Work Package 4: Energy harvesting and management
Lead Academic: Dr Bernard Stark (University of Bristol)
With Professor Steve Beeby (University of Southampton)
WP4 is investigating methods for powering autonomous body-worn healthcare devices. The material science is being carried out at Southampton University, and antenna design, power electronics, and low-power systems work is carried out in the Electrical Energy Management Research Group at Bristol University.
We are investigating a number of functional materials and the associated power electronics:
Inductive power transfer
Our domestic application is challenging as it implies getting power to moving targets, where there is a low magnetic coupling between coils, and where the coils' quality factors are low due to the use of textile and miniaturised coils. We have shown in the lab how to obtain sufficient power transfer levels whilst respecting the strictest exposure limits proposed by ICNIRP . We are currently exploring how to adapt designs to real domestic daily routines.
One such prototype is a kit that allows the user to set up convenient "power zones" in which their devices are charged whilst being worn. It uses a large area textile coils, a Class D coil-driver, and switched-mode power-conditioning that provides regulated d.c. and energy storage.
2.45 GHz RF power transfer
In theory, power can be radiated to devices by wireless routers. The integration of this concept into body-worn products is challenging: the received power varies rapidly over a range of microwatts to milliwatts, depending on the movement of the receiver, and antenna designs that are optimised for efficiency are generally too large and rigid for body worn applications. We are developing increasingly smaller and more efficient rectifying antennas for smart watches, and the associated ultra-low power electronic system to provide continuous power to a sensor.
Textile antennas would increase flexibility and comfort, and give designers the ability to integrate large area antennas into garments, furniture, and other items in the user’s environment. We are investigating how to make materials that provide both the mechanical and electrical properties required. For example we have characterised the electrical losses of numerous commercial textiles and other flexible materials, in order to produce a suitable composite structure.
The textile antenna shown to the right was fabricated using screen printing. This technology is low cost, simple and suitable for patch fabrication. The fabric substrate is PES/cotton (65/35) which is widely used in the garment industry. As the fabric surface is rough and not suitable for direct deposition of conductive layers, an interface layer using UV-curable ink was printed onto the fabric surface, to reduce surface roughness.
Textile energy harvesters
We are developing ferroelectret and triboelectric textiles that generate a voltage when compressed or bent. This voltage only lasts for milliseconds, and therefore only around 1 μJ is available per harvesting event. These μJ events need to be accumulated in order to power up a sensor intermittently. This process is carried out by a power management circuit. A significant technical challenge that we are currently investigating is how to activate power management circuity precisely when a pulse occurs, but without spending power on pulse monitoring circuits. Also, once a pulse is detected, the power management needs to operate with sub-microwatt conversion losses, and optimally load the energy harvester during the very short pulse period.
The ferroelectret harvester textiles are manufactured from cellular polypropylene (PP), in order to contain cells fixed electrical charges. When the cells are compressed, the distance between charges changes and an electric field is produced. This field drives a current pulse through silver contacts to external circuits. We have constructed several prototypes integrated into clothing, including multi-layer insoles that generate over 50 μJ per step.
We are investigating the printing of photovoltaic inks onto textiles and flexible materials for use in body-worn applications. The challenge of working with non-planar and flexible materials is substantial, and a lot of work is needed to get this to the stage where it could be applied. We have created intact organic cells on woven textiles, albeit of low efficiency. The goal is to obtain textiles that power wearable sensors from natural and artificial indoor lighting.
More information is available on the group's website.