Optical energy harvesting in a smart materials-based micro-actuator using a vertical multi-junction PV cell

This paper illustrates the energy harvesting principle established in an optical micro-electro-mechanical system (MEMS) using a vertical multi-junction photovoltaic cell (PV cell: 3*3*0.1 mm). The micro-system is a bistable micro-actuator which includes two active shape memory alloy elements (SMA: 3*1*0.1 mm), heated by a laser beam. The SMA elements are used as a biasing spring to activate the micro-system. In this study, the focus is only on actuating the SMA while harvesting energy converted from laser lighting. First, the laser is homogenized using an N-BK7 light pipe homogenizing rod (75*2 mm). Then, the uniformity is verified experimentally using an optoelectronic system able to measure the power on every 800 mu m of the surface; resulting an average output power of 100 mW with a variation of +/- 9%. Next, the current/voltage (IV) curve of the PV cell is extracted, for an irradiance of 1.05 W/cm(2), giving a maximum electric power of 32.5 mW. The set-up for the system is modelled using Creo software and executed by resin 3D printing. Finally, the actuation of each SMA element is done alternately (period = 6 sec) using a MEMS active mirror which steers the homogenized laser onto them. While cycling, the unused optical energy from the laser is detected by the PV cell resulting to harvest around 60 mJ per cycle. This energy is stored in a solid state thin film micro-battery. In the future work, the SMA elements will actuate the bistable micro-actuator and the quantity of energy harvested will be extracted.