To further improve the stability and renewable energy utilization of the compound heat source heat pump (CHSHP) system under complex operating conditions, a multi-mode solar heat pump (MMSHP) system was proposed and a mathematical model was established in this paper. The accuracy of the model is verified by experimental data, with the relative error within {plus minus}15%. The effects of the number of solar photovoltaic/thermal (PV/T) collectors (area), environmental parameters, and refrigerant flow rate on system performance are discussed separately for different models by the validated model. The results show that (1) PV/T-water source heat pump (PV/T-WSHP) mode: the number of PV/T collectors in parallel (area) increases to 3 (3.84 m2), and the system performance improves the most, subsequently weakening; (2) PV/T-water & air composite heat pump (PV/T-W&ASHP) model: When the refrigerant distribution ratio in the double-pipe heat exchanger1 is increased from 55% to 95%, the collector's heat collecting capacity, power generation efficiency, and system coefficient of performance (COP) increase by 8.99%, 1.22%, and 6.4%, respectively. Moreover, the increase in solar irradiance can promote more refrigerant flow into the double -pipe heat exchanger 1, and improve heat pump COP more than the effect of increased ambient temperature. The research results can provide an effective reference for further optimization of solar heat pump systems and full utilization of renewable energy.

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Copper-graphite brush and slip-ring systems are often used to transmit electrical energy and signal between moving and stationary parts. At present, the research on the brush-ring system mainly focuses on power transmission, while on signal transmission is relatively lesser. In this study, the effects of friction and wear behavior on signal waveform distortion were in-situ analyzed using a custom-designed tribotester, which can specially synchronize the slip-ring rotation period with the input signal waveform. Results were analyzed comprehensively from tribological and electrical aspects to find out the key factors, including friction coefficient, friction temperature, contact resistance, surface morphology, roughness, wear particles, and compositions. It was found that the distortion of signal waveform is mainly affected by the friction film. During the sliding electrical contact motion, the increase of friction film will increase the contact resistance, which in turn increases the signal waveform distortion. At the same time, under the normal load and shear force, the friction film moves along the sliding direction, hence causing the signal phase angle to shift. From a tribological point of view, the friction coefficient and temperature decrease with increasing the friction film. So, for the brush-ring system, the formation of friction film is good for the friction stability but bad for signal transmission.

With the aim to analyze the flow pattern and heat transfer characteristics of the working fluid in a flat plate heat pipe (FPHP) which was sealed by a transparent tempered glass plate, ethanol, acetone, and R141b were taken as the working medium, and visual experiments were performed at different heat flux when the inclination angle was 90°. The vapor-liquid distribution and the heat transfer characteristics of the FPHP were investigated at different liquid filling ratios. According to the experimental results and the recording of high-speed cameras, some important conclusions had been drawn as follows: (i) As the power increases, the vapor-liquid interface in the FPHP declines and the effects of dryout is significantly intnsified, leading to a sharp increase in temperature. The FPHP with a filling ratios of 25.7% owns better thermal performance than that with the filling ratios of 11.8% and 66% at different heating power; (ii) the bubble generation inside the FPHP became more intense with increasing heat flux, and various bubble movement patterns were found at different the liquid filling ratios; (iii) As the liquid film flowed downward, the thickness of the liquid film increased at first and then decreases. The condensation of steam was reduced due to the thickening of the liquid film on the wall. The liquid film became thinner when it was entrapped and evaporated in the downward flow.

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