changing boiler to make better use of solar.

pinnks

There are of course a couple more variables which make the maths model more complex.

First there is the heat loss in the pipes between boiler and tank, which in my case means I use about three times as many kWh of gas to heat the water as I do with diverted PV (some of this will be in the boiler efficiency calculation of course as not all of that extra gas will be lost as heat in the pipes.)

Second there is the fact that the diverter siphons off small amounts of excess generation over long periods. I often see my diverter running at a few hundred watts and even on days like today when one achieves maximum production figures, my water will be hot before lunch when my big system comes into direct sun. So, my water gets hot when I am generating a max of about 1.5kW and using about 300W as base load.

If my 2.5kWh per day went back to the grid I don't think it would be reasonable to suggest that the grid could be adjusted to not generate that amount elsewhere. I don't know what excess the grid runs at to make sure our power never goes off but I imagine that all of the drip feeds into immersion heaters will always be lost in that grid overhead.

So, there is an argument that diverted PV for water heating is the most carbon efficient. It has no impact on wider grid production and of course replaces gas.

Not sure how you could build that into a model or whether, given the grid excess that is always maintained it just makes trying to model this pointless as PV is always the better option...

I'll duck now and await the onslaught telling me why I am talking out of my ...

:beer:

Ben84

pinnks wrote: »

If my 2.5kWh per day went back to the grid I don't think it would be reasonable to suggest that the grid could be adjusted to not generate that amount elsewhere. I don't know what excess the grid runs at to make sure our power never goes off but I imagine that all of the drip feeds into immersion heaters will always be lost in that grid overhead.

Your single system won't make any great difference, but the accumulated influence of many does reduce demand during the day. Adding input is basically the same as turning something off - and that too when many people do it changes the demand on the grid and the amount of electricity that has to be generated.

Martyn1981

Ben84 wrote: »

Hi zeupater,

I've gone back and done the maths again. You're right, I made a mistake in the numbers! Gas would in many cases produce more CO2 than using electricity in the UK to heat water.

Hiya Ben, fascinating discussion, and my, how fast things have changed.

Watching Gridwatch yesterday, coal was running very, very low, down to 0.28GW at one point. I suspect the massive reduction we've seen in coal generation (presumably replaced by gas) has shifted the CO2 figure.

I've always liked the idea of diverters as they develop technology for handling excess, and may just swing a decision in favour of PV, so diversion of PV is better than no PV at all. However, I too felt that overall, CO2 emissions were better off without them.

However, I was working on 1.5kWh gas to 1kWh leccy originally, but as pinnks and others have found, the difference is probably greater.

My position is starting to shift now, from 'not really worth worrying about' to 'might actually be better'. Interesting!

@ pinnks, hi, as Ben points out, individual diverters won't impact the grid, but imagine (in the future) 1m households diverting 0.5kW each. That works out at 0.5GW of demand the grid would see (as it's no longer being offset from the neighbours of diverter owners). Demand yesterday was in the low 20GW's most of the sunny period (ok, it was a Sunday), but that's still a noticeable chunk of demand, and equal to a medium sized gas generation plant.

Note - I should probably point out (just for clarification) that the lowest (daylight) demand period yesterday was the expected sunny Sunday afternoon. I'd guess (and it is a guess) that diversion to water heating, or batteries (in the future) would probably cease by 2pm or so, as they'd be mostly 'full'. Irrelevant info regarding a day, but may be of interest when considering demand patterns through the day.

Mart.

ASavvyBuyer

A diverter from REUK for £30 that monitors the red LED on the import meter and a dimmer in the circuit for the immersion heater works well for us.
Had all the "free" hot water we need since the beginning of March. Gas boiler now turned off and don't expect to turn it back on until the Autumn unless we get a very cold spell and need the heating back on again.

Martyn1981

Ben84 wrote: »

So, I calculated it the wrong way. I might go back to using the immersion heater elements in my hot water tank after all.

Just had a thought, and in fairness to Z he sticks with CO2 from gas, not grid CO2, as the diversion shouldn't be compared to the grid average, but to gas alone, as that is what will go up and down as a result of diversion.

Mart.

zeupater

Martyn1981 wrote: »

Just had a thought, and in fairness to Z he sticks with CO2 from gas, not grid CO2, as the diversion shouldn't be compared to the grid average, but to gas alone, as that is what will go up and down as a result of diversion.

Mart.

Hi Mart

Agree ... the main reasoning for doing that is that the other forms of generation could/should be classified as being fixed(ish) on any particular day due to their relative inability to track demand at short notice (pumped hydro excluded). Using the overall average mix CO2/kWh.t would tend to skew the effect of balancing power provision, this simply leaves CCGT as being the main grid-balancing power source over a reasonable period when considering the overall environmental impact of pv diversion .... it's not exactly what happens, but for a very simplistic CO2 comparison it's not likely to be that far out to even worry about ...

HTH
Z

Cardew

The most important factor for many in buying an Immersun or similar was deciding what was best for the environment; financial return was of little importance!

Indeed for many the same noble motive applied to purchasing PV panels themselves.

zeupater

Cardew wrote: »

The most important factor for many in buying an Immersun or similar was deciding what was best for the environment; financial return was of little importance!

Indeed for many the same noble motive applied to purchasing PV panels themselves.

... as is the case for those who, over the years, have spent many £thousands of their own money on alternative & environmentally 'friendly' technologies with little/no possibility of financial return ...

Z

DREKLY

Hi SavvyBuyer, can you tell me where to get a "Dimmer" that will handle 3Kw
immersion elements please ?
All the dimmers I have are only suitable for lighting, i.e. a few hundred watts !
I have used one a a drill speed controller in the distant past, but am puzzled
as to how one will operate an immersion at half load, say 1.5Kw....
thanks !

Cardew

zeupater wrote: »

... as is the case for those who, over the years, have spent many £thousands of their own money on alternative & environmentally 'friendly' technologies with little/no possibility of financial return ...

Z

Perhaps the Guru would have the figures for solar PV installations in the periods before and after the introduction of FIT?

Methinks FIT suddenly made people a lot more concerned about the environment;)