ASHP - E7 or not E7?

[ProDave]

Thanks @Bramco

 

My PV diverter is partly my own design based on another design I saw published some time ago.  My works on a half second time slice, on the basis that at full power a 3kW immersion will deliver 1Wh in just under a second, so at half second time slicing I should remain comfortably under 1Wh regardless of how much power is going to the immersion heater.  In that half second interval the immersion can be at anything from 0% to 100% power in 2% increments.

 

I have not looked at how the OpenEnergyMonitor one works but I guess it's a similar principle. 

 

With mine I only read the import and export power once every half second and update the immersion power percentage to be applied to the next half second.  And my time slicing is synchronised to the grid, I have an ac input rectified but not smoothed and read by a digital input to give me 100 time events per second at close to zero crossing of the ac waveform.

 

I know it works correctly with the present standard electricity meter.  There are times when the PV surplus is more than the immersion heater can absorb (in practice I find mine consumes 2.8kW at full power) so I also turn on a radio controlled remote switched socket when it is getting close to 100% and in the shoulder months I have a 700W electric convection heater plugged into that.  In the summer such surplus just gets exported.

 

Good to know a smart meter does appear to work on a similar measurement principle to a standard meter.

[canalsiderenovation]

1 hour ago, Bramco said:

You may ask how I'm so sure the smart meter isn't working to a more accurate standard since I can't find a spec for the charging units.  Well, once the smart meter was installed, I waited until we had a really sunny day where all of our excess PV would be diverted to the immersion heater.  On the Octopus web site you can see the usage per hour and sure enough for several hours when we didn't use any big appliances we were charged 0.0p and I could see from my monitoring of the heat bank that it was being heated as normal.

 

So I'm not absolutely certain but I do know that the diverter is working as designed and it's not drawing any power from the grid when it's working full tilt.  I should add, we only have 1.75kWp on the roof, so all of the maximum generation can be diverted to the 3kW immersion heater.

 

I think we are almost identical to @Bramco. On the Octopus app I can see the usage per half hour and it's not drawing power from the grid when it's working. We can see from the lights on our Solic 200 exactly what is happening.

 

So now for instance, we have surplus power and it is being offered to the immersion as well as other things (washing machine likely). If I was to turn the washing machine off the sensor light would flash red/green as we'd have all excess going to the immersion. Out DHW is only at 34 degrees as we both had baths last night (water was at 52 degrees last night after a sunny day). I'd expect if we continue with the mixed cloud/blue skies the temp will be high 40s by the end of the day from the solar alone (as has been the case for the past few months).

 

 

Edited September 19, 2021 by canalsiderenovation

[SteamyTea]

On some standard electrical meters, the LED flashes when power is drawn from the grid i.e every Wh. It goes in permanently when exporting. 

If that is the case, a simple optical sensor could pick up the difference and divert power.

 

A more expensive option would be battery storage in parallel with thermal storage. But that would require a BMS of some sort, and probably a secondary inverter.

 

Has anyone actually looked at the fraction of time that their systems are importing, exporting and diverting. May find that the cost if diverting is greater than the savings over a typical year. That does depend on the kWh prices if imported energy and individual attitude to sharing green power with unknown consumers.

[ProDave]

Different meters behave differently.  At our last house the red light would flash when exporting power, but not count it on it's display.  I believe even the basic dumb meters have an export register but you can't read it on the display but you can read it via the IR port.

 

Our present meter goes solid red when exporting.  And now the display alternates between the reading and RED (Reverse Energy Detected) which was intended as a function to detect if you have been tampering with the metering.

 

I know my diverter is doing a good job, i have installed my own export meter, and it so far in 3 years has clocked up a total of 275kWh of export, which is mostly when the surplus is more than the immersion can absorb, or when away and not using hot water and the HW tank temperature has maxed out.

 

I built a crude data logging function into my diverter.  I know when the immersion is on, it draws 2.8kWh so simply counting each on period and a simple summation tells me last time I checked, that about 1/3 of what we generate ends up in the immersion heater.

 

 

[Bramco]

22 hours ago, SteamyTea said:

Does your diverter electrically isolate the 3 kW immersion heater from the grid when your 1.75 kWp system in generating?

No, they don't work like that.  The isolation from the grid occurs when there if there is a power cut - the inverter handles that.   

 

There is an isolation switch in the circuit to the immersion heater (as normal), so we can isolate it if needs be.  To turn off the diverter, we can simply unplug it and there's another isolation point in the feed to the diverter.  But these are all manual.

 

22 hours ago, SteamyTea said:

Would you not be better off with a 1 kW and a secondary 0.75 kW element (of maybe a 0.5 kW one as max power dies not happen that often).

 

No. The immersion in normal operation is only connected to the diverter. So it sops up as much energy as the diverter can divert, even very small amounts.  Essentially these things work by continuously monitoring the mains flow (samples at about 50 times per mains cycle, so 2500/s). If reverse flow is detected, then the diverter starts diverting, even if it is a small amount. The 'bucket' algorithm, make sure that the 1wH charging unit threshold of the meter isn't transgressed, so the meter doesn't register a unit used while the diverter is diverting. 

 

This is important whenever the amount of energy that can be diverted is lower than the rating of the immersion. If you didn't have the bucket algorithm, you would be importing the difference to make up the full 3kW of the immersion rating. 

 

Simon

 

 

[Bramco]

20 hours ago, ProDave said:

Different meters behave differently.  At our last house the red light would flash when exporting power, but not count it on it's display.  I believe even the basic dumb meters have an export register but you can't read it on the display but you can read it via the IR port.

 

Our present meter goes solid red when exporting.  And now the display alternates between the reading and RED (Reverse Energy Detected) which was intended as a function to detect if you have been tampering with the metering.

 

Superficially they do behave differently but in terms of units registered for charging purposes they all seem to work the same - I think this must be in a statute somewhere but as I said I can't find this.

 

Our previous meter used to go solid red when it had detected reverse energy - which happens when the immersion has heated up the tank to it's target temperature, i.e. it can't take any more surplus energy.  The smart meter doesn't do this.

 

The old meter flashed 1000 times per kWh. the smart meter 2000 times.

 

But these are just the way the meters display info for us users - the key diverters all working with different meters is the regulatory charging unit of 1wH which allows diverters to work by avoiding repeatedly going over the next unit.

 

Simon

[SteamyTea]

12 minutes ago, Bramco said:

The old meter flashed 1000 times per kWh. the smart meter 2000 times

So when I have my 3W kitchen light on, it flashed every 10 minutes instead of every 20.

 

[Bramco]

1 hour ago, SteamyTea said:

So when I have my 3W kitchen light on, it flashed every 10 minutes instead of every 20.

Yes, it will flash twice as fast.

[George]

On 14/09/2021 at 15:02, joe90 said:

I presume you also have a back up fridge and a backup freezer, they are very similar in construction.

Immersion heaters (like I have).

 

Imo the best low carbon back up space heating for a heat pump - especially in a victorian house - is a log burner. 

 

Probably a bit more motivation this year if lighting the fire will save on the electricity bill... 

[SteamyTea]

3 hours ago, George said:

Imo the best low carbon back up space heating for a heat pump - especially in a victorian house - is a log burner. 

It may be low carbon in your mind, but the CO₂ emissions are worse than a gas boiler and now grid electricity.

And that is before starting on all the other risks of timber combustion.

[George]

29 minutes ago, SteamyTea said:

It may be low carbon in your mind, but the CO₂ emissions are worse than a gas boiler and now grid electricity.

And that is before starting on all the other risks of timber combustion.

Well the carbon hasn't been dug out of the ground. It was fixed from the atmosphere a few years ago. If I let the tree rot away, most of it would have gone back to being carbon dioxide. 

Edited September 21, 2021 by George

[SteamyTea]

8 hours ago, George said:

If I let the tree rot away, most of it would have gone back to being carbon dioxide. 

No.  This is a myth.

Most will become carbon in the soil.

[George]

40 minutes ago, SteamyTea said:

No.  This is a myth.

Most will become carbon in the soil.

I'm no biologist so explain How? Presumably the cells of bacteria and fungus decomposing still undergo respiration so will produce carbon dioxide.

 

Still, all the carbon released has only recently been removed from the atmosphere. Making the emissions from digging up fossilised carbon equivalent to those which can be considered part of the carbon cycle (although concentrated for human benefit) is a bit of a con that only serves to benefit the likes of BP, ExxonMobil and Shell et al. 

 

Edit - remember I'm not digging up roots to burn those, just the above ground stuff. 

Edited September 22, 2021 by George

[SteamyTea]

54 minutes ago, George said:

I'm no biologist so explain How

It is to do with energy transfer at the cell level.

Normal biological mechanisms are very low energy and can only break down a limited amount of the cellular carbon.  Combustion on the other hand is a high energy process and quickly breaks the carbon carbon bonds and allows oxygen to react easily.  This is why gardeners compost.

There is also the issue with quickly putting in a large amount of CO₂ into the atmosphere in a short time (which is what we have currently been doing for 250 years).  The associated temperature rise does not fall back to the previous level in the same amount of time (to do with energy levels of electrons and the associated photon release).  This is why we have a legacy problem with atmosphere CO₂.  We need to store, for a long time, CO₂ or sequestered CO₂, not just assume that we can take out this years emissions, next year.

 

A quick reality check is to count the rings in the log you are about to burn, each ring is a years growth.  Then time how long that log takes to burn.  If the years it takes to burn matches the number of rings, then you are part way to becomes carbon neutral.

I would like to see that log.

 

 

[George]

Right, so, now you are not saying the carbon isn't released?

 

I understand and accept the argument that there is a short term advantage in not burning biomass for our current problem. Although I've gone from biomass being my principle source of heating to a ashp back up so I'm not going to lose sleep over it. 

 

And I think you are failing to see the forest for the trees. Yes one tree took a long time to store that carbon, but the whole forest fixed a fire's worth in a fraction of the time (the whole point of coppice/woodland management. If you kept burning more than your trees can produce then you'll have a problem). Ditto for the whole absorption capability of the carbon cycle. So long as we used carbon that is an active part of the cycle we aren't going to cause much more than a few hiccups in the balance. Looking at the history of man made global warming, despite burning millions of tonnes of biomass for thousands of years, it's only when humans discovered burning fossil carbon does the CO2 levels really pick up.

 

I stand by thinking that as a back up for a air source heat pump it is still a better solution than any fossil carbon based heating. 

Edited September 22, 2021 by George

[ProDave]

Anything you burn sends CO2 up your chimney.  The atmosphere does not differentiate between new CO2 and millions of years old CO2.  It does the same regardless.

 

From the planets point of view the best thing you can do is make something from your wood rather than burn it.

 

If you want to pretend that burning recently grown wood is carbon neutral then go ahead, after all that is what they believe with DRAX but the rest of us won't be convinced.

 

P.s I do burn wood mostly from our own trees but I have never claimed it is carbon neutral, I know it is NOT.  Wood burning is a useful source of secondary heat, particularly if you have the trees to collect it from and it all comes as part of normal management of the trees, but please don't kid yourself you are helping to save the planet by burning some of it.

[George]

Carbon is only temporarily stored as wood. It does need to be released as carbon dioxide at some point (via decay or burning) else photosynthesis will stop. 

 

The carbon emissions as part of the carbon cycle are different to the carbon emissions from fossil fuels. Nature emits about 780GT of carbon dioxide every year (compared to 30GT from fossil fuels), but is pretty much all is reabsorbed via plants and oceans. 

 

This is a fundamental concept which has been part of the story of global warming since the start. Like I say, considering them as equivalent is a massive massive sop to coal, oil and gas companies. 

 

Edit -fully accept the atmosphere doesn't care about the source of CO2. This is about how you account for emissions and how we reduce them long term - the only answer is a relentless focus on fossil carbon. 

 

Also accept that sequesting biomass is a good idea. But burning biomass is also just a slow form of solar power (solar fixes and concentrates carbon, then burn it). It's the oldest and most reliable form of energy concentration. 

Edited September 22, 2021 by George

[joe90]

1 hour ago, ProDave said:

P.s I do burn wood mostly from our own trees but I have never claimed it is carbon neutral, I know it is NOT.  Wood burning is a useful source of secondary heat, particularly if you have the trees to collect it from and it all comes as part of normal management of the trees, but please don't kid yourself you are helping to save the planet by burning some of it.

Ditto, 

[Geoff-Belgraver]

On 19/09/2021 at 01:13, canalsiderenovation said:

 

 

We moved earlier in the year to Octopus Go Faster for the same reason. Their normal day rate was cheaper than any competitor at 14.63 KwH AND we got to pick 3, 4 or 5 hours of cheap electricity with 4.5p, 5p or 5.5p per KwH depending on duration/time you pick. Note that this is the Go Faster Tarrif which has more options than the standard Go one. We went for the maximum 5 hours from 8:30pm until 1:30am at 5.5p. These times suit us as it covers our DHW in the evenings when I'm most likely to be having a bath and the solar tops it up in the day (though granted since May/June the solar has primarily done all our hot water) and its at a time when I am still likely to be using electricity with TV, housework like hoovering etc.  @ProDave these earlier hours may work better if you don't want the ASHP running through the night. You can even just have 3 hours of cheap electricity at 4.5p from 8:30-11:30pm?

 

I'm looking forward to making full us of it in winter when we won't have much solar when I'll programme all appliances like washer, dryer, dishwasher to come on those times as well as our heating. I know we will still have the heating timed at various points to come on in the day to maintain a temperature I'm happy with but any cheaper rate at anytime is a bonus!

 

I think my 12 months renewal is up at the beginning of March and I'm hoping I can stay on this or a similar tarrif though I expect the normal day rate will have shot up, as it has with other competitors. Whilst I did have major issues with the app and double recording I did eventually get compensation from Octopus and I do actually now like the app now it's working so I can see the electricity use every half an hour and indeed it made me realize we had a STP pump issue which we had a sudden spike in electricity!

I’m sure we’ll all be on load-shifting tariffs like OtopusGo and others  before long and they will all be delivered by smart meters....Whilst I don’t like the idea of replacing a simple well-tried technology with new-fangled electronics, we will miss out on load-shifting deals if we stand still. The current low-wind problems exacerbated by the problems with an interconnector and gas prices illustrate how important it will be to store kW somehow. – see how gas is used to turn up the volume as the demand increases as we all wake up https://www.energydashboard.co.uk/fourtyeight

 

 So, just to keep the idea of using an ASHP and cheap off-peak electricity and a large thermal store rolling, I have arranged for a full MCS Survey of my house to see what we can achieve for RHI purposes. See https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/

 

I’m having this done free under the Flex Community scheme in Partnership with Bath & West Community Energy (BWCE) https://www.bwce.coop/about-us/ by Stemy Energy https://www.stemyenergy.co.uk who have contracted out the survey to a local ASHP installer.   

 

I've also talked at length with the MD of Stiebel-Eltron re ASHP and large domestic thermal stores and find that they are definitely not well known in the UK, but there are lots on the continent, so coupled with the MCS data, I should get a better feel for what's practical and cost effective.

 

My best delivered price so far for a fully insulated 5000 litre thermal store is £4.4k. The payback will depend on how hot we can get if from the best ASHP at night....

 

Of course, once I have all the data, I could do it all with second-hand kit.

[TonyT]

Where will you store 5000 litres of hot water?

 

[Geoff-Belgraver]

5 minutes ago, TonyT said:

Where will you store 5000 litres of hot water?

 

I have a good solid concrete pad at the end of the garden next to the back lane for the tank and the ASHP - it will need a 20m run of insulated pipe to the house

[SteamyTea]

11 minutes ago, TonyT said:

Where will you store 5000 litres of hot water

Will be a large ASHP as well.

5000 litres raised by 15°C (35° to 50°C) is 87.5 kWh.

On E7 you get 7 hours, so that will be a 12.5 kW, so realistically a 15 kW unit.

 

You also have almost 11m² of surface area in that size TS, so it better be very thick insulation.

If you get it down to 0.15 W/m.K, then over 24 hours, with a dT of 40°C, that will be 1.5 kWh/day.

But I suspect the insulation will be more like 0.5 W/m.K, so around 5 kWh/day.

Edited September 23, 2021 by SteamyTea

[Geoff-Belgraver]

4 minutes ago, SteamyTea said:

Will be a large ASHP as well.

5000 litres raised by 15°C (35° to 50°C) is 87.5 kWh.

On E7 you get 7 hours, so that will be a 12.5 kW, so realistically a 15 kW unit.

yep - probably - that's why the MCS survey is so important (if they are up to doing the calc for the thermal store)

[SteamyTea]

3 minutes ago, Geoff-Belgraver said:

yep - probably - that's why the MCS survey is so important (if they are up to doing the calc for the thermal store)

Just added in some heat losses for a thermal store.

[Dan F]

I don't know if it's availabe yet, but this is an potential alternative to a thermal store: https://tepeo.com/thezeb.  It seems there is a bit more concrete information available now then when it was last discussed here on the forum. See:  https://drive.google.com/file/d/1T1TS-RQhHnBYV5KgPm-dgAaXog0wHl_k/view