Optical wireless energy transfer for self-sufficient small cells

Wireless backhaul communication and power transfer can make the deployment of outdoor small cells (SCs) more cost effective; thus, their rapid densification can be enabled. For the first time, solar cells can be leveraged for the two-fold function of energy harvesting (EH) and high speed optical wireless communication. In this thesis, two complementary concepts for power provision to SCs are researched using solar cells – the optical wireless power transfer (OWPT) in the nighttime and solar EH during daytime. A harvested power of 1 W is considered to be required for an autonomous SC operation. The conditions of darkness – worst case scenario – are initially selected, because the SC needs to harvest power in the absence of ambient light. The best case scenario of daytime SC EH from sunlight is then explored to determine the required battery size and the additional power from optical sources. As a first approach, an indoor 5 m experimental link is created using a white light-emitting diode for OWPT to an amorphous silicon (Si) solar panel. Despite the use of a large mirror for collimation, the harvested power and energy efficiency of the link are measured to be only 18.3 mW and 0.1%, respectively. Up to five red laser diodes (LDs) with lenses and crystalline Si (c-Si) cells are used in a follow-up study to increase the link efficiency. A maximum power efficiency of 3.2% is measured for a link comprising two LDs and a mono-c-Si cell, and the efficiency of all of its components is determined. Also, the laser system is shown to achieve an improvement of the energy efficiency by 2.7 times compared with a state-of-the-art inductive power transfer system with dipole coils. Since the harvested power is only 25.7 mW, an analytical model for an elliptical Gaussian beam is developed to determine the required number of LDs for harvesting 1 W; this shows an estimated number of 61 red LDs with 50 mW of output optical power per device. However, a beam enclosure of the developed Class 3B laser system of up to a 3.6 m distance is required for eye safety. A simulation study is conducted in Zemax for the design of an outdoor 100 m infrared wireless link able to harvest 1 W under clear weather conditions. Harvesting 1.2 W and meeting eye safety regulations for Class 1 are shown to be feasible by a 1550 nm laser link. The required number of laser power converters is estimated to be 47 with an area of 5 × 5 mm2 per device. Also, the dimensions of the transmitter and receiver are considered to be acceptable for the practical application of SC EH. In the last part of this thesis, two multi-c-Si solar panels are initially used for EH in an outdoor environment during daytime. The power supply of at least 1 W is shown to be achievable during hour periods under sunny and cloudy conditions. A maximum average power of 4.1 W is measured in the partial presence of clouds using a 10 W solar panel. Since the variability of weather conditions induces the harvested power to fluctuate with values of mW, the use of optical sources is required in periods of insufficient solar EH for SCs. Therefore, a hybrid solar/laser based EH design is proposed for a continuous annual SC provision of 1 W in ‘darker’ places on earth such as Edinburgh, UK. The 10 W multi-c-Si solar panel and the 1550 nm laser link are considered; thus, the feasibility of supplying the SC with at least 1 W per hour monthly using a battery with energy content of only 60 Wh is shown through simulations. A maximum monthly average harvested power of 824 mW is shown to be required by the 1550 nm laser system that has already been overachieved through simulations in Zemax.