Solar Thermal Energy Storage Applications Research Articles

Recent advancements in sorption technology for solar thermal energy storage applications

Sorption thermal energy storage (STES) technology, belonging to the wider class of thermochemical TES, represents a promising alternative to common sensible and latent TESs, especially for applications at low-medium temperature (i.e. below 130 °C). The interest towards this technology is confirmed by the huge amount of research and development activities ongoing, represented by scientific publications as well as funded projects and international working groups.In such a context, the present paper reports about the most recent activities in the STES field. Particularly, it focuses on the analysis of innovative sorbent materials currently under development, comprising liquid absorption, physical adsorption, chemical reactions in pure salts and composite sorbents. Analysis of critical issues like achievable storage density, stability and corrosiveness were reported for each sorbent material.Furthermore, prototypes recently developed for each of the sorbent classes above reported were presented, highlighting the achievable performance, with particular attention towards the TES density attained under typical working conditions for heat and cold storage applications, both with open and closed TES systems.Finally, the most recent international activities focusing on the further development of STES, to help achieving higher Technology Readiness Level (TRL) were reported. These mainly comprise funded R&D projects and international working groups supported by intergovernmental organizations and research and innovations alliances.

Application of nanomaterials in solar thermal energy storage

Solar thermal conversion technology harvests the sun’s energy, rather than fossil fuels, to generate low-cost, low/zero-emission energy in the form of heating, cooling or electrical form for residential, commercial, and industrial sectors. The advent of nanofluids and nanocomposites or phase change materials, is a new field of study which is adapted to enhance the efficiency of solar collectors. The concepts of thermal energy storage technologies are investigated and the role of nanomaterials in energy conversion is discussed. This review revealed that although the exploitation of nanomaterials will boost the performance of solar collectors almost in all cases, this would be accompanied by certain challenges such as production cost, instability, agglomeration and erosion. Earlier studies have dealt with the enhancement of thermal conductivity and heat capacity; however, less attention has been given to the facing challenges. Moreover, no exact criteria can be found for the selection of appropriate nanomaterials and their properties for a specific application. In most research studies, the nanoparticles’ material and properties have not been selected based on estimated values so that all the aspects of desired application could be considered simultaneously. The wide spread use of nanomaterials can lead to cost effective solutions as well. Therefore, it seems there should be a sense of techno-economic optimization in exploiting nanomaterials for solar thermal energy storage applications. The optimization should cover the key parameters, particularly nanoparticle type, size, loading and shape which depends on the sort of application and also dispersion technology.

Accurate Isomerization Enthalpy and Investigation of the Errors in Density Functional Theory for Dihydroazulene/Vinylheptafulvene Photochromism Using Diffusion Monte Carlo

We investigate the isomerization enthalpy of the dihydroazulene/vinylheptafulvene (DHA/VHF) molecular photoswitch system derivatives using electronic structure calculation methods including density functional theory (DFT), quantum Monte Carlo (QMC), and coupled cluster (CCSD(T)). Recent efforts have focused on tuning the isomerization enthalpy of the photoswitch for solar thermal energy storage applications using substitutional functional groups on its five- and seven-membered carbon rings, predominantly using DFT for the energy predictions. However, using the higher accuracy QMC and CCSD(T) methods, we show that in many cases DFT incorrectly predicts the isomerization enthalpy, and the errors depend on the functional groups substituted and the choice of the DFT functional. Isomerization of the DHA to VHF molecule is an electrocyclic ring-opening reaction on the five-membered ring of the DHA isomer. We find that the DFT errors are correlated to the electrocyclic ring-opening reactions of cyclobutene and c...

Investigation on thermal reliability and corrosion characteristics of glutaric acid as an organic phase change material for solar thermal energy storage applications

The successful implementation of the latent heat solar thermal energy storage system depends on the long term thermal stability and corrosion characteristics of phase change materials (PCM). Glutaric acid (GA) is an organic PCM with phase transition temperature 94.97–99.20 oC and latent heat of fusion 184.8 J/g. Thermo-physical properties and corrosive nature of glutaric acid is investigated using accelerated thermal cycle method under controlled conditions. The study was conducted upon 2000 thermal cycles to explore the feasibility of material for long term usage in solar thermal applications. Relative percentage difference (RPD%), gravimetric analysis and scanning electron microscope (SEM) techniques were used to evaluate the thermal stability and compatibility of glutaric acid. The test results reveal that glutaric acid tends to be a potential PCM for solar thermal energy storage applications due to its prolonged thermal stability and less corrosive nature on containment materials.

Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures

Organic phase change materials (PCMs) have many properties that make them desirable for integration in latent-heat solar thermal energy storage (TES) systems operating in the ambient-to-moderate temperature range, but there are significant gaps concerning their material properties, and their reliability. We present a comprehensive study of thermal and related properties (transition temperature, transition enthalpy change, heat capacity, thermal conductivity, thermal diffusivity, density), thermal stability (on cycling through the transition up to 3000 times) and chemical stability (when in contact with 16 common containment materials at 75°C) for six organic phase change materials: decanoic acid (aka capric acid, [CH3(CH2)8COOH]), dodecanoic acid (aka lauric acid, [CH3(CH2)10COOH]), tetradecanoic acid (aka myristic acid, [CH3(CH2)12COOH]), hexadecanoic acid (aka palmitic acid, [CH3(CH2)14COOH]), octadecanoic acid (aka stearic acid, [CH3(CH2)16COOH]) and 1-octadecanol ([CH3(CH2)17OH]). All show melting transitions in the temperature range 30–70°C which is suitable for solar thermal applications, with substantial enthalpy changes (>145Jg-1), and all are thermally stable over 3000 cycles. These materials all have significant potential for solar thermal energy storage applications.

A new composite sorbent based on SrBr2 and silica gel for solar energy storage application with high energy storage density and stability

The excellent matching between the sorption and desorption temperatures of hexahydrated SrBr2 and those required for solar heat storage for building applications, the high heat of reaction (67.5kJ/mol of water) coupled with the gain/loss of 5mol of water per mole of salt make this salt an appealing sorbent for solar thermal energy storage applications coupled to space heating. Due to the morphological instability of this salt, it is necessary to incorporate it in a porous matrix as a composite sorbent. A new composite material for thermochemical energy storage applications was developed. It consists of a mesoporous silica gel impregnated by strontium bromide with salt content equal to 58wt.%. The structure and the sorption properties of the composite were characterized by SEM-EDX, temperature dependent XRD, XRF, and N2 sorption measurements. The salt is homogeneously distributed inside the pores of the silica gel. Water sorption isotherms were measured between 20°C and 80°C, which enabled us to understand the sorption mechanism. A mathematical model was developed and used to fit the experimental data in order to predict the sorption behavior of the composite at different conditions (influence of temperature and pressure conditions on the cycle loading lift and energy storage density). The interest of using such a composite for thermal energy storage application is then discussed (thermal energy produced by solar collector and used for space heating). A high cycle loading lift of 0.22g/g is obtained corresponding to an energy storage capacity of 230Wh/kg and an energy storage density of 203kWh/m3 of packed bed composite (between 30°C and 80°C at 12.5mbar) is reported, with an excellent stability over 14 sorption/desorption cycles. The sorption kinetics of this composite is enhanced compared to pure salt. Test on a laboratory scale open type reactor gives a maximum specific thermal power of 200W/kg and a mean specific thermal power of 92W/kg at 30°C and 12.5mbar for an extent of reaction of 0.68.

Experimental process investigation of a latent heat energy storage system with a staggered heat exchanger with different phase change materials for solar thermal energy storage applications

This work investigates melting and solidification processes of four different Phase Change Materials (PCM) used as latent heat thermal storage system. The experimental rig was consisted of an insulated tank, filled with the under investigation PCM, a staggered heat exchanger to supply or extract heat from the PCM cavity and a water pump to circulate Heat Transfer Fluid (HTF). Both charging (melting) and discharging (solidification) processes were conducted for two different HTF flow rates. The main scope of this work was to develop a first approach and to investigate the behaviour of PCM under various load conditions (different HTF flow rates). Results show that different HTF flow rates affect melting and solidification time periods; in both processes time was reduced while HTF flow rate was increased but in differentways due to the transition from conduction to convection heat transfer mechanisms.

Effect of nanoparticle dispersion on enhancing the specific heat capacity of quaternary nitrate for solar thermal energy storage application

Molten salt is considered to be part of the main direction for studying high temperature heat transfer and storage medium in concentrated solar power due to its good thermo physical properties. Specific heat capacity performs a significant role in increasing the overall cycle efficiency and reducing the cost of heat transfer and storage. In this paper, in order to enhance the specific heat capacity, 1wt% SiO2 nanoparticles are added into a quaternary nitrate (Ca(NO3)2-KNO3-NaNO3-LiNO3 with a low melting point called the low melting point salt (LMPS), which are mixed through ultra-sonication. Experimental results show that the specific heat capacity can be enhanced when nanoparticles are evenly distributed in the quaternary nitrate. The optimal ultra-sonication time with a frequency of 45kHz is researched. The average specific heat capacity of nano-LMPS can reach up to 1.87J/(gK) and the average enhancement reaches 19.4%. There is a good linear relationship between the specific heat capacity of nano-LMPS and ultra-sonication time. Scanning electron microgram images reveal that lamellar networks with higher density phase on the solid nano-LMPS surface exist and may cause the specific heat capacity enhancement of nano-LMPS.

Preparation and thermal energy storage properties of d-Mannitol/expanded graphite composite phase change material

A d-Mannitol/expanded graphite(EG) composite phase change material(PCM) was prepared for solar thermal energy storage or waste heat recovery applications performed at 180–240°C. EG was produced by microwave irradiation performed. It was found that the EG prepared exhibited the maximum sorption capacity of 85wt% for d-Mannitol. Scanning electron microscopy (SEM) images showed that d-Mannitol was uniformly dispersed in the micro holes of EG. Differential scanning calorimeter (DSC) analysis indicated that the phase change temperature of the composite PCM was close to that of d-Mannitol, and its latent heat was equivalent to the calculated value based on the mass fraction of d-Mannitol in the composite. With the assistance of hot disk analyzer, the thermal conductivities of the composite PCM first rise and then drop with increasing of its densities, and similar linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite were obtained. Thermal energy storage performance of the composite PCM was tested in latent thermal energy storage (LTES) system.The heat transfer in the composite PCM during the thermal energy storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the mass fraction of EG and the apparent density of the composite PCM.

Measuring Thermochemical Energy Storage Capacity with Redox Cycles of Doped-CaMnO3

Our team has explored redox cycles of doped CaMnO3-δ between air and low O2 partial pressures (~10-4 bar) for high-temperature thermochemical energy storage (TCES) applications. In this study, we have explored both A-site and B-site doped compositions using earth-abundant cations to identify perovskites for cost-effective TCES in concentrated solar and other high-temperature, thermal storage applications. Reduction of doped CaMnO3-δ above 800 °C in the low P O2 requires some amount of dopant to avoid irreversible decomposition of the perovskite structure observed for reduction of undoped CaMnO3-δ [1]. This study shows that small amounts (5%) of A-site or B-site dopant can stabilize the CaMnO3-δ perovskite structure during reduction at temperatures up to 1100 °C and P O2 = 10-4 bar. For selected, stable doped CaMnO3-δ compositions, total specific TCES (Δh tot) was defined by the sum of specific sensible energy (Δh sens) and chemical energy (Δh chem) captured during heating and reduction from air at a cool temperature (T C fixed at 500 °C in this study) to low P O2 at varying high temperatures ( T H). B-site doping with 5% Cr and A-site doping with 5% Sr provided thermodynamic limits of Δh tot over 720 kJ kg-1 and 790 kJ kg-1 respectively for T H = 1000 °C. These materials were also tested kientically to assess the rates at which energy storage is captured during reduction and released during oxidation as relevant for concentrated solar energy storage applications. The results indicate that the doped CaMnO3-δ and in particular A-site doped Ca0.95Sr0.05MnO3-δ have promise as a TCES storage media with Δh chem providing just under half of the total energy stored during the combined heating and reduction. For the perovskite redox cycles, the heat of oxide reduction -ΔH O can vary with non-stoichometry δ [2]. To find the integrated Δh chem, the functional dependence of ΔH O with δ must be detemined. Two methods have been undertaken to explore this functional dependence: 1) combined TGA-DSC calorimetry measurements with incremental changes in δ, and 2) point-defect model fitting to equilibrium δ vs. P O2 data from TGA measurements [3,4]. The DSC measurements showed that the magnitude of ΔH O decreases to a near constant value for δ > 0.1 as illustrated for Ca0.95Sr0.05MnO3-δ in Figure 1. Similar trends were observed for the other perovskite compositions. The alternative method for finding ΔH O as a function of δ by fitting the equilibrium δ vs. P O2 involved modeling the perovskite reduction with two reversible point-defect reactions -- oxide-ion vacancy formation and Mn cation disproportionation from 4+ to 3+ and 5+ states [4]. The model fits originally assumed that these two reactions had enthalpies independent of δ but the contribution of the disproportionation reaction increased with δ thereby causing the combined ΔH O to have a minor dependence on δ as also shown in Figure 1. The point defect modeling did not show the same trends for ΔH O vs. δ but provided very similar values at high δ > 0.1. The integration of ΔH O over a change in δ (i.e. Δδ as in Table 1) provides a basis for calculating the integrated Δh chem as a function of T H. Integrated Δh chem values for CaCr0.05Mn0.95O3-δ, Ca0.95Sr0.05MnO3-δ, and Ca0.9Sr0.1MnO3-δ are shown in Table 1 along with the total energy stored which includes Δh sens derived from integration of CP with respect to T over the redox cycle heating. The higher Δh chem and associated Δh tot for Ca0.95Sr0.05MnO3-δ stems from its increase reducibility without a loss in heat of reaction and makes it a more promising material for TCES applications in these temperature ranges.For thermal storage application, kinetic rates of TCES capture are important, particularly for concentrated solar applications where solar receiver residence times are limited. To explore kinetics, reduction and oxidation rates of porous perovskite particle beds were measured for fitting thermodynamically consistent kinetic models to the observed rates. Kinetic testing and model fitting for the Ca0.95Sr0.05MnO3-δ indicates that it achieves approximately 80% of its Δh chem thermodynamic limit in 60 s exposure of low P O2 gas for reduction at T H = 900 °C. On the other hand, at the same temperature reoxidation occurs rapidly and releases over 90% of the Δh chem limit in 30 s. The thermodynamic consistent model involving surface kinetics and ionic bulk diffusion in the perovskite particles for Ca0.95Sr0.05MnO3-δ and for CaCr0.05Mn0.95O3-δ provide a basis for designing reactors for redox cycles to be used in TCES for concentrated solar and other potential thermal energy storage applications.

Destabilization of lithium hydride and the thermodynamic assessment of the Li–Al–H system for solar thermal energy storage

2LiH(s) + 2Al(s) → 2LiAl(s) + H2(g).

Influence of accelerated thermal charging and discharging cycles on thermo-physical properties of organic phase change materials for solar thermal energy storage applications

Integration of appropriate thermal energy storage system plays a predominant role in upgrading the efficiency of solar thermal energy devices by reducing the incongruity between energy supply and demand. Latent heat thermal energy storage based on phase change materials (PCM) is found to be the most efficient and prospective method for storage of solar thermal energy. Ensuring the thermal reliability of PCM through large number of charging (melting) and discharging (solidification) cycles is a primary prerequisite to determine the suitability of PCM for a specific thermal energy storage applications. The present study explains the experimental analysis carried out on two PCM’s namely benzamide and sebacic acid to check the compatibility of the material in solar thermal energy storage applications. The selected materials were subjected to one thousand accelerated melting and solidification cycles in order to investigate the percentage of variation at different stages on latent heat of fusion, phase transition temperature, onset and peak melting temperature. Differential Scanning Calorimeter (DSC) was used to determine the phase transition temperature and heat of fusion upon completion of every 100 thermal cycles and continued up to 1000 cycles. Relative Percentage Difference (RPD%) is calculated to find out the absolute deviation of melting temperature and latent heat of fusion with respect to zeroth cycle. The experimental study recorded a melting temperatures of benzamide and sebacic acid as 125.09°C and 135.92°C with latent heat of fusion of 285.1(J/g) and 374.4(J/g). The maximum RPD for melting temperature and heat of fusion for benzamide was calculated as 0.02% and −14.83% whereas sebacic acid it was −0.85% and −6.06% upon 1000 thermal cycles. The study revealed that both benzamide and sebacic acid are potential candidates for thermal energy storage due to their superior thermal reliability characteristics even after one thousand thermal cycles.

Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets

In this study, the thermal conductivity of form-stable phase change materials (PCMs) is improved by the addition of graphene nanoplatelets (GNPs). In the first step, the dispersed GNPs were mixed with palmitic acid (PA) particles using an ultra-sonication method. Then, the prepared material was added to form-stable phase change materials (PA/PPy) obtained by a polymerisation technique. The amount of PA particles remained constant in the prepared form-stable phase change materials. FT-IR and SEM analyses were used to investigate the structure and surface morphology of form-stable PCMs, respectively. The thermal properties, stability and conductivity of the materials were also characterised using DSC, TGA and thermal conductivity analysis, respectively. The results indicated that GNPs had a significant effect on improving the thermal conductivity of form-stable PCMs (PA/PPy). By doping 1.6% of GNPs, the thermal conductivity and thermal capacity of PA/PPy/GNPs form-stable PCMs could reach up to 0.43W/mK and 151J/g, respectively. In other words, thermal conductivity increased by 34.3% in comparison with PA/PPy form-stable PCMs. The prepared PA/PPy/GNPs form-stable PCMs are anticipated to show improved performance for solar thermal energy storage applications.

Palmitic acid/polypyrrole composites as form-stable phase change materials for thermal energy storage

In this study a novel palmitic acid (PA)/polypyrrole (PPy) form-stable PCMs were readily prepared by in situ polymerization method. PA was used as thermal energy storage material and PPy was operated as supporting material. Form-stable PCMs were investigated by SEM (scanning electron microscopy) and FTIR (Fourier transform infrared spectrometer) analysis that illustrated PA Particles were wrapped by PPy particles. XRD (X-ray diffractometer) was used for crystalline phase of PA/PPy composites. Thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) were used for investigating Thermal stability and thermal energy storage properties of prepared form-stable PCMs. According to the obtained results the form stable PCMs exhibited favorable thermal stability in terms of their phase change temperature. The form-stable PCMs (79.9wt% loading of PA) were considered as the highest loading PCM with desirable latent heat storage of 166.3J/g and good thermal stability. Accelerated thermal cycling tests also showed that form stable PCM had an acceptable thermal reliability. As a consequence of acceptable thermal properties, thermal stability and chemical stability, we can consider the new kind of form stable PCMs for low temperature solar thermal energy storage applications.

Thermal property measurement and heat storage analysis of LiNO3/KCl – expanded graphite composite phase change material

A LiNO3/KCl-expanded graphite (EG) composite phase change material (PCM) was prepared for solar thermal energy storage application at high temperature (∼200°C). In such composite material, eutectic system LiNO3/KCl is characterized by high phase change latent heat and EG serves as the heat transfer promoter. Investigations by means of differential scanning calorimetry (DSC), hot disk analyzer and heat storage performance tests in a latent thermal energy storage (LTES) unit were devoted to the thermal property measurement and heat storage performance analysis of the LiNO3/KCl–EG composite. Experimental results revealed that the melting temperature of the composite material was close to that of the eutectic LiNO3/KCl, and the phase change latent heat ranging from 142.41 to 178.10J/g was dependent on its mass fraction of EG. The thermal conductivities of the composites were 1.85–7.56 times higher compared with the eutectic LiNO3/KCl, and the conductivity value varied with the EG mass content and the apparent density of the composite. In addition, the heat transfer in the composite material during the heat storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the phase change latent heat and the apparent density of the composite material.

Shaping adsorption properties of nano-porous molecular sieves for solar thermal energy storage and heat pump applications

The water adsorption properties of modified porous sorbents for solar thermal energy storage and heat transformation have been investigated by measurements of water adsorption isotherms, microcalorimetry and storage tests. A chabazite type SAPO, a dealuminated faujasite type zeolite, and a mesostructured aluminosilicate, have been synthesized and compared with common zeolites NaX, NaY and silica gel. It has been found that optimized lattice composition and pore architecture contribute to well adapt hydrophilic properties and a beneficial steep isotherm.

Preparation and thermal energy storage behaviour of stearic acid–TiO2 nanofluids as a phase change material for solar heating systems

This paper investigates the phase change behaviour of newly prepared stearic acid–TiO2 nanofluids as composite phase change materials (PCMs). TiO2 nanoparticles of 0.05, 0.1, 0.15, 0.2, 0.25, 0.3wt% were dispersed in stearic acid, individually. The phase change temperatures and latent heats of nanofluids for melting and solidification processes were determined by differential scanning calorimetry (DSC). The complete melting and solidification times of nanofluids were reduced by 7.03, 12.56, 19.59, 28.64, 35.17, 43.72% and 6.62, 13.57, 20.53, 26.82, 34.11, 41.39% for 0.05, 0.1, 0.15, 0.2, 0.25, 0.3wt% TiO2 nanoparticles, respectively. Time reductions of both the processes proved thermal conductivity enhancement of nanofluids and it was ascertained with laser flash analyzer (LFA) measurements. Based on the results, stearic acid based composites could be recommended as potential candidate for low temperature solar thermal energy storage applications due to their better thermal reliability, chemical stability and heat transfer characteristics.

Adsorption properties of porous materials for solar thermal energy storage and heat pump applications

The water adsorption properties of modified porous sorbents for solar thermal energy storage and heat transformation have been investigated by thermogravimetry (TG) differential thermogravimetry (DTG), microcalorimetry, measurements of water adsorption isotherms, and storage tests. A chabazite type SAPO, a dealuminated faujasite type zeolite, and a mesostructured aluminosilicate, have been synthesized and compared with common zeolites X, Y and silica gel. It has been found that optimized lattice composition and pore architecture contribute to well adapt hydrophilic properties and a beneficial steep isotherm.

Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications

In this study, we report the anomalous enhancement of specific heat capacity of high-temperature nanofluids. Alkali metal chloride salt eutectics were doped with silica nanoparticles at 1% mass concentration. The specific heat capacity of the nanofluid was enhanced by 14.5%. Dispersion behavior of the nanoparticles in the eutectic was confirmed by scanning electron microscopy (SEM). Three independent competing transport mechanisms are enumerated to explain this anomalous behavior.

Eudragit S (methyl methacrylate methacrylic acid copolymer)/fatty acid blends as form‐stable phase change material for latent heat thermal energy storage

AbstractBy composing (Eudragit S) with fatty acids (stearic acid (SA), palmitic acid (PA), and myristic acid (MA)), form‐stable phase change materials (PCMs), which can retain the same shape in a solid state even when the temperature of the PCMs is over the melting points of the fatty acids, are prepared. The compatibility of fatty acids with the Eudragit S is proved by microscopic investigation and infrared (FTIR) spectroscopy. The melting and crystallization temperatures and the latent heats of melting and crystallization of the form‐stable PCMs are measured by Differential Scanning Calorimetry (DSC) method. The maximum mass percentage of all fatty acids in the form‐stable PCMs is found as 70%, and no leakage of fatty acid is observed at the temperature range of 50–70°C for several heating cycles. Thermal properties obtained from the DSC analysis indicate that the Eudragit S/fatty acid blends as form‐stable PCM have great potential for passive solar latent heat thermal energy storage (LHTES) applications in terms of their satisfactory thermal properties and utility advantage. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1402–1406, 2006