Solar DHW with UniQ and PV Diversion

[DamonHD]

OK, I'm probably going straight to somewhere bad for this thought experiment, but once I have DHW sorted as per my project...

 

1) The council is determined to pull our place down in a few years, for redevelopment.

 

2) We have already greatly reduced gas consumption, from ~9MWh/y (2007) to more like 3.5MWh/y (for a family of four now) of which ~1MWh/y is DHW by my estimates.

 

3) Thus there is ~2.5MWh/y of space heat demand over winter.

 

4) It would likely be a tremendous pain getting an ASHP installed for reasons, including planning.  I'd like to do a heat pump if I sensibly could for many reasons.

 

5) Highest mean recent gas demand was 28kWh/d over Jan 2017 with a HDD (12C base) of 248, the highest over several years.  Probably 24kWh/d was space-heat.

 

6) I want to stop burning stuff, eg gas.  I'd like to be grid friendly, eg move demand away from peak.

 

Ignoring DHW, why shouldn't I replace the combi gas boiler with a 3kW Willis heater and circulation pump, guaranteed NOT to run 4pm to 7pm (would probably be 6am to 4pm and 7pm to 10pm ish enabled, called on demand by individual Radbots) and switch to Octupus Agile or similar?  Carbon intensity probably up a bit in the short term, OPEX costs probably similar to Ecotricity (eg matching SVT locally, no gas needed any more), CAPEX costs very low.  System complexity fairly low. Uses more electricity than strictly necessary, which is a clear downside.

 

Be gentle-ish please!

 

Rgds

 

Damon

Edited February 3, 2020 by DamonHD

[SteamyTea]

Do you know, and terminology gets hard here, how long a kWh of thermal input lasts. The decrement delay.

What I mean by this is, if you put 1 kWh of heat into your building in say 5 minutes and raise the temperature by 1°C (or whatever), how long does it take to fall back by that 1°C (or whatever it was).

 

Alternatively, just plumb the Willis in and see what it does.  Monitoring will be very easy.

[DamonHD]

I don't have the abive number, but eyeballing a graph of live tempratures as measured by Radbots overnight, and ignoring the period immediately after a Rabot turns off a rad, 1C fall in 4h to 8h may be a reasonable first guess.

 

I'd have to disturb the plumbing quite a lot to try it out, so I won't do that just yet.

 

Rgds

 

Damon

Edited February 4, 2020 by DamonHD

[SteamyTea]

Assuming you could live with a 2°C temperature swing (1°C above and 1 below ideal) and the off period is during the day, when houses get naturally warmer (with the odd exception), you could probably get away with it.  I only use a 4 hour heating period between 3 AM and up to 7 AM, and get away with it most of the time.

 

Have you tried to mimic your timings with your gas boiler.  Just get it to switch on for a known period of time to match the 3 kW output.  Or fan heaters.

 

February is a good month to test things as it is often the coldest month.

Edited February 4, 2020 by SteamyTea

[Ed Davies]

5 hours ago, SteamyTea said:

Do you know, and terminology gets hard here, how long a kWh of thermal input lasts. The decrement delay.

 

I think what you're looking for is the house's thermal time constant.

[SteamyTea]

2 minutes ago, Ed Davies said:

 

I think what you're looking for is the house's thermal time constant.

Yes, it was posted before I had finished my first mug of tea.

[DamonHD]

9 hours ago, SteamyTea said:

Have you tried to mimic your timings with your gas boiler.  Just get it to switch on for a known period of time to match the 3 kW output.  Or fan heaters.

 

No, because it is usually modulating and I have no idea how efficient it is, etc.  I do have hourly gas consumption readings from which I manually pick out DHW and SH use from time to time.

 

I suspect that my SH demand is low enough that my combi is massively inefficient when satisfying it.  Maybe 50%.  Official efficiency ~78% (SEDBUK) though for what operation and conditions I am not clear.

 

Rgds

 

Damon

[DamonHD]

Any space heat component may be a long way away yet, but I've attempted to reformulate that question as "for the last ~5 years of the life of this house would it be better to keep the current ~80% efficient gas combi for ~2.5MWh/y of space heat, or switch to low-capex direct resistance, assuming a heat-pump with CoP ~3 is not reasonably do-able?"

 

Rgds

 

Damon

[Nickfromwales]

20 minutes ago, DamonHD said:

Any space heat component may be a long way away yet, but I've attempted to reformulate that question as "for the last ~5 years of the life of this house would it be better to keep the current ~80% efficient gas combi for ~2.5MWh/y of space heat, or switch to low-capex direct resistance, assuming a heat-pump with CoP ~3 is not reasonably do-able?"

 

Rgds

 

Damon

Best to see if your beloved project will still be standing long enough to gather data? ?

[Jeremy Harris]

If the gas boiler is working OK, and is still supported with spares, then my inclination would be to leave it as-is, as that's probably going to be the least expensive option.  Tempting as it is to look at replacing it to reduce your carbon footprint, there's also an argument for retaining it until the house gets demolished, as that way its manufacturing carbon footprint gets stretched out over a longer working lifetime.

 

Always hard, these moral vs. economic decisions.

[Nickfromwales]

2 minutes ago, Jeremy Harris said:

If the gas boiler is working OK, and is still supported with spares, then my inclination would be to leave it as-is, as that's probably going to be the least expensive option.  Tempting as it is to look at replacing it to reduce your carbon footprint, there's also an argument for retaining it until the house gets demolished, as that way its manufacturing carbon footprint gets stretched out over a longer working lifetime.

 

Always hard, these moral vs. economic decisions.

+1

[DamonHD]

Putting aside the moral aspect, I still wonder which is loweest carbon.  Of course we do not know how fast the GB grid will continue to decarbonise, especially in winter when this really atters.

 

Rgds

 

Damon

[Jeremy Harris]

One thing that I think we can probably be pretty sure about, is that in five years time the grid is probably going to be far less carbon intensive than it is now.  We also can't be sure about what new products might become available as new housing stock gets weaned off gas and on to electricity for heating and hot water.  I have a feeling that we may well see some new products emerge, as well as a price drop as niche products (like heat pumps and thermal storage) become more main stream.

 

It could be that in five years time your decisions might be simpler/easier, especially if the move of house creates opportunities to further improve household overall energy performance.  For us, reducing energy use was right at the heart of our reason for self-building, rather than anything else.  It might be that your choice of future home could be driven by a similar wish.

[DamonHD]

OK, given feedback here, and as soon as everything settles down from the funding round we just closed, I'll try and get this moving again.

 

UniQ6 + CombiSol + Eddi + hub + Combimate phosphate doser plus possibly a little extra wiring to make a following phase easier plus just possibly some kind of flow meter is about the long and the short of it.

 

Hurrah!

 

Rgds

 

Damon

[MrMagic]

A quick question here to the smarter folk - I've also been looking at options for DHW, keeping it separate from household heating.

 

Data gathered so far - 

- 2x showers in the morning and 1 bath in the evening

- Actual measured water usage -

- 2x showers = 180L total

- 1 bath = 60L (yes, the bath really uses less water than 1 shower, really need to fix that)

- Current delivery temperature target set on combi boiler - 50degC (actual seems to vary between 48 - 53)

 

So in theory I need 180 litres of 50degC water to cover the morning and then a reheat to do the bath in the evening, or a capacity of 240 litres at 50degC - Obviously this is mixed at the end so the hot water is probably less.

 

If I've found the correct formula it should be - 

Calculate the kilowatt-hours (kWh) required to heat the water using the following formula: Pt = (4.2 × L × T ) ÷ 3600. 

Pt is the power used to heat the water, in kWh. L is the number of liters of water that is being heated and 

T is the difference in temperature from what you started with, listed in degrees Celsius.

 

Pt = (4.2 x 240 x 30) / 3600 = 8.4kWh of DHW input per day (minimum!)

 

(is 20degC incoming too high? any ideas on what is the correct design temperature?)

 

Looking at the Sunamp website it seems to be pushing me towards a Uniq9 (10.5kWh / 210L equiv)

 

On the financials it doesn't win over Gas (assuming 100% of electric brought in) - ...but like @DamonHD that isn't the point here...

Gas price - 2.9p/kWh

Gas total @ 75% eff = (8.4 * 1.25) * 2.9 = 30.45p

 

Elec price - 5p/kWh

Elec total @ 100% eff = (8.4 * 1) * 5 = 42p

 

So I guess my question(s) are..

does this all make sense? (technically not financially)

can a single Sunamp Uniq9 really cover all our DHW needs? (do I oversize/undersize)?

whats the recovery time of a Sunamp? (is it 1:1 recovery, i.e. 2.8kWh in = 2.8kWh of charge, running the internal heater for 1 hour)?

 

Anything else I should be looking at / thinking about?

 

P.S My wife likes 'unlimited' hot water, so may have to look at the Stiebel as well

[Jeremy Harris]

16 minutes ago, MrMagic said:

A quick question here to the smarter folk - I've also been looking at options for DHW, keeping it separate from household heating.

 

Data gathered so far - 

- 2x showers in the morning and 1 bath in the evening

- Actual measured water usage -

- 2x showers = 180L total

- 1 bath = 60L (yes, the bath really uses less water than 1 shower, really need to fix that)

- Current delivery temperature target set on combi boiler - 50degC (actual seems to vary between 48 - 53)

 

So in theory I need 180 litres of 50degC water to cover the morning and then a reheat to do the bath in the evening, or a capacity of 240 litres at 50degC - Obviously this is mixed at the end so the hot water is probably less.

 

If I've found the correct formula it should be - 

Calculate the kilowatt-hours (kWh) required to heat the water using the following formula: Pt = (4.2 × L × T ) ÷ 3600. 

Pt is the power used to heat the water, in kWh. L is the number of liters of water that is being heated and 

T is the difference in temperature from what you started with, listed in degrees Celsius.

 

Pt = (4.2 x 240 x 30) / 3600 = 8.4kWh of DHW input per day (minimum!)

 

(is 20degC incoming too high? any ideas on what is the correct design temperature?)

 

Looking at the Sunamp website it seems to be pushing me towards a Uniq9 (10.5kWh / 210L equiv)

 

On the financials it doesn't win over Gas (assuming 100% of electric brought in) - ...but like @DamonHD that isn't the point here...

Gas price - 2.9p/kWh

Gas total @ 75% eff = (8.4 * 1.25) * 2.9 = 30.45p

 

Elec price - 5p/kWh

Elec total @ 100% eff = (8.4 * 1) * 5 = 42p

 

So I guess my question(s) are..

does this all make sense? (technically not financially)

can a single Sunamp Uniq9 really cover all our DHW needs? (do I oversize/undersize)?

whats the recovery time of a Sunamp? (is it 1:1 recovery, i.e. 2.8kWh in = 2.8kWh of charge, running the internal heater for 1 hour)?

 

Anything else I should be looking at / thinking about?

 

P.S My wife likes 'unlimited' hot water, so may have to look at the Stiebel as well

 

 

Looks about right, but Pt is really the energy needed to heat the water, not the power, plus the incoming water temperature may vary a bit between summer and winter, but tends to be around 1m to 2m deep ground temperature, which for most of the UK is usually about 8°C.

 

We just run two showers a day and rarely use the bath, and we find that our hot water energy use is around 3 kWh to 3.2 kWh per person per day, so between 6 kWh and 6.4 kWh per day in total.  Our UniQ 9 kWh unit can just about meet that OK when heated from either excess PV generation during the day, or boosted from off-peak electricity at night.  It would probably not be able to run a bath in the evening without boosting at peak rate some days.

 

A directly electrically heated hot water system, even if run from off-peak electricity, will always cost more to run than a gas boiler.  The advantage of the Sunamp is really that it has lower losses than a water tank, so is more efficient at storing hot water, particularly in summer when heat losses aren't useful in terms of helping keep the house warm.  The Sunamp is also a good match if you have a PV system, for much the same reason as it's a bit better than a hot water tank.

 

I doubt that a 9 kWh Sunamp would meet your needs without being boost heated during the day, to provide the hot water for the evening bath.

 

The recovery time is just the input power divided into the energy taken out, so if 6 kWh was taken out for showers, then the recovery time would be a bit over 2 hours (the 2.8 kW power rating is at 230 VAC, and the average UK mains voltage is about 240 VAC, so the power input will probably be a bit closer to 3 kW).

 

We find that we always have enough hot water, and it's not often that the Sunamp doesn't get a bit of charge during the day from the PV system.  Today, for example, our Sunamp was fully charged by lunchtime, thanks to the bright sunshine we're having.

[ProDave]

@MrMagic where ate you getting your electricity at 5p perkWh?  Even off peak that is incredibly cheap.

[Jeremy Harris]

3 minutes ago, ProDave said:

@MrMagic where ate you getting your electricity at 5p perkWh?  Even off peak that is incredibly cheap.

 

 

I'd assumed that was the Octopus Go cheap rate (that's currently about 5p/kWh).

 

Our cheap rate (E7) is currently 8.148p/kWh, and peak rate is 15.729p/kWh.

[MrMagic]

Thanks @Jeremy Harris, I must make a pilgrimage up your way some time to see it all in action.

 

@ProDave Yes, Octopus Go @ 5p/kWh cheap rate, although may be switching to Agile as it looks like it'll work out cheaper on our usage profile. I've also added Bulb E7 to my calculator after JSH mentioned it on another thread and at the moment it's coming out at a half way house price wise between Agile and Go, but early days on that calculation.

 

Boosting during the day would be OK - PV here would be diverted and it's been a lovely day for it today.

 

On a more technical note (and I think this has been mentioned before)... even with the Sunamp control system you've still got an 'input' of sorts direct to the immersion? I think someone mentioned here that it just goes through a contactor in the control box which I guess just opens when the system is fully charged?

 

Should add looking at this again in detail over a normal or Mixergy tank partly based on space constraints - like a lot of households all the water tanks were ripped out years ago when going to a combi... now looks like we need to go back in the other direction to effectively make the most of renewables/ToU tariffs.

Edited February 12, 2020 by MrMagic

[MrMagic]

As a random aside - heres the mains voltage here today (60s sampling)

 

[SteamyTea]

1 minute ago, MrMagic said:

heres the mains voltage here today

What happens at just after 7 AM, and just before 9 AM, and again just after 10 AM.

Kettle, showers, or something more sinister? 

[MrMagic]

7 & 9 are certainly the kettle, just after 10am is probably the uranium enrichment kicking in...

[DamonHD]

Completely annoying that I've not been able to progress this, but it occurs to me that I might double down and be able for little extra money and effort to divert some space heat demand to electricity when intensity is low:

 

http://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html#returnWillis

 

Tell me why this won't work, or isn't a good idea (other than exergy)?

 

Rgds

 

Damon

[ProDave]

I made life even simpler.  My home made solar PV diverter now switches on a wireless relay for an auxiliary load when it is approaching 100% power to the immersion heater.  I just have a 750W electric convection heater plugged into that. 

 

It just ensures I can self use more of my own generation as at mid day with not much else on in the house, more can be generated than the immersion heater on it's own can absorb.

 

I don't need much extra heat in the house now, so in summer I might put the wireless relay and heater out in the garage, just out of spite to stop it getting exported for no payment.

[DamonHD]

Interesting!

 

But when we want space heat we just don't have any PV to divert!  This is going to be almost entirely sucking in grid electricity.  (I *will* divert any excess PV to DHW, but given mid-winter total generation is ~1.5kWh/d of which only a tiny amount is exported anyway given an a/c coupled battery...)

 

Rgds

 

Damon