Temperature Coefficients Of Solar Cells Research Articles

Theoretical study of a novel intermediate temperature photovoltaic/thermal system equipped with heat pipe and evacuated tube

Solar photovoltaic/thermal (PV/T) technology has enormous promise in the field of renewable cogeneration as a key technology to increase the utilization rate of solar energy. The structural restrictions of PV/T and the high power temperature coefficient of solar cells make PV/T mostly used in low-temperature situations. However, the combination of intermediate temperature PV/T and low-grade energy utilization devices can create a wider range of application values, including absorption refrigeration, seawater desalination, and the organic Rankine cycle. To increase the overall efficiency and thermal energy grade of the PV/T system, a novel heat pipe evacuated tube PV/T (HE-PV/T) system is proposed. The heat transfer is modeled using distributed parameters, and the thermoelectric performance and temperature uniformity are computed through numerical simulation. The impacts of different parameters on the thermodynamic performance of the HE-PV/T system are examined. Compared with traditional flat plate PV/T, the system's overall energy utilization efficiency and exergy efficiency have been significantly increased. When the inlet temperature is 80.0 °C, the overall energy and exergy efficiency of the HE-PV/T system can reach 33.55 % and 7.92 %, which is 29.66 % and 21.97 % higher than that of flat plate PV/T. Besides, the temperature distribution of the HE-PV/T is more uniform, which is beneficial for reducing thermal stress and mechanical damage. The property superiority and thermodynamic feasibility of the HE-PV/T system at medium temperature are demonstrated.

A parametric study on the performance characteristics of an evacuated flat-plate photovoltaic/thermal (PV/T) collector

An evacuated flat-plate photovoltaic/thermal (E-PV/T) collector was proposed. The inner space of the E-PV/T collector is vacuumed to suppress non-radiative heat losses, thus increasing thermal efficiency of the collector. Therefore, the E-PV/T collector has the potential to simultaneously deliver electricity and heat at high temperatures. A mathematic model was developed to evaluate the performance of the E-PV/T collector. The effect of some key parameters (e.g., initial water temperature in the water tank, vacuum degree, long-wave panel emissivity, and temperature coefficient of solar cells) on the performance of the E-PV/T system was investigated and the results were compared with a normal flat-plate PV/T (N-PV/T) system. Results suggest that the vacuum helps to enhance the total efficiency by nearly 10% points in high-temperature conditions (>80 °C). The vacuum degree of the upper space exerts a greater effect on system efficiencies compared to that of the lower space. Lower long-wave panel emissivity and greater temperature coefficient of the solar cell promote the performance of the collector. By lowering the long-wave panel emissivity from 0.95 to 0.05, the total efficiency soars from 26.82% to 61.20%. This study may help to guide parametric optimization and operation strategy of flat-plate PV/T collectors for high-temperature applications.

A detailed study on loss processes in solar cells

Only a small part of the incident solar energy converts to the electrical power in photovoltaic devices. The majority of the energy loss contributes to the heat generation in devices and thus leads to a temperature rise, causing an inevitable impact on the performance of photovoltaic devices. Hence, loss processes in solar cells play very important roles in solar-electric conversion process. This paper systematically studies both the intrinsic and extrinsic losses in solar cells. Energy distributions of solar cells with different kinds of parameters are presented to characterize the different kinds of loss processes in detail. The sensitivities of loss processes to the structural and operating parameters of solar cells such as external radiative efficiency, solid angle of absorption and operating temperature are discussed, for the parameters have significant impact on the loss processes. The external radiative efficiency, solid angle of absorption (e.g., the concentrator photovoltaic system), series resistance and operating temperature are demonstrated to greatly affect the loss processes. Furthermore, based on the calculated thermal equilibrium states, the temperature coefficients of solar cells versus the bandgap Eg are plotted.

Physics of the temperature coefficients of solar cells

Physics ruling the temperature sensitivity of photovoltaic (PV) cells is discussed. Dependences with temperature of the fundamental losses for single junction solar cells are examined and fundamental temperature coefficients (TCs) are calculated. Impacts on TCs of the incident spectrum and of variations of the bandgap with temperature are highlighted. It is shown that the unusual behavior of the bandgaps of perovskite semiconductor compounds such as CH3NH3PbI3-xClx and CsSnI3 will ultimately, in the radiative limit, give PV cells made of these materials peculiar temperature sensitivities. The different losses limiting the efficiency of present commercial cells are depicted on a p–n junction diagram. This representation provides valuable information on the energy transfer mechanisms within PV cells. In particular, it is shown that an important fraction of the heat generation occurs at the junction. A review of the loss mechanisms driving the temperature coefficients of the different cell parameters (open circuit voltage Voc, short circuit current density Jsc, fill factor FF) is proposed. The temperature sensitivity of open circuit voltage is connected to the balance between generation and recombination of carriers and its variation with temperature. A general expression that relates the temperature sensitivity of Voc to the External Radiative Efficiency (ERE) of a solar cell is provided. Comparisons with experimental data are discussed. The impacts of bandgap temperature dependence and incident spectrum on the temperature sensitivity of short circuit current are demonstrated. Finally, it is argued that if the fill factor temperature sensitivity is ideally closely related to the open circuit voltage temperature sensitivity of the cell, it depends for some cells strongly on technological issues linked to carrier transport such as contact resistances.