• Energy Research

This study presents the unsteady exergy transfer analysis of an aluminum holding furnace with new heating resistance scheme. This holding system consists of four multilayer refractory walls and one resistance heating system which is responsible of maintaining the appropriate aluminum temperature and composition for further casting. The purpose of this analysis is to understand and identify heat losses and irreversibilities of the holding process of an aluminum furnace by means of the First and Second Law of Thermodynamics. In this study, bi-dimensional temperature and exergy fields during heat and exergy transfer processes are presented. The exergy balance is completely computed for this system, obtaining: exergy transfer, exergy variation rate, and destroyed exergy rate.

The energy considered as waste heat in industrial furnaces owing to inefficiencies represents a substantial opportunity for recovery by means of thermal energy storage (TES) implementation. Although conventional systems based on sensible heat are used extensively, these systems involve technical limitations. Latent heat storage based on phase change materials (PCMs) results in a promising alternative for storing and recovering waste heat. Within this scope, the proposed PCM-TES allows for demonstrating its implementation feasibility in energy-intensive industries at high temperature range. The stored energy is meant to preheat the air temperature entering the furnace by using a PCM whose melting point is 885 °C. In this sense, a heat transfer model simulation is established to determine an appropriate design based on mass and energy conservation equations. The thermal performance is analysed for the melting and solidification processes, the phase transition and its influence on heat transference. Moreover, the temperature profile is illustrated for the PCM and combustion air stream. The obtained results prove the achievability of very high temperature levels (from 700 to 865 °C) in the combustion air preheating in a ceramic furnace; so corroborating an energy and environmental efficiency enhancement, compared to the initial condition presenting an air outlet at 650 °C.

Reductions in energy consumption, carbon footprint, equipment size, and cost are key objectives for the forthcoming energy-intensive industries roadmaps. In this sense, solutions such as waste heat recovery, which can be replicated into different sectors (e.g., ceramics, concrete, glass, steel, aluminium, pulp, and paper) are highly promoted. In this line, latent heat thermal energy storage (TES) contributes as an innovative technology solution to improve the overall system efficiency by recovering and storing industrial waste heat. To this end, phase-change material (PCM) selection is assisted through a decision-support system (DSS). A simplified tool based on the MATLAB® model, based on correlations among the most relevant system parameters, was developed to prove the feasibility of a cross-sectorial approach. The research work conducted a parametric analysis to assess the techno-economic performance of the PCM-TES solution under different working conditions and sectors. Additionally, a multicriteria assessment was performed comparing the tool outputs from metal alloys and inorganic hydrated PCM salts. Overall, the inorganic PCMs presented higher net economic and energy savings (up to 25,000 €/yr; 480 MWh/yr), while metal alloys involved promising results, shorter cycles, and competitive economic ratios; its commercial development is still limited.

Experimental tests of a small polygeneration plant unit based on hybrid renewable energy and desalination have been used to validate a novel model simulation of this plant. It provides electricity by coupling photovoltaic/thermal collectors and a micro-wind turbine, fresh water by means of hybrid desalination (membrane distillation, and reverse osmosis), and sanitary hot water coming from the photovoltaic/thermal collectors and an evacuated tubes collector. Plant was designed to operate in off-grid conditions and conventional energy storage systems were used (lead acid batteries and hot water tank). Simulation model was performed in TRNSYS environment and it was fully validated by comparing several key plant measurements. The analysis was focused on five typical days of summer, fall and winter. Some differences found in experimental and simulated values were analysed. To reduce those gaps, some modifications have been suggested to the model or the pilot unit respectively. As a result, a validated model in TRNSYS of the plant was obtained. This model can be used for the further scale up of new projects to cover any other scheduled demands of power and water in isolated areas, whenever is based on the combination of the abovementioned technologies.

The mathematical model of exergy transfer in cullet glass heated by microwave inside of a cubical cavity with the aid of a susceptor is presented. Part I of this paper presented a numerical combined electromagnetic and heat transfer model by applying both transient Maxwell’s equations and heat transfer equations. Then, the electromagnetic and temperature fields were used to obtain the exergy transfer analysis in the oven. Exergy transfer analysis informs us about the efficiency of energy transformations taking place during the heating process, since it explains how the quality of the energy behaves along the heating process. The rate of internal exergy, exergy flowing and destroyed exergy were obtained and presented for this transient process. Part I showed that the susceptor location could change the temperature fields of cullet glass. So, an exergy analysis is important to understand the irreversibilities produced by a susceptor during preheating (microwaves activation) and heating process of the cullet glass, and how they could be minimized. Exergy transfer analysis shows how both, electromagnetic and heat transfer, are responsible of the irreversibilities generated in the heating process.

Abstract Exergy transfer analysis is proposed as a complementary method for analyzing and better understanding of the efficiency of microwave heating systems, which play a key role in medicine, chemical engineering, food industry and material processing. Since this method is able to detect and quantify in detail where irreversibilities occur, it can be a useful first step in the improvement of microwave systems. First, the different approaches for modeling microwave systems and interactions wave-thermal system are reviewed. Then, relation between Poynting vector, energy and exergy in microwaves is analyzed in depth. Afterwards, a 2-D model of energy and exergy transfer for a microwave heating system is presented. The model is based on Lambert's Law and is applied for the analysis of a potato heating. A comparison with a conventional heating is also presented, where the irreversibility evolution of both process, microwave and conventional, is analyzed with the presented methodology, presenting advantages and disadvantages for each process. This simple example proposed for illustration demonstrates how the analysis of exergy transfer results allows one not only to characterize the global exergy efficiency of the system but also analyze in depth the time and space distribution of exergy flows, exergy variation and irreversibility.

In this paper, the analysis of the local exergy costs of a cullet glass heated by microwaves in a cubical cavity activated by a susceptor is presented. The analysis is based on a previously validated 3-D electromagnetic model, but goes further by applying the concepts of local exergy efficiency and local unit exergy consumption, what enables a local analysis (in time and space) of the process efficiency. Furthermore, local exergy cost quantifies in detail the path of the exergy cost formation during microwave heating, which is determined by the local irreversibilities taking place in this transient process. Four different susceptor positions have been also compared, in order to find out not only which one is the most efficient but also to justify in detail this result by the time and space evolution of efficiency, unit exergy consumption (both external microwave power and conduction contributions) and unit exergy cost. The best conclusion of the paper is that the local exergy cost approach can contribute to the design of more efficient energy conversion systems, as it could be noted in its application to a complex process like this 3-D example of microwave cullet heating.