Salad is a solar panel you can eat

Wind turbines and solar panels are energy conversion devices, their “fuel” is the weather prevailing at their location.  The terms we use to describe the weather also describe the energy we might expect to harvest with a given device.  If the sun is high in a clear sky, solar panels will work well, this will not be the case under a thick overcast sky in winter.  For wind, Munn’s third law can be a useful guide, it states that “If you can wear a skirt or kilt without embarrassment, this might not be a good place for a turbine”.

When I first became interested in sustainability, I wanted to understand the solar energy available in my back yard.  This involved spending a lot of time with a very small PV panel and cheap multimeter.  On a fine summer day, this was a pleasant task, less so when knee deep in December snow.  Builders working on a nearby roof questioned my sanity, as did my family, dog and friends.

In addition to rain sodden notes written with freezing fingers, there was also an observation of the obvious, that plants growth is related to the amount of energy they get from the sun.  In a conversation with an allotment holder I described a lettuce as a solar panel you could eat and he too doubted my sanity.  The relationship between solar energy and crop yield is well known to farmers.

Agriculture was the first solar energy business.  In many respects it is better model for wind and solar energy producers than the utility companies.  Seasonality is fundamental to farming.  Seeds have to be sown when the soil is warm and wet enough for them to germinate and crops are harvested  when the plants have done enough photosynthesis to produce something edible.  Storage is built into the system, for example the grain harvest takes place in late summer, the grain is then stored in silos and used at a more or less constant rate through the year.  This analogy can be extended to wind, the late summer harvest would be followed by higher winds around the autumn equinoxes which turned the wind mills to produce flower.

To try and understand the relationship between plant growth and solar energy, I started a crude experiment.  For a few weeks each Sunday afternoon, I sowed 5 ml of cress seeds in a shallow circular pot.  During the week I tried to ensure that it had a good supply of water.  At the end of each week I “harvested” the cress with a pair of scissors and weighed the result.  I chose cress because it will crop within the space of a week, whilst I made a point of eating the harvest, it is not a substantial meal and trips to the supermarket continued as normal.  The photos below show the difference in the plants after a “good” week and a “bad” week.

Whilst I was focussing on solar irradiance, I was aware that temperature was also important and that temperature is the result of irradiance.

During each week I made an estimate of the cumulative solar irradiance recieved cress and by the time I had used all my seeds, obtained this graph:

This exercise was not a model of experimental design and there were numerous sources of error, but a relationship between solar irradiance and cress yield emerged.

This work was done in 2011, since them I have become aware of solar irradiance data collected by satellites such as Ceres and made available as part of the NASA Earth Observation programme and at some point I want to rework the results using that type of data.

During this period, if the weather during the preceding week had been fine and sunny, Monday’s lunch was either an egg and cress sandwich or some form of salad.

A slightly more detailed description of the experiment can be found at the end of this link:

The Solar Cress Experiment



About SolarBucket

I trained as a mechanical engineer in the 1970's and then spent most of the following 25 years doing sums and software for Oil and Gas Exploration. Current interests are the study of wind and solar resources.
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