Mimicking plant transpiration to scavenge electricity (Image: Image Source / Rex Features)

Artificial photosynthesis has yet to be cracked, but electrical engineers in the US think that synthetic leaves could be used to generate electricity in a different way – by sweating.

Natural leaves constantly lose water through evaporation in a process called transpiration, which draws water from the roots to the very top of even the tallest trees.

The new synthetic leaves also lose water through evaporation to create that mechanical water pump effect, and use it to generate power.


Flowing bubbles

Michel Maharbiz at the University of California, Berkeley, working with Ruba Borno at the University of Michigan and Joe Steinmeyer at MIT, built their leaves from glass wafers shot through with a branching network of tiny water-filled channels arranged like the veins of a leaf.

The smaller channels extend to the edge of the plate and have open ends that allow water to evaporate, drawing fluid along the leaf’s central stem at a rate of 1.5 centimetres per second.

The researchers added metal plates to the walls of the central stem and connected them to a circuit. The charged plates and the water within the stem create a sandwich of two conducting layers separated by an insulating layer – in effect, a capacitor.

The leaf is transformed into a source of power by periodically interrupting the water flowing into the leaf with air bubbles. Thanks to the different electrical properties of air and water, every time a bubble passes between the plates the capacitance of the device changes and a small electric current is generated, which passes to an external circuit where it’s used to pump up the voltage on a storage capacitor.

“We use the mechanical energy in the liquid flow to change the capacitance and add energy to the capacitor,” says Maharbiz.

Energy scavenger

Each bubble results in an increase in output voltage of some 2 to 5 microvolts, and the device has a power density of some 2 microwatts per cubic centimetre. “I think we could easily reach hundreds of microwatts per cubic centimetre [with modifications],” he says.

That is still a fraction of the power density of power systems such as fuel cells or batteries, but it’s a respectable figure for an energy scavenging system, Maharbiz says.

The device could be scaled up to produce artificial trees that generate power entirely through evaporation wherever there’s a cyclical change in humidity. Although the modest power output is not enough to rival solar technology, Maharbiz thinks it could act as a complementary technology – the sunlight that generates solar power could also drive transpiration to boost the electricity generated.

Breaking the tension

Abraham Stroock at Cornell University in Ithaca, New York, thinks this is the first attempt to generate electricity from evaporation-driven flow. Although he points out that one US firm Voltree has succeeded in generating tiny quantities of power from the pH difference between soil and the roots of real trees.

“One challenge with the new study is that a bubble is used to generate the current in the capacitor,” Stroock says. Bubbles prevent transpiration taking place over long distances because they break the tension that allows the water column to be pulled along like a piece of string.

Maharbiz says he can get round this issue by using solid insulators instead of bubbles, that spin in place as the water is pulled passed like a water wheel to create the permittivity differences needed to generate power.

Journal reference: Applied Physics Letters (DOI: 10.1063/1.3157144)