Calculation for carbon saved per kWh generated by solar PV?

jamesingram

rogerblack wrote: »

In addition, calculating internal energy displacement is hard.

If I put my washing machine on at midday, and use free solar to run it - then only if I was going to run the exact same cycle nomatter what the weather or time of day does it simply subtract from the grid use.

If you have a 4kw panel, that you have setup so that when it's sunny, 3kW goes to water heating, rather than using gas, your solar water heating does _not_ have zero carbon footprint.

sorry dont quite follow that ? ,
once embodied energy has been recovered with renewable kit then why wouldn't it be a net energy gain?
cheers Jim

rogerblack

jamesingram wrote: »

sorry dont quite follow that ? ,
once embodied energy has been recovered with renewable kit then why wouldn't it be a net energy gain?
cheers Jim

If I use 1kWh at midday, produced from my solar panels, rather than using 1kWh of gas at night to heat water, there is a net cost in doing so in carbon terms, as the kWh of electricity that I might export during the day would have displaced carbon 1kWh worth of carbon from power stations.

1kWh of gas water heating may use less carbon than the amount that you have not displaced due to not exporting the 1kWh in the middle of the day.

jamesingram

Sorry , yes i was just thinking energy so,

from http://www.nottenergy.com/energy_cost_comparison/

gas 0.224 kg/kwh
electric 0.594

approx 2x CO2 if your using Your PV for HW
thats of course if the N.grid is capable of offsetting your export ?:)

celerity

rogerblack wrote: »

If you have a 4kw panel, that you have setup so that when it's sunny, 3kW goes to water heating, rather than using gas, your solar water heating does _not_ have zero carbon footprint.

I'm not sure I follow this reasoning either?

For the sake of argument, let's assume the panels have already been used efficiently to pay for their manufacture, which we seem to agree on this thread may be approx four years.
At that point, it doesn't really matter how efficiently you use your generated energy, as it is essentially free, with virtually no carbon footprint.
Yes, using it to displace gas usage is not as carbon-efficient as displacing mains electricity, but you can't claim that as a carbon "footprint" as it's already free. In other words, more or less carbon may be saved depending on your usage, but you're still not generating any carbon no matter what you do (ok, you could use it to burn toast )

I'm prepared to be shot down in flames about this, as I could well be misunderstanding your argument, it's quite late after all .

/\dam

Martyn1981

celerity wrote: »

I'm not sure I follow this reasoning either?

/\dam

You're not the only one, but I think I'm learning something interesting.

If I understand what Roger is saying correctly, then consuming the electricity to heat water will reduce a households demand for gas, but effectively increase the neighbours demand for high carbon electricity (since you could have helped supply them).

Have I got that right guys?

Looking at the numbers, do I assume it takes about 2kWh (equivalent) of gas to generate 1kWh of electricity, so it's more efficient to burn the gas in a domestic boiler for heat, whilst setting spare PV electrons free to roam the wild, and help out in the local neighbourhood.

Morals and money are a funny mix! Personally I take some strange pride in exporting as much as possible, ie switching stuff off regardless of how much excess is being generated. Roll on accurate export measurement. If we were getting a more reasonable 4/5p for export (no distribution losses) that would encourage people to export more. Doubt it would (or could) be done, but I'd be happy to see the FIT reduced and export increased. Had the export been increased with energy price inflation last year, then it would have gone from 3.1p to approx 3.6p rather than rounding down from 3.2488p.

Mart.

celerity

Martyn1981 wrote: »

If I understand what Roger is saying correctly, then consuming the electricity to heat water will reduce a households demand for gas, but effectively increase the neighbours demand for high carbon electricity (since you could have helped supply them).

I agree with this, but I don't see that it logically follows that this means the solar PV owner no longer has a zero carbon footprint, which is what Roger is opining.

If someone uses their excess solar PV to power a neon sign in their attic which lights up to read "I'll be damned if I'll let my neighbour use my energy!" then granted, they are being a wasteful git, but the carbon footprint is still basically zero, as the energy being squandered hasn't required carbon to produce. The fact that it could have been given to someone else to save them burning carbon is correct, but doesn't change the owner's carbon footprint. If it did, we'd all have to calculate our carbon footprints based on the theoretical ideal of only using resources ourselves if there was nobody else on the planet that couldn't use them more efficiently. This would be an interesting utopia to live in (maybe!) but would in practise be quite difficult...

Another way of looking at it (to play with Game Theory for a moment), is that if Roger were correct, our hypothetical "non-optimal" solar PV owner's neighbour would be better off if they assassinated him. I would say this doesn't seem to be the case. At best, they would have the same amount of energy as they did while he was alive, at worst, they would have less (maybe the neon sign would have failed at some point )

/\dam

zeupater

Hi

The issue at hand is whether you're considering the national/global carbon displacement or simply the personal/household one, so both sides of the debate are equally correct .....

HTH
Z

Martyn1981

celerity wrote: »

I agree with this, but I don't see that it logically follows that this means the solar PV owner no longer has a zero carbon footprint, which is what Roger is opining.

Another way of looking at it (to play with Game Theory for a moment), is that if Roger were correct, our hypothetical "non-optimal" solar PV owner's neighbour would be better off if they assassinated him.

/\dam

Fascinating little chat this.

One thing I've learned is to try to pick my neighbours more carefully (if one can). Not sure if the PV revenue will afford me a bodyguard!

zeupater wrote: »

The issue at hand is whether you're considering the national/global carbon displacement or simply the personal/household one, so both sides of the debate are equally correct .....

Nothings ever simple, is it when you try to look at the big picture. Oh well, fun mental exercise. As always, money talks, so 'playing' with the export rate could help to pull peoples habits into line.

Can't believe how social/liberal I've become, I'm sure I used to be a capitalist back in my youth.

Mart.

rogerblack

Martyn1981 wrote: »

You're not the only one, but I think I'm learning something interesting.

If I understand what Roger is saying correctly, then consuming the electricity to heat water will reduce a households demand for gas, but effectively increase the neighbours demand for high carbon electricity (since you could have helped supply them).

Something like that, yes.

If you use 1kWh of solar electricity, not gas, your home will of course produce less CO2 from this - around 200 grams less.

But the country, who incidentally is paying for this electricity emits an extra 400 grams. (600-200g)

(Of course, one home choosing to do this may not cause any effect - but at some point someone will need to turn up the wick on a power-station.)

celerity

rogerblack wrote: »

If you use 1kWh of solar electricity, not gas, your home will of course produce less CO2 from this - around 200 grams less.

But the country, who incidentally is paying for this electricity emits an extra 400 grams. (600-200g)

By my way of thinking, I would rephrase this as the country only saves 200g as opposed to saving 600g. But they are still saving thanks to the clean, free energy of solar PV.

(Of course, one home choosing to do this may not cause any effect - but at some point someone will need to turn up the wick on a power-station.)

But they'll need to turn up the wick less than they would have to without the solar PV contribution in the first place. In that scenario the country has to produce 200g for my gas plus the original 600g my neighbour uses, so 800g in total.
With me using my energy to displace gas (which as you rightly say is less efficient in terms of offsetting carbon), the total is still only 600g, so there is a benefit.

If my chickens lay enough eggs that I can regularly give my neighbour half a dozen for free, but one day I decide to feed one to my dog so only donate five, I'd be a bit put out if he accused me of costing him an egg .

/\dam