Raindrop sandRecently a study about a solar cell that also works in the rain crossed news threshold. Authors start modestly “All-weather solar cells are promising in solving the energy crisis.” Why this statement would be true, they do not tell. Stories about the paper seemed to spread widely in the geek press. Some samples…

Rain is normally a solar energy cell’s worst nightmare, but a team of Chinese scientists could make it a tremendous ally. They’ve developed a solar cell with an atom-thick graphene layer that harvests energy from raindrops, making it useful even on the gloomiest days. Jon Fingas (Endgadget)

Solar energy panels that can also generate power from raindrops have been been designed, offering a possible solution for UK homeowners looking to invest in renewable energy.  The all-weather solar panels that can create electricity from light on sunny days and rain on cloudy days could be the perfect solution for the UK“:  Cara McGoogan (The Telegraph)

Most of the bad press solar panels get has to do with their limited efficiency in bad weather. Sure, they might be great in New Mexico, but you wouldn’t use them as much in England. Well, a new innovation might soon change that. Chinese scientists have developed a way for solar panels to produce electricity using rain water.Alfredo Carpineti (IFLScience)

Rain means clouds and clouds mean less sunlight. That’s bad news for most solar cells, but a new design can actually make use of rain drops that fall on its surface, allowing it to generate electricity even when the weather’s bad.Jamie Condliffe (Gizmodo)

This all seemed very silly to me. It is very important to grasp which things are large and which are small if you aim to have viewpoints worth listening to. Notice that two critical questions are not raised by anyone.

  1. How much energy is there in rain water?
  2. How much of the energy the gadget harvests?

Both questions are simple enough to answer if the curiosity is there to raise the questions. How much energy is there in rain and how does this compare with the power produced by a solar panel? Take UK which is not known for great weather. It rains roughly 1 meter/year on each square meter of the country. The terminal velocity of water drops depend on their size, but if I estimate 4 m/s I am probably fairly accurate. This means that kinetic energy of rain contains about 8000 Joules which means on average about 0.25 mW power hitting at a surface over the year. This is almost 100000 times smaller that average electrical power from a solar panel in the same spot. Harvesting this seems like a waste of time if your goal is to “solve the energy crisis”.

Well there is this thing about ions in the raindrops working some miracles together with graphene…Maybe I should actually glance at the paper. So their “raindrops” turn out to contain 0.6-2M of NaCl i.e. table salt. Sea water contains about 35 grams of salt per liter i.e. about 0.6M. Authors “raindrops” actually contained as much salt as seawater or more. Compared to sea water, rain water contains essentially no salt (give or take tiny amounts of impurities). What happens if we replace authors sea water raindrops with something more like the real thing? Nothing…that is what happens as demonstrated by the figure 5 authors had hidden in the supplementary.

Ok, well let us just spread salt all over clouds, because solar power. How much power was actually generated? Next figure shows that. When they dropped a drop every 10 seconds, they got about 40 picowatts in a pulse lasting maybe 100 ms. If they dropped water faster, power was reduced although since they had more events, the overall average power was probably about constant…so more rainfall didn’t end up as higher output. Kind of strange and makes UK screwed I guess (see McGoogan earlier).

Energy in each of those pulses was few picojoules (pico=10-12). How does that compare with the kinetic energy of a raindrop? Let us say we have a raindrop with a radius of 2 mm and velocity 4m/s. Then it will have a kinetic energy of about 0.3 mJ or roughly 100 million times more than the energy authors extracted from their seawater drops. Authors probably observed a tiny electrical effect due to small variation in ion concentrations and then proceeded to hype the result till kingdom come with the kind assistance from the media.

To make it more clear how tiny the effect is, let us say we want to generate that 20W/m2 with this thing. So 20 ml/h gave about 0.4 pW on average.  To get 20 Watts we need a flow rate of about 300000 m3/s or about 100 Niagara Falls through 1 m2 area. Not at all crazy. (Incidentally, I am pretty sure I can come up with ways to extract more than 20W even from a single Niagara Fall. I know it sounds arrogant, but there you go…) Did I already tell you that the pointless device deteriorated after having experienced one drop every 4 second for about 1000 seconds? That is after 250 drops in about 15 minutes. We are saved!

100 of these through one square meter. Vested interests are the only reason we are not doing that already.

I conclude with a short (slightly vulgar, sorry) language lession. Finns have a jingoistic concept “Venäläinen perseensuristin“. I have noticed that the term doesn’t translate well and often with foreigners it takes awhile before it sinks in. Direct translation is “Russian ass buzzer”. It is an answer to a question: “What is it that doesn’t buzz and which you cannot fit into your ass.” It is an unnecessary product which doesn’t even work. Many Finns are of the opinion that Soviet Union excelled at producing such products. I will file this “solar panel in the rain” into the category “perseensuristin” and I am happy to observe that such products seem to be a universal human skill.