• Energy Research

This paper investigates the cryogenic heat transfer phenomena of nitrogen flowing in helically coiled tubes under the combined effects of pseudocritical conditions, buoyancy, and coil curvature. The ultimate goal was to design optimum heat exchangers for liquid air energy storage. Local heat transfer coefficients were evaluated peripherally across tube cross sections. The pressure, mass flux, and heat flux effects on the heat transfer were examined. The dual effect of buoyancy and coil curvature on heat transfer coefficients was interpreted via a dimensionless number Ψ, which denotes a ratio between the two effects. Results reveal that the heat transfer coefficients increase with increasing mass flux but decreasing pressure and heat flux. The buoyancy effect dominates the heat transfer at fluid temperatures below the pseudocritical temperature (e.g., −146.3 °C at 35 bar), while the coil curvature-induced centrifugal effect dominates at higher temperatures. The heat transfer coefficients for the helical coil were approximately 13% lower compared with those in straight tube at fluid temperatures below the pseudocritical temperature, but their difference shrinks (<±6%) at higher temperatures. The reason is that the benefits of coil curvature and improved turbulent mixing on heat transfer are counteracted by the thermophysical property variation and buoyancy effect.

Calcium carbonate is promising thermochemical heat storage material for next-generation solar power systems due to its high energy storage density, low cost, and high operation temperature. Researchers have tried to improve energy storage performances of calcium carbonate recently, but most researches focus on powders, which are not suitable for scalable applications. Here, novel granular porous calcium carbonate particles with very high solar absorptance, energy storage density, abrasive resistances, and energy storage rate are proposed for direct solar thermochemical heat storage. The average solar absorptance is improved by 234% compared with ordinary particles. Both cycle stability and abrasive resistances are excellent with almost no decay of energy storage density over 25 cycles nor apparent particle weight loss over 24 h of continuous operation insides a planetary ball mill. In addition, the decomposition temperature is reduced by 2.8%–5.6% while the reaction rate of heat storage is enhanced by 80%–205% depending on the CO2 partial pressure. The decomposition process of doped granular porous CaCO3 particles is found to involve three overlapping processes. This work provides new routes to achieve scalable direct solar thermochemical heat storage for next-generation high-temperature solar power systems.

Abstract The indoor solar simulator is an excellent supply device for high solar flux applications, but usually suffers from the low flux under a large light area and unadjustable light distribution. In this work, a 130 kWe indoor solar simulator with tunable ultra-high flux in a projection area of 200 mm diameter is designed and established. 13 of 10 kWe xenon short-arc lamps and reflectors are coupled to make the flux be concentrated to a spot. Besides, a direct measure system consisting of a Gardon gauge and a two-dimensional moving unit is proposed to map the flux distribution. The expanded uncertainty is 7.46% with a converge factor of 2, compared with the conventional indirect camera-based method with an error up to 50%. Up to 7 lamps are simultaneously measured. The Monte Carlo method is adopted to analyze flux distributions of individual lamps as well as the whole lamp group, which coincides well with the experimental data. When the input electric power is 11.12 kW, the electric-to-light conversion efficiency is 31.60% ± 2.36% on the focus plane. The peak flux reaches 11.267 ± 1.571 MW/m2 and a mean flux amounts to 1.054 ± 0.079 MW/m2 in a projection area of 200 mm diameter. Both the distribution and non-uniformity of irradiation flux are tunable. By reducing the power of the central lamp to the half and defocusing the target, the distribution non-uniformity of the light intensity can be remarkably reduced from 68.87% to 18.59% in a projection area of 60 mm diameter. The developed device will provide a reliable source for indoor experiments.

Abstract In this paper, we synthesized bionic bone hierarchical porous AlN ceramics which encapsulate polyethylene glycol2000 (PEG2000) to form a new composite phase change material . Argon adsorption and SEM demonstrate that the AlN ceramic has hierarchical structure with most probable diameters of 2.4 nm and 27.3 nm. Raman spectroscopy show that no aluminum vacancies are formed inside the material, which effectively reduce the probability of phonon scattering and improve the thermal performance. EDS and XRD demonstrate that the main crystalline phase of the ceramics is AlN with a small amount of impurity peaks. The thermal conductivity of AlN ceramics with a porosity of 75.2% and 67.5% are 5.10 and 13.86 W/m·K, and that of the composite material is 17.16 W/m·K. The latent heat of melting of the composite material is 88.73 J/g and the melting point is 54.75 °C. The material breaks through the defects of slow dynamic response time and low thermal conductivity of traditional energy storage materials .

Abstract We investigated the microstructures and their formation mechanisms of carbonate salt based composite phase change materials (CPCMs). Such materials typically consist of a carbonate salt as the phase change material (PCM), a thermal conductivity enhancement material (TCEM) and a ceramic skeleton material (CSM) for structure stabilisation, and are mainly for medium and high temperature thermal energy storage applications. Two carbonate salt based composites were studied with one being eutectic NaLiCO3 and the other Na2CO3. MgO and graphite flakes were used respectively as the CSM and TCEM for fabricating the composite modules. A scanning electron microscope with energy dispersive spectrometer (SEM-EDS) was used to observe the microstructures and the salt distribution and redistribution within the composite structures during repeated melting-solidification cycles. The results showed salt migration within the composite structure during the thermal cycling. Such a microscopic motion led to a more homogenous distribution of not only the salt but also the CSM and TCEM. At a low graphite flake loading, breakage of the graphite flake was observed, suggesting stress generation during thermal cycling. The extent of the breakage reduced with increasing graphite flake loading, suggesting the stress generation be related to microscopic motion. The MgO based CSM particles were likely to be sintered, forming a porous structure. Such a structure reduced the swelling effect partially due to the use of graphite on which the salts had a poor wettability.

Abstract As a promising energy storage technology, liquid carbon dioxide energy storage has become a hotspot due to its high energy density and less restriction by the geographical conditions. A new liquid carbon dioxide energy storage system with cold recuperator and low pressure stores is presented in this paper. Mathematical model of the system is established and parametric analysis is conducted to investigate the influences of some crucial variables on the system performance. Moreover, advanced exergy analysis is utilized to reveal the interactions among components and the system’s real improvement potential. Results demonstrate that the system embraces a 7.3% improvement of round trip efficiency compared to the system without cold recuperator. More importantly, the total volume of tanks is reduced by 32.65%. Advanced exergy analysis indicates that the interconnections between system components are not very strong. Both conventional and advanced exergy analysis suggest that the low pressure compressor should be given the first priority for optimization. The difference is that the high pressure compressor and latent cold storage is in the third and fourth place for the conventional method while it is located in the second and fifth priority in the advanced method.

In two-step solar thermochemical CO2-to-fuel conversion reactions, an ultrahigh isothermal cycle CO yield (376.1 μmol g−1) at no more than 1300 °C is reported based on the proposed Sm0.6Sr0.4MnO3.

Abstract Conventional solar thermochemical heat storage based on indirect surface-heating usually suffers from high heat losses and low solar-chemical efficiency. Here, a different solar thermochemical heat storage system based on direct solar illumination on fluidized black CaCO3 pellets is proposed. A Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) model considering irradiation ray tracing, granular flow, heat and mass transfer, and chemical reaction, is built. Black CaCO3 pellets are fabricated via a facile template mixing method, and the solar absorptance is enhanced to 63.9% from 27.9% of traditional pure CaCO3. Effects of gas velocity and irradiative flux on thermochemical heat storage performance in a fluidized volumetric bed are investigated by incorporating measured kinetic and solar absorptance properties of designed black CaCO3 pellets. The peak solar-chemical efficiency reaches a value higher than 43% benefiting from enhanced solar absorptance, higher gas velocity and irradiative flux. This work guides the design of the high-efficiency direct solar thermochemical heat storage system.