Working Pair Research Articles

Improvement of the properties of ionic liquids for the absorption refrigeration cycle

Ionic liquids (ILs), recognized as environmentally friendly green reagents, have good application prospects in many fields. However, their high viscosity has hindered their widespread application. Currently, an effective solution is to introduce a solvent as an additive. In this work, ethanol with different molar fractions was chosen to form working pairs with three ILs ([emim]BF4, [bmim]BF4 and [hmim]BF4), to reduce their viscosities. The densities and viscosities of the three working pairs were obtained in a temperature range of 293.15∼338.15 K, thereby determining the thermal expansion coefficients. The effects of ethanol on the volumetric and viscometric characteristics of the three ILs are discussed. To further analyze the interactions between ethanol and the ILs, excess mole volume and viscosity deviations were introduced. The interactions between different molecules were stronger than those between the same molecules in the working pairs. This is the main reason why the addition of ethanol reduces the viscosity of the three ILs. Finally, the hard-sphere model was applied to predict the viscosities of the working pairs. Upon introducing an adjustable argument, the average absolute relative deviation of the prediction for the aforementioned three ILs decreased to 4.4%, 5.9%, and 5.0%, respectively.

Augmentation strategy toward superior water absorption performance: An attempt utilizing [EMIm]Ac ionic liquid/water pair

Ionic liquids (ILs) offer significant advantages as alternative absorbents in absorption refrigeration cycles due to their superior physicochemical properties. Assessing the performance of new IL-based working pairs in absorption systems is essential for their rapid application in industrial applications. This study evaluates the water absorption performance of an aqueous solution of 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) under both saturated atmospheric and vacuum conditions. Additionally, the performance of an absorption chiller utilizing the [EMIM]Ac/H2O working pair was analysed using the Aspen Plus process simulator. A discrepancy was observed between the water vapor pressure in the air and the saturated vapor pressure of the aqueous solution at equilibrium. A theoretical criterion for the minimum relative humidity required for effective water vapor absorption by this absorbent at a given mass fraction is proposed. Under vacuum conditions, lower absolute pressure or higher mass fraction enhances absorption. Surfactants significantly improve absorption performance, with 2-octanol and 1-octanol demonstrating approximately double the absorption capacity compared to [EMIM]Ac. Higher mass fractions of the aqueous solution enhance the absorption rate but reduce the desorption rate. The coefficient of performance (COP) of the system decreases with increasing pressure and mass fraction, reaching a peak of 0.9 at a mass fraction of 20 % and a pressure of 3 kPa. Lower pressure and mass fraction enhance exergy efficiency.

Study on the Performance of a Novel Double-Section Full-Open Absorption Heat Pump for Flue Gas Waste Heat Recovery

Open absorption heat pumps are considered one of the most promising methods for efficiently utilizing low-grade waste heat, reducing energy consumption, and lowering greenhouse gas emissions. However, traditional heat pumps have significant limitations in the range of flue gas temperatures they can recover, and their relatively low system performance further restricts practical applications. In this study, we propose a novel double-section full-open absorption heat pump driven by flue gas from the desulfurization tower. By designing the absorber with a double-layer structure, the system can recover more latent and sensible heat from the flue gas, significantly enhancing its thermal recovery capability. Additionally, replacing the traditional LiBr/H2O working pair with LiCl/H2O significantly reduces the risks of solution crystallization and equipment corrosion. Through comprehensive research, the strengths and weaknesses of the system were explored. The results indicate that this system effectively recovers flue gas waste heat within the temperature range of 30–70 °C. Specifically, at a flue gas temperature of 70 °C and a flow rate of 3 kg/s, the system achieves a COP of 1.838, along with a heating capacity of 158.83 kW and a ROI of 34.1%. These metrics demonstrate that the system not only delivers high performance but also exhibits excellent economic viability. Additionally, when the solution temperature is lowered to 10 °C, the system’s maximum COP reaches 1.96, reflecting a significant 30.67% improvement over traditional heat pumps. These findings highlight the system’s potential for application in coal-fired power plants, where varying levels of power output can benefit from enhanced thermal recovery and efficiency.

Investigation of a novel combined ab- and ad- sorption based thermal energy storage and upgrade system

This paper presents a novel sorption-based cycle combining energy storage with energy upgrade to store low-grade heat for long periods and transform it on demand to usable energy at elevated temperatures. The system consists of an ammonia-water absorption system coupled to a resorption thermal transformer using manganese chloride and calcium chloride as the working pair with an ammonia absorbate. The absorption and adsorption systems are charged by separating the constituent sorbent-sorbate pairs, and they remain separated during the transition season. The discharge consists of a two-stage sorption process between the two systems. This work presents a transient thermodynamic model and analysis of the two-stage sorption system. For a typical solar thermal energy input of 106 °C during charging, and ambient temperatures of 5 °C and 25 °C in the winter and summer, respectively, the system is capable of discharging at an average elevated temperature of 62 °C with an energy storage efficiency of 27 %. This system is capable of simultaneously producing heat at a lower temperature by varying the mass flow rate through the high-temperature salt. The energy storage density of the combined system under different conditions is between 71 and 185 kJ kg−1. In comparison with other conventional single-stage thermal energy storage systems, this combination of absorption and adsorption-based storage provides a higher coefficient of performance for a comparable temperature output.

State of the art on solid–gas sorption based long-term thermochemical energy storage

Solid-gas sorption thermochemical heat storage technology is an innovative and promising solution for storing heat over long periods. The review focuses on the construction of composite sorption thermochemical heat storage materials and binary mixed salt materials with porous matrix as the supporting materials, which can further improve the hydration rate and cycle stability of single hydrated salt. Furthermore, the heat storage capabilities of the metal halide/ammonia working pair make it suitable for extreme environments, surpassing the limitations of water-based working pairs in terms of low-temperature adaptability. It is worth noting that common solid–gas reactors and measures to enhance heat and mass transfer are also reviewed. Meanwhile, the efficient open and closed heat storage systems and their optimization schemes are introduced, and the feasibility of the practical operation of the heat storage systems is evaluated comprehensively from the technical and economic perspectives, and the future research challenges are prospected.

Prediction method of adsorption thermal energy storage reactor performances based on reaction wave model

Adsorption thermal energy storage (ATES) is one of the most important ways to achieve efficient utilization of solar energy. The lack of effective prediction methods of reactor performance severely restricts the application of the ATES system. In this paper, the prediction method of reactor performance was proposed based on the adsorption reaction wave model. The expressions between the reactor performances and the design parameters were derived by using the wave parameters of the adsorption rate wave. The mathematical relationships of design parameters-wave parameters and wave parameters-reactor performances were established, respectively. The experiments on adsorption thermal energy storage were performed, in which the zeolite-water vapor was determined as the working pairs. The air temperature and specific humidity at the inlet and outlet of the reactor were measured, and corresponding adsorption amount, thermal energy and stable output power were obtained. A comparison of the predictions for the reactor performances with experiments was carried out. The results indicated that the proposed prediction method was capable of accurately predicting the reactor performances, with a maximum deviation of less than 6.0 %. Moreover, the proposed prediction method is superior to available methods in terms of simplicity and generality.

Deep eutectic solvents and traditional refrigerants in absorption refrigeration cycles using molecular approaches

Absorption Refrigeration Cycles (ARC) represent a sustainable technology capable of effectively producing cold with modest energy consumption from waste heat or renewable sources. However, the right choice of working fluid pair absorbent–refrigerant is essential for the system to work properly. This contribution is devoted to analysing the performance of an ARC using a wide variety of refrigerants in combination with selected deep eutectic solvents (DES) as absorbents. Three choline chloride-based and one ionic liquid (IL)-based DESs as absorbers are combined with sixteen refrigerants. The PC-SAFT equation of state is employed to assess the thermophysical properties. Accurate modelling of the absorption cycle has revealed that the phase behaviour of the working mixture significantly constrains the temperature and pressure range within which the cycle can operate. Idealised ARCs are insufficient for accurately representing the capabilities and limitations of these systems. Of the sixty-four working pair mixtures, only eighteen are suitable for ARCs based on technical and thermodynamics criteria. The Coefficient of Performance (COP) has been updated with additional key performance indicators that consider environmental and safety factors. The maximum values of the COP for this mixture are around 0.92, while many options lie in ranges around 0.60. This comprehensive analysis helps establish a ranking of the best options. The results have shown that the IL-based DES using R227ea or R236ea is the most efficient system among all the systems studied, with performances of 0.92 and 0.74.

High-density and anti-clogging three-phase absorption heat storage with crystallization management

Three-phase sorption heat storage can significantly enhance the energy storage density (ESD) through the crystallization of salt-water working pair. However, the crystals will cause the clogging of solution circulation, making it difficult to be realized in absorption heat storage with liquid absorbent. In this work, the crystal management strategy is proposed for three-phase absorption heat storage. By controlling the crystallization ratio, energy storage enhancement and clogging prevention can be achieved simultaneously. Both theoretical and experimental analyses were carried out. A detailed thermodynamic model of the three-phase absorption heat storage cycle was established, by considering the change of crystallization ratio. A prototype of LiBr-water three-phase absorption heat storage with crystallization management which includes crystal filtering, high-level solution intake, crystal fusion by electric heating rods, and water flushing, was designed and fabricated. The anti-clogging design ensures the circulation of solution, efficient vapor absorption and crystal dissolution in discharging process, thus achieving high energy storage density and stable heat output simultaneously. The three-phase absorption heat storage prototype demonstrated high ESD of 220 kWh/m3 and 178 kWh/m3 with crystallization ratio of 0.41 and 0.26 under the space heating and hot water supplying conditions, respectively. Comparing with the absorption heat storage prototype without crystallization, this represents ESD enhancement of 102% ∼ 210%, which confirms the feasibility and superiority of the proposed crystal management strategies.

Review on Absorption Refrigeration Technology and Its Potential in Energy-Saving and Carbon Emission Reduction in Natural Gas and Hydrogen Liquefaction

With the requirement of energy decarbonization, natural gas (NG) and hydrogen (H2) become increasingly important in the world’s energy landscape. The liquefaction of NG and H2 significantly increases energy density, facilitating large-scale storage and long-distance transport. However, conventional liquefaction processes mainly adopt electricity-driven compression refrigeration technology, which generally results in high energy consumption and carbon dioxide emissions. Absorption refrigeration technology (ART) presents a promising avenue for enhancing energy efficiency and reducing emissions in both NG and H2 liquefaction processes. Its ability to utilize industrial waste heat and renewable thermal energy sources over a large temperature range makes it particularly attractive for sustainable energy practices. This review comprehensively analyzes the progress of ART in terms of working pairs, cycle configurations, and heat and mass transfer in main components. To operate under different driven heat sources and refrigeration temperatures, working pairs exhibit a diversified development trend. The environment-friendly and high-efficiency working pairs, in which ionic liquids and deep eutectic solvents are new absorbents, exhibit promising development potential. Through the coupling of heat and mass transfer within the cycle or the addition of sub-components, cycle configurations with higher energy efficiency and a wider range of operational conditions are greatly focused. Additives, ultrasonic oscillations, and mechanical treatment of heat exchanger surfaces efficiently enhance heat and mass transfer in the absorbers and generators of ART. Notably, nanoparticle additives and ultrasonic oscillations demonstrate a synergistic enhancement effect, which could significantly improve the energy efficiency of ART. For the conventional NG and H2 liquefaction processes, the energy-saving and carbon emission reduction potential of ART is analyzed from the perspectives of specific power consumption (SPC) and carbon dioxide emissions (CEs). The results show that ART integrated into the liquefaction processes could reduce the SPC and CE by 10~38% and 10~36% for NG liquefaction processes, and 2~24% and 5~24% for H2 liquefaction processes. ART, which can achieve lower precooling temperatures and higher energy efficiency, shows more attractive perspectives in low carbon emissions of NG and H2 liquefaction.

Screening HFC/HFO and ionic liquid for absorption refrigeration at the atomic scale by the prediction model of machine learning

Absorption refrigeration is a highly effective method for utilizing renewable energy, as it can be driven by low-grade heat sources such as industrial waste heat, solar energy, and geothermal energy. The development of new working pairs, particularly hydrofluorocarbon/hydrofluoroolefin refrigerants combined with ionic liquids, has been pivotal in enhancing the cooling efficiency of absorption refrigeration systems. These systems rely on the solubility difference between the generator and absorber, making solubility a crucial factor in determining their efficiency. In this context, we have established an advanced solubility estimation model. This model employs the Attention E(n)-equivariant Graph Neural Network (AEGNN) applied to disconnected graphs, enabling comprehensive learning from both topological and Euclidean structural information. Our atomic-scale model demonstrates significantly higher accuracy than traditional group contribution methods, with an average absolute deviation of 0.003 mol/mol from experimental data. Moreover, it encompasses a much broader range of working pairs. Through extensive screening, we have identified working pairs with high estimated solubility differences. Compared to the high-efficiency working pair identified in the literature, the best-screened working pairs exhibit an improvement in solubility differences by more than 0.3 mol/mol under common operating conditions.

Comparative analysis of thermodynamic performance of high-temperature absorption heat transformers using various ionic liquids as working pairs

The study investigated six sets of ionic liquid (IL) working pairs based on high-temperature vapor-liquid equilibrium data. These sets were modeled using the NRTL activity coefficient model and integrated into a single-stage absorption heat transformer (AHT) for system cycle simulation analysis. A comparison was made with traditional H2O + LiBr working pairs commonly used in AHTs. The study aimed to assess the feasibility of using IL working pairs in high-temperature AHTs to achieve higher output temperatures when dealing with heat sources exceeding 120 °C. Compared to the traditional H2O + LiBr working pair, which has strong COP and ECOP but a limited temperature range, IL pairs offer advantages under various conditions. For instance, H2O + [HMIM][Cl] and H2O + [BMIM][Br] can operate at higher condensation temperatures, providing broader temperature ranges. H2O + [HMIM][Cl] has a wider operational temperature range, suitable for unstable waste heat sources. It also has the highest optimal ECOP value of 0.64 at 197 °C absorption temperature, and shows good cyclic performance, achieving temperature rises of about 78 °C. ILs can maintain stable COPs and circulation ratios over a wide range of absorption temperatures, thus achieving higher temperature rises or meeting the need for higher absorption temperatures. Notably, although the H2O + IL combination exhibits slightly higher exergy loss than the traditional H2O + LiBr pair when comparing exergy losses in the AHT system among different working pairs, its low corrosiveness, lack of crystallization, and wide operating temperature range make these drawbacks insignificant.

Compression-assisted absorption refrigeration cycle employing organic working pairs for ultra-low grade heat recovery

Solar absorption refrigeration technology is considered an ideal alternative to energy-intensive refrigerated warehouses due to its decarbonization and environmental friendliness. However, it faces challenges in competing with vapor compression refrigeration, such as high driving heat source temperature (>100 °C) and a long payback period. In this paper, we address these challenges by focusing on cycle structure and working pair, and propose a compression-assisted absorption refrigeration cycle (CARC) using R32/dimethylacetamide (DMAC) as working pair. The aim is to significantly reduce driving heat source temperature by adjusting absorption pressure utilizing an auxiliary compressor, enabling the cycle to be connected to low-cost and efficient non-concentrating solar collectors, thereby effectively shortening its payback period. This cycle includes two operating modes: CARC mode powered by solar energy during the daytime and vapor compression refrigeration cycle mode driven by off-peak electricity at night, ensuring 24-hour cooling capacity for refrigerated warehouses. The results indicate that this cycle can flexibly utilize 60∼80 °C solar hot water easily obtained as driving heat source and its evaporation temperature decreases to -20 °C. To assess its performance benefits, a compression-assisted absorption refrigeration system is developed and implemented. By utilizing the limited roof area of a refrigerated warehouse, it can achieve an 18.2 % reduction in electricity consumption and a 24.2 % reduction in carbon emissions compared to the vapor compression one annually. Additionally, during peak electricity demand periods in summer, the system exhibits significant energy-saving effects, and its payback period is about 4.2 years. Ultimately, this cycle provides a viable solution for energy-saving and emission reduction in refrigerated warehouses.

Study on the thermodynamic performance of solar absorption refrigeration combined with a ‘seasonal cold storage’ system

Inter-seasonal thermal storage technologies are focused on storing and transitioning abundant solar energy from summer to winter for heating, often ignoring the fact that abundant cold energy in winter can also be transferred to summer for cooling. In this paper, a novel solar absorption refrigeration combined with a ‘seasonal cold storage’ system is proposed to store abundant ambient cold energy in the form of chemical potential in winter and is utilized to discharge energy for refrigeration in summer. Moreover, based on the thermophysical properties of LiBr/H2O, LiBr-[BMIM]Br/C2H5OH and LiBr-[BMIM]Cl/CH3OH, the thermodynamic performances under various operating conditions were calculated via MATLAB. The results show that the system using LiBr-[BMIM]Cl/CH3OH obtained a larger energy storage density of 339.22 kJ·kg−1 below the ice point without freezing compared to traditional ice storage. The systems using LiBr-[BMIM]Br/C2H5OH and LiBr-[BMIM]Cl/CH3OH achieved higher coefficients of performance of 0.94 and 0.90, respectively. Moreover, the generation temperature of the system using both the ternary working pairs was below that of LiBr/H2O by more than 5 K, which was beneficial for enhancing the utilization of solar energy and saving the collector area. Overall, this new system using LiBr-[BMIM]Cl/CH3OH exhibited the shortest payback period of 2.73 a, with considerable economic benefits.

Study of metal–organic framework (MOF)/water pairs for adsorption heat transformer applications

Metal-organic framework (MOF), an advanced category of porous coordination polymers, has recently gained prominence as a novel class of adsorbent materials, captivating the attention of researchers worldwide engaged in diverse applications related to adsorption. An adsorption heat lifter or adsorption heat transformer (AdHT) plays a crucial role in elevating the temperature of a waste heat source. This is achieved by harnessing the adsorption enthalpy or isosteric heat of adsorption. The system performance of the AdHT system is affected by the working pairs, which leads to a possible application for MOF/water working pairs in such a system. In this study, 2-tank and 3-tank systems are invoked to investigate and compare the coefficient of performances (COPs) of three MOFs: MOF–801, aluminum fumarate, and MIL–100(Fe) paired with water while considering an atmospheric temperature of 308.15 K. With a waste heat source and cooling water temperature difference (TD) of 70 K in the 3-tank system, MOF–801/water outperforms aluminum fumarate and MIL-100(Fe). Considering a temperature driving force that explains the temperature change of heat transfer fluid during the flowing procedure inside pipes, ΔT = 5 K, the MOF–801/water pair can provide a gross temperature lift of around 30 K at a relatively good COP of 0.4867. There are very few articles that studied the MOFs/water pair for AdHT 2-tank and 3-tank system applications. Hence, the results of this study highlight their viability for designing environmentally friendly and energy-efficient heating systems.

Novel working fluid pair of methanol/betaine-urea for absorption refrigeration system driven by low-temperature heat sources

An absorption refrigeration system (ARS) designed for low-temperature heat sources like industrial waste heat, solar-heated water, or geothermal heat can offer substantial energy-saving benefits. This study explores a novel working fluid pair comprising methanol and betaine-urea (BE-UR), forming a deep eutectic solvent (DES) system. This design capitalizes on methanol's high enthalpy of vaporization, the low heat capacity of the working pair, and the appropriate intensity and strength of the hydrogen bonding network within the solution. The proposed fluid pair shows full solubility of BE-UR in methanol at a mole ratio of 1.81 to 1 (methanol to BE-UR), with an equilibrium vapor pressure dropping to 8.59 kPa at 303.15 K. Molecular dynamics simulations unveil a microscale solution structure, showcasing a hydrogen bond network and unbonded methanol in the concentrated working fluid pair. This configuration enhances the absorption process of methanol vapor in the ARS absorption unit. Aspen Plus simulations predict a coefficient of performance (COP) exceeding 0.85, comparable to H2O/LiBr in single effect-ARS and outperforming NH3/H2O in SE-ARS by approximately 30 %, while maintaining a lower cycle ratio of about 2.

Experimental studies on the impact of adsorbent particle size on the adsorption chiller performance

Adsorption chiller dynamics and its effect on the performance are greatly influenced by the adsorbent particle size because they govern both the micro (intra-particle) and macro (inter-particle) heat and mass transfer resistances within the adsorber bed. The net effect of adsorbent particle size and shape on the overall cooling system performance is addressed here. Initial CFD analysis of columnar adsorber bed with silica gel + water as the working pair, assuming uniform spherical adsorbent, showed increased uptake capacity with smaller sized (0.23 mm radius) particles over bigger sized (0.8 mm radius) ones despite smaller vapor penetration depth of the former. Subsequently, the effect of adsorbent particle size and shape on the overall cooling system performance is investigated experimentally in a single-stage one-bed mode for RD 2060 (0.8 mm radius) and RD 3070 (0.23 mm radius) silica gel specimens. Though the smaller size of RD 3070 is favourable for the adsorption kinetics at the particle level, its non-spherical shape and highly non-uniform size distribution resulted in significantly lower permeability and thermal diffusivity of the packed adsorber bed, thus hindering the cyclic steady state throughput. For the same operating conditions, RD 2060 and RD 3070 yield a cooling capacity (CC) of 107 W and 22 W, respectively. Further, the coefficient of performance (COP) drops by 73% in the case of RD 3070. The present study indicates that the adsorber bed design must consider both the adsorbent size and shape, in addition to other system operational parameters to enhance performance indicators.

Two-Stage Solar–NaOH Thermochemical Heat Pump Heating System for Building Heating: Operations Strategies and Theoretical Performance

Heating buildings with solar energy is challenged by the seasonal mismatch between solar availability and heating demand. Thermochemical energy storage is a promising technology to overcome this challenge because of its high energy density. In building applications, space requirement is also an important consideration. Therefore, both the storage space and collector areas are important considerations, with only the latter often being neglected in previous studies. This paper proposes a novel two-stage thermochemical heat pump heating system based on the working pair of NaOH/H2O. We demonstrate that this system can work with a concentration difference (70% wt–30% wt) for the climate in hot summer and cold winter regions in China. The energy storage density based on the discharged solution is 363 kWh/m3. With this solar-driven thermochemical heat pump heating system, 35.13 m2 of collectors and 10.48 tons of 70% wt NaOH solution are sufficient to complete a full charge–discharge cycle and meet the heating demand of a single-family house (winter space heating + DHW: 9370 kWh, summer DHW: 2280 kWh). The theoretical maximum storage for solution (discharged + water tank) is 32.47 m3. Compared with the sensible seasonal storage alternative, the collector area is reduced by 12.5% and the storage space is reduced by 59%, with a possible further reduction through optimization. With the potential to be further optimized for space saving, the two-stage solar–NaOH heat pump heating system is an energy-efficient and space-efficient heating system for buildings in the hot summer and cold winter regions of China.

Performance investigation on dual bed adsorption chiller integrated with a partially covered N-PVT collector

The theoretical performance investigation of the adsorption chiller system integrated with a partially covered N-PVT Collector is mathematically modeled and presented in the paper. The study employs a MATLAB-Simulink integrated interface, and the silica gel-water combination serves as the working pair. The average weather conditions of Shiv Nadar Institute of Eminence for the month of May are taken into account the study. The investigation shows that solar energy integrated with an adsorption cooling system could efficiently provide a cooling effect. The results suggested that, due to the direct integration of the PVT with the chiller system, the influence of cycle operation is significant at the minimum and maximum solar radiation times. An optimization research for the optimal number of collectors for silica gel regeneration revealed that using N = 5 collectors results in an outlet hot water temperature range of 110 0C. During operation, the maximum adsorption capacity of 0.21 kg of water/Kg desiccant is obtained, for providing an adequate cooling effect. The system achieved a specific cooling power of between 50 and 200 W/kg during the process, and a coefficient of performance between 0.15 and 0.23.

Investigating maximum temperature lift potential of the adsorption heat transformer cycle using IUPAC classified isotherms

Adsorption heat transformer (AHT) cycle is capable of upgrading the low-grade waste heat to a higher temperature. The maximum temperature lift of the AHT cycle can represent its theoretical performance limit. However, such a metric is currently absent from the literature due to the scarcity of fundamental studies on the heat upgrading sorption cycles. Therefore, in the present study, three models are proposed to derive the ‘maximum temperature lift’ of a typical AHT cycle: (i) heat engine heat pump representation, (ii) the 2nd law of thermodynamic formulation, and (iii) complete preheating. The first two models are developed based on the reversible cycle approach, whereas the 3rd model incorporates adsorbed phase properties. Thus, the first two models might be considered as the formulations for the thermodynamic temperature limit (lift) of an AHT cycle while the 3rd model is specific to the nature of a particular adsorbent + adsorbate pair which might be close to practical applications. The reversible models predict a maximum temperature lift of 22∘C to 58∘C for heat source temperatures between 50∘C to 80∘C. The 3rd model exhibits lower values of maximum temperature lift compared to the reversible models, owing to the inclusion of material properties in its formulation. The performance of the models is demonstrated by determining the maximum temperature lift of four water-based adsorption working pairs, each featuring distinct IUPAC (International Union of Pure and Applied Chemistry) isotherm types. This study will help propel the working pair selection and the thermodynamic modeling of sorption cycles to achieve its near maximum capability.

Adsorption behavior of H2O on the strontium bromide surface: first-principles and molecular dynamics calculations.

Thermochemical adsorption heat storage based on gas-solid interaction is an energy storage technology for the effective recovery of industrial waste heat and renewable energy sources such as solar energy. Currently, hygroscopic salts and water are commonly used as the working pairs in thermochemical thermal storage systems. Strontium bromide (SrBr2) is a highly potential material for low-grade thermal energy storage and building applications due to its high sorption capacity and reaction enthalpy. In order to better understand the microscopic mechanism of adsorption, the water adsorption behavior of strontium bromide surfaces on the atomic scale is investigated in this study by using density functional theory (DFT). The effects of doping Ca or Mg into strontium bromide on water adsorption are analyzed by the comparison of the energy barrier, density of states (DOS) and crystal orbital Hamilton population (COHP) obtained before and after metal atom doping. The energy barrier of the SrBr2/H2O reaction system is 5.916 kcal mol-1, and the energy barrier of Ca-doped SrBr2 reduces to 3.432 kcal mol-1, whereas the energy barrier of Mg-doped SrBr2 increases to 13.394 kcal mol-1. The thermodynamic properties of strontium bromide are significantly improved by doping with calcium atoms. The absolute value of the COHP for Ca-doped SrBr2 decreases, which indicates that Ca-doped SrBr2 can adsorb the H2O molecules more easily at the same temperature. The doping of Ca atoms has a positive effect on the adsorption heat storage process. This study provides insight into the adsorption mechanism of water molecules on strontium bromide and could facilitate the design of efficient composite adsorbents.