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Thermal oils are omnipresent in processes using heat at high temperature as heat transfer fluids (HTFs) as well as thermal storage materials (TESMs), particularly in concentrating solar plants. In ...

Absorption systems have considerable potential to fight global warming and help transition to renewable energies. These machines use multifunctional exchangers (heat and mass transfer), which have been pointed out as the “bottleneck” in the performance of such systems, especially the absorber. The present review offers an up-to-date overview of the different exchanger technologies that have been studied and tested for small-capacity single-stage continuous absorption chillers operating on binary working fluids. The review focuses more particularly on the experimental studies of innovative exchanger technologies with conventional or special geometries, with or without mechanical treatments, tested on the three most experimented working pairs (NH3–H2O, H2O–LiBr, and NH3–LiNO3). The use of chemical treatments is not covered. The main objective is to provide information to improve our understanding of how the exchanger technologies used in the mentioned systems have evolved over the past 40 years as well as the advantages and disadvantages for their different uses.

Synthetic and mineral oils are widely used in high temperature processes (above 200°C) as heat transfer fluids or thermal storage materials. Mineral oils are essentially derived from petroleum that...

Thermodynamic and exergoeconomic assessment of a combined cooling and power cycle  Study based on an existing ammonia water experimental prototype  Scale-up of the considered plant predicted to reach exergy efficiency around 24%  Unit cost of produced exergy of 28.

International audience; Ejector refrigeration cycles offer an alternative to traditional systems for the production of cooling using low temperature heat. In this paper, a real gas thermodynamic model based on the mass flow rate maximisation is presented. This model has the advantage of simplifying the calculation algorithm and avoiding a complex description of the double choking mechanism taking place within the ejector. First, the model hypothesis and calculation algorithm are presented. The impact of each efficiency is evaluated and a tuning procedure is developed to calibrate the model on experimental data. Validation is performed on multiple datasets relative to two different fluids: R600a and R134a. The ejector model is then used to simulate a SERS (single ejector refrigeration system) cycle, to validate its robustness and capability to be used in the prediction of thermodynamic cycles performance. A comparative analysis of different fluids is carried out on the SERS, highlighting the important role played by the choice of the superheating. Finally, the model is used to predict performance in the case of a two-phase primary flow pointing out the limits of the model and the need of further experimental studies for the inclusion of appropriate semi-empirical corrections.

International audience

The use of biogas has been proven to be an interesting alternative energy source. However, biogas usually contains trace compounds, which can lead to damage in the valorization installation. Conseq...

Absorption chillers require precise models for their control and optimization. The present work analyzes the behavior of a 10-kW absorption chiller by means of four different methods that use the external fluid temperatures to situate the machine operating conditions, either through a characteristic temperature difference or through the COPcarnot. The studied models are of different physical insight levels: a semi-empirical method (the characteristic equation method), two empirical methods (the adapted characteristic equation method and the Carnot function model), and a thermodynamic model, here called the “effectiveness model,” based on thermal and mass effectivenesses. The COPcarnot is used for the first time as a common parameter to situate the results from the different models and the effectivenessses of the components. Different from previous models presented in the literature, the proposed effectiveness model is able to account for the non-equilibrium conditions of the solution with the vapor phase at the inlet and outlet of the sorption exchangers. Therefore, owing to a better understanding of the internal machine behavior, it allowed for the identification of thermal and electric COP improvement opportunities.

International audience; The present work is focused on the investigation of an absorption cycle integrated with an impulse axial turbine for the combined production of cooling and electric power. This technology holds great promise for its ability to harness low-temperature heat sources, more effciently in comparison to separate production with simple cycles. By developing a 1D model of the expander, and integrating it into a 0D model of the complete cycle, it is possible to evaluate the performance of the cycle and its variation with respect to the operating parameters, namely the temperature of the external resources. Pending an experimental validation of the results, this study showed the importance of correctly defining the temperature of the sources - namely the generator temperature - in order to satisfy the technological needs while also maximising the effciency of the cycle. Finally it was highlighted how the integration of a supersonic impulse turbine strongly limits the flexibility during operation given the constant mass flow rate treated by the expander.

An innovative low-energy, low-tech, and low-cost cooling system for buildings is presented. This cooling system simultaneously makes use of three available heat sinks: the ground, evaporation of water, and radiation to the sky. A terra-cotta tank is placed along a northern wall of the building to achieve the last two phenomena. This simple, inexpensive, and energy-saving system was simulated on a 100-m2 house in Bordeaux climatic conditions. The cooling capacity of the system varies between 4500 and 1000 W depending on the period. The average COP is 24 in these climatic conditions, which is approximately five times greater than a conventional commercial air conditioner. Further research remains necessary to analyze long-term behavior and performance of this system.

This article aims to study the opportunity to integrate absorption heat pumps (AHP) in substations of district heating networks (DHN). Three different operating modes of the AHP are proposed to match the varying building needs throughout the year from the heat provided by the DHN. The upgrade mode allows to increase locally the temperature of the DHN to cover high temperature needs with low temperature DHN. The cooling mode allows to cool down buildings from DHN heat in summer. And the heating mode allows to improve the heat transfer from the DHN to the building. To compare the performance reached with several absorbent/refrigerant couples, a steady state model was developed to simulate the AHP in DHN substations in typical operating conditions for the three operating modes considered. The results of the simulations confirm the thermodynamic opportunity to use AHP in DHN substations and the opportunity to switch between the operating modes during the day or the year to improve the profitability of the system. However, this study showed that attention should be paid to crystallization risks. Regarding the performance and the crystallization occurrence, couples using ammonia as refrigerant should be preferred to ones using water.

To face the current increase of building cooling demand and the concerns related to climate changes, an energy-efficient evaporative cooling system using porous material has been developed. This ar...

Solar thermal energy storage is used in many applications, from building to concentrating solar power plants and industry. The temperature levels encountered range from ambient temperature to more than 1000 °C, and operating times range from a few hours to several months. This paper reviews different types of solar thermal energy storage (sensible heat, latent heat, and thermochemical storage) for low- (40–120 °C) and medium-to-high-temperature (120–1000 °C) applications.

Heating of buildings is a highly energy-demanding task. Therefore, improving energy management is crucial for more environmentally friendly development of future constructions, and the use of thermochemical heat storage technologies in solid materials is one of the most promising solutions. Nevertheless, certain drawbacks such as the impact of air pollutants on the long term durability of thermochemical heat storage materials (ageing up to 30 years) need to be studied to implement installations. In certain thermochemical heat storage systems, air passes through porous materials to carry water and heat, and the air's pollutants can interact with the solid material, thus decreasing its storage capacity. In the present work, adsorption tests were performed (under dynamic conditions) on zeolite-base materials, using different model molecules (i.e. toluene, styrene and hexaldehyde) representative of air pollutants. Strong competition between the adsorption of pollutants and water on the storage material was demonstrated. Water molecules were able to displace the molecules of pollutants, previously adsorbed on the material surface, thus delaying the saturation of the material and extending its life. Nevertheless, a lowering of the water adsorption and heat storage capacity was measured for a high number of cycles. The cycled materials have been thoroughly characterised in their physicochemical properties to identify the modifications of the materials (i.e., de-alumination) and correlate them with the heat storage capacity. The impact of pollutants was demonstrated to be stronger on the pure zeolite, while the salt present on the MgSO4/13× composite seems to protect the material surface from the pollutant effect.

The global energy demand is increasing at alarming rates. Until 2040, most of this energy will still be coming from fossil fuels, which generate pollution and global warming, thereby increasing the need for cooling (higher energy demand) and creating a vicious circle. Absorption systems represent a possibility for contributing to the reduction of fossil fuels consumption and CO2 emissions. These systems can run on renewable energies and heat waste; however, to date they possess high initial investment costs. The present review offers an up-to-date global overview of the small-capacity (≤ 50 kW) single-stage continuous absorption systems operating on binary working fluids for cooling from the theoretical, experimental, and commercial points of view, with a special focus on the comparison of cycle performance between the different families of refrigerants and working pairs. The main objective is to provide information for a good understanding of the evolution of these systems over the past 40 years as well as the main advantages and disadvantages of using different working pairs.

An experimental study of an innovative low-energy and low-cost cooling system for buildings is presented. To achieve low-energy use, this cooling system simultaneously makes use of three available heat sinks: the ground, evaporation of water and radiation to the sky. This simple, inexpensive and energy-saving system was installed on a single-family 100-m2 house in Bordeaux (South-West of France) and tested during summer 2015. On the test sequence presented (10–25 August), this system dissipated an average of 8.5 kWh day−1 of heat from the building, with a maximum of 19.3 kWh. This energy was dissipated mainly into the ground and crawl space (4.5 kWh day−1) and by evaporation and radiative cooling (4.7 kWh day−1). The system COP over the period was 19 and it maintained an inside air temperature under 27 °C even during warm summer days, with approximately 42 L day−1 water consumption.

Thermochemical systems are a good alternative to current technologies for long-term heat storage, since the energy is stored as a chemical potential and there is no heat loss during the storage phase. A large number of studies have now been conducted on the development of integrated thermochemical reactors, but fewer studies have investigated separate reactor technologies. The latter present the advantage of dissociating the thermal power and storage capacity of the system, which also increases the energy storage density of the process. This paper investigates the functioning of an open continuous thermochemical reactor with falling solid flow and humid air cross-flow. A 2D model was developed and set up with the finite element simulation software COMSOL Multiphysics to represent heat and mass transfer phenomena in the reactor. This study focuses on the influence of the inlet pressure and vapour fraction of the air on the reaction behaviour and the reactor performance (power, temperature). This study also investigates the influence of the reaction front on the pressure drop in the reactor. It highlights the presence of a sharp reaction front between one part of the reactor (bottom) where the solid is completely hydrated and the other part (top) where the solid has not reacted yet. This sharp front is strongly dependent on the inlet air conditions. Also, there is an inertia phenomenon at the entrance of the unreacted solid, which is influenced by inlet air pressure, creating a particular air velocity field.

This paper investigates heat and mass transfer occurring in an interseasonal absorption heat storage system using LiBr/H2O as the sorption couple. It focuses on the poor performances of the falling...

International audience; This study presents a new plate-type falling film absorber design, consisting in a vertical grooved falling film absorber. The grooves are designed to obtain good absorber plate wettability, even at a low solution flow rate, resulting in a laminar solution flow regime. Using experimental and numerical tools, the vapor absorption on a LiBr falling film solution is characterized for different operating conditions. The impact of absorber length, cooling water inlet temperature, absorber water vapor pressure, solution inlet temperature, LiBr mass fraction and flow rate is investigated. Experimentally, a high absorption rate is achieved: as high as 7·10 −3 kg s −1 m −2. Moreover, a 1D stationary model of water vapor absorption in a laminar vertical falling film is introduced and validated. Numerical investigations allow defining the absorber effectiveness for a wide range of operating conditions.

A heat storage process by absorption is studied in this paper. It is devoted to solar domestic systems. Energy and exergy studies are performed on the ideal cycle, and prove the contribution of the solution crystallisation to the system storage density, with an improvement of 22%, without degradation of the exergetic efficiency of the process. A prototype has been built and tested in conditions compatible with domestic solar thermal collectors. The process has been proved successful for heat storage. The heat charging was more efficient than the discharging phase, with respective heat transferred in the range of 1–2 kW and 0.3–0.5 kW, in typical solar domestic conditions. Crystallisation has been observed, and will increase the storage density but discrepancies were found between the ideal solution and the global prototype crystallisation behaviour, possibly due to some impurities presence, corrosion products and a slow dissolution kinetic.

International audience; This study investigates a moving-bed thermochemical reactor with hydrated salt and humid air flow, suitable for building heating applications. A well-instrumented reactor prototype, developed to produce a heating power up to 1 kW, allowed analysing the functioning of the reactor. The SrBr$_2$.H$_2$O/SrBr$_2$.6H$_2$O pair was used and allowed reaching reactor temperatures up to 41 °C and specific heating powers of 1.7–4.6 kW/m$^3$ of reactor bed. Once the thermal steady state was reached, the temperature was homogeneous throughout the bed. This study shows the impact of the air humidity at the reactor inlet on the reactor performances and shows the feasibility of continuous thermochemical heat storage in a moving bed reactor with hydrated salt. Finally, this study highlighted the advantages and limitations of moving bed reactors for thermochemical storage applications.

With the possible depletion of oil resources and environmental considerations, vegetable oils are considered with great interest in high-temperature applications especially for solar plants. Many parameters affect vegetable oil stability, namely temperature, oxygen, moisture, duration of exposure to heat, etc. Conventional markers such as peroxide or free fatty acids do not always reliably indicate oil quality at high temperature. Evaluating the content of other degradation products such as polymers provides a better estimate. Reducing exposure to oxygen limits oil degradation. Vegetable oils have proven to be a favourable alternative to mineral oils in some applications, and several vegetable oils are commercially available. Their current use in solar cookers gives a good overview of their potential. Several reasons advocate for the use of vegetable oils in concentrating solar power plants: they are renewable resources, environmentally friendly, non-hazardous, highly available and less flammable products. Moreover, their thermophysical properties are similar, if not better, than those of thermal oils commonly used in solar plants. Despite these advantages, vegetable oils face some challenges, as for example their low oxidation stability. If the major challenge of using vegetable oils at high temperatures is their oxidation stability, the numerous environmental, economic and technological advantages make them innovative fluids for high temperature applications, particularly for solar applications.

International audience; This paper presents experimental results obtained with a prototype of a solar heat storage absorption process using the KCOOH/H2O couple. The process principle and the prototype tested are described, and both charging and discharging results are presented and discussed. The parameters influencing the results the most are water and solution flow on the reactor heat exchangers as well as the heat source and sink temperatures. The system's main limitation is the wetting of the exchanger plates. However, the tests prove the interest of the KCOOH/H2O couple in this type of low-temperature heat storage process.

The development of efficient long-term heat storage systems could significantly increase the use of solar thermal energy for building heating. Among the different heat storage technologies, the absorption heat storage system seems promising for this application. To analyze the potential of this technology, a numerical model based on mass, species, energy, and exergy balances has been developed. The evolution over time of the storage imposes a transient approach. Simulations were performed considering temperature conditions close to those of a storage system used for space heating coupled to solar thermal collectors (as the heat source), with ground source heat exchangers (as the cold source). The transient behavior of the system was analyzed in both the charging and discharging phases. This analysis highlights the lowering of energetic and exergetic performance during both phases, and these phenomena are discussed. The thermal efficiency and the energy storage density of the system were determined, equal to 48.4 % and 263 MJ/m3, respectively. The exergetic efficiency is equal to 15.0 %, and the exergy destruction rate is 85.8 %. The key elements in terms of exergy destruction are the solution storage tank, the generator, and the absorber. The impact of using a solution heat exchanger (SHX) was studied. The risk of the solution crystallizing in the SHX was taken into account. With a SHX, the thermal efficiency of the system can reach 75 %, its storage density was 331 MJ/m3, and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively.

Thermoelectric (TE) modules are possible reversible pre-cooling and pre-heating devices for ventilation air in buildings. In this study, the opportunity of direct coupling of TE modules with photovoltaic (PV) cells is considered. This coupling is evaluated through a numerical simulation depending on the meteorological conditions of Chambery, Alpine region in France, and on the cooling or heating use of the TE modules, through annual energy and exergy efficiencies. For the considered conditions, TE module performances are of the same order as the ones of the vapour compression heat pumps, with a TE coefficient of performance higher than 2 for low values of input DC current. The PV–TE coupling efficiency varies between 0.096 and 0.23 over the year, with an average value of 0.157. Evolutions of the exergy effectiveness of PV and TE elements follow the same trends as the corresponding energy efficiencies but with steeper variations for the coupling exergy yield that varies between 0.004 and 0.014, with an annual average value of 0.010. The direct PV–TE coupling does not seem to be a sustainable option for the summer cooling purpose particularly. A case study with indirect coupling under a warm climate is considered and shows that the use of TE devices could be efficient in housing to ensure summer thermal comfort, but the corresponding necessary PV area would induce a high investment. Copyright © 2008 John Wiley & Sons, Ltd.

The IEA joint Task 42 / Annex 29 is aimed at developing compact thermal energy storage materials and systems. In Working Group B, experts are working on the development of compact thermal energy storage applications, in the areas cooling, domestic heating and hot water and industry. The majority of application projects were in the field of room heating and domestic hot water. In this article, an overview is given of a large number of applications. The storage technologies used in the applications are latent heat storage, open and closed solid sorption, liquid sorption and salt hydrates and composites thereof. On a broad front, a lot of progress was made in the development of components and systems, providing knowledge and experience regarding the design, numerical modeling, building, testing and economical assessing of components and storage systems. Most important findings are that the interaction of storage materials with the materials of components can be deciding for the technical feasibility, that a number of components, like reactor, heat exchangers and evaporators are less understood than initially thought and need more development, that the inclusion of storage materials in systems generate new challenges like the occurrence of non-condensable gases and thermo-mechanical effects and that standardized and simplified system approaches are needed.

This paper addresses the thermal performances of a zeolite-based open sorption heat storage system to provide thermal energy for space heating needs. The study focuses on the experimentation of a significant scale prototype using zeolite 13X/H 2 O as the reactive pair, and on the development of a 1D mathematical model used to predict both the charging (desorption) and the discharging (adsorption) processes occurring inside the storage unit. The experimental campaigns and the numerical results lead to some promising conclusions on the thermal performances of such a storage unit. With 40 kg of zeolite, a temperature lift of 38 °C on average at the outlet of each zeolite’s vessel during 8 h was achieved during the discharging with an airflow inlet at 20 °C, 10 g/kg of dry air of specific humidity and a flow rate of 180 m 3 /h. Some discrepancies between the experimental and simulation results were observed during both the charging and discharging tests, and were explained.

International audience; An exergy-based analysis applied to ideal thermochemical dipoles allowed to design an original process that could use low-grade energy, produced from a thermal solar collector at around 70 °C, to provide low-temperature cold, below −23 °C, in order to store deep-frozen food. The ideal coefficient of performance (COP) of this system is 0.5 and the exergetic yield is 1. Taking into account the process enthalpies and the sensible heat of the reactants, the COPthermo is 0.17. The process functioning is described in this paper. It alternates between a regeneration mode during daytime and cold production mode during night-time. An experimental prototype was designed and built. It proved the feasibility of the concept and showed an experimental COP of about 0.06, which is similar to the up-to-date solar cooling systems, but at higher cold temperatures. The mean annual exergetic yield of the process is about 0.06.

International audience; A solar deep-freezing process has been designed. It aims at cooling down a cold box to about −20 °C, using simple flat plate solar collectors operating at 70 °C. This original process involves two cascaded thermochemical systems based on the BaCl2/ammonia reaction. Its working mode is discontinuous as it alternates between a regeneration mode during daytime and a cold production mode during nighttime. A global dynamic model involving the various system components allows the simulation of the process; it predicts the evolution of the components temperatures and the rates of chemical reactions of the system. It also allows the dimensioning of the system components to maintain a 500 l cold box at −20 °C during the 6 sunniest months of the year under typical Mediterranean weather conditions and provide over 80% of the total yearly cooling needs of this box. This requires a solar collector area of 5.8 m2 and 39 kg of reactive salt. The predicted coefficient of performance (COP) is about 0.1 over the year, and the net solar COP, taking into account the collector efficiencies, is 0.05.

This chapter presents the thermal energy storage technologies suitable for low temperature (up to 150 °C) solar thermal applications in buildings. In the first part, insight is provided into recent developments about systems technologies and materials, their classification, limitations, and potential solutions for their application. In the second part, special focus is given on an innovative process using absorption cycles for long-term solar energy storage. A detailed dynamic model of the system has been developed for the simulation in order to evaluate the process performance and to optimize its design. A prototype of the system has also been presented, as well as experimental results in the process charging and discharging modes.

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National audience; Cet article présente la démarche mise en oeuvre dans le projet collaboratif ANR Hygrobat, qui vise à donner des outils pour quantifier de manière fiable l'impact des transferts de masse sur les transferts de chaleur dans les parois de bâtiments comprenant des matériaux fortement hygroscopiques. Les matériaux sélectionnés dans ce projets (fibre de bois, bois massif, OSB) ont été soigneusement caractérisés. Leurs propriétés hygrothermiques ont été mesurées en laboratoire, puis ils ont été mis en oeuvre dans des parois, soumises à des conditions aux limites de variées. L'originalité de l'étude réside dans : (i) la démarche " pas à pas " de complexité croissante, partant des caractérisations d'un seul matériau en conditions stationnaires, jusqu'aux mesures sur une paroi multicouche en climat réel ; (ii) les vérifications croisées de mesures expérimentales, qui sont effectuées dans deux des laboratoires partenaires du projet, parfois sur des dispositifs différents ; (iii) association des benchmarks expérimentaux et numériques. MOTS-CLÉS : transferts couplés de chaleur et de masse, matériaux hygroscopiques, benchmarks