Optimum use of PV

[Triassic]

I’m still trying to get my head round the optimum design for my DHW and UFH system.

 

My current thought is to have an ASHP providing heat into a 70l buffer, connected to the UFH, and to use the heat within the buffer to preheat the DHW going to a Sunamp unit (still not sure what size I need, is there a size guide when using preheat?). I’m also planning to install 4Kw of solar PV and use the energy produced in the above set up. So which would be more efficient, to heat the Sunamp unit or to heat an immersion within the buffer tank, I have a feeling I’m asking a silly question here!

Edited September 20, 2018 by Triassic

[PeterW]

70 litres has very little heat capacity - it will also be at a relatively low temperature for a preheat.

 

What other dump loads are you planning for the PV..?

[Triassic]

6 minutes ago, PeterW said:

70 litres has very little heat capacity - it will also be at a relatively low temperature for a preheat.

 

What other dump loads are you planning for the PV..?

I keep hearing about problems with the heat losses from large thermal stores, so had in mind something similar to J C Harris current setbup. I also have around 500m of UFH pipe buried in my ground floor insulated slab.

 

Could you explain dump loads? I suppose I have two options, immersion heaters in a large body of stored water and the immersion’s in the Sunamp unit.

Edited September 20, 2018 by Triassic

[Ed Davies]

I'm assuming a reasonably well-insulated house (above building regs) and hence a fairly small ASHP. The only time I can imagine it would make sense to use an immersion in the buffer tank would be if the PV is producing only a trickle of power, a lot less than the ASHP needs, and the Sunamp is full. Seems a bit of a marginal case.

 

Most of the time it's a balancing act, how much of the available output of the PV to feed to the ASHP → buffer tank → slab and how much to the Sunamp. Presumably you wouldn't worry about doing both at the same time as that'd only work when the PV's producing close to peak power which will only happen for long on days when space heating isn't an issue and there's plenty of heat for DHW.

 

Similarly, in January the PV isn't going to provide much towards your heating requirement, most will have to come from the grid anyway so not worth worrying about (other than thinking whether or not economy 7 or the like makes sense).

 

So, what you're trying to do is get the right mix in the shoulder months. There's a certain amount of electricity that'll be needed, some will come from the PV and the rest will come from the grid. I guess, then, that the objective is to minimize the amount that comes from the grid. That'll depend on the capacities of your heating systems. E.g., does the Sunamp have just a day's worth of DHW or multiple days? Does that vary depending on guests, kids being at home, etc? Does the house cool down significantly in a day or can it be left for a couple of days. The failure would be starting a sunny day with the house as warm as is comfortable and the Sunamp full. You want to leave some storage capacity (Sunamp and slab) to use sunshine when it's available.

 

Not an answer, I know, but hopefully a useful way of looking at the question.

[le-cerveau]

You can get PCM34 SunAmp Units in the UniQ range, USe that as the buffer tank and DHW pre-heat, that is what I have, ASHP feeds the SunAmp and  the UFH takes heat from the same circuit.  The DHW is then on the other heat exchanger.  No wourry about losses and you can up size as required for DHW pre heat.

 

Cost is higher than a Buffer tank though!

[Triassic]

Part of my problem is that we will be two people living in a large four double bedroomed house. So DHW has to range from two people for 80% of the time,  to 8 people for 20% of the time!

[ragg987]

...

Edited September 20, 2018 by ragg987
deleted - incomplete post

[Ed Davies]

50 minutes ago, Triassic said:

So DHW has to range from two people for 80% of the time,  to 8 people for 20% of the time!

Does the Sunamp help track its heat content? I.e., does it have any electronic output which tells you what percentage of its capacity is in use?

[ragg987]

Are you intending the ASHP to heat the Sunamp? (Bearing in mind you need a high-temp ASHP to do that, an ordinary one will not reach the phase change temp)

 

Some considerations:

• 70l of buffer water is tiny. One shower will deplete most of it
• Heating using ASHP / PV / direct electrical will struggle to maintain more than 1 shower when your hot water store runs out
• Hence you need to size your store based on a likely scenario
  • 8 showers within 1 hr means storage for 8 showers -> approx 20-25kWh
  • but 4 showers in the morning and 4 in the evening means you can halve your storage
• I understand Sunamps are modular units and allow partial heating, e.g. you can 'switch off' cells when only 2 of you and switch them on when you have guests. Probably worth checking with Sunamp how this works.

PV becomes a bonus source of electrical energy that you can optimise as follows:

1. First priority is to use the PV in your house - appliances timed to run on sunny days, or EV car charging
2. Next priority is to supply the Sunamp - use circuit 1 on your solar diverter
3. Last priority is to pre-heat the buffer - use circuit 2 of solar diverter
4. If all above are satisfied, you export the excess

You can experiment with buffer temperature to avoid overheating your house. Your ASHP will support 2 circuits, so you can have DHW higher (to allow Sunamp to store the energy) and buffer lower (to supply UFH or pre-heat DHW).

 

The solar diverter is important. For instance the Immersun has ability to sequence between 3 loads. The example above uses 2 loads. On sunny days, you sequence Sunamp > buffer. On gloomy days any spare PV drives the ASHP.