• Energy Research

Abstract Sunlight Concentrated and transmission for daylighting via optical fibers is a booming technology of direct utilization of solar energy. It uses optical fibers to introduce sunlight deep into building interior. In this work, a sunlight concentrating and transmitting system via plastic optical fibers was developed and tested. This study addressed the cooling problem of the plastic optical fibers under highly irradiation. Polymethylmethacrylate (PMMA) plastic fibers have a strong absorption effect on infrared light, which means that plastic fibers heat up so much when they transmit focused sunlight that they burn. In this study, a comprehensive cooling approach was developed to solve the overheating problem of plastic optical fibers, which consists of an infrared filter, cooling water and a homogenizer. The transmission efficiency of the system was measured to be about 15%. Experiments show that the comprehensive cooling measures developed can ensure that PMMA plastic optical fibers are in a safe temperature range. This shows that it is feasible to use low-cost plastic optical fibers instead of expensive silica fiber to transmit high flux for daylighting.

AbstractA new single-axis solar tracking device is designed and explored, which is able to lift and lower the photovoltaic panels. The photovoltaic panels can be tilted to east-west directions in the process of tracking the sun. In windy weather, the solar panels can be placed close to horizontal rail by using stent, which can minimize the frontal area. What's more, the mechanical strength of this device is better than traditional single-axis solar tracking system, so as to enhance wind resistance in windy weather. The device in this paper is suitable for PV power plants on building roofs because it can meet the strict requirements of wind resistance capacity and safety.

Abstract Based on the symmetries of the parabolic trough concentrating system and the solar radiation, a descending dimension algorithm has been developed with a computational complexity of O ( N 2 ) , whereas that of the Monte Carlo ray tracing (MCRT) method and the finite element method are of O ( N 4 ) The formula of the energy function ψ ( θ ) for solar disk slice was constructed to solve the integration of the flux distribution. The numerical results indicated that the results obtained by the presented algorithm have a good agreement with results produced by MCRT, meanwhile CPU time requirement of the algorithm is about a few seconds, much shorter than that of MCRT. The formulas for calculating the number of reflections of ray tracing within a homogenizer and the exact coordinates of the landing location on the absorber of each ray are established from geometrical aspect. The effect of the homogenizer for flux density uniform is clearly characterized.

Abstract Parabolic trough concentration (PTC) systems are widespread technologies for the large-scale exploitation of solar energy. The flux density on the wall of the absorber of a PTC is non-uniform and complicated, and the calculation of flux distribution is key for estimating the optic performance of a PTC and analyzing the flow characteristic of the fluids in the tubular absorber. Traditional 3D ray-tracing methods for the calculation of flux distribution consume many computational resources and involve long CPU running time. This paper presents a 2D method for the fast calculation of the flux distribution of a PTC based on a descending dimension algorithm. By converting the PTC optical model from 3D space to 2D space, the calculation consumption required by the presented 2D method shrinks by two orders of magnitude compared to traditional 3D ray-tracing methods. Upon assessment, the method demonstrates a capacity to work out the flux density distribution within 0.22 s, compared to approximately 40 s required by traditional 3D ray-tracing methods, using a standard personal computer with a level of good accuracy, with a standard deviation of approximately 0.3 suns. Mathematical proof of this method was also provided. The numerical results were compared with those from the literature and a good agreement was observed, with the average standard deviation of 0.359 suns under different incident angles, proving the reliability of the method presented here. Based on this 2D method, a software tool was developed to facilitate the analysis of characteristics of solar PTC systems.

Abstract In practical applications, there are many optical errors in the parabolic trough concentrators (PTC), such as mirror slope errors and tracking errors. Image detection is a no contact technique that has a potential to diagnose the optical errors at large plants in use. In this study, a CCD (Charge Coupled Device) camera was used to directly observe the absorber, and the images of the flux peaks were successful obtained. The CCD image showed two bright peaks on the outer wall of the tubular absorber. By comparing the CCD image with the simulation results, the concave deformation of the parabolic mirror was diagnosed, and the slope error of the parabolic mirror was figured out. The images also revealed that sun tracking errors can cause a synchronous shift in the flux. The flux image method does not influence the normal operation of PTC and is an on-line method, which facilitates the field application.

Abstract A daylighting system consisting of optical fibers and a sun-tracking model has been developed. The system has a concentrating level of 2500 suns and a tracking precision of better than 0.1° to ensure that the amount of overlap of the focal spot and the entrance face is greater than 80%. The device includes two feedback circles, coarse and fine adjustments, using an angle encoder and a special photodiode array, respectively. The coarse adjustment process relies on predictable a priori information of the sun’s trajectory, and discrete features of the output signals from the photodiode array are used by the control program for the fine adjustment. The system then operates in a predictive control mode and exhibits good tracking performance. The optical transmission efficiency is maintained between 37% and 40%, which is close to the theoretical maximum value of 42%. The fluctuation range of illuminance on the working face is less than 20%, which meets visual demands. Five lenses (100 mm in diameter) and five 10-m, 2-mm-diameter plastic optical fibers can provide an illumination of 26.7 lx for an underpass (4.6 m × 4.2 m) 10 m away. Of a direct normal irradiance of 514 W/m2, about 70% of the light illuminates the floor.

The strain on the cell casing can serve as an indicator of the internal state of the cell. Monitoring strain during the charging and discharging processes aids in determining optimal charging and discharging operations, thereby maximizing the lifespan and performance of the battery. Herein, strain dynamic curves are obtained for lithium‐ion batteries at low, medium, and high charge and discharge rates by affixing strain gauges to individual cells. The investigation delves into parameters such as strain relaxation time, maximum strain, and residual strain at various charge rates and states of charge. The experimental findings reveal distinctive patterns, indicating that the strain curve during high‐rate charging resembles a second‐order function, exhibiting more pronounced fluctuations as the rate increases. This stands in stark contrast to the strain exponential decay observed during conventional medium and low‐rate charging. Notably, the strain residual resulting from high‐rate charging proves to be several times higher than that observed in low‐rate charging, hinting at potential differences in the dynamic distribution of lithium ions within batteries during high‐rate versus medium‐low‐rate charging modes.

Abstract Heliostat daylighting systems, used to transmit sunlight deep into rooms where natural light cannot reach, are increasingly applied in buildings. A roof-mounted heliostat with an area of 22.95 m2 was developed in this work to verify the feasibility of high flux and long distance daylighting in large building interior. The developed heliostat system consists of a heliostat, a secondary reflector, and glass windows forming the light path within the building. The problem of gravitational deformation of the steel beams base of the heliostat was solved by a rectification algorithm embedded into the computer program, to realize vertical daylighting. The spectrum and chromaticity of the heliostat daylighting system developed was measured, and the results verify the good visual quality of the interior illumination. The light transmission distance is more than 70 m, and the system can provide a level of 20–80 klux daylighting illuminance in the daytime. An economic analysis was carried out, and data indicates a good cost-effectiveness of the heliostat daylighting system developed. It is hoped that this research will be of some reference value to the design of heliostat daylighting systems in large buildings.