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Comment by lhopki01 | Hacker News Reader

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.

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People still do good things with greenhouses. I recently watched a video from a guy growing veggies through the winter in his greenhouse. He did have a non-glass southern wall but it wasn't the type for storing heat. Instead, he was using drums of water inside the greenhouse as thermal mass, which kept the temperature up during the night.

And people still grow plants by hand. The future is still going to be automated farms just as the present day is mechanised farms.

The advantages that automated warehouse farms give will be too great to not do. Faster grow cycles, no need for fertiliser, pesticides, or herbicides, closer to consumers, space efficiencies and so on.

First, sugar cane converts 8% of the light absorbed by the plant is preserved as chemical energy.

Second, photosynthesis is actually fairly efficient, the issue is plants are living organisms and we only care about energy stored not energy used. Net result, plants still need about 45% of sunlight even if you stick with the 'perfect' bands.

Sugar cane is very much the exception. Most crop plants (which have over the years been selected for efficiency indirectly) manage 1-2%. No one is suggesting growing sugar cane indoors yet, just purely on the basis of it's height you won't gain much by stacking it.

Also you haven't addressed stacking, heat retention or for that matter nighttime light.

The important point was Sugar cane is not better at photosynthesis than other plants. It simply 'wastes' less energy on growth and 'life'.

Break even would take 50% efficient solar collectors and 90% efficient lights including transmission and conversion losses. Clearly, we are nowhere near that point.

PS: The only reasonable option would be solar collectors that focus on different parts of the spectrum and let light through to the plants under them.

You don't need 50% efficient solar collectors. That'd be the case if 1 meter squared of plants had 1 meter squared of leaves. Even at full growth there are large gaps and for most of the growing time there are even larger gaps. 1 meter squared of solar panels is always going to be 1 meter squared. Also plants can't use light above 10,000 lux while solar panels will just generate more power.

And again how do you stack plants? How do you keep them warm enough? How do you grow them during the night?

Your solar collectors are not operating at night either. Break even means having 10 square miles of solar farm and 1 square mile of dense indoor farming needs to be more efficient than 11 square miles of farming and that extra needs to cover infrastructure costs.

You can play with the arrangements of plants in a greenhouse just as easily as a factory. EX: Stacking plants using clear trays and nutrient bath allows for using more light in the same space.

Anyway, the reason why this 'works' is crazy subsidies in Japan which actually has quite a bit of farm land, plus a market for ultra premium lettuce for decorative reasons.

Where are you getting 10 square miles of solar farm to support 1 square mile of dense indoor farming? Energy can be stored or brought in from other areas during the night.

And no you can't stack in a greenhouse for the same reason that there are hardly any plants growing on the jungle floor.

This sort of thing is already being done in London and Sweden. It's the level of automation that is making this news. Of course it starts at the high end but eventually all food will be grown like this just because it'll be the most efficient overall.

You focus entirely on the efficiency of photosynthesis in plants when there are so many other variables as for why this is a good idea.

Running these for decorative lettuce in a big city is one thing, when farmers use this for corn in Iowa you might have a point.

Shipping bulk rice from US to China is cheaper in terms of energy than shipping electricity required to grow that same Rice.

PS: Another sanity check. The US has 922 Million Acres of farmland. At 1kw/m * 8h / day that's vastly more than the world total energy usage. You can't even replace that with fusion power as the waste energy would directly produce significant global worming. (4046 sq M / Acre * 8kwh/day = 32 Megawatt hours per Acre * .92 billion = 30,000 Twh per day, world electricity is under 30,000Twh per year. ) https://en.wikipedia.org/wiki/World_energy_consumption#/medi...

Grains will be the last thing to change and that won't be for a very long time.

Also you have to take into account increased yields, faster grow times, lower fertiliser, pesticide, herbicide and tractor fuel costs.

Your sanity check is way off. Solar panels only get about 22W/m^2 though out the year on average and they're more efficient than plants. Second the maximum plants can use is about 100W/m^2 and that is at maximum midday strength. Now lets take into account varying intensity of sunlight, growing seasons, leaf orientation, gaps between plants and other factors and your energy requirements are much lower.

Research is still being done but most plants grow very well with the full red spectrum and 10-20% of the blue spectrum. This vastly reduces the energy required.

22W/m^2 is either really out of date or your thinking far north.

If you really want more farmland and have a lot of energy there are simply much better options, desalination and irrigation of west Texas for example.

PS: You can bump plant efficiency from 10 to 20% from using a narrow frequency of light, but that's a fairly low cap. And 10%(plant) > {20%(panel) * 20%(plant) =} 4%, which is why I said you need to have 50% panels to 'break even'.

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