Sunamp for UFH

[LA3222]

I've had a scoot around for the obvious and can't seem to find a thread where this is covered.  So my understanding is that the current crop of SunAmp's can be used for domestic heating, my query is can the UFH run directly from the SunAmp and if so how/has anyone done this yet.

 

I assume that if the SunAmp were to be directly connected to your UFH it can't be used for anything else then - it is forever more stuck supplying warm water to the slab, so there is the cost implications there.  Also how would you size it appropriately?  If your peak heat loss is 5kWh and you bought a 5kWh SunAmp it would be depleted after an hour, or would it charge back up as you use it?  So do you then have to oversize it - double, so 10kWh?

 

I intend to make use of a large amount of solar and intend to use SunAmp's for my DHW, I'm just pondering the best way to approach the UFH.

 

Is it a case of an ASHP is a better fit for the UFH?  Sometimes, although something 'can' be done it probably shouldn't - so am I barking up the wrong tree here?

 

Thanks

Edited September 11, 2019 by LA3222

[Jeremy Harris]

A Sunamp is only a replacement for a buffer tank, really, as it's just a thermal store.  I guess one could be used for an UFH buffer, but as they no longer make the low temperature one, the 58°C one that's currently available wouldn't seem to be a good fit for UFH.  It's also a pretty expensive way to provide a buffer, I think.

 

As a Sunamp can't be easily charged from an ASHP, due to the need to charge at around 65°C, what were you thinking of using to charge the Sunamp?  Using an electrically heated Sunamp for heating would seem to be expensive, as even at cheap rate overnight charging the cost would still be around double the operating cost of an ASHP.  There won't be any useful solar charging opportunities in the heating season, we've found that there isn't enough spare generation to provide more than a small amount of our winter hot water (works fine for hot water the rest of the year though).

 

 

[LA3222]

It's something I had yet to turn my mind to tbh @JSHarris.  I have been in contact with Wunda ref UFH as I will need the pipe soon to put into the slab and they asked what I would be using.

 

I remember reading that SunAmp can be used for DHW and space heating hence the query.  Your answer settles in my mind that it's a 'pump' idea which suggests that in a house that doesn't have gas the ideal setup is to leave SunAmps to the DHW side of life and an ASHP for the UFH.     

[Jeremy Harris]

Talking of using excess PV to charge a Sunamp, by coincidence I've just noticed that our Sunamp hasn't fully charged today.  That's the first day since about the end of March that we will end up getting some of tomorrow's hot water from an E7 boost charge tonight.  Still, we've had completely free hot water for nearly six months, so I can't really complain.

[SteamyTea]

15 hours ago, JSHarris said:

Talking of using excess PV to charge a Sunamp, by coincidence I've just noticed that our Sunamp hasn't fully charged today.

Were you charging the car as well?

Or was it just a very dull day.

[Jeremy Harris]

4 minutes ago, SteamyTea said:

Were you charging the car as well?

Or was it just a very dull day.

 

 

Just a very dull day, first one for ages where we've exported nothing at all.  We generated around 5.5 kWh in total, so some went to heat hot water, but not a lot, as the house tends to use around 300W or so.

[SteamyTea]

7 minutes ago, JSHarris said:

 

 

Just a very dull day, first one for ages where we've exported nothing at all.  We generated around 5.5 kWh in total, so some went to heat hot water, but not a lot, as the house tends to use around 300W or so.

Just had a quick look at my usage for the 10^th Sept.

Mean of 243W, total 6.080 kWh

It was washing day, so a bit higher than normal.

 

Your 300W is pretty good considering that you run your water systems as well.

[Jeremy Harris]

10 minutes ago, SteamyTea said:

Just had a quick look at my usage for the 10^th Sept.

Mean of 243W, total 6.080 kWh

It was washing day, so a bit higher than normal.

 

Your 300W is pretty good considering that you run your water systems as well.

 

 

Just checked, we used about 8 kWh over the 24 hours period. so a bit over 330 W average.  Our inverter started up just before 08:30 (which is late, it's usually come on an hour or so earlier recently).  The 5.5 kWh was generated from then until about 16:30, when the inverter turned off, so we generated around 690 W on average when the system was on, so it will have put around 2.9 kWh into the hot water, maybe half our usage or thereabouts.

 

I've just checked, and right now (at 08:45) we are generating about 590 W, so not a lot.  Looks like a repeat of yesterday.

[Ed Davies]

36 minutes ago, JSHarris said:

Looks like a repeat of yesterday.

 

This is a key thing when thinking about PV into electric or heat batteries: what's the distribution of sunlight over short runs of days. E.g., if two consecutive days are dull, what's the probability that the third day will be dull also. My impression, not backed up by any statistics, is that consecutive days tend to be correlated (if it's dull one day it's likely to be dull the next) but over about 3 or 4 days it tends to be anti-corrolated: if it's dull today it's more likely to be bright the day after next, and vice-versa of course. So if you're wanting to use PV to cover a lot of your energy use (for DHW, say) then it's worth having about three days of storage.

[Jeremy Harris]

I agree.  We have just under 2 days worth of DHW storage, but also top up the Sunamp overnight, at the E7 rate, if it needs it.  The cost of doing that is pretty modest, the worst case of topping it up overnight when there's been no excess PV generation (very rarely happens) would cost around 50p.  I'd guess that most of the paid for top ups cost maybe half to two thirds that.

 

If I had to guess, based on the ~9 months worth of data I've got on the split between free hot water and paid for hot water, I'd say that our annual bill for hot water will be around £60 or so.  I can live with that, TBH, especially as we get paid around £1000 a year for the FiT and exported electricity.

[Ed Davies]

It'll be interesting to see your statistics when you've 12 months of data, as a counter argument to the “what about when the sun isn't shining and the wind isn't blowing” brigade, if nothing else.

[Jeremy Harris]

25 minutes ago, Ed Davies said:

It'll be interesting to see your statistics when you've 12 months of data, as a counter argument to the “what about when the sun isn't shining and the wind isn't blowing” brigade, if nothing else.

 

 

I'll make sure to do an annual review next January, when I have 12 months worth of data.

 

Just to illustrate how fickle the weather can be, we are now generating around 1.9 kW, with a total for the day so far of 2 kWh, so it looks like today's going to be a bit better than yesterday.

[SteamyTea]

1 hour ago, Ed Davies said:

E.g., if two consecutive days are dull, what's the probability that the third day will be dull also

There used to be a saying that the weather tomorrow will be within 20% of the weather today.

You can take your pick of any aspect to measure and predict.

For a laugh, over at the other place, I predicted the sunlight for a day, a year in advance. I just squeaked in, once standard error was taken into account.

My first Masters was meant to be 'The stochastic nature of clouds on PV generation'.

As usual for a Master's program, that idea was crushed.

May still have the data, so could revisit it.

[Jeremy Harris]

When I said the weather can be fickle I wasn't joking.  The Sunamp was fully charged as of about half an hour ago, we're back to normal (generated about 7.5 kWh so far today) and there's just about enough export for me to think about plugging my car in for a top up.

 

Bit of a contrast to yesterday.

[Ed Davies]

Just done some scatter plots of sunlight for days against the sunlight 1, 2, and 3 days later for June, July and August this year. Sunlight is in “Mfot·s” (a joke unit described in this blog post).

 

It needs a bit more statistics but just looking at the plots it doesn't feel like there's much correlation one way or the other at any of the day differences. Be amusing if somebody with daily PV production data could do something like this.

[SteamyTea]

I took a slightly different approach.

I looked for a rain free day during the hours of daylight, then looked to see if the next two days were rain free (left axis, blue diamond).

If they were, I looked at the mean and standard deviation of the solar power, during the hours of daylight, in watts (right axis red square and purple cross) for the full 3 days.

There is not a full years worth of data (got to drag some more out), but it shows a rise in mean power as summer approaches.  The 'gap' in the data during July and August is not missing data, it is the rainy season down here, better known as school holidays.

I have not done a proper correlation yet, truth is, it is late, and I can't think of an easy way to show it.

But a quick visual scan shows more power during times of rain free days.

Edited September 12, 2019 by SteamyTea

[ragg987]

If it helps I can share my spreadsheet of daily PV generation, shows kWh per day. I also have some weather info like temperature and rainfall, but this is a bit hit and miss in places. Approx 2 years worth.

[ragg987]

Example visualisation - not directly related to your correlation question. Hopefully self-explanatory, "Diverted" refers to spare energy sent to my immersion heaters rather than being exported. As subsidised by @SteamyTea.

 

Edited September 13, 2019 by ragg987

[Ed Davies]

6 hours ago, ragg987 said:

If it helps I can share my spreadsheet of daily PV generation, shows kWh per day.

 

Yes please. The size, orientation and slope of your PV would be interesting background, too.

 

BTW, the reason I only did June/July/August was that if you do a whole year there's an obvious correlation added because January tends to be darker than July. On the other hand, glancing at my data that doesn't actually seem to be the case (the brightest days are in March) probably as a result of the sensor being indoors and very dependent on the sun angle. So real outside information would be helpful to look at.

[SteamyTea]

Let's see if I can upload my data.

It is a zipped file, so extension needs changing from txt to zip

Format is csv.

It fats out to about 32mb

Solar_Perranporth.txt

Edited September 13, 2019 by SteamyTea

[willbish]

On 12/09/2019 at 09:38, JSHarris said:

I agree.  We have just under 2 days worth of DHW storage, but also top up the Sunamp overnight, at the E7 rate, if it needs it.  The cost of doing that is pretty modest, the worst case of topping it up overnight when there's been no excess PV generation (very rarely happens) would cost around 50p.  I'd guess that most of the paid for top ups cost maybe half to two thirds that.

 

If I had to guess, based on the ~9 months worth of data I've got on the split between free hot water and paid for hot water, I'd say that our annual bill for hot water will be around £60 or so.  I can live with that, TBH, especially as we get paid around £1000 a year for the FiT and exported electricity.

 

Do you have a way of reviewing the state of charge of the Sunamp and automatically controlling if it needs overnight E7 grid charging?

 

If you had at least two days DHW storage you could allow the Sunamp to remain partly discharged overnight in the hope that the PV will generate enough excess the following day

[Jeremy Harris]

8 minutes ago, willbish said:

 

Do you have a way of reviewing the state of charge of the Sunamp and automatically controlling if it needs overnight E7 grid charging?

 

If you had at least two days DHW storage you could allow the Sunamp to remain partly discharged overnight in the hope that the PV will generate enough excess the following day

 

 

Sadly not.  One of the major issues we had with the Sunamp was that it wouldn't accept a charge until it was significantly discharged, and there is no easy way to find out the actual state of charge (temperature doesn't tell you anything very useful as it's a phase change storage system),  The new controller has fixed the charge acceptance threshold problem, but there is still no easy way to determine the remaining state of charge.

 

To be fair to Sunamp, trying to do this would be complex and error prone, anyway.  I think the only way to attempt it would be to measure input charge (easy for an electrically charged unit, not so easy for one charged by hot water) and also measure discharge energy.  To do the latter would mean measuring the temperature and flow rate of the outgoing hot water, together with a factor to allow for heat loss (a fixed factor dependent on time since last charge would probably work OK) and then subtract that from the last know fully charged state.

 

The fickle nature of UK weather, plus the variability in the house demand (some days are a lot higher, when the washing machine, dishwasher etc are run) would make any attempt to predict PV generation relative to consumption challenging.

 

Given that I think we will only end up paying about £60 a year for hot water, if it ends up costing us £10 more a year just because we're not squeezing the last bit out of our available PV generation I'm not going to lose any sleep over it.

[ragg987]

3 hours ago, Ed Davies said:

Yes please. The size, orientation and slope of your PV would be interesting background, too.

Attached, file suffix is .TXT and will need to be renamed to .XLSX

 

4kWp array, 16 panels, each with a Solaredge microconverter. Solaredge 4kW inverter.

Due south facing (within a few degrees) on 40 degree angle roof.

Mounted flush with roof using GSE system.

Aylesbury, Bucks

Tall trees on east side provide shading soon after sunrise, but completely clear after that

Weedon energy.xlsx.txt

Edited September 13, 2019 by ragg987

[Ed Davies]

I've started some code to look at @SteamyTea's data. Will do more on that and similar for @ragg987's later.

[ragg987]

2 hours ago, Ed Davies said:

On the other hand, glancing at my data that doesn't actually seem to be the case (the brightest days are in March) probably as a result of the sensor being indoors and very dependent on the sun angle. So real outside information would be helpful to look at.

I think the only way to do it is real PV data as there are other variables with brightness, examples:

• PV efficiency will drop with external temperature - this seems pretty significant e.g. 2 full sun days 1 year apart
  • 7 Feb 2018, avergae temp -1C, PV generation 16.7kWh
  • 7 Feb 2019, average temp 8C, PV generation 13.6kWh
• length of day - brightness in spring may be same as in summer, but summer days are longer
• sensor brightness orientation - it can easily be moved by a few degrees which could have a big impact on readings