Vol 5 - Special issue IREEC 2

Entropy: Thermodynamics – Energy – Environment – Economy

List of Articles

Le laboratoire d’énergétique et environnement de l’école nationale d’ingénieurs de Tunis et le centre d’énergétique et thermique de Lyon se sont associés en mars dernier pour organiser le second colloque international froid énergie et environnement (IREEC2). Ce colloque qui a réuni une centaine de participants, a été organisé avec l’appui de la société tunisienne de dessalement et de l’association tunisienne de réfrigération et climatisation sous l’égide de l’institut international du froid et de l’Association française du froid. Plus de cinquante communications ont été présentées en trois ateliers parallèles ainsi que six plénières. Un franc succès a été remporté par ces présentations dont nous avons retenus quelques exemples marquants qui font l’objet de ce numéro spécial d’Entropie.

Photovoltaic (PV) panels are an integral part of solar energy systems, converting sunlight directly into electricity. With the increasing demand for renewable energy sources, PV panels have gained significant attention due to their ability to generate clean and sustainable power. However, the performance of PV panels is influenced by various factors, including their design, materials, operating conditions, and environmental factors. Cooling is a crucial aspect in the operation of PV panels, as high temperatures can significantly affect their efficiency and overall performance. Water and nanofluid cooling have emerged as promising strategies to mitigate temperature-related issues and enhance the energy output of PV panels. This abstract focuses on the application of water and nanofluid cooling techniques in PV panels and their impact on performance. This work explores the influence of parameters such as mass flow rate, nanofluid concentration, and nanofluid type on cell temperature reduction and resulting thermal and electrical efficiencies of a PV panel situated in the Gabes region. The study considers three cases: a standalone PV panel, a PV/T system with water cooling, and a PV/T system with nanofluid cooling. To maximize the interaction between the cooling fluid and the back surface of the solar panel, the tested fluids are circulated through a rectangular heat exchanger.

Solar Ponds are large areas consisting essentially of three density-stratified zones: an energy storage zone (Layer Convective Zone ’LCZ’), a thermal insulation zone (Non-Convective Zone ’NCZ’), and a protection zone against natural hazards such as wind and dust (Upper Convective Zone ’UCZ’). The performance and efficiency of solar ponds depend on the stability of the gradient layer. The latter is subject to double molecular diffusion of heat and mass and hydrodynamic movements (double convective diffusion) tend to be homogenized. The coexistence of different diffusions in the stratified system constitutes complex instability phenomena whose physics is poorly known until today. In this study, preliminary experiments showed that the stability of a solar pond could be increased by using phase change materials (PCM) placed at the bottom of the pond.

Among the thermal applications of solar energy, solar cooking is considered one of the simplest, most viable and most attractive options in terms of using solar energy. In isolated mountainous regions, or in the desert where wood energy resources are constantly decreasing while heat needs are constantly growing, solar cooking of food products then appears to be the ideal way to remedy this problem. issue. This study focuses on the numerical simulation of a prototype solar cooker. This device is designed to ensure food cooking thanks to solar radiation which will be captured and trapped to reach temperature levels favorable to healthy food cooking. The modeling of the prototype of the solar cooker studied is carried out by introducing the different heat exchanges involved between the different elements of the solar cooker. We present an analysis of the results of the numerical simulation of the system processed using MATLAB software with a calculation algorithm based on the Runge-Kutta method. An analysis of the exergy of the studied solar oven is carried out to analyze and optimize the operation of the oven.

This paper presents a novel thermodynamic system developed to generate heat and cold for industrial processes. The efficiency is improved compared to commercialized ones and current state-of-the-art systems. The proposed hydro-CO₂ piston combines three counter-intuitive innovations with the operation of discontinuous and slowed thermodynamic cycles where mechanical work is transferred to the refrigeration through a hydraulic circuit based on modified transcritical Carnot and Rankine cycles. This allows to operate unusual thermodynamic transformations such as isothermal compression and two-phase isentropic expansion. The cycles are tailored to the demand and irreversibilities are minimized to make cold/heat generation valorisation highly efficient and cost-effective. This study focuses on the technology energetic efficiency for commercial refrigeration producing negative (-20°C) and positive (0°C) cold. Based on numerical analysis and validated models using numerical tools (EES, Python) and thermodynamic data bases (REFPRP), simulations results compare standard CO₂ transcritical cycles (STC) to the hydro-CO₂ piston in two different case studies. The results show an increase in COP between 38 and 112% showing the energy efficiency and environmental impact decrease potential of the technology.

In this paper, a numerical study of the radiation flux and temperature distribution of a parabolic dish receiver is carried out. The SOLTRACE code is used to predict the radiation flux distribution and the FLUENT software is used to study the temperature distribution. In a first step, the concentrated solar heat flux distribution is calculated using the SOLTRACE software The calculated heat flux distribution obtained is then used as wall heat flux boundary conditions for the receiver. In a second step of this study, the receiver’s thermal performance is optimized by a simulation using the commercial CFD code FLUENT. Different receiver configurations are studied in order to define the optimal configuration to obtain the best performance. The first is a cylindrical receiver with a tangential inlet located at the bottom and a normal concentric outlet located at its upper surface and the second is a spiral absorber of 19 turns with an inlet across its periphery and an outlet across the central spiral. The effect of the different inlet-outlet positions of the working fluid is carried out. Two positions are studied: the first position is characterized by an inlet through the periphery and an outlet through the central spiral and the second by an inlet through the central spiral and an outlet through the periphery. The thermal analysis has proven that the 19-turn spiral tube with an inlet through the periphery and an outlet through the central spiral is the best configuration that improves the efficiency of the solar receiver and the overall performance of the solar dish.

The thermochemical storage based on the solid-gas sorption process is used in a temperature-controlled transport container. The principle is based on the coupling of a liquid-to-gas phase change of a natural fluid (ammonia) and a reversible reaction between this gas and a reactive solid. It allows producing a controlled refrigeration effect, offering a stand-alone solution (container) for transport maintaining the cold chain requirements. This article presents the principle of the liquid/gas sorption and the results of the thermal performance tests carried out on standalone isothermal containers for different temperature ranges. This solution allows to transport heat-sensitive products between +2 °C and +8 °C or between 0 °C and +4 °C or below -18 °C. Depending on the outdoor temperature profile, the required temperature, the door openings and the thermal inertia of the products, the autonomy of the container varies from 8 to 48 hours.

To investigate the performance characteristics of a Double Evaporator Ejector Refrigeration System, a mathematical model based on one-dimensional ejector theory and including the friction effect of the mixing chamber, is established and simulated by the software EES. For refrigeration and freezing temperatures compatible with household refrigeration applications, the resulting code is used to investigate the influence of suction chamber pressure drop ΔP, and the nature of the refrigerant on the performance characteristics of DEERS equipped with a liquid-vapor separator and using environmentally friendly fluids. The simulation results show that the pressure drop, ΔP, has an important influence on the performance of DEERS. For all the fluids tested, apart from the ejector entrainment ratio, U, all the other performance characteristics of the system decrease as ΔP increases. According to these results, it is R717 which leads to the best performance (COP_imp =29.2 %, CR_red =27.9 %, and VCC_imp =38.6 %). The results also show that COP_imp decreases when the condensing temperature, T_C increases and reaches a minimum. However, VCC_imp and CR_red are not affected by the change in T_C. In addition, COP_imp, VCC_imp and CR_red increase when the refrigeration temperature, T_ev1, increases and the freezing temperature, T_ev2, decreases.