To save energy and reduce CO₂ emissions, the utilization of solar energy and waste heat using latent heat storage (LHS) has emerged as an attractive solution because of advantages such as large density of heat storage, constant-temperature heat supply, and repeatable utilization without degradation. This review describes research trends in LHS technologies using phase-change materials (PCMs) based on papers published from 2001–2009, and state-of-the-art LHS technologies for high-temperature applications over 100°C, such as solid–solid PCM, encapsulation of PCMs, PCM composites, solar power generation with LHS, and waste heat recovery systems.

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Fe base Phase Change Materials (PCM) for high temperature around 953–1273 K using multiple solid–solid transformations were developed. The amount of latent heat of PCM samples were measured by Differential Scanning Calorimetry. The amount of latent heat of Fe–xCo samples increased with increasing Co addition. The reducing atmosphere was suitable for the samples developed in this work. The durability performance of samples was confirmed by heat treatment experiments. From the viewpoint of heat accumulation/supply ability, cost and durability performance, Fe–Co(–Cr) system alloy is one of the most suitable Phase Change Materials for heat recovery from high temperature exhaust gases.

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Carbon dioxide reduction is an important problem for steel industries. Steelmakers in Japan are trying to reuse and recycle waste energy and resources. Slag is a potential resource of energy and materials because it contains a lot of elements and is at the high temperature of around 1500°C when exhausted.
The RCA (Rotary Cup Atomizing) method is a popular process in the slag atomizing method. However, when there is a slip between the slag and cup, the energy of cup rotation is not used effectively. In the present study, we developed a rotary cylinder atomizing (RCLA) method that can efficiently use the rotation energy for atomizing the molten slag. Two kinds of RCLA were developed and the slag atomizing behaviors were investigated.
The minimum diameter of the slag particle was from 10 to 50% of the nozzle diameter. The obtained slag particles were amorphous spheres having high sphericity. The higher rotation speed and smaller nozzle diameter could make smaller particles.

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Slag is a potential resource of energy and materials because it contains a lot of elements and is at the high temperature of around 1500°C when exhausted.
In the previous study, we developed a rotary cylinder atomizing (RCLA) method that can efficiently use the rotation energy for atomizing the molten slag. The minimum diameter of the slag particle was from 10 to 50% of the nozzle diameter. The obtained slag particles were amorphous spheres with high aspect ratios. The higher rotation speed and smaller nozzle diameter could make smaller particles. In the present study, the mechanism of slag particle formation from spouting slag string through the nozzle was investigated using the high-speed camera and the theoretical approach was performed.
The slag was string-shaped when spouted from the nozzle. The relationship between the particle diameter (d) and nozzle diameter (2a) was derived as follows:
[Equation]
Where ρ is density, L is the distance from the center of rotation to the tip of the slag string, Z is rotation speed and γ is surface tension of slag.
The flow rate of slag was evaluated using Hagen–Poiseuille's equation and the relationship between the particle diameter and the slag string diameter was obtained using Weber's equation. By comparison between the experimental and calculated results, we concluded that a string diameter of 0.2 mm for a 1.3mm nozzle diameter was adequate in this experiment.

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In a previous study, a rotary cylinder atomizing (RCLA) method for slag was developed and the slag spouting behavior from a nozzle was investigated. The theoretical approach of the previous study agreed well with the observed result.
In this study, the potential of slag particles obtained by RCLA method as a phase changing material (PCM) was investigated.
The latent heats and the temperature ranges of heat generation were measured for two kinds of amorphous slags (slag particles and water granulated-slag from blast furnace) using DTA (differential thermal analysis).
Mathematical simulation of a countercurrent heat exchanger was performed and the effect of slag latent heat on gas temperature was estimated. The temperature range of the latent heat of amorphous slag was estimated to range from 800 to 950°C. The values of latent heat for slag particles and the water-granulated slag were 294 kJ/kg and 235 kJ/kg, respectively.
Simulation results indicated that the latent heat of slag can decrease the temperature of waste gas which might increased the usable kind of waste gas.

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Supercooling and subsequent solidification behavior of phase change materials (PCM) of sodium acetate trihydrate (SAT) and erythritol were studied by using a thermal analysis technique. A molten SAT specimen easily supercooled below 0°C and the observed maximum degree of supercoolimg was 89°C. Three changes due to phase transformation were observed in the cooling curve of the SAT. First change corresponds to the precipitation of sodium acetate (CH₃COONa), and second change is the recalescence due to the solidification of supercooled SAT. It was considered that third change corresponds to the solid state transformation of SAT. From the maximum degree of the supercooling of SAT, the solid/liquid interfacial energy of SAT was evaluated as 5.56×10⁻² J/m². The observed maximum degree of the supercooling of erythritol was 91°C. An experiment to induce the nucleation in supercooled liquid of SAT was carried out and the effectiveness of the combination of the ultrasonic irradiation and the addition of nucleation catalysts was demonstrated.

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Latent heat storage is one of the key technologies to utilize waste heat because this technology enables to store thermal energy in high density and for long time. This study focuses on flow characteristics in heat storage bath in form of direct contact since it has great effects on operation limit or thermal efficiency. The two-phase flow behavior of immiscible fluids was measured in a small cylindrical heat storage bath which had a nozzle at the center of the bottom and was settled in a rectangular transparent tank. A heating medium oil was fed from the nozzle to the bath half filled with a phase change material (PCM). Two series of the experiments were carried out. One was cold isothermal experiment which used water as model PCM, and the other was heat storage experiment which used sodium acetate trihydrate as the PCM. The formation behavior of the heating medium droplet changed from single droplet at nozzle outlet, single droplet from liquid column, multiple droplet from liquid column to atomization at the nozzle with increase in inflow velocity of heating medium. At the interface between the PCM and the heating medium, the heating medium droplets accumulated, coalesced, enlarged and disrupted, then the heating medium was released from the PCM layer. Furthermore the variation of flow behavior in the heat storage bath with progress in phase change of the PCM was elucidated.

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In recent years various approaches to reduce carbon dioxide emission from iron- and steelmaking industry have been made, and recovery of the waste heat is one of these approaches. Heat exchanger is one of key facilities to recover the waste heat released as sensible heat of fluids. In this study a new concept to raise efficiency of heat exchanger has been proposed. The temperature boundary layer is scraped from moving heat transfer surface by rigid blades to remove the heat transfer resistance of the boundary layer. Some prototypes of the heat exchangers of double-tube type with this concept were made. The inner tube of the heat exchanger rotates and the blades are set on both sides of the inner tube. The performance of the heat exchanger was examined in various combinations of working fluids. The results showed that the overall heat transfer coefficients, which is a key parameter to expresses heat exchanger performance, increased with increase in revolution of the inner tube regardless of working fluids. Thus the effectiveness of this new concept was confirmed. The effect of the boundary layer scraping is remarkable in the liquid–liquid or liquid–steam systems, and the heat transfer coefficient increased by about ten times in some cases. For the combination including gas, although this concept is useful, there still is a room to optimize the design of the heat exchangers to improve the performance. It is expected that practical use of this heat exchanger is great help to recover the waste heat from the industries.

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Industries consisted of high temperature processes discharge a certain amount of waste heat in a wide temperature range. In such industries, on the other hand, a large amount of primary energy is still used for drying processes of raw materials and wastes such as wet dust and sludge. In this study, a new drying process was proposed by employing metallic heat storage materials (HSMs) as drying media, which can store waste heat from low to mid temperatures (250–500°C) as both sensible and latent heat. The process possesses several advantages that the process size and exhaust gas volume are significantly small and heat recovery from a dusty gas is possible.
The cold model experiments understanding the motion of HSM balls and powder inside the rotary dryer and its numerical simulations were carried out. The behavior of HSM balls was simulated by using the friction coefficient as a fitting parameter. Further, assuming that a HSM ball forms a composite particle with a powder layer, the numerical simulations of its drying process were conducted. They confirmed that, drying time can be shortened significantly when the latent heat was considered. Design of the drying process of wet dust in a practical scale showed that the size of the dryer will be several times smaller than that of a conventional rotary dryer because of higher volumetric heat transfer coefficient between HSM and dusts. Since the heat exchange tower between waste gas and HSM balls is also compact, the proposed process shows a certain potential to be applicable as an actual drying process.

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Application of LaGaO₃ based electrolyte for steam electrolysis was studied for the useful utilization of unused heat energy from steelmaking process. It was found that the H₂ formation rate almost obeys the Faraday's law suggesting unity of oxide ion transport number in LaGaO₃ perovskite under an electrolysis condition up to 2.0 V applied potential. Among the examined cathode catalyst, nickel shows the smallest cathodic overpotential and the addition of Fe to Ni is highly effective for increasing the H₂ formation rate in the steam electrolysis at 873 K. The highest H₂ formation rate is obtained at the composition of Ni : Fe = 9 : 1. Short stack of the steam electrolysis cell consisting of 25 cells in series connection was also studied and the formation rate of H₂ was achieved as large as 70 L/min at 1.8 V applied potential. Heat balance was calculated based on the developed stack level. It became clear that the heat source of 223 K level is enough for sustaining cell temperature because of large Joule heat.

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The thermoelectric properties of a nonstoichiometric titanium oxide (TiO_1.1) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of TiO_1.1 increases to 9000 S/m at 1073 K, is showing semiconducting behavior. The Seebeck coefficient, α, of TiO_1.1 shows a general trend in which the value increases gradually from 0.4 mV/K at 573 K to 1.0 mV/K at 1223 K. As a consequence, the power factor, α²σ, reaches 8.6×10⁻³ W/(m·K²), the largest value of all reported oxide materials. The thermal conductivity, κ, of TiO_1.1 increases with temperature, from 1.3 W/(m·K) at 573 K to 7.1 W/(m·K) at 1223 K. In spite of the considerably large values of κ, the figure of merit, Z = α²σ/κ, reaches 1.6×10⁻³ K⁻¹ for TiO_1.1 at 973 K. The extremely large power factor of TiO_1.1 compared to other metal oxides can be attributed to the optimal carrier density. The dimensionless figure of merit, ZT, of 1.64 attained by TiO_1.1 at 1073 K is the largest value of all reported other thermoelectric materials in this temperature region. And that TiO_1.1 has ZT values of nearly unity or greater in the range of 773 to 1223 K, demonstrates the usefulness of the nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.

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Thermoelectric properties of combustion synthesized and spark plasma sintered rare-earth-doped (La, Sm, Gd, Dy and Y) SrTiO₃ was investigated from room temperature to 870 K from viewpoint of energy and time saving without deterioration in thermoelectric properties. All single phases of rare-earth-doped SrTiO₃ were successfully synthesized and sintered with high densities. With temperature increasing, the absolute value of Seebeck coefficient of all the samples increased and the electric conductivity decreased; the power factor of all the samples decreased except Y-doped sample in the considering temperature range. In all the samples, the La-doped sample and the Y-doped sample had the highest and lowest power factor, respectively. The figure of merit of La-doped samples with different doping amounts was evaluated and the maximum figure of merit 0.22 was obtained at 800 K from Sr_0.92La_0.08TiO₃ sample. Comparing Y and La-doped samples prepared by our synthesis method with that of conventional solid-state reaction method, the thermoelectric properties of our samples were relatively higher. Thus the combination of combustion synthesis and spark plasma sintering has a potential to prepare perovskite-oxide materials with relatively higher thermoelectric properties for high-temperature application.

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This paper describes the experimental investigation of a new methane steam reformer utilizing waste heat through the latent heat storage material called “phase change material (PCM)”. The intermittently released waste heat such as steelmaking off gas was first converted to a continuous, constant temperature heat source in the form of latent heat of the PCM. Then, the stored heat in the PCM was supplied to methane to induce an endothermic steam-reforming reaction. In the experiments, a laboratory scale reformer in which packed beds of nickel porous catalyst was circumferentially covered by PCM of copper was intermittently heated from outside by a burner and the transient local temperatures were monitored. The results revealed that the PCM temperature was constant in spite of intermittent heating and that hydrogen was generated continuously. The proposed system could possibly produce hydrogen with less CO₂ emission than that produced by a conventional hydrogen generator. This could allow the integration of two processes in the steelmaking and hydrogen production industries.

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System analysis was conducted on a proposed combined system for methane steam reforming comprising conventional hydrogen production and waste heat recovery from steelmaking. Operating data for a conventional methane steam reforming system were collected and analyzed. The results showed that the conventional system utilized only 60% of the natural gas as raw material and the rest is consumed for supplying the reaction heat for methane steam reforming. On the basis of this data, the proposed system was evaluated on five factors—natural gas consumption, enthalpy flow, CO₂ emission, cost, and exergy loss. For the proposed system, the factors were only 59.6%, 59.7%, 62.8%, 86.5%, and 65.8% of those of the conventional system, respectively. This supports the feasibility of hydrogen production from recovered waste heat. Furthermore, the proposed system is expected to contribute to the production of ‘green’ hydrogen that incurs less CO₂ emission.

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This paper principally presents a process analysis of the production systems of blast furnace cement (BFC) based on exergy analysis and its carbon dioxide emission. The analysis was first carried out by using exergy balances of actual operating data in the cement industry. The results revealed that a large sum of net exergy losses was found on the conventional BFC production; this was contrary to the preliminary expectations. In the BFC production, the recovery of the thermal exergy of the molten slag should reduce the total exergy losses by up to 20%. In contrast, the emission of CO₂—488.2 kg/ton—in BFC production was lower than 797.5 kg/ton emission in portland cement production; this was because portland cement consumes more carbonaceous fuels such as coal. In conclusion, to reduce exergy loss, save energy and minimize CO₂ emission, it is imperative that the BFC production process should be improved with the recovery of the thermal exergy of the molten slag, e.g. the direct mixing of raw material of limestone with molten slag is an innovative and attractive solution because limestone can be easily decomposed by sensible heat of the molten slag. This means thermal combination of portland cement and BFC for the effective use of waste heat, in which waste heat in the conventional BFC process is recovered and used for limestone decomposition in the clinker production to produce BFC.

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A waste-heat transportation (HT) system whose operation depends on the latent heat (LH) of high-temperature phase change material (PCM) is effective in reducing carbon dioxide (CO₂) emission from industries. This paper describes 1) the use of the binary eutectic mixture NaOH/Na₂CO₃ as a PCM to realize the HT system, 2) the feasibility of HT system using this PCM from viewpoints of energy requirements, exergy loss, and CO₂ emissions. In this study, we examined the thermophysical properties of the PCM and its chemical stability with reference to the heat transfer medium of the HT system by differential scanning calorimetry and thermogravimetry-differential thermal analysis. We observed that NaOH/Na₂CO₃ had a LH of fusion of 252 kJ/kg and a melting point (MP) and a freezing point (FP) of 285±1°C that was suitable for the HT system. There were no significant changes in the chemical and physical properties after aging for 500 h during phase change when dibenzyltoluene was used as the heat transfer medium. On the contrary, in the system analysis, the operating data in the proposed system—as well as in a conventional heat supply system—were calculated based on heat and material balances. The results show it has only 9.5% of the energy requirements, 39.7% of the exergy loss, and 19.6% of the CO₂ emissions of conventional systems that lack heat-recovery capabilities.

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