• Energy Research

Abstract A plate-type thermoelectric generation (TEG) system is typically used in engine exhaust waste heat recovery systems because of its appropriate structure. To obtain an effective design of these TEG systems, this study focuses primarily on optimal matching performance analysis, by building a complete numerical TEG model with the finite element method using FORTRAN. A commercial-type thermoelectric material is used in the numerical calculation. Moreover, all types of work conditions with different exhaust parameters (mf = 10–50 g s−1 and Tfin = 300–600 °C) and cooler’s heat transfer processes are included. When peak net power is achieved, all corresponding optimal features are analyzed, where both air-cooling and water-cooling methods are considered. The results indicate that, for any type of work condition, the optimal height remains the same (Bopt = 7.0 mm for the air-cooling method and Bopt = 4.0 mm for the water-cooling method) and the other optimal parameters can be expressed by fitting correlations, which can deduce the optimal length (Lopt) and width (wopt) of an exhaust heat exchanger (for example, they are Lopt = 1.55 m and wopt = 0.78 m when mf = 50 g s−1, Tfin = 500 °C, and hc = 80 W m−2 K−1 for the air-cooling method). The introduced fitting correlations are verified to have a high accuracy.

Abstract A plate-type thermoelectric generation (TEG) system is typically used in engine exhaust waste heat recovery systems because of its appropriate structure. To obtain an effective design of these TEG systems, this study focuses primarily on optimal matching performance analysis, by building a complete numerical TEG model with the finite element method using FORTRAN. A commercial-type thermoelectric material is used in the numerical calculation. Moreover, all types of work conditions with different exhaust parameters (mf = 10–50 g s−1 and Tfin = 300–600 °C) and cooler’s heat transfer processes are included. When peak net power is achieved, all corresponding optimal features are analyzed, where both air-cooling and water-cooling methods are considered. The results indicate that, for any type of work condition, the optimal height remains the same (Bopt = 7.0 mm for the air-cooling method and Bopt = 4.0 mm for the water-cooling method) and the other optimal parameters can be expressed by fitting correlations, which can deduce the optimal length (Lopt) and width (wopt) of an exhaust heat exchanger (for example, they are Lopt = 1.55 m and wopt = 0.78 m when mf = 50 g s−1, Tfin = 500 °C, and hc = 80 W m−2 K−1 for the air-cooling method). The introduced fitting correlations are verified to have a high accuracy.

Abstract Calcium oxide/water reaction is a promising thermochemical energy storage system owing to its high reaction enthalpy, wide availability at low cost, and favorable reaction temperature. In this research work, a calcium carbonate material was developed, featuring nano-order diameter with a silane coupling agent on the surface, and was kinetically evaluated for use in calcium oxide/water thermochemical energy storage. The kinetics analysis of the material was conducted and compared with other conventional calcium carbonate materials in which particle diameter was 2–3 μm as a reference material. Calcium oxide materials prepared through a decarbonation of calcium dioxide were used for 20 repeated cycle operations. In comparison with the conventional calcium carbonate material, the developed material showed high durability for the cycle operation and maintained its reaction conversion as almost 100% between the 8th and 20th cycle. The presence of silicate produced from the thermal decomposition of the silane coupling agent on the material surface could prevent particle aggregation. It maintained vapor diffusivity between the particles, and enhanced cyclic reaction durability of the developed material. The reaction conversion directly affects thermochemical energy storage performance, and these results indicated that the developed material had sufficient performance compared to conventional calcium carbonate materials as a thermochemical energy storage material.

Abstract Calcium oxide/water reaction is a promising thermochemical energy storage system owing to its high reaction enthalpy, wide availability at low cost, and favorable reaction temperature. In this research work, a calcium carbonate material was developed, featuring nano-order diameter with a silane coupling agent on the surface, and was kinetically evaluated for use in calcium oxide/water thermochemical energy storage. The kinetics analysis of the material was conducted and compared with other conventional calcium carbonate materials in which particle diameter was 2–3 μm as a reference material. Calcium oxide materials prepared through a decarbonation of calcium dioxide were used for 20 repeated cycle operations. In comparison with the conventional calcium carbonate material, the developed material showed high durability for the cycle operation and maintained its reaction conversion as almost 100% between the 8th and 20th cycle. The presence of silicate produced from the thermal decomposition of the silane coupling agent on the material surface could prevent particle aggregation. It maintained vapor diffusivity between the particles, and enhanced cyclic reaction durability of the developed material. The reaction conversion directly affects thermochemical energy storage performance, and these results indicated that the developed material had sufficient performance compared to conventional calcium carbonate materials as a thermochemical energy storage material.

The increase of the capacity factor of thermal processes which use renewable energies is closely linked to the implementation of thermal energy storage (TES) systems. Currently, TES systems can be classified depending on the technology for storing thermal: sensible heat, latent heat, and sorption and chemical reactions (usually known as thermochemical energy storage). However, there is no standardized procedure for the evaluation of such technologies, and therefore the development of performance indicators which suit the requisites of the final users becomes an important goal. In the present paper, the authors identified the energy density as an important performance indicator for TES, and evaluated it at both material and system levels. This approach is afterwards applied to prototypes covering the three TES technologies: a two-tank molten salts sensible storage system, a shell-and-tube latent heat storage system, and a magnesium oxide and water chemical storage system. The evaluation of the energy density highlighted the difference of its value at the material value, which presents a theoretical maximum, and the results at system level, which considers all the parts required for operating the TES, and thus presents a significantly lower value. Moreover, the proposed approach captured the effect of the complexity and overall size of the system, showing the relevance of this performance indicator for evaluating technologies for applications in which volume is a limiting parameter. The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. Jaume Gasia would like to thank the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2018 FI_B2 00100). Aran Solé would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI2015-25741. The authors would also like to thank the participants of IEA ECES Annex 30 for their critical view and feedback during the development of the methodology.