ASHP & Sunamp - no boiler no PV

[Jeremy Harris]

23 minutes ago, andy said:

 

On that front, the batteries are fully recyclable:

 

https://www.tesla.com/en_GB/blog/teslas-closed-loop-battery-recycling-program

 

The snag is that the embodied energy in manufacturing them is still very high, although that will come down as battery manufacturers both reduce the energy intensity of the whole manufacturing and supply chain (including the mining side of it) and as they include a greater proportion of renewable energy to the manufacturing process (the Chinese are investing massively in this, and Tesla has done a bit by roofing Gigafactories with PV).

 

 

16 minutes ago, andy said:

 

I will need to do the calculations to see if the off-peak is worthwhile as like you say it may not be.  This area could change with demand based pricing which the Powerwall will hopefully be aware of, so it would fill up at the cheapest time possible and then everything else draws off that.

 

Another benefit of the Powerwall is you aren't tied to using the PV energy when the sun is actually shining, so you're able to shift that utilisation to fit your realistic usage patterns but it still makes sense to use such appliances when there's an excess of energy available.

 

The sums are pretty easy.  A Powerwall 2 will cost around £7500 to £8000 installed (best guess at the moment) and the cells will have a calendar life of between 10 and 15 years, with the inverter probably lasting between 8 and 10 years.  If off-peak energy is used to charge the battery pack, at a typical unit price today of around £0.07/kWh, and allowing for a typical round-trip efficiency of about 90%, then the cost per unit of energy delivered from the battery pack will be around £0.077/kWh.  If the house consumes 2/3rds of it's total energy use during peak rate time, and if this can be met 100% by the battery pack, say 10 kWh/day (and that's probably an optimistic figure) then the cost saving per day from off-peak battery charging to offset peak rate consumption will be somewhere around £0.08/kWh, or around £0.8 per day. 

 

To recover the cost of the Tesla Powerwall 2 just using off-peak savings would take around 25 years, but the battery will be effectively scrap after around 15 years at the most, so will never pay for itself this way.

 

If you add in excess PV charging as well, then that figure comes down a bit, but not a lot, given that peak energy use is during the period when PV will be generating very little.  Last time I ran the sums for the Tesla Powerwall 2 against our requirements, with our 6.25 kWp PV array and lower overall energy consumption than your prediction, I came up with a return on investment time of around 20 years.  Anything over 10 years doesn't stack up, as by then the battery capacity will have reduced significantly and the inverter will be at the end of its life (power inverters seem to have a typical life of around 8 to 10 years).

 

For us, the 7.2 kWh Sofar battery system almost breaks even at the 10 year point, but not quite, as of a few weeks ago when I looked at the costs again.  Prices of those systems are dropping though, whilst Tesla Powerwall prices seem to be increasing.

[andy]

On 03/11/2018 at 13:39, ProDave said:

The idea of charging them with off peak electricity is equally dubious.  To get an off peak tariff you pay a much higher daytime rate and a higher standing charge. It is not just viable for this imho.

 

I want battery storage for my own house eventually. I am still thinking that will eventually be a DIY built system with NiFe batteries. At least I know with those there is a reasonable chance of the batteries lasting until I fall off my perch so it should be a buy once system.

 

One of the best things you can do to self use as much PV as possible is use the big appliances like Dishwasher, washing machine and tumble dryer one at a time close to the middle of the day. If you are out at work you would need to do that with timers.

 

I will need to do the calculations to see if the off-peak is worthwhile as like you say it may not be.  This area could change with demand based pricing which the Powerwall will hopefully be aware of, so it would fill up at the cheapest time possible and then everything else draws off that.

 

Another benefit of the Powerwall is you aren't tied to using the PV energy when the sun is actually shining, so you're able to shift that utilisation to fit your realistic usage patterns but it still makes sense to use such appliances when there's an excess of energy available.

 

Going back to my whole design though and having received revised figures for heat/DHW requirements as circa 3,000kWh/a and DHW probably about 2,000kWh/a what I'm thinking is:

• ASHP running underfloor heating - would a buffer tank be needed or is that only needed if we went for a gas boiler?
• Sunamp for DHW - topped up via PV & mains electricity immersion, leave ASHP out of the mix?  Our DHW usage during the week is pretty much nil so keen to minimise standing loses here
• Could I get 2 Sunamps, one to act as lower temp buffer for UFH which the ASHP feeds into and also pre-heats DHW then DHW goes into a second Sunamp to raise up to required temp?

 

I'd like to be able to get enough simultaneous hot water to run both showers but not sure which models fit the above.

 

Gas boiler will no doubt be cheaper but by not bothering with it I save re-connection fee for gas (£1,000 or so) plus standing charge.  If we did go gas boiler route, would there be any point with Sunamps?

 

Yours confused...

 

Andy

[joe90]

I am running An ASHP for UFH with a buffer tank (which has an immersion as a back up). The ASHP will also do our DHW with maybe inline or immersion topup (if required). Yet to use the system over a winter so will see how it goes.

[ProDave]

I am at the other end of the KISS scale, ASHP directly driving UFH  ASHP does a good job of regulating the flow temperature so not much work fir the UFH mixing valve to do.  No problem encountered so heating demand is very low at the moment and the ASHP runs at a very low power, or just stops for a while if the water is up to demand temperature.

 

Unvented cylinder for HW and from past experience that will run 2 showers at the same time though we only have 1 here at the moment so can't test that theory.

[joe90]

1 hour ago, ProDave said:

I am at the other end of the KISS scale, ASHP directly driving UFH  ASHP does a good job of regulating the flow temperature so not much work fir the UFH mixing valve to do.  No problem encountered so heating demand is very low at the moment and the ASHP runs at a very low power, or just stops for a while if the water is up to demand temperature.

 

Unvented cylinder for HW and from past experience that will run 2 showers at the same time though we only have 1 here at the moment so can't test that theory.

 

+1.

[andy]

Thanks for the input all, appreciate it.

 

Like the idea of ASHP direct to UFH but what about the DHW bit?  KISS suggest Sunamp(s) with mains electric/PV immersion would be good?

[ProDave]

Someone else will have to advise on the Sun amp's.  Can they take heat in the form of hot water from an ASHP?

[joe90]

I intend to use the ASHP for the DHW as well, may need direct lecky to top up the temp but there are some on here that don’t need to . If you have an ASHP why not use it if only to prewarm the DHW.

Edited November 15, 2018 by joe90

[Gav_P]

2 hours ago, joe90 said:

I intend to use the ASHP for the DHW as well, may need direct lecky to top up the temp but there are some on here that don’t need to . If you have an ASHP why not use it if only to prewarm the DHW.

This is exactly what I do. ASHP directly to UFH, radiators & DHW (UVC with immersion top up)... I’ve also got Willis heaters now just in case of another issue with the ASHP or if it’s not man enough in the worst weather.

 

 So no storage, no SA, no PV. So significantly more basic system than most people on here, but cost me about £1k (plus UFH of about £250) in total so difficult to justify more expensive options as the payback would be significantly longer. 

 

My view is keep it simple, unless you enjoy it as a hobby - as that’s the only way of keeping the costs from being prohibitive. 

[Jeremy Harris]

2 hours ago, ProDave said:

Someone else will have to advise on the Sun amp's.  Can they take heat in the form of hot water from an ASHP?

 

Yes, there's a hot water version, but it needs water that's a bit hotter than the phase change material melting point (58 deg C for the DHW Sunamps) in order to work.  Phase change storage isn't like hot water storage, in that the temperature of the phase change material (PCM) stays constant whilst it melts and absorbs heat energy as it changes phase.  This means you can keep pumping heat into it at 58 deg C until all the PCM has melted, when it will stop absorbing any more heat without the temperature increasing.

 

This constant temperature during phase change is what makes the Sunamp such a damned good hot water system, as it delivers water at around 58 deg C from fully charged to fully discharged, whereas a hot water storage system will tend to reduce the output temperature as it is used.

[Barney12]

2 hours ago, JSHarris said:

 

Yes, there's a hot water version, but it needs water that's a bit hotter than the phase change material melting point (58 deg C for the DHW Sunamps) in order to work.  Phase change storage isn't like hot water storage, in that the temperature of the phase change material (PCM) stays constant whilst it melts and absorbs heat energy as it changes phase.  This means you can keep pumping heat into it at 58 deg C until all the PCM has melted, when it will stop absorbing any more heat without the temperature increasing.

 

This constant temperature during phase change is what makes the Sunamp such a damned good hot water system, as it delivers water at around 58 deg C from fully charged to fully discharged, whereas a hot water storage system will tend to reduce the output temperature as it is used.

 

Interesting. I’m being a bit slow to catch on with how the Sunamp actually work.

 

So when they are ‘charged’ the material is in its liquid state?

 

So how on earth do the electrically charged ones work. Surely if the inbuilt immersion’s are firing 2.8kw of heat into this ‘liquid’ it’s going to heat up? If it doesn’t where on earth does that generated heat go?

[ProDave]

9 minutes ago, Barney12 said:

 

Interesting. I’m being a bit slow to catch on with how the Sunamp actually work.

 

So when they are ‘charged’ the material is in its liquid state?

 

So how on earth do the electrically charged ones work. Surely if the inbuilt immersion’s are firing 2.8kw of heat into this ‘liquid’ it’s going to heat up? If it doesn’t where on earth does that generated heat go?

Think of a bucket of ice. You put heat into the bucket and the ice starts to melt, so you have a bucket of ice, and a bit of very cold water (0 degrees).

 

The water won't start to heat until all the ice has changed state to water.  The energy is going into changing state. The latent heat of fusion

 

The stuff in the sun amp works the same, but at 58 degrees, not 0.

[Barney12]

1 minute ago, ProDave said:

Think of a bucket of ice. You put heat into the bucket and the ice starts to melt, so you have a bucket of ice, and a bit of very cold water (0 degrees).

 

The water won't start to heat until all the ice has changed state to water.  The energy is going into changing state.

 

The stuff in the sun amp works the same, but at 58 degrees, not 0.

 

Yes, understood. But continue to heat said bucket of ice (which is now water) and it eventually boils! 

[ProDave]

Just now, Barney12 said:

 

Yes, understood. But continue to heat said bucket of ice (which is now water) and it eventually boils! 

Yes indeed, once all the ice has melted the water will heat up.  The Sun amp exploits the relatively large amount of heat needed to freeze and thaw a substance which enables so much heat to be stored in such a small space, and released at a near constant temperature.

[Barney12]

1 minute ago, ProDave said:

Yes indeed, once all the ice has melted the water will heat up.  The Sun amp exploits the relatively large amount of heat needed to freeze and thaw a substance which enables so much heat to be stored in such a small space, and released at a near constant temperature.

 

So, my question remains unanswered.

 

Jeremy said this “This means you can keep pumping heat into it at 58 deg C until all the PCM has melted, when it will stop absorbing any more heat without the temperature increasing.”

[ProDave]

In the case of the ice bucket, it you were heating it with a water loop with the water in the loop at say 2 degrees, once the ice had melted and the water reached 2 degrees, it would stop absorbing any more heat.

[Barney12]

Just now, ProDave said:

In the case of the ice bucket, it you were heating it with a water loop with the water in the loop at say 2 degrees, once the ice had melted and the water reached 2 degrees, it would stop absorbing any more heat.

 

Yes, I get that too. But that wasn’t my question! 

 

“So how on earth do the electrically charged ones work. Surely if the inbuilt immersion’s are firing 2.8kw of heat into this ‘liquid’ it’s going to heat up? If it doesn’t where on earth does that generated heat go?“

 

 

[Jeremy Harris]

22 minutes ago, Barney12 said:

 

Interesting. I’m being a bit slow to catch on with how the Sunamp actually work.

 

So when they are ‘charged’ the material is in its liquid state?

 

So how on earth do the electrically charged ones work. Surely if the inbuilt immersion’s are firing 2.8kw of heat into this ‘liquid’ it’s going to heat up? If it doesn’t where on earth does that generated heat go?

 

Initially, the heater slowly heats the solid PCM around the heating element until it changes phase to a liquid in the region around the heater.  To prevent local overheating around the element, the heat is moderated initially by being turned on and off by the relay in the control box.

 

As soon as enough PCM has melted around the element enough for there to be convection currents in the liquid phase, then the heater can run at full power, heating the liquid pool which then melts the remaining PCM in the cell.  The temperature stays constant during this stage where the PCM is still changing phase.  As soon as all the PCM has changed phase, then adding additional heat will raise the temperature of the PCM to store a little more heat, and then the heating element will be shut down.  The PCM remains liquid until it's triggered to nucleate and start forming solid crystals.  As these solid crystals of PCM form, they release heat, which can then be collected by the heat exchangers.

 

 

[Barney12]

13 hours ago, JSHarris said:

 

Initially, the heater slowly heats the solid PCM around the heating element until it changes phase to a liquid in the region around the heater.  To prevent local overheating around the element, the heat is moderated initially by being turned on and off by the relay in the control box.

 

As soon as enough PCM has melted around the element enough for there to be convection currents in the liquid phase, then the heater can run at full power, heating the liquid pool which then melts the remaining PCM in the cell.  The temperature stays constant during this stage where the PCM is still changing phase.  As soon as all the PCM has changed phase, then adding additional heat will raise the temperature of the PCM to store a little more heat, and then the heating element will be shut down.  The PCM remains liquid until it's triggered to nucleate and start forming solid crystals.  As these solid crystals of PCM form, they release heat, which can then be collected by the heat exchangers.

 

 

 

Thanks Jeremy. The one bit I still don't like about the Sunamps (but it didn't stop me purchasing them) is the non serviceable nature of immersion heaters. Its a hell of a lump of heavy goop to send all the way back to Scotland if the immersion fails .

 

So to feed my interest.....what happens if you boil the sunamp goop? Does it do spectacular things (I.e explode!) and would such an overheat scenario kill (or reduce) the goops ability to phase change efficiently?

 

(Soory goop is easier to type than phase change )

 

 

 

 

[Ed Davies]

No, I don't think it gets explody (I'd assume they have a PRV of some sort) but it does kill the goop (PCM) which would be a disappointment.

 

This is part of why, for the DIY control system I have in mind, I'd only load it through the low-power hot-water exchanger. It's easy to use a TMV and/or a NC pipe stat to provide a backup overheat protection to make sure the hot water input never goes over the safe temperature.

 

PRV - pressure relief valve.

PCM - phase change material.

TMV - thermostatic mixing valve.

NC - normally closed.

[Jeremy Harris]

51 minutes ago, Barney12 said:

 

Thanks Jeremy. The one bit I still don't like about the Sunamps (but it didn't stop me purchasing them) is the non serviceable nature of immersion heaters. Its a hell of a lump of heavy goop to send all the way back to Scotland if the immersion fails .

 

So to feed my interest.....what happens if you boil the sunamp goop? Does it do spectacular things (I.e explode!) and would such an overheat scenario kill (or reduce) the goops ability to phase change efficiently?

 

(Soory goop is easier to type than phase change )

 

 

 

 

 

 

It boils at a mighty 880 deg C, so the plastic housing of the cell would have melted long before that, as would the cable, so I think the risk is pretty low.  It's debatable whether the 2.8 kW heater could actually get the thing up to this temperature, as I suspect the heat losses would exceed the input power long before then

 

The particular mix of sodium acetate and other compounds in the PCM will degrade if overheated, though, or so I've been told, so there is over heat protection built in.

 

Immersions often (in my experience always) fail as a consequence of corrosion or scaling, and neither is really likely in the PCM cell.

Edited November 16, 2018 by JSHarris
Edited in the light of Ed's correction below

[Ed Davies]

1 minute ago, JSHarris said:

It boils at a mighty 880 deg C,

 

Are  you sure? Anhydrous sodium acetate boils at about that temperature but the trihydrate form boils at 122 °C. It's the trihydrate which has a melting point of 58 °C whereas that of the anhydrous is 324 °C so I assume the base material is more like the trihydrate form.

 

https://en.wikipedia.org/wiki/Sodium_acetate

[Jeremy Harris]

Good point, I checked the wrong variation of the base compound.  There is a PRV on the top of the PCM cell, so the cell itself is protected from over-pressure.

[readiescards]

The system works!

A ASHP feeds a PCM34 Sunamp that acts as a preheat for a PCM58 Sunamp and also a ufh buffer. The PCM58 has an electric immersion element  to 

charged it ( to be solar PV at some point in future)

 

Though the late addition of a back up pair of Willis units added some complications:

 

@Nickfromwales can explain it in detail if anyone wants to know more. 

 

 

[Barney12]

That’s a really disappointing picture.........

 

 

....... @Nickfromwales prefers to work in a small dark cramped plant room, it reminds him of his youth down the pits