Solar Cress

In an attempt to understand the maths and stats of renewable energy, I often sit in the garden around solar noon, watch the sky and take measurements of irradiance.  I became curious to know if this relationship could be quantified.  At the beginning of April 2011, I started a simple experiment.  Each Sunday evening, I sow 5ml of cress seed in a circular tray of seed compost.  The following Sunday, any cress is harvested and weighed.  There are some experimental design issues, thus the results are a combination of learning to grow cress and the effects of temperature and irradiance, so it may take a while to make sense of the results.  At the time of writing, I have only just started to correlate the results.

The graph below shows the average temperature and cumulative GHI for each week:

The harvest including guessed/estimated values is shown in this graph:

After eight weeks, a pattern is emerging, the warmer and sunnier the weather, the better the harvest (something any farmer or gardner could have told me).  The graph below shows the plot of cumulative GHI vs. harvest.

Whilst cumulative irradiance is important, the seeds require warm weather in the early part of the week to germinate, so that there are some leaves to receive the solar radation.

The Pictures

Week 23 (06-Jun/12-Jun)
Yield: 23 gm (estimated), Est. Cumulative GHI: 37.0 kwh/m2, Mean Temp: 12.3 deg. C.  (Provisional).  This week's photos were taken indoors due to the rain outside, hence, the colour change.  A cold, damp week for the time of year.  However, at this time of year the sun is high in the sky (low air mass) which minimises the attenuating effect of clouds, thus the cumulative GHI was reasonably high.  The harvest took place after the tray had spent a day in the rain, thus the harvest weight might migh include some water.
Week 22 (30-May/05-Jun)
Yield: 25 gm (estimated), Est. Cumulative GHI: 42.6 kwh/m2, Mean Temp: 15.1 deg. C.  The week started and finished cool and cloudy, but the days in between were hot and sunny.  On the Sunday of the harvest, it had rained, the weight of the wet cress was 35 gm, after drying out over night this reduced to 27gm, 2 gms were knocked off to account for bits of compost stuck in the leaves.
Week 21 (23-May/29-May)
Yield: 7-9 gm (estimated), Est. Cumulative GHI: 31.4 kwh/m2, Mean Temp: 12.4 deg. C..  The weather was changable, clear, cloudy, cool and warm.  The sowing was a little uneven.  Mixing the seeds with dry sand to improve the distribution was not a success and the yellow surface may have reflected some sun light.
Week 20 (16-May/22-May)
Yield: 6 gm (estimated), Est. Cumulative GHI: 30.5 kwh/m2, Mean Temp: 12.9 deg. C. The week was cool and cloudy. Germination did not complete until Friday.
Week 19 (09-May/15-May)
Yield: 5 - 7 gm (estimated), Est. Cumulative GHI: 37 kwh/m2, Mean Temp: 12.3 deg. C.  Cooler and clouder than preceeding weeks, the seeds did not fully germinate until Friday.
Week 18 (02-May/08-May)
Yield: 11 gm (estimated), Cumulative GHI: 37 kwh/m2, Mean temp: 12.9 deg. C.  Generally a week of clear skies with few or scattered clouds putting in an appearance but cooler than the preceding week.  Getting better at harvesting.
Week 17 (25-Apr/01-May)
Yield 9 gm (estimated), Est. Cumulative GHI: 38.8, Mean temp: 13.5 deg. C.  A warm sunny week, but lower temperatures early in the week during the germination period.  The yield maybe an underestimate.
Week 16 (18-Apr/24-Apr)
Yield 15 gm (estimated), Estimated Cumulative GHI: 42 kwh/m2, Mean temp: 14.7 deg. C.  The week was warm and sunny with temperatures above average.  The warm start to the week resulted in rapid germination and the high levels of irradiance stimulated plant growth.
Week 15 (11-Apr/17-Apr)
Yield: N/A, Est cumulative GHI: 27 kwh/m2, Mean temp: 9.8 deg. C.  Few and Scattered cloud around on most days, except Sunday with was clear.
Week 14 (04-Apr/10-Apr)
Yield: N/A, Est. cumulative GHI: 36 kwh/m2 (maybe an overestimate), Mean temp. 10.7 deg. C.  Apart from Monday when the sky was over cast, the sky was generally clear.

Footnote

Engineering students are generally discouraged from eating their labwork.  However, in this instance, not to eat the results would be a waste.  It also provides an incentive to ensure the crop is harvested efficiently and that the leaves are free from compost.