal or less than 1.8 microns and the JX Crystals Inc GaSb infrared sensitive thermophotovoltaic cells [1. 2] can convert 30% of this radiant energy into electric power. This means that at least 1 W/ciu' of electric power could be generated from the now wasted radiant energy in a steel mill. In a recent visit to a steel mill in Xuan Gong China, we were told that they have 2000 nr of glowing steel at temperatures above 1127 C in process 24 hours per day and 7 days a week. See figure 1. At 1 W/cm'. this means that it is potentially possible to generate 20 MW of electricity with TPV at this steel mill alone.

A recent visit to a steel mill in China provided an interesting perspective. That steel mill produces 10 million Metric-Tons (MT) of steel a year. A typical billet has a square cross section of 16 cm x 16 cm and a length of 5.6 m and weighs 1 MT. This equates to 1.250 billets in process every hour. If TPV converter circuit arrays are placed along the two 15 cm x 5.6 m faces adjacent to each of these billets, the area of these TPV arrays would be 1.250 x 0.15 x 2 x 5.6 = 2100 m2. This calculation is consistent with the input from the operator of the mill.

One can now extrapolate to world wide TPV electric power production potential from the steel industry. In 2012, the world steel production was 1,548 million MT [3]. So, the world wide potential for electricity production could be 3.1 GW. In theory, one could double this number by utilizing all four facets from the billet. Furthermore, if one notes that each billet of steel gets heated to melting twice during production, once for casting and a second time for shaping, the potential TPV electric power production could then approach 10 GW.

Next, one might ask, at what cost will TPV be affordable? The fact that this potential TPV electric power facility would operate for 24 hours per day is a distinct advantage over solar PV where the sun is only available on average for 8 hours per day. One can estimate the potential value of a TPV plant from the potential annual revenues. Assuming the value of electricity to be 8 cents per kWh and noting that there are 365cx24 = 8760 hours per year, 1 kW of TPV electric power capacity will produce 8765x0.08 = S700 dollars per year. If one asks for a 3 year payback, the TPV power plant might be worth S2100/kW or $2.1 per W. Figure 2 shows an estimate of the cost of GaSb TPV circuits [4]. As is shown, the costs are a function of volume but will come down to affordable levels at volumes above 1 MW.

# Economic Potential for Thermophotovoltaic Electric Power Generation in the Steel Industry

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