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Carbon Footprints and maths
There' s a very sensible article here looking at what is likely to make a different in carbon terms and what is pure greenwash just designed to make you feel good.
His approach is very simple. He looks at everything in terms of kilowatt hours. All forms of energy consumption are measured by the same yardstick.
My favourite quote: Turning phone chargers off when they are not in use is a feeble gesture, like bailing the Titanic with a teaspoon.
But the writer has very positive things to say about electric cars and solar hot water systems. (and pours scorn on roof-mounted windmills)
His approach is very simple. He looks at everything in terms of kilowatt hours. All forms of energy consumption are measured by the same yardstick.
My favourite quote: Turning phone chargers off when they are not in use is a feeble gesture, like bailing the Titanic with a teaspoon.
But the writer has very positive things to say about electric cars and solar hot water systems. (and pours scorn on roof-mounted windmills)
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My parents were poor, we turned *everything* off because we had to make the electric bill as small as possible and I still do it. Anyone staying with me will notice me going around turning lights off. My evening lighting is a single halogen uplighter, chosen for it's aesthetic light rather than any other reason, I'm afraid, I'm quite sensitive to the quality of light.
FF
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Phone chargers are very low power anyway and left plugged in any power brick continues to dissipate up to 10% of it's normal operating power. For example an old style vcr uses approximately 40W whilst playing/recording a tape and in standby consumes approximately 4W despite the fact the electronic require less than 1W which means 3W is dissipated as heat. Over the course of a week that 3W is approximately half a kilowatt hour.
(only the latest generations of power supply can operate efficiently to deliver less than a watt whilst delivering the 30W or so for an operating DVD recorder)
Yes and his numbers have slipped - doesn't anyone proofread for that?
Let's take hot water first
3 square metres facing South, incident energy in UK approximately 1 kilowatt per square metre, effeciency of system - practically 80% thermal efficiency but for this assume 100%.
Therefore energy absorbed equals 3 Kilowatt hours per hour given assorted factors ~(angle of incidence, amount of cloud etc whether it's summer of winter) you ought to get the equivalent of 4 to 6 hours of energy harvesting which equates to 4 time 3 which is 12 kilowatt hours not 3.8 kilowatt hours.
Reverting to micro wind turbines the errors here are egregious though in general terms the point is valid. Wind is far too fickle and a site wind survey ought to be undertaken before contemplating such a device.
Quoting from the article
OK 600W is peak power NOT average. energy yield at normal wind speeds (normally assumed to be over 4.5 m/s to 8 m/s) is around one third to one half of this - so 200W
Amount of time the wind speed exceeds 4.5 m/s varies enormously from around 10% to 40% of the time, dependant on area. BUT and it's a big but this isn't spread evenly I can remember one winter (two/three years back) a high pressure zone established over England meant there was no perceptible breeze for over two weeks!
OK assume 10% or just over two hours. This is 0.4 Kilowatt hours NOT 0.04 kilowatt hours, which is sufficient to run two CF lights for 24 hours or one 60W incandescent for 6 hours NOT one twentieth of a light bulb!
Oh and normally accepted figures for heat pumps used to be 1 unit of energy in yields 3 to 3.5 units out - dependant upon the source of low
grade heat. So I'd love to know where he gets 4.9 out!
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All four watts are dissipated as heat. You mean the other three are doing it pointlessly.
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Yes, I suppose when I start nitpicking I should expect that!
if I modified the sentence to read
"For example an old style vcr uses approximately 40W whilst playing/recording a tape and in standby consumes approximately 4W despite the fact the electronic require less than 1W which means 3W is pointlessly dissipated as heat. Over the course of a week that 3W is approximately half a kilowatt hour."
Would this would satisfy you?
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Roughly speaking the energy yield for both solar thermal and PV follows a sine wave over the course of a year. if you know maximum and minimum values you have a fairly good rule of thumb for what to expect at other times of the year
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My main light is an energy-saving circular flourescent tuned to daylight frequencies. I like that one a lot.
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We're being sold this idea that if we don't leave our TVs on stand-by and our phone chargers plugged in, then we're "doing our bit" to Save The Planet(tm) see the latest Act on C02 for example.
Of course what it *doesn't* say is that the guy in the ad probably drove his own car to and from work instead of taking public transport and the family took a holiday abroad on a cheap flight and BTW there's no draught excluder on the letterbox and, oh, are those single-glazed windows they have...
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One other noticeable thing is that the hot water comes through faster. I can't give quantitative figures as I didn't measure the speed of the hot water before the solar was fitted, so I only have my memory of how long it took as a comparison.
The system we have is a combi-boiler which is fed by pre-heated water from the solar hot water system. On a good day the boiler is completely factored out.
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30 degrees equates to 2 hours. For the sake of argument I'm assuming the roof facing about South East by South so the array would be most effectively aligned at 10:00 am GMT. Given that for six weeks either side of the solstice, sunrise is between 7:15 and 8:00 am (and sunset 16:00 and 16:45) and that the sun would be too low on the horizon for an hour or so after sunrise to have much effect (let's say 9:00 am for convenience) you'd lose about a quarter of the most effective solar heating time. which is two hours or 30 degrees either side of perpendicular to the solar array (i.e. 10:00 am in this example).
At 30 degrees off perpendicular you still get about 80 to 85% of the energy. At 45 degrees, or one more hour this drops to around 50% (a combination of 70% incident energy due to angle and reduced incident cross section of the array exposed).
Ok this is a long winded way to get to the point that for about twelve weeks centred around the winter solstice your putative array would yield about 60% -70% of an array facing due South. In part because you're losing an hour of effective solar heating in the morning and making less effective use of the highest incident energy between 12:00 and 14:00.
If we assume that during the same period the ability to harvest energy is also at it's lowest at around 30% to 40% of summer levels (higher levels of overcast etc) and take Professor David MacKay's value of 3.8 kWh as a mean you ought to be able to expect between 2.2 and 3.0 kWh as a mean. Not a lot, but...
Outside of this twelve week period day length and solar intensity changes rapidly such that by the end of February the array would have gained back hour of effective heating in the morning sacrificed by it's sub optimal orientation for the winter quarter, and benefit from increased solar radiation to lift this value appreciably. From mid April onwards I'd expect it to yield sufficient that it ought to cope with the bulk of your hot water needs (assuming you aren't taking baths each day (from my records a bath needs approximately 3 to 4 kWh to heat the water)).
Sorry, I said long winded didn't I?
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IN summer, I use around 10kwh per day, presumably laundry and dish-washing.
I'm reducing my laundry temperatures which will help a little, but it looks as though even in summer I'll probably only halve my bills.
I'll have to look at the installation costs, as they're pretty high.
Thanks for the figures.
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During the summer quarter a 3 square metre array should be good for 8 to 10 kWh per day and so could virtually eliminate hot water costs. I'd love to see
How old is your clothes washer? the difference in energy consumption between old style and new efficient models is eye widening. two years ago we were shopping for one and the cost difference between standard and efficient models (from the same manufacturer and similar appearance!) was only around £20.00 whilst energy usage differed by 3.5kWh per cycle. At current costs that's more than 50 pence per wash!
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For us it's currently theoretical as the roof faces due East/West. The only time a fixed solar water system would function here is from about now to end of September and at reduced efficiencies (roughly 2/3 of a kilowatt as opposed to 1 kilowatt per square metre - late afternoon solar flux) for about two to three hours yielding say 3 to 3.5 kWh for six to seven months of the year. So 200 days times 3kWh equals 600kWh. Payback measured in decades!
In your case I approximate 90 days of 2.5 kWh say another 90 days 5 kWh, 90 around 7 to 8 kWh and 90 days of around 10 kWh. Total somewhere between 2000 and 2200 kWh
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Because the 60 degree cycle is the one used to calculate the energy efficiency rating, our machine actually used more energy on a 40 degree wash than 60 degree one.
Also, it doesn't have a spin on the 30 degree cycle, which is a real nuisance now I'm used that more often.
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