Holst Centre and imec have identified all the subsystems that make up a far-field RF energy transport system. Such a system may be used to (re)charge the batteries of wireless sensors that are used in e.g. smart building integration applications.
By optimizing both the transmitter radiation pattern and the receiving rectifying antenna, the RF energy transport efficiency can be greatly improved. In particular, it will allow a significant increase of the operating distance between the RF source and the rechargeable batteries.
For smart building integration, hundreds of wireless sensors are needed per building, sensing light, temperature, presence etc. Cabling all these sensors is too costly, as is the replacement and disposal of the prime batteries. An alternative solution is provided by remotely RF charging rechargeable batteries by using a far-field RF energy transport technology. Such a technology uses a dedicated RF energy source, transmitting RF radiation in the 2446-2454MHz ISM frequency band. But restrictions posed on the effective isotropic radiated power (EIRP) make it challenging to realize DC power levels in the order of 100µW over distances of multiple meters. Holst Centre and imec now propose measures to improve the energy efficiency of the RF energy transport system. This will allow an increase of the operating distance between source and battery.
First, the researchers show that by characterizing the indoor environment and adapting the transmitting radiation pattern to these characteristics, more power may be obtained at larger distances using less transmit power. The transmitting radiation pattern can be optimized by using a geometrical optics based model of the environment. As an illustration, simulations reveal that more power can be received by using a multi-beam antenna that points at the receiving antenna and at the first order reflection points of e.g. a corridor, instead of an antenna that is directed to the receiver antenna only.
Since the transmit antenna beam shape depends on the environment, a transmit antenna with beam-shaping capabilities is required. Holst Centre and imec propose the use of a cost-effective switched array antenna, in which one dipole/monopole element will be driven and parasitic dipoles/monopoles will be switched open or short-circuited/grounded. By modeling, the best switching scheme to illuminate the receiver and the wall positions for in-phase reflected wave contributions can be determined.
Besides maximizing the power incident on the receiver, the subcomponents of the receiving rectifying antenna – also called rectenna – need to be optimized as well. The functional blocks of the rectenna are the antenna, the rectifier, the DC-to-DC voltage boost converter and the load. An optimization consists in identifying the impedance matching efficiencies in between the blocks and the conversion efficiencies over the blocks. As an illustration, a packaged remote 2.45GHz RF battery charger and a commercially available 433MHz wireless temperature and humidity sensor are considered, with an operating distance currently limited to 5m. By optimizing all the subsystems and interconnects, it is estimated that this distance can be doubled.
