Super-renewable bamboo turbines
Meera talked to Jim Platts of the Univeristy of Cambridge about bamboo in wind turbine blades
Jim - Because wind turbines are a rotating machine but they are in the turbulent boundary layer of the atmosphere they suffer very much from fluctuating loads, the materials which are best for dealing with that are fibre reinforced materials, metals aren't so good. So we can easily think of glass fibre, carbon fibre, or fibre-reinforced composite materials but actually wood is a naturally occurring fibre-reinforced material.
Meera - What makes bamboo more appropriate than other types of wood?
Jim - Many common species of wood have fibres running in lots of different directions, so a block of wood is strong in all directions, but some materials, bamboo is one of them, but you can also think of fir trees, the fibres in the wood are running up the trunk of the tree or up the stem of the bamboo.
Meera - Running in one direction?
Jim - Yes, if they are all running in one direction so you get better properties in that direction
Meera - So why is it more beneficial for a blade to have unidirectional fibres?
Jim - Well a blade is itself rather like a tree. It is fastened at the root end of the blade, and then it is a long cantilevered beam picking up what are heavy aerodynamic loads from the wind, so it is bending the tip of the blade, just like when a tree bends when the wind blows. So you want the fibres running along the blade to give it the tension and compression strength where the beam is bending.
Meera - What actually happens to incorporate wood from bamboo into a wind turbine?
Jim - When we are talking about bamboo we mean Mosa bamboo, which typically grows 12-15m high, the stem is 120mm in diameter and the wall is about 15mm thick, so this is big stuff. What we want is the skin of the bamboo, the outer 1-2mm at the outside of the bamboo which is where the fibres are most densely packed. So that is where we get our highest strength. The rest of the bamboo can be used for furniture or whatever, but this is the really hight tech bit. We then take those strips and stick them one on top of the other to make a plank and then to make the aerodynamic shape and strength of the blade we lie all these planks side by side and end to end to give us the structural strength. Then we cover it with a polythene bag, pump the air out and let resin flow in to stick them all together, which makes one completed structure with a fibreglass skin on the outside to give it a hard surface but all these bamboo strips stuck together inside to give it the technical strength to do the job.
Meera - You actually have some samples of the planks here, put side-by-side covered with resin, and it is very solid, you can't even feel any gaps between the planks
Jim - The resin is doing two jobs, sticking the planks together, but also wood has its best properties when dry so we have dried out the bamboo before we use it, and when the epoxy resin soaks around it the epoxy resin is a complete vapour barrier. A bit like all the chips in the silicon chips in your computer which are encased in epoxy to keep the moisture off them.
Meera - And of course bamboo has a negative carbon footprint
Jim - Yes if we compare bamboo to glass fibre or carbon fibre, we use a lot of energy melting the sand to make the glass to make the fibres, and this energy produces CO2. Because bamboo is a natural product it is actually a carbon sequestration itself. It is taking CO2 from the atmosphere and it is making a high quality structural material out of it. And over the 20 years operating life of the wind turbine it will give you as electricity 400 times the energy content of the bamboo to make the blades. So it has a negative carbon footprint and a huge energy payback.
Meera - So to get a few facts and figures about this then, a typical turbine made of bamboo, what would it look like, how big would it be and how much energy would it produce
JIm - A typical wind turbine is a tower 80-90m high the rotor on the top 80m in diameter, and wind turbine blades 40m long. 3 of those blades on a hub which will produce 1.5MW of electricity. And you would normally have those in a wind farm of 10s or even hundreds of wind turbines making a significant sized power station.
Meera - Where in the world is bamboo going to be used, as it isn't native to the UK?
Jim - There are several major countries with big bamboo resources. The industry is beginning to develop the technology in China.