Water, water everywhere…

By Jeff Hardy

This week I have been inspired by water and energy, in fact specifically by a paper by French scientists [1] on harvesting energy from raindrops. In the authors own words “Our system recovers the vibration energy from a piezoelectric flexible structure impacted by a water drop”. What?!? Put more simply, some materials (in this case polyvinylidene fluoride) can convert mechanical energy into electrical energy. This is called the piezoelectric effect, and is similar to that which I described previously in power dressing. So as rain drops hit the material it generates an electrical current (naturally it’s a bit more complicated than this).

So how much power can you generate? The authors estimate that the available rain power in French regions with a continental climate to be almost 1 Wh per square metre per year. For comparison, in Scotland a south-facing roof receives between 700 - 1100 kWh/m2 of solar radiation during a year – oh dear. It’s probably a bit unfair to make this comparison now since this is very early in the development path of this technology. I also like this idea since I’m from the North West of England where it is very wet indeed.

This idea got me wondering about other novel ways in which water could be used to generate electricity. I thought it best to steer away from the classics such as watermills, hydroelectricity, wave and tidal power and generation of hydrogen through electrolysis or thermochemical methods. Instead I have dug out a couple of examples from the literature which interested me.

It appears that engineers at the University of Alberta in Canada have found that pumping water through microchannels in a glass disk can generate an electrical current [2]. In fact they claim “[that it is] the first new way to produce sustainable electricity in 160 years”.

How does it work? Forcing water through tiny glass channels is known to be tough because the channel walls become charged which creates an electric field that hinders the flow of charged ions through the channel. For example a negatively charged channel wall will result in negatively charged ions being forced to the centre of the channel where they will move faster than their positively charged colleagues which are attracted to the walls (because opposites attract). Over time this means a positive charge is built up at one end of the channel and a negative charge at the other – not unlike a battery! By wiring up the ends of the channel a (rather small) current can be produced. It needs some further work as the current is so small that it would take years to charge a mobile phone, but it is an interesting idea.

It is possible to generate electricity from estuaries where fresh water streams flow into the sea. This is known as salinity-gradient energy but thankfully is also referred to as blue energy. Blue energy can work either on the principle of osmosis (the movement of water from a low salt concentration to a high salt concentration) or electrodialysis (the movement of salt from a highly concentrated solution to a low concentrated solution) where the saline water and fresh water be separated by a selectively permeable membrane. In the osmosis process water pressure is created that can drive a turbine. In the electrodialysis case the movement of ions creates the electricity. The only by-product of blue energy is brackish water which would naturally occur in an estuary anyway. The global energy output from estuaries is estimated at 2.6TW, which represents a whopping 20% of the current worldwide energy demand. With figures like these it sounds rather exciting, but once again it is early days in the development of this technology and I think only a couple of test units exist in the Netherlands.

It is amazing what you can turn up when you look into a subject. Of these three topics, blue energy was the only one I had come across before this week. I’m sure if I looked a little harder I’d be able to find other interesting examples. It seems reassuring that there is so much work going into future low-carbon energy technologies. If only we could make better use of the ones available today…

[1] R. Guigon et al., Smart Mater. Struct., 17, (2008), 015038-9
[2] J. Yang et al., J. Micromech. Microeng., 13, (2003), 963