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The numerical simulation, design, and construction of a double glazed integrated collector storage (ICS) with compound parabolic concentrators (CPC) reflectors are presented. The design of the double glazing is optimized by means of numerical simulations under Tunisian climatic conditions. The numerical program used for the simulation consists on the transient resolution of the balance energy equations of each component of the solar water heater. Different proposed models of a double glazing are studied and compared with a reference case of a single glass ICS model. The simulation results have shown that the use of double glazing with argon layer and low-e coated lower glass instead of one glass cover significantly improves the thermal behavior of the ICS heater and visibly reduces its thermal losses during night. Taking into account these numerical results, a double-glazing ICS prototype-collector is constructed. The details of the construction, geometry, optical properties, and dimensions of the prototype are presented. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical study of a steam ejector using CFD with the objective of identifying suitable experimental conditions that allows for a reliable ejector cycle operation. The flow structure and the mixing process inside the ejector were assessed for a range of operating conditions typical for a solar driven air conditioning system. The effect of varying the condenser pressure was studied. It was found that for relatively low condenser pressures (<2.5 kPa), two distinct shock waves occurred in the diffuser section resulting in a reduced re-compression efficiency, although it did not have a direct impact on the entrainment ratio. The effect of generator pressure was also studied. It was found that the entrainment ratio showed a maximum for constant condenser conditions. A near optimal condition of 70 kPa generator and 3 kPa condenser pressures were identified and tested in a preliminary experiment carried out with the solar driven ejector system installed in Tunis. [Copyright &y& Elsevier]

Abstract: The aim of this work is to develop a numerical code able to predict the thermal behavior of a double tank integrated collector storage system (ICS) with compound parabolic concentrator (CPC). The developed numerical model is based on the detailed analysis of the different forms of heat transfers occurring in the ICS system. The balance equations of each element of the system have been established and solved by means of a transient algorithm. A prototype of an ICS device was constructed and experimentally tested outdoors in order to observe the variation of water temperature in the storage tanks. The experimental results are presented and the validity of the model is examined by comparison of the theoretical results with experiments which demonstrates a good agreement. The numerical model is then used to perform theoretical study on the present ICS solar heater. The simulation results of the variation of the thermal efficiency are presented. The results of the yearly parametric study of the effect of the concentrators reflectivity, the absorber emissivity and the use of double glazing on the thermal performance of the ICS system are also presented and discussed. The developed numerical tool within this work can be considered as important for the study of double tanked ICS solar water heater regarding its transient thermal behavior. [Copyright &y& Elsevier]

Abstract: The glass curtain walls have been recently introduced in Tunisia; they are seen as a new fashion and are highly appreciated by some for their pleasing aesthetics. The objective of this paper is to investigate whether the glass curtain walls are appropriate for the Tunisian local climate and context and if it is so, to give recommendations concerning the kind of glass to be used. A TRNSYS [Klein SA. TRNSYS a transient system simulation Program V5 14.2. Solar Energy Laboratory, University of Wisconsin Madison, July 1996] simulation was conducted on a typical administrative building. The investigation concerns only the building heating and cooling load. The building was split in five thermal zones; for each thermal zone, all the windows have the same orientation. The single zone model TYPE19 of TRNSYS [Klein, 1996] was used to model each thermal zone. An additional convection heat transfer between the different thermal zones of the building was modelled according to the Brown and Solvason law [Brown WG, Solvason KR. Natural convection through rectangular openings in partitions, Part 1: vertical partitions. Int. J. Heat Mass Transfer 1962; 5: 859–68]. This particular law was used because it has been validated in the Tunisian context by Bouden . We assume that the glass curtain wall will be implemented only on the main building facade; this is why it was simulated with different glazing sizes and glass types. The other facades remain unchanged. The results of this simulation have shown that, in relation to space heating, the glass curtain wall can be very interesting in the Tunisian context if the orientation as well as the kind of glazing are carefully selected. [Copyright &y& Elsevier]

The development of a dynamic model using the TRaNsient System Simulation program (TRNSYS) for the performance assessment of a solar-driven air conditioning system with integrated PCM cold storage is presented. The simulations were carried out for satisfying the cooling needs of a 140 m 3 space during the summer season in Tunis, Tunisia. The model is composed of four main subsystems including: solar loop, ejector cycle, PCM cold storage and air conditioned space. The effect of varying the solar collector area ( A sc ) and the hot storage capacity (V hs ) on the solar fraction are investigated. It was found that the application of a relatively small hot storage tank (700 l) led to the highest solar fraction (92%). A collector area about 80 m 2 is needed to assure a solar fraction of 70%. Increasing A sc beyond this value has only a small effect on the overall system efficiency. The influence of applying cold storage is also investigated. The results without cold storage indicated that the comfort temperature was exceeded in more than 26% of the time. With cold storage the periods of high indoor temperatures reduced significantly. An optimal storage volume of 1000 l was identified resulting in the highest cooling COP and excellent indoor comfort (95% of the time with a room temperature below 26 °C). The maximum COP and solar thermal ratio (STR) were 0.193 and 0.097, respectively. [ABSTRACT FROM AUTHOR]

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy. [ABSTRACT FROM AUTHOR]

In the present paper, the performance of a microencapsulated phase change material (in 45% w/w concentration) for low temperature thermal energy storage, suitable for air conditioning applications is studied. The results are compared to a sensible heat storage unit using water. Thermo-physical properties such as the specific heat, enthalpy variation, thermal conductivity and density are also experimentally determined. The non-Newtonian shear-thickening behavior of the phase change material slurry is quantified. Thermal energy performance is experimentally determined for a 100 l horizontal tank. The heat transfer between the heat transfer fluid and the phase change material was provided by a tube-bundle heat exchanger inside the tank. The results show that the amount of energy stored using the phase change material is 53% higher than for water after 10 h of charging, for the same storage tank volume. It was found that the heat transfer coefficient between the phase change material and the tube wall increases during the phase change temperature range, however it remains smaller than the values obtained for water. [ABSTRACT FROM AUTHOR]

• The numerical model of Parabolic Trough Collectors for Direct Steam Generation is presented. • The model is based on a step by step finite volume method. • The validation of the numerical results with experiments has shown good agreements. • A sensitivity analysis is performed and optimal working conditions are derived. Direct Steam Generation (DSG) has attracted considerable attention in comparison with conventional technologies based on Heat Transfer Fluids (HTF) for electricity generation due to its higher temperatures. In this context, a detailed numerical model has been developed, not only for comparison and assessment purposes but also for optimization analysis, in order to obtain an accurate comprehensive analysis of this type of systems. This paper explains in detail a numerical model developed to accurately predict the phase change and the heat transfer in the DSG Parabolic Trough Collector (PTC) system. The whole numerical model has been validated using experimental data of Direct Solar Steam (DISS) test facility from scientific literature. The model results have been compared also to the measurements of an experimental DSG PTC setup installed at Ecole Nationale d'Ingénieurs de Tunis (ENIT) running under different working and climate conditions. In all cases, both numerical results and experimental data present a reasonable agreement. The numerical code is used to perform a parametric study pointing out the influence of inlet conditions on the performance of DSG PTCs. Emphasis is placed on the determination of the optimal mass flow rate range leading to a maximum thermal efficiency of the solar collectors. Annual thermal performance of the solar collectors is featured including useful energy and thermal efficiency evaluated under Tunisian climatic conditions. [ABSTRACT FROM AUTHOR]

The electricity mix in Tunisia mainly relied on conventional energy sources for over 50 years. Recently, due to fossil fuel prices oscillations and national reserves shortage, the need arose for restructuring the energy supply system. Targeting the integration of renewable energies could be a plan for satisfying the increasing demand and the supply independence. However, several macroeconomic conditions and policies present barriers for the integration of Renewable Energy Sources (RES), despite their abundance, availability and environmental benefits. This paper presents a long-term model of Tunisia electricity system, based on OSeMOSYS (Open Source energy MOdelling SYStem), aimed at unveiling potential benefits of increasing RES in electricity production. The paper first investigates peculiarities of Tunisia electricity system, arguing the necessity to include them in the electricity system model. Then, it explains the choice of OSeMOSYS and brought modifications, including peculiar system characteristics. Finally, the model is applied to two scenarios, a Business As Usual case and a 30% RES target in electricity production case, for time horizon 2010–2030. Results demonstrate the importance of system features detailed modelling. Specifically, they show that targeting RES state-invested integration in the electricity mix may allow higher energy independence to be reached, without increasing significantly system costs. [ABSTRACT FROM AUTHOR]

Due to the economic development and occupancy requests, building thermal comfort reached higher levels during the last years. Energy consumption rates have become excessive and engendered an increasing reliance on fossil-fuel reserves. Hence, the conception of energy-efficient buildings as well as applying solar cooling techniques has become a promising solution. In this context, the current work dealt with the appraisal of a solar system that drives the cooling process in an office building located in the Center of Researches and Energy Technologies in Tunisia. The solar system consisting of linear parabolic trough solar collectors' field coupled to a 16 kW double effect Lithium Bromide absorption chiller, supplies chilled water to a set of fan coils installed in the 126 m 2 laboratory building. A dynamic model that couples the solar cooling system with the building was developed using the TRNSYS tool and several simulations were performed to assess the case study and improve its performance. The model results were compared to the data collected during the experimental campaign conducted in summer 2015 and showed that the collectors efficiency was at the range of 26–35%, the COP ranged between 0.65 and 1.29, the daily maximum solar COP was approximately at 35%. However, the solar system was unable to cover 32.3% of the cooling requirements, the absorption chiller was switched on only during 53.8% of its total operating time. An improved system configuration was then studied; the integration of an auxiliary heater prior to the chiller as well as the increase of the aperture area guaranteed high driving temperatures and more suitable conditions to the absorption chiller. As a result, the chiller operating time increased to 75.8%, the cooling power increased by 75.6%, the solar COP reaches 57% and the solar fraction averaged 87%. The summer season performances predict that the improved system configuration achieves primary energy savings that reach 82.3% compared to a classic air conditioning system producing the same cooling power, the yearly avoided CO 2 emissions are estimated to 2947 kg. [ABSTRACT FROM AUTHOR]

Highlights: [•] Numerical simulation model of honeycomb TIM is presented. [•] The coupled conduction and radiation problem is solved in three dimensional coordinates. [•] The validation with experimental results shows acceptable agreements. [•] A parametric study is performed. [ABSTRACT FROM AUTHOR]