Thermochemical Storage Research Articles

Characterisation of magnesium, zinc and iron sulfates for thermochemical storage

Thermochemical heat-storage materials such as magnesium, zinc and iron sulfates offer high energy-storage densities and a reliable means of long-term storage of solar energy. In this research study, the dynamic properties of these three salts were investigated based on their hydration and dehydration characteristics. To investigate the effect of regeneration temperatures on the physio-chemical properties of the materials, four sets of temperatures were used. At 100°C, zinc sulfate showed a significant loss of water as compared to magnesium and iron salts, while in the hydration process at lower temperatures, zinc sulfate showed the highest sorption enthalpy and energy density. Furthermore, the crystallinity of salt hydrates was characterised using powder X-ray diffraction measurements, which revealed that dehydration of water molecules occurred only in magnesium sulfate, whereas partial distortion occurred in iron sulfate with a rise in temperature. Collating the results together indicated that zinc sulfate is more energy efficient than both magnesium and iron sulfate. With a calculated hydration energy density at 100°C of 1·26 GJ/m3, zinc sulfate is the best option among the three salts for its use as a thermochemical material.

Thermochemical storage performance of a packed bed of calcium hydroxide composite with a silicon-based ceramic honeycomb support

The thermal decomposition of calcium hydroxide (Ca(OH)2) into calcium oxide (CaO) and water vapor has been suggested as a reversible gas–solid reaction suitable for thermochemical energy storage. This study aims to develop a composite material to enhance the heat transfer in a reaction bed. We focus on a novel material using CaO/Ca(OH)2 and a ceramic honeycomb support composed of silicon carbide and silicon. A laboratory-scale evaluation of the thermochemical storage performance of the composite using honeycomb support has not been sufficiently reported in the literature. In this study, a 100 W-scale packed bed reactor was used to evaluate the performance of the composite. We found that the composite had a volumetric energy density of 0.76 MJ L-bed−1 and exhibited a heat output rate of 1.6 kW L-bed−1 (for the first 5 min at the highest hydration pressure of 85 kPa), which was 1.8 times higher than the heat output rate previously reported for a pure Ca(OH)2 pellet bed. The composite also retained its high reactivity during repetitive reactions. Therefore, it was concluded that the composite using the ceramic honeycomb support is practically more useful for thermochemical energy storage than conventional pure CaO/Ca(OH)2 materials.

K2CO3 in closed heat storage systems

Potassium carbonate, K2CO3, has been identified as one of the most promising thermochemical storage materials for the built environment. Where a lot of knowledge has been gained on hydration/dehydration behavior at atmospheric (open system) conditions, little is known of this process under pure water vapor conditions (closed vacuum system). In this paper, for the first time, the equilibrium behavior and reaction kinetics of a K2CO3 composite are investigated under pure water vapor conditions, as present in closed vacuum systems. In this work the metastable behavior of a K2CO3 composite is investigated under vacuum conditions and compared to its metastable behavior under atmospheric conditions. It is found that the metastable zone is also present in vacuum conditions, however induction times in the metastable zone are much shorter which indicates a faster nucleation rate in vacuum conditions.Moreover the effect of inert gasses in a closed system is studied and it is shown that it is critical to remove all sources of non-condensable gasses. Finally in cyclic measurements it is shown that K2CO3 is stable in multi cyclic experiments, concluding that it is a suitable material for a heat battery based on the concept of a closed reactor.

Double hydrates salt as sustainable thermochemical energy storage materials: Evaluation of dehydration behavior and structural phase transition reversibility

Currently, thermochemical energy storage (TCES) offers the opportunity to relieve the enormous gap of energy demand and supply in the solar energy field. This method has its specific requirements, such as high energy storage density, good thermal stability, high reaction enthalpy, high heat capacity and switched temperature range of thermochemical storage material (TCM) for low to medium temperature building applications. This paper reports the synthesis of blödite-type structure Na2M(SO4)2·4H2O (M = Zn, Mg) and copper kröhnkite phase Na2Cu(SO4)2·2H2O. Their structural and thermal behavior were also investigated to explore a new temperature range using incorporated salts. An evaluation of potential energy storage density, structural reversibility, thermal cycling stability and storage efficiency of double salts compared to pure hydrates salts as TCM was carried out. The preliminary results show a good structural matching of synthesized double salts confirmed by XRD, FTIR, and Raman spectroscopy, while the transition phase investigation since dehydration process was reported by XRD and Raman spectroscopy. Moreover, Na2Zn(SO4)2·4H2O exhibited a suitable charging range temperature (<105 °C), good cycling stability at the first ten hydration/dehydration cycles with an excellent energy storage density of 4.7 GJ m−3 and an interesting theoretical efficiency up to 77.4%.

Investigation of calcium hydroxide powder for thermochemical storage modified with nanostructured flow agents

Thermal energy storage is a key component for power supply based on concentrating solar power. A promising thermochemical storage material in terms of cost-efficiency, storage density and reaction temperatures is based on the chemical reaction of calcium oxide with water. Handling of the solid materials, however, has proven to be difficult due to their powdery state particularly in dynamic storage systems. It is therefore proposed in the presented work to modify calcium hydroxide powder with nanostructured flow agents to improve the powder's flowability. Therefore, five additives consisting of nanostructured silicon and/or aluminium oxide were mixed systematically with calcium hydroxide powder and the flowability of the mixtures was determined using a ring shear cell. Four additives caused an improved flowability of the powder with additive weight fractions of 6–12%. After thermochemical cycling of the mixtures, however, flowability of the mixtures was clearly decreased while that of the pure powder was increased. Analysis of the samples showed a correlation between growth in particle size and increased flowability. Additionally, formation of calcium silicate and/or aluminate phases led to a decrease in absolute heat release of up to 50% as measured by thermal flux calorimetry, although some of the side products seemingly add to the measured heat release by hydrating exothermally. Based on these results, stabilisation of a moderately increased particle size preferably without the addition of additives is recommended, as a sole prevention of agglomeration by means of flow agents does not result in a manageable storage material.

Industrial carnallite-waste for thermochemical energy storage application

The key to successful development and implementation of thermochemical storage systems is the identification of high energy density and low-cost storage materials. In this work, an industrial waste based on a double salt hydrate, coming from non-metallic mining was studied for thermochemical storage applications. Initially, chemical characterization was performed and determined that carnallite-waste material consists of 73.54 wt% of KCl·MgCl2·6H2O and impurities such as NaCl (23.04 wt%), KCl (1.76 wt%) and CaSO4 (1.66 wt%). Using thermal analyses methods, the operating conditions such as temperatures and partial pressures, were optimized for seasonal thermochemical storage applications to PHy = 1.3 kPa and ϑHy = 40 °C, and to PDe = 4.0 kPa and ϑDe = 110 °C. Under these conditions, the reaction reversibility over 10 cycles (10 years) was significantly high, with only 8.5% decrease in chemical reversibility. Furthermore, the duration of dehydration and hydration isotherms was optimized to 15 and 360 min, respectively. Finally, 1.129 GJ/m3 energy storage density was calculated after the tenth cycle of hydration/dehydration for this material. Hence 7.1 m3 of carnallite was estimated to meet the demand of 8 GJ of energy for an average household during the six months of cold seasons. These results are comparable and competitive with an energy storage density of materials such as K2CO3 and MgCl2, reported as promising for seasonal thermochemical storage applications. It should be noted that carnallite-waste material has no commercial value so far and its use contributes to developing sustainable low-cost thermochemical energy storage systems.

Experimental analysis of encapsulated CaO/Ca(OH)2 granules as thermochemical storage in a novel moving bed reactor

The advantages of Ca(OH)2 such as low cost and high energy density make it a promising material for thermal energy storage. However, the development of cost efficient large scale storage systems remains challenging. One reason is that the low thermal conductivity and cohesiveness of the powder bulk material impede the reliable operation of moving bed reactors. One approach to address these drawbacks is the macro encapsulation of pre-granulated storage material.In this work, a newly moving bed reactor was developed and the performance of two encapsulated storage materials was investigated in lab scale and under application relevant boundary conditions. The two tested material samples were ceramic encapsulated CaO granules (sample 5D1F) and Ca(OH)2 granules coated with Al2O3 nanostructured particles (sample Al2O3). The reaction performance, cycling stability and ability to flow were experimentally examined and the operation of the moving bed reactor was successfully demonstrated. It was found that both encapsulated materials retained their shape after sixfold cycling. After the experiment series, the sample 5D1F flowed freely out of the reactor while the sample Al2O3 clogged the reactor tubes due to the volume expansion during hydration. Further, the experiments revealed that the reaction performance of the sample 5D1F is reduced, while the performance of the sample Al2O3 is comparable to unmodified Ca(OH)2 granules. Overall, this study shows that encapsulated granules are stable under operation in larger reactors and the granules further facilitate the reactor design and operation. Additionally, the volumetric energy density of compacted granules is higher compared to the loose powder bulk. In particular, the encapsulation with Al2O3 nanostructured particles showed promising performance for the cost efficient development of a scalable thermochemical energy storage system and therefore needs to be further investigated.

Hydration mechanism in Ce-exchanged zeolites and heat release performances upon adsorption of water vapour in support of their potential use in thermochemical storage of energy under mild conditions of adsorbent regeneration and saturation

Abstract The potential use of commercially available 13X zeolite, modified by ion-exchange with cerium compensating cations possessing high charges and high hydration energies, has been tested in view of low-temperature storage of solar energy performed under mild operating conditions of low regeneration temperatures and low pressures of water vapour during the adsorption step. Structural and textural properties, sorption behaviour towards water vapour of three selected samples containing various proportions of Ce3+ and Ce4+ compensating cations and the pristine Na+-13X zeolite were studied by a variety of experimental techniques including Wavelength Dispersive X-Ray Fluorescence, Energy Dispersive X-ray Spectroscopy, X-ray diffraction, Thermogravimetric analysis, as well as measurements of the adsorption of gaseous nitrogen at 77 K and water vapour at 313 K. Based on the structure refinement procedure applied to the experimental XRD patterns, it was demonstrated that extra-framework cerium cations were preferentially located on sites I’ and II in dry and hydrated zeolites, showing relatively little dependence on the hydration level. Monte Carlo simulations were used to determine the limit values of the amount adsorbed and differential heat of adsorption, which could be obtained experimentally if the zeolite samples were completely dried. The potential of Ce-containing zeolites as adsorbents for the thermochemical energy storage was finally determined under flow conditions by firstly dehydrating samples at 353 K or 423 K and then saturating them at 296 K with water vapour at a mole fraction of 0.03. The choice of the operating conditions was decided so as to maintain the stability of the zeolite structure while taking the risk of reduced thermal performance of zeolite adsorbents undergoing incomplete regeneration-dehydration. Under such mild conditions, the modified 13X zeolites exhibited enhanced thermal performance in comparison with that of the pristine 13X, by releasing between 700 and 1100 kJ per kg of the adsorbent during a period of 6–8 h. Through a complementary study based on calorimetry measurements and molecular simulations, the understanding of the hydration-dehydration steps in Ce-exchanged zeolites and cation displacement upon hydration has allowed to establish the best compromise for the conditions of zeolite regeneration and saturation in the case of heat long-term storage applications.

Optimal Indirect Integration of Steam Rankine Cycles in Concentrated Solar Power Coupled with Thermochemical Storage

Storing energy in thermo-chemical form is a promising alternative for renewable energy sources and one of the fields that could exploit its advantages is constituted by Concentrated Solar Power (CSP). High efficiencies, high energy densities, long-term storage and dispatchability in power generation could be tackled with the employment of a Calcium-Looping in a central tower CSP plant. Although Brayton cycles based on sCO2 and He probably represent the most efficient option, it can be interesting to investigate the integration of Steam Rankine cycles because of the lower costs and the higher stage of development that characterize this technology. The present work has the purposes of studying this kind of integration and analyze the system in both energy and economic terms. To perform a comprehensive investigation, the power block layout and its thermal feeding strategy are included in the analysis and optimized. In addition, a multi-objective optimization is performed to evaluate possible compromises between the two most important aspects related to the entire power plant.

Thermal energy storage with phase change materials: Application on coaxial heat exchanger with fins

For storing energy in thermal form, there are three types of storage: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical storage (TCS). In the third millennium, LHS technology has been the most attractive technology for researchers and specialists, because of its enormous storage density compared to sensitive storage (high energy storage density, greater than 100 kJ/kg) in a small temperature range. Nevertheless, due to the low thermal conductivity of phase change materials (PCM), the thermal performance of the LHTES system is generally unsatisfactory. Therefore, this study highlights the role of annular fins on coaxial heat exchanger LHTES with annular fins, examined using software Comsol Multiphysics based on the finite element method. The present investigation is only for the process of charging. DelcoTerm is used as heat transfer fluid (HTF), and NaNO2-NaNO3 is used as PCM. In fact, the results show that the coaxial system with annular fins has a significant effect on the charging time compared to the reference case without fins. As the number of fins increases, the performance of LHTES increases.

Evaluating energy and greenhouse gas emission footprints of thermal energy storage systems for concentrated solar power applications

Abstract Greenhouse gas emissions from the power generation sector contribute significantly to climate change. The use of thermal energy storage systems can reduce the sector's impact depending on factors such as plant energy performance and environmental friendliness. In this study, an Excel-based model was developed to evaluate the greenhouse gas emissions and net energy ratio for thermal energy storage technologies used in concentrated solar power applications. Five thermal energy storage systems were considered: two-tank indirect sensible heat storage, two-tank direct sensible heat storage, one-tank direct sensible heat storage, latent heat storage, and thermochemical storage. To capture the uncertainties in the results for each type of storage, a Monte Carlo simulation was performed by varying key operational and model parameters. With uncertainty taken into consideration, it was determined that the mean greenhouse gas emission values for two-tank direct sensible heat storage and one-tank direct sensible heat storage are 15 gCO2-eq/kWh and 11 gCO2-eq/kWh, respectively. The two systems offer higher energy performances and lower emissions than the other storage systems and thus the potential to be implemented commercially for concentrated solar power applications.

Experimental investigation of a moving-bed heat storage thermochemical reactor with SrBr2/H2O couple

Abstract 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 SrBr2.H2O/SrBr2.6H2O pair was used and allowed reaching reactor temperatures up to 41 °C and specific heating powers of 1.7–4.6 kW/m3 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.

Numerical modelling for the solar driven bi-reforming of methane for the production of syngas in a solar thermochemical micro-packed bed reactor

High temperature heat transfer and thermochemical storage performances of the solar driven bi-reforming of methane (SDSCB-RM) in a solar thermochemical micro-packed bed (ST-μPB) reactor are numerically investigated under different operating conditions along ST-μPB reactor length. A pseudo-homogeneous mathematical model is developed to simulate the heat and mass transfer processes coupled with thermochemical reaction kinetics in ST-μPB reactor with radiative heat loss. The effect of several parameters including the gas flow rate (Qg), effective thermal conductivity (λs,eff), operating time (ti) and operating temperature (Top.) were investigated. The simulated results shown that the pressure drop increases with the increase of Qg. When the Qg is increased, the temperature profiles at the surface of the solid phase as well as the temperature profiles of the gas phase are remarkably decreasing. The consumption of reactants (CH4, H2O and CO2) is increased when the λs,eff is gradually increased. On the other hand, the production of products (H2, and CO) is remarkably increasing with the increase of the λs,eff. According to simulated results, the overall conversions of reactants (CH4 and CO2) and the dimensionless flow rate (DFR) of H2 reach the maximum values of 98.18%, 75.61% and 1.6278 at the operating time of 2.50 h. The thermochemical energy storage efficiency (ηChem) remarkably increases with the operating temperature and the maximum value of the ηChem can be as high as 74.21% at 1123 K. The overall conversions of reactants (CH4 and CO2), DFR of H2 and the energy stored as chemical enthalpy (QChem) were also evaluated in relation to the operating temperature and their maximum values of 99.43%, 89.03%, 1.6383 and 1.3745 kJ/s are obtained at 1225 K.

Water heating by Parabolic Trough Collector with storage in the Ouargla region of Algerian Sahara

Abstract The PTC is one of the most important solar concentrating systems, the most effective in producing steam in thermal power plants. In order to increase the efficiency of PTC, we did an experimental study has been performed based on the design and completion of a 1.8 m × 1.6 m PTC solar collector, with the addition of a flat glass cover in order to maintain the optical properties of the reflector and the glass envelope of dust, and moisture due to wind and rain on the one hand, and to increase thermal insulation and increase the thermal efficiency on the other hand. According to the solar radiation, and climatic conditions of Ouargla, we recorded the temperature of the outlet water and then calculate the thermal efficiency during the day, for different water flows. Where the experimental results showed the effectiveness of the glass cover by increasing the thermal efficiency to 9% during the peak period of radiation solar system (Soudani et al., 2017). In order to extend the results to different dimensions of PTC length and larger mass flows, we prepared a numerical simulation of the PTC model. After verifying the reliability of this model by comparing the numerical simulation results with the experimental results, we record the effect of PTC length and mass flow of water on outlet water temperature. To store this heat for the absence of the sun, we used a heat pump which is a thermo-chemical storage system, consisting of water as sorbet and sodium chloride as the sorbent, and then study the effect of different parameters on the performance of the thermo chemical heat pump.

Clean Energy and Fuel Storage

Clean energy and fuel storage is often required for both stationary and automotive applications. Some of the clean energy and fuel storage technologies currently under extensive research and development are hydrogen storage, direct electric storage, mechanical energy storage, solar-thermal energy storage, electrochemical (batteries and supercapacitors), and thermochemical storage. The gravimetric and volumetric storage capacity, energy storage density, power output, operating temperature and pressure, cycle life, recyclability, and cost of clean energy or fuel storage are some of the factors that govern efficient energy and fuel storage technologies for potential deployment in energy harvesting (solar and wind farms) stations and on-board vehicular transportation. This Special Issue thus serves the need to promote exploratory research and development on clean energy and fuel storage technologies while addressing their challenges to a practical and sustainable infrastructure.

Numerical Study for Open Reactor Design Using Salt Hydrate

There are various methods to store thermal energy from season to season. Because the density of energy storage in thermochemical materials is higher than the latent and sensible heat storage method, the thermochemical heat storage is considered the sufficient type. This paper provides a mathematical model of thermochemical storage process for thermochemical materials; potassium aluminum sulfate 12-hydrate (KAl(SO4)2.12H2O). The software used is COMSOL Multiphysics Modelling Software. The model data is validated and compared with the previous experimental result. Furthermore, the temperature difference and the pressure over the material inside the reactor are investigated. There are seven designs for the bed. However, the inlet and outlet diameter of the bed is changed; there is any change in the volume and height of the bed. The model results present that the rise in entrance area decreases the charging time and rises the pressure drop inside the material. Case 4 is the best design from the charge time.

Application of a mid-/low-temperature solar thermochemical technology in the distributed energy system with cooling, heating and power production

A new solar-assisted cooling, heating and power (CCHP) system is developed for improving the energy conversion efficiency in this work. Solar thermal energy (250–350 °C) collected by a parabolic trough solar collector is used to drive the thermochemical reaction of methanol decomposition, then the generated solar fuel in the form of syngas is fed into an internal combustion engine (ICE) to generate electricity. The high-temperature exhaust gas released by the ICE is used to produce cooling and heating energies via a double-effect LiBr-H2O absorption refrigerator and a heat exchanger. Solar energy is converted to chemical energy in the form of syngas, the distinct advantages of energy cascade utilization and thermochemical storage can be realized. Numerical simulation results show that in the proposed CCHP system the energy and exergy efficiencies at the designate condition are 82.0% and 58.72%, respectively. The off-design performances are investigated by deploying the proposed CCHP system to a shopping center building, the annual efficiency and the solar fraction reach 53.6% and 9.81%, respectively. In comparison with the reference solar-assisted CCHP system, the annual methanol consumption of the proposed CCHP system decreases to 874.31 tons with the fuel saving ratio of 4.56% and less CO2 emission. This novel solar CCHP system presents a promising cost-effective performance and the system life cycle cost saving ratio reaches 2.84%. The research findings provide an alternative route towards efficiently utilizing solar energy.

Doping Magnesium Hydroxide with Ce(NO3)3 : A Promising Candidate Thermochemical Energy Storage Materials

Development of advanced materials with high energy density for thermochemical energy storage may contribute to the efficient utilization of solar energy and waste heat. In this work, Ce(NO3)3/Mg(OH)2 as a new heat storage material for TCEs was investigated. The effect of Ce(NO3)3 on dehydration of Mg(OH)2 was investigated by differential scanning calorimetry and thermogravimetry techniques. The addition of Ce(NO3)3 can significantly reduce the dehydration temperature and accelerate the dehydration of Mg(OH)2. The temperature reduction depends on the Ce(NO3)3 content W and reaches 29 °C at W = 10 wt%, while the dehydration heat was reduced only slightly (1086 J/g). The new material exhibits an adjustable decomposition temperature, fast dehydration rate and large heat storage density, and may be considered as a promising candidate for thermochemical storage of concentrated solar energy.

Validated model of thermochemical energy storage based on cobalt oxides

Thermal Energy Storage (TES) can play a critical role through provision of reliable energy supply and increase the market penetration of renewable energy sources. Thermochemical Energy Storage (TCES) based on reversible reactions offers distinguished advantages in comparison with sensible and latent heat storage: higher energy density, higher temperature range and possibility of seasonal storage. TCES systems based on the redox cycle of metallic oxides shows significant potential for integration with Concentrated Solar Power (CSP) plants using air as the heat transfer fluid, which also acts as a reactant for the redox reaction. A pilot scale thermochemical storage reactor designed for a CSP plant has been developed and tested in the framework of a collaborative European funded project “RESTRUCTURE” at the Solar Tower Julich (STJ). TCES system is proposed with the aim of achieving higher energy storage capacity and higher storage temperature. Numerical modeling of a TCES prototype presented in this study is a contribution towards this effort. The present work is focused on the innovative one-dimensional modeling of a TCES system based on the redox cycle of cobalt oxides (Co3O4/CoO), coated on the ceramics honeycomb structures. The numerical model for TCES involved the energy balance and reaction kinetics describing the redox reaction of cobalt oxides, to simulate the phenomena of thermochemical storage. The simulation results were presented as the temperature profiles at different positions inside the storage vessel and they were validated against experimental data published in literature by other groups. This validation proved that this model can simulate the overall thermochemical storage process with reasonable accuracy. The simulation tool was also used to perform the parametric analysis of the storage module, which provides guidance to optimize the performance of the storage system. Moreover, due to its good compromise between reliability and computational time, the established 1-D thermochemical storage model can be integrated with the CSP plant model for dynamic analysis of the whole system, which is the aim of this study.

Adapting the MgO-CO2 Working Pair for Thermochemical Energy Storage by Doping with Salts: Effect of the (LiK)NO3 Content

The MgO-CO2 working pair has been regarded as prospective for thermochemical energy storage (TCES) due to its relatively high heat storage capacity, low cost, and wide availability. This study is aimed at the optimization of the molar salt content, α, for the MgO modified with the eutectic mixture of LiNO3 and KNO3 (Li0.42K0.58NO3) which was earlier shown to provide high conversion, Δx, in heat-storage/release processes at 300–400 °C. The composites that have different salt content were prepared and carbonation kinetics was investigated under various conditions (carbonation temperature, Tcarb., is 290–360 °C and CO2 pressure, P(CO2), is 50–101 kPa). Significant accelerating effect was revealed at α ≥ 0.05, and the Δx value was maximized at α = 0.10–0.20. The largest conversion of 0.70 was detected at α = 0.10 and Tcarb. = 350 °C that corresponds to the specific useful heat (Qcomp.) is 1.63 MJ/kg-composite. However, the salt content of 0.20 ensures the high conversion, Δx = 0.63–0.67 and Qcomp. = 1.18–1.25 MJ/kg-composite in the whole temperature range between 290 and 350 °C. The (LiK)NO3/MgO composite with an optimal salt content of 0.20 exhibits reasonable durability through cyclic experiment at 330 °C, namely, the stabilized reacted conversion Δx = 0.34 (Qcomp. = 0.64 MJ/kg-composite). The studied (Li0.42K0.58)NO3 promoted MgO-CO2 working pair has good potential as thermochemical storage material of middle temperature heat (300–400 °C).