Search

A number of models and modelling approaches for phase transitions of phase change materials (PCMs) have been proposed in recent years. However, many of these models have not been thoroughly validated with experimental data. This is in particular the case of the models for thermal hysteresis and partial phase transitions of PCMs, where the design and execution of relevant experiments is difficult. The most widely used experimental techniques for characterization of PCMs - Differential Scanning Calorimetry (DSC) and T-history method – require minimization of the temperature gradients in the test samples and thus the obtained results do not represent very well the behavior of PCMs in thermal energy storage (TES) systems (where large temperature gradients in PCMs are commonplace). An experimental set-up for the acquisition of data suitable for validation of phase change models have been proposed and assembled. The set-up can be used for the model validation in case of both the direct and inverse heat transfer problems. The set-up is based on the monitoring of the phase change front propagation in a rectangular cavity, where the positive or negative heat flux is introduced at one of the cavity walls. Such an arrangement results in (often significant) temperature gradients in a PCM. Unlike in similar experimental set-ups, where a heat transfer fluid (HTF) is used to introduce the heat flux at the wall, the Peltier cells are used in the proposed experimental set-up for this purpose. Also, most experiments reported in the literature only addressed the melting process (heating of a PCM) with the positive heat flux introduced at the wall. The Peltier cells allow for relatively quick switching between the positive and negative heat flux (heating/cooling) and as a result the thermal processes similar to real-life operation of TES systems can be investigated. The cubical cavity with 250 mm long internal edges is made of PMMA. The wall, at which the heat flux is introduced, is made of a 15 mm thick aluminum plate with embedded RTD temperature sensors for wall temperature monitoring. A heat flux sensor is installed on the side of the aluminum plate facing the PCM (the heat flux sensor covers the entire surface of the plate). An extended heat transfer surface in the form of a finned aluminum sink is installed on the Peltier cells to improve the heat transfer between the cells and the ambient environment.

Thermal energy storage with phase change materials (PCMs) has attracted a lot of attention in the last several decades. Most PCMs do not change phase at a constant temperature but rather in a certain temperature range. It means that the PCM need to transit through its phase change temperature range to fully change phase from the solid state to the liquid state and vice-versa. The situation, in which the phase transition begins and/or ends within the phase change temperature range (in the mushy zone), is usually called a partial phase transition (or a partial phase change). The partial phase transitions occur quite often in real-life thermal energy storage systems with PCMs; especially when a PCM has a wide phase change temperature range. The behavior of PCMs during the partial phase transitions is poorly understood at the moment, because the experimental techniques used for the characterization of PCMs (such as the differential scanning calorimetry – DSC) are difficult to apply for the study of partial phase transitions. The lack of knowledge in this area influences the accuracy of phase change simulation models. The main goal of the experimental investigations, described in the paper, was to obtain data for the development of a simulation model for partial phase changes. The experimental set up for the investigation of partial phase changes of PCMs has been proposed, assembled, and the pilot measurements have been conducted. The experimental set up consists of two water storage tanks (that can be maintained at different water temperatures), a water-PCM concentric tube type heat exchanger and a data acquisition system. The water flows through the central tube of the heat exchanger while the PCM is located in the annular space of the exchanger. The water storage tanks, maintained at the temperatures within the phase change temperature range of a PCM, allow for the investigations of the heat storage cycles consisting of partial phase changes.

This paper presents a numerical modelling approach to investigate convective heat transfer in a rectangular cavity filled with a Phase Change Material (PCM), specifically organic paraffin-based Rubitherm RT 35 HC. Natural convection occurs during both the melting and solidification processes due to the changes in the density of a liquid PCM. Therefore, a significant enhancement of the heat transfer in the upper part of the cavity can be observed. The study aims to investigate, both experimentally and numerically, the effect of convective flow on the overall heat transfer performance of a PCM in the cavity. The computational analysis was performed using a two-dimensional explicit finite difference model implemented in MATLAB. To accurately represent the thermal behaviour of the PCM enclosed in a cavity, the proposed modelling approach incorporates the effects of natural convection by considering the effective thermal conductivity of the PCM as a function of both temperature and position within the cavity. By doing so, heat transfer in the upper part of the cavity is enhanced while heat transfer in the bottom part is mitigated, resulting in a more precise representation of the actual thermal behaviour of the PCM. The comparison between the constant thermal conductivity and the effective thermal conductivity with a linear increase has been made, resulting in a significant improvement in the model accuracy. The analysis demonstrated that the effective thermal conductivity approach led to a significant improvement in the accuracy of the numerical model. Specifically, a reduction of 47.5 % was observed in the root mean square error (RMSE) value, decreasing from 4.0 K to 1.9 K. [ABSTRACT FROM AUTHOR]

The paper is aimed atto a study of a possibility to use PCM (Phase Change Materials) to increase the thermal storage properties of buildings. For this purpose, both the experimental testing and computer modelling were performed. The PCM DuPont Energain panel was used for the study of its thermal technical parameters under laboratory conditions. Laboratory testing of the effect of PCM surface treatment showed a double increase in the heat transfer coefficient, but also a decrease of 17 % in the time constant under given conditions. The computer modelling was used for the study of the influence of walls sheathing on the temperature comfort inside the tested room of the wooden house by its passive cooling in summer. Results of the numerical simulations proved influence of the PCM covering. But in the presented model, the PCM covering caused only a slight improvement in the thermal comfort inside the room.

The article deals with the economic evaluation of investment and optimization of the renewable energy sources for family houses. For example, from the point of view of solar systems, the optimal solution is based on the specific application of it. The design is dependent on the location of the panels and ration between active aperture area and real daytime consumption. Common calculations according to actual standards often give overstated results, which also reflected in the value of the investments. The article shows the research of optimal parameters of the thermal solar system for preparing of domestic hot water. A combination of related standards and software TRNSYS are used to find optimal parameters. Thanks to created and verified simulation models, it is possible to design parameters so as to avoid under-dimensioning or over-dimensioning of the each system. Energy price is another factor affects the payback period of investments. This is affected by the used energy sources and their combination. For example, buildings that use electricity to heat water or heating have different energy charges than a building that uses natural gas. So, the aim is to find optimal solution of the combination of renewable energy sources.

The paper deals with computer testing of the temperature distribution in buildings by using COMSOL Multiphysics software. It is devoted to a description of a computer application created in the Application Builder user interface for simulation of the temperature distribution in a room heated by two heat sources. The application allows you to change geometric dimensions of all elements of the studied model and their spatial distribution, as well as a choice of physical properties needed to access the distribution of temperature in the room depending on the ambient temperature and the heat input of the considered sources. Main functions of the application are presented by simulation of cyclic heating and cooling of the tested room.

The article deals with the economic evaluation of investment and optimization of the solar water heating system for family houses. From the point of view of solar systems, the optimal solution is based on the specific application of it. The design is dependent on the location of solar thermal collectors and ration between active aperture area and real daytime consumption. Common calculations according to actual standards often give overstated results, which also reflected in the value of the investments. The article presents the research of optimal parameters of the thermal solar system for preparing of domestic hot water. A combination of related standards and software TRNSYS are used to find optimal parameters. Thanks to created and verified simulation models, it is possible to design parameters so as to avoid under-dimensioning or over-dimensioning of the solar system. Energy price is another factor affects the payback period of investments. This is affected by the used energy sources and their combination. For example, buildings that use electricity to heat water or heating have different energy charges than a building that uses natural gas. So, the aim is to find technically and economically efficient solution.

The article deals with the use of modern technologies that can improve the thermal comfort in buildings. The article describes the usage of thermal energy storage device based on the phase change material (PCM). The technology improves the thermal capacity of the building and it is possible to use it for active heating and cooling. It is designed as a “green technology” so it is able to use renewable energy sources, e.g., photovoltaic panels, solar thermal collectors, and heat pump. Moreover, an interesting possibility is the ability to use thermal energy storage in combination with a photovoltaic system and thermoelectric coolers. In the research, there were made measurements of the different operating modes and the results are presented in the text.

The paper presents the interim results of our research on the developing methodological procedure which could be used for assessment of a thermal stability of buildings with regards to its thermal accumulative parameters. The principle of testing is based on a combination of computer simulation of cooled room model developed in COMSOL Multiphysics software and on theoretical calculations with respect to compliance with valid European and Czech technical standards used in building industry and architecture under conditions obtained by real measurement for the room to be tested. The presented example shows the effect of the heataccumulation properties of the outside wall insulation materials on the course of the cooling room for winter conditions.

Energy storage is going to play an important role in the transition to carbon-neutral energy production. Heat is the form of energy that is relatively easy to store, even over long periods of time. Despite their relatively slow take off as a thermal energy storage medium, the phase change materials (PCMs) can play an important role in the future of thermal energy storage (TES). Some PCMs exhibit behaviours that are difficult to model accurately (such as supercooling or phase change hysteresis). Computer modelling of the PCM behaviour is further complicated by the fact that the PCMs sometimes do not undergo complete phase transitions in real-life operation of TES. Three approaches to modelling of partial phase changes of a PCM, obtained from the literature, were assessed on a simple test case. The test case was a wall with a PCM layer, which was designed as transient with the adiabatic boundary condition on one side of the wall and the sine wave heat flux on the other side. The phase change hysteresis shift of 5 °C was investigated. The results obtained with the partial phase change modelling approaches varied on average by 2.8 °C and 1.84 °C for the 'curve switch' model and the 'curve scale' model (when compared to the single curve model). The increase in thermal conductivity (from 0.2 to 0.3 W m-1 K-1) led to 34.4 % reduction in size of the phase change interruption point interval. [ABSTRACT FROM AUTHOR]

Many engineering problems involve heat transfer with phase change and their solution often lead to challenging heat transfer problems having no direct solution. A direct solution in this respect means the determination of the thermal behaviour of a system under imposed initial and boundary conditions. The direct solution is not possible in problems where those initial and boundary conditions are unknown. In such cases, an inverse approach has to be used. However, most of the methods available for the solution of inverse heat transfer problems have been applied to heat transfer problems without the phase change. In this respect, soft computing methods seem to be a promising approach. The reason is that soft computing methods build on artificial intelligence, nature-inspired mechanisms and other principles, which enable to effectively find a sufficiently accurate solution to even very complex problems for which hard computing approach fails. In this paper, a computer heat transfer model accounting for the phase change was created and a neural network approach, which also belongs to the soft computing family, was applied to the solution of an inverse heat transfer problem. The identical problem was also solved by means of a well-established (traditional) Beck's method and the two inverse solutions were compared to each other, including the assessment of the overall computational procedure. The results showed that the approach based on neural networks was efficient and qualitatively led to similar results as in case of the Beck's method and was computationally more efficient. [ABSTRACT FROM AUTHOR]

This article describes the usage of thermal energy storage in the form of heat and cold with an adaptation of the special device which is composed of the thermal panels. These panels are based on the phase change materials (PCM) for normal inner environment temperature in buildings. The energy for the thermal energy storage is possible to get from built-in electric heating foil or from the tube heat exchanger, which is build in the thermal panels. This technology is able to use renewable energy sources, for example, solar thermal collectors and air-to-water heat pump as a source of heat for heating of the hot water tank. In the cooling mode, there is able to use the heat pump or photovoltaics panels in combination with thermoelectric coolers for cooling.

This paper deals with the study of a non-stationary heat conduction in the solid wall. It is focused on mathematical modelling of its asymmetric heating and cooling by imperfect heat transfer to both sides of the wall. It describes method used for deriving of the long time analytical solution describing temperature distribution in the heated (cooled) wall by use Laplace transform and verification of its validity by numerical calculation with COMSOL Multiphysics software. In the tested example, the maximum difference between analytical and numerical solution was about 3.5 % considering the possible maximum and minimum temperatures in the wall under the given conditions.

Some engineering heat transfer problems cannot be solved directly as certain parameters influencing heat transfer are unknown. This is where the solution of inverse heat transfer problems might be utilized. The paper explores application of the Particle Swarm Optimisation (PSO) method to heat transfer problems with phase change. In order to assess the robustness and accuracy of the method, the study was conducted on simulation results. The explored case is an air-PCM heat exchanger. In the first step, the simulations with various values of parameters influencing heat transfer were performed. All cases were simulated as transient with timedependent boundary conditions. In the second step, some of these parameters (phase change temperature, density, thermal conductivity, etc.) were assumed to be unknown and they were obtained from the simulation results with the use of the PSO method. The cost function was defined in the form of a root mean square error between the simulation results (the outlet air temperature from the heat exchanger) and the results of the optimized scenario. The number of unknown parameters varied from one to seven (in case of the description of the relationship of effective heat capacity on temperature, which was parameterized in the form of an asymmetric Gaussian function). The results have shown that the PSO is a robust and accurate tool with only up to 2 % difference in the enthalpy of fusion and no significant discrepancies during the higher dimension optimisation. [ABSTRACT FROM AUTHOR]

Solar radiation is a clean and renewable source of energy, which can be employed in various forms. In contrast to electricity, the use of solar energy in the form of heat is simple and straightforward. Solar air collectors (SAC), which convert solar radiation into heat and transfer it to the air, represent a way how to use solar energy for space heating in buildings. In the paper, the operation and optimal design of a solar air collector integrating a phase change material (PCM) for thermal energy storage is computationally investigated. A computer model of a front and back pass solar air collector with a PCM-based absorber was developed and validated against experimental data. The energy balance approach coupled with the control volume method was implemented for solving conduction heat transfer inside the PCM, and the effective heat capacity method was used for phase change modelling. The developed model was consequently coupled with the self-adaptive differential evolution optimization method. The cost function was defined as the root mean square error between the outlet SAC temperature and the set temperature. Using PCM parameters and its width as variables, the optimal set of parameters was determined. The optimal temperature of phase change was equal to 64.8 °C, PCM thickness 0.08 m and material parameter c1 = 69,997 J/kg·K. [ABSTRACT FROM AUTHOR]