Boya Zhang

Epoxy-based model insulators were manufactured and fluorinated under a F2/N2 mixture (12.5% F2) at 50 °C and 0.1 MPa for 15 min and 60 min. Surface charge accumulation and decay behavior were studied with and without dc voltage application. The effect of direct fluorination on surface charge migration as well as on flashover voltage was verified. The obtained results show that the charge decay of epoxy-based insulators is a slow process, but the decay rate increases when an outer dc electric field is applied. The surface charge distribution is changed when a streamer is triggered on the insulator surface. The existence of heteropolarity surface charges can decrease the dc surface flashover voltage to some extent, while the surface flashover voltage is almost unchanged when charges of the same polarity accumulate on the insulator surface. The short time fluorinated insulator can modify the surface resistivity, and the rate of surface charge dissipation is greatly increased under a dc electric field.

Charge accumulation on a solid insulator surface is one of the critical factors for the development of dc gas-insulated equipment since it will lead to the overstress of polymeric insulation due to local field distortion and enhancement. Therefore, it is important to study the charge accumulation phenomenon on spacer surface under dc field. For decades, researchers have made tremendous progress on this subject by measurement and simulation. However, measurement results are quite different by different researchers due to various electrode configurations and experimental conditions. Further, most researchers use potential to represent charge density, which is not rigorous in that many charge density distribution details are hidden behind the potential. As for pure numerical simulation, reports are rather academic and sometimes cannot accord with the real fact. In this paper, attempts are made to characterize the charge accumulation patterns on spacer surface in HVDC gas-insulated system. Surface charge distributions on a model GIL spacer in 0.1 MPa air under DC voltage are obtained by an advanced measurement method, from which the dominant uniform charging pattern and random charge speckles are separated. Mechanism responsible for the dominant uniform charging pattern is discussed with the aid of a simulation model. Results indicate that, in a well-cleaned system, the electric current through the spacer bulk is the principal factor, but gas conduction is not negligible due to some inevitable ion sources. Highly localized pockets of charge are also observed, which are referred to as speckles. They may originate from micro discharges due to tiny metal particles on the spacer surface or microscopic protrusions on the electrodes.

Microcapsulation of phase change materials (PCMs) within a shell is one of the most feasible methods to explore their applications for thermal energy storage. Here, a facile method to microencapsulate PCMs within polystyrene/cellulose nanocrystal (CNC) hybrid shell via Pickering emulsion polymerization was developed. CNCs, as biobased and sustainable materials hydrolyzed from wood pulp, were employed as emulsifiers of the PCM Pickering emulsion and shell components of the PCM microcapsules as well. CNCs displayed a high efficiency in the stabilization of paraffin wax (PW) Pickering emulsion, and the heat capacity and stability of PW microcapsules with CNC shell (PW@CNC) increased dramatically with the amounts of CNCs. PW microcapsules with polystyrene and CNC hybrid shell (PW@PS/CNC) were prepared via Pickering emulsion polymerization of styrene from the CNC stabilized PW Pickering emulsion droplets. The PW@PS/CNC slurries possessed a latent heat capacity of 31.9 J/g with stability as high as 99.4% after 100 heating and cooling scans. The PW@PS/CNC powder possessed a latent heat capacity of 160.3 J/g, corresponding to a high encapsulation ratio of 83.5%. Moreover, coconut oil (CO), as an example of biobased PCMs, was also microencapsulated within polystyrene and CNC hybrid shell (CO@PS/CNC) via a similar method. Both PW@PS/CNC and CO@PS/CNC slurries displayed excellent temperature regulation ability and offered promising potentials for thermal energy storage systems.