has anyone compared energy using LED used in growing plants? sure sun is free but in winter conditions or at night for example? would it be more cost effective to keep plants in the sun during the day and give artificial light at night so that they keep growing 24x7?
If the electricity for the grow lights is produced locally via solar power, it is actually more efficient than using sunlight directly (provided LED grow lights are used). This is because the grow lights provide only the narrow bands of wavelengths that plants absorb most efficiently. Only about 45% of visible sunlight is usable by plants, compared to around 95% for photovoltaics, which more than makes up for conversion losses. You also get the ability to stack the plants vertically, and the lack of windows makes for lower heat loss in the winter.
Since plants absorb certain wavelengths of light, and LEDs produce certain wavelengths, it can be more efficient to grow plants with solar charged LEDs than with solar directly.
I think it is a pretty cool fact that because of the ability to use LED lights in this manner where they emit only the ranges of light wavelengths that are beneficial to plants, you can actually capture sunlight with solar panels. Convert that light to electricity. Then re-emit that electricity as light through these narrow range LEDs as well as store it and emit it over longer periods of time. You actually end up with more plant growth than would be possible if the plants were directly in the sun.
You don't use daylight LEDs to grow plants though. The "Green" spectrum is completely wasted. I don't know about weed (I was growing algae in a photobioreactor for a time) but you need a single-wavelength red LED to get biomass growth and blue LEDs to make the thing 'flower'. By using only the wavelengths that photosynthesis requires, you save a substantial amount of energy per photon. It was more of a concern for enclosed PBRs but not having to dissipate all of that heat was a big boon for energy saving too.
> Artificial lighting saves land because plants can be grown above each other, but if the electricity for the lighting comes from solar panels, then the savings are canceled out by the land required to install the solar panels.
This doesn't have to be the case. Plants don't use all the sunlight that hits them, because available light isn't generally the bottleneck in plant growth. Note how most plants are green, which is to say they're content with reflecting the most energy-dense range of the visible spectrum. Solar panels can in theory (and possibly in practice, I'm unsure of the current state of the art) yield more efficient utilization of solar energy than plants do. (Of course we also need to consider efficiency losses from reconverting the energy back into light, but I recall the reading that the overall system efficiency can still beat direct sunlight in theory (consider that the grow lights can be precisely tuned to only emit energy in the frequencies that plants crave).)
Thanks for the numbers on LED lighting. As a comparison, photosynthesis efficiency is about 5% [1]. One could at first think that 12% is better than 5%, but the plant still need to grow even with LED lighting.
The first step in the photosynthesis is the light absorption. The loss there is due to the limited light absorption (range 400 - 700nm). So maybe this step could be improved by using LED lighting dialled in to the range of the plant. The rest of the loss chain would remain mostly the same.
Doing the math with the wikipedia numbers
Plant growing in open field
===========================
0.53*0.7*0.76*0.32*0.6 = 0.05413632
==> 5.4%
Plant growing with LED lighting
===============================
Considering 100 % LED light absorption by the plant (which is an exaggerated hypothesis) and the 12% LED lighting efficiency
0.12*1.0*0.7*0.76*0.32*0.6 = 0.01225728
==> 1.2% efficiency
It will be more efficient for plants to capture free energy from the sun directly than to convert it into electricity, transmit that over a long distance, possibly store it for a while, and then convert it back to light before plants capture it, for a very long time. The energy losses by solar panels and led lights are very significant.
However, you also get the uplift in efficiency from the fact plants prefer certain wavelengths of light, and have nearly no use for others (eg. green). And LED's are good at producing certain wavelengths of light.
The past few years has seen dramatic shift to LED lights, which use less electricity and put off less heat. Often not considered is the burning of natural gas to purposefully create CO2 (which plants love to consume).
This can theoretically be more efficient than growing plants under sunlight. Chlorophyll mostly absorbs light in the 400nm to 700nm band which is about half of all the photons that hit lights. Even in that band ~24%[1] of the energy in them in lost due to the higher energy photons (closer to the 400nm side) being converted to lower energy photons.
So right there plants are only 25% efficient at collecting energy from photons. This is why you see indoor plants grown under red LEDs. Modern commercially available solar panels are about 21% efficient so that is actually getting close to plant can do (I'm not going to include inefficiencies in power conversion right now). Solar panels in the high 40% efficiency range exist, but they are crazy expensive and only used on things like satellites.
So using current (very advanced) technology we could actually get about double the plant output per area land by using the most advanced solar panels we have to collect light energy to power LEDs.
Most plants photosynthesise on a very narrow band of light. It's actually more efficient to generate electricity using solar panels and reemit using specific wavelength leds. Plants are around 2% efficient while solar panels are around 20%.
Also as has been said below it allows you to stack plants in a way that would be impossible if you were using sunlight.
Additional thoughts are that greenhouses are terrible at retaining heat (interestingly the origin of greenhouses used to have a thick southern facing brick wall to act as a heat soak to maintain an even heat throughout the night) and so if you're trying to keep a certain heat level having glass might end up costing you way more in energy because of the need to heat than you gain from the sun.
> No, but it is possible to convert that sunlight into solar energy and use that to power LEDs. More efficient too; since you can use LEDs that emit the proper spectrum for the plants you're growing.
Citation needed on the "more efficient" part. Sure, if you assume that the photovoltaics, transmission systems, and LED lights themselves are all 100% efficient (and that there is no energy cost to maintain any of those systems) so that you are transforming the spectrum for free, it would be more efficient, but none of those things are true, and some of them are very far from true.
Photovoltaic cells can take in a broader range of energy from the sun than plants can through photosynthesis. LEDs can then emit that energy at specific wavelengths such as red or blue that the plants can take in.
The whole system can end up being more efficient.
Furthermore the prospect of growing plants in a completely controlled environment where you can control all the variables from light intensity, light cycle, temperature, humidity and remove the pest factor and therefore the cost of pesticides can result in a much better final product.
Interestingly, solar panels and LEDs are more efficient than plants directly in the sun, due to the spectrum inefficiency of the green light absorbing dye.
>The other is that the quantity of sunlight varies throughout the year in everywhere that isn't on the equator. Grow lights allow you to go from seed to harvest faster
Why not have both? Grow lights when the sun isn't optimal, and the sun when it is. Wouldn't it be a huge cost saving measure to use free light instead of burning energy on grow lights when you don't need it?
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