• Energy Research

Abstract This work reports a detailed experimental study that is aimed at investigating the Thermal Energy Storage (TES) performance of cementitious systems containing Microencapsulated Phase Change Materials (MPCMs). New spherical-shaped test specimens for TES measurements were produced following an innovative casting technique, developed at the Institut fur Werkstoffe im Bauwesen – TU Darmstadt. Three water-to-cement ratios and three MPCMs volume fractions, leading to a total of nine different mixtures, were investigated. The thermal experiments were accompanied by mechanical tests to observe the effect MPCMs have on the resulting strengths in both compression and bending. The analysis and discussion of the TES results are employed for calibrating an enthalpy-based model. The experimental data have been applied to evaluate the corresponding temperature-based material parameters like specific heat, conductivity, or more in general, the energy storage capacity of a system under transient heat conduction conditions.

Thermal energy storage using phase change materials (PCMs) is a promising technology for improving the thermal performance of buildings and reducing their energy consumption. However, the effectiveness of passive PCMs in buildings depends on their optimal design regarding the building typology and typical climate conditions. Within this context, the present contribution introduces a novel multiobjective computational method to optimize the thermophysical properties of cementitious building panels enhanced with a microencapsulated PCM (MPCM). To achieve this, a parametric model for PCM-based cementitious composites is developed in EnergyPlus, considering as design variables the melting temperature of PCMs and the thickness and thermal conductivity of the panel. A multiobjective genetic algorithm is dynamically coupled with the building energy model to find the best trade-off between annual heating and cooling loads. The optimization results obtained for a case study building in Sofia (Bulgaria-EU) reveal that the annual heating and cooling loads have contradictory performances regarding the thermophysical properties studied. A thick MPCM-enhanced panel with a melting temperature of 22 °C is needed to reduce the heating loads, while a thin panel with a melting temperature of 27 °C is required to mitigate the cooling loads. Using these designs, the annual heating and cooling loads decrease by 23% and 3%, respectively. Moreover, up to 12.4% cooling load reduction is reached if the thermal conductivity of the panels is increased. Therefore, it is also concluded that the thermal conductivity of the cement-based panels can significantly influence the effectiveness of MPCMs in buildings.

Solar energy is a promising renewable option to provide energy demands in combination with conventional energy sources. However, to enhance the integration of renewable energies into the industrial section, it is necessary to employ thermal energy storage media. Among different thermal energy storage options, concrete has proven to be a very effective sensible thermal energy storage solution. However, its main raw material (i.e., the Portland cement), is the cause of huge CO2 emissions to the environment. This study focuses on the design, manufacture, assembly and experimentation of thermal energy storage blocks made of alternative binders in substitution of Portland cement: i.e., hybrid materials and alkaline activated materials. The thermal energy storage blocks have been thermally cycled, with a charging temperature of 228 °C and a discharging temperature of 30 °C, in order to analyze the operational times and temperatures. The results obtained are very promising and comparable to those of the Portland cement system. Charging times have improved, with the alternative materials achieving temperature increases of up to 4%, while greater stabilization has been achieved during discharge, with the fluid reaching temperatures up to 6% higher than the reference system. These advancements support efficient, sustainable thermal energy storage technologies.

Abstract This paper reports the results of an experimental program on sustainable cementitious composites made with Recycled Brick Aggregates (RBAs) employed as carriers for Phase-Change Materials (PCMs). The PCM-RBAs were produced by following an advanced incorporation and immobilization technique, developed and patented at the Institut fur Werkstoffe im Bauwesen – TU Darmstadt. Spherical mortar samples made of plain cement paste plus porous RBAs, filled with liquid paraffinic wax, were used for monitoring the thermal behavior of various combinations of PCM-RBAs. The experimental campaign also comprised mechanical tests aimed at observing the effect of PCM in the RBAs, on the resulting mortar strengths under both compression and bending. Promising results have been obtained for the RBA mortars, showing a high thermal energy storage capacity at an almost negligible and acceptable strength loss due to PCM addition.

De nouvelles avenues pour le stockage de l'énergie thermique (tes) doivent être étudiées en raison du manque de compétitivité des technologies d'énergie solaire concentrée (CSP). Des solutions doivent être trouvées pour remplacer les réservoirs de sel fondu qui ont un impact économique majeur et sont difficiles à entretenir en raison de problèmes de corrosion. En ce sens, le béton représentait un candidat attrayant en prouvant un excellent tes sensible en CSP. Cependant, sa phase principale, en ciment Portland (PC), a des conséquences environnementales importantes. La production de PC est connue pour émettre des niveaux élevés de gaz polluants, en particulier le CO2. On estime qu'il est responsable de 5 % à 7 % des émissions mondiales de CO2, ce qui en fait un contributeur majeur au changement climatique. Ce travail présente des matériaux cimentaires plus verts, fabriqués à partir de ciments alcalins et de ciments hybrides, destinés à être utilisés comme supports tes alternatifs respectueux de l'environnement dans les usines CSP. Une campagne expérimentale est présentée qui montre que ces matériaux éco-efficaces peuvent avoir de meilleures propriétés mécaniques, que le mortier PC ordinaire, lorsqu'ils sont exposés à des températures élevées, en plus, peuvent offrir des améliorations de leurs propriétés thermiques (conductivité thermique ou chaleur spécifique). La deuxième partie du travail est consacrée aux simulations par éléments finis, dans le but de trouver la meilleure configuration, en termes de sélection des matériaux et de géométrie, qui sont plus efficaces que le système tes. Les travaux montrent les avancées suivantes dans la technologie CSP en utilisant des liants alternatifs respectueux de l'environnement : le volume d'installation peut être réduit de 17 % par rapport à un réservoir de sel fondu, tandis que la surface de l'échangeur de chaleur peut être redimensionnée de 29 % par rapport au système de référence utilisant un PC. Ces améliorations permettent des variations plus importantes de l'efficacité opérationnelle et des capacités dynamiques du CSP et représentent des progrès importants vers le développement de technologies CSP plus efficaces et durables. Es necesario investigar nuevas vías para el almacenamiento de energía térmica (tes) debido a la falta de competitividad de las tecnologías de energía solar concentrada (CSP). Se deben encontrar soluciones para reemplazar los tanques de sal fundida que tienen un gran impacto económico y son difíciles de mantener debido a problemas de corrosión. En este sentido, el concreto representó un candidato atractivo al demostrar una excelente tes sensible en CSP. Sin embargo, su fase principal, hecha de cemento Portland (PC), tiene importantes consecuencias ambientales. Se sabe que la producción de PC emite altos niveles de gases contaminantes, en particular CO2. Se estima que es responsable de entre el 5% y el 7% de las emisiones de CO2 del mundo, lo que lo convierte en un importante contribuyente al cambio climático. Este trabajo presenta materiales cementosos más verdes, hechos de cementos alcalinos y cementos híbridos, para ser utilizados como medios alternativos de tes ecológicos en plantas CSP. Se presenta una campaña experimental que muestra que estos materiales ecoeficientes pueden tener mejores propiedades mecánicas, que el mortero PC ordinario, cuando se expone a altas temperaturas, además, puede ofrecer mejoras de sus propiedades térmicas (conductividad térmica o calor específico). La segunda parte del trabajo está dedicada a las simulaciones de elementos finitos, con el objetivo de encontrar la mejor configuración, en términos de selección de materiales y geometría, que sean más eficientes como sistema tes. El trabajo muestra los siguientes avances en la tecnología CSP mediante el uso de aglutinantes alternativos ecológicos: el volumen de instalación se puede reducir en un 17%, en comparación con un tanque de sal fundida, mientras que la superficie del intercambiador de calor se puede redimensionar en un 29%, en comparación con el sistema de referencia que utiliza PC. Estas mejoras permiten variaciones más amplias en la eficiencia operativa y las capacidades dinámicas de la CSP y representan un progreso importante hacia el desarrollo de tecnologías de CSP más eficientes y sostenibles. New avenues for thermal energy storage (TES) need to be investigated due to the lack of competitiveness of concentrated solar power (CSP) technologies. Solutions must be found to replace molten salt tanks which have a major economic impact and are difficult to maintain due to corrosion problems. In this sense, concrete represented an attractive candidate by proving excellent sensible TES in CSP. However, its main phase, made of Portland cement (PC), has significant environmental consequences. The production of PC is known to emit high levels of polluting gases, particularly the CO2. It is estimated to be responsible for between 5% and 7% of the world's CO2 emissions, making it a major contributor to climate change. This work presents greener cementitious materials, made of alkaline cements and hybrids cements, to be used as alternative eco-friendly TES media in CSP plants. An experimental campaign is presented which shows that these eco-efficient materials can have better mechanical properties, than the ordinary PC mortar, when exposed to high temperatures, in addition, can offer improvements of their thermal properties (thermal conductivity or specific heat). Second part of the work is devoted to Finite Element simulations, with the aim to find the best configuration, in terms of selection of materials and geometry, which are more efficient as TES system. The work is showing the following advancements in CSP technology by using alternative eco-friendly binders: the installation volume can be reduced by 17%, compared to a molten salt tank, while the heat exchanger's surface area can be resized by 29%, compared to the reference system using PC. These improvements enable wider variations in CSP operational efficiency and dynamic capabilities and represent important progress towards developing more efficient and sustainable CSP technologies. يجب التحقيق في طرق جديدة لتخزين الطاقة الحرارية (TES) بسبب الافتقار إلى القدرة التنافسية لتقنيات الطاقة الشمسية المركزة (CSP). يجب إيجاد حلول لتحل محل خزانات الملح المنصهر التي لها تأثير اقتصادي كبير ويصعب الحفاظ عليها بسبب مشاكل التآكل. وبهذا المعنى، مثلت الخرسانة مرشحًا جذابًا من خلال إثبات أنها تجربة معقولة ممتازة في مجال الطاقة الشمسية المركزة. ومع ذلك، فإن مرحلته الرئيسية، المصنوعة من أسمنت بورتلاند (PC)، لها عواقب بيئية كبيرة. من المعروف أن إنتاج PC ينبعث منه مستويات عالية من الغازات الملوثة، وخاصة ثاني أكسيد الكربون. ويقدر أنها مسؤولة عن ما بين 5 ٪ و 7 ٪ من انبعاثات ثاني أكسيد الكربون في العالم، مما يجعلها مساهما رئيسيا في تغير المناخ. يقدم هذا العمل مواد أسمنتية أكثر اخضرارًا، مصنوعة من الأسمنت القلوي والأسمنت الهجين، لاستخدامها كوسيط TES بديل صديق للبيئة في محطات الطاقة الشمسية المركزة. يتم تقديم حملة تجريبية توضح أن هذه المواد ذات الكفاءة البيئية يمكن أن يكون لها خصائص ميكانيكية أفضل، من ملاط PC العادي، عند تعرضها لدرجات حرارة عالية، بالإضافة إلى ذلك، يمكن أن تقدم تحسينات في خصائصها الحرارية (الموصلية الحرارية أو الحرارة النوعية). الجزء الثاني من العمل مخصص لمحاكاة العناصر المحدودة، بهدف العثور على أفضل تكوين، من حيث اختيار المواد والهندسة، والتي هي أكثر كفاءة كنظام TES. يُظهر العمل التطورات التالية في تقنية الطاقة الشمسية المركزة باستخدام مواد رابطة بديلة صديقة للبيئة: يمكن تقليل حجم التركيب بنسبة 17 ٪، مقارنة بخزان الملح المنصهر، في حين يمكن تغيير حجم مساحة سطح المبادل الحراري بنسبة 29 ٪، مقارنة بالنظام المرجعي باستخدام الكمبيوتر. تتيح هذه التحسينات اختلافات أوسع في الكفاءة التشغيلية والقدرات الديناميكية للطاقة الشمسية المركزة وتمثل تقدمًا مهمًا نحو تطوير تقنيات أكثر كفاءة واستدامة للطاقة الشمسية المركزة.

This paper reports a comprehensive experimental investigation of cement pastes enhanced with Microencapsulated Phase Change Materials (MPCM) for Thermal Energy Storage (TES) purposes. The experimental plan considers three water-to-binder ratios and three MPCM volume fractions, for a total of nine different MPCM paste mixtures. The water-to-binder ratios of the pastes are 0.33, 0.40 and 0.45, which were mixed with a commercial MPCM, namely Nextek 37D® having a melting/solidification temperature of 37 °C, with volume percentage substitutions of 0%, 20% and 40%, respectively. Thermal, physical and mechanical tests were performed to investigate the effect MPCM have on the resulting TES, strengths and conductive properties of the considered mixtures by employing DSC, Hot-Disk, and mechanical tests. The measured latent heat of MPCM was 197.3 J/g and 194.6 J/g for heating and cooling, respectively. The volumetric latent enthalpies for the MPCM-based composites showed an almost constant average of 20–25 MJ/m3 for samples with 20% MPCM and 55–60 MJ/m3 for samples with 40% MPCM, independently of the w/b ratio. Thermal conductivity values measured at 25 and 45 °C ranged between 0.93 and 0.44 W/m × K. MPCM substitution turned out to significantly affect the overall porosity of the composite resulting in a lower thermal conductivity for the MPCM-pastes in comparison to the plain cement matrix. Finally, mechanical tests were conducted that showed a strength loss due to either increasing w/b ratios or for enhanced amounts of MPCM (e.g., up to a 74% and 69% of strength loss were registered for bending and compression, respectively). The thermo-physical and mechanical characterizations were conducted according to an experimental plan that provided a wide set of research results for both sole MPCM and MPCM-cement systems analyzed by SEM, EDS/elemental mapping, contact angle tests, particle size distribution analysis and Mercury Intrusion Porosimetry technique.

The transition to sustainable energy highlights the importance of thermal energy storage (TES) systems, particularly in concentrated solar power plants. While Portland cement has shown potential in TES applications, its high CO₂ emissions limit its sustainability. Therefore, this research examines alternative cementitious materials, specifically alkali-activated (AAM) and hybrid alkaline materials (HM), which use blast furnace slag as a binder and incorporate recycled aggregates such as glass waste and electric arc furnace slag. These alternatives not only demonstrate enhanced thermal and mechanical stability up to 500 °C but also exhibit improved energy efficiency. Finite Element Method simulations indicate that these alternatives can reduce TES system volume and improve heat transfer efficiency. Additionally, Life Cycle Assessment highlights significant reductions in carbon and water footprints. This study provides insights into the use of AAM and HM mortars as viable, lower-impact alternatives that align with sustainability goals in renewable energy applications.