Thermochemical Heat Storage Systems Research Articles

New hybrid thermal energy storage unit using dual hydrides with cascaded PCMs: Application to CSP plants

Thermal energy storage is necessary for concentrated solar power (CSP) plants; it's a useful technique for reducing fluctuations in the energy supply and aids in peak demand management. Therefore, in the present paper, a novel Hybrid Cascaded Thermal Energy Storage (Hyb-CTES) unit is proposed for use in solar-driven Rankine steam power plant. The concept of the heat storage hybridization examined in this study is to couple metal hydride-based thermochemical heat storage system with cascaded phase change materials-based latent heat storage. To assess the dynamic behavior of the Hyb-CTES unit, a two-dimensional transient model is established, and a computational code is developed. The mathematical model has been validated successfully by comparing it with experimental data and by verifying both the mass and energy balance. To choose the appropriate PCM's melting temperature, four approaches are proposed and tested. Numerical simulations were performed with the help of the developed computer code, and the findings demonstrated that the supplied hydrogen pressure and the temperatures of the metal hydride beds are the crucial parameters for determining the PCM melting temperature. In addition, it is found that the proposed cascaded PCMs configuration enhances the heat storage unit's thermal performance and can reduce the heat charge/discharge cycle duration by 19 %. Also, the results indicate that the new Hyb-CTES unit is a promising alternative for thermal storage for CSP plants.

Thermochemical heat storage performance of Fe-doped MgO/Mg(OH)2: Experimental and DFT investigation

MgO/Mg(OH)2 thermochemical heat storage system can realize large-scale energy storage by repetitive hydration/dehydration reactions to enhance the stability of solar energy utilization. However, slow interconversion and incomplete reactions caused by agglomeration restrict the cyclic heat storage capability of MgO/Mg(OH)2. In this work, a low-cost efficient Fe-doped MgO was prepared by a simple wet-mixing method and its energy released performance and kinetics in MgO/Mg(OH)2 heat storage progress were examined in a twin fixed-bed reactor and a simultaneous thermal analyzer. Additionally, density functional theory calculations were used to examine the impact of Fe on the mechanisms of MgO/Mg(OH)2 heat storage process. The results shows that when the molar ratio of MgO to Fe is 95:5, the energy released density of Fe-doped MgO is about 400 % higher than that of MgO after 10 heat storage cycles. In preparation process, the existence of Fe leads to the generation of MgFe2O4, which forms layered microstructure with the high specific surface area and pore volume and mitigates agglomeration behavior of MgO. It improves the cyclic stability of MgO and reduces the dehydration temperature of Mg(OH)2, and simultaneously the activation energy of the dehydration stage is reduced by 33.2 %. Fe enhances the interaction of H2O with the MgO surface and promotes the dissociation of H2O on Mg(OH)2 surface. Combined with economic analysis, Fe-doped MgO is a suitable material for MgO/Mg(OH)2 heat storage cycles.

Sodium acetate-based thermochemical energy storage with low charging temperature and enhanced power density

The electrification of heat necessitates the development of innovative domestic heat batteries to effectively balance energy demand with renewable power supply. Thermochemical heat storage systems show great promise in supporting the electrification of heating, thanks to their high thermal energy storage density and minimal thermal losses. Among these systems, salt hydrate-based thermochemical systems are particularly appealing. However, they do suffer from slow hydration kinetics in the presence of steam, which limits the achievable power density. Additionally, their relatively high dehydration temperature hinders their application in supporting heating systems. Furthermore, there are still challenges regarding the appropriate thermodynamic, physical, kinetic, chemical, and economic requirements for implementing these systems in heating applications. This study analyzes a proposal for thermochemical energy storage based on the direct hydration of sodium acetate with liquid water. The proposed scheme satisfies numerous requirements for heating applications. By directly adding liquid water to the salt, an unprecedented power density of 5.96 W/g is achieved, nearly two orders of magnitude higher than previously reported for other salt-based systems that utilize steam. Albeit the reactivity drops as a consequence of deliquescence and particle aggregation, it has been shown that this deactivation can be effectively mitigated by incorporating 10 % silica, achieving lower but stable energy and power density values. Furthermore, unlike other salts studied previously, sodium acetate can be fully dehydrated at temperatures within the ideal range for electrified heating systems such as heat pumps (40 °C – 60 °C). The performance of the proposed scheme in terms of dehydration, hydration, and multicyclic behavior is determined through experimental analysis.

Sodium thermal electrochemical converter coupled with organic Rankine cycle and thermochemical heat storage for power-heat-power application

The inherent intermittency of solar PV and wind sources makes the power supply from renewables unstable. To mitigate this issue, power-heat-power (PHP) systems have been developed in which surplus/cheap electricity can be stored in the form of high-temperature heat, and whenever needed it can be used for electricity generation using a power cycle. The present work focuses on the integration of an innovative combined cycle consisting of a sodium thermal electrochemical converter (Na-TEC) in combination with an organic Rankine cycle (ORC) as a bottoming cycle, with a redox oxide-based thermochemical heat storage (TCHS) system for PHP application. A Multiphysics model is developed to simulate and analyze the performance of the proposed combined cycle. Parametric optimization is performed to achieve optimal system performance. The proposed Na-TEC/ORC achieved a maximum power density of 0.98 W/cm2 and a maximum conversion efficiency of 40 % which is 21.2 % higher in comparison to a standalone Na-TEC device. A maximum exergetic efficiency of 53 % can be achieved while keeping the evaporator temperature at 1150 K and the condenser temperature at 550 K for this cycle. For the maximum conversion efficiency of Na-TEC, a round-trip-efficiency of 36.1 % is calculated for the proposed PHP system provided the TCHS system is 95 % efficient.

Simulation-based analysis of thermochemical heat storage feasibility in third-generation district heating systems: Case study of Enschede, Netherlands

In this article a dual heat storage system comprising thermochemical heat storage (TCS) and hot water storage for managing the mismatch between heat generation and demand in district heating systems (DHs) is evaluated. TCS is known as technology suitable for long-term heat storage due to its high energy density and negligible heat losses over a longer period. However, the integration of TCS in DHs is significantly influenced by the operating conditions of DHs. Here we evaluate the feasibility of integrating TCS into DHs in the Enschede region of the Netherlands. DHs models are established to simulate heat generation, demand, and storage, and a control strategy is designed to manage storage coordination. The obtained results show that the dual storage system outperforms the single hot water storage system in reducing peak load generation. Depending on the TCS's operational condition, annual energy generation from peak load in dual storage systems could drop by 30–60 % compared to the single hot water storage system. It is achieved mainly by managing the energy capacity remaining in the storage system. The technical feasibility and benefits of implementing a TCS system in DHs with a dual storage system are shown to be more energy efficient.

Bi-salts composites to enhance the hydration kinetics and heat storage capacity

In this study, salt hydrates were deposited on a porous carbon (PC) support to address the challenges associated with maintaining heat capacity and slow hydration kinetics in thermochemical heat storage (THS) systems. Mono-salt (MgSO4) or binary salt (MgSO4-MgCl2) composites on the PC were analyzed by thermogravimetry/differential scanning calorimetry (TG/DSC). The results show that binary salt composites exhibit faster hydration kinetics and higher water adsorption capacity than monosalt composites. The addition of MgCl2 overcomes the kinetic drawbacks caused by the hydrophobic nature of the support and the presence of salt aggregates. The PC_60binary composite achieves a water uptake of 0.75 g/g, exceeding the water uptake of PC_60MgSO4 (0.48 g/g). Furthermore, the heat released by the composites follows a similar trend to water sorption. PC_60MgSO4 produces 52 % (1356 J/g) of the heat released by pure MgSO4, while PC_60binary achieves 70 % (1840 J/g) of the heat released by pure mixed salts. These results suggest that the newly developed binary salt composites are promising for long-term heat storage applications. These experimental results highlight the improved performance of binary salt composites in terms of hydration kinetics, water adsorption and heat release. This study provides valuable pointers for the development of more efficient THS systems.

A design optimization method for solar-driven thermochemical storage systems based on building performance simulation

The challenge of the temporal mismatch between energy supply and demand in buildings is growing with the increasing share of renewable energy in total energy consumption. Among all the state-of-the-art energy storage solutions, thermochemical heat storage shows a unique potential thanks to its considerable energy density, acceptable cost, and negligible heat loss. For this reason, it becomes a promising alternative to common sensible heat storage solutions for building applications. The integration of such a novel technology in buildings necessitates a method for the assessment of its potential impact and benefit, the comparison to common alternatives, and the optimization of the system design. This work proposes a method based on modeling and simulation of the interaction between the thermochemical heat storage system and the building using a data-driven surrogate model of the storage system in combination with a building performance simulation engine. The data-driven model was developed and validated based on laboratory measurements of a novel closed-loop thermochemical heat storage system, the heat battery (HB). The method was demonstrated in a case study to identify the optimal size of the HB in a solar-driven configuration based on a residential building use case. The results show that the heat battery can digest the thermal energy transferred from the solar thermal collector to reduce the original electricity consumption for heating the detached house (0.7 MWh to 1.0 MWh in considered cases) without any obvious sacrifice in thermal comfort and that the small-scale HB (with a storage volume below 160 l) shows efficient usage of the designed storage capacity.

Corn Cobs' Biochar as Green Host of Salt Hydrates for Enhancing the Water Sorption Kinetics in Thermochemical Heat Storage Systems.

Heat storage technologies are essential for increasing the use of solar energy in the household sector. Their development can be achieved by designing new storage materials; one way is to impregnate a porous matrix with hygroscopic salts. In this article, the possibility of using biochar-based composite sorbents to develop promising new heat storage materials for efficient thermal storage is explored. Biochar-based composites with defined salt loadings (5, 10, 15, and 20%) were produced by impregnating MgSO4 into a biochar matrix derived from corn cobs. The new materials demonstrated a high water sorption capacity of 0.24 g/g (20MgCC). After six successive charging-discharging cycles (dehydration/dehydration cycles), only a negligible variation of the heat released and the water uptake was measured, confirming the absence of deactivation of 20MgCC upon cycling. The new 20MgCC composite showed an energy storage density of 635 J/g (Tads = 30 °C and RH = 60%), higher than that of other composites containing a similar amount of hydrate salt. The macroporous nature of this biochar increases the available surface for salt deposition. During the hydration step, the water molecules effectively diffuse through a homogeneous layer of salt, as described by the intra-particle model applied in this work. The new efficient biochar-based composites open a low-carbon path for the production of sustainable thermal energy storage materials and applications.

Numerical Simulation of the Ca(OH)2/CaO Thermochemical Heat Storage Process in an Internal Heating Fixed-Bed Reactor

Using a Ca(OH)2/CaO thermochemical heat storage system is an effective way to promote the utilization of renewable energy. However, poor thermal conductivity restricts the application of a widely used fixed-bed reactor. To improve the heat storage rate, the internal heating mode, which heats the reactant via the internal heating tube instead of the external wall, was adopted, and the heat storage process in the fixed-bed reactor was investigated numerically. The results show that the number and location of tubes have a significant impact on heat storage performance. Compared with the external wall heating mode, the optimized scheme of six internal heating tubes can shorten the reaction time by 21.78%. The temperature and reaction extent distribution reveal that as the reaction proceeds, the optimized scheme has a higher temperature and reaction extent. Additionally, the effects of different conditions, such as solid particle porosity, wall temperature, outlet pressure, and solid particle size, were also analyzed. The study demonstrates that increases in solid particle porosity, wall temperature, and solid particle size as well as a decrease in outlet pressure can improve the heat storage rate.

Biomass power plant integrated with a thermochemical heat storage system using a fluidized bed reactor worked by variable renewable energy: Dynamic simulation, energetic and economic analyses

This study focused on a biomass power plant (Organic Rankine Cycle: ORC, 1 MWe) integrated with a thermochemical heat storage (TCS) system as a Carnot battery. In the TCS, a fluidized bed reactor using CaO/Ca(OH)2/alumina particles was used, and steam was used as a fluidizing and reacting gas. The surplus electricity from variable renewable energy (VRE) is converted to heat and stored during daytime, and the stored energy is utilized to generate power in the evening. In this study, there were three objectives. First, process performances were compared when R245fa or R1233zd was used as a working fluid of ORC, and influences of ORC parameters such as the turbine inlet temperature, superheat temperature, and the scale of the power generation, were evaluated. In addition, effects of the fluidized bed volume and the reduction of biomass fuel combustion were also investigated. Second, a potential of the proposed process that can absorb the VRE fluctuation was evaluated by the dynamic calculation. Third, the economics of the TCS system was evaluated. Results show that the round-trip efficiency of the process using R1233zd was 8.77 %, which was slightly lower than that of the process using R245fa (8.84 %). The increases of a turbine inlet temperature led to the increase of heat recovery from the outlet steam of the TCS reactor, meanwhile, the round-trip efficiency decreased. The efficiencies of the cases changing the superheat temperature (5 → 15 °C) were slightly changed (<0.5 %). To achieve high energy efficiencies, a large-scale biomass power plant and a small-fluidized bed volume was effective. It was found that the VRE fluctuation was mostly absorbed by the TCS with a fluidized bed and ORC process. In the economic evaluations, the levelized cost of storage regarding only the TCS system was 0.92–2.37 USD/kWhe when the charging electricity cost is 0.05 USD/kWhe. LCOS also decreases to 0.50–1.14 USD/kWhe if the electricity cost is free during daytime in the future and power output is two times in a day.

Modelling of 4th generation district heating systems integrated with different thermal energy storage technologies – Methodology

A model was established that allows the simulation and analysis of the performance of a district heating (DH) system supplied with heat generated from renewable sources (RHS) and a selection of different types of thermal energy storage (TES) systems. The model uses hourly weather data, building specifications and occupancies to calculate i) hourly domestic heat demands for both space and domestic hot water heating and ii) hourly heat produced by RHS. The heat flow to and from sources to loads and TES systems is calculated on an hourly basis. The model calculates hourly heat losses from dwellings, TES and pipes, and takes into consideration recent research related to latent and thermochemical heat storage systems. The model developed enables the analysis of the effects of different district heating system operating parameters (including installed capacity and penetration of different RHS, thermal energy storage capacity, number and distribution of TES) on system cost and system energy efficiency. The ultimate goal of developing the model was the determination of a system specification and operating conditions that i) minimises cost and ii) maximises the overall energy efficiency of the network while ensuring that domestic heat demands are met on an hourly basis for a specified time-period in an existing or planned town or urban area.

Experimental investigation on a strontium chloride/ammonia sorption heat storage system

• A thermochemical sorption heat storage system has been established. • The total highest heat storage density is 1871.03 kJ/kg SrCl 2 composite sorbent. • The total highest heat storage efficiency of the sorption system is 92.74%. Thermochemical sorption heat storage based on solid-gas interaction is considered as a cutting-edge technology for harvesting solar energy and industrial waste heat due to the significant advantages of high heat storage density, flexible working modes and long-term storage capacity with negligible heat loss. As a vital technology to enhance energy infrastructures, the thermochemical sorption heat storage has aroused extensive interest in the past decade. In this work, a solid-gas thermochemical sorption system was established and investigated for recovering the thermal energy. Working pair of strontium chloride/ammonia was selected, and strontium chloride composite sorbents were synthesized using the expanded graphite as matrix. The heat storage performances of strontium chloride/ammonia sorption system at the different temperatures were experimentally investigated. Under the condition of 92 °C charging temperature, 40 °C discharging temperature and -5 ℃ condensation/evaporation temperature, the highest sorption heat storage density of the sorption system obtained is up to 1630.78 kJ/kg SrCl 2 (or 1386.16 kJ/kg composite sorbent), and the total heat storage density can reach 2201.21 kJ/kg SrCl 2 (or 1871.03 kJ/kg composite sorbent). The total energy efficiency of the sorption system increases from 74.71% to 92.74% with the heat storage temperature rising from 68 ℃ to 92 ℃. Results indicated that strontium chloride and its composites are promising heat storage materials, and revealed great potentials of thermochemical sorption heat storage technology for rational and efficient utilization of thermal energy.

Investigation of the impact of zeolite shaping and salt deposition on the characteristics and performance of composite thermochemical heat storage systems

13X zeolite in powder displays higher storage performances and faster hydration kinetics than 13X in beads. Salt-doped zeolite composite in powder form presents lower pore blockage and higher storage performances than composites in beads.

Life Cycle Assessment of Sensible, Latent and Thermochemical Thermal Energy Storage Systems for Climate Change Mitigation – A Systematic Review

Renewable energy sources coupled with thermal energy storage (TES) systems offer a better hope in mitigating climate change. But, in order to integrate TES systems into the grid, it is important to understand their environmental performances. Life cycle assessment (LCA) serves as a leading methodological tool for environmental decision-making processes that allows one to identify the critical areas of improvement in a product life cycle, and hence can be used effectively in climate change mitigation strategies. Due to the scarcity of review articles that provide useful information on the LCAs of TES systems, a total of 23 papers were reviewed in this study. These were reviewed under three categories: sensible heat storage (SHS) systems, latent heat storage (LHS) systems, and thermochemical heat storage (THS) systems. Further, the greenhouse gas (GHG) emissions arising from TES systems were evaluated, giving special attention on the global warming potential impact category. The production stage was found to be the major contributor to GWP in all three TES systems. Following this review study, it can be concluded that the environmental performance can be greatly enhanced through TES systems, due to significant reductions in GHG emissions. Keywords: LCA, thermal energy storage, sensible heat, latent heat, thermochemical heat, PCMs DOI: 10.7176/JETP/12-5-02 Publication date: November 30 th 2022

Crystal growth, crystal structure determination, and computational studies of a new mixed (NH4)2Mn1–xZnx(SO4)2(H2O)6 Tutton salt

Tutton salts have been extensively explored in recent decades due to their attractive physical and chemical properties, which make them potential candidates for thermochemical heat storage systems and optical technologies. In this paper, a series of new mixed Tutton salts with the chemical formula (NH4)2Mn1–xZnx(SO4)2(H2O)6 is reported. Crystals are successfully grown by the solvent slow evaporation method and characterized by powder X-ray diffraction (PXRD) with Rietveld refinement. In particular, the crystal structure of the mixed (NH4)2Mn0.5Zn0.5(SO4)2(H2O)6 crystal is solved through PRXD data using the DICVOL06 algorithm for diffraction pattern indexing and the Le Bail method for lattice parameter and spatial group determination. The structure is refined using the Rietveld method implemented in TOPAS® and reported in the Cambridge Structural Database file number 2104098. Moreover, a computational study using Hirshfeld surface and crystal void analyses is conducted to identify and quantify the intermolecular interactions in the crystal structure as well as to determine the amount of free space in the unit cell. Furthermore, 2D-fingerprint plots are generated to evaluate the main intermolecular contacts that stabilize the crystal lattice. Density functional theory is employed to calculate the structural, thermodynamic, and electronic properties of the coordination [Zn(H2O)6]2+ and [Mn(H2O)6]2+ complexes present in the salts. Molecular orbitals, bond lengths, and the Jahn–Teller effect are also discussed. The findings suggest that in Mn-Zn salts several properties dependent on the electronic structure can be tuned up by modifying the chemical composition.

Investigation of open-type 1 kW-scale thermochemical heat storage system with zeolite 13X for power-to-heat applications

Investigation of open-type 1 kW-scale thermochemical heat storage system with zeolite 13X for power-to-heat applications

Experimental investigation of a solar-charged sorption thermal battery

In the present study, an integrated solar-driven sorption energy storage unit was investigated under real climatic conditions in Famagusta, North Cyprus. Vermiculite–CaCl2 was selected as the working material due to its high energy density, good cyclic ability and low cost. Three series of charging–discharging experiments were performed employing the developed solar-driven thermochemical heat storage system. The discharging tests were performed during the night for duration of 5 h. During the discharging experiments, temperature and relative humidity of inlet air were between 21 and 24°C and 80% and 90%, respectively. Under these conditions, the average energy output over three cycles was found to be 2.1 kWh, with an average energy storage density of 156 kWh/m3. A manufactured parabolic solar concentrator was utilized to provide the required thermal energy in the charging process. Hence, the average surface temperature of the reactor was in the range of 78°C–83°C. The average solar energy supply and desorption energy utilized in the charging process were 5.56 and 1.75 kWh, respectively. The rate of moisture desorption was 6.5 g/min while the thermo-cyclic efficiency was 38%.

Thermo-physical properties measurements of hygroscopic and reactive material (zeolite 13X) for open adsorptive heat storage operation

The determination of the thermo-physical properties (density, specific heat capacity, and thermal conductivity) of hygroscopic and reactive solid materials, as those applied in sorption thermochemical heat storage systems, is not trivial. Lack of precision in the measurement and contradictory results make it difficult to enter these thermal parameters in the programs used for simulating the functioning and operating scenarios of thermochemical heat storage devices. For this reason, different techniques have been applied and compared in order to improve the accuracy of the measurement. In particular, there was a difference in thermal conductivity (about 30%) between the hot disc experiments and the others, as well as a higher dispersion of the experimental data (MAD > 19%). The same was found for specific heat capacity (MAD ≈14%). Hence, the transient hot disk method is not suitable for estimating thermal conductivity and the specific heat capacity of this type of material. Nevertheless, this technique enables a good estimation of the thermal effusivity. The thermal conductivity and the specific heat capacity were investigated by the hot wire method and by TG-DSC, respectively. A slight increase in specific heat with temperature was observed, but in the range of 70–95 °C, this value is constant at 913 J kg−1 K−1. The applied methodology was validated by comparing the experimental and calculated thermal effusivity values.

Investigation on a Vermiculite-Based Solar Thermochemical Heat Storage System for Building Applications

Investigation on a Vermiculite-Based Solar Thermochemical Heat Storage System for Building Applications

Energy and exergy analyses of ammoniated salts based thermochemical sorption heat storage system

Thermal energy storage (TES) is a critical technology to improve energy infrastructures by harvesting low-grade heat and renewable energy. Thermochemical sorption heat storage (TSHS) is considered to be the most promising TES technology due to excellent energy storage performance and flexible working mode. Ammoniated salts with high adsorption capacity and reaction enthalpy are favorable heat storage materials. Using energy and exergy analyses to evaluate heat storage performance is conducive to the highly utilization of the energy and optimization of the energy system. In this work, the thermodynamic performance of the sorption heat storage cycle with ammoniated salts was surveyed through the energy and exergy analysis methods. The general efficiencies of four salts for the sorption heat storage during charging, discharging and the whole system were estimated. Results showed that SrCl2 and SrBr2 are expected to be the most promising materials for the sorption heat storage among the four chemical salts. The overall energy efficiencies of SrCl2-NH3 and SrBr2-NH3 sorption heat storage systems are 39.8% and 41.6%, respectively, and the corresponding overall exergy efficiency are 17.3% and 13%. The Carnot factor-induced efficiency of SrCl2-NH3 and SrBr2-NH3 sorption heat storage systems are 0.123 and 0.087, respectively.