Can you actually explain what makes LEDs so special?

First things first: LEDs?

LED stands for light emitting diode.  So what’s a diode?

A diode, pictured below, is an electronic component with two points of opposite charge.  In between those two points is a semiconductor.  The semiconductor acts on the electric current as that current passes across the two points of the diode, dropping down the energy levels of the electrons in the current.  It may help to think of the semiconductor as a permeable surface that allows the electrons to fall down out of their normal orbits, much like a sieve.  The energy released in that reaction is what produces light, and in more wasteful systems, heat.  The differences between semiconductors will produce various colors of light like the blue ones the scientists who won the Nobel Prize discovered. 

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What really matters more than the explanation is why that technology is better.  The answer to that question lies in its efficacy.

Efficacy is measured by the number of lumens per watt a light source produces.  To break it down a bit further, a lumen is a measurement of the amount of light a source produces while a watt is the amount of energy a source uses.

Efficacy proxies how efficient a light source is at converting energy to light.  The higher the efficacy, the more efficient the bulb is, and the more efficient the bulb is, the less energy wasted as heat loss.

So let’s first look at traditional horticultural lighting.  These are the long, fluorescent lights greenhouses use in winter months or people may use indoors to start seedlings.

Another common type of horticultural light are high-intensity discharge (HID) bulbs.  I can talk more specifically about how they compare to LEDs at a later time, but for now I’ll just mention that I chose not to analyze HIDs in this piece because of a number of variables that make that analysis difficult over time.  Those variables include uneven life-cycle watt usage, fading of the light, and disposal issues derived from certain chemicals integral to the HID’s function.

Here’s the top result on my google search for fluorescent lights and a bit of the sales copy from that page in bullet form:

  • Four 2’ Compact T5 High Output Bulbs 6400K GROW BULBS Included (PLL-55W)
  • Compact T5 bulbs have an outstanding 20,000 hour life expectancy (one year is 8,760 hours) and unlike sodium or halide bulbs, a T5 loses very little of its’ light output over its’ life.
  • High Output bulbs have extremely high lumen per watt rating – 5,000 lumens per bulb!  This system emits an incredible 20,000 lumens!
  • Lamps emit very little heat, allowing you to hang the fixture very close the the plant canopy.  THIS DRAMATICALLY INCREASES YOUR YIELDS!

Further reading shows the efficacy: 92-lumens/watt.  To produce those 92-lumens/watt, this unit takes up 24”x24” feet of space at a total cost of 134.95.

Now we have a frame of reference for the fluorescent lights.  I’ve never used this product, so can no way comment on its productive capabilities, but this information is useful for comparative purposes nonetheless.

Compared to the stagnant development of fluorescent lights, records are being set in the world of LED lighting as recently as March.  Manufacturer CREE set the record for the most efficient bulb ever, producing over 300-lumens/watt.  That’s three times the efficiency of the fluorescent lighting system.  Unfortunately, these lights aren’t expected to hit the market for at least another year.

In terms of products out now, check out this ranking fromhttp://plantozoid.com/.

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The top-ranked product is the Diamond Series XML 350 and it will cost you $549; more if you want to scale up production for commercial purposes.  Regardless, these LED stats are so much better than the fluorescent bulb we looked at.

First of all, we are looking at a light that is going to last more than twice as long as the fluorescent model above.  Unfortunately, instead of lumens, the Diamond Series lists coverage in sq. footage so we don’t have an exactly equal comparison here, but one rule of thumb says that you need around 2000 lumens per square foot for light intensive crops.  So if we are covering 20+ square feet, we are looking at 40,000 lumens giving us a rate of 121-lumens/watt.

That’s certainly more efficient than fluorescent, but there are trade-offs in areas like initial cost.  However, once that cost is overcome, and especially as the technology improves, it will be inconceivable not to use LEDs in vertical farms.

So let’s take a brief look at the pros and cons of LED lights as the technology currently stands:

Pros

  • Can produce dual band color spectrum (red and blue) at the same time.  Plus by tailoring the light to the specific part of the spectrum that plants use to make energy, LEDs ensure no light is left unused
  • Highly energy-efficient—nearly all wattage is converted into usable light
  • Minimal heat production reduces A/C and heat sink costs

Cons

  • Expensive to purchase—LED rigs can cost as much as large HID assemblies yet deliver light to less square-footage (note, in my above comparison, I looked at two lighting units that were in the same size range of the actual hardware, not the size of the space they lit)

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I want to highlight again cost is only a short term problem.  As shown in the graph above, LEDs are only getting cheaper and the energy savings that they already provide let them pay for themselves in just a few years.  This brings us to our next point- the implications of increasing growth efficiency through LEDs.

11 thoughts on “Can you actually explain what makes LEDs so special?

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  8. There is no doubt that LEDs are the future, but are they the present?

    No. Not at all.

    You will save yourself around 20 watts for the same square foot coverage. Sounds great, doesn’t it? Unfortunately it will take 50 hours to reach a single kWh (that is at 12 hours a day, which is quite common for leafy greens).

    Here comes the tricky part: How much are you paying for your kWh? The higher the cost of your electricity, the more sense it will make to get LEDs, but let’s say the standard price in the US is 10 cents (In Spain I get them for 0.06€).

    Every 4 days you will save yourself 10 cents in electricity. Every 40 days you’ll save yourself an entire dollar.

    How much does a 4-foot fluorescent tube cost? Around 1 dollar getting them in bulk from the manufacturer (which all vertical farms should obviously do). The cheapest 4-foot LEDs you can find will be 100 bucks.

    How long will it take to get that initial cost back due to your energy savings? Those extra 99 dollars, saving 1 dollar every 40 days means … 99 times 40 days … 3,960, over 10 years.

    Yes, you’ll also have to get reflectors for your fluorescents – 99.85% reflective anodized aluminum cost less than 4 euros each, and will last decades providing proper maintenance. Those 4 euros won’t be a problem either.

    As for HVAC costs: LEDs and fluorescent tubes aren’t any different in heat (BTU) production. The reason LEDs are usually said to “emit less heat” is simply because less watts run through them. Of course 35 watts will emit less heat than 54w, but again, looking at how much it will cost to cool your growing area (each watt will mean 3.41 BTU of heat) it would take many, many years to get back the money you invested in your LEDs. The exact numbers are difficult to get, since it depends on your climate and the temperature you want your grow room to be at, but summing up, saving yourself 68 BTU per 4-foot luminaire isn’t going to offset those extra 99 bucks you paid for your LED (and that’s going with the cheapest available)

    Sorry for the rant but the numbers still don’t work out. Let’s talk again in 2020, when I’m sure LEDs will be cheap enough to finally kick fluorescents out of vertical farms forever.

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