Straw

[SteamyTea]

To save me high jacking someone's quite sensible topic, I thought I would have a quick look at straw as a building material.

 

It is often touted as a sustainable, low energy input material, so I thought I would look up how much straw is harvested from a hectare (10,000 m²) of land, a nice size garden, but only 1/13,293,000 of the land area of England.

Taking wheat straw, it is an average if 4 tonnes/hectare, the wheat itself was 7.8 tonnes (both 2021 figures and from here).

There are some standards for straw bails and the density has to be at least 80 kg/m³ at <20% moisture.

Now I think that 'construction' bails have a higher density, but I shall assume 120 kg/m³ for this (weights and measure can be adjusted as better data comes to light).

 

So at 4,000 kg/10,000 m² and 120 kg /m³, you get 33 m³ out of a hectare.

 

Thermal conductivity has been tested in this paper to be approximately, which goes into different k-values at different densities.  They estimated, via testing, that at 120 kg/m³ the k-value was 0. 75 W.m^-1,K^-1.

To get a U-Value of 0.18 W.m^-2.K^-1 would need 0.41 m thickness (well maybe a little less depending on what it is covered in).

 

That works out at 80 m² of wall area per hectare.

 

Taking a two storey house 10 m by 7 m by 5 m high, with 25% window and door openings, that works out at 127 m² external wall area.  Or, 1.6 hectares of land (16,000 m²).

 

Just to put that into perspective, on the same area of land you could put a solar farm of approximately 1.5 MW installed capacity.  That would yield about 1.5 GWh of electricity each year.  That is enough power for about 100 houses.

 

Ok, so you do loose the usefulness of selling some wheat, about £1600/year and the ability to 'grow' enough straw to insulate one house a year.

But even 3p/kWh you get £45,000 a year for selling the power.

 

So is building from straw really a good idea?

 

(it is late and I may have made a mistake on my arithmetic, but I am sure others will spot it and put me in my place)

[JohnMo]

6 minutes ago, SteamyTea said:

Just to put that into perspective, on the same area of land you could put a solar farm of approximately 1.5 MW installed capacity.  That would yield about 1.5 GWh of electricity each year.  That is enough power for about 100 houses.

Or you could put vertical panels at less density and get wheat or what ever from the same area and solar electricity.

[SteamyTea]

Just now, JohnMo said:

Or you could put vertical panels at less density and get wheat or what ever from the same area and solar electricity.

Or a 8 MW windturbine.

 

Be worth working out how much PV needs to be fitted (with a bit of storage) to run a low energy house all year round, and at what sort of housing density to get around shading problems.

Though building one larger solar farm is more efficient as it can be put on class 3b and class 4 farm land, which we have a lot of in Cornwall, and then there are all those old mining sites (I work on one and it is no good for building as too polluted and has holes in it).

[Nick Thomas]

The maths looks ballpark right to me 👍. I'm confused by trading off the land use against solar panels, though. It's not like we're short of land for either, after all.

 

Straw bale gets some critique from the permaculture side - it's a byproduct of an agricultural process they want to go away, and so, they argue, building from it isn't a sensible idea in the long term because the supply of straw will dry up

 

But in the here and now, it is available as a byproduct - a lot of it goes into incinerators at the moment, for instance - so an alternative way of tackling this is to ask how many houses/year we could build with our existing surplus straw. A brief search tells me we currently burn 200,000 tonnes of straw per year - at 120kg/M³, that's 1,666,666 M³ of straw, or about twice that in M².

 

If we say a 100M² house needs ~150M² of exterior wall, that gives us ~22K houses/year, just from straw that would otherwise be burnt.

 

The people who like burning it suggest they burn 3 million tonnes/year of straw without disturbing agricultural uses; we could take that and build ~330K houses/year - significantly more than are actually built. I suppose there would be a concomitant reduction in kWh generated, but I don't see much point in trading one off against the other. There are better ways to generate electricity.

 

Ecococon / modcell means the stuff used for burning could equally be used for building - it doesn't need to be very much other than dry.

[Nick Thomas]

https://terravesta.com/news/the-worlds-first-miscanthus-bale-house/ is an experimental build trying to satisfy the permaculture lot by saying "look, we have this perennial crop we can cultivate for housebuilding material instead of straw". In a world where we're producing significantly less straw, though, we've also freed up a great deal of land through no longer growing wheat or barley on it.

[MikeSharp01]

Fascinating stuff.

29 minutes ago, SteamyTea said:

U-Value of 0.18 W.m^-2.K^-1 would need 0.41 m thickness

0.18 seems quite high to me but I guess the bales might be more than 410mm deep.

[joe90]

burning straw in the fields was banned and straw is a waste product (although used as bedding etc) so why not use it in building if it works. Most people think it’s flammable but when coated in lime (which in effect is limestone) it becomes fire proof. I once saw a photo of houses burnt down in forest fires in the USA but the only structure still standing (minus its timber parts) was a straw bake house.

[Nick Thomas]

11 minutes ago, MikeSharp01 said:

0.18 seems quite high to me but I guess the bales might be more than 410mm deep.

 

Typically ~450mm.

[SteamyTea]

4 hours ago, MikeSharp01 said:

0.18 seems quite high to me

I think that is what building regs require as a minimum for walls now.

 

4 hours ago, joe90 said:

burning straw in the fields was banned and straw is a waste product

It is often ploughed back into the ground as it improves water retention and carbon sequestration.

4 hours ago, joe90 said:

although used as bedding

Yes, it is where its main value is.

 

[sam]

Sleaford Renewable Energy Plant is a straw-fired plant, I think the largest in the UK. 240 thousand tonnes of straw per year produces 38MW.

[SteamyTea]

16 minutes ago, sam said:

240 thousand tonnes of straw per year produces 38MW

Let's do some arithmetic.

 

240,000 [tonnes] / 4 [tonnes/hectare] = 60,000 hectares.

 

That could support a 50 GW solar farm, which would yield 50 TWh/year of electricity.

 

38 [MW] x 8760 [hours/year] = 333,000 MWh or 0.33 TWh/year of electricity.

 

You could produce the same amount of electricity, using solar, with 400 hectares of land.

 

In 2021, the UK produced 8.6 million tonnes of straw.

 

 

 

[sam]

I agree with your calculations but straw is a by product. Until recently often a waste product. The primary crop is the wheat (or other cereal).

 

The observation that solar panels are more effective, by land area, use of land than growing biomass to burn for electricity is unsurprising. They are also significantly more capital intensive.

 

Drax is another interesting comparison because the wood harvested is often from land which would be difficult to use for other purposes, such as solar. Although the reality of important biomass from North America is questionable.

 

The potential to capture the co2 and be carbon negative adds another dimension.

 

Straw is plentiful and produced near house building areas so an interesting building material to explore.

[SteamyTea]

19 minutes ago, sam said:

Until recently often a waste product

I am not sure it has every really been a waste by product, as I mentioned earlier, it is ploughed in to improve soil, and most has gone for bedding.

Recent straw prices are here.

 

 

[joe90]

The European Commission has listed straw as waste that can be used to produce “advanced biofuel” to decarbonise the transport sector. But the EU farmers’ association begs to differ. It says straw is an agricultural co-product and not waste.

[SimonD]

34 minutes ago, joe90 said:

The European Commission has listed straw as waste that can be used to produce “advanced biofuel” to decarbonise the transport sector. But the EU farmers’ association begs to differ. It says straw is an agricultural co-product and not waste.

 

Bless the politicians, so they're doing a similar thing to how they classified wood pellets and subsidised their use only to find out the cost was the massive loss of ancient forest and biodiverity around the whole of Europe. So the farmers will be forced to sell their waste product but buy more fossil fuel derived products to replace the natural co-product?

Edited April 23 by SimonD

[MikeSharp01]

52 minutes ago, SimonD said:

So the farmers will be forced to sell their waste product but buy more fossil fuel derived products to replace the natural co-product?

Yep - everything is connected to everything else!