After David Mackay

[fivemack]

Every other day, I have a bath.

It is 50cm wide, 130cm long, 15cm deep, say 0.1 cubic metres or a hundred kilos - the water weighs slightly more than I do - and made of water at 40C, heated to that temperature from the 15C at which it arrives in the house.

So that's about ten megajoules - about three kilowatt-hours - of heat that had to be applied to the water. I've got a reasonably modern boiler of say 60% efficiency, the energy content of natural gas is 37 megajoules per cubic metre, so I'm using about half a cubic metre of gas to heat the bath, say twenty moles of methane. I've turned it into twenty moles of CO2 - 880 grams.

So my bathing habit produces 160 kilos of CO2 annually. Easyjet produces 100 grams of CO2 per passenger-kilometre, so my bathing habit is equivalent to an annual return flight to Berlin.

One ton of CO2 emission is equivalent to three hot baths a day for a year - that's a nice human-scale unit.

It doesn't seem unreasonable to hope that, as civilisation progresses, everyone in the world would be able to share my bathing habits. That would be a billion tons of CO2 annually, slightly under 4% of current planetary CO2 output and a little under the present output of the Chinese cement industry; not entirely unreasonable.

It is, however, also three billion cubic metres of natural gas a day, or say a round trillion a year (about 30% of the planetary consumption of 2.819Tm^3/year from reserves of about 200Tm^3); if the water was heated electrically, it's thirty petajoules a day - a third of a terawatt, three times the output of all the nuclear power stations in France, or the power produced by covering Luxembourg in solar panels.

This sounds as if the world can have a bath every other day in an entirely sustainable fashion for an infrastructure input of around fifty billion dollars a year (nuclear power stations costing $3 per watt and lasting twenty years); large but doable. I'm glad of this, I didn't know at the start of the calculation whether my ablutory habits alone would be enough to make my lifestyle unsustainable on planetary scale.

Comments

[randwolf.livejournal.com]

[Correction]

Or one could gather solar energy on the roof without bothering about conversion to electricity at considerably less expense, probably, than using nuclear-generated electricity. Naw...

[bohemiancoast.livejournal.com]

I think you're entirely ignoring the fact that not all of us *want* a bath every other day.

I have a shower, which probably delivers 5 litres per minute (using data from the environment agency). But my shower is typically about five minutes; three if I'm in a tearing hurry, seven if I'm really lazing. So I'm using about a quarter of the water you are, and hence a quarter of the carbon (assuming I'm heating to the same temperature; I might be having my shower a little hotter than the typical bath, I don't know). Assuming it's a 10kw shower (again from the EA site), I'll be using about .8kwh, which again feels like about a quarter as much.

Now. I shower most days. With the 2.2kwH I'm saving by not using a bath, I could, for example, run a 150 watt bulb in my living room for 15 hours, far more than the two to five hours I'm actually likely to want to use it.

I would much, much rather have adequate light in my house than baths. But for some reason, wanting to light my home properly is considered a great sin, currently in the process of being outlawed.

[tigerfort.livejournal.com]

This does of course ignore the potentially substantial energy cost of providing 100kg of clean water to your house. If you live in an area with high rainfall (per capita, which is often not the same thing as per unit area; Britain is *very* dry per capita because of the high population density), this probably isn't too bad. But if you need to thoroughly clean initially nasty water, or transport it hundreds of miles, it's going to start to add up.

[ex-robhu.livejournal.com]

There are good reasons to believe that nuclear power will be cheaper in the future too, when fourth generation (http://en.wikipedia.org/wiki/Generation_IV_reactor) reactors are available (or just the generation 3+ reactors (http://en.wikipedia.org/wiki/Generation_III+_reactor) that will be online 'soon'). In particular I'm thinking of designs like the the supercritical water reactor (http://en.wikipedia.org/wiki/Supercritical_water_reactor) that would have a thermal efficiency of 45% (versus 33% for current light water reactors).

[j4.livejournal.com]

So if I only bathed once a week, I could fly to Canada!!!!

The only flaw in this otherwise perfect plan is that nobody would want to spend a 12-hour flight sitting next to somebody who only bathed once a week. ^_^

minor correction

[paul.dean.myopenid.com (from livejournal.com)]

It seems to me that your problem is not with having lots of baths, but with having lots of hot baths. I can't diagnose from this distance whether such a habit is due to a general proclivity for luxury or to being unaware of the alternative of a (necessarily) short and (extremely) refreshing cold bath ;-)

FWIW I have about 2 baths a year, but only because my bath is so huge it takes over an hour to fill and my solar water heater doesn't contain enough water to fill it...

[ex-robhu.livejournal.com]

Solar Water heaters now required by law in Hawaii: http://www.enn.com/pollution/article/37518

[mobbsy.livejournal.com]

We could just stop enjoying ablutions and start using the sort of showering techniques they use on ships (wet self, turn off shower, soap and scrub self, rinse).

Alternatively we could, as you say, just build a lot of nuclear power stations.