• Energy Research

The study presents an original implementation of an algorithm capable to describe the thermal behavior of buildings. The envelope structure and the climatic data are used as input data. The algorithm identifies shares of heating and cooling loads for walls, windows and fresh air. The results were obtained for a real hospital based on hourly calculations. Based on the provided results, energy efficient solutions, including shielding technologies, can be hierarchized considering the thermal losses and gains corresponding to each component.

The study presents a procedure for precise evaluation of the apparent heat capacity of phase change materials (PCM), based on differential scanning calorimetry (DSC). A mathematical model capable to predict the thermal behavior of a PCM within a latent heat thermal storage system was developed. The model takes into account the PCM’s apparent heat capacity variation with temperature. The model was validated with an experimental latent heat storage system presented in this study. It was found that the model is most accurate for PCMs with little or no subcooling and that validation of such models, on the designed laboratory equipment, should be carried out with reduced air flow rate. The average value of the mean bias error for the PCM and air temperature predictions were found to be 6.12% and 5.21% respectively.

One of the greatest challenges of humanity is to reach world climate-neutrality by 2050. Using renewable energy and in particular solar energy instead of fossil fuels, falls in this direction. A quest for the use of the solar energy as much as possible throughout the year becomes very important in this paradigm. A solution for the use of solar energy during the shading of the solar installation caused by clouds, for example, or even at night, is the use of the phase change materials (PCM). Also, to use the solar equipment as much as possible, the use of cascaded PCM appear as necessity. A transpired solar air collector equipped with cascaded PCM was numerically studied using a validated mathematical model, developed in MATLAB, for one specific autumn day in the temperate climate of Romania. Three different PCM with phase change temperatures of 24 °C (RT24), 26 °C (RT26) and 28 °C (RT28), were used for the PCM cascade. The air, the PCM spheres temperatures and the liquid mass fraction in three different zones of the thermal energy storage, at low, intermediate, and high temperature zones were studied. It is also noticeable the phase shift of each PCM zone, the first to release the heat is the RT28, then the RT26 and finally the RT24. The results showed that cascaded PCM storage could be used for an optimized heating/drying condition allowing to store an amount of energy during the first part of the day that could be released during drying time in the second part of the day. During daytime the LTHES stores thermal energy leading to a maximum decrease of air temperature of 4 °C. During night-time the LTHES released the thermal energy into the outlet air flow leading to a maximum increase of 8.4 °C. Analysing the liquid mass fraction of the PCMs, the results showed that only the RT24 zone is totally melting, with a maximum liquid mass fraction of 1, while the RT28 and RT26 reach a maximum liquid fraction at the centre of the sphere of 0.23 and 0.54, respectively.

Abstract The study provides novel and validated explicit equations of the apparent heat capacity variation with temperature for three phase change materials (PCM): RT20, RT25 and RT27. The developed models of PCM thermal behavior were successfully applied in the energy efficiency analysis of the fresh air cooling system with PCM latent heat storage system, serving a virtual office building considered to be located in different climatic conditions. Following the study, a practical guideline for estimation of the required quantity of PCM in fresh air cooling systems was provided. Two novel specific energy indicators were also proposed, which are useful for feasibility studies. The evaluation of PCM energy efficiency in fresh air cooling systems revealed that savings in the electric energy consumption of (7–41) % can be achieved, depending on the particular local conditions. Limits of the use of PCM in fresh air cooling systems were also provided.

Abstract Thermal energy storage tanks have a fundamental role in the preparation and simultaneous distribution of domestic hot water. These tanks are supplied with thermal energy via classical means, based on electrical energy and fuel combustion, and via renewable energy sources. This study investigates the possibility of reducing the storage tank’s accumulation volume by employing phase change materials to store high densities of thermal energy at constant temperatures. A mathematical model was developed to simulate the operation of storage tanks equipped with immersed solar coil, also taking into account the addition of phase change materials encapsulated in cylindrical containers. The mathematical model was validated based on literature data. The simulation program also determines the temperature of the primary heat transfer fluids, prepared by a solar collector system and a flat plate heat exchanger coupled with a gaseous fuel boiler. The program was used to study the performance of storage tanks that serve a public swimming pool, a type of building that is characterized by a high demand of domestic hot water. To the best knowledge of the authors, this scenario was not reported in the literature. The study revealed that the nominal volume of the storage tanks can be reduced by 25% in comparison to the classic alternative, by using phase change materials as thermal storage. Also, the tanks equipped with phase change material achieved a reduction of fuel consumption and CO2 emissions by (5.00-11.97) % in comparison to the classic solution.

The study presents a new analytical model capable to reveal the thermal behaviour of all the components of the solar ammonia-water absorption system, powered by parabolic trough collectors, serving different types of food storages: refrigeration chamber, refrigerated food storage, freezing chamber and frozen food storage. The heat inputs, that determine the total cooling load, for each food storage spaces consist of: heat gains through walls, heat gains through ventilation (fresh air), heat that must be dissipated from the stored products (technological cooling load required to cool down the products) and heat gains through operation. The influence of the number of solar parabolic trough collectors and of the storage tank size on different parameters of the refrigeration plant are investigated under low and high storage temperatures.•Food cooling with solar absorption refrigeration system.•Hourly based variation of NH3-H2O solar absorption system performances.•Long term simulation of solar absorption cooling for refrigeration and cooling.

A key component of the concept of Smart City is the use of energy efficient technology for household and nonresidential building applications. In this context, the study presents an experimental setup with phase change material (PCM) encapsulated in spherical shells and an analysis of phase change materials for energy efficient cold storage applications using air as heat transfer fluid. The solidification and melting processes are investigated and compared with the thermal behavior indicated by the differential scanning calorimetry (DSC) signals generated at cooling and heating rates of 0.1 K/min. The solidification and melting experiments were carried out at a mean air velocity of 0.837 and 0.833 m/s, respectively. The temperature differences between the peak phase change temperature and the mean air temperature are 20.87 and 17.21 °C, for solidification and melting, respectively. It was observed that melting occurred at a higher rate than solidification due to natural convection. Melting completes (3–12) minutes faster than solidification depending on the position of the sphere in the thermal setup. Because of the natural convection some deviations between the experimental and differential scanning calorimetry heating rates were observed. The experimental thermal storage is capable to reduce the air temperature with approximately (7.54-14.61) °C, at melting.

Abstract The study presents a numerical model that uses the enthalpy method in order to solve the moving boundary problem and to predict the thermal behaviour of phase change materials during solidification and melting. The commercially available paraffin RT21 is considered as phase change material, and the thermophysical properties of the paraffin were determined by differential scanning calorimetry. An inverse enthalpy-temperature function was determined based on the results of the differential scanning calorimetry measurements in order to model the phase change process. The model was experimentally validated, and good agreement was found between the computed and measured results.

The study carried out by simulation, concerns the thermal behavior of an office building’s solar fresh air cooling system, based on a LiBr-H2O absorption chiller in different climatic conditions. The coefficient of performance (COP) and the solar fraction were considered performance parameters and were analyzed with respect to the operating limits: risk of crystallization and maintaining at least a minimum degassing zone. A new correlation between the required solar hot temperature and the cooling water temperature was established and then embedded in another new correlation between the COP and the cooling water temperature that was used in simulations during the whole cooling season corresponding to each location. It was found that: the solar hot water should be maintained in the range of (80-100) °C depending on the cooling water temperature, the COP of the solar LiBr-H2O absorption chiller with or without cold storage tank can reach (76.5-82.4) % depending on the location, and the solar fraction can reach (29.5-62.0) % without cold storage tank and can exceed 100 % with cold storage tank, the excess cooling power being available to cover other types of cooling loads: through the building envelope, from lighting, from occupants, etc.