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

[fivemack.livejournal.com]

Ooh, excellent, I didn't know that. I had an uninformed belief that plumbing wasn't very reliable, and I was wrong.

I suppose I have had a luxuriant bath using solar-heated water in Scotland in May - admittedly after a quite clement day in May.

I rent, which makes a lot of these permanent house modifications (even insulation) somewhat impractical. I've found a seller of solar-water-heating-panels who will charge four thousand pounds for a set, including installation, but I don't think that breaks even in under a hundred years.

[pavanne.livejournal.com]

Bear in mind that it takes practically as much energy to heat your water from 15 degrees Centigrade to 20 degrees Centigrade as it does to heat it from 40 to 45 - so even if you let the solar do the basic legwork and then give it an hour-long blitz with the electric heater when you get home, you are saving relative to using electricity the whole time. This means you effectively do get some heating on a chilly-but-sunny winter's day, as long as the whole family times their showers nicely so the water temperature in the bottom of the tank is more than 5 degrees below the temperature in the collecting tubes (there's an electric display in the airing cupboard) for the longest amount of time. Of course electricity vs gas is another question, electricity seems quite an inefficient way to heat water to me!

I point out that a grant from the Low Carbon Buildings Programme would reduce the payback period considerably, and I'd happily bet on gas price hikes over the next 20 years. In short, I'd probably apply for a grant for my own solar passive water heating if I owned a house with a suitable roof (unlike photovoltaics, I don't think the price of solar water heating systems is going to fall dramatically in the next 2-3 years), but of course I'm renting too. And renting, with all the sharing of space and heat and such, is possibly more environmental and economic than owning.

[fivemack.livejournal.com]

I agree that electricity is less efficient than gas for heating water, but there was an implicit 'carbon-free' in my argument. I don't know how likely it is to be sensible to have fossil-carbon-free gas coming through the pipes into the house to be burned for heating water directly, so I think that means the choice is between {nuclear, hydro, wind, tide, solar}-generated electricity heating resistors heating water, or sun heating black pipes heating water. The sun has it.