I’m optimistic in general about the prospects of solar energy; as should be well known, the total amount of energy arriving at the earth from the sun is orders of magnitude more than is needed to supply all our energy needs. The problem currently is reducing the price and hugely scaling up the production areas of photovoltaics. But, as I live in the not notoriously sunny country of Britain, someone will always want to make some sarcastic comment about how we’d be better off trying to harvest energy from rain rather than sun here. So I was pleased to see, in the midst of a commentary from Philip Ball on the general concept of scavenging energy from the environment, a reference to generating energy from falling raindrops.
The research, described in an article in physorg.com: Rain power: harvesting energy from the sky, was done by Thomas Jager, Romain Guigon, Jean-Jacques Chaillout, and Ghislain Despesse, from Minatec in Grenoble. The original work is described in two articles in the journal Smart Materials and Structures, Harvesting raindrop energy: theory and Harvesting raindrop energy: experimental study (subscription required for full article). The basic idea is very simple; it uses a piezoelectric material, which generates a voltage across its faces when it is deformed, to convert the energy imparted on the impact of a raindrop onto a surface into a pulse of electrical current. The material chosen is a polymer called poly(vinylidene fluoride), which is already extensively exploited for its piezoelectric properties in applications such as microphones and loudspeakers.
So, should we abandon plans to coat our roofs with solar cells and instead invest in rain-energy harvesting panels? It’s worth doing a back of an envelope sum. The article claims that a typical raindrop’s velocity is about 3 m/s. Taking Sheffield’s average annual rainfall of about 80 cm, we can estimate the total kinetic energy of the rain landing on a square meter in a year as 3600 J, corresponding to a power per unit area of about 3 milliwatts. This isn’t very impressive; even at these dismal northern latitudes the sun supplies about 100 W per square meter, averaged over the year. So, even accounting for the fact that PVDF is likely to be a lot cheaper than any photovoltaic material in prospect, and energy conversion efficiencies might be higher, its difficult to see any circumstances in which it would make sense to try and collect rainwater energy rather than sunlight.
Another way to do it would be to collect rainwater on the roof and let it flow down to ground level (which of course you’re doing anyway), collecting energy from the flow. If you get 80 cm per year, over 1 square meter that would be .8 cubic meters or 800 liters of water, weighing 800 kg. If you could extract the potential energy of this weight falling 3 meters to the ground, mgh would be 800*10*3 or 24000 Joules, 7 times greater than the kinetic energy of the raindrops. Unfortunately this still puts power in the milliwatt range.
Of course – and in this way we’ve rediscovered personal hydroelectric plants! This makes clear what you need for meaningful hydro plants – a very large catchment area with high rainfall so you collect enough water, and somewhere hilly so you have a large potential energy to capture. The problem with rain is that, although you’d think the potential energy you were capturing is very large because clouds are rather high up, the drops the rain comes in are so small that their terminal velocity is rather low. One can imagine the limit of infinite amounts of rain falling in the form of infinitesimally small drops, at velocities approaching zero, from which you could extract no useful energy at all. Which pretty much describes the climate of my nearby city of Manchester, of course.