Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

Working fluids for high-temperature organic Rankine cycles

Alkanes, aromates and linear siloxanes are considered as working fluids for high-temperature organic Rankine cycles (ORCs). Case studies are performed using the molecular based equations of state BACKONE and PC-SAFT. First, “isolated” ORC processes with maximum temperatures of 250 °C and 300 °C are studied at sub- or supercritical maximum pressures. With internal heat recovery, the thermal efficiencies η_th averaged over all substances amount to about 70% of the Carnot efficiency and increase with the critical temperature. Second, we include a pinch analysis for the heat transfer from the heat carrier to the ORC working fluid by an external heat exchanger (EHE). The question is for the least heat capacity flow rates of the heat carrier required for 1 MW net power output. For the heat carrier inlet temperatures of 280 °C and 350 °C are considered. Rankings based on the thermal efficiency of the ORC and on the heat capacity flow rates of the heat carrier as well as on the volume and the heat flow rates show cyclopentane to be the best working fluid for all cases studied.     

Highly conductive composites made of phase change materials and graphite for thermal storage

Conventional phase change materials (PCMs) are already well known for their high thermal capacity and constant working temperature for thermal storage applications. Nevertheless, their low thermal conductivity (around 1 W m⁻¹ K⁻¹) leads to low and decreasing heat storage and discharge powers. Up to now, this major drawback has drastically inhibited their possible applications in industrial or domestic fields. The use of graphite to enhance the thermal conductivity of those materials has been already proposed in the case of paraffin but the corresponding applications are restricted to low-melting temperatures (below 150 °C). For many applications, especially for solar concentrated technologies, this temperature range is too low. In the present paper, new composites made of salts or eutectics and graphite flakes, in a melting temperature range of 200-300 °C are presented in terms of stability, storage capacity and thermal conductivity. The application of those materials to thermal storage is illustrated through simulated results according to different possible designs. The synergy between the storage composite properties and the interfacial area available for heat transfer with the working fluid is presented and discussed.     

The preparation and properties of multi-component molten salts

This paper was focused on thermal stability of molten salts and their thermo-physical properties at high temperature. In this experiment, multi-component molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate with 5% additive A of the chlorides were prepared by statical mixing method. The experiments found molten salt with 5% additive A had higher thermal stability and its best operating temperature would be increased to 550 °C from 500 °C when comparing to ternary nitrate salt. Meanwhile, thermal stability and thermal cycling analysis showed molten salts with 5% additive A had lower freezing point and loss of nitrite content and deterioration time of molten salts were reduced at the same time. DSC tests also indicated loss of latent heat in molten salts with 5% additive A was decreased. Besides, thermo-physical properties measured showed molten salt with 5% additive A had a heat capacity of 2.32 kJ/kg °C, lower than 4.19 kJ/kg °C for water between 0 °C and 100 °C and a low viscosity range from 3.0 to 1.4 cp between 150 °C and 500 °C, analogous with 1.8–0.3 cp for water between 0 °C and 100 °C. Other thermo-physical properties, such as thermal conductivity, density and linear thermal expansion, were also determined here.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.     

Thermal decomposition of pyrotechnic mixtures containing sucrose with either potassium chlorate or potassium perchlorate

The thermal behavior of sucrose, KClO₃, KClO₄, and KClO₃ + sucrose and KClO₄ + sucrose mixtures was studied experimentally using differential thermal analysis (DTA) and thermogravimetry (TG). The mixtures (KClO₃ or KClO₄ with sucrose) are sometimes used as pyrotechnic mixtures in military activities. The results demonstrate that the ignition temperatures for KClO₃ and KClO₄ are 472 and 592 °C, respectively. On the other hand, DTA-TG of pure sucrose indicates that this compound decomposes at 180 °C before it initiates. Also, when KClO₃ is replaced by KClO₄ as oxidizer of the fuel, sucrose, the sensitivity of the mixture decreases. In addition, the ignition temperature for a mixture of KClO₃ and sucrose can be as low as 180 °C. However, replacing KClO₃ with KClO₄ elevates the ignition temperature of the mixture by at least 200 to ∼410 °C, which is a safe temperature for preventing activation of the mixture by accidental factors such as static electricity.     

Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte

La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd_0.1Ce_0.9O_1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm² at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm² was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm² at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm² at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm² at 650, 600, 550 and 500 °C, respectively.     

Thermal properties of phosphoric acid-doped polybenzimidazole membranes in water and methanol-water mixtures

The thermal properties of phosphoric acid-doped poly[2-2′-(m-phenylene)-5-5′ bi-benzimidazole] (PBI) and poly[2,5-benzimidazole] (ABPBI) membranes, ionomeric materials with promising properties to be used as electrolytes in direct methanol and in high temperature polymer electrolyte membrane (PEM) fuel cells, were studied by means of differential scanning calorimetry (DSC) technique in the temperature range from −145 °C to 200 °C. The DSC scans of samples equilibrated in water at different relative humidities (RH) and in liquid water-methanol mixtures were analyzed in relation to glass transition, water crystallization/melting and solvent desorption in different temperature regions. The thermal relaxation observed in the very low temperature region could be ascribed to the glass transition of the H₃PO₄-H₂O mixture confined in the polymeric matrix. After cooling the samples up to −145 °C, frozen water was detected in PBI and ABPBI at different RH, although at 100% RH less amount of water had crystallized than that observed in Nafion membranes under the same conditions. Even more important is the fact that the freezing degree of water is much lower in ABPBI membranes equilibrated in liquid water-methanol mixtures than that observed for PBI and, in a previous study, for Nafion. Thus, apart from other well known properties, acid-doped ABPBI emerges as an excellent ionomer for applications in direct methanol fuel cells working in cold environments.     

Thermal characteristics of a FeF3 cathode via conversion reaction in comparison with LiFePO4

The thermal stability of a FeF₃ cathode via a conversion reaction was quantitatively studied using differential scanning calorimetry (DSC). Mixtures of charged and discharged FeF₃ electrodes and electrolyte were measured by changing the ratio of electrode to electrolyte. A mild exothermic peak was observed at temperatures ranging from 210 to 380 °C for the mixtures of charged electrode and electrolyte even if the electrode/electrolyte ratio was changed. Moreover, the cycling depth had no effect on the thermal stability of the charged electrode in the electrolyte. For the mixtures of discharged electrode and electrolyte, exothermic reactions occurred in the range of 250-350 °C, which varied with the electrode/electrolyte ratio. Although the exothermic reactions of the mixtures varied with the electrode/electrolyte ratio, the thermal risk for both charged and discharged electrodes coexisted with the electrolyte appeared to be mainly due to electrolyte decomposition. By comparing the heat values of mixtures of the charged and discharged electrodes and electrolyte, the FeF₃ electrodes in the electrolyte demonstrated better thermal stability than LiFePO₄ electrodes at elevated temperatures.     

Carbon-doped Sb2S3 thin films: Structural, optical and electrical properties

We report the modification of electrical properties of chemical-bath-deposited antimony sulphide (Sb₂S₃) thin films by thermal diffusion of carbon. Sb₂S₃ thin films were obtained from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on glass substrates. A carbon thin film was deposited on Sb₂S₃ film by arc vacuum evaporation and the Sb₂S₃-C layer was subjected to heating at 300 °C in nitrogen atmosphere or in low vacuum for 30 min. The value of resistivity of Sb₂S₃ thin films was substantially reduced from 10⁸ Ω cm for undoped condition to 10² Ω cm for doped thin films. The doped films, Sb₂S₃:C, retained the orthogonal stibnite structure and the optical band gap energy in comparison with that of undoped Sb₂S₃ thin films. By varying the carbon content (wt%) the electrical resistivity of Sb₂S₃ can be controlled in order to make it suitable for various opto-electronic applications.     

High performance composite interconnect La0.7Ca0.3CrO3/20 mol% ReO1.5 doped CeO2 (Re = Sm,Gd, Y) for solid oxide fuel cells

This paper reports and discusses composite interconnect materials that were modified from La_0.7Ca_0.3CrO_3−δ (LCC) by addition of Re doped CeO₂ (Re = Sm, Gd, Y) for improved conductivity at relative low temperatures. It is found that the addition of small amounts of RDC (ReO_1.5 doped CeO₂) into LCC dramatically increased the electrical conductivity. For the best system studied, LCC + 5 wt% SDC (Sm_0.2Ce_0.8O_1.9), LCC + 3 wt% GDC (Gd_0.2Ce_0.8O_1.9) and LCC + 3 wt% YDC (Y_0.2Ce_0.8O_1.9), the electrical conductivities reached 687.8, 124.6 and 104.8 S cm⁻¹ at 800 °C in air, respectively. The electrical conductivities of the specimens, LCC + 3 wt% SDC, LCC + 1 wt% GDC and LCC + 2 wt% YDC in H₂ at 800 °C were 7.1, 3.8 and 5.9 S cm⁻¹, respectively. With the increase of RDC content, the relative density increased, indicating that RDC served as an effective sintering aid in enhancing the sinterability of the powders. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with the increase of the RDC content. The oxygen permeation measurements indicated a negligible oxygen ionic conduction, indicating that the efficiency loss of a solid oxide fuel cell by permeation is negligible for the general cell design using LCC + RDC as interconnect. Therefore, the composite materials La_0.7Ca_0.3CrO₃/20 mol% ReO_1.5 doped CeO₂ are very promising interconnecting ceramics for solid oxide fuel cells (SOFCs).     

Assessment of PrBaCo2O5+δ + Sm0.2Ce0.8O1.9 composites prepared by physical mixing as electrodes of solid oxide fuel cells

The performance of PrBaCo₂O_5+δ + Sm_0.2Ce_0.8O_1.9 (PrBC + SDC) composites as electrodes of intermediate-temperature solid oxide fuel cells is investigated. The effects of SDC content on the performance and properties of the electrodes, including thermal expansion, DC conductivity, oxygen desorption, area specific resistance (ASR) and cathodic overpotential are evaluated. The thermal expansion coefficient and electrical conductivity of the electrode decreases with an increase in SDC content. However, the electrical conductivity of a composite electrode containing 50 wt% SDC reaches 150 S cm⁻¹ at 600 °C. Among the various electrodes under investigation, an electrode containing 30 wt% SDC exhibits superior electrochemical performance. A peak power density of approximately 1150 and 573 mW cm⁻² is reached at 650 and 550 °C, respectively, for an anode-supported thin-film SDC electrolyte cell with the optimal composite electrode. The improved performance of a composite electrode containing 70 wt% PrBC and 30 wt% SDC is attributed to a reduction in the diffusion path of oxygen-ions within the electrode, which is a result of a three-dimensional oxygen-ion diffusion path in SDC and a one-dimensional diffusion path in PrBC.     

Hydrogen storage properties of destabilized MgH2–Li3AlH6 system

To improve the dehydrogenation properties of MgH₂, a novel hydrogen storage system, MgH₂–Li₃AlH₆, is prepared by mechanochemical milling. Three physical mixtures containing different mole ratios (1:4, 1:1 and 4:1) of MgH₂ and Li₃AlH₆ are studied and there exists a mutual destabilization effect between the components. The last mixture shows a capacity of 6.5 wt% H₂ with the lowest starting temperature of dehydrogenation (170 °C). First, Li₃AlH₆ decomposes into Al, LiH and H₂, and then the as-formed Al can easily destabilize MgH₂ to form the intermetallic compound Mg₁₇Al₁₂ at a temperature of 235 °C, which is about 180 °C lower than the decomposition temperature of pristine MgH₂. Finally, the residual MgH₂ undergoes a self-decomposition whose apparent activation energy has been reduced by about 22 kJ mol⁻¹ compared with pristine MgH₂. At a constant temperature of 250 °C, the mixture can dehydrogenate completely under an initial vacuum and rehydrogenate to form MgH₂ under 2 MPa H₂, showing good cycle stability after the first cycle with a capacity of 4.5 wt% H₂. The comparison between 4 MgH₂ + Li₃AlH₆ and 4 MgH₂ + LiAlH₄ mixtures is also investigated.     

Enhanced sinterability of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ by addition of nickel oxide

The effect of nickel oxide addition on the sintering behavior and electrical properties of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as an electrolyte for solid oxide fuel cells was systematically studied. Results suggest that the addition of a small amount (∼1 wt%) of NiO to BZCYYb greatly promoted densification, achieving ∼96% of the theoretical density after sintering at 1350 °C in air for 3 h (reducing the sintering temperature by ∼200 °C). Further, a sample sintered at 1450 °C for 3 h showed high open circuit voltages (OCVs) when used as the electrolyte membrane to separate the two electrodes under typical SOFC operating conditions, indicating that the electrical conductivity of the electrical conductivity of the BZCYYb was not adversely affected by the addition of ∼1 wt% NiO.     

Highly conductive epoxy/graphite composites for bipolar plates in proton exchange membrane fuel cells

Carbon-filled epoxy composites are developed for potential application as bipolar plates in proton exchange membrane (PEM) fuel cells. These composites are prepared by solution intercalation mixing, followed by compression molding and curing. Electrical conductivity, thermal and mechanical properties, and hygrothermal characteristics are determined as function of carbon-filler content. Expanded graphite and carbon black are used as synergistic combination to obtain desired in-plane and through-plane conductivities. These composites show high glass transition temperatures (T_g ∼ 180 °C), high thermal degradation temperatures (T₂ ∼ 415 °C), in-plane conductivity of 200–500 S cm⁻¹ with 50 wt% carbon fillers, in addition to offering high values of flexural modulus, flexural strength, and impact strength, respectively 2 × 10⁴ MPa, 72 MPa, and 173 J m⁻¹. The presence of carbon fillers helps reduce water uptake from 4 to 5 wt% for unfilled epoxy resins to 1–2 wt%. In addition, morphology, electrical, mechanical, and thermal properties remain unchanged on exposure to boiling water and acid reflux. This data indicate that the composites developed in this work meet many attributes of bipolar plates for use in PEM fuel cells.     

High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells

Novel aromatic polyether type copolymers bearing side chain polar pyridine rings as well as combination of main and side chain pyridine units have been evaluated as potential polymer electrolytes for proton exchange membrane fuel cells (PEMFCs). The advanced chemical and physicochemical properties of these new polymers with their high oxidative stability, mechanical integrity and high glass transition temperatures (T_g's up to 270 °C) and decomposition temperatures (T_d's up to 480 °C) make them promising candidates for high and medium temperature proton exchange membranes in fuel cells. These copolymers exhibit adequate proton conductivities up to 0.08 S cm⁻¹ even at moderate phosphoric acid doping levels. An optimized terpolymer chemical structure has been developed, which has been effectively tested as high temperature phosphoric acid imbibed polymer electrolyte. MEA prepared out of the novel terpolymer chemical structure is approaching state of the art fuel cell operating performance (135 mW cm⁻² with electrical efficiency 45%) at high temperatures (150-180 °C) despite the low phosphoric acid content (<200 wt%) and the low platinum loading (ca. 0.7 mg cm⁻²). Durability tests were performed affording stable performance for more than 1000 h.     

Electrocatalytic properties of new spinel-type MMoO4 (M = Fe,Co and Ni) electrodes for oxygen evolution in alkaline solutions

New spinel-type oxides with molecular formulae MMoO₄ (M = Fe, Co and Ni) have been prepared by a thermal decomposition method at 650 °C and investigated as electrocatalysts for the O₂ evolution reaction (OER) in KOH solutions. The results show that the new oxide electrocatalysts are highly active for the OER, but they have very low electrochemically active areas, the relative oxide roughness factor being <2. The Tafel slope values for the OER on each oxide in 1 M KOH are found to be ∼40 and ∼60 mV at low and higher potentials, respectively. The reaction order with respect to OH⁻ concentration has been observed to be 1.18, 1.51 and 1.94 on CoMoO₄, FeMoO₄ and NiMoO₄ electrodes, respectively. Both in 1 M KOH and practical cell solution (30 wt% KOH) at 25 °C, the performance of the FeMoO₄ electrode is observed to be better than the other two electrodes.     

Effects of the addition of ZnO and Y2O3 on the electrochemical characteristics of a Ni(OH)2 electrode in nickel–metal hydride secondary batteries

This study examined the effects of the addition of ZnO and Y₂O₃ on the electrochemical characteristics of a Ni(OH)₂ electrode in nickel–metal hydride (Ni–MH) secondary batteries. The discharge capacity of the electrode was less affected by the addition of ZnO and Y₂O₃ at a 0.2 C-rate and 25 °C. However, the addition of Y₂O₃ deteriorated the discharge capacity and the cycle life of the electrode by increasing the charge transfer resistance of the electrode at an increased C-rate of 1 C and 25 °C. Under severer conditions at 1 C-rate and 60 °C, the electrode materials were separated from the current collector and, accordingly, the discharge capacity was abruptly degraded with cycling for the electrodes comprising only 4 wt% ZnO or 4 wt% Y₂O₃. In contrast, the electrodes containing both 2 wt% ZnO and 2 wt% Y₂O₃ exhibited stable discharge capacity with cycling and excellent cycle life due to the high overvoltage for oxygen evolution. The present results indicate that the addition of ZnO and Y₂O₃ with an optimum composition suppresses oxygen evolution in the interfaces between active materials and the current collector and improves the cycle life of the electrode.     

Role of MgCo compound on the sorption properties of the Mg-Co milled mixtures

The influence of MgCo on the reaction paths during hydriding and dehydriding processes of Mg-Co mixtures was studied using a combined HP-DSC and XRD approach. Mg-Co mixtures with different compositions were mechanically milled under argon to prepare Mg-Co nanocomposites and then submitted to thermal treatment at 300 °C for 5 days to induce MgCo formation. The local Mg-Co composition in the milled and milled-heated samples determines the nature of the phases obtained after hydriding/dehydriding cycling. The formation of Mg₆Co₂H₁₁, Mg₂CoH₅ and MgH₂ hydrides occurs after the first hydriding stage of the 2Mg-Co and Mg-Co milled mixtures due to kinetic restrictions. On the contrary, Mg-Co milled-heated mixture exhibits the selective formation of Mg₂CoH₅ during first hydriding via two-step reaction. In the first one, MgCo disproportion to MgH₂ and Co takes place simultaneously with Mg hydriding (<200 °C). The second step involves MgCo hydriding to Mg₂CoH₅ through MgH₂ as intermediate phase (>200 °C). Dehydriding reaction is enhanced by dispersion of Co into Mg-matrix, which reduces more than 100 °C the hydrogen desorption temperature when compared with the Mg-Co milled sample without previous heating.     

Effects of inclination angle on conduction—natural convection in divided enclosures filled with different fluids

Laminar natural convection in inclined enclosures filled with different fluids was studied by a numerical method. The enclosure was divided by a solid impermeable divider. One side of partition of enclosure was filled with air and the other side had water. The enclosure was heated from one vertical wall and cooled from the other while horizontal walls were adiabatic. The governing equations which were written in stream function–vorticity form were solved using a finite difference technique. Results were presented by streamlines, isotherms, mean and local Nusselt numbers for different thermal conductivity ratios of solid impermeable material (plywood or concrete), inclination angle (0° ≤ ? ≤ 360°) and Grashof numbers (10³ ≤ Gr ≤ 10⁶). The code was validated by earlier studies, which are available in the literature on conjugate natural convection heat transfer. Analytical solutions were obtained for low Grashof numbers. Obtained results showed that both heat transfer and flow strength strongly depended on thermal conductivity ratio of the solid material of partition, inclination angle and Grashof numbers. The heat transfer was lower in the air side of the enclosure than that of the water side.