Trees for fuel

[SteamyTea]

Some of the longer standing members may remember that I got into a debate about how useless biomass was as a fuel.  There is a bloke over an the other places that decided that Dr. David MacKay was talking total rubbish and that the UK could be run from a few grass clippings, and another bloke that claimed that he had fitted a domestic biodigester that could run a house off a few poos, food waste and anything else that could be digested.

I was not alone in thinking that all this did not add up, so I planned an experiment.

I found a very small sycamore sapling in my garden, put it in a pot and let it grow for a year.  Then every year of 5 years I pulled it out the pot, leaned up the roots and weighed it.  I also measured the height and diameter.

I also did the same with a second sapling starting a year later.

So that gives me 3 full years worth of data, but on a small sample and not grown in ideal conditions.

I am not worried by this as it would take whatever magic was in Jack's bean to get to a decent mass to be worth burning.

 

So here are the mean masses for 2015, 2016, 2017,2018.

41, 88, 193, 330.

They are grams not kilograms.

 

Now at the moment I don't know what the dry mass will be, but if it is 20%, that gives me 66gm to burn.

If it has a specific energy content of 5 kWh/kg (quite optimistic that is), then I have grown, in 5 years, 1/3^rd of a kWh, and it has taken up a couple of square metres of my garden.

So that is about 20,000 kWh of insolation to get a whopping 0.33 kWh back at winter time.

That is a conversion rate of 1.65%, which oddly enough, is pretty high for a plant (but I have not included leaf fall, but have included the root).

 

So if anyone thinks that it is best to grow timber for fuel, it just isn't.  PV is better by a factor of 10.  And if you want to store PV in a battery, you don't need a very large one to store 0.165 kWh.m^-2.

[DavidFrancis]

Haven't looked at any of the other figures, but I make 0.33 to be 0.00156% of 20,000.  Perhaps that's not so high for a plant?  But then I guess the bigger the tree the more weight it puts on every year (up to a certain point)?

 

Not disagreeing with your fundamental point, though.

[MikeSharp01]

Ok so that seems somewhat incontrovertible then. 

[Jeremy Harris]

Over on the other place I remember debating the same point with the same person as @SteamyTea, and relating my experience indirectly gained from an experiment that was being conducted at my first place of work (what was then The Radiochemical Centre, and then a part of the UKAEA).

 

I've found some of the posts I wrote years ago about this, and will paste some them here for added interest.  These are from September 23rd 2011:

 

Quote

Biomass cannot ever, no matter how the figures are tweaked, be a major part of our energy supply. If you take the most efficient plants (in terms of conversion ratio of sunlight to usable biomass potential chemical energy) then the upper limit for photosynthetic energy conversion is around 11%. In practice few plants (including algae) get better than around 5%.

If you were to assume that all of the UK's available land were given over to growing the most efficient biomass crop possible (say, 5% efficient) then it would only produce around 1/3rd of the energy of that same land area covered in solar panels, or about 1/9th of that same land area covered in solar thermal collectors.

The UK uses around 9.9 x 10^18 J of energy per year. A hectare of biomass might be able to produce about 250MJ with a perfect crop of a high conversion ratio species. To grow enough biomass to meet the current UK annual energy need would require around 40,000,000,000 hectares. We currently have about 8,000,000 hectares of arable land in the UK and around 19,000,000 hectares of agricultural land in total, about 75% of our total available land area. If all our arable land was dedicated to growing biomass for energy, instead of food, we could produce about 0.002% of our total energy needs.

I don't see how the potential to provide an absolute maximum of 0.002% of our energy needs from biomass can ever be considered to be a "major part".

 

I was then questioned on whether I was using the oil yield from the biomass crop (which I was in the context of the debate back then) and gave this reply):

 

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Indeed, I did use the oil yield, because the specific point being made was regarding fuel oil, not total biomass yield. Around 50% of rape seed goes to animal feed as waste from oil extraction, I believe, plus there are large volumes of rape straw that can be burned to recover energy. The total energy yield from a rape crop may be significantly greater than the oil yield, I agree, perhaps around three times greater at a guess.

Other crops are, I believe, significantly better than rape in terms of energy produced per hectare. Willow is potentially around 45 MWh/ha/yr, but there are large areas of the UK where the conditions aren't suitable for its growth so average production rates will be a lot lower than this.

 

I followed up with this:

 

Quote

• I started my career as a chemist, working in organic radiochemistry for what was then the UKAEA, in the early 70's. I spent around a year studying carbon uptake in a wide range of plant species, grown in controlled, sealed environments where we could vary the amount of C14 labelled CO2 in the atmosphere and could accurately control and measure irradiance, both in terms of magnitude and wavelength.
  
  I found that the best photosynthetic energy conversion ratio I obtained was around 6%, and was highly dependent on light level, availability of nutrients and water and temperature. Others have reached the same conclusion, that 3 to 6% efficiency is typical for plant energy conversion. Your own conclusion that a warm, controlled environment produces better yields in some species tallies with my own findings, and isn't surprising.
  
  A colleague was doing work on the photochemistry of plant leaf cells, specifically the efficiency of chloroplasts in being able to convert light to chemical potential energy. She found that chloroplasts were theoretically able to convert about 11% of the total sunlight they were exposed to into chemical energy, but that loss factors reduced this considerably when considering the whole plant. These loss factors include such fundamentals as the angle of a leaf towards the illuminating source, shading from other leaves and stems etc.
  
  If we were able to recover and use all of the potential energy that plants can derive from the sun over their lifespan, then we could reasonably expect to obtain around 5% of that from the sunlight that had shone on them through life, assuming that they were kept at a reasonable temperature and were provided with adequate nutrients and moisture.
  
  This sets an upper bound on what we might theoretically be able to obtain in terms of energy yield per hectare, a figure that cannot be exceeded because to do so would imply that all the work done on understanding photosynthesis over the years is wrong. We know how much sunlight falls on the surface of the earth and have pretty good data for it by region. East Anglia has the greatest area of arable land in the UK, I believe, and has an annual irradiance figure of around 995kWh/m² (3,582 MJ). If we were able to grow crops all year around, with no break for harvesting or sowing, and if those crops were able to convert available sunlight into usable potential energy at a constant rate of 5% throughout the year, then in theory we could produce around 179 MJ/m², or 1,791,000 MJ per hectare. In practice the yield will be significantly lower, because the growing season may only be half of the year, not all the energy from sunlight gets converted to usable potential chemical energy in the plant and there will be an energy input in the form of producing seed, preparing the land, providing nutrients and water, harvesting etc.
  
  Nevertheless, if we ignore all the practical issues that may affect energy yield, we can still set the absolute upper bound for biomass, in terms of the theoretical potential energy per hectare, at this figure of about 1.8 TJ. Using this upper bound (which is admittedly far from achievable in practice) we can calculate that we would need approximately 5.5 million hectares of perfectly efficient crops, with no energy input other than sunlight, to meet the UKs energy needs. I believe this is the sort of figure that may be convincing some that biomass can meet our needs, but this is a flawed assumption, as the real, practical, date from crop yield testing seems to show.
  
  If we look at some real data for biomass crop yields we find that they are very significantly lower than the theoretical upper bound the above calculations suggest. Willow is a commonly suggested biomass crop, and in suitable conditions can give a maximum yield of about 165GJ/ha. Waste wheat straw can give a maximum of around 60GJ/ha. The average yields across the country will be much lower than these maximum figures, so it would be reasonable to assume that we might get around 100 to 110 GJ/ha for willow (in the right environment - it needs a lot of moisture) and perhaps 40 GJ/ha for wheat straw.
  
  If we could grow willow over all of the UK arable land area for fuel (an unlikely assumption) then it looks like we could get around 8.8 x 10^17 J, against the UK energy requirement of 9.9 x 10^18 J, around 8.8% of our requirement. However, willow won't grow well over large swathes of the UK, so we cannot get close to this figure in practice.
  
  Wheat straw might be a better example to use, as even though the energy yield per hectare is lower, it will grow pretty much anywhere. If we were able to recover wheat straw with no energy input (other than sunlight) and use it as fuel, and if all of the 8 million hectares of UK arable land were growing wheat, then it could provide around 3.2 x 10^17 J p.a. or around 3.2% of our energy requirement.
  
  Even these figures are very optimistic though, as they ignore the significant energy input, in the form of land preparation, sowing, irrigation, pesticides, harvesting, crop preparation into fuel etc. In the specific case of growing oil seed rape for biofuel, some researchers are indicating that the net energy output can be only a small percentage of the gross energy output, because of relatively high energy input in growing and harvesting. Some have even indicated that growing crops like these can produce a negative overall energy output.
  
  I'll accept that the estimates I made in that post to which you so vehemently expressed objection may be overly pessimistic, but suggest that even a very optimistic view of biomass potential shows that it simply cannot produce more than a tiny amount of our energy needs, even if we stopped growing food on all our arable land.

 

@SteamyTea then asked:

 

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Re John's statement about heat and light on growth, does the upper and lower bounds of a plants temperature range just set one of the conditions that make it grow. Outside of those bounds yield will drop off rapidly regardless of light intensity. Within those temperature bounds, as long as there is enough light, yields are fairly stable. Or is it the other way around. Serious question that needs a solution, I suspect that the term 'solutions to partial differential equations' will soon creep in.

 

and I replied:

 

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It's a long time ago now, but I recall that my data for carbon uptake (analogous to total energy) were highly variable. 6% was the best I saw, but even small variations in temperature, moisture etc made massive differences, with it not being uncommon to see uptake rates down around 1% at times. I was able to change the total CO2 ratio in the growing cabinets, and this, too made a significant difference in uptake rate, although I wouldn't expect the predicted change in atmospheric CO2 to have a massive impact on plant growth at our latitude for some years.

I'm not a plant biologist, so can't claim to fully understand the response of plants to temperature, but I strongly suspect that each species has an optimum temperature for best growth and even small changes either side of that are likely to result in poorer yields. This paper <url>http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1569569/</url> seems to support that view. I know from experience that yields of common arable crops can vary by 20% - 30% or more, just from seasonal variations in temperature, sunshine and rainfall, so I think we also have to factor that level of variability into any biomass crop that we might be reliant upon.

 

I was then denounced on the basis that I was a scientist, so "great in theory and the lab but when they get down to the practical application there common sense goes out the window".

 

It's fun debating with those who are so narrow-minded and unwilling to face facts for a time, but it does get tedious after a while.  Needless to say, the owner of that forum was a great supporter of burning wood, and convinced it would save the planet, so I was booted off a while later for being a heretic (well, not quite, but not far off!).

[Dudda]

2 hours ago, SteamyTea said:

......could run a house off a few poos, food waste and anything else that could be digested......

A timber fire is nice to curl up at around during a cold winter. Wouldn't want to curl up around a burning sh*t

[jamieled]

I wouldn't dispute the overall point, as there's plenty of evidence linked  above.  But....

 

I'm not sure I'd expect a five year experiment to illustrate much with trees.  You wouldn't expect most tree stock to put on much biomass in the early years due to the typical growth curves (Sycamore is faster than most though). 

Even though it doesn't work from a practical point of view, if we were to use timber, then growing a tree, cutting it down and then planting a new one would be a pretty inefficient way of doing it - there's a reason coppicing became popular when wood was in high demand as the developed root system allows for much faster regrowth (although not enough to negate the general conclusions above).

 

Chopping wood is quite satisfying

[SteamyTea]

43 minutes ago, jamieled said:

there's a reason coppicing became popular when wood was in high demand

All I have really done is grown enough for a bit of coppicing in 5 years.  And to root ball probably accounts for half the mass (can't ever check now as I went to the woods and planted them to see what happens in their natural environment).

I think the point is that a mature tree has greater environmental benefit alive than burnt.

Edited March 23, 2018 by SteamyTea

[Jeremy Harris]

When I was asked to give a talk about our house in the village hall, I had to think of a way of trying to make the numbers mean something.  Having a SAP EI rating of -0.9 tonnes of CO₂ is OK, but doesn't mean much to most people.  Finding out that trees have a lifetime CO₂ sequestration rate of around 20kg per year on average through life, meant I could represent our house in terms of the equivalent number of trees grown on our small plot.  It turns out that we effectively sequester the same amount of CO₂ as around 40 to 45 trees.  At most we could grow around 10 to 15 mature trees on our plot if it was intensive woodland.  As our solar array is only on a small area of the whole plot, it further illustrates just how poor trees are at turning CO₂ into biomass.

[jamieled]

17 minutes ago, SteamyTea said:

All I have really done is grown enough for a bit of coppicing in 5 years.  And to root ball probably accounts for half the mass (can't ever check now as I went to the woods and planted them to see what happens in their natural environment).

I think the point is that a mature tree has greater environmental benefit alive than burnt.

Indeed.  I wonder what it would look like if you calculated the energy required to manufacture and install solar vs the energy expended in felling, transporting, processing and drying timber.  Even on a local basis where the trees are next to your house.

[SteamyTea]

There have been studies done on the embodied energy and CO₂ for PV modules.

I can't remember the figures, but it is pretty low.

http://info.cat.org.uk/questions/pv/what-energy-and-carbon-payback-time-pv-panels-uk/

That report seems to say that it is about the same time as I have taken to grow those twigs.

[Nickfromwales]

19 hours ago, jamieled said:

Chopping wood is quite satisfying

Not if your a tree

[SteamyTea]

Is it as satisfying as punching out a politician.  There should be a day set aside for that, April 1^st is coming up.

[Ferdinand]

9 hours ago, Nickfromwales said:

Not if your a tree

 

Never heard any complaining...

[SteamyTea]

3 hours ago, Ferdinand said:

Never heard any complaining...

You not read your Tolkien then.

[Ferdinand]

52 minutes ago, SteamyTea said:

You not read your Tolkien then.

 

Taurelilómëa-tumbalemorna Tumbaletaurëa Lómëanor.

 

Quite.

[StructuralEngineer]

On 3/23/2018 at 10:02, SteamyTea said:

So here are the mean masses for 2015, 2016, 2017,2018.

41, 88, 193, 330.

They are grams not kilograms.

Looks to me that this is increasing exponentially. Plotted on a log scale it looks like this (almost a straight line):

 

So if that's the case, in ten years you could have a 24 tonne tree (24,000 grams).

 

[James H]

All these figures make my brain hurt. 

There is one biomass power sation in Yorkshire that imports its woodchips from south American so I am told. They are brought in by ship to Tyneside kept dry and trucked down to Yorkshire. 

That one plant consumes more than the total of timber felled in the Uk in one year. Which is why it has to be imported.

Apparently this makes sense. 

 

It raining and I am trying to draw a design for a treehouse. 

[SteamyTea]

2 hours ago, StructuralEngineer said:

Looks to me that this is increasing exponentially. Plotted on a log scale it looks like this (almost a straight line):

Most things in nature, and in mathematics look better on a natural log scale.  No one knows why it is that way.

I have planed the saplings in the local wood, so shall keep an eye on them over the next few years and see how they grow in their natural environment.