The ever-increasing use of wireless devices, such as mobile phones, computing, and remote sensing systems, have led to increased demand and dependence on the use of batteries.
Currently, most electronic devices, primarily in the consumer market (mobile phones, wearables), are powered by batteries which have several disadvantages: they must be replaced or recharged periodically and mostly they are not practical with their size and weight.
One of the market challenges is the potential reduction of large batteries and new power management systems to improve device efficiency. One possibility is to extract energy from the environment to recharge a battery or to directly power the electronic device. RF energy harvesting uses the idea of capturing transmitted electromagnetic waves and storing them for later use. The design includes an antenna with a circuit capable of converting RF signals into DC voltage. The efficiency of an antenna depends mainly on its impedance and that of the conditioning circuit. If the two impedances do not match, then it will not be possible to receive all the available power from the free space (Figure 1).
General layout of an RF Energy Harvesting system
Understanding electromagnetic waves is essential for designing an energy harvesting RF system. The behavior of electromagnetic waves varies according to distance, frequency and the working environment.
The engineering of the system consists therefore in the design of an antenna whose purpose is to capture the electromagnetic waves (RF in this case). Friis's equation regulates transmission/reception in free space:
Where Pi are the powers in transmission and reception (the latter being the most interesting to evaluate the energy harvesting power supply), Gi are the gains of the two antennas, λ the RF wavelength, and R the distance between the two antennas. The general layout of an RF energy harvesting system consists of a receiving antenna with a suitable design, a rectifier and a dc-dc circuit connected to the load. In general, an impedance matching circuit is applied between the antenna and the rectifier. In its most general form, we find a DC-DC conversion circuit connected to an energy storage system (battery or capacitor) which is connected to the load.
Various antennas have been used in RF energy harvesting applications, from simple dipoles to more complex designs, such as the spiral antenna. Although the latter offers good polarization performance, they are generally limited to broadband designs with a few hundred MHz widths. Currently, multi-frequency narrowband designs are generally limited due to the need for a complex feeding mechanism for each antenna element (figure 2).
Simple RF energy harvesting circuit
A buck/boost converter is a power management architecture suitable for energy harvesting from many artificial sources, from movement or vibration via a piezoelectric transducer through RF energy. A protection shunt on the front-end allows a variety of different piezoelectric elements to be housed (Figure 3).
Application diagram for RF energy harvesting with the LTC3588
The performance characteristics of the ICs required for inclusion in these applications are as follows:
- low quiescent currents: generally, less than 6 μA;
- low start-up voltage: starting from 20 mV;
- ability to manage AC inputs;
- compact solutions with a minimum of external components.
Since the output from the energy collection devices is generally small and intermittent, a system must be carefully designed and can include more than one controller. Moreover, digital control is an essential part of any energy collection system, allowing a single device to manage a wide range of energy sources that have very different requirements. Power conversion and management are generally the core of modern energy harvesting systems. A careful selection of integrated power management circuits and energy storage devices, taking into account the energy budget and collection efficiency, are fundamental in the design of any electronic device.
Through thin-film batteries and the new transducer technology (in addition to the antenna we also have piezoelectric), energy harvesting could be the next big frontier in the renewable energy sector.