Universidad de Zaragoza, Prensas de la Universidad
Abstract
The energy considered as waste heat in industrial furnaces owing to inefficiencies represents a substantial opportunity for recovery and storage. Nevertheless, the application of thermal energy storage (TES) systems based on phase change materials (PCM) in energy-intensive industries (EII) at very high temperatures is scarce and restricted by technological and economic barriers. The topic of this PhD thesis is framed on the study and analysis of PCM-TES to be used as a waste heat recovery and storage unit for high temperature applications (up to 1000ºC). The main objective of PCM-TES integration is recovering and storing waste heat from combustion gases or other surplus sources, currently unused, to preheat the air temperature entering the furnace, or other heat demanding processes. In this vein, implementing PCM-TES is a sustainable and innovative option to increase energy efficiency (5-10%) and to reduce the environmental impact associated. The combustion air preheated with the recovered thermal energy reached an increase up to 200-300ºC in the cases analysed in the dissertation.Design of latent heat TES requires knowledge of the heat transfer process, as well as the phase change behaviour of the PCM used. On the one hand, the configuration design is specifically adapted to the plant operational requirements, by a methodology combining the search of the best conceptual design and a proper PCMs selection. To that end, key technical, energy and economic factors are weighted by an in-house multiple criteria decision analysis (MCDA) to define the most promising design configuration. The final chosen conceptual design consists of a shell-and-tube system, where the exhaust gases flow inside the tubes, the air combustion is placed in the shell side and the PCM is contained in doubled concentric tubes. On the other hand, thermal characterisation and stability cycle tests were performed on the candidate storage materials for two representative application cases in the ceramic and steel industries. Both metal alloys and inorganic salts were analysed to select the most suitable alternative of PCMs working at high temperature. To investigate the operation of PCM systems, computational simulations can assess the thermal behaviour and expected operational performance. In this sense, temperature profiles of the PCM and the heat transfer fluids are defined by means of 3D numerical model implemented in MATLAB® and COMSOL Multiphysics®. In both models, the energy equation considers both heat conduction and natural convection to predict its effect on the behaviour of the PCM. The first approach is the MATLAB® in-house-developed modelling of the melting and solidification processes. This tool sets the basis for an appropriate system design and sizing, thermal stress resistance and material selection ensuring the technical feasibility of these systems working at critical temperature ranges. The results are reliable and less time consuming; thus, it is a useful tool during the early design stages and for practical application in the engineering and industry. Specifically, for the ceramic sector, the design resulted in a shell-and-tube system with 1188 kg of a PCM melting at 885ºC involving a latent storage capacity of 227 MJ. In this case, it was demonstrated the achievability of very high temperature levels in the combustion air for preheating (over 700ºC, higher than conventional sensible heat exchangers). Similarly, 1606 kg of PCM, whose phase-change temperature is 509ºC, is considered for the steel sector providing a latent capacity of 420 MJ. The combustion air was preheated from 300 to 480°C, matching the intermittent heat treatment and batch processes of the steel plant.In the second model approach, the obtained results from the COMSOL Multiphysics® modelling aims at simulating multiphysics problems and allows predicting the thermal performance with high precision; conversely, it presents a higher computational time cost. This model is used to simulate the industrial prototype and to perform a prospective validation of the MATLAB® model. This thesis aims at promoting and facilitating the integration of PCM-TES systems at industrial scale. In this line, technical documentation and process specifications for the PCM-TES prototype were established to achieve the level of reliability, efficiency and safety required. As a result, the configuration of the system was adapted to the plant requirements and the procedures for working operation and the instrumentation of the monitoring and control system were developed. Regarding simulated PCM-TES prototype performance, the combustion air received 338 kWh of heat from the PCM within 3 hours. During the charging, the PCM absorbed 351 kWh from the flue gas stream for 6 hours. In total, the annual energy savings are 230 MWh. The predicted thermal behaviour provides the PCM-TES design validation and reduces the uncertainty risks in the operational performance and its on-site implementation at large scale.With the aim of proofing the feasibility of a cross-sectorial approach by enlarging its replicability in many industrial sectors, a simplified tool based on the MATLAB® model was developed based on correlations among the most relevant system parameters. Along this line, the thesis conducted a parametric and sensitivity analysis to assess the techno-economic performance of the PCM-TES solution under different working conditions and sectors. This assessment highlighted that a suitable design, material selection and sizing are crucial parameters to obtain energy and economic benefits. Additionally, a multicriteria assessment was conducted with the tool outputs comparing metal alloys and inorganic hydrated PCM salts. Overall, the inorganic PCMs presented NG savings up to 2.6%, which means a higher net economic and energy savings (26,400 €; 480 MWh/year); while metal alloys involved shorter charge/discharge cycles and competitive economic ratios, its commercial development is, conversely, still limited. Finally, acceptable payback periods are observed when operating under 800ºC (between 5-8 years in the steel sector). This fact highlighted the technical and economic barriers existing in working at high temperature levels.All things considered, this thesis aims at demonstrating the feasibility of implementing, at industrial scale, a PCM-TES system of recover wasted energy from EIIs and overcoming the current lack of information, especially at high temperatures. The results obtained are a starting point for consolidating and promoting novel technological solutions and materials towards a more sustainable and efficient industry.<br /
