Solar diverting excess to tank or using solar to power the ASHP to heat water

[canalsiderenovation]

I'm having a bit of a debate with someone about heating our hot water from mid March til October. We normally turn our heat pump off from now til autumn as we aren't using heating or hot water. They are saying rather than using the Solic to divert excess solar to the water tank we should be using the solar for the heat pump  to heat the hot water as it will always be cheaper. The problem I have with this is that it would rely on us turning the hot water programme on for the heat pump when we were generating solar and off when we weren't or having a set schedule when we may or may not have solar and we only have just under 3.68Kw of solar so combined with us using the solar for other things I'm not sure it would power the heat pump with other stuff.

 

They are saying heating via the heat pump (using the solar to power the heat pump) will always be cheaper than the immersion (via solar). They are saying on average we are putting as much heat into the tank as you take out giving an example that  assuming we use 200litres of water at 50degrees which came in originally at 15 degrees, we need just over 8kWh of energy which can either be delivered directly via your Solic200, or through the heat pump which means it will only actually use about 2.5kWh of electricity, leaving 5.5kWh to export.

 

We aren't exporting (yet) and actually we probably use everything we generate as we work from home and are quite savvy with making the most of the solar. My question is have I been doing this wrong? Should I be using the solar to power the ASHP and let the ASHP heat the water?

 

[S2D2]

It can be clearer to think about it in terms of price if you have a reasonable export tariff. If you can export 1kWh solar for 15p and add 1kWh of heat to the tank with an ASHP for ~11p (33p/CoP of 3) then worst case (no solar) you are still 4p/kWh better off. 

 

That only gets better if you happen to have excess solar during the ASHP schedule, as the ASHP will then add 1kWh of heat for 5p (15p missed export/CoP of 3). No need to change the schedule regularly, just set it for when you're most likely to have excess.

 

Different export rates will change the calculations, it's a fine balance at the moment with actual reasonable export rates starting to tip the balance. I hope that's clear!

[JohnMo]

Two ways to look at it

Diverer - if you have 1kWh going spare, you get 1kWh of water heating.  If you time your your ASHP to do DHW late morning you will likely get 3kWh or more instead of 1kWh.  Once you ASHP has finished it's heating cycle, any further excess can charge to a higher temperature for free, via the diverter or you can export.

[canalsiderenovation]

1 hour ago, S2D2 said:

It can be clearer to think about it in terms of price if you have a reasonable export tariff. If you can export 1kWh solar for 15p and add 1kWh of heat to the tank with an ASHP for ~11p (33p/CoP of 3) then worst case (no solar) you are still 4p/kWh better off. 

 

That only gets better if you happen to have excess solar during the ASHP schedule, as the ASHP will then add 1kWh of heat for 5p (15p missed export/CoP of 3). No need to change the schedule regularly, just set it for when you're most likely to have excess.

 

Different export rates will change the calculations, it's a fine balance at the moment with actual reasonable export rates starting to tip the balance. I hope that's clear!

 

We aren't exporting at the moment so I guess until that's sorted I'll carry on using the solar to divert excess to the hot water tank unless it makes sense to use the heat pump. My issue with using the solar to do the heat pump is I'd have to guess when we would have excess solar (likely mid morning onwards) where with the solar diverting excess I just know it automatically does it and I would hazard a guess our export would be so low with just under 4kw of panels. Once we know we have solar, the dishwasher, washer, laptop charging etc goes on. Need to get this export sorted .....

[ProDave]

My solution is I have the ASHP heating timer set to tun on at 11AM.  The assumption is by 11AM the solar PV should be well up if the sun is out and so the ASHP should be able to soak up a lot of free PV energy.

 

I then use other appliances like dishwasher etc or washing machine after mid day when the ASHP will have usually finished it's HW heating.

 

The PV diverter will send excess power to the immersion at any time of day, often quite early on a sunny day, so that usually will have pre heated the tank a bit.  And even after the ASHP has finished heating the tank to 48 degrees, there is still plenty of room for the immersion to heat it hotter than that with surplus PV in the afternoon.

 

The best thing you can do for self useage if you can tolerate it, is look at the weather forecast.  e.g. It's going to be a cloudy day today but sunny tomorrow so I will hold off using the washing machine until tomorrow etc.

[Wil]

With using the HP, unless you have some sort of direct and intelligent link between solar generation, house consumption and the HP, you will almost always end up consuming some grid electricity. This argument has been had on another thread recently and I get the proponents saying for the 'greater good' we should be exporting whatever we can, paid or otherwise.

 

However to keep it simple, you have a solution whereby you can charge your HW tank using excess solar whenever it is available, from the moment the solar generation becomes greater than the total consumption. From that point on, all HW is free until you get a piping hot tank at 75/80 degrees. 

 

If you run your HP, you'll get whatever it's DHW setpoint is (50degrees?) but also run the risk of import an cost to do so. each time the sun goes behind a cloud or turn something else on or ((HP consumption + House) > Solar Generation) then you will be importing the difference.

 

Personally I'd be sticking with the Solic until the PV can't cover your DHW load and then the HP comes back on. This way you also run the moving and serviceable parts of your expensive HP half as much and prolong it's life as well.

 

Edited to say ^ ProDave's approach is ideal and could even by automated using forecasting software, but requires a fair bit of continuous input from you (him).

Edited March 31, 2023 by Wil

[ProDave]

I find the biggest barrier to solar hot water, however you organise it, is when you want to use the hot water.

 

I am a "shower in the evening" person.  All I need in the morning is tepid water to wash my face.  So for me starting the day with a completely cold tank and only heating the HW somehow when the sun is up, and then using that hot water in the evening would be fine.

 

That plan goes out of the window when someone else wants a morning shower on any day of the week she feels like.  So the only compromise for that is leave the ASHP on to heat the tank when it needs until bed time and there is always enough HW in the tank for that morning shower.

 

I have not succeeded in changing that behaviour, but feel if you could, better use would be made of solar PV for heating more of the water.

[SteamyTea]

It is really down to the difference between power and energy.

Energy is power multiplied by time.

 

To run a heat pump you need a certain amount of power, for a fixed time, otherwise you are importing the difference.

This is basically down to having a delay between the HP starting up, and then delivering power.

This is not so much of a problem with a purely resistive load like an immersion heater. These can take a few seconds of excess power, heat the water a bit, then shut down till the sky is clear again.

 

The easy way to find out is to monitor every minute or so. Then work out how often the HP would, on average, be importing power.

[Conor]

For us, it makes sense to use ASHP and export. This is because we are on an economy 7 tariff and we buy off-peak at about 10p a unit. We sell our PV for 8.5p a unit. So with a conservative COP of 2.5-3 in warmer months, it "costs" half as much to heat with the ashp then solar diverter.

 

What I'll probably do is turn the immersion stat down from 56c to the same or a bit below as the heatpump, 45c, so that way if we are ever running low during the day we get a free top-up so we don't run out. While the bulk of the heating will be off-peak. 

 

Another aspect is that the off-peak period ends at 9am... So conceivably on bright mornings between 7-9 the PV will cover a fair chunk of the ashp demand.

Edited March 31, 2023 by Conor

[joth]

For me overnight electricity is cheaper than midday solar (7.5p cheap rate import vs 15p to export excess) so it's a no brainer to get the tank ASHP hot overnight, then top up from ASHP at midday (or as soon as the battery is full). The only question is whether to then use the immersion diverter for >55°C top off or just export the rest. 

Edited March 31, 2023 by joth

[ReedRichards]

When my heat pump heats the hot water cylinder it heats it to 50 C and draws up to 6 kW of power.

 

When my immersion heater heats the hot water it heats it up to around 65 C and draws 3 kW of power.

 

So I can make my hot water hotter with the immersion heater and since 3 kW of output is within the capability of my solar panels (aided by my battery)  can do this completely free.  Or I can supplement my solar power with imported electricity, use the heat pump to get cooler hot water but then have a lot more solar electricity to export.  In my case I am paid for deemed rather than actual export so the immersion heater option is a clear winner.   

[ProDave]

17 minutes ago, ReedRichards said:

When my heat pump heats the hot water cylinder it heats it to 50 C and draws up to 6 kW of power.

 

When my immersion heater heats the hot water it heats it up to around 65 C and draws 3 kW of power.

 

So I can make my hot water hotter with the immersion heater and since 3 kW of output is within the capability of my solar panels (aided by my battery)  can do this completely free.

So you have an 18kW ASHP do you?

 

My 5kW ASHP draws no more than 2kW of electricity and is well withing the capacity of the solar PV when the sun is out.

 

But yes the chief USP of a solar PV diverter to an immersion heater is it can send anything from a few watts up to the full power of the immersion heater (mine is in practice 2.8kW) depending on what is available.

 

On a sunny day is is common to see the ASHP running full tilt and still some going to the immersion heater at the same time.

[ReedRichards]

3 hours ago, ProDave said:

So you have an 18kW ASHP do you?

 

No, it's a 12 kW ASHP

 

3 hours ago, ProDave said:

My 5kW ASHP draws no more than 2kW of electricity and is well within the capacity of the solar PV when the sun is out.

 

That's good for you but it's going to take a lot longer to heat the same amount of hot water compared to mine.

 

3 hours ago, ProDave said:

But yes the chief USP of a solar PV diverter to an immersion heater is it can send anything from a few watts up to the full power of the immersion heater (mine is in practice 2.8kW) depending on what is available.

 

I don't actually have a solar diverter, they're expensive I didn't think I could justify the cost.  I just switch the immersion heater on and off using a switch that is controlled by WiFi and programmable.  If my battery is near to full charge and the sun is shining I switch it on; the battery covers when the sun goes behind a cloud.

[Marvin]

11 hours ago, canalsiderenovation said:

Should I be using the solar to power the ASHP and let the ASHP heat the water?

 

Hi @canalsiderenovation

 

Yes. As you are not being paid for the power. We do. New ASHP £5k? New immersion £50? Also we have super insulated our tank and loose little heat so heat the water higher than required and it lasts more than a day (a day that could be no sun and we would have to use the ASHP).

 

We use weather compensation mode in the winter and I have adjusted the hot water temp to the lowest practical hot water setting. Basically any time the water drops to the level set the ASHP will heat the hot water, otherwise the solar diverter will take over and raise the temp to 60°C.

 

Good luck

 

Marvin

 

 

[Dave Jones]

I'm thinking on this as well on our current build. It needs a programmable controller of sorts to make it all work.

 

There is a small firm who make a box to trigger the MVHR boost automatically (temp sensor on shower hot feeds, CT clamp on hob connection) I'm going to email them and see if they can make an addon for it.

 

Just needs a bit of logic: when spare electric is available AND the temp of the tank is below 45C turn of immersion and turn on ASHP. Once 45C is hit turn off ASHP and turn on Immersion.

 

 

[ProDave]

6 hours ago, Dave Jones said:

Just needs a bit of logic: when spare electric is available AND the temp of the tank is below 45C turn of immersion and turn on ASHP. Once 45C is hit turn off ASHP and turn on Immersion.

Simple to make your self with an Arduino or Raspbery Pi, whichever you find easiest to program (Arduino for me)

[Dave Jones]

1 hour ago, ProDave said:

Simple to make your self with an Arduino or Raspbery Pi, whichever you find easiest to program (Arduino for me)

 

if i cant make it using a hammer then its beyond me im afraid.

[SteamyTea]

Generally, for 8 months of the year, PV can supply a normal 2 person household with all its DHW. Possibly for 5 or 6 months of the year, the PV will export as well.

One has to ask if it is really worth the time, effort and complexity to save a few quid of imported energy.

You could just look at a PVGIS hourly report and take the integral, 40% either side of the mean (around noon for most people with south facing PV).

Then run things in that window.

That window will vary about an hour a month, so you are adding or subtracting 30 minutes to each 'tail', but only during March, April, September and October, the summer can look after itself.

I have cheap timers on my DHW and storage heaters. Think they cost less than 20 quid each, they are set up to lock out connection when it is unnecessary.

A simple solution to a complicated and variable problem.

[ReedRichards]

6 hours ago, SteamyTea said:

Generally, for 8 months of the year, PV can supply a normal 2 person household with all its DHW.

Well here we are on 1st April and there have been 2 days so far this year when my solar generation was enough to charge the battery and still leave some left over to heat the DHW.  On most days my solar power is being eaten by my heat pump almost as fast as it is generated.  Granted, the ASHP is heating the hot water as necessary, as well as the house.  But if it was only heating the house I don't think I would be exporting any more electricity.  Perhaps my 2 person household is not normal?  

[sharpener]

I have been wondering about this. ATM I am charging to ~50% at night rate and the sun does the rest most days and there is enough left over for the HW.

 

I don't think when I install the HP I will put it downstream of the battery system, as in a power cut it would exhaust the battery very rapidly leaving nothing for lights or cooking. If I tee it in before the battery CT then the battery will have priority but the HP will still have the benefit of any residual PV before it is exported. Like @SteamyTea I have a timer so the immersion heater can come on to top up the HW temp after the HP has done most of the heavy lifting with the benefit of the CoP.

 

Currently I do the reverse, the immersion stat is set higher than the tank stat so the oil heating only comes on at the end of the day if the PV was insufficient.

 

[Marvin]

1 hour ago, sharpener said:

Currently I do the reverse, the immersion stat is set higher than the tank stat so the oil heating only comes on at the end of the day if the PV was insufficient.

Excellent! We do the same but with our immersion and ASHP.

[canalsiderenovation]

Seems we all do slightly different things! I'm not going to worry too much at the moment as we aren't exporting and the solar is doing ample hot water for us just with the solic and we don't have to worry that if the ASHP was heating our hot water at certain times we may or may not be using the grid if it was a bit cloudy etc and other electric things were on too and with us both working from home the coffee machine is always going, laptops etc charging. At the moment we just know it automatically diverts when there is excess and it's enough for our two person household. If we sort our export out I'll reassess but it's only practical if we are being paid more to export than the electric would cost us and that seems unlikely unless we go on an E7 tarrif which doesn't suit out use (particularly with the ASHP heating in winter).

 

If we end up getting batteries later in the year I'll probably need to completely reassess but that's for another day!