Of 3D printing and electricity from walking

It amazes me how easily people can fall for ideas that don't stand the test of just a little mathematics: there are people who believe that a 3D printer in every garage is as good an idea as generating electrical energy from tiles that people walk on.

As a proposed replacement for mass-produced products, the 3D printer represents an environmental disaster.

Let's take a Lego block as the example. My calculation is that a 2 x 1 block has about 400 cubic millimetres of plastic in it, which a 3D printer like the Up Plus can run off in a best-case 14 seconds.

The Up Plus is specified at 200 Watts, and that gives us a nice, convenient calculation: 200 Watts for 14 seconds is 2,800 joules of energy (since one watt second is one joule).

How does that compare with an extrusion process?

I don't have a number for Lego bricks, so I'll have to use public data and treat it as an “average” plastic. According to this document, from Europe's plastics industry, the whole industry produced 52 megatonnes of plastic in 2007, at an energy consumption of 3.5 million gigajoules.

That works out to 67 joules per gram of plastic produced – and if a 2 x 1 block weighs 1.6 grams, its energy budget is nearly 108 joules.

The 3D printer, on that measure, is 1/26^th as efficient as the extrusion process.

Sure, printers will get better, more efficient, quicker – but a 26 times efficiency deficit is a hell of a handicap to start with. Some things aren't subject to Moore's law – like the basic physics that a lot of the energy is needed to move the print head around, and you can't create a massless print head, so it's always going to need energy to move. It heats the plastic input mass to melting point, and heating element efficiency is capped by physics. And so on.

3D printers are interesting and exciting and they'll make huge changes out in the real world. It's just that its place in the hype world is bugging me.

Now, over to the “generate electricity walking idea, which also boils down to joules. The idea is so exciting that Adam Spencer abandoned his maths on Friday (May 3) to give it a kick along.

Whatever else it's generating, Pavegen is generating buzz. And what for? Generating real electricity, about seven joules of it, every time someone steps on a Pavegen tile.

I'm going to make two assumptions: that it takes about a kilowatt-hour to actually fabricate the tile, transport it, install it, and extract electricity out of it; and that each step generates seven watts for one second. At seven joules per step, it will take that tile about half a million steps to pay back its energy budget: if it's in a high-traffic spot, with 1,000 people stepping on it each day, that's 500 days.

After that, it's generating free electricity. At a thousand steps per day, you're getting a whole 7,000 watt-seconds out of the tile – enough to run one 11 watt compact fluoro lamp for about ten minutes. Turn the lamp off for a while and you've got the same environmental benefit as from the Pavegen tile, without the capex.

Of course, if it takes more energy to create the Pavegen, it gets worse – if it takes 10 kW to make, deliver and install the Pavegen tile, then the payback is five million steps.

And once again, I strongly suspect, physics is going to constrain the whole idea. There's only so much energy you can extract out of a footfall. Most of the available energy has to be available for the person to walk with, after all!