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Customer Check-In

This time I used a small tri-pod to hold the camera.  Not ideal, but does show the plants ok.  The plants near the fan are getting a nice air flow.  The Stevia leaves (plants closer to the camera) are curling in. My first guess is the leaves are either getting too much light and/or the temperature is too high.  The leaves themselves are all green, so at this point I do not believe the curling is due to too much nutrients.  I’ll be adjusting the lights based on the discussion in this post.  I’ll address the temperature in a subsequent post.

Increase CO2, Decrease Light Costs

Isn’t it FANTASTIC that increasing the CO2 can decrease the cost of lighting?

I am practicing sustainability.  I am not only interested in the $ cost,  but also how much energy is currently being used by the Leaf Spa and if there are things I can tweak to minimize both costs without compromising healthy leaf growth.

The current amount of power used by the LEDs is 95.8 W.  The amount of energy used up within a 24 hour period = 95.8 W * 20 hours / 1000 = 1.9kWh. From a $ perspective, Chart 2 from the Bureau of Land Statistic’s web page notes 1kW costs us folks in Seattle around $.10. So 1.9kW $.19 per day.

I have not optimized the settings for light intensity in the presence of higher CO2 levels.  Indeed, I haven’t evaluated the current light intensity with what is considered a healthy light intensity for the leaves.

The goal of this post is to see if I can tweak the light intensity to a lower amount such that costs in both energy and money are decreased.

Thanks To Those That Went Before

Dr. Kubota – The lectures in the course –  Greenhouse Plant Physiology & Technology from Dr. Kubota (University of Arizona), inspired me to understand the how and why CO2 benefits plant growth…after all, I want to give plants a Spa experience – so ultimately, only the best for the plants! 🙂

Increasing CO2 Increases Photosynthesis

Dr. Kubota showed us this image when she gave the lecture on the effects of CO2 on plants.  What this image is saying:

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given the same amount of light, the photosynthetic response is increased when the CO2 concentration is higher than the normal (of ~400ppm).   Since CO2 level + light drives the photosynthetic response rate, we can get to the same leaf mass with less light if light is supplemented with CO2.  YIPPEE!

Dr. Kubota goes on to note the effects of higher CO2 are somewhat thwarted by the plant’s (partial) closing of stomata.  Which is a bit of a bummer.  However, I was encouraged to see this slide:

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 Given the research results for lettuce – and making a wild extrapolation…a guesstimate I most likely shouldn’t be making but will experiment with is using 30% less light. 

Photoperiod and Amount DLI

Currently, the photoperiod is 20 hours.  The DLI is much higher than what  I discussed in a previous post, of a 10 mol·m-2·d-1 target.  Now that I have had time to explore the current set up, I’m going to adjust photoperiod and DLI to accommodate the learnings discussed in the article: Managing Electrical Conductivity (EC) For Hydroponic Basil Production.  While the Leaf Spa is being designed to accommodate many leafy plants, I am assuming  for this round of exploration, basil leaves can represent the leaves of the other plants when it comes to photoperiod, DLI, and CO2 concentration. The article states:

Basil had comparable growth (fresh and dry mass, branches, nodes) across nutrient solution ECs and there were no significant differences (Figure 1).

Figure 1. Sweet basil grown with ECs ranging from 0.5 to 4 mS∙cm−1 under low (~7 mol∙m−2∙d−1) or high (~15 mol∙m−2∙d−1) daily light integrals (DLIs). This photo was taken three weeks after transplanting seedlings into hydroponic systems and treatments.

Although EC has no effect on basil growth, growth increased for all three basil species grown under ~15 mol∙m−2∙d−1 compared to those grown under 7 mol∙m−2∙d−1. There was no interaction between the EC and DLI on basil growth.

New Targets for Photoperiod and DLI

Based on the information in this article and background information that has accumulated in my brain over the past year, assuming a CO2 level of ~ 1200ppm, the new targets are:

  • photoperiod of 16 hours.
  • DLI of 70% 15 mol∙m−2∙d−1 = 10.5 , rounding up to 11 mol∙m−2∙d−1
This amount of DLI + ~1200ppm CO2 should get to the equivalent photosynthetic rate as a DLI of 15 mol·m-2·d-1 + ~ 400ppm.

Current DLI

I’ll be using the calculation for DLI I discussed in this post: DLI = PAR reading  x (3,600 * photoperiod)/1,000,000.  In this post, I gave my initial readings for the Leaf Spa PAR levels.  The readings were far from uniform, ranging from 288µmol/s at an edge to 737µmol/s at the center.

Good better best Using the COB LEDs makes for uneven PAR as shown in the table below.  COBs are perhaps best for lighting when the light source is significantly higher than the canopy.  For the small height of the leaf shelf, smaller LEDs spread uniformly across the bottom of the shelf – or other mechanism (such as using lenses, i.e.: exploring optics) to smooth out the LED’s emittance – should be explored.

I just took measurements at each plant base and then at ~the top of the Stevia plant.

Base       Average Average
  290 280 380 317  
540 680 460 300 495 391
  430 400 250 360  
           
~Top of Stevia          
540 640 780 350 578  
930 600 810 700 760 613
  540 500 460 500  

The numbers are ball park.  I did not pay attention to getting exact measurements.  Rather these were what I read holding the PAR meter “to about where” I thought was the right spot and taking a reading.  Totally unscientific and not reproducible, but for my needs, good enough.  The top of Stevia refers to the distance seen in this image: 

Pasted Image 3 18 17 7 11 AM

Calculate Current Values

Links for how to calculate DLI when the LED is constant:  

First, I’ll calculate the DLI (DLI = PAR reading (in µmol m-2 d-1) x (3,600 (seconds/hour) * photoperiod in hours )/1,000,000 µmol/mole) for:

  • the base: DLI = (391  * 3600  * 20 )/1000 000 = 28.15 moles/day
  • the top of the Stevia Leaf: DLI = (613 * 3600 * 20)/1000000 = 44.14 moles/day
Whoa!  That’s more than is required.
Good better best I didn’t have a way (or at least couldn’t figure out a way) to calculate the DLI based on the LED’s data sheets.  At best, some vendors such as Cree provide instantaneous PPF – but this tells me how much the LED emits.  It doesn’t tell me how much PPF will smack onto a leaf.  I’m seeing measurements to be in it’s infancy with debates around definitions of acronyms such as PPFD.  Growmau (during this video) came up with a calculation based on a 4’ x 4’ grow space (1.48m2), using a 650w light @ 2.2 µmol/J.  650 * 2.2 = 1,430 µmol/sec emitted. 1,430/1.48 = 966 PPFD avg (¨mol/s/m2)…
 

Calculate When DLI ~ 11 moles/Day

Currently, the COBs are being run at 100% with a constant current of 1.05A.  If I continue to run the COBs at 100%, the photoperiod – which is currently at 20 hours would be:

  • the base = (319 * 3600 * X )/1000000 = 11 moles/day -> (11 * 1000000)/ (319 * 3600) = 9.6 hours
  • the top of the Stevia Leaf: (11 * 1000000)/(613 * 3600) = 5 hours.
Given unevenness of PAR – both from a plant location within the Leaf Spa as well as how far the leaves are from the LEDs as they grow, I need to pick an approximate value for the PAR to base calculations on.  I want to use a conservative approximate PAR value.  Looking at the measured values, I’m choosing 400 PAR to base calculations at.  
  • PAR = 400 -> (11 * 1000000)/ (400 * 3600) -> ~ 8 hours
hmm….I want the lights to be on for 16 hours. 

Adjusting the LED Driver

The LED Driver I am using is the Meanwell 709-ELG100-C1050A.  I purchased this Meanwell driver from Mouser (product link).  I am very happy with this driver.  However, up until now I hadn’t thought much about lowering the light intensity.

Good better best Future LED driver designs should take into consideration how the light intensity can be adjusted.

As noted in the data sheet:

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A models can adjust the current through a built-in potentiometer.  This is the model I bought.  Perhaps a better choice would have been the B model (for about $4 more).  The B model has connections to dim the intensity.  The range the current can be adjusted to is noted within the data sheet:

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The C1050A can be adjusted from 525 – 1050mA.

Wouldn’t you know it? The drawing in the data sheet shows the current adjustment POT on the bottom of the case:

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To get to 15 moles/day on a 16 hour photoperiod, the (average) PAR should be ~ 200.  This means I can lower the current to the low end.

Amount of Energy – Down by Half

I noted earlier the current amount of power used by the LEDs is 95.8 W.  The amount of energy used up within a 24 hour period = 95.8 W * 20 hours / 1000 = 1.9kWh. $ amounts vary.  Given $.10 / kWh, the cost = $.19 per day.

Lowering the current brought the power used down to 46.7 W.  I also adjusted the photoperiod to 16 hours.  With this setup, the amount of energy used within a 24 hour period = 46.7W * 16 hours / 1000 = .75kWh at $.10 / kWh, the cost = $.08 per day, half as much.

 

I don’t know if the lower settings are too low.  I’ll keep observing.  I certainly love the idea of 1/2 the energy and have the $ amount to run the LEDs.  Also, the lower setting will have other benefits such as:

  • decreasing the temperature of the Grow Chamber.
  • increasing the lifespan of the LEDs.

That’s it for now.  Time to move on to another adventure.

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