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[en] Highlights: •A novel high temperature solar thermal energy storage system is presented. •The system employs a molten metal oxide for energy storage. •The system can achieve a high energy density of 5 GJ/m3. •Oxygen can be also produced as a valuable by-product. -- Abstract: A novel cycle, the chemical looping of molten copper oxide, is proposed with the thermodynamic potential to achieve sensible, latent and thermochemical heat storage with an energy density of approximately 5.0 GJ/m3, which is approximately 6 times more than the 0.83 GJ/m3 of molten salt. This cycle avoids the technical challenges associated with the application of solid materials (especially multivalent metals) for thermochemical energy storage such as attrition, agglomeration, particle breakage and structural change in successive reduction and oxidation reactions, although it brings alternative challenges associated with the handling of molten metal oxides. A process path for the concept is proposed based on data from the literature for the equilibrium composition of copper and oxygen at different temperatures and gas phase pressures. The process has been modelled with codes developed in MATLAB. The calculations estimate that from the total input concentrated solar thermal energy into the system, about 73% can be absorbed, while the rest is lost through re-radiation heat loss. Furthermore, it is estimated that of the absorbed heat, approximately 95% is stored, while the rest leaves the system as high temperature gas. The calculations also predict that approximately 20% of the inlet solar thermal energy is partitioned as the chemical storage, which is also employed for oxygen production. Also reported is the sensitivity to the effects of key operating parameters.
[en] Highlights: • Impact of non-displaced feedwater heater on plant’s performance has been evaluated. • Two operation strategies for non-displaced feedwater heater has been proposed. • Constant temperature strategy is generally better. • Constant mass flow rate strategy is suit for rich solar thermal input. - Abstract: Solar Aided Power Generation is a technology in which low grade solar thermal energy is used to displace the high grade heat of the extraction steam in a regenerative Rankine cycle power plant for feedwater preheating purpose. The displaced extraction steam can then expand further in the steam turbine to generate power. In such a power plant, using the (concentrated) solar thermal energy to displace the extraction steam to high pressure/temperature feedwater heaters (i.e. displaced feedwater heaters) is the most popular arrangement. Namely the extraction steam to low pressure/temperature feedwater heaters (i.e. non-displaced feedwater heaters) is not displaced by the solar thermal energy. In a Solar Aided Power Generation plants, when solar radiation/input changes, the extraction steam to the displaced feedwater heaters requires to be adjusted according to the solar radiation. However, for the extraction steams to the non-displaced feedwater heaters, it can be either adjusted accordingly following so-called constant temperature strategy or unadjusted i.e. following so-called constant mass flow rate strategy, when solar radiation/input changes. The previous studies overlooked the operation of non-displaced feedwater heaters, which has also impact on the whole plant’s performance. This paper aims to understand/reveal the impact of the two different operation strategies for non-displaced feedwater heaters on the plant’s performance. In this paper, a 300 MW Rankine cycle power plant, in which the extraction steam to high pressure/temperature feedwater heaters is displaced by the solar thermal energy, is used as study case for this purpose. It was found that plant adopting the constant temperature strategy is generally better than that adopting the constant mass flow rate strategy. However, if rich solar energy is available, adopting the constant mass flow rate strategy can achieve better performance.
[en] Highlights: • SAPG with concentrating and non-concentrating collectors has been compared. • Non-concentrating collectors could be superior to concentrating collectors in SAPG. • Using non-concentrating collectors is more effective in low latitude. - Abstract: The preheating of the feedwater in a Regenerative Rankine Cycle power plant with solar thermal energy, termed Solar Aided Power Generation, is an efficient method to use low to medium temperature solar thermal energy. Here, we compared the use of medium temperature (200–300 °C) energy from concentrating solar collectors (e.g. parabolic trough collectors) to displace the extraction steam to high temperature/pressure feedwater heaters with that from low temperature (100–200 °C) non-concentrating solar collectors (e.g. evacuated tube collectors) to displace the extraction steam to low temperature/pressure feedwater heaters of the power plant. In this paper, the in terms of net land based solar to power efficiency and annual solar power output per collector capital cost of a Solar Aided Power Generation using concentrating and non-concentrating solar collectors has been comparted using the annual hourly solar radiation data in three locations (Singapore; Multan, Pakistan and St. Petersburg, Russia). It was found that such a power system using non-concentrating solar collectors is superior to concentrating collectors in terms of net land based solar to power efficiency. In some low latitude locations e.g. Singapore, using non-concentrating solar collectors even have advantages of lower solar power output per collector capital cost over using the concentrating solar collectors in an SAPG plant.
[en] There is now strong evidence that long-duration gamma-ray bursts (LGRBs) are preferentially formed in low-metallicity environments. However, the magnitude of this effect and its functional dependence on metallicity have not been well characterized. In our previous paper, we compared the metallicity distribution of LGRB host galaxies to that of star-forming galaxies in the local universe. Here we build upon this work by in effect dividing one distribution by the other, and thus directly determine the relative rate of LGRB formation as a function of metallicity in the low-redshift universe. We find a dramatic cutoff in LGRB formation above a metallicity of in the KK04 scale, with LGRBs forming between 10 and 50 times more frequently per unit star formation below this cutoff than above. Furthermore, our data suggest that the rate of LGRB formation per unit star formation continues to fall above this break. We estimate that the LGRB formation rate per unit star formation may drop by as much as a factor of 100 between one-third solar and solar metallicity.
[en] Light emission under MeV hydrogen and oxygen ions in stoichiometric SrTiO_3 are identified at temperatures of 100 K, 170 K and room-temperature. MeV ions predominately deposit their energies to electrons in SrTiO_3 with energy densities orders of magnitude higher than from UV or x-ray sources but comparable to femtosecond lasers. The ionoluminescence (IL) spectra can be resolved into three main Gaussian bands at 2.0 eV, 2.5 eV and 2.8 eV, whose relative contributions strongly depend on irradiation temperature, electronic energy loss and irradiation fluence. Two main bands, observed at 2.5 eV and 2.8 eV, are intrinsic and associated with electron–hole recombination in the perfect SrTiO_3 lattice. The 2.8 eV band is attributed to recombination of free (conduction) electrons with an in-gap level, possibly related to self-trapped holes. Self-trapped excitons (STEs) are considered suitable candidates for the 2.5 eV emission band, which implies a large energy relaxation in comparison to the intrinsic edge transition. The dynamics of electronic excitation, governs a rapid initial rise of the intensity; whereas, accumulated irradiation damage (competing non-radiative recombination channels) accounts for a subsequent intensity decrease. The previously invoked role of isolated oxygen vacancies for the blue luminescence (2.8 eV) does not appear consistent with the data. An increasing well-resolved band at 2.0 eV dominates at 170 K and below. It has been only previously observed in heavily strained and amorphous SrTiO_3, and is, here, attributed to transitions from d(t 2g) conduction band levels to d(e g) levels below the gap. In accordance with ab initio theoretical calculations they are associated to trapped electron states in relaxed Ti"3"+ centers at an oxygen vacancy within distorted TiO_6 octahedra. The mechanism of defect evolution monitored during real-time IL experiments is presented. In conclusion, the light emission data confirm that IL is a useful tool to investigate lattice disorder in irradiated SrTiO_3.
[en] Palladium (Pd) is considered as a possible candidate as catalyst for proton exchange membrane fuel cells (PEMFCs) due to its high activity and affordable price compared to platinum (Pt). However, the stability of Pd is known to be limited, yet still not fully understood. In this work, Pd dissolution is studied in acidic media using an online inductively coupled plasma mass spectrometry (ICP-MS) in combination with an electrochemical scanning flow cell (SFC). Crucial parameters influencing dissolution like potential scan rate, upper potential limit (UPL) and electrolyte composition are studied on a bulk polycrystalline Pd (poly-Pd). Furthermore, a comparison with a supported high-surface area catalyst is carried out for its potential use in industrial applications. For this aim, a carbon supported Pd nanocatalyst (Pd/C) is synthesized and its performance is compared with that of bulk poly-Pd. Our results evidence that the transient dissolution is promoted by three main contributions (one anodic and two cathodic). At potentials below 1.5 VRHE the anodic dissolution is the dominating mechanism, whereas at higher potentials the cathodic mechanisms prevail. On the basis of the obtained results, a model is thereafter proposed to explain the transient Pd dissolution.
[en] Recent Atacama Large Millimeter/submillimeter Array observations present mounting evidence for the presence of exocometary gas released within Kuiper Belt analogs around nearby main-sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J = 2-1 emission colocated with dust emission from the cometary belt in the 440 Myr old Fomalhaut system. Through spectrospatial filtering, we achieve a 5.4σ detection and determine that the ring’s sky-projected rotation axis matches that of the star. The CO mass derived () is the lowest of any circumstellar disk detected to date and must be of exocometary origin. Using a steady-state model, we estimate the CO+CO2 mass fraction of exocomets around Fomalhaut to be between 4.6% and 76%, consistent with solar system comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct interstellar medium inheritance. In addition, we find tentative evidence that % of the detected flux originates from a region near the eccentric belt’s pericenter. If confirmed, the latter may be explained through a recent impact event or CO pericenter glow due to exocometary release within a steady-state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.
[en] Highlights: • ORR activity and selectivity are tested using a RRDE for different AuPd compositions. • Size-controlled nanoparticles are immobilized directly on to the electrode. • Au rich particles favour 2-electron reduction with low activity. • Au nanoparticles can maintain high selectivity to H2O2 over time. The electrochemical synthesis of hydrogen peroxide (H2O2) represents a promising alternative to the anthraquinone process, as it combines on-site chemical and electrical production. The design of selective electrocatalysts is challenging and is commonly based on the alloying of elements to generate a synergistic effect and increase activity. In the present work, we report the electrochemical activity of Au-Pd nanoparticles immobilized directly onto an electrode as a model to study H2O2 electrochemical synthesis from fundamentals to continuous production. The impact of composition on the oxygen reduction reaction (ORR), the selectivity, as well as the peroxide reduction and oxidation reactions (PROR) are studied.