Search

Phase change materials (PCM) are promising technology to store thermal energy at a constant temperature. A large amount of energy can be stored or released in latent heat form during the transition of material from one phase to another. Despite the great benefits, most PCMs have their own limitations i.e., low phase change enthalpy, poor specific heat and thermal conductivity, supercooling, volume change, phase segregation, etc. Consequently, efficient thermal energy storage requires improving the thermophysical properties of PCMs. The present study is a comprehensive review of existing techniques for PCMs thermophysical properties enhancement. The research progresses on adding zero, one, two, and three-dimensionally structured additives to PCM is assessed to improve the thermal transport by enhancing the PCM effective thermal conductivity. The enhancement of latent heat of fusion and specific heat using various additives is also discussed. Further, the latest techniques on supercooling and phase segregation reduction are also presented. Last, the modelling of the novel composite materials formed by combining a PCM with other materials is presented. Despite the fact that the majority of these methods are still in the research and development stage, some of them have the potential to be commercialized in the near future. Reliable and efficient PCMs are exceptionally useful for storing solar energy and industrial waste heat, especially for constant temperature applications.

International audience

The application of an innovative translucent superinsulated latent heat storage wall, combining transparent insulation material and phase change materials (TIM-PCM wall), on the envelope of a typical office under different climates is evaluated. Energy and economic analysis related to this application are presented. The simulation process is carried out using an experimentally validated numerical model. The results show that the incorporation of the TIM-PCM wall, on the south orientation, is more efficient than the use of a double-glazed in all considered climates. The optimum TIM-PCM wall area is evaluated economically through life-cycle cost and payback period analysis. The purpose is to ensure effective performance of the wall in each climate and at the same time to ensure economic viability. The results show that, in polar and subarctic climates, the application of the TIM-PCM wall has a high economic value and the investment appears to be attractive, the payback period being 10.5 years and 7.8 years respectively. In Dras (continental climate), the use of the wall is found economically unfeasible due to low energy prices and high discount rates. At current prices, the TIM-PCM wall investment in Sacramento (Mediterranean climate) and Toronto (Humid continental) does not offer economic benefits.

Worldwide, the residential buildings are consuming a considerable amount of energy. The high potential of buildings towards energy efficiency has drawn special attention to the passive design parameters. A comprehensive study on optimal passive design for residential buildings is presented in this paper. Twenty-five different climates are simulated with the aim to produce best practices to reduce building energy demands (for cooling and heating) in addition to the life-cycle cost (LCC). The occupants' adaptive thermal comfort is also improved by implementing the appropriate passive cooling strategies such as blinds and natural ventilation. In this respect, the implemented methodology is composed of four phases: building energy simulation, optimization, Multi-criteria Decision Making (MCDM), sensitivity study, and finally an adaptive comfort analysis. An optimal passive solution of the studied building indicates the potential to save up to 54%, 87% and 52% of the cooling demands (Qcool), heating demands (Qheat) and LCC respectively with respect to the initial configuration. The obtained optimal passive parameters are validated with the National Renewable Energy Laboratory NREL benchmark for low energy building's envelope. Additionally, the integrated passive cooling strategies have demonstrated its competency since it leads to a significant overheating decrease.

Net-zero energy buildings (NZEBs) have been considered as an efficient solution to limit the growing energy consumption and pollution emissions from buildings. The configurations and the capacities of the implemented renewable energy systems in NZEBs should be wisely selected to ensure the intended performance objective. This study aims to optimize, investigate and compare six renewable energy solution sets for designing NZEBs in three different climates: Indore (cooling dominant), Tromso (heating dominant), and Beijing (mixed climate). The optimization is carried out using a multi-criteria decision-making methodology. The implemented methodology is composed of two phases. In the first phase, the optimal sizes of solution sets in each climate are derived and analyzed. The effectiveness of optimal solution sets is evaluated with respect to economy, environment, energy and grid stress. In the second phase, recommendations for each region are offered according to the overall performance evaluation results. The evaluation criteria include life cycle cost, payback period, levelized cost of energy, CO2eq emissions, grid interaction index, load matching index, and total energy consumption. The analyses show that, in Indore (hot climate), it is recommended to utilize the solution set composed of air source heat pump for cooling and flat plate solar collectors for domestic hot water (DHW) production. In Tromso (cold climate), the use of a biodiesel generator is promising to produce both electricity and hot steam for heating as well as DHW use. In Beijing (mixed climate), it is recommended to utilize electric chillers for cooling and natural gas condensing boiler for heating and DHW usage.

International audience; Integrating heat exchanger pipes with structural foundations in one system has created a new renewable solution for buildings’ thermal loads. However, the interaction between thermal and geotechnical loads makes their design more complex and challenging. This review-study represents the current state of knowledge about the thermal and thermo-mechanical behaviors of energy piles. It also investigates the key parameters that affect their design concerning the piles’ dimensions, the arrangement of pipes, concrete admixture, and fluid characteristics. It is found that the thermal efficiency improves significantly by increasing the number of pipes inside the piles and by adding thermally conductive materials to the concrete within acceptable limits. Besides, this paper reviews most of the studies conducted on optimizing vertical ground heat exchangers coupled with heat pumps. Objective functions, decision variables, design constraints, and optimization methods are specified and listed. It is concluded that a multi-objective optimization is highly recommended to enhance the dual performance of an energy pile system coupled with a heat pump using the 4E evaluation criteria (energy, exergy, economy, and environment) while ensuring the safety of the foundation under thermal cyclic loads.

International audience; Cooling demand in the building sector is growing rapidly; thermal energy storage systems using phase change materials (PCM) can be a very useful way to improve the building thermal performance. The right use of PCM in the envelope can minimize peak cooling loads, allow the use of smaller HVAC technical equipment for cooling, and has the capability to keep the indoor temperature within the comfort range due to smaller indoor temperature fluctuations. This article presents an overview of different PCM applications in buildings for reducing cooling loads under different climate conditions, and the factors affecting the successful and the effective use of the PCM. Many drawbacks have been found in PCM applications, mainly the intense impact of summer weather conditions over the PCM performance, which prohibits its complete solidification during night, and thus, limiting its effectiveness during the day. Proposed solutions are reviewed in this article. Finally, a topology diagram is presented to summarize the steps leading to an effective use of PCM in building applications.

International audience; This paper investigates the thermal performance of a translucent solar wall providing, concurrently, storage and restitution of heat, super thermal-acoustic insulation and daylighting to the interior environment. The wall is composed of glazing, silica aerogel used as a transparent insulation material (TIM) and glass bricks filled with fatty acid, an eutectic phase change material (PCM). To assess the TIM–PCM wall thermal behavior, experimentations were conducted in-situ in a full-sized test cell located in Sophia Antipolis, southern France. Experimental data shows that the tested wall is more effective in winter and might cause overheating during the summer mainly due to solar gains and un-cycling behavior of PCM which remains in liquid state. To enhance the energy performance of the wall in summertime, a numerical model describing the heat transfer mechanisms occurring in the PCM layer in combination with the other transparent wall layers is developed. Then, the model of the wall is linked to TRNSYS software to assess the thermal performance of the whole building. The numerical model is validated experimentally and a good agreement is shown comparing the simulated values with the measured data for seven consecutive days in summer and winter. The importance of considering the natural convection effect in the liquid PCM is also demonstrated. Moreover, it was shown that shading devices can effectively reduce overheating while natural night ventilation decreases the indoor temperature without affecting the PCM performance since the outdoor temperature is always higher than the phase change temperature. The use of a glass with selective solar reflection properties depending on the season instead of the ordinary glazing is shown also to be very effective way to overcome the overheating problem. Finally, the TIM-PCM wall is tested under different climate conditions and passive solutions are given to ensure thermal comfort in summer season.

International audience; Electrical energy demand is continuously increasing due to the global population growth and improved living standards. This proposes a growth in the potential for the use of tri-generation systems in residential sector, due to their ability to produce both useful thermal energy and electrical power from a single source. In Lebanon, the electrical capacity shortage has created an informal back-up self-generation systems, mostly are diesel power generators, estimated to represent about 30% of all electricity generated. This paper presents a simulation study, using TRNSYS software, of tri-generation systems working on diesel engines to supply the different residential energy demands. The recovered percentage of the exhaust gas wasted energy is determined and an optimizing study is carried out to find the optimum storage tanks sizes of the recovered thermal energy. Results show that tri-generation systems are found to make diesel power generators more energy-efficient and thus reduce greenhouse gases emissions.

International audience; In this article, a simplified model for melting of a phase change material (PCM) in presence of natural convection and radiation is presented. A modified enthalpy method is adopted to solve the phase change problem, the natural convection occurring in the liquid PCM is accounted for using the enhanced thermal conductivity approach coupled with the scaling theory, and the absorbed shortwave radiation flux is added into the energy equation as a source term using a simplified solution algorithm. Two dimensional implicit finite volume method is used to solve the energy equation. First, the simplified model for melting with natural convection is validated using a CFD model, in addition to experimental and numerical benchmark solutions for a test case. Then, the simplified model for melting with combined natural convection and radiation is applied to the melting of a fatty acid eutectic filled in glass bricks, which will be used later to model the annual thermal behavior of a special translucent façade. This complete model is validated against the lattice Boltzmann-discrete ordinate method LBM-DOM. It was shown that (1) the proposed simplified model is simple to implement and its simulations run significantly faster than those of CFD models and LBM-DOM model. Consequently, it can be easily integrated into an energy simulation tool for yearly performance evaluation, (2) during PCM melting process, natural convection has a noteworthy role as it enhances the average fraction of liquid and the position of the melting front, (3) shortwave radiation enhances the average liquid fraction.

International audience; The conception of net zero energy buildings (NZEB) has been introduced to limit energy consumption and pollution emissions in buildings. Classification of NZEB is based on renewable energy (RE) supply options, energy measurement process, RE-sources location, and balances whether are energetic or exergetic. In general, it is traditionally agreed that there are three main steps to reach the NZEB performance, starting through the use of passive strategies, energy efficient technologies, and then RE generation systems. Then, these three steps could be accompanied with the smart integration of advanced efficient energy technologies. A state of the art shows that the main ZEB studies are related to: energy savings, reduce electric bills, energy independence, pollution reduction, and occupants comfort, in addition, others are more interested in the aesthetic aspect by combining modern technologies with innovations to achieve high energy and sustainability performance. Building optimization is a promising technique to evaluate NZEB design choices; it has been adopted to choose the perfect solution to reach the zero energy performance through the optimization of an objective function related to energy (thermal loads, RE generation, energy savings) and/or environment (CO2 emissions) and/or economy (life-cycle cost (LCC), net-present value (NPV), investment cost). This paper starts by presenting the global energetic and pollution challenges the world faces. Moreover, it shows, to the best to the author’s knowledge, the existing NZEB definitions and the corresponding case studies investigated in 8 different climatic zones (humid continental, humid subtropical, Mediterranean, moderate continental, moderate continental, marine west coast, tropical, semi-arid and hot), the paper also focus on the importance to treat each climate separately. Even in the same country, two or more climates may co-exist. NZEBs drawbacks are also presented. Furthermore, different optimization problems are reviewed in the last section. Building energy optimization methods are employed to obtain the ideal solution for specific objective functions which are either related to energy, and/or environment and/or economy. Optimization variables are distributed between passive and/or RE generation systems. Finally, a table summarizing the most commonly used electric and thermal RE applications which yield to the zero energy balance in each climate, as well as three flowcharts are presented to summarize the whole three-stage procedure, to reach NZEB, starting from building designing, passing through the optimization procedure, and lastly categorizing the zero energy balance.

International audience; Enhancing efficiency, meeting environmental standards, and preserving fuel resources are highly important objectives in power generation systems, at a time where world is endangered by several energetic, environmental, and health crises. Thus, better designs are constantly sought to increase the performance of such systems. In this study, an environment friendly trigeneration system based on solid oxide fuel cell (SOFC) is selected and designed for domestic applications. Negligible emissions are thus recorded as the SOFC is fueled solely by hydrogen. The system is modeled following three steps: the energy simulation of residential building to determine its demands, the system’s prime mover–SOFC, and the trigeneration recovery system ensuring the maximum coverage of heating, cooling, and domestic hot water loads respectively. The system is then evaluated under the 4-E assessment criteria: energy, exergy, economy, and environment. Depending on these criteria, the system is multi-objectively optimized. Two operation strategies are adopted: off-grid following electrical load and on-grid base load operations. Optimization results show that the trigeneration system is energetically and economically superior and performs well under both strategies. The maximum energy and exergy efficiencies (65.2% and 45.77%) and minimum system cost rate (22.2 cents/kWh) are obtained under on-grid base load operation.

International audience; The challenge in Zero energy building (ZEB) design is to find the best combination of design strategies that would face the energy performance problems of a particular building. This paper outlines the methodology and the cost-effectiveness potential for optimizing the design of net-zero energy building (NZEB) in a cold climate region in Lebanon; Cedars. Specifically, the non-sorting genetic algorithm (NSGA-II) is chosen in order to minimize thermal, electrical demands and life cycle cost (LCC) while reaching the net zero energy balance; and thus getting the Pareto-front. A ranking decision making technique (ELECTRE III) is applied to the Pareto-front so as to obtain one optimal solution. A wide range of energy efficiency measures are investigated, besides solar energy systems are employed to produce required electricity and hot water for domestic purposes. The results clearly indicate that, for designing a residential NZEB in cold climate, it is essential to minimize the space thermal load through a building envelope with high thermal performance. Envelop high level of insulation is an essential step to decrease the high heating demand. Building thermal loads are decreased by 33.19%. Moreover the LCC is decreased by 31.09%.

International audience; Energy demands and fuel costs are continuously increasing which necessitates either finding new energy resources or improving current energy systems. Multi-generation systems as cogeneration (CHP) and trigeneration (CCHP) are interesting solutions that can enhance energy generation performance and fix some interrelated reliability, safety, and flexibility issues. In this regard, many prime mover technologies are available in which the choice between each is greatly dependent on end-user conditions and preferences. Yet, it is prior to choose whether cogeneration or trigeneration is more suitable. Thus, this paper reviews the main differences between CHP and CCHP systems in most available prime mover technologies, after which a selection table is proposed in order to make appropriate multi-generation system installation choices, depending on specific case study parameters. In general, CHP and CCHP systems yield positive technical and environmental performance impacts.

International audience; This paper presents a review of the energy status, conventional and renewable, in Lebanon and illustrates their problems with the suggested recommendations. In addition, a detailed review of the principal directorates of the electricity of Lebanon Company (EDL) is also presented all along with the existing problems and recommended solutions. An economic, environmental optimization of different power sources is studied, where three scenarios are introduced based on the fuel source of different CCGT power plants. The results emphasized on the maximum possible use of wind energy and natural gas in electricity generation. Based on the optimization study, a five-year master plan for electricity generation is modeled. The suggested plan has an investment capital cost of 5553 M$ with the savings and additional incomes being 5900 M$ compared to EDL financial status in the adopted base year, 2009. In addition, this plan exceeds the trend to introduce a 12% share of renewable energy in the power sector by 2020, where the share is supposed to be about 15% out of the total installed capacity before this date. The concluding remarks highlight the role of politics in the development of the energy sector.

International audience; A simple model for a detached house in Beirut is optimized using a building energy simulation program to minimize two objective functions: the annual thermal demand; and the difference between the total electrical load and the on-site electricity generated from the renewable energy system. The optimization variables are: insulation thickness of external walls and roof, thickness of roof reinforced concrete, size of windows on each façade, number of solar collectors of the domestic hot water system (SDHW), SDHW circulating pump flow rate, and number of panels of the Photovoltaic system (PV). The main optimization constraint adopted is the thermal comfort criterion. Results of the optimization shows that the exclusion of cost parameter may lead to unfeasible solutions even if the net zero balance is theoretically achieved.

International audience; Electricity, heating, and cooling are the three main components constituting the tripod of energy consumption in residential, commercial, and public buildings all around the world. Their separate generation causes higher fuel consumption, at a time where energy demands and fuel costs are continuously rising. Combined cooling, heating, and power (CCHP) or trigeneration could be a solution for such challenge yielding an efficient, reliable, flexible, competitive, and less pollutant alternative. A variety of trigeneration technologies are available and their proper choice is influenced by the employed energy system conditions and preferences. In this paper, different types of trigeneration systems are classified according to the prime mover, size and energy sequence usage. A leveled selection procedure is subsequently listed in the consecutive sections. The first level contains the applied prime mover technologies which are considered to be the heart of any CCHP system. The second level comprises the heat recovery equipment (heating and cooling) of which suitable selection should be compatible with the used prime mover. The third level includes the thermal energy storage system and heat transfer fluid to be employed. For each section of the paper, a survey of conducted studies with CHP/CCHP implementation is presented. A comprehensive table of evaluation criteria for such systems based on energy, exergy, economy, and environment measures is performed, along with a survey of the methods used in their design, optimization, and decision-making. Moreover, a classification diagram of the main CHP/CCHP system components is summarized. A general selection approach of the appropriate CCHP system according to specific needs is finally suggested. In almost all reviewed works, CCHP systems are found to have positive technical and performance impacts.

International audience; This paper presents the dynamic modeling of a domestic hybrid water heating system. The system is composed of a solar collector, a heat pump water heater, a wind turbine, a battery and a hot water storage tank. Both air-source and geothermal-heat pumps are investigated. Detailed mathematical dynamic models of the individual components are presented and validated. Simulations for typical days in summer and winter for Beirut and Cedars, two Lebanese locations with different meteorological and demographic conditions, are conducted using Matlab software. A renewable coverage factor (RCF) is defined, representing the renewable energy share with respect to the total delivered energy. Results reveal that the proposed hybrid system is capable of securing all hot water needs in all case studies and RCF is always above 63%. Furthermore, an energetic, economic optimal management model is developed for the proposed hybrid system. It is applied to the considered case studies, where results illustrate the optimum size of each of the system components as well as the optimum energy-flow distributions among them over two investigated time periods, five and ten years. It is noticed that the initial cost of the hybrid system is acceptable and important annual savings are obtained.

International audience; Water heating contributes an important proportion of residential energy consumption all around the world. Different kinds of domestic hot-water production systems exist. The operational cost, environmental effect and performance of these systems differ according to various energy sources, climates, system types and system designs. Hence, the proper choice of a domestic hot-water system could save energy, protect nature and reduce operational costs, significantly. This paper illustrates, to the best of the author's knowledge, the existing water-heating systems all along with the principle, advantages, disadvantages and state-of-the-art for each. Six different categories were presented, namely wood, oil/gas, electric, heat pump, solar and instantaneous systems. The heat-pump systems were further classified into several groups, namely air source, ground source, solar assisted, ground source-solar assisted, photovoltaic–thermal and gas-engine driven systems. In addition, concerning solar water heating, different types of systems and collectors were presented and reviewed. Principal conclusions from the review are outlined and a general approach to recommend the appropriate water-heating system is proposed.

International audience; This paper presents a dynamic simulation model to predict the performance of an ASHPWH (air source heat pump water heater). The developed model is used to assess its performance in the Lebanese context. It is shown that for the four Lebanese climatic zones, the expected monthly values of the average COP (coefficient of performance) varies from 2.9 to 5, leading to high efficiencies compared with conventional electric water heaters. The energy savings and GHG (greenhouse gas) emissions reduction are investigated for each zone. Furthermore, it is recommended to use the ASHPWH during the period of highest daily ambient temperatures (noon or afternoon), assuming that the electricity tariff and hot water loads are constant. In addition, an optimal management model for the ASHPWH is developed and applied for a typical winter day of Beirut. Moreover, the developed dynamic model of ASHPWH is used to compare the performance of three similar systems that differ only with the condenser geometry, where results show that using mini-condenser geometries increase the COP (coefficient of performance) and consequently, more energy is saved as well as more GHG emissions are reduced. In addition, the condenser “surface compactness” is increased giving rise to an efficient compact heat exchanger.

International audience; This paper presents a brief review of the conventional and renewable energy statuses, in Lebanon, as well as of the principal problems facing the electricity of Lebanon Company (EDL). In addition, an analytical-critical review of the latest three official Lebanese electricity plans is presented. Furthermore, two future electricity generation plan-scenarios for Lebanon are investigated, where multi variables are examined, namely: cost, environment and tariff. First, an economical-environmental optimization is carried out, where two reasonable scenarios are introduced based on the fuel source of CCGT power plants. Results revealed that the investment in wind energy and natural gas for power production should be a main concern in the future. Second, an optimization of the average electricity tariff for each of the two investigated generation scenarios is studied, where three different approaches are illustrated. Results confirm that all tariff plans are acceptable and convincing, and that the use of natural gas in CCGT plants is always preferred against gasoil. Moreover, the optimized average tariff is maximally a double of the current EDL average tariff, which is very convincing compared with the actual cost of electricity for the current Lebanese electricity sector.