The continuous growth of the concrete industry requires an increased quantity of cement and natural aggregates year after year, and it is responsible for a major part of the global CO₂ emissions. These aspects led to rigorous research for suitable raw materials. Taking into account that these raw materials must have a sustainable character and also a low impact on environmental pollution, the replacement of the conventional components of concrete by residual waste can lead to these targets. This paper’s aim is to analyze the density, compressive strength and the thermal conductivity of nine concrete compositions with various rates of waste: four mixes with 10%, 20%, 40% and 60% chopped PET bottles aggregates and 10% fly ash as cement partial substitution; a mix with 60% waste polystyrene of 4–8 mm and 10% fly ash; a mix with 20% waste polystyrene of 4–8 mm, 10% waste polystyrene of 0–4 mm and 10% fly ash; a mix with 50% waste polystyrene of 4–8 mm, 20% waste polystyrene of 0–4 mm and 20% fly ash two mixes with 10% fly ash and 10% and 40% waste sawdust, respectively. Using 60% PET aggregates, 60% polystyrene granules of 4–8 mm, or 20% polystyrene of 0–4 mm together with 50% polystyrene of 4–8 mm led to the obtainment of lightweight concrete, with a density lower than 2000 kg/m³. These mixes also registered the best results from a thermal conductivity point of view, after the concrete mix with 40% saw dust. Regarding compressive strength, the mix with 10% PET obtained a result very close to the reference mix, while those with 20% PET, 40% PET, 30% polystyrene, and 10% saw dust, respectively, registered values between 22 MPa and 25 MPa, values appropriate for structural uses.

One of the biggest challenges the world is facing these days is to reduce the greenhouse gases emissions in order to prevent the global warming. Since a significant quantity of CO₂ emissions is the result of the energy producing process required in industry or buildings, the waste heat recovery is an important aspect in the fight for preserving the planet. In this study, an innovative waste heat recovery system which can recover waste heat energy from cooling liquids used in industry or in different processes, was designed and subjected to experimental investigations. The equipment uses heat pipes to capture thermal energy from the residual fluids transiting the evaporator zone and transfer it to the cold water transiting the condenser zone. The efficiency of the heat exchanger was tested in 9 scenarios, by varying the temperature of the primary agent to 60, 65 and 70 °C and the volume flow rate of the secondary agent to 1, 2 and 3 L/min. The temperature of the secondary agent and the volume flow rate of the primary agent were kept constant at 10 °C, respectively 24 L/min. The results were later validated through numerical simulations, and confirmed that the equipment can easily recover waste thermal energy from used water with low and medium temperatures at very low costs compared to the traditional heat exchangers. The results were promising, revealing an efficiency of the equipment up to 76.7%.

The subject of buildings energy efficiency has gained increased interest in modern society. Recently, researchers aiming to reduce the energy demand of buildings have studied the implementation of phase change materials (PCMs) in various building elements. At the same time, researchers studied the implementation of unconventional technologies such as heat pipes (HP) in the buildings’ heating system. This paper combines both of these technologies in order to take advantage of the thermal storage properties of PCMs and overcome their reduced conductivity with heat pipes. Through computational fluid dynamics (CFD) simulations, this paper studies and highlights that with the implementation of a PCM, a reduction of the daily energy demand is achieved through the increase in the discharge phase time.

In the modern society the concept of energy efficiency of buildings has become a subject of great interest, due to the negative impact of buildings upon the environment, society and economy. One of the methods of overcoming this is to provide a better efficiency of the energy storage because it is the key to resolve the difference between supply and demand. In comparison to electro-chemical batteries which provide limited energy storage at a high financially cost, thus in this regard, thermal energy storage technologies with high energy density have increased market potential. PCMs present objective advantages in comparison to classic construction materials due to their characteristics of being able to store and release higher quantities of thermal energy during the phase change process. A PCM radiant floor heating system can provide a reduction of the energy consumption for heating by up to 43%, trough the higher discharge time of the PCM compared to regular heating systems.