We present and compare six simulation codes for positive streamer discharges from six different research groups. Four groups use a fully self-implemented code and two make use of COMSOL Multiphysics ®. Three test cases are considered, in which axisymmetric positive streamers are simulated in dry air at 1 bar and 300 K in an undervolted gap. All groups use the same fluid model with the same transport coefficients. The first test case includes a relatively high background density of electrons and ions without photoionization. When each group uses their standard grid resolution, results show considerable variation, particularly in the prediction of streamer velocities and maximal electric fields. However, for sufficiently fine grids good agreement is reached between several codes. The second test includes a lower background ionization density, and oscillations in the streamer properties, branching and numerical instabilities are observed. By using a finer grid spacing some groups were able to reach reasonable agreement in their results, without oscillations. The third test case includes photoionization, using both Luque's and Bourdon's Helmholtz approximation. The results agree reasonably well, and the numerical differences appear to be more significant than the type of Helmholtz approximation. Computing times, used hardware and numerical parameters are described for each code and test case. We provide detailed output in the supplementary data, so that other streamer codes can be compared to the results presented here.

published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User

This paper proposes an integrated model of power-toammonia (P2A) to exploit the inherent operational dispatchability of nitrogen-ammonia (N2-NH3) cycles for high-renewable multienergy systems. In this model, the steady-state electrolytic process is mathematically formulated into a thermodynamic system based on thermo-electrochemical effects, and the long-term degradation process of P2A is transformed as the short-term degradation cost to characterize its cost-efficiency. Furthermore, the enhanced utilization of P2A is explored to form a renewable energy hub for coupled multi-energy supplies, and a coupling matrix is formulated for the optimal synergies of electrical, ammonia and thermal energy carriers. An iterative solution approach is further developed to schedule the hub-internal multi-energy conversion and storage devices for high-efficiency utilization of available hybrid solarwind renewables. Numerical studies on a stand-alone microgrid over a 24-hour scheduling periods are presented to confirm the effectiveness and superiority of the proposed methodology over regular battery and power-to-gas (P2G) storages on system operational economy and renewable energy accommodation. Index Terms--Energy hub, integrated energy system, microgrid, power-to-ammonia, renewable energy. NOMENCLATURE Indices and sets k Index of scheduling time n Index of electrolysis cell branch ∆k Time interval Parameters a1, a2, a3, a4 Coefficients of battery cycle life Cz,n, CW,n Thermal capacitances of electrolyte and wall (kWh/°C) DLe,k,max, DLh,k,max Maximum limits of interruptible loads for electricity and heat (kW) dhZ,n, dhW,n Geometric characteristic length of heat transfer surface of electrolyte and wall (m) ER Energy storage capacity of battery (kWh) F Faraday constant (C/mol) fA,ramp,n, fA,max,n Ramp rate and maximum production of electrolysis cells per time interval (m 3 ) This work was jointly supported by the National Natural Science Foundation of China (51877072) and Huxiang Young Talents Programme of Hunan Province under Grant 2019RS2018.

No abstract

published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User

Leader discharge is one of the main phases in long air gap breakdown, which is characterized by high temperature and high conductivity. It is of great importance to determine thermal characteristics of leader discharges. In this paper, a long-optical-path Mach-Zehnder interferometer was set up to measure the thermal parameters (thermal diameter, gas density, and gas temperature) of positive leader discharges in atmospheric air. IEC standard positive switching impulse voltages were applied to a near-one-meter point-plane air gap. Filamentary channels with high gas temperature and low density corresponding to leader discharges were observed as significant distortions in the interference fringe images. Typical diameters of the entire heated channel range from 1.5 mm to 3.5 mm with an average expansion velocity of 6.7 m/s. In contrast, typical diameters of the intensely heated region with a sharp gas density reduction range from 0.4 mm to 1.1 mm, about one third of the entire heated channel. The radial distribution of the gas density is calculated from the fringe displacements by performing an Abel inverse transform. The typical calculated gas density reduction in the center of a propagating leader channel is 80% to 90%, corresponding to a gas temperature of 1500 K to 3000 K based on the ideal gas law. Leaders tend to terminate if the central temperature is below 1500 K.

The propellers and turbines used in aeroplanes are typically powered by fossil-fuel combustion. Recently, Xu et al (2018 Nature 563 532–5) demonstrated a successful flight of an aeroplane using ionic-wind propulsion, which does not require combustion or moving parts. The ionic wind phenomenon induced by electrical discharges has been revealed since 17th century, but it was the first practical example of the so called solid-state propulsion. The detailed capabilities of such ionic wind based or electroaerodynamic driven aeroplanes have aroused the interest of both researchers and industries, detailed modeling works are required for deeper insights. In this paper, a 2D unipolar ion drift model coupled with Navier–Stokes equations is developed and validated by experiments. The electric field, space charge distribution, ionic wind velocity and body force are obtained. The flight velocity, the lift and distance of the ionic wind based aeroplane in the experiment and in an extreme case are analyzed theoretically based on the modeling results. The results show that the performance of an ionic wind based aeroplane depends on the matching between electrical parameters (discharge geometry, voltage, etc), flight parameters (initial velocity, weight control, airfoil, wingspan, etc) and power storage (battery storage, battery weight, etc).

One of the main problems in the Ultra High Voltage (UHV) transmission project is to choose the external insulation distance, which requires a deep understanding of the long air gap discharge mechanism. The leader-streamer propagation is one of most important stages in long air gap discharge. In the conductor-tower lattice configuration, we have measured the voltage, the current on the high voltage side and the electric field in the gap. While the streamer in the leader-streamer system presented a conical or hyperboloid diffuse shape, the clear branch structure streamer in front of the leader was firstly observed by a high speed camera in the experiment. Besides, it is found that the leader velocity, width and injected charge for the branch type streamer are greater than those of a diffuse type. We propose that the phenomenon results from the high humidity, which was 15.5-16.5 g/m 3 in our experiment.