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Bivalent (or even trivalent!) system design options for a new GSHP installation

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(@algienon)
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Hi all, we live in rural Aberdeenshire. I have two interests in joining the forums...

  1. I'm on a journey to make our house as energy-efficient and self-reliant as possible (without going completely off-grid). More on that below
  2. I'm currently completing an Open University Energy and Sustainability Engineering Degree and my final project is on optimising the design of heat pump-based heating systems to maximise the changing electricity price dynamics. Given the stress this is causing, I'm seriously wondering if this was worth doing in my 50s 😢. Anyway, I'll start another post on that elsewhere in the forums, as I have questions for the collective hive mind.

We live in a 4-bedroom house in the middle of the countryside. It was built in 1970, and its construction is as energy-efficient as you'd expect of a house from that bygone era! In the last five years, we have:

  • clad the house in 100mm of PUR external wall insulation
  • replaced the double glazing with triple glazing
  • replaced the loft insulation with 100mm of PUR between the rafters and 300mm of rock wool on top
  • hung scallops of vapour membrane between the floor joists and filled it with 150mm of rockwool. In all but one room so far, you can tell when you go into that room.
  • installed a mechanical ventilation and heat recovery (MVHR) system
  • Installed 6 kV of solar panels, although only 3.6 kV are connected to an inverter at the moment as the installer needs to sort out the G99
  • "Upgraded" our heating system so that the oil boiler feeds a 500-litre thermal store (which then feeds all the radiators) and installed a boiler stove that also feeds the thermal store. This has been awesome at keeping our oil use down in winter and has been a god-send in multi-day power cuts. The pumps won't feed the rads, but we have one warm room and as much hot water as we need.

Going forward, the plan is to:

  • Upgrade the PV array to 8kW, add battery storage and a solar diverter to the immersion in the thermal store - the plan is to prevent any locally generated electrons escaping to the grid.
  • Replace the oil boiler with a GSHP. I've had an extensive heat loss survey and geological survey completed, so I'm confident I know what size of heat pump we need and the depth of the borehole.
  • Then relax!

Being in the north of Scotland, IΒ  decided to go for GSHP over ASHP. I know ASHP will work around here - plenty of new houses are getting them - but the delta T in winter is going to be significantly lower with GSHP and so the electricity bills should be significantly lower too. My biggest struggle is finding an installer who can design a system that doesn't completely abandon our current set-up - the high-temp vs. low temp. problem. Removing the boiler stove is non-negotiable. However, I'm not precious about having one integrated system. Again, I'll start another post elsewhere to ask for opinions on how we might solve this.

I look forward to pouring over the wealth of knowledge and experience that is undoubtedly captured in the forum posts, and if anyone knows a good installer in the north-east of Scotland, please let me know.

Algie

This topic was modified 2 months ago by Mars

   
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Mars
 Mars
(@editor)
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@algienon, welcome to the forums. @graeme-r is a highly rated installer in Scotland – not sure if covers Aberdeenshire, but might be worth reaching out to him via his website.

Buy Bodge Buster – Homeowner Air Source Heat Pump Installation Guide: https://amzn.to/3NVndlU
From Zero to Heat Pump Hero: https://amzn.to/4bWkPFb

Follow our sustainability journey at My Home Farm: https://myhomefarm.co.uk


   
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(@bontwoody)
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@algienon

Welcome Algie, it sounds like you have great backbone for your installation with all the insulation you have added. A couple of comments on what you have mentioned:

1. Thermal stores are generally thought not to be the best match for a heat pump due to the higher water temperature needed in the store compared to a cylinder. That said, I used one very successfully in my previous house and I note that you (perhaps deliberately) are proposing to heat it with an immersion heater. Given my previous experience I would suggest that the heat pump could do that more efficiently as long as you dont need the store temperature over 55-60C

2. I too have a solar diverter, but have it disconnected. The reason for this is that I can more profitably heat the water with an immersion during off peak and then export the electricity I would have used to heat it. I appreciate this depends on your tariff. I also appreciate that despite what I said above Im using an immersion rather than a heat pump to heat my DHW πŸ™‚ The reason is that the ROI on installing a heat exchanger to my Mixergy cylinder is quite long!

If you decide that you still want a diverter, I have a almost unused one I could sell 😆Β 

This post was modified 2 months ago by bontwoody

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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(@dr_dongle)
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@algienon much kudos for all your insulation measures. We used IMS in Perth who I am pretty certain operate up north. They were perfectly happy to install a hybrid system though mine is simpler than yours and I could give them a clean start. I learned things from talking to their guys which is always a good start.


   
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Transparent
(@transparent)
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Posted by: @algienon

[....] Installed 6 kV of solar panels, although only 3.6 kV are connected to an inverter at the moment as the installer needs to sort out the G99

Going forward, the plan is to:

  • Upgrade the PV array to 8kW, add battery storage and a solar diverter to the immersion in the thermal store - the plan is to prevent any locally generated electrons escaping to the grid.

Woah.... Not so fast @algienon !

Great aspirations, but may I bring this conversation back to a strategy which is more likely to be achievable within the laws of physics?

I'm going to be giving you information which is directly relevant to your OU Degree course, and your final year project in particular.

For others arriving here in future, G99 is a set of rules for exporting electricity to the grid when households have more than one source of generation/storage.

G98 is the certification which applies to the component parts which are permitted to be connected to the grid.
It covers safety and operational specifications which affect not only the householder property, but also the risk of adverse effects on the grid itself.

We must assume here that the installer is being asked to 'sort out G99' stuff because @algienon wants to

  • have both a solar inverter and a storage battery (both of which can export)
  • seek a higher generation level than 3.6kW (16A) which is the notional limit for a single-phase property

Β 

But @algienon doesn't actually want to pump Scottish electrons onto the grid anyway!

So why choose to fight a battle with your DNO over G99 when its whole purpose is to regulate a strategy which goes in a different direction? 😖Β 

In (north) Scotland there is already a surplus of generation from renewable sources.
Since a high proportion of that is from hydro, that can at least be controlled.
Over-supply of electricity from wind & solar, in contrast, has to be thrown away.

Worse still, the National Grid Transmission Network in Scotland is mainly 275kV, whereas England and Wales is predominantly 400kV.
The Scottish 'pipes' are too small, and additional generation can only be accepted if the country is re-plumbed.
Those costs will be borne by the consumers through increased TUoS (Transmission Use of Service) charges within electricity bills.

But if the network infrastructure upgrades are mainly for exporting power southwards towards London & the South-East, why would the population of Scotland want to pay for that? 🤔Β 

Let's look at the main energy flows in that Transmission grid, using a helpful diagram from National Grid ESO:

ScotlandGridFlow2

So my recommendation is to change strategy and run storage batteries "off-grid".

That allows you to have your forthcoming GSHP and other critical circuits running directly from the stored electricity.
Unlike the G98/G99 strategy, you'll still have power if there's a grid outage.

Off Grid battery2c

You'll need to start the new topic by identifying what solar inverter you've just had installed.

Try to give it a generic title rather than something too restricted to your own set-up.
"Running a heat pump from stored electricity" would suffice.

This post was modified 2 months ago by Transparent
This post was modified 2 months ago by Mars

Save energy... recycle electrons!


   
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(@bontwoody)
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@transparent @algienon

If the batteries were DC coupled rather than AC that might help get it past the DNO as only the inverter could export. It did in my case. I have a small Huawei battery which is modular and can with the aid of an extra component work during a power outage.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Transparent
(@transparent)
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Ah... we've just been relocated! 😎Β 

So, apart from wanting to know the model of inverter being used to export that 3.6kW I have two other questions:

1. I'm interested to know how 'hands on' you are @algienon
How much of the renovation work do you do, excluding the stuff for which a professional is mandatory of course?

Β 

2. How much of this response which I'm writing to @bontwoody do you understand, and with which of us do you tend to agree?
(Note that my strategy might not be able to use the existing inverter.)

I ignored the possibility of having only the inverter exporting to the grid because it's already deficient in what it's trying to achieve.
There's 2.4kW of solar array present which won't get used on full-sun days.

And there's an intention to add a further 2kW to that total.

Whatever steps are taken towards adding solar panels and battery storage, it's unlikely that the constraints with existing inverter can be overcome.
But we can only tell that once we know which model it is of course.

This post was modified 2 months ago by Transparent

Save energy... recycle electrons!


   
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(@bontwoody)
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Posts: 418
 

@transparentΒ 

Good points! Do you know if because of the over supply of renewables in Scotland the export prices are reduced there? If they were, that could radically change the strategy to favour fully 'off grid' batteries.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Transparent
(@transparent)
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No.
The only part of a tariff which changes according to your geographical location is the portion which goes to the DNO for your region for the electricity you import.
That's made up of

  • the daily standing charge
  • the Distribution Use of Service charge (DUoS)

Β 

This section of Ofgem's charges spreadsheet shows the annual amounts per DNO Region for zero consumption (just the standing charge) and for a consumer using 3100kWh

ElecCosts 3e

Export prices are set entirely by your chosen Energy Supplier for each tariff they offer.

So they can change on a whim.
It's not a useful indicator with which to calculate payback time of an investment into storage batteries.

Β 

DESNZ made a formal announcement on 12th March which ruled out the use of Nodal Pricing (based on your location) for end-user tariffs.

Instead, the Secretary of State, Claire Coutinho MP, said that Nodal Pricing would be used on the wholesale market, where our Energy Suppliers buy in the electricity they sell on to us.
I have no idea how that could work!
It makes nonsense of the very concept of having lower costs to consume electricity closest to where it's being generated.

Of course there will be a new Secretary of State at DESNZ sometime before 28th Jan 2025, and energy policies will change according to who's in power.

@algienon will need to make decisions for storage batteries, inverters and a GSHP based on the laws of physics rather than prevailing government opinion.

If Nodal Pricing does get introduced at some future point in time, the difference it makes to Scottish consumers will be less than other parts of GB.

"Why?" I hear you ask!

Well, where I live in SW England, our main sources of renewable power are from wind turbines and solar.
So I'm likely to be offered lower prices when they are producing peak output.

But in Scotland, the major source of renewable electricity is from hydro, which can be adjusted manually.
So if the wind turbines off the Aberdeen coast are giving peak-power, then they can turn down the output from hydro and save the water for later.
Scottish fluctuations in the cost of renewables will be much less than elsewhere.

Save energy... recycle electrons!


   
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(@algienon)
Eminent Member Member
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Joined: 2 months ago
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Topic starter  

@transparent thanks for taking the time to provide so much information. To answer your questions...

  • The inverter is a Solis S5-GR1P3.6K-M. It's sort of on loan from the installer until we are ready to upgrade and connect the rest of the panels. Interestingly, your question had me checking the app for the model number and I realised that the datalogger or inverter (or both) have been offline since the end of March! The panel on the inverter is unresponsive. I'm hoping this is just because it's dark now. I'll check again in the morning.
  • When it comes to electrics, I'm only as hands-on as swapping out sockets and light fittings. I've occasionally repositioned sockets, but that's my limit. I leave plumbing and electrics to the experts. I'm happy to have a go at most other things if time allows. If not, I bring in those who can do it in a fraction of the time and don't use the job as an excuse to buy more power tools 😉 Our renovation was a huge job, and I was travelling a lot when it happened, but I did do all the loft insulation myself.
  • I understand all of your reply to @bontwoody. I used the degree to get a job with a DNO last year before I finished. After a year I still have huge amounts to learn about the industry (I'm in a very niche role), having spend 25 years in Oil and Gas, but between the degree and the job I have a better understanding of the basics than your average Joe.

I suppose I should clarify what I want from a PV/Battery system.

  1. Use all I generate
  2. If it's not used at the point of generation, then it is stored in the battery for later use
  3. If the battery is at capacity, it is diverted to water heating. A heat pump will be more cost-effective, but if the electrons are going to waste anyway, they may as well be put to use
  4. At some point we'll undoubtedly end up with an EV, so it will either fit between 1 and 2 or 2 and 3
  5. Be able to island the system so that we can run a few small devices, charge phones and use lights when we have another power cut

I'm more than happy not to export, as the tariffs are so poor compared to the cost of importing, so your schematic looks like a great starting point for designing the system I want.

I'll have a look at it in more detail and get back with questions.

Algie


   
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(@algienon)
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Posts: 7
Topic starter  

Hi all,Β 

I'm extracting this series of questions from my introduction post above as that topic is starting to coalesce around the design of my PV/battery system.

As stated in my intro...

We "upgraded" our heating system 7 years ago so that the oil boiler feeds one of the coils in a 500-litre 4-coil thermal store (which itself in the same circuit as the radiators) and installed a boiler stove to one of the other coils to also heat the thermal store. This has been awesome at keeping our oil use down in winter and has been a godsend in multi-day power cuts. The pumps won't feed the rads, but we have one warm room and as much hot water as we need.

At some point, oil boilers will be outlawed or prohibitively expensive to run due to carbon taxes. This is a concern, and I'm looking for sustainable alternatives. Due to our location in the northeast of Scotland, I'd rather go with a GSHP to keep the kWh consumed to a minimum. My issue comes in finding someone to design a system that doesn't completely abandon what we already have. I'm not precious about all the kit, but the principle of having back-up heating and HW is a must. Not reliant on a battery-powered heat pump either, though this is likely to be the first fallback in a power cut. However, we have had a number of multi-day outages in the last few years, and the battery storage for that is likely to be reassuringly expensive.

So my question is, what are people's views on bivalent systems? I can see there being issues with the different operating temperatures. Would it be easier to have two separate systems?

All thoughts are appreciated. We're on a learning journey here 😀Β 

Algie

This post was modified 2 months ago by Mars

   
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Abernyte
(@abernyte)
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I too have wrestled with the same problem. I am also in NE Scotland, on a cold and windy hillside, subject to power outages too, and was heating with log burner and lpg system boiler feeding a thermal store, which heated the rads. I knew that fossil fuels days are numbered and also felt the moral obligation to stop burning and polluting my own patch. I also run a series of air quality sensors, but that is a different issue!Β 

A heat pump being the logical alternative I looked at bivalent with the LPG boiler but that requires a buffer tank to work and I was not prepared to risk the associated loss in efficiency in the heat pump when I felt that I was already taking a leap of faith in keeping warm.Β  A long discussion with my trusted installer assured me that, properly installed, an ASHP would comfortably heat the house under all conditions. I went with that advice and out went the log burner, lpg boiler and thermal store and he was proved correct. Such is the ability of the ASHP I see now that I might have got away with a bivalent system but I am glad that it has worked as it has now.

What about the power outages?Β  Yes there have been a few, two days was the last one, but as back up I wired in a small number of power sockets on a generator only circuit. Now if required I can spin up a 3kW generator, connect it to that circuit which terminates in an outside socket and heat a couple of rooms with wee portable heaters.Β  Be bold!

This post was modified 2 months ago by Abernyte

   
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