What is Wireless Energy Harvesting?

Wireless Energy Harvesting (WEH) is a technology that enables the harvesting of energy from radio frequency (RF) signals. All RF Signals simultaneously carry both information and energy. In most cases, receivers only process the information carried by the signals. Wireless Energy Harvesting Technology can be used by receivers to harvest some energy from the RF signals. This energy can then be used to power devices or charge batteries making it ideal for low-power sensor networks, IoT, RFID and a range of other applications.

A WEH-enabled sensor device usually consists of an antenna, a transceiver, WEH unit, power management unit (PMU), sensor/processor unit, and possibly an onboard battery. The WEH unit and PMU are the two most important devices for energy harvesting. The WEH unit receives the transmitted radio waves with an antenna and converts the received RF energy into a stable direct current (DC) energy source which can be used to power a device or charge a battery. The conversion of the received RF power to usable DC supply comes with a certain amount of power loss in the matching circuit and the internal circuitry of the power converter. The power conversion efficiency (PCE) of the converter is the ratio of the generated usable DC output power to the input RF power. State-of-the-art RF-to-DC converters (also known as rectifiers) are used in a WEH unit to achieve high PCE values, up to 70% or more. The PMU controls the storage of the harvested energy. It also manages the distribution of the available energy among different consumers to maximize the lifetime of the device while maintaining a high quality of service.

In the context of IoT, wireless sensor networks (WSNs) and radio-frequency identification (RFID) tags etc. wireless energy sources can be classified into two categories: dedicated and ambient sources. Dedicated RF sources are deployed to provide a predictable energy supply to the device and the WEH unit is typically optimized to harvest energy from these sources. Ambient sources are transmitters that may radiate stable/unstable power over time periodically or randomly and thus the WEH sensor can not be optimized to harvest energy from specific sources. Harvesting energy from these sources requires an intelligent WEH solution that monitors the channel for harvesting opportunities. Different ambient sources transmit at different frequency bands therefore, harvesting wireless energy at multiple frequency bands complicates the antenna geometry requirements and demands a sophisticated power converter. WEH units are usually configured to use ambient energy harvesting as an auxiliary source only.

IoT’s intelligent infrastructure aims at including as many devices as possible in a network, communicating with each other wirelessly. To make this possible IoT transceivers need to be integrated into several small devices too which creates the additional challenge of integrating a power source for these transceivers in the same device. Hence, low power consumption is an important factor when designing IoT devices. WEH enables these devices to have smaller batteries to power them and uses RF energy received by them to recharge these batteries. This extends the recharge cycle of these devices to a great extent by supplying adequate power for a longer period. WEH technology also makes it possible to include more sensors in the same package thus increasing the functionality of these devices. 

Wireless Energy Harvesting technology is still relatively new and is expected to have a wide range of upcoming applications in wireless sensor networks, Machine-to-Machine (M2M) communications, RFID devices etc.