Is industrial hemp the ultimate energy crop?

[n2doc]

BY THOMAS PRADE
ON JANUARY 4, 2014

Bioenergy is currently the fastest growing source of renewable energy. Cultivating energy crops on arable land can decrease dependency on depleting fossil resources and it can mitigate climate change.

But some biofuel crops have bad environmental effects: they use too much water, displace people and create more emissions than they save. This has led to a demand for high-yielding energy crops with low environmental impact. Industrial hemp is said to be just that.

Enthusiasts have been promoting the use of industrial hemp for producing bioenergy for a long time now. With its potentially high biomass yield and its suitability to fit into existing crop rotations, hemp could not only complement but exceed other available energy crops.

Hemp, Cannabis sativa, originates from western Asia and India and from there spread around the globe. For centuries, fibers were used to make ropes, sails, cloth and paper, while the seeds were used for protein-rich food and feed. Interest in hemp declined when other fibers such as sisal and jute replaced hemp in the 19th century.

more

http://pando.com/2014/01/04/is-industrial-hemp-the-ultimate-energy-crop/

[ms liberty]

1. K&R...n/t

[mike_c]

2. hemp's photosynthetic efficiency is not particularly remarkable...

...as far as I'm aware, especially among C₃ plants. If I recall correctly, the primary problem with biofuel approaches is that the energy density of plain (unreacted) primary production is simply not sufficient to replace more than a small fraction of the world's dependence upon high density fossil fuels. There was a Dukes paper from 2000 or so titled "Burning buried sunshine" that estimated that in 1997 or '98-- I'm citing from memory, I'm afraid-- worldwide energy consumption was something like 400 times the total photosynthetic energy capture globally, i.e. capturing all of the world's primary production, which would cause immediate biosphere collapse, would meet less than 0.01% of global energy demand. Stated another way, in that year humans consumed the equivalent of 400 years of paleo-primary production. If I'm not mistaken, Dukes calculated that increasing human demand upon global photosynthetic efficiency to 50% of annual primary production, which would also have devastating ecosystem consequences, would only cover the annual growth in energy use in the transportation sector, again for the late 1990s benchmark.

I'm likely misremembering the details, but still. I can think of lots of reasons to grow cannabis, and any excess biomass should certainly be used for something, but it doesn't sound like anything that depends upon the photosynthetic efficiency of extant biomass will do much more than dent the problem.

[NYC_SKP]

4. IIRC, plants generally are under 1% efficient in transforming sunlight into stored chemical energy.

And then there's an additional significant loss turning that energy into useful electricity or liquid fuel.

Meanwhile, solar photovoltaics are improving constantly and exceed 20% efficiency in many commercial applications.

No dice for hemp.

[jeff47]

5. Depends on the plant

http://en.wikipedia.org/wiki/Photosynthetic_efficiency

Short version: 0.1% to 8%.

But keep in mind that's "fully lifecycle" - Photons in versus carbohydrates produced. And the sugars burned by the plant to survive count against that efficiency rating. When people give much higher numbers for things like solar panels, that doesn't include the energy required to make the panel. Only photons in versus electrons out.

[NYC_SKP]

7. True, but neither are energy needed to plant, water, fertilize, etc. said plants included in calcs.

Which would drop those figures back to below what I believe PV is including considerations of manufacture and transport, etc.

Numbers below seem to assume plants are planted, cutivated, watered, etc., free of energy input or loss.

Stated another way:
100% sunlight → non-bioavailable photons waste is 47%, leaving
53% (in the 400–700 nm range) → 30% of photons are lost due to incomplete absorption, leaving
37% (absorbed photon energy) → 24% is lost due to wavelength-mismatch degradation to 700 nm energy, leaving
28.2% (sunlight energy collected by chlorophyl) → 32% efficient conversion of ATP and NADPH to d-glucose, leaving
9% (collected as sugar) → 35–40% of sugar is recycled/consumed by the leaf in dark and photo-respiration, leaving
5.4% net leaf efficiency.

[jeff47]

8. That's what's in hemp's favor in this case.

Doesn't need much irrigation and fertilizer. Hence the nickname "weed".

As for PV? Not even close. Plants utterly crush the efficiency of PV when you account for construction costs. Have to remember PV requires things like smelting ore.

[NYC_SKP]

9. It might be a draw. I'd guess that the pre-production energy costs for PV electrons...

Are offset by the post harvest costs and inefficiencies of hemp.

I haven't seen a study, or set of studies, that could confirm that.

Both are better than coal or oil!

[kristopher]

11. Electricity and liquid fuels are different energy carriers with different attributes

PV has a much greater energy payback but that doesn't help an airplane to fly or a ship to cross the ocean.

The energy balance for cellulosic ethanol (which is what hemp would be processed as) isn't particularly good, just barely above break even when you consider all inputs (including sunshine) against only the liquid fuel delivered. There will be some other energy you could credit to the by-products of the ethanol production process, but they aren't used as a fuel so it isn't really necessary to count them for this discussion. So, in the respect of it being break even on energy, it is best thought of as a battery.

However electricity in a 1000kg of batteries isn't nearly as energy dense as 1000kg of hydrocarbon fuel. Even when we account for the inefficiencies of combusting fuel for work vs using electricity for work, the greater energy density can still be required when really heavy lifting is required.

No one should think of this as a 1:1 replacement for liquid fossil fuels in our culture. It's use is expected to be limited to things that batteries and hydrogen fuel cells aren't adequate for.

As noted there are other benefits when compared to corn ethanol such as less water use, and the use of less desirable acreage.

This is all from memory but it's essentially correct. The part that might be subject to change, IIRC, is that there were some chemical processes that gave hope the input energy for processing could be reduced with an need result of a positive energy balance 3 or 4 times the input. If the details are critical, you should investigate "cellulosic ethanol" for more accurate numbers.

ETA: The OP says biofuels are the fastest growing form of renewable energy:

[aristocles]

3. Oh yeah...

Wow.

Whatever.

Is there any more wine?

[TeamPooka]

10. yes it is. nt

[AAO]

12. No brainer. It's also the new (biodegradable) plastic.