Effect of Optical–Electrical–Thermal Coupling on the Performance of High-Concentration Multijunction Solar Cells

In the process of high-concentration photovoltaic (HCPV) power generation, multijunction cells work in the conditions of high radiation and high current. Non-uniformity of focusing, the mismatch between the focusing spectrum caused by the dispersion effect and the spectrum of multijunction solar cell design and the increase in cell temperature are the key factors affecting the photoelectric performance of the multijunction solar cell. The coupling effect of three factors on the performance of multijunction solar cell intensifies its negative impact. Based on the previous research, the light intensity and spectral characteristics under Fresnel lens focusing are calculated through the optical model, and the optical–electrical–thermal coupling model under non-uniform illumination is established. The results show that obvious changes exist in the concentration spectrum distribution, energy and non-uniformity along different optical axis positions. These changes lead to serious current mismatch and transverse current in the multijunction solar cell placed near the focal plane which decreases the output power. The lost energy makes the cell temperature highest near the focal plane. In the condition of passive heat dissipation with 500 times geometric concentration ratio, the output power of the solar cell near the focal plane decreases by 35% and the temperature increases by 15%. Therefore, optimizing the placement position of the multijunction cell in the optical axis direction can alleviate the negative effects of optical–electrical–thermal coupling caused by focusing non-uniformity, spectral mismatch and rising cell temperature, and improve the output performance of the cell. This conclusion is verified by the experimental result

The III–V Triple-Junction Solar Cell Characteristics and Optimization with a Fresnel Lens Concentrator

At present, the Fresnel lens are commonly used as the condenser in high-concentrating photovoltaic (HCPV) modules. It is ideally believed that the output power of a III–V triple-junction solar cell which is placed on the focal plane of a Fresnel lens is the largest, because the intensity of the sunlight on the focal plane is the largest. Actually, according to our work, the dispersion of sunlight through a Fresnel lens and the nonparallelism and divergence of the incident light will lead to changes in the spectrum and the homogeneity of illumination, and cause a drop of the solar cell output. In this paper, the influence of the dispersion and nonparallel incidence of the light on the output of a triple-junction solar cell at different positions near the focal plane were theoretically studied, combined with the light-tracing simulation method and triple-junction solar cell circuit network model. The results show that the III–V triple-junction solar cell has the highest output power in both sides of the focal plane positions. The output power can be increased by about 15% after being optimized. The simulation results were verified by the experiments

Experimental Study and Mechanism Analysis of the Flow Boiling and Heat Transfer Characteristics in Microchannels with Different Surface Wettability

In this paper experiments have been conducted to investigate the flow boiling and heat transfer characteristics in microchannels with three different surface wettability. Three types of microchannels with a super-hydrophilic surface (θ ≈ 0°), a hydrophilic surface (θ = 43°) and an untreated surface (θ = 70°) were prepared. The results show that the average heat transfer coefficient of a super-hydrophilic surface microchannel is significantly higher than that of an untreated surface microchannel, especially when the mass flux is high. The visualization of the flow patterns states that the number of bubble nucleation generated in the super-hydrophilic microchannel at the beginning of the flow boiling is significantly more than that in the untreated microchannel. Through detailed analysis of the experimental data, flow patterns and microchannel surface SEM images, it can be inferred that the super-hydrophilic surface microchannel has more active nucleation cavities, a high nucleation rate and a large nucleation number, a small bubble departure diameter and a fast departure frequency, thereby promoting the flow and heat transfer in the microchannel. In addition, through the force analysis of the vapor-liquid interface, the mechanism that the super-hydrophilic microchannel without dryout under high heat flux conditions is clarified

Evaluation of performance of MPPT devices in PV systems with storage batteries

In the PV system with storage batteries, as a maximum power point tracking (MPPT) device is used to enhance battery charging, the enhancement must be greater than the internal loss of the device itself, or there will be no net gain at all. To evaluate the MPPT device benefits under different climate, the theoretical calculation models have been constructed. By simulation, a comparative study between two types of PV charge controllers with and without a MPPT device under different atmospheric conditions was presented. The comparison was made by means of the energy production obtained from the PV generator of each system. The climatic conditons of Beijing and Guangzhou in China have been regarded. From the results obtained it can be concluded that the effectiveness of the MPPT device in Guangzhou is not very obvious, however the MPPT device did greatly enhance the output in Beijing in the winter. (c) 2006 Elsevier Ltd. All rights reserved