tek, thinks & la strada ☯ॐ☢ csmr@kapsi

Why not power houses with poo?

In high-school, I used to wonder about the rumored urine-powered-engines, that the Big Oil was always buying and shelfing away, suppressing all the free fuel. There seemed to be some energy in H2O, electrolysis had been invented - might be legit!

Howcome we shipped in tons of ancient plant goo? Must be fake! The concept of energy stored as hydrogen-carbon chains was hard to grasp. Still is at times, but oil certainly looks strong when it comes to stored energy density.

So, long after other things forced me to leave the global hydrogen-car-Big-Oil -conspiracy for others to uncover, it occurred to me to ask: why buildings don't use the habitants natural waste output to generate power, on site? Is it even possible? How would that happen?

Which technology could be used to generate power from poo?

The technology that comes up is biogas, from a reactor that uses anaerobic bacterial fermentation to generate biogas. A mix likely to contain CH4, CO2, H2S, H2 and more. The little bacteria turn carbs and oils into their simplest building blocks, then into fatty acids, that they digest down to acetic acid and gases.

Simply: waste is put into an air-proof tank, enough water and warmth is added. The bacteria then convert solid waste into sludge and gas. Much like brewing beer!

If its a mom-and-pop -operation, the resulting biogas is used as-is. But if its fuel for industy, engines or otherwise more serious business, the biogas can be scrubbed to remove carbon dioxide to achieve bit better energy density.

How good is biogas?

Biogas looks to be versatile, and can power piston engines, turbines, ovens and gas-flame-stoves, and more. It only contains bit over half the energy of natural gas, bit under half the energy of diesel. Ton of brown-waste input produces between 20 kWh (pig slurry) and 250 kWh (household biowaste container) - ~21 MJ/m3.

So, why isn't biogas made in our our basement?

Quick, get the cellar full of biogas and warm the house for free?! Well the problem is energy use for typical household. It is so much higher than the energy in Your person's waste.

One persons poops daily biogas output is only 0,02 cubic meters. Thats going to contain 120 watt-hours of energy.

In a real-world scenario this just might boil one liter of room tempereature water in 5 minutes - if the stove is efficient and doesn't leak gas or heat. Typical person easily uses 10 times this energy during the day.

The math only works for huge building complexes

Your shit ain't strong enough. Its won't power much. Biogas from twenty peoples waste might be enough for one modest persons energy consumption at home. It's not going to amount to much.

But in buildings with tens of thousands of users, this biogas business begins to add up: 20000 people would generate megawatt-hours worth of poop-energy - every day.

The daily biogas from humanure of 20 000 would equal 1/4 m3 of diesel, or 1/3 m3 of gasoline. That might equal or exceed 9000 euros worth monthly, and this income might be enough for payback-period of less than 20 years, given a bio-reactor was built.

Solid plant or food waste work better

Solid plant waste, vegetable waste, food waste contain significant energy, order of magnitude more than manures. Biogas digester processing may be ideal for:

  • Large bakeries
  • Food plants
  • Other type of plant-matter processing-plant

Energy, so easy to consume, so hard to make, even harder to get to consumers. I hope this makes sense to you – in the same way as it seems to me - as why there isn't a huge multi-cubic-meter biogas-fermentation-barrel built-in under the house.

This has been rewritten 2020-04-21. Feedback welcome, see details on contacts page.

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