fivemack | After David Mackay

After David Mackay

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.

Flat | Top-Level Comments Only

[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...

Edited 2008-06-30 22:59 (UTC)

You could gather solar energy on the roof, but when something goes wrong with the secondary-loop heat exchangers at Sizewell B nuclear power station, there are teams of engineers on the premises at the Suffolk coast ready to fix it at a moment's notice, and there are twenty more power stations ready to ensure that the supply of electricity to England isn't much interrupted.

When something goes wrong with the primary-loop solar heat exchangers on my roof, I need to take time off work to pay personally for people with ladders and spanners to come and fix it, I go three days without a bath and possibly with water coming out of the ceiling, and I still need the gas or electric heating to cover the common case on rainy Wednesdays in January when there wasn't any solar heat for the exchangers to collect, so I need a small plumber's nightmare of storage tanks and interchange valves too.

I don't think it's unreasonable to prefer one mighty plumber's nightmare of inconel tubing and directional-solidified nickel turbine blades in Suffolk to one small kludged-together plumber's nightmare of plastic pipes assembled by the lowest bidder in my house and every other house in the street.

Edited 2008-06-30 23:17 (UTC)

Nuclear power is a decent way to generate electricity, if we really need that much, but using electricity to heat water is usually a hugely wasteful practice. I suspect that when all costs are tallied your nuclear-electric water heater is a lot more expensive than gathering solar energy and using it to heat water. If you still want to spend some extra money on your heating system, you can lease 24x7 maintenance on your local system, and probably still save money overall.

http://www.inference.phy.cam.ac.uk/mackay/presentations/SEWTHA11/mgp00088.html

I like using solar hot water heaters, but if you want a solution without local plumbers and leaks on your roof, go for a japanese-made heat pump, which is 4.9 times more efficient than a plain electric water heater.

David MacKay

I don't think that is a fair criticism of solar water heating- my parents have had a system for 15 years now which has had precisely no maintenance (bar some boiler work not directly related to the solar heat exchanger) and replaces nearly all our water heating energy use in the summer, quite a lot in autumn and spring, and a little bit in the winter. They're extremely simple devices (especially the ~80%* of the world's total installation which is on Chinese roofs). Professionally, I have never heard that high maintenance is a problem with solar passive panels, but then I am usually speaking with industry people who would say that.

Not that I'm especially anti-nuclear, or that I'm arguing with your point really, but I suspect you could cut your estimate in half by using solar passive sensibly and then need fewer nuclear power stations.

*ESTIF, Chinese National Development & Reform Committee

Where are these objects bought from? (and are they currently a reasonable price?). Not that I own a house I could attach one to :-( (yet another flaw of renting).

You would buy from an installer - the manufacture of these things is quite easy and so the manufacturing industry is very fragmented. My parents' cost (iirc) £6k and they got a £2k grant under the Clear Skies programme, which has been replaced with the Low Carbon Buildings Programme, here:
http://www.lowcarbonbuildings.org.uk/home/
for which the applications process is apparently a nightmare, with relatively low caps for total applications processed each month. The site gives the standard cost of a system as £3,200-4,500, with a 10-year warranty and the suggestion of a yearly check by householder and a professional check-up every 3-5 years.

The best way to find an installer is probably by looking at their 'certified installers' list, or at the Solar Trade Association, http://www.solar-trade.org.uk/

It's hard to tell if these are expensive. Depends on far too many factors (for example how many people are taking baths or showers, how good you are about taking cool showers when it's only a bit sunny, whether you quit with the dishwasher etc). My parents think it's probably paid for itself, but are not really sure.

Oooh, cool. Thanks.

I'm sure I read somewhere that dishwashers use less hot water than washing up manually does. I guess it depends how much water you use...

This may very well be true; however, dishwashers (at least the one my parents used to have - this is somewhat limited market research!) heat their own water as they use it, like power showers, instead of taking it from the hot water tank.

Ah right, yes. And obviously that doesn't use the handy hot water...

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.

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.

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.

when something goes wrong with the secondary-loop heat exchangers at Sizewell B nuclear power station, there are teams of engineers on the premises at the Suffolk coast ready to fix it at a moment's notice

Interesting choice of example (http://www.guardian.co.uk/business/2008/may/28/power.cuts), Mr. W. *resists unprofessional comments about British Energy*

there are twenty more power stations ready to ensure that the supply of electricity to England isn't much interrupted

...typically at the very reasonable price of £420/MWh. (Working-day baseload currently trades (http://www.prebonenergy.com/mkt_ukpower.aspx) at about £90/MWh.)

People predicting gas price rises have good reason to do so; currently Winter '08 is trading at about a pound a therm.

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.

But you can get compact fluroescents that use far less energy. I have 3 150 watt equivalents that use approximately 100 watts total when all turned on at once.

Except that they don't appear to provide light that my eyes find useful for ordinary tasks such as cooking, reading, or, indeed, bathing. My bedroom is lit by 3 '60 watt equivalents' that between them provide a sort of dim glow, a little like a nightlight. And these are the 'modern' lights that we're supposed to find adequate.

I can only conclude that I am not fooled by whatever trick the compact fluorescents pull; a 12 watt '60 watt equivalent' appears to me to be about as bright as, well, a 12 watt bulb.

Plus they are green and flash on and off, which I find rather disagreeable. And I despise turning a light on and having to wait for a second or two before it comes on. I fear that it is the thin end of the wedge; they have gone from being instantly available, like a PDA, to awaking about as slowly as a Mac laptop. The next generation of domestic lighting will no doubt be dimmer still, and take as long as a Windows laptop to boot.

I appreciate that as a child, I laughed at the stories of people who insisted on sticking with gaslight years after it had been completely overtaken by electricity. I think my current predicament is merely karma.

My experience of compact fluroescents has been the same as yours, but not with these current ones.

The current ones I have are not dim at all - they're blinding in fact - I had to get those chinese paper ball things to cover them to diffuse the light as guests would complain that they were being blinded by them.

They do still 'warm up' to go to full brightness, but are immediately as bright as a 100w bulb, and within about 1 minute are very close to their full brightness.

Your comments about the light not looking right may be due to you using 2400K fluroescents. The ones I have are 6400K, and look perfectly fine (in fact the colour reproduction of things is noticeably better as the light is whiter than the lightly yellow light I had before).

You should probably get one and try it out: http://www.ecofriendlylightbulbs.co.uk/product.php?productid=16180&cat=278&page=1

Oh, and about your mac comment - my macbook takes about a quarter of a second to wake :P

OK, I have ordered two of the 30w daylight spirals. I'm hoping they're fab.

I look forward to a full review :->

LED lighting is on the edge of possible-for-houses these days (as in, you can get but it costs lots); almost certainly going to be masses better than fluorescents. So a brighter but also more efficient future there.

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.

LED lights are suitable for spotlights now, but not very good for generally lighting your home as it's hard to make them anything other than spotlights.

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.

I have trouble believing that you can't un-spot a spot-light by careful use of defusing filters and mirrors. Possibly expensive though.

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).

Probably expensive and bulky I'd imagine.

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.

In what way is he ignoring it? His conclusion is that it's sustainable for the whole world to share his bathing habits; your bathing habits being more efficient than his hardly invalidates that.

(If you meant "you are writing a post about something other than what I wanted you to write a post about", then my confusion is removed.)

Hardly. He's explaining that he has a wildly luxurious bathing habit that would, if replicated globally, take 4% of current carbon output. I was just pointing out that if you were thinking of spending 4% of the world's carbon output, there might be better things to spend it on.

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.

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).

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. ^_^

Clearly the correct way to finesse this is to get the people sitting nearby to lobby the cabin crew to bump you up to first class ...

That sounds like a good way to get a double seat to yourself to sleep on, not a flaw ;-)

(or take your weekly bath just before the flight).

home-grown carbon offsets. Why yes these are potatoes that have taken root in my ears.

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...

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

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.

Flat | Top-Level Comments Only

After David Mackay

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Better than using plain electricity is to use electricity with heat pumps

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minor correction

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