Absorption Thermal Energy Storage Research Articles

Thermodynamic performance of CaCl2 absorption heat pump thermal energy storage system with triple storage tanks

Renewable energy source is playing an increasingly important role in district heating based on cogeneration units. Absorption thermal energy storage has the characteristics of high thermal energy storage density and low heat loss in long-term storage. In this paper, an absorption heat pump thermal energy storage system with CaCl2-water solution as the working fluid is proposed for solving the problem of insufficient wind power accommodations due to coal-fired cogeneration heat-power coupling. A steady-state thermodynamic model of the system is constructed in the Aspen Plus software. Under the considered design conditions, the volume of the tank required for one hour of operation of the system, thermal energy storage density, thermal energy storage ratio, and heating ratio are 464.61 m3, 29.81 kW h/m3, 46.50%, and 53.54%, respectively. The thermal energy storage density is 1.43 times and 1.25 times, and the tank volume is 0.7 times and 0.8 times, of those of a dual tank thermal energy storage system with H2O and CaCl2-water solution as the working fluids respectively. The effects of the system parameters on the thermal energy storage performance are simulated to obtain the optimal performance. At the optimum system performance, the tank volume required to operate the system for one hour, thermal energy storage density, heating ratio, and thermal energy storage ratio are found to be 439.56 m3, 37.39 kW h/m3, 44.83%, and 55.17%, respectively. Accordingly, the thermal energy storage performance of this system is also found to be superior to that of the traditional thermal energy storage system, which will be beneficial for reducing the volume of storage tanks in cogeneration units and increasing the thermal storage temperature.

Daily sorption thermal battery cycle for building applications

The study propose firstly an absorption thermal energy storage (ATES) system designed for repeated operations on a daily base without any additional manipulation. The performance of the sorption thermal battery was analyzed by varying four parameters (mass flow ratio, number of exchanger heat transfer units (NTU), heat source, and cooling water temperature). The lower the cooling water temperature, the better was the performance, particularly for improving the energy storage density (ESD). Additionally, if low-temperature cooling water is used, the coefficient of performance (COP) with a 65 °C heat source is comparable to that with 95 °C. ESD increased with the increasing the heat source temperature, and the maximum COP was obtained when using a 75–85 °C heat source. To solve the crystallization problem, [DMIM][DMP], a type of ionic liquid, was applied as an absorbent. However, the COP of H2O-[DMIM][DMP] with a heat source temperature of 120 °C was lower that of H2O–LiBr with 95–100 °C. Therefore, it is advantageous to use H2O–LiBr to improve the sorption thermal battery performance. ESD and COP of the sorption thermal battery are 451.13 kJ/kg and 0.728, respectively, when using a 95 °C heat source and 32 °C cooling water.

A double-effect/two-stage absorption refrigeration and thermal energy storage hybrid cycle using dual working fluids

• The hybrid cycle can get a generation temperature of 385 K and a COP of 2.37. • This cycle can work below 273.15 K with a high ESD over 300 kJ·kg −1 . • It realized combined cooling and heating for 24 h using dual working fluids. To improve the flexibility of absorption thermal energy storage (ATES) cycle, including lower the generation temperature, larger the operating temperature region and combined cooling and heating for 24 h, a double-effect/two-stage absorption refrigeration and thermal energy storage hybrid cycle using LiBr/H 2 O and LiBr-[BMIM]Br/C 2 H 5 OH working fluids is proposed in this paper. The cycle includes an absorption heat pump (AHP) sub cycle and an ATES sub cycle. Based on the measured thermophysical properties, the thermodynamic performance of this new cycle under different working conditions was calculated by MATLAB. According to the results, the hybrid cycle obtained a low generation temperature of 385 K, attributed to the utilization of the low-temperature working fluid LiBr-[BMIM]Br/C 2 H 5 OH. Moreover, the evaporation temperature of 268.15 K expanded the operating region of the hybrid cycle, and the highest COP and ESD of 2.37 and 394 kJ·kg −1 were obtained at that temperature, respectively. More importantly, the cycle realized combined cooling and heating all day long, with a cooling output temperature range of 268.15–278.15 K and a heating output temperature range of 321–344 K, which can fully meet the demands of commercial and civilian applications. To further verify the feasibility of the cycle, the model sensitivity and chemical potential achieved in the hybrid cycle were analyzed.

Evaluation of a high-performance evaporative cooler-assisted open three-phase absorption thermal energy storage cycle for cooling

Absorption thermal energy storage is promising for long-term storage of solar energy with both cooling and heating output purposes. Recently, the three-phase absorption thermal energy storage attracts significant attention due to its high energy storage density. However, its operation suffers from the vacuum operation and crystals blockage of piping systems, especially for the cooling output scenario. To resolve these issues, a three-phase absorption thermal energy storage in humid air for cooling is proposed, in which the dehumidification via three-phase absorption drives the evaporative cooling. Airflow is used for heat and mass transfer, which avoids vacuum operation and crystal blockage simultaneously. Comprehensive analysis considering the thermodynamic status of both working pair and humid air is carried out, with LiCl-H2O, LiBr-H2O, and CaCl2-H2O. Results showed that high ESD (energy storage density) of 563 kWh/m3 and COP (coefficient of performance) of 1.22 can be achieved under typical summer operating conditions, which are 3.2 and 2.1 times higher than that of the conventional cycle, respectively. Such simultaneous enhancement in ESD and COP has not been achieved by low-grade energy-driven sorption thermal storage yet, showing its great potential in real applications.

A novel modified LiCl solution for three-phase absorption thermal energy storage and its thermal and physical properties

• A novel LiCl solution for three-phase absorption thermal energy storage is proposed. • The energy storage density of the solution is enhanced by adding two types of additives. • The LiCl crystal slurry overcomes the fluidity problem caused by crystallization. • Thermal and physical properties of the modified solution are measured by experiments. Absorption thermal energy storage (TES) is gaining increasing attention due to its large energy storage density (ESD), mobility and long-term thermal storage capability. Expanding the working concentration difference of a solution can significantly enhance its ESD; however, this may result in crystallization, influencing fluidity and blocking flow channels in a TES device. Furthermore, bulky crystals are difficult to dissolve during energy discharge. In this paper, a novel modified LiCl solution is proposed for three-phase absorption TES, which allows the growth of fine crystals and the formation of well suspended slurry. For the first time, two types of additives with competing mechanism are introduced into the solution: ethylene glycol as a crystallization inhibitor and SiO 2 nanoparticles (SNPs) as a nucleating agent. Compared with traditional working fluids, the proposed solution has a larger ESD and good fluidity. The expanded concentration difference is determined by experiments on solubility and suspension. The optimal mass ratio of the LiCl solution, ethylene glycol and SiO 2 nanoparticles are obtained considering both ESD and fluidity. Heat and cold storage density using this novel LiCl crystal slurry can be enhanced by 24.8% and 156.0% respectively. Measurements on thermal and physical properties including vapor pressure, density and viscosity are also carried out.

Dynamic performance analysis of a solar driving absorption chiller integrated with absorption thermal energy storage

The use of a solar-powered absorption chiller for residential cooling is impeded by the instability and intermittency of solar energy with time. Integrating the absorption heat storage into the absorption refrigeration alleviates the mismatch between solar sources and the cold demand. This paper investigated a solar-powered absorption chiller with an absorption thermal energy storage system using a dynamic numerical model. The dynamic model is solved according to the nonlinear programming and the second-order iterative numerical integration program. Based on simulation and comparison results, the performance characteristics for the proposed system are discussed. The results indicate that the combination of an absorption chiller and absorption thermal energy storage is feasible. Compared with the reference system, the total refrigerating capacity and average performance coefficient for the proposed system are increased by 4.44% and 3.5%. Finally, the total cold storage capacity of the integrated system can reach 6890.4 MJ/day, which can extend the cooling time to about 14 h. This research provides insight into future work regarding the wide application for solar cooling.

Comparative dynamic performance of hybrid absorption thermal batteries using H2O/1,3-dimethylimidazolium dimethylphosphate

The basic absorption thermal energy storage cycle suffers from low energy storage efficiency and density, while the conventional H2O/salt working fluids risk crystallization problems. To achieve crystallization-free thermal batteries with improved energy storage performance, a hybrid compression-assisted absorption thermal energy storage cycle using H2O/1,3-dimethylimidazolium dimethylphosphate is proposed. The fluid property is modeled using non-random two liquid equations and the hybrid cycle is modeled using heat/mass transfer/conservation equations; both models are validated against measurement data with good accuracies. The effect of pressure ratio on the transient behaviors and energy storage density/efficiency is investigated to characterize the performance enhancement. Besides, different hybrid cycles are compared with the basic cycle to identify the best-performing thermal battery. Results show that, with both generation and absorption enhancement, the concentration difference is significantly enlarged from 0.230 to 0.480 by the hybrid cycle with a pressure ratio of 2.0. Meanwhile, with energy storage efficiencies maintained at 0.715–0.794, the energy storage density is effectively increased from 110.3 to 199.2 kWh/m3. Through valve switching, three hybrid cycles are realized, i.e., hybrid cycle with charging/discharging compression, hybrid cycle with charging compression, and hybrid cycle with discharging compression. Comparisons indicate that the hybrid cycle with discharging compression is the most energy-efficient, with the highest energy storage efficiency of 0.816, compared to 0.794 of the hybrid cycle with charging/discharging compression and 0.790 of the hybrid cycle with charging compression. In terms of energy storage density, the hybrid cycle with discharging compression performs quite close to the hybrid cycle with charging/discharging compression, only slightly lower by 1.6–4.3%. However, compared to the hybrid cycle with charging compression, the energy storage density of the hybrid cycle with discharging compression is significantly enhanced by 11.4–52.0%. In summary, the hybrid cycle with discharging compression is the best cycle comprehensively considering the energy storage efficiency and density. This study aims to provide theoretical supports and suggestions for the development of advanced thermal battery cycles.

A hybrid compression-assisted absorption thermal battery with high energy storage density/efficiency and low charging temperature

With worsening of global warming, environmental pollution, and energy crisis, the effective storage of renewable/waste energy has become a widely focused research topic. As an emerging thermal battery technology, absorption thermal energy storage aims to utilize low-grade energy for flexible applications (e.g., cooling, heating, dehumidification), which facilitates the matching between the energy supply and the energy demand. However, the current absorption thermal battery cycle suffers from high charging temperature, slow charging/discharging rate, low energy storage efficiency, or low energy storage density. To further improve the storage performance, a hybrid compression-assisted absorption thermal energy storage cycle is proposed in this work. Four thermal battery cycles, with/without compression in the charging/discharging processes, have been designed for comparisons. Dynamic characteristics and storage performance have been comparatively investigated by simulation using an experimentally validated model. Results show that the cycles with auxiliary compression can achieve a higher energy storage efficiency and density with a faster charging/discharging rate under a lower charging temperature. With a charging temperature of 80 °C, the energy storage efficiency and density are as high as 0.67 and 282.8 kWh/m3 for the proposed compression-assisted cycle, while they are only 0.58 and 104.8 kWh/m3 for the basic cycle. Moreover, the average charging and discharging rates of the compression-assisted cycle are 6.78 kW and 4.88 kW, respectively, which are also enhanced significantly compared to 1.88 kW and 1.27 kW of the basic cycle. This study could facilitate the development of absorption thermal battery with lower charging temperatures.

Double-effect/two-stage compression-assisted absorption thermal energy storage using LiNO3-ionic liquids/H2O working fluids

A novel double-effect/two-stage compression-assisted absorption thermal energy storage system using LiNO3-[BMIM] (1-butyl-3-methylimidazolium) NO3/H2O for combined cooling and heating is proposed to enhance thermal performance under a larger concentration glide without crystallization. The thermodynamic calculation is conducted by Matlab based on the measured thermophysical properties, and its performance is compared with that of a system using other working fluids. The results show that this proposed system using LiNO3-ionic liquids/H2O with compressor assistance achieves a higher coefficient of performance and energy storage density at a larger concentration glide without crystallization. In comparison to LiBr/H2O, the generation temperature of this system using LiNO3-ionic liquids/H2O is reduced by over 50 K at a larger concentration f, which is beneficial for the utilization of solar energy. As the discharging temperature increases from 321.15 K to 333.15 K and f from 0.08 to 0.20, this novel system using LiNO3-[BMIM]NO3/H2O obtains the largest coefficient of performance of 1.36 and exergetic coefficient of performance of 0.46 at generation temperature below 354.2 K, compared to LiNO3/H2O and LiNO3-[DMIM][DMP]/H2O. Moreover, the maximum energy storage density based on LiNO3-[BMIM]NO3/H2O reaches 695.4 kJ/kg at a discharging temperature of 333.15 K and a f of 0.20. In comparison to the existing absorption thermal energy storage systems, the proposed system using LiNO3-[BMIM]NO3/H2O also shows outstanding performance in various aspects. This new system with the LiNO3-[BMIM]NO3/H2O alternative working fluid has a great potential for utilizing solar energy for combined cooling and heating.

Dynamic characteristics and performance improvement of a high-efficiency double-effect thermal battery for cooling and heating

Recently increasing interests have been attached to the thermal batteries based on absorption thermal energy storage for renewable and waste energy utilization to address the mismatch between the energy supply and demand. The existing studies mainly focused on the improvement of energy storage density, while there are rare researches on lifting energy storage efficiency which is also a challenge. The motivation of this paper is to improve energy storage efficiency to promote practical applications of the thermal batteries. A novel double-effect absorption thermal energy storage system is proposed. With the assumption of well-conditioned external fluids, a mathematical model is established and verified by experimental data. The dynamic characteristics are presented and the results show that the charging time of the double-effect system is significantly shortened from 1.91 h to 0.68 h in the high-temperature (160 °C–200 °C) range, being shorter than that of the single-effect system when the charging temperature is 200 °C. Besides, the energy storage efficiency varies from 1.27 to 1.17 and from 1.44 to 1.33 for cooling and heating, respectively, being enhanced significantly compared to the single-effect systems, by 57.1%–61.6% for cooling and 58.2%–61.8% for heating. The slight reductions in energy storage density are 3.2%–6.9% and 2.7%–6.0% for cooling and heating, respectively. The solution mass distribution of the double-effect system has also been explored, indicating that the equal-distribution is comprehensively the best scheme. This study can provide theoretical references and suggestions for the applications of absorption thermal energy storage with high-temperature heat sources.

Low-temperature compression-assisted absorption thermal energy storage using ionic liquids

Abstract Thermal energy storage technologies play a significant role in building energy efficiency by balancing the mismatch between renewable energy supply and building energy demand. The absorption thermal energy storage (ATES) stands out due to its high energy storage density (ESD), high coefficient of performance (COP), low charging temperature and wider application flexibility. A hybrid compression-assisted ATES (CATES) using ionic liquid (IL)-based working fluids is investigated to address the problems of the existing ATES cycle. Models for mixture property and cycle performance are established with verified accuracies. Four ILs ([DMIM][DMP], [EMIM][Ac], [EMIM][DEP], and [EMIM][EtSO4]) are compared with H2O/LiBr. Results show that the CATES effectively avoid the crystallization, decreases the circulation ratio, lowers the charging temperature, and improves the COP/ESD. H2O/[DMIM][DMP] has the highest COP and performs better than H2O/LiBr with generation temperatures above 86 °C, while H2O/[EMIM][EtSO4] shows the highest COP with generation temperatures below 75 °C. Among the H2O/IL mixtures, H2O/[EMIM][Ac] shows the highest ESD with generation temperatures above 86 °C, otherwise H2O/[EMIM][EtSO4] shows the highest. The optimal compression ratio is 1.6–2.8 for H2O/[DMIM][DMP] under the generation temperatures of 90–70 °C with the maximum COP of 0.758–0.727. The ESD increases significantly with the compression ratio.

Screening of novel water/ionic liquid working fluids for absorption thermal energy storage in cooling systems

Screening of novel water/ionic liquid working fluids for absorption thermal energy storage in cooling systems

Charging and discharging characteristics of absorption thermal energy storage using ionic-liquid-based working fluids

The absorption thermal energy storage (ATES) systems using H2O/ionic liquid (IL) mixtures as novel working fluids are explored to avoid the crystallization problem. The property model and cycle model are established and validated against experimental data. The dynamic charging/discharging characteristics and overall cycle performance are compared for four ILs ([DMIM][DMP], [EMIM][Ac], [EMIM][DEP], and [EMIM][EtSO4]). Under a typical condition, [DMIM][DMP] yields the highest coefficient of performance (COP) of 0.722 while [EMIM][DEP] yields the lowest COP of 0.603; [DMIM][DMP] shows the highest energy storage density (ESD) of 94.1 kW h/m3 while [EMIM][DEP] shows the lowest ESD of 77.5 kW h/m3; [EMIM][Ac] needs the longest charging time of 107.5 min, while [EMIM][EtSO4] needs the longest discharging time of 207.0 min. Being the best-performing IL in terms of high COP and ESD, [DMIM][DMP] has been further investigated, with charging temperatures of 85–100 °C, cooling water temperatures of 25–35 °C and discharging temperatures of 9–15 °C. The highest COP is 0.761 and the highest ESD is 149.5 kW h/m3 in the investigated operating conditions. In summary, it is feasible to use H2O/ILs as crystallization-free working fluids of the ATES systems. This study aims to provide theoretical references and suggestions for the selection of novel working fluids for the ATES systems.

Performance analysis of absorption thermal energy storage for distributed energy systems

In recent years, distributed energy systems (DES) have attracted worldwide attention. Distributed generation unit (DG) in DES usually works under part load during night, which results in low efficiency and the waste heat of DG cannot be fully utilized. This study proposes a novel absorption thermal energy storage system together with electric energy storage for distributed energy systems. The proposed absorption thermal energy storage system which is a combination of absorption chiller and liquid storage tanks, has a higher energy storage density. During off-peak hours, the extra electricity of DG can be used by electric chillers (EC) and the extra waste heat of DG is stored by the proposed absorption thermal energy storage system. The stored thermal energy is released in peak hours to meet the cooling loads of buildings. A case study of DES in a campus under cooling-dominated climate is conducted to evaluate the performance of the proposed system. The results in a typical summer day indicate that the DG utilization rate increases from 80% to 92.9%, meanwhile the required capacities of electric chillers can be obviously reduced. The operating cost of DES also reduces by 12.9% compared with the DES without energy storage. Through appropriate operation strategy, off-the-grid operation for DES can be achieved without energy waste by applying the proposed energy storage method.

A New Perspective of Analysis and Optimization for Absorption Thermal Energy Storage System Based on Entransy Theory

Abstract Absorption thermal energy storage system is a new technology with promising future. This contribution proposes a new perspective to analyze this kind system and introduces the entransy dissipation-based thermal resistance theory to optimize the system. The analogy between heat transfer and electric conductance, integration of the heat transfer and the phase change yield a new physical image and a direct mathematical model to describe the physical relation between the design requirements and the unknown design parameters, i.e. the heat transfer area, the heat capacity of each heat exchanger. Then the optimization problem converts to a conditional extremum problem applying the mathematical model. Finally, two examples are taken to testify the validity of the new method.

Experimental investigation on charging and discharging performance of absorption thermal energy storage system

Because of high thermal storage density and little heat loss, absorption thermal energy storage (ATES) is known as a potential thermal energy storage (TES) technology. To investigate the performance of the ATES system with LiBr–H2O, a prototype with 10kWh cooling storage capacity was designed and built. The experiments demonstrated that charging and discharging processes are successful in producing 7°C chilled water, 65°C domestic hot water, or 43°C heating water to meet the user’s requirements. Characteristics such as temperature, concentration and power variation of the ATES system during charging and discharging processes were investigated. The performance of the ATES system for supplying cooling, heating or domestic hot water was analyzed and compared. The results indicate that the energy storage efficiencies (ESE) for cooling, domestic hot water and heating are 0.51, 0.97, 1.03, respectively, and the energy storage densities (ESD) for cooling, domestic hot water and heating reach 42, 88, 110kWh/m3, respectively. The performance is better than those of previous TES systems, which proves that the ATES system using LiBr–H2O may be a good option for thermal energy storage.

Study on LiBr-H<sub>2</sub>O Absorption Refrigeration System with Integral Storage

The absorption thermal energy storage (TES) system stores the energy in the form of potential energy of solution and is a promising technology for efficient energy transformation process. The performance of the absorption refrigeration system with integral storage for cooling applications using LiBr-H2O as working pair under the condition without crystallization was analyzed on the basis of the first law of thermodynamics. Simulation was employed to determine the coefficient of performance (COP) and energy storage density (ESD) of the absorption TES system under different conditions such as the absorption temperature and storage temperature. The results show that the COP of the system is 0.7453 and ESD is 169.853 MJ/m3 under typical operation conditions in summer. A low absorption temperature yields both a higher COP and ESD. The solution heat exchanger could improve the COP of the system while has no effect on ESD. Results also showed that system has a good advantage when compared to other storage methods since it is do no need thermal insulation. The absorption TES may be considered as one of the promising thermal energy storage methods.

Performance analysis of pressurized absorption thermal storage equipment

Due to its high thermal storage density and little heat loss, absorption thermal energy storage (ATES) is known as a potential thermal energy storage technology. However, current equipment has poor absorption ability and suffers from low efficiency. In this paper, a pressurized absorption thermal storage technology is proposed and the principle is introduced. Effects of pressurization on the thermodynamic performance are analyzed under different operating conditions. The results indicate that the utilization of pressurization can largely enhance the coefficient of performance of ATES system when the evaporating and generating temperatures are low and condensing temperature is high.comparing with the absorption storage cycle without pressurization, a cycle with pressurization can increase the energy storage density by 30%-295% at the pressure ratio of 3. © All Rights Reserved.

吸收式化学蓄能的研究综述

Absorption thermal energy storage technology has the advantages of high energy storage density and negligible heat loss. It is a promising thermal energy storage method that can be applied in renewable energy effective utilization such as solar thermal energy and low temperature waste heat utilization such as industrial waste heat and waste heat from combined cooling, heating and power system. The basic principle and characteristics of the Absorption thermal energy storage are introduced firstly. The research status and developments in working pairs selection, cycles investigation, theoretical and experimental studies are summarized in detail. Lastly, the key technologies on absorption thermal energy storage needed to research are pointed out.