Solar Energy Storage Applications Research Articles

Thermal Conductivity of Eutectic Nitrates and Nitrates/Expanded Graphite Composite as Phase Change Materials.

Nitrates and eutectic nitrate mixtures are considered as potential phase change materials (PCMs) for the middle-temperature-range solar energy storage applications. But the extensive utilization is restricted by the poor thermal conductivity and thermal stability. In the present study, sodium nitrate-potassium nitrate eutectic mixture was used as the base PCM, and expanded graphite (EG) was added to the mixture so as to improve the thermal conductivities. The elaboration method consists of a physically mixing of salt powders with or without EG, and the composite PCMs were cold-compressed to form shape-stabilized PCMs at room temperature. The thermal conductivities of the composite PCMs fabricated by cold-compression were investigated at different temperatures by the steady state method. The results showed that the addition of EG significantly enhanced the thermal conductivities. The thermal conductivities of pure nitrates and nitrates/EG composite PCMs in solid state showed the behavior of temperature dependant, and they slightly decreased with the increase of the temperature.

Photochemical Upconversion: The Primacy of Kinetics.

Incoherent photochemical upconversion is a process by which low-energy light can be converted into a higher-energy form with promising applications in solar energy conversion and storage, photocatalysis, biological imaging, and photochemical drug activation. Despite intensive research in recent years, there remains an underappreciation of the chemical kinetics that controls the efficiency of the upconversion process. Here, we provide a brief overview of research into photochemical upconversion and provide a tutorial to guide the design of efficient upconversion compositions. We further provide our perspective on where this area of research is heading and how very efficient systems will be developed.

Thermal performance enhancement of shell and tube Latent Heat Storage Unit using longitudinal fins

Latent Heat Storage Unit (LHSU) employing Phase Change Materials (PCMs) is an effective means of thermal energy storage for solar applications. The practical use of such energy storage unit is however limited by the low thermal conductivity of the available PCMs. Significant augmentation in the heat transfer rate of PCMs is possible by installation of longitudinal fins. The augmentation in heat transfer for a shell and tube type LHSU is estimated by carrying out experimental analysis with three longitudinal fins installed on the heat transfer fluid (HTF) tube. The heat transfer augmentation is established in terms of melting and solidification time for varying fluid inlet temperatures and flow rates of heat transfer fluid (HTF). Experimental results show that the heat transfer augmentation is more sensitive to increase in HTF inlet temperature as compared to increase in mass flow rate of HTF. Solidification time has been observed to reduce up to 43.6% by installation of three fins.

Thermal enhancement of paraffin as a phase change material with nanomagnetite

Paraffin is a common phase-change-material (PCM) exploited in many thermal and solar energy storage applications. Its relatively large latent heat with a stable phase change, nontoxicity and noncorrosivity are the main reasons. Paraffin׳s low thermal conductivity may become a drawback for high power applications. In this study, the effect of adding nanomaterials to enhance paraffin׳s thermal performance is investigated. Nanomagnetite (Fe3O4) particles were prepared by sol–gel method for this investigation. Once characterization of nanomagnetite was done, it was added at 1%, 5%, 10%, and 15% ratios to paraffin (melting range: 46–48°C) to obtain a paraffin nanocomposite. Thermal characterization with Differential Scanning Calorimeter (DSC) shows that the latent heat storage capacity of paraffin has been increased for 10% nanomagnetite composite, while the melting temperature range remained the same.

Poly(fluorene-co-thiophene)-based ionic transition-metal complex polymers for solar energy harvesting and storage applications

This paper describes the synthesis, electrochemical and photophysical properties of two poly(fluorene-co-thiophene)-based polymers featuring pendant Ru(II) polypyridyl-based ionic transition metal complexes. These systems combine long-lived excited states of the Ru(II) chromophores and the large optical cross-sections of the conjugated backbones, potentially allowing for polymer-assisted solar radiation-harvesting and storage functions via ultrafast energy and charge transfer processes. Modified azide–alkyne “click” cycloaddition chemistry conditions are demonstrated as an effective approach to obtain quantitatively functionalized conjugated polymer–ionic transition metal complex assemblies. Introduction of variable fractions of electron-rich thiophene units into these architectures induces a bathochromic shift in the polyfluorene absorption maxima, while simultaneously stabilizing the oxidized states of the conjugated scaffolds, as determined by electrochemical and in situ spectroelectrochemical experiments. The hybrid assemblies exhibit ultrafast energy flow (700 fs to 4.8 ps) and photoinduced charge-separation (1.8–2.0 ps) between the conjugated backbone and the Ru(II) moieties. Notably, the chemical composition of the main-chain repeat unit determines the dominant pathway for decay of the conjugated backbone excited states with the fraction of electron transfer increasing from 25% to 75% upon incorporation of an additional thiophene heterocycle.

Preparation and characterization of form-stable paraffin/polyurethane composites as phase change materials for thermal energy storage

Polyurethane phase change material (PUPCM) has been demonstrated to be effective solid–solid phase change material for thermal energy storage. However, the high cost and complex process on preparation of PUPCMs with high enthalpy and broad phase transition temperature range can prohibit industrial-scale applications. In this work, a series of novel form-stable paraffin/PUPCMs composites (n-octadecane/PUPCM, n-eicosane/PUPCM and paraffin wax/PUPCM) with high enthalpy and broad phase transition temperature range (20–65°C) were directly synthesized via bulk polymerization. The composites were prepared at different mass fractions of n-octadecane (10, 20, 25, 30%w/w). The results indicated that the maximum encapsulation ratio for n-octadecane/PUPCM10000 composites was around 25%w/w. The chemical structure and crystalline properties of these composites were characterized by Fourier transform infrared spectroscopy (FT-IR), polarizing optical microscopy (POM), wide-angle X-ray diffraction (WAXD). Thermal properties and thermal reliability of the composites were determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). From DSC analysis, the composites showed a typical dual phase change temperature. The enthalpy for the composite with 25%w/w n-eicosane was as high as 141.2J/g. TGA analysis indicated that the composites degraded at considerably high temperatures. The process of preparation of PUPCMs and their composites was very simple, inexpensive, environmental friendly and easy to process into desired shapes, which could find the promising applications in solar energy storage and building.

Synthesis and Crystallographic Analysis of Shape-Controlled SnS Nanocrystal Photocatalysts: Evidence for a Pseudotetragonal Structural Modification

Tin sulfide, SnS, is a narrow band gap semiconductor comprised of inexpensive, earth abundant, and environmentally benign elements that is emerging as an important material for a diverse range of applications in solar energy conversion, energy storage, and electronics. Relative to many comparable systems, much less is known about the factors that influence the synthesis or morphology-dependent properties of SnS nanostructures. Here, we report the synthesis of colloidal SnS cubes, spherical polyhedra, and sheets and demonstrate their activity for the photocatalytic degradation of methylene blue. We also study their morphology-dependent polymorphism using an in-depth crystallographic analysis that correlates high-resolution TEM data of individual nanocrystals with ensemble-based electron diffraction and powder XRD data. These studies reveal that the crystal structure adopted by the SnS cubes and spherical polyhedra is expanded along the a and b axes and contracted along c, converging on a pseudotetragonal cell that is distinct from that of orthorhombic α-SnS, the most stable polymorph. All of the peaks observed in powder XRD patterns that are often interpreted as originating from a mixture of metastable zincblende-type SnS and α-SnS can instead be accounted for by this single-phase pseudotetragonal modification, and this helps to rationalize discrepancies that exist between theoretical predictions of SnS polymorph stability and interpretations of experimental diffraction data. This same crystallographic analysis also indicates the morphologies of the nanocrystals and the facets by which they are bound, and it reveals that the SnS cubes form through selective overgrowth of spherical polyhedral seeds.

Thermally enhanced paraffin for solar applications

Compact thermal energy storage for solar applications requires new phase change materials with higher thermal energy storage capacities. In this study, nano magnetite (Fe2O3) particles were prepared with the sol-gel method to be used for enhancing thermal properties of paraffin. 10% nano particle was added to paraffin (melting range: 56-58°C and latent heat: 119J/kg) to prepare a nano composite. SEM analysis shows that nano magnetites are homogeneously distributed in paraffin structure with particle size in the range of 15-25μm. Latent heat of the paraffin nano composite measured by DSC was 144J/g. This result indicated that thermal storage capacity of paraffin can be increased by about 20% by addition of 10% nano magnetite. Further analysis on determining thermal conductivity and thermal stability is ongoing.

Ruthenium based redox flow battery for solar energy storage

The technical performance for the operation of a stand alone redox flow battery system for solar energy storage is presented. An undivided reactor configuration has been employed along with porous graphite felt electrodes and ruthenium acetylacetonate as electrolyte in acetonitrile solvent. Limiting current densities are determined for concentrations of 0.02 M and 0.1 M ruthenium acetylacetonate. Based on these, operating conditions for 0.02 M ruthenium acetylacetonate are determined as charging current density of 7 mA/cm 2, charge electrolyte superficial velocity of 0.0072 cm/s (through the porous electrodes), discharge current density of 2 mA/cm 2 and discharge electrolyte superficial velocity of 0.0045 cm/s. An optimum power output of 35 mW is also obtained upon discharge at 2.1 mA/cm 2. With an increase in the concentration of ruthenium species from 0.02 M to 0.1 M, the current densities and power output are higher by a factor of five approximately (at same superficial velocities) due to higher mass transport phenomenon. Moreover at 0.02 M concentration the voltage efficiency is better for battery full of electrolytes prior to charging (52.1%) in comparison to an empty battery (40.5%) due to better mass transport phenomenon. Voltage efficiencies are higher as expected at concentrations of 0.1 M ruthenium acetylacetonate (55% when battery is full of electrolytes and 48% when empty) showing that the all-ruthenium redox flow battery has some promise for future applications in solar energy storage. Some improvements for the system are also discussed briefly.

A novel shape-stabilized PCM: Electrospun ultrafine fibers based on lauric acid/polyethylene terephthalate composite

Recently, ultrafine fibers of PCM/polymer composites have been developed as a novel shape-stabilized polymer-matrix phase change material (PCM) via electrospinning technique. In this study, ultrafine fibers of lauric acid/polyethylene terephthalate (LA/PET) composite (1:1, w/w) were successfully prepared and characterized by field-emission scanning electron microscopy (FE-SEM), differential scanning calorimetry (DSC) and tensile testing. The results indicated that the electrospun fibers showed smooth surfaces and cylindrical shape with diameters ranging from several tens to several hundreds nanometer, and the latent heat of fusion of the fibers is about 70.76 J/g. Although the tensile properties of the electrospun composite fibers were lower than that of the electrospun pure PET fibers, they showed suitable and competent tensile strength for the potential applications in solar energy storage and thermo-regulating textile.

Lauric and myristic acids eutectic mixture as phase change material for low-temperature heating applications

Lauric acid (m.p.: 42.6°C) and myristic acid (m.p.: 52.2°C) are phase change materials (PCM) having quite high melting points which can limit their use in low-temperature solar applications such as solar space heating and greenhouse heating. However, their melting temperatures can be tailored to appropriate value by preparing a eutectic mixture of lauric acid (LA) and myristic acid (MA). In the present study, the thermal analysis based on differential scanning calorimetry (DSC) technique shows that the mixture of 66.0 wt% LA forms a eutectic mixture having melting temperature of 34.2°C and the latent heat of fusion of 166.8 J g−1. This study also considers the experimental establishment of thermal characteristics of the eutectic PCM in a vertical concentric pipe-in-pipe heat storage system. Thermal performance of the PCM was evaluated with respect to the effect of inlet temperature and mass flow rate of the heat transfer fluid on those characteristics during the heat charging and discharging processes. The DSC thermal analysis and the experimental results indicate that the LA–MA eutectic PCM can be potential material for low-temperature solar energy storage applications in terms of its thermo-physical and thermal characteristics. Copyright © 2005 John Wiley & Sons, Ltd.

Ion‐induced luminescence control in pyrene‐tethered bipyridine ligand systems

A series of tunable fluorophores combining the pyrene nucleus and the 2, 2′‐bipyridine ligand unit are presented. They were designed to display luminescence tuning upon metal ion complexation. The effects are strongly affected by the nature of the covalent bridge linking the two subunits. The intensity, the color, but also the lifetime of the luminescence emission are features that can be finely modulated depending on the molecular structure, the nature of the solvent and the metal cation. This approach provides accesss to a new class of photosensitive receptors for applications in chemosensing, optoelectronic, or solar energy storage and conversion.

Thermophysical properties of some paraffins applicable to thermal energy storage

Abstract An experimental study has been conducted with the aim to investigate and evaluate thermophysical properties of technical grade paraffins appropriate for solar energy storage applications. The results obtained involve the kinetics of phase transition and latent heat of phase change, the thermal cycling stability at heating and cooling and specific heat for the temperature range of application. A method for the automatic computer controlled thermal cycling has been developed. The dynamic mode of precise uniform rising and lowering of the temperature at various rates has been accomplished. Nine hundred thermal cycles have been carried out on two different mixtures of technical grade paraffins, varied by their composition. The change of the DSC kinetics measured by Mettler TA 3000 and the relative thermodynamic values of the phase transitions were not registered after cycling. The calculated enthalpies of three paraffin mixtures were found to depend on their oil content and the distribution of atoms defined by chemical and gas chromatographic analyses.

Thermal stability of sodium salt hydrates for solar energy storage applications

Thermal stability of salt hydrates of sodium cation namely sodium hydroxide monohydrate, sodium acetate trihydrate, sodium thiosulphate pentahydrate, sodium carbonate decahydrate, sodium sulphate decahydrate and di-sodium hydrogen phosphate dodecahydrate has been evaluated at elevated temperatures using thermogravimetric technique. The energy of activation of dehydration of these salt hydrates have been reported. A new criterion based upon the ratio of percent water loss and energy of activation has been reported to determine the relative thermal stability of sodium salt hydrates.

Ruthenium, rhodium and palladium complexes of pyridines

The syntheses, chemical reactivity, physicochemical properties and applications in solar energy conversion and storage of ruthenium, rhodium and palladium complexes of pyridines are described.

Energetic aspects of the syngas production by solar energy: Reforming of methane and carbon gasification

The syngas (CO+H 2 ) production from methane or carbon has been studied from the energetic point of view for solar energy storage applications. Results indicate that, at operating temperatures about 900–1000 K a net enthalpy change, or stored energy, of 192 KJ (CH 4 +H 2 O) or 272 KJ (CH 4 +CO 2 ) is obtainable, per mol of reformed methane. For the carbon gasification a maximum enthalpy change of 131 KJ is aspected. Further developments are submitted to the choice of more selective catalysts and to the building of suitable chemical reactors-heat exchangers designed for solar plants.

Modeling and Simulation of a Heat Regenerator in Cyclic Equilibrium With Radiation

Heat exchangers containing a heat storing mass are called generators. The heat storing mass, usually formed of prous packing material, offers a large heat transfer surface to the fluids passing through it. Regenerators are reversed at fixed, usually regular, time intervals. The thermal storing mass removes heat from the hot fluid passing through through it and after the reversal releases it to the cold fluid. The passages of the fluids, between which heat isexchanged, are separated in time but not spatially. Regenerators are clearly distinguished from recuperators by the fact that their operation depends not only on position but also on time, and an exact calculation is considerably more involved for regenerators than it is for recuperators. A comprehensive review article by Shah (1983) encompasses all aspects of regenator theory. However, one finds that the radiation mechanism has not been directly considered, except as part of the heat exchange coefficient (Razelos and Benjamin, 1978) or by including an apparent thermal solid conductivity involving a solid-solid-fluid contact radiation effect (Olale et al., 1980). With a high fluid temperature, it would seem appropriate to consider radiation energy transfer directly into the model equations, as is done in this work. Furthermore, in viewmore » of the attention focused in the last few years on the use of a porous bed as a solid matrix for energy storage in solar applications, this work also considers the effect of void fraction on the various heat transfer mechanisms.« less

Solar Energy: When, Where, How?

Nomenclature H = enthalpy ReD —Reynolds number, based on diameter S = entropy T = temperature Wp = peak Watt AG = Gibbs free energy change 7] = efficiency I. Introduction S OLAR energy can be converted into a number of useful end-products via a wide spectrum of conversion technologies. A helpful way of categorizing technologies is to develop a morphology of solar conversion systems. Figures 13, first developed by Grosskreutz,1 illustrate a morphological breakdown of solar conversion modes by generic type. When solar radiation is absorbed by a material, two primary processes ensue: 1) vibrational excitation of atoms and molecules in the absorber produce heat which leads to endproducts via the thermoconversion path; and 2) selected quantum processes in the absorber, through electronic excitation and charge transfer, lead to useful end-products via the photoconversion path. The detailed morphology of these two paths is exhibited in Figs. 2 and 3. The salient features of each process are described below. A. The Thermoconversion Path There are five primary products associated with the thermoconversion path illustrated in Fig. 2. Of these, hot fluids, with or without concentrators, represent a direct conversion mechanism which is the most commonly known solar conversion technology. The other four are often classified as indirect conversion technologies in that solar radiation first energizes other media, such as the atmosphere (winds) and the oceans, to which man applies a conversion technology in order to extract useful power. The boundary that separates the cross-hatched area from the open area designates the cutting edge of research and development. To the left of this boundary the conversion technology is mature and is either in an advanced engineering development stage or is commercially available. Electricity and space heating represent the most familiar end-products of thermoconversion. Process heat and fuels and chemicals are more recent entries into the arena of solar thermoconversion applications. Because of its end-use matching flexibility, process heat may have a larger quad impact than electricity, whereas fuels and chemicals will add a new dimension to energy storage in solar applications, namely, spatial displacement of energy. B. The Photoconversion Path There are three primary products associated with the photoconversion path illustrated in Fig. 3. The common features to all three products are electronic excitation processes (in contradistinction to vibrational excitation for the thermoconversion path), followed by charge separation, and charge transfer mechanisms that eventually lead to electricity generation or to energy storage for fuel production. In semiconductor s, photon absorption and electronic excitation involve primarily photosensitive atoms, whereas in