Enhancing the performance of dielectric capacitors toward higher energy density and higher operating temperatures has been drawing increased interest. Therefore, in this investigation, research efforts were dedicated to the fabrication and characterization of nanocomposites in order to enhance the energy density at both room temperature and elevated temperature. The dielectric capacitors are fabricated using nanocomposites composed of BaTiO3 nanoparticles with polyimide (PI) matrix aiming at combining the high relative dielectric permittivity of the ceramic filler and the high breakdown strength of the polymeric matrix. Dielectric energy storage performance is assessed for nanocomposites with volume fractions ranging from 0 to 20% under operating frequency from 20 Hz to 1 MHz and temperatures ranging from 20 to 120∘C. It is observed that with the increase of temperature, the capacitance increased while the energy density slightly decreased but significantly higher than pure polymer samples. The highest energy density was found for BaTiO3/PI nanocomposites with 20% volume fraction, 9.63 J/cm3 at 20∘C and 6.79 J/cm3 at 120∘C. Overall, testing results indicate that using nanocomposites of BaTiO3/PI as a dielectric component shows promise for implementation to preserve high energy density values up to temperatures of 120∘C.
High-temperature receiver designs for solar powered supercritical CO2 Brayton cycles that can produce ∼1 MW of electricity are being investigated. Advantages of a supercritical CO2 closed-loop Brayton cycle with recuperation include high efficiency (∼50%) and a small footprint relative to equivalent systems employing steam Rankine power cycles. Heating for the supercritical CO2 system occurs in a high-temperature solar receiver that can produce temperatures of at least 700 °C. Depending on whether the CO2 is heated directly or indirectly, the receiver may need to withstand pressures up to 20 MPa (200 bar). This paper reviews several high-temperature receiver designs that have been investigated as part of the SERIIUS program. Designs for direct heating of CO2 include volumetric receivers and tubular receivers, while designs for indirect heating include volumetric air receivers, molten-salt and liquid-metal tubular receivers, and falling particle receivers. Indirect receiver designs also allow storage of thermal energy for dispatchable electricity generation. Advantages and disadvantages of alternative designs are presented. Current results show that the most viable options include tubular receiver designs for direct and indirect heating of CO2 and falling particle receiver designs for indirect heating and storage.
The overall efficiency of a concentrating solar power (CSP) plant depends on the effectiveness of thermal energy storage (TES) system (Kearney and Herrmann, 2002, “Assessment of a Molten Salt Heat Transfer Fluid,” ASME). A single tank TES system consists of a thermocline region which produces the temperature gradient between hot and cold storage fluid by density difference (Energy Efficiency and Renewable Energy, http://www.eere.energy.gov/basics/renewable_energy/thermal_storage.html). Preservation of this thermocline region in the tank during charging and discharging cycles depends on the uniformity of the velocity profile at any horizontal plane. Our objective is to maximize the uniformity of the velocity distribution using a pipe-network distributor by varying the number of holes, distance between the holes, position of the holes and number of distributor pipes. For simplicity, we consider that the diameter of the inlet, main pipe, the distributor pipes and the height and the width of the tank are constant. We use Hitec® molten salt as the storage medium and the commercial software Gambit 2.4.6 and Fluent 6.3 for the computational analysis. We analyze the standard deviation in the velocity field and compare the deviations at different positions of the tank height for different configurations. Since the distance of the holes from the inlet and their respective arrangements affects the flow distribution throughout the tank; we investigate the impacts of rearranging the holes position on flow distribution. Impact of the number of holes and distributor pipes are also analyzed. We analyze our findings to determine a configuration for the best case scenario.
Conversion of direct solar energy, in particular the Concentrated Solar Power (CSP) technologies, has a significant role on conventional energy cost and efficiency. A single tank thermocline Thermal Energy Storage (TES) system is accountable for the overall efficiency of this conversion system. A single tank TES system has a thermocline region that produces the temperature gradient between hot and cold storage fluid by density difference. The overall energy storage capacity depends on sustaining of this region at uniform manner. This paper analyzes how the difference in the percentage of porous medium influences the effectiveness of the flow-distribution and hence, the overall performance of the TES system. The effectiveness is assessed by the optimal flow distribution. The optimal distribution is obtained by examining the velocity profile at any horizontal plane. This plane should be uniform for sustaining the thermocline region during the operation period. To achieve a uniform velocity distribution, two symmetric perforated plate flow distributors were placed in the tank. The distributors were positioned near the inlet and outlet, and checked the performance by varying the percentage of porous medium since the distribution is influenced by the porosity. Porous distributors with hexagonal shape pore were considered and Hitec® molten salt was used as a heat transfer fluid. These respective percentages of porosity affect the flow distribution throughout the tank during the flow distribution. The standard deviations of the velocity field at different positions along z-plane and thermal diffusivity were analyzed. The analyses of our cases were done to distinguish a configuration for the minimum thermal diffusivity and velocity deviation from the mean flow. A finite volume based computational fluid dynamics software was used to execute the computational analysis.
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