Transient performance of a nanowire-based near-field thermophotovoltaic system

Great improvement has been made in the optimization on the near-field thermophotovoltaic system at the steady state, whereas system safety evaluation under near-field thermal radiation and transient output performance considering the energy consumption of the cooling device are lacking. Therefore, it is important to extend the investigation on the system to the transient state. Based on the law of energy conservation and theory of near-field radiative heat transfer, a transient energy transfer model of a W nanowire-based near-field TPV system considering the energy consumption of the cooling device was established. The heating process of each component in the system was explored, and the effects of the input power density and cooling medium's velocity on system performances were analyzed. Moreover, a new evaluation index named the start-up power consumption was proposed to characterize the start-up performance of the system. The results show that based on the common working conditions, the maximum heating rate of the emitter in the transient process is up to 1500 K/s, which makes a serious threat to the experimental safety of the system. However, only the maximal operating temperature of 1752 K can be obtained according to the steady-state model and this temperature is within safe range, indicating the shortcomings of the steady-state model. The maximum heating rate of the emitter can be reduced to 304 K/s by adopting a progressive heating mode with a power supply rate of 20 W/cm(2)/s, showing that the safety operation of the system can be ensured. The highest cell efficiency occurs at the transient moment due to the increasing cell temperature. Increasing the cooling medium's velocity is expected to maintain the highest cell efficiency until the steady state to improve the net system efficiency. However, the system needs external energy in the start-up stage because the cell is insufficient to produce enough electricity to drive the cooling device during this time, and the start-up power consumption of the system increases sharply with the increasing velocity of the cooling medium, which can be as high as 23405.9 W/m(2). By comprehensively considering the net system efficiency and start-up power consumption, the alternative velocity was determined as 4 m/s. It can be seen that the transient energy transfer model of the near-field TPV system can be used to better guide the experimental safety design and comprehensively evaluate the system performance compared with the steady-state model.