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Summary: Using phase change material (PCM) in the building envelope can provide energy saving advantages by shaving peak heating and cooling loads. However, the poor thermal conductivity of PCMs limits their application and potential benefits because of inadequate heat storage/release rates. Increasing effective thermal conductivity by adding metal nanoparticles with high thermal conductivity to PCM (NPCM) could be a promising method to accelerate the phase change process, thereby exploiting latent heat more effectively. The potential impact of the utilization of NPCM technology has not yet been adequately explored for building external walls. This study aimed to reveal whether the dispersal of highly conductive nanoparticles in PCM in external building walls helps conserve energy or not. PCM enriched with aluminium oxide nanoparticles (Al2O3) with a content of 1, 2, and 3 vol% was used. The outputs showed that the nanoparticle addition decreased the energy‐saving performance of PCM since the reduction in the thermal resistance and latent heat capacity caused by the nanoparticles loading was more profound than the enhancement provided by the improvement of latent heat exploitation caused by the increased thermal conductivity. For example, heating energy saving was reduced by 0.6% when a 3‐cm PCM with 1 vol% Al2O3 was used instead of a pure PCM. The negative impact increased to 1.7% by increasing the nanoparticle concentration to 3 vol%. Thus, augmenting the thermal conductivity for higher latent heat activation by adding nanoparticles was not beneficial for building wall applications in a hot‐summer Mediterranean climate. Highlights: Effect of nanoparticle‐enhanced PCM on building energy saving.PCM with Al2O3 content of 0, 1, 2, and 3 vol% embedded in external wall.Yearly energy saving of 20.7% was achieved with pure PCM in the wall.Adding 3 vol% Al2O3 to PCM reduced heating energy saving by 1.7%.Negative effect on energy saving increased with increasing nanoparticle concentration. [ABSTRACT FROM AUTHOR]

From the thermodynamic perspective, the term temperature is clearly defined for ideal physical systems: A unique temperature can be assigned to each black body via its radiation spectrum, and the temperature of an ideal gas is given by the velocity distribution of the molecules. While the indoor environment is not an ideal system, fundamental physical and chemical processes, such as diffusion, partitioning equilibria, and chemical reactions, are predictably temperature‐dependent. For example, the logarithm of reaction rate and equilibria constants are proportional to the reciprocal of the absolute temperature. It is therefore possible to have non‐linear, very steep changes in chemical phenomena over a relatively small temperature range. On the contrary, transport processes are more influenced by spatial temperature, momentum, and pressure gradients as well as by the density, porosity, and composition of indoor materials. Consequently, emergent phenomena, such as emission rates or dynamic air concentrations, can be the result of complex temperature‐dependent relationships that require a more empirical approach. Indoor environmental conditions are further influenced by the thermal comfort needs of occupants. Not only do occupants have to create thermal conditions that serve to maintain their core body temperature, which is usually accomplished by wearing appropriate clothing, but also the surroundings must be adapted so that they feel comfortable. This includes the interaction of the living space with the ambient environment, which can vary greatly by region and season. Design of houses, apartments, commercial buildings, and schools is generally utility and comfort driven, requiring an appropriate energy balance, sometimes considering ventilation but rarely including the impact of temperature on indoor contaminant levels. In our article, we start with a review of fundamental thermodynamic variables and discuss their influence on typical indoor processes. Then, we describe the heat balance of people in their thermal environment. An extensive literature study is devoted to the thermal conditions in buildings, the temperature‐dependent release of indoor pollutants from materials and their distribution in the various interior compartments as well as aspects of indoor chemistry. Finally, we assess the need to consider temperature holistically with regard to the changes to be expected as a result of global emergencies such as climate change. [ABSTRACT FROM AUTHOR]