Keywords

Introduction

The integrated steel industry in Japan has significantly reduced its energy use for the past several decades by investing in energy efficient processes and facilities, and has kept the highest energy efficiency in the world. However the steelmaking industry is strongly required to develop new technologies for further energy conservation in view of energy security, high and volatile energy prices, and climate change. One of the key technologies to achieve the requirement is waste heat recovery. Figure 17.1 shows the waste heat for various processes of steelworks [1]. We have been investing in many energy efficient processes and facilities. However, a large amount of heat still remains wasted. For waste heat such as radiant heat from steel products which has not been efficiently used, thermoelectric generation is one of the most effective technologies to recover it, because thermoelectric generation can convert heat directly into electric power using the Seebeck effect. This environmentally friendly technology is expected to have an important role for energy conservation, waste heat recovery and CO2 reduction. Thermoelectric generation has many advantages such as no mechanical moving parts, compact, no CO2 emissions and long lifetime. This paper describes the thermoelectric generation system which was installed at East Japan Works of JFE Steel Corporation, using waste radiant heat from the butt welded pipe.

Fig. 17.1
figure 1

Waste heat energy in the various processes in steelworks [1]

Full size image

Thermoelectric Generation System at Steelworks

Thermoelectric Generation Module

Figure 17.2 shows a Bismuth-Telluride thermoelectric generation module (TEG module), which is made from a number of n-type and p-type thermoelectric elements [2]. It has high conversion efficiency and is a high-powered commercial thermoelectric generator. The size of TEG module is 50 mm × 50 mm × 4.2 mm. Its maximum output power is 24 W, with a maximum conversion efficiency of 7.2 %, when the hot-side temperature is 553 K and the cold-side temperature is 303 K respectively [3]. The output P can be obtained from the external load r connected to the TEG module. P brings the maximum output P gmax in impedance matching when R i = r.

Fig. 17.2
figure 2

Bi-Te thermoelectric generation module [2]

Full size image

Thermoelectric Generation Unit

Figure 17.3 shows an illustration of a partial cross section of the thermoelectric generation unit (TEG unit). The TEG unit consists of 16 TEG modules. These thermoelectric generation modules are separated into four groups of four thermoelectric generation modules connected in series, and each group is connected in parallel. The TEG modules were connected to the MPPT control unit to achieve maximum power at any temperature conditions. The output of 16 thermoelectric generation modules is used to charge storage batteries and supply power to LED lamps through the MPPT controller. Each TEG module is sandwiched between the heat collection plate and the water cooled plate by a spring structure, meaning that almost constant pressure is applied to the modules, even if a temperature difference is generated. The pressure is set at 1 MPa, with a cooling water flow rate of 10 l/min. The size of the heat collection plate is about 400 mm × 280 mm. Copper is used for the heat collection plate. The surface treatment of the heat collection plate is an electroless nickel plating (black) [3]. The temperature of heat collection plate T h and water cooled plate T c were measured by thermocouples. Figure 17.4 shows the thermoelectric generation unit installed in the JFE’s butt welded pipe mill. The heat collection plate of the TEG units was heated by radiant heat from the butt welded pipe.

Fig. 17.3
figure 3

Thermoelectric generation unit [3]

Full size image
Fig. 17.4
figure 4

Thermoelectric generation unit installed to the JFE’s butt welded pipe mill

Full size image

Verification Tests

JFE started verification tests to generate electric power by using radiant heat from butt welded pipes in 2011. The performance of the TEG unit had been investigated under various operating conditions in steelworks. Figure 17.5 shows the results of the measured temperature difference ∆T TEGU(=T h − T c). Here, ∆T TEGU is the temperature difference of the heat collection plate (T h) and the water cooled plate (T c) of the thermoelectric generation unit.

Fig. 17.5
figure 5

One of the examples of the verification test data

Full size image

When the welded pipe mill is under operation, the TEG unit is warmed up by radiant heat inducing a temperature difference, and the power output is generated by TEG unit.

The Eqs. (17.1) and (17.2) are the basic equation of thermoelectric generation:

$$ {Q}_{\mathrm{a}}={\alpha}_{\mathrm{e}}{T}_{\mathrm{hj}}I-\frac{1}{2}{r}_{\mathrm{e}}{I}^2+{K}_{\mathrm{e}}\varDelta {T}_{\mathrm{j}} $$
(17.1)
$$ {Q}_{\mathrm{d}}={\alpha}_{\mathrm{e}}{T}_{\mathrm{cj}}I+\frac{1}{2}{r}_{\mathrm{e}}{I}^2+{K}_{\mathrm{e}}\varDelta {T}_{\mathrm{j}} $$
(17.2)

where the heat Q a is supplied by the thermal source at the hot surface, Q d is the heat flowing out at cold surface, α e is the Seebeck coefficient, r_e is the internal electrical resistance, K e is the thermal conductance, I represents the electric current [4], ∆T j(=T hj − T cj) is the temperature difference of thermoelectric element, T hj is the hot side temperature, and T cj is the cold side temperature. These equations are derived from the heat equation subject to the Peltier effect and Joule heating with boundary conditions of the hot side temperature and the cold side temperature.

The power generation output P g is given by Eq. (17.3):

$$ {P}_{\mathrm{g}}={Q}_{\mathrm{a}}-{Q}_{\mathrm{d}}=\left({\alpha}_{\mathrm{e}}\varDelta {T}_{\mathrm{j}}-{r}_{\mathrm{e}}I\right)I={R}_{\mathrm{L}}{I}^2 $$
(17.3)

where R L is the external electric resistance.

Also, the maximum generation output P gmax is given by Eq. (17.4).

$$ {P}_{\mathrm{g} \max }=\frac{1}{4}\frac{{\left({\alpha}_{\mathrm{e}}\varDelta {T}_{\mathrm{j}}\right)}^2}{r_{\mathrm{e}}} $$
(17.4)

In order to obtain high power generation output, a large temperature difference ∆T j is required. On the other hand, T hj must be under the maximum tolerance temperature. Therefore, it is very important to simulate T hj for various conditions. In this system, the heat input Q a is mainly the radiant heat from the pipe. The heat input Q a is expressed by Eq. (17.5):

$$ {Q}_{\mathrm{a}}=\varepsilon \cdot F\cdot \sigma \cdot A\left({T}_{\mathrm{s}}^4-{T}_{\mathrm{h}}^4\right)+h\cdot A\cdot \varDelta T $$
(17.5)

where ε is the emissivity, σ is Stefan-Boltzmann constant, F is the view factor, T s is pipe temperature, T h is the hot side temperature of the thermoelectric generation unit, and h is heat transfer coefficient.

The view factor F is a function of the distance from the pipe to the TEG unit and it is inversely proportional to the distance. Therefore, the heat input Q a becomes larger as the TEG unit approaches the pipe. With a larger heat input, the heat collection plate temperature rises and the temperature difference ∆T TEGU increases.

Using the thermal resistance of TEG unit and Eqs. (17.1)–(17.5), the relationship between ∆T TEGU and the distance from the pipe to the TEG unit is numerically investigated. The result is shown in Fig. 17.6. The experimental results are compatible with the simulation results.

Fig. 17.6
figure 6

Relationship between the distance from the pipe to TEG unit and the temperature difference ∆T TEGU

Full size image

Figure 17.7 shows the power output of TEGS units as a function of ∆T TEGU. The measurements of the power output of the TEG units seem to confirm the simulated output. The TEG output increases with ∆T TEGU and the thermoelectric generation system output is about 250 W when the ∆T TEGU is 250 K.

Fig. 17.7
figure 7

Electric power output of TEG units as a function of ∆T TEGU

Full size image

Conclusion

A thermoelectric generation unit was tested in the butt welded pipe mill at the East Japan Works in the Keihin District of JFE Steel Corporation to recover waste heat. The thermoelectric generation unit which has 16 thermoelectric generation modules provides about 250 W when the temperature difference between the heat collection plate T h and water cooled plate T c at the thermoelectric generation unit is 250 K. The resulting electric output is a good fit between the simulation and the experimental data.