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Expectations for high-resolution observations by very low Earth orbit (VLEO, below 300 km) satellites are growing. However, the density of atomic oxygen (AO) is much higher in VLEO than in LEO (300 km or higher), so severe degradation of materials is a concern due to high-flux AO collisions. To clarify the degradation, we developed and deployed the material degradation monitor (MDM) mission onboard the super-low altitude test satellite (SLATS). In the MDM, several materials were exposed to the environment at 160–560 km from 23 December 2017 to 1 October 2019 and observed optically by a CCD camera. In general, AO fluence is evaluated by measuring the volume or mass loss of a polyimide, but this was difficult to do because the samples could not be returned to the ground in this mission. This study evaluated AO fluences using the CCD images of a polyimide, Vespel. The sample had nine non-penetrating holes with different bottom-plate thicknesses and was monitored by the CCD while being illuminated by LED light from the back. The CCD images reveal the intensity of the light transmitted through the holes in the bottom plate. The erosion depths of the polyimide, corresponding to the AO fluences, can be evaluated from the pixel values at the hole positions on the images. Here, the correlation between the pixel values and thickness was investigated for pristine and AO-exposed Vespel samples using the MDM engineering models. The total green pixel values followed the Beer-Lambert law for the bottom-plate thicknesses, regardless of AO exposure. Applying the correlation to the MDM flight images, we found that the AO fluence increased significantly after the end of March 2019, finally reaching (6.5–9.8) × 1021 atoms/cm2. This optical imaging method would be useful for evaluating the AO effects on the other MDM samples and high AO fluences in future VLEO missions. • In MDM, 13 materials were exposed to LEO and VLEO and observed by a CCD camera. • A polyimide resin, Vespel, was included in the samples to evaluate AO fluences. • This study analyzed optical images of Vespel exposed to AO in ground and MDM. • AO fluences could be estimated using the images based on the Beer-Lambert law. • The final AO fluence was estimated to be (6.5–9.8) × 1021 atoms/cm2 at the MDM. [ABSTRACT FROM AUTHOR]

Atomic oxygen (AO) is the dominant constituent of the residual atmosphere in low Earth orbit (LEO). AO collides with spacecraft at a velocity of 8 km/s, causing oxidation and erosion of polymeric materials. Particularly in very low Earth orbit (VLEO) at altitudes below 300 km, the AO density is at least ten times that at 500–700 km, and the concentration of molecular nitrogen (N 2) is 10% or more. Such high-flux AO and N 2 can cause nonlinear processes and mass losses for polymers. This study aims to clarify the optical property changes for polymeric materials exposed to LEO and VLEO in the Material Degradation Monitor (MDM) mission onboard the Super-Low-Altitude Test Satellite (SLATS). The AO and N 2 fluences and the UV irradiances were estimated using the NRLMSISE-00 atmospheric and solar radiation models. The relationships were investigated between the changes in CCD images and the estimated fluences of the samples. For silsesquioxane (SQ)-coated polyimide film, the diffuse reflection of visible light increased noticeably with an AO fluence above 1 × 1021 atoms/cm2. The formation of a silica layer on the coating induces cracks. For silver-coated FEP films, the diffuse reflection increased entirely at AO fluences above 3 × 1021 atoms/cm2. The AO and N 2 collisions would erode and roughen the FEP surfaces. Visible transmission through silver-coated FEP films also increased locally with increasing AO fluence, indicating that some AO oxidized the silver layer despite the presence of an Inconel (Ni alloy) layer. The visible reflection from Beta Cloth was decreased by its reaction with UV. Further, the competitive reactions with UV and AO resulted in two-step changes in the visible reflection from the ETFE polymer (cable covering). • Density of atomic oxygen (AO) is exceptionally high in very low Earth orbit (VLEO). • Material Degradation Monitor (MDM) exposed 13 spacecraft materials to LEO and VLEO. • Cracks were seen for SQ -coated polyimide at the AO fluence above 1 × 1021 atoms/cm2. • Surfaces of FEP films were roughened by the AO fluence above 3 × 1021 atoms/cm2. • Silver coatings of the FEPs would be oxidized by AO despite Inconel coating. [ABSTRACT FROM AUTHOR]

• Demand uncertainty greatly affects the appropriateness of fuel cell system operation. • Use of demand forecast aimed at reducing expected error is not always crucial. • Estimating the impact of uncertainty on operational results is the key point. • Planning approach directly estimating the expected operational result is effective. [Display omitted] Polymer electrolyte fuel cell cogeneration systems (PEFC-CGSs) provide hot water by utilizing exhaust heat produced in electricity generation process. The energy saving potential of PEFC-CGSs can be maximized by optimal operational plans, and most state-of-the-art approaches implement operational planning function (OPF) based on energy demand time-series prediction by using machine learning techniques. In general, prediction of demand time-series with small expected average errors is regarded as the most important point in obtaining appropriate operational plans; however, several recent studies have revealed that other complex factors such as the direction and timing of forecast errors greatly affect the quality of operational plans in some cases. Core ideas proposed in these previous studies are broadly classified into seven types. The purpose of this study is to characterize these OPFs from the two aspects: the output form of prediction model and prediction target variable, and to clarify "what kind of uncertainty should be focused on" and "how this uncertainty should be handled" in designing OPF. The seven kinds of OPFs were comprehensively evaluated via numerical simulations using real-world data. The results show the significance of OPF based on prediction of expected operational cost surface using multiple output prediction model. [ABSTRACT FROM AUTHOR]

Satellites in low Earth orbit (LEO) are subject to several environmental factors, atomic oxygen (AO) having the greatest effect on space materials, especially polymers. Recently, the possibility of utilizing orbits at an altitude lower than 300 km (super LEO or S-LEO) has been explored, but one complication in using these orbits is that AO fluence is expected to far exceed that in an ordinary LEO. The effect of S-LEO AO on materials used in space is not well understood. To investigate the influence of S-LEO AO on space materials, we developed the Material Degradation Monitor (MDM), which is installed on a JAXA spacecraft, the Super Low Altitude Test Satellite (SLATS, also known as TSUBAME). The SLATS is the first S-LEO satellite to operate at an altitude under 200 km and the MDM is the first near real-time experiment to examine the effects of AO on materials in this orbit. The aim of the MDM is to clarify degradation behaviors of the space materials according to AO fluence. We developed the instruments needed to detect AO fluence and the changes it causes to samples of various materials in S-LEO. The MDM performed as expected until the end of the SLATS operation. Some changes in materials were found from the images taken from front and back LEDs. Although there was no breakage by AO, optical properties changed that are assumed to be due to material erosion by AO. • The super-low altitude test satellite was demonstrated at an altitude under 200 km. • A near-real-time space material monitoring system was designed for the satellite. • Material-degradation images that show change with time were successfully acquired. • The behaviors of the materials on exposure to atomic oxygen can be analyzed. [ABSTRACT FROM AUTHOR]

When a solid polymer water electrolyzer (SPWE) is operated under high-pressure conditions, a large pressure difference occurs between the anode and cathode. This causes crossover of produced gas, especially hydrogen gas, leading to a decrease in the production efficiency of an SPWE. As a countermeasure against gas crossover, water should be supplied into the cathode channel, as well as into the anode channel, because the water flow will facilitate the drainage of hydrogen gas outside of the cell, resulting in decreased crossover and increased efficiency of the SPWE. This countermeasure is evaluated by observing SPWE operation at a pressure of 2 MPa, with a visualization of hydrogen bubbles in the cathode channel. The evaluation revealed that supplying water into the cathode channel increases the efficiency by several percent at 0.33 A/cm 2 . Further, the visualization of the hydrogen bubbles revealed an enhancement in the separation of hydrogen bubbles from the surface of the current supplier. This suggests that additional water flow can increase the hydrogen production efficiency through promoting bubble detachment. [ABSTRACT FROM AUTHOR]