Thoughts on a Chimney

While I am currently spending a significant amount of my time promoting the idea that blog technology can be, and should be used for serious scientific material, I thought I would make a post of a different and perhaps more traditional vein: that is, a light-weight idea, with no serious research behind it, but Years ago now, I created an Energy Wiki full of daft ideas for making energy. I last revisted this in 2009, with an idea for storing energy at sea. I’d actually forgotten that part of the reason for this was to try out Inkscape, which is part of the reason for this post. I wanted to try a bit of multi-media, that is, a blog post with an image in it. High tech.

So, the idea. One form of renewable is the Solar Updraft Tower, also known as a solar chimney. This works straightforwardly enough: you build a large greenhouse in a desert, with a very large chimney in the middle. The top of the chimney is in cold air, the bottom in hot, and an updraft results; stick a turbine in or at the base of the chimney, and you get energy out.

The problem is to work at all efficiently, you need a big temperature differential, so a tall chimney. This in turn means a wide chimney, both to support a substantial updraft, and for mechanical reasons. Tall means 500m or more. The bottom line of this is that a pretty significant capital expenditure is required, followed by a relatively long pay-back period, which in turn means that the biggest single expense of the project is likely to be interest charges, rather than anything else.

So, my idea, is to use an inflatable chimney instead. Initially, I thought about some kind of helium lifting scheme, but then I realised that this makes no sense; why not use hot air, which after all is what the whole system is designed to generate. Consider, for instance, the following organisation:

Inflatable Chimney

Essentially, it’s a traditional balloon with a hole in the middle. Obviously the whole system is stackable — a second balloon could be placed on top of the first and so on. The whole structure could be assembled or dissassembled as desired. Unfortunately, though this would probably take quite a bit of work.

My second thought came from the idea that, while most designs for solar chimneys have the chimney in the middle of the greenhouse, it doesn’t really need to be. A horizontal pipe to the middle would be enough. The chimney could be outside of the greenhouse. The advantage that this brings is that the tower could be raised or lowered in-situ, without the risk of it falling on, and damaging the greenhouse. So my second idea was to build the chimney as a two cylinders, with the gap between the serving as the inflatable, buoyant structure. By pleating the cylinders in opposite directions like so:

Concertina Chimney

the whole structure should concertina up and down. By inflating from the top and deflating from the bottom, it should be possible to raise or lower the entire system by opening and shutting vents at the bottom or top of each section to the inside of the chimney.

One advantage with this system, is that as the chimney gets higher, the temperature differential between the inside and the outside gets greater, which should mean that the taller the tower, the more bouyant the sections get; this should help to keep the entire thing as upright as possible, as will the air travelling through the middle, like some gigantic party blower.

Another addition that cames to mind would be to add inflatable half-toroids around the chimney at regular intervals. With a curve on the top, and a flat bottom-side, the entire thing should operate like an aerofoil, lifting the tower up; so, the windier it gets, the greater the lift, which is just what is needed to keep it as upright as possible. This should mean that the chimney can operate in relatively high wind levels.

This kind of system could even work in concert with a fixed chimney — extending the height by 500m say, and increasing it’s efficiency. It could also act as a supplement — operating only on very hot days when the greenhouse has excess capacity. Or, finally, it could operate while the main chimney was being built, meaning that a plant can start generating income earlier, which should reduce the cost of interest payments.

Of course, this all comes with drawbacks: the ongoing running costs are likely to be a significant; wind will remain a significant factor regardless; and, finally, inflating the tower will using hot air, which will reduce the efficiency of the whole system. Are these flaws significant? Well, as I said, this post is light-weight with no serious research behind it. I have no idea, nor any really clear idea about how to work out these costs. Answers on a postcard please.