DATE2020.07.02 #Press Releases

Development of long-term heat-storage ceramics storing waste heat from power plants and factories through hot water

－Absorbing heat energy from hot water and releasing the energy by applying pressure－

Presenters

Shin-ichi Ohkoshi (Professor, Department of Chemistry, School of Science, The University of Tokyo )
Yoshitaka Nakamura (Industrial Solutions Company, Panasonic Corporation）
Masaki Azuma (Professor, Laboratory for Materials and Structures, Tokyo Institute of Technology）
Yuki Sakai (Researcher, Kanagawa Institute of Industrial Science and Technology)

Key points of the work

• We discovered a heat-storage ceramic that eternally preserves heat energy from hot water below 100ºC.
• This long-term heat-storage ceramic can release heat energy on demand by applying pressure.
• The present material is expected to be used for effective reuse and storage of waste heat from thermal power plants, nuclear power plants, factories, etc.

Overview of the work

A collaborative research group of Professor Shin-ichi Ohkoshi of School of Science, the University of Tokyo, discovered a long-term thermal storage ceramic that can permanently store the thermal energy of hot water or warm water from 38ºC to 67ºC. This new compound is scandium-substituted lambda trititanium pentoxide (λ-Sc_xTi_3−xO₅), which can store the energy of heat of 100°C or less such as hot water, and by applying pressure, the energy can be extracted. Such long-term thermal storage ceramics absorbing low-temperature exhaust heat are effective in storing the thermal energy of hot water discharged from thermal power plants and nuclear power plants. It is also expected as a material for reusing waste heat from factories and automobiles.

The results of this research published in the online version of Science Advances on July 2, 2020, Japan time.

Details of the work

In thermal power plants and nuclear power plants, it is difficult to convert all of the generated thermal energy into electric power. In fact, 70% of the generated thermal energy is lost to the surroundings as waste heat. The waste heat is mainly cooled by water and released to the sea as hot water (100°C or less), which cannot be used effectively. If such waste heat could be stored and reused without being lost, it could not only improve the energy efficiency but also prevent adverse effects on the surrounding environment due to the release of hot water into rivers or sea.

In this study, we synthesized a new compound of scandium-substituted lambda trititanium pentoxide (λ-Sc_xTi_3−xO₅), in which a part of titanium in lambda trititanium pentoxide (λ-Ti₃O₅) ^[１] is replaced by scandium (Sc). This compound was synthesized by arc melting method ^[２] and has a composition of Sc_xTi_3−xO₅ (x=0.09, 0.105, 0.108). Synchrotron X-ray diffraction^[３] measurement at Spring-8 revealed that the present compound has the same monoclinic (space group C2/m) crystal structure as the non-substituted λ-Ti₃O₅ (Fig. 1a). Transmission electron microscope image indicated that the obtained sample is an aggregation of stripe-shaped domains of about 100 nm × 200 nm size. This scandium-substituted lambda trititanium pentoxide has very high stability and does not change even after 367 days (1 year). When pressure was applied to the scandium-substituted lambda trititanium pentoxide, a pressure-induced phase transition to scandium-substituted beta trititanium pentoxide (β-Sc_xTi_3−xO₅) was observed instantaneously. (Hereinafter, λ-Sc_xTi_3−xO₅ is called the λ phase and β-Sc_xTi_3−xO₅ is called the β phase.) Heat-storage properties of the pressure-produced β phase were examined. The sample with the composition of x = 0.09 showed an endothermic peak at 67 ºC, revealing that the present material is a solid-solid phase transition type heat-storage material absorbing heat below 100 ºC (Fig. 1b). We also confirmed that the phase transition between the λ and β phases occurs repeatedly by the application of pressure and heat (Fig. 1c). Thus, we succeeded in the development of a long-term heat-storage ceramic for low-temperature exhaust heat.

The mechanism of long-term heat storage and pressure-induced heat release is explained as follows. As shown in the enthalpy curves of the λ and β phases (Fig. 1d), after the β phase absorbs heat and transforms into λ phase in the heating process, the λ phase is maintained when the temperature is lowered again, even to very low temperatures. Since there is an energy barrier between the λ phase and the β phase, the λ phase is maintained at low temperatures without transition to the β phase with lower energy. When pressure is applied to this λ phase, the energy barrier between the two phases disappears, and transition to the β phase occurs (Fig. 1e).

Figure 1 ： Heat-storage and heat-release properties of scandium-substituted lambda trititanium pentoxide.
a, Crystal structure of scandium-substituted lambda trititanium pentoxide and photograph of the sample obtained by arc melting method. b, Calorimetric data of Sc_0.09Ti_2.91O₅ showing a heat absorption peak at 67°C (340 K). Prior to the measurement, the sample was pressurized by 1.7 GPa. c, Reproducibility of pressure-induced and heat-induced phase transitions. In the pressurization process (blue line), a pressure of 1.7 GPa was applied to the λ phase at room temperature, and the λ phase fraction was analysed from the XRD data measured after releasing the pressure. In the heating process (red line), the sample was heated to 473 K (200°C) after pressure release and then cooled to room temperature. The phase fraction of the λ phase showed good reproducibility. d, Temperature dependence of the enthalpy of λ (blue line) and β (red line) phases of Sc_0.09Ti_2.91O₅ calculated from the transition enthalpy obtained from calorimetry and the temperature dependence of the enthalpy obtained from phonon mode calculations. The curves in the graph are offset to the formation energy of the β phase at 0 K obtained from first-principles calculation. (i) Pressure-produced β phase accumulates heat energy and undergoes a phase transition to λ phase at 67°C. (ii) Temperature of the λ phase is increased and then decreased again. (iii) λ phase is maintained even at low temperatures. e, Gibbs energy curves versus λ phase fraction for pressures of 0.1 MPa, 400 MPa, and 700 MPa. The phase transition from λ to β phase occurs due to the disappearance of the energy barrier between the two phases.

Fig. 2 shows an application idea of a heat-storage system using scandium-substituted lambda trititanium pentoxide. At a power plant, water is drawn from the river or the sea to cool the turbine. After cooling the turbine, the water becomes hot water. When this hot water is carried to a heat storage tank containing the long-term heat storage ceramic, the long-term heat storage ceramics absorb the energy from the hot water and undergoes a phase transition from the β phase to the λ phase to store the energy. The stored energy can be reused because it can be released on demand by applying pressure. For example, the heat energy can be carried by water drawn from rivers or seas to nearby factories and buildings. Also, by taking advantage of the characteristic of storing energy until pressure application, long-term heat-storage ceramic in a heat storage state (λ phase) can be transported by trucks or other transportations, and heat can be extracted by applying pressure at a required time, enabling the reuse of heat energy even in remote locations. In other words, this ceramic makes it possible to reuse the energy of hot water from thermal and nuclear power plants at anytime, anywhere. Furthermore, since the heat energy of hot water generated at thermal and nuclear power plants is removed and cooled by the heat-storage ceramics, the temperature rise of rivers and sea will be suppressed, thus reducing the environmental load. In addition, the heat-storage temperature of scandium-substituted lambda trititanium pentoxide can be controlled by changing the Sc content: 45°C for λ-Sc_0.105Ti_2.895O₅ and 38°C for λ-Sc_0.108Ti_2.892O₅.

Figure 2 ： Application of scandium-substituted lambda trititanium pentoxide in a power plants.
Schematic of a thermal energy recycling system using scandium-substituted lambda trititanium pentoxide. Blue and red pipelines carry cold and hot water, respectively, as the heat transport medium.

Scandium-substituted lambda trititanium pentoxide can store heat energy from waste heat in a temperature range that has not been reused before, and thus, it is expected to open new possibilities for thermal management. In addition to power plants, this material has the potential for a variety of applications, including heat-storage applications for reusing waste heat from factories, transportation vehicles, mobile phones, and electronic devices.

This work was supported in part by JSPS Grant-in-Aid for Specially Promoted Research (grant 15H05697), JSPS Grant-in-Aid for Scientific Research(A) (grant 20H00369), and Collaborative Research Projects, Laboratory for Materials and Structures, Tokyo Institute of Technology.

Publication journal

Journal	Science Advances
Title	Long-term heat-storage ceramics absorbing thermal energy from hot water
Authors	Yoshitaka Nakamura, Yuki Sakai, Masaki Azuma, Shin-ichi Ohkoshi
DOI No	10.1126/sciadv.aaz5264

 

Glossary

[1]  Lambda trititanium pentoxide (λ-Ti₃O₅)

Lambda trititanium pentoxide is a titanium oxide materials with a new crystalline structure, which was discovered by Prof. Ohkoshi and colleagues in 2010 [Nature Chemistry, 2, 539 (2010)]. Recently, a new concept of heat storage ceramics has been proposed based on its physical properties [Nature Communications, 6, 7037 (2015)]. In addition to its metallic characteristic, this material has been found to exhibit a variety of phase transitions, including photo-induced phase transition, pressure-induced phase transition, and current-induced phase transition. ↑

[2]  Arc melting method

A method for synthesizing alloys and other materials by melting multiple materials at high temperatures. In vacuum or in inert gas atmosphere, an arc discharge between the electrode and the water-cooled copper mold is used to melt the raw materials placed on the copper mold to obtain a uniformly mixed solid solution.↑

[3]  Synchrotron X-ray diffraction (SXRD)

A structure diffraction measurement using synchrotron radiation, which is generated when electrons are accelerated close to light speed and bent by a magnetic field. Compared to laboratory X-ray diffraction, it is possible to obtain more precise data in a shorter time.↑