changing boiler to make better use of solar.

pinnks

Mart/Ben

I appreciate that my diverter will, on its own, not make any material difference (analogous to those who justify not recycling, say newspapers, on the basis that recycling their newspaper will not save the planet...) but if all diverters were active at the same time then they would be seen by the grid. But my point was that they won't all be active at the same time because your cloud shades your system at a different time of the day to my cloud shading mine, or your load fluctuates differently to mine, or your PV points in a different direction and so on.

So because diverters react immediately to small excess generation and ping whatever they can into the immersion, a large number of diverters could be turning on and off again to quickly and essentially randomly to allow the grid to react a watt-for-watt basis. Clearly, on a sunny day when they are all going hammer and tongs and pumping 3kW into thousands of immersions the grid would react but I don't think the large generators would react to the reality of how diverters work on an average day.

But I am no expert on how flexibly the grid can manage generation to demand. If I am right, you would need to factor that into your CO2 savings calculation as I could not guarantee that sending leccy to the grid and using gas to heat my hot water would mean a net CO2 saving.

Ben84

Martyn1981 wrote: »

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

Hi Martyn,
You're right, things have changed quite a bit with the pollution made by electricity in the UK. For a long time I've been pretty confident that gas beats electric for heating things, which is probably why I trusted my mistaken numbers earlier. But the gap has narrowed, and in the specific case of heating water, which gas boilers take a large efficiency drop when they do, electric looks better now. I'm thinking I could go back to using my immersion heater elements with a time switch to turn them on late at night when nuclear power is making the most contribution to the grid. It would also mean during the summer I could turn off the boiler's pilot light - but I do want to be sure that won't be bad for the boiler before doing that.

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.

The way I'm seeing it is that the excess solar electricity can either displace gas by heating the water, or electricity from the grid by being fed in to it - so in purely environmental terms I think it makes sense to displace the most polluting fuel? In the case of heating hot water, it would seem it's the gas, at least in terms of CO2 emissions.

Ben84

pinnks wrote: »

But I am no expert on how flexibly the grid can manage generation to demand. If I am right, you would need to factor that into your CO2 savings calculation as I could not guarantee that sending leccy to the grid and using gas to heat my hot water would mean a net CO2 saving.

Hi pinnks,
To be clear, I changed angle along the way when I realised I'd made a mistake with the numbers, and I now think that using the excess to heat the water is a better choice.

However, if you were to send it to the grid, I do think it would also make a difference. Not all by itself from one household, but if lots of people were doing it. The power stations should be able to respond to large numbers of solar panels introducing electricity, even if they're varying output over time, because they already respond to usage patterns that are doing exactly the same thing. Thousands of appliances must be turning on and off all the time. in fact probably more abruptly than solar varies as I can add and drop multiple kW in fractions of a second when I turn on the kettle or turn off the vacuum cleaner, but all this noise of everyday use evens out between households in to something somewhat smoother with general trends over the day, and they manage to maintain the voltage and frequency within good limits throughout it all. Adding solar energy to the grid is no different in effect to turning off an appliance using the equivalent amount of energy, so if it can respond to the overall effect of many appliances turning on and off, I expect it to equally respond to many solar panels doing it too.

silverwhistle

I've got a diverter and am on course to save 1100 units a year, which is slightly limited as I'm single so don't use that much hot water, particularly in summer when long, hot post playing football baths aren't needed! Savings would be more with a family.

My unit rate is very high (5.19p p kWh) as I'm a very low user on a standing charge free tariff and my two summer bills are zero (or a few pence). Often on a summer's day starting with a warm tank I get the 'HOT' message up before midday. Even in the shoulder periods (early March/late October) I only turn the gas boiler on to constant for a specific need, so I get no issues with cycling, and a favourable impact on boiler maintenance costs. If we're talking about aggregate demand I'd imagine the grid would notice this as most tanks begin to stop taking charge as the day wears on, based on the average insolation for the day.

I can't comment on the economics for the OP, particularly if there's an additional cost of a tank and pipework, but for me it's been worth it, and as I'm on a fixed and limited income the security of reasonable and predictable energy bills is also important.

Sterlingtimes

If within the 20 year contract term for the solar PV subsidy you are moved on to metered export having a smart meter installed, you need to compare the price of 1 kWh of gas with 1 kWh of diverted electricity.

For me currently price of gas is 2.44p. At 50% efficiency in heating the hot water cylinder, the price is 4.88p per kWh of hot water.

The value in not diverting solar generated electricity is 4.85 p per kWh.

The saving is zero. Should the gas price rise there would be a marginal improvement.

It is the current deemed export facility that distorts the model and makes diversion just about viable financially. But will that last for 20 years?

ASavvyBuyer

DREKLY wrote: »

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 !

The element in our hot water tank is only 1.2kw but in the base of the tank. I connected it through a 2kw dimmer bought on ebay. There may be ones for 3kw or higher. Another alternative is to use a high power diode, so that only half of each cycle is fed to the element.

EricMears

ASavvyBuyer wrote: »

Another alternative is to use a high power diode, so that only half of each cycle is fed to the element.

Wouldn't that 'use' all the power - half to heater element and the other half lost as heat from diode ?

zeupater

Sterlingtimes wrote: »

If within the 20 year contract term for the solar PV subsidy you are moved on to metered export having a smart meter installed, you need to compare the price of 1 kWh of gas with 1 kWh of diverted electricity.

For me currently price of gas is 2.44p. At 50% efficiency in heating the hot water cylinder, the price is 4.88p per kWh of hot water.

The value in not diverting solar generated electricity is 4.85 p per kWh.

The saving is zero. Should the gas price rise there would be a marginal improvement.

It is the current deemed export facility that distorts the model and makes diversion just about viable financially. But will that last for 20 years?

Hi Sterling ...

Does your cost comparison include the value of the gas standing charge?. Of course, it can be argued that the standing charge is a fixed sum and should therefore be irrelevant for comparison, however, considering that zero standing charge offerings with higher unit costs actually do exist which, in low annualised usage conditions, can deliver substantial savings the irrelevance argument proves to be invalid.

It's not the that everyone would benefit from solar pv diversion as it all comes down to the aggregated effect of capital outlay, DHW usage and other energy efficiency measures which are applicable to a particular household. Those currently consuming large amounts of gas would logically have a lower consolidated cost/unit, whereas for many others who have already taken considerable efficiency measures the consolidated cost/unit could well be within the region where a zero standing charge tariff becomes viable ... the issue is therefore consumption led - and importantly, average energy consumption in the UK is reducing ....

Others may have (or be looking at) diverting excess pv, but we have solar thermal which provides the majority of our annual DHW. Today we will use no gas and pay nothing for the privilege of consuming nothing and having a neutral environmental impact ... yesterday we used nothing either, as was the case for the whole of last month as well as the vast majority of the month before and likely will continue to be until late October/mid November when top-up assistance from the GCH boiler becomes necessary .... Our own headline cost/unit may be high, but it's fully transparent, which is completely different to the murky world of over-complexity traps which the energy industry utilise to defend their margins and sow the seeds of confusion as to what their customers are actually paying per unit (ie unit cost+standing charge+disloyalty charge-loyalty bonus-dual fuel-paperless billing-self meter reading-early payment discount ... etc) - apart from those on similar zero standing charge offerings to ours, does anyone else really know or check what their actual cost/unit is ? ...

HTH
Z

ASavvyBuyer

EricMears wrote: »

Wouldn't that 'use' all the power - half to heater element and the other half lost as heat from diode ?

No, that's not the way diodes work. They let the current flow in only one direction and virtually act as an insulator for the other direction. Therefore, for each AC cycle only half of the power gets through to the element.

silverwhistle

zeupater wrote: »

does anyone else really know or check what their actual cost/unit is ? ... Z

Yep! Think we've both posted variations on that theme. In my post above I should have noted that my unit cost was actually 5.1975p. per unit (incl. VAT) and am currently in my >6months period when there's nothing to pay for gas. As I'm on a diverter the period isn't quite as long as for thermal for which I don't have space at my ex-council terrace.

I get the bill, I pay the bill, normally on the same day on-line, and the simplicity, transparency, predictability (and for me) low cost are ideal.

On a more general point (and this isn't in response to Sterlingtimes) in other contexts or on other threads I find that quoting unit prices doesn't help me as I'm not a standard user, and the comparison sites are even more frustrating as they don't supply the information I want in an easily available form.