This paper proposes a photovoltaic (PV) string-level isolated DC–DC power optimizer with wide voltage range. A hybrid control scheme in which pulse frequency modulation (PFM) control and pulse width modulation (PWM) control are combined with a variable switching frequency is employed to regulate the wide PV voltage range. By adjusting the switching frequency in the above region during the PWM control process, the circulating current period can be eliminated and the turn-on period of the bidirectional switch of the dual-bridge LLC (DBLLC) resonant converter is reduced compared to that with a conventional PWM control scheme with a fixed switching frequency, resulting in better switching and conduction loss. Soft start-up control under a no-load condition is proposed to charge the DC-link electrolytic capacitor from 0 V. A laboratory prototype of a 6.25 kW DBLLC resonant converter with a transformer, including integrated resonant inductance, is built and tested in order to verify the performance and theoretical claims.
The role of polydimethylsiloxane (PDMS) as a compatibilizer of polyimide/silica hybrid composites was investigated. Introduction of PDMS into a polyimide matrix retards the phase separation of hybrid composites and also prevents the formation of high-molecular-weight silicate. PDMS interacts with silica because of the similarity of its structure with the sol-gel glass matrix of the silica precursor, indicating that poly(imide siloxane)/silica might be a good candidate material for organic/inorganic hybrid composites.
The conversion of 5-(hydroxymethyl)furfural (5-HMF) into 2,5-dimethylfuran (2,5-DMF) via cascade hydrogenation and hydrogenolysis over metallic catalysts has been considered a promising method to produce high-energy-content biofuel. Understanding the adsorption of reactants, cascade reactions, and desorption of byproducts is essential for developing efficient and selective catalysts. Herein, the most plausible reaction mechanisms for the conversion of 5-HMF to 2,5-DMF over the Pd(111), Cu(111), and Cu₃Pd(111) surfaces are investigated using density functional theory calculations. The reaction pathways for the formation of reaction intermediates (2,5-bis(hydroxymethyl)furan, 5-methylfurfural, and 5-methylfurfuryl alcohol (5-MFA)), 2,5-DMF, and byproducts (water, 2,5-dimethyltetrahydrofuran (2,5-DMTHF)) are established. The overall reaction barrier on the Pd(111) surface, which is governed by the hydrogenolysis of the C–OH bonds in 5-MFA, is larger (1.96 eV) than that on the Cu₃Pd(111) surface (1.68 eV). In addition, the significantly higher adsorption energy of 2,5-DMF on the Pd(111) surface (−2.47 eV) than on the Cu₃Pd(111) surface (−0.18 eV), which is caused by the flat adsorption geometry with a η²-(C–O)-aldehyde configuration, leads to the formation of 2,5-DMTHF via furan ring saturation. Even though the overall energy barrier on the Cu(111) surface (0.84 eV) is much lower than those on the Pd(111) and Cu₃Pd(111) surfaces, the weak perpendicular adsorption of 5-HMF in a η¹-(O)-aldehyde configuration, highly unfavorable dissociative H₂ adsorption, and high energy required for the formation of H₂O hinder the conversion of 5-HMF and its intermediates. The favorable adsorption of 5-HMF (−0.54 eV), low overall reaction barrier, facile desorption of 2.5-DMF (−0.18 eV), and low energy barrier for dissociative H₂ adsorption render the Pd–Cu alloy catalyst a promising candidate for the selective conversion of 5-HMF to 2,5-DMF.
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