Saturday, 10 March 2018

How much energy can you get from rain?

Researchers in China have created a hybrid solar panel that can also make electricity from rain [1]. This has obvious advantages, especially for regions where you get more rainfall in winter than in summer because it will help to even out generation through the year. But how much energy can you actually get from rain? How does it compare with solar energy - and is it worthwhile? Here is a back of the envelope calculation.

The short answer is: the likely yield in Cambridge is about 2.5% of the electricity from solar panels of the same area. For the UK average rainfall, say 5%. For Wales, maybe 7%.

Whether this is worthwhile or not depends on how much energy and materials are needed to build the extra equipment into the panel, and how much (if at all) it decreases the efficiency of the solar power conversion.

It may be more efficient to build specialist rainfall energy converters, separate from solar panels. You could have solar panels on the South side of your roof and rainfall panels on the West/North. However the Chinese researchers find integration with the solar panels an advantage, as they can share wiring and some of the electronics. It might be more expensive in energy and materials to build completely separate panels.

Is this worth it? I am not convinced.

Here are my assumptions.

  • The kinetic energy of a raindrop, as with anything else is ½ x mass x velocity².
  • The terminal velocity of large raindrops is 10 m/s. Smaller raindrops go slower, as little as 2 m/s [2]. Let's say the average is 8 m/s.
  • Here in Cambridge, the long term average rainfall over the year is 560 mm/year [3]. That means on one square meter of area, we would get 560 kg water over the year. This is much less than the UK average which is 1150mm, about twice Cambridge [4].
  • Let's assume we can convert the kinetic energy into electricity with 50% efficiency. (The press reports do not say what the efficiency of the Chinese system is but I have found another example using a TENG device reporting 22% efficiency so 50% is probably optimistic at the moment [5].

Using these numbers ( ½ x 560 x 8²) I get an annual energy yield per square meter of 2.5 kWh.

This compares with a typical yield from solar panels (in this area) of 100 kWh/m2 (based on 800 kWh/kWp and 1kWp occupying 8 m²).

[1] Energy harvester collects energy from sunlight and raindrops ( March 2018
[2] How fast do raindrops fall? (The weather guys)
[3] Climate and soils (Cambridge University Botanic Garden)
[4] Climate Averages table (Met Office)
[5] Jinsung Chun, Byeong Uk Ye, Jae Won Lee, Dukhyun Choi, Chong-Yun Kang, Sang-Woo Kim, Zhong Lin Wang, and Jeong Min Baik Boosted output performance of triboelectric nanogenerator via electric double layer effect (Nature Communications) 2016


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