Quote:
Originally Posted by Winkelix
Haven't you ever wondered how many coffee grounds some of those coffee stores (tim hortons, starbucks) they produce. And it probably all goes to waste. This isn't really much of a gadget yet, but I am thinking of working on some large scale coffee stove. There are some hurdles to cover yet, such as drying. But in the future, it might be cool to see a Tim Hortons running on it's coffee grounds. I just wanted to ask whether this is a practical idea to get into. It still seems pretty far away, but I had to ask for an opinion.
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We can make a few quick estimates, to put the idea into perspective.
First, a bit of coffee data, gathered from my kitchen and the internet
- Coffee required to make a 12 cup carafe of coffee (From coffee package instructions – I usually make it a good bit stronger
): .084 kg
- Energy to operate a 12 cup “Mr Coffee” coffee maker (directly measured using my handy P4400 Kill A Watt): 720,000 J (Incidentally, The coffee maker draws about 950 W while it’s brewing, then alternates between 15 s bursts of 950 and 45 s bursts of 0 W, so uses about the same amount of energy keeping the coffee warm for an hour as it did brewing it)
- Energy density of coffee (From the wikipedia article “Energy Density” assuming the same approximate average value as for wood): 10,000,000 (1e7) J/kg
With this data, we can calculate the energy available in one basket of grounds (ignoring mass loss from brewing and drying) as 840,000 J, to conclude that it should be just barely possible to make a coffee maker that runs on its own grounds. You’d need an efficient system to dry and collect the grounds, but in principle, it looks feasible.
In addition to coffee makers, I’d estimate coffee shops draw at least a continuous average of 1000 W for their lights, heat/cooling, etc. – assuming they use energy efficient lighting, etc, and don’t, as many of my favorites do, bake bread, etc. So it looks unfeasible to power them on coffee grounds alone. Most food businesses produce a lot of paper waste that could easily be used as fuel – many times more, I estimate, than they produce coffee grounds, even if coffee is all they sell.
A problem with burning fuels such as dried beans (coffee grounds), or paper, is that without a lot of emission-reduction devices (exhaust filters, electrostatic scrubbers, etc.), they’re terribly bad air-polluters (think of 19th century London). Emission-reduction devices could fix that, but scale poorly – small ones tend to use more power than their burner produces. So, in practical engineering terms, I think it’s a better idea to just continue throwing coffee grounds in the trash, and put your efforts into large-scale
Waste-to-energy technology.
Looking at it globally, world coffee production is about 7,000,000,000 (7e9) kg/yr, so if all coffee grounds could be reprocessed as fuel, at 100% efficiency, it could supply about 2 GW (2e9 W) – about as much as a large hydroelectric dam. Assuming it could be burned about as efficiently as excellent fuels like coal – a world average of about 30% efficiency, and that the grounds by available vehicles to plants averaging a couple of hundred km from consumer collection points (grocery stores would be a good place, I’d think – works well for plastic sacks), we could expect to see about 20% total efficiency, for total of 400 MW (4e8 W), about as much as a large passenger or warship, or about 0.02% of the world’s energy needs.
Coffee appears to have little potential as a fuel. As Buffy notes, coffee grounds make a good addition to a compost heap. I think that’s a better option than burning them.
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Some quick estimates
