Posted by: @davidalgarve

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I have a Mitsu PUHZ SW12YHA heat pump supplying to a radiator system and a Bosch 490L DHW tank. House is in the Algarve, Portugal

The DHW tank is currently served by the ASHP on night time tariff in the winter and a combination of this and mid day ASHP in the summer.

I am just having a 12 panel Growatt Solar EV system installed which includes, initially, a 5 kW battery.

I am not at all sure how I should power the DHW tank after the solar PV is available.

e.g. Should I have the DHW tank immersion heater on a timer to power from the panels/ battery during the sunniest hours or should I run the ASHP during these times.

I give these as two examples, but I am sure there are other possibilities.

Your ideas would be much appreciated.     

Hi David.

You could have a power diverter installed along with your PV system. 

The diverter monitors the current flow at your electricity supply, and when power starts to be exported the diverter sends the excess power to the immersion heater to heat the DHW cyclinder. If your PV system is generating 3kW or more then a 3W immersion heater would be operating at full power. If there is say only 1 kW of excess power available then the diverter will only send 1 kW to the immersion heater. It that way it heats the water without importing power from the grid.

I have a power diverter and it’s works great but now I don’t use it. 

The reason why is because the tariff I’m on pays me more to export my excess than it does to import at night. 

I would check out if you can get something similar as now it’s just an expensive piece of wall art. 😁

Thanks to both Derek M and bontwoody.

My original concept was a power diverter, but they seem expensive for what they are and reading up on them, there seemed to be a view that if you have a battery, they are a waste of money, i.e. "If you already have battery storage as part of your solar panel system – like 73% of UK installations, according to Flexi-Orb – investing in a diverter is unlikely to be a cost-effective choice.".

So if this is correct, how do I optimise by using the battery?

Would I be correct in assuming that using the ASHP to heat the water (as I do currently) is going to be less efficient than powering the DHW immersion heater from the panels/ battery in some way?

@davidalgarve No. the ashp if the most efficient way of heating your domestic hot water. Immersion heaters only have a COP of 1. The battery just gives you flexibility of when you use your power (or grid)

Are they any off peak tariffs in Portugal?

Hi bentwoody.

We have a two part " Bi-Horario" tariff. The peak (generally 7:00 to 12:00 weekdays) and off peak (0:00 to 7:00 weekdays 2:00 to 18:00 Saturday and all day Sunday). the Peak rate is €0.2473 and the off peak is €0.1447

In Summer, I am using the heat pump around the hottest part of the day for DHW, but in winter, I depend more on off peak grid power.

Sorry, I am having difficulty working out the logical approach to this. I was thinking that I should substitute the ASHP in summer for solar PV with the immersion heater controlled by a smart switch. Obviously I would have to watch that no other big demands occur at this time, e.g. swimming pool pump etc. I do take your point about COP, but worry a little about the demand exceeding solarPV/ battery capacity when running quite a big ASHP and thereby possibly drawing on the grid.

@davidalgarve is that a 12kw heat pump David? 

Do you know how many kWh you use to heat your tank? If so divide it by you hot water COP and see if the 5kWh battery is big to do it all. 

If you are having problems working it all out let me know and I’ll do it for you. 

@davidalgarve

As Bontwoody has pointed out, the most efficient way to produce hot water is by means of your heat pump. This may also be the most cost effective way, provided that the heat pump is powered solely by solar PV generation. If the heat pump is being powered by a mix of solar PV and mains supply then it will be less cost effective.

With the addition of battery storage it may be possible to charge the batteries using solar PV, and then use both solar PV generation and battery power to run the heat pump for hot water production. This reduces the likelihood of any mains power import so minimises the likely cost. This method is slightly less efficient than running the heat pump directly from solar PV generation, since charging and discharging the batteries reduces overall efficiency by approximately 10%.

Maximum cost effective operation will be dependent on the type and price of the various tariffs that may be available, coupled with how much solar PV generation is available for use from day to day.

Just a thought...is the Algarve not a no brainer for solar thermal?

Another couple of linked questions, @davidalgarve; how many of you are there living together and how much hot water do you actually use?

In our house we found we showers only account for around 3kWh per day and it's very rare that people will need showers both in the evening and the morning. As a result, I put a rule in place that stopped the ASHP heating DHW during the night in the sunnier months and waiting until mid morning when the ASHP can run off solar energy. Someone showering before bedtime would be using water from a full hot tank after a day of free heating and someone showering in the morning would find the water still hot enough from overnight. The only issue comes on the rare occasions when someone showers before bedtime and then someone else needs a shower in the morning, but even so the tank is not depleted by just one shower (unless it's my youngest son in which case....).

That's certainly a scenario that could relatively easily be tweaked to your circumstances.

Firstly my apologies to Bontwoody for misspelling the name - Not having an easy time with my office disrupted and the solar contractor wanting something every five minutes.

I am checking again on the PUHZ-SW120YHA, because I understood it to be rated at 16kW but the manual is ambiguous at either 12 or 16kW.

I don't have any idea of kW used to heat tank, but it can't be very much with just two people rattling round in a big house (taking showers in the morning) and the big tank was chosen for when visitors come or for when the house is sold on to a bigger family.

We took out a rather old solar water heater on the roof terrace to make space for the solar PV.

Thanks to Derek M for his input.  Daytime tariff is 1.7 x more expensive than Nighttime

Majordennisbloodnok's experience sounds relevant and it seems that I should probably forget using solar/ battery for the immersion heater and just experiment with solar/ battery >ASHP for both winter and summer.

@davidalgarve Lol no problem re the name.

I done some back of the fag packet calculations and if you were heating your tank from 35 to 50 C each day (probably a massive overestimate for two of you) it would take about 8 kWh of energy.

The two of us in our house use about 3-4 kWh, so I would think you would be about the same.

The COP for a heat pump doing a hot water run could be as low as 1.5 or as high as 3 depending on a number of factors so its definitely the way to go (unless you have thermal solar)

The only limitation I can think of would be the output of your battery. My 5kWh pack can only output 3kW max, so if your heat pump was using more than the battery can provide then the rest would either come from your solar or from the grid.

Do you get paid for export to the grid and if you do is the rate better than the off peak rate. If it is then it might be more cost effective to heat your hot water during the cheap rate tariff.