After David Mackay
Jun. 30th, 2008 10:50 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
fivemackEvery 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.
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.
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no subject
Date: 2008-07-01 11:20 am (UTC)I also prefer showers to baths (for speed) - I find that putting the plug in when taking a shower (our shower is over the bath) gets me a bath with maybe 2 inches of lukewarm water in it; compared to taking a bath which gets me a bath almost entirely full of very hot water. So I'm confident that showers use less water than baths.
Showers that do their own heating (with electricity) are possibly bad, because gas water/heating is more efficient; because electricity generation is done by heating water with gas and has inefficiencies. Except that if you want to move away from using *any* gas then electrically heating water is better (because electricity can come from wind farms and nuclear power).
The passive solar heating idea is probably the best bet for water-heating efficiently. But it's a bit expensive.
no subject
Date: 2008-07-01 12:02 pm (UTC)They are narrow band light sources, with poor correlated colour temperatures (CCT) which means if you light your whole home with LEDs, (for most people) the 'quality' of the light will look bad. This was a problem on older compact fluroescents too, but modern ones come with 'daylight' (6400K) CCTs.
Of course LEDs are very efficient. The ones I have in my home are 2 watt GU10s, being equivalent to about 20 watt incandesents.
no subject
Date: 2008-07-01 12:18 pm (UTC)Normal lightbulbs aren't "daylight" though, you have to pay extra. At least LEDs don't flicker. I'm not sure what's so great about "daylight" bulbs (and I don't pay for them).
no subject
Date: 2008-07-01 12:24 pm (UTC)You do have to pay extra for 6000K+ compact fluorescents, but not that much more (http://www.ecofriendlylightbulbs.co.uk/product.php?productid=16180&cat=278&page=1), particularly when you consider you're going to be saving an enormous amount compared to an ordinary incandescent.
Older compact fluorescents flickered quite badly. Modern ones don't so much. Ones with a modern electronic ballast flicker at about 30,000Hz, which is too high for people to detect.
I've tried both 2000K and 6400K compact fluorescents out, and the difference in colour quality is really quite obvious, not just in looking at the bulb but in looking at the lit room. I'd never go back to a 2000K fluorescent now.