Let me abuse of your time to share my problem and know your view.
I have an off-grid system as there is not option to acces the grid from there.
I have a MP II 5kw, 3kw solar string, smartsolar 250/70 can bus and 2 x Pylontech US5000 (10kw)
I want to install an aerotermia device for heating and SHW that most probably will have 8 or 9kw power. This will requiere a high PV production during the day, not less than 12kw, to cover the aerothermia demand and also to charge the battery bank. Of course the battery capacity will have to be big to procide the requiered power during the night. Not less than 30kw.
As 30kw of a Pylontech bank is very expensive, I was thinking to add a Fronius PV inverter with its own 30kw battery (a cheap DIY battery) but Fronius supports few batteries. Of course, i would have to increse MP II with another 5kw unit in parallel.
My view is that I have 2 options:
1 - As VE systems does not support 2 different battery banks and we can not mix diferent battery brands in a bank, I should Increase battery capacity by adding 30kw of Pylontech batteries. Increase VE solar chargers (most probably a MPPT RS 200A) and add another MPII 5kw in parallel
2 - install a separated off-grid system for the aerothermia demand. 1 or 2 Multi RS Solar (soon parallel configuration will be supported) with a 30kw or more DIY battery with a 200A JK Inverter BMS that works quite well with VE inverters.
Actually I don’t think that your maths will work out that easy…
With a heat pump in an off-grid situation, the main concern is start-up current (A).
Your Victron system needs to be able to suport that…When starting, the inverter could draw up to 40 times the current from what it would normally draw when up and running. Modern heatpumps normally have a soft-start function. Examine the data-sheet carefully.
The number of 8-9kW Power draw for that heatpump seems huge. Again, check the data-sheet … maybe this is the peak during startup. A heatpump that consumes around 2kW electric power will produce around 2.5 to 4 times the value of heat, so a 2kW heatpump will output around 5-8kW of heat … a heatpump consuming 8kW of electroic power will produce 20-32kW heat (which seems a huge demand). Also with 8-9kW power draw, You’ll need a 3-phase system to support that.
heating is important during winter, when PV intake is low…depending on where you live, you will see maybe only a 10th of what you would see during summer peaks. hence in order to sustain a 2kW heatpump for a day, with maybe only 6-8hrs PV intake - your PV array needs to be much larger than 30kWp. I have a very efficient, 2kW heatpump and I’ve seen days during winter season, where it’s consumption is about 40kWh (in a good isolated home). So your Battery needs to support 30kWh spread along 14hrs and your PV needs to produce 40kWh at least during 6-8hrs of light. My 20kWp PV-Array only produces 6-8kWh at winter days, instead of 40-110kWh during Summer).
a battery of 30kWh nominal capacity will provide you with only 70% of this capacity, because of conversion losses and min SoC not under 20% in order to sustain high loads without a low battery alarm
Without knowing all your details, my gutt feeling is, that you will need a generator, at least for some hours during Nov-Feb, too
Thanks hominidae. Very useful feedback. Most probably i used heating power instead of electrical power. I will review carefully the tech specs of the heat pump.
Regarding winter PV production, Looking for some solar maps of my area (i am based in catalonia), they tell me that my actual 3kw panels should provide in average around 10kw per day. Most probably i have to consider a lower value, something around 7 kw, for my final pv sizing.
Regarding heat pump consumption i have a doubt, does the 2kw electricsl power of the hest pump is the average hourly consumption or it is the hourly consumption assuming that the heat pump will be runing all the hour? I am planning to install a oversized inertial tank to help heat pump power during the night. I also have a wood stove to support hest pump in cold/cloudy winter days.
On the other hand, do you believe that a 5kw MPII can handle a 2kw heat pump assuming that it has a soft-start option? I am not worried about inverter as I can install a second one in parallel if required.
Very large PV grid worries me as solar power conversion became a headache. I was thinking in a MPPT RS 200A that can manage 11kw. How do you manage your huge solar PV power conversion?
Ah, catalonia…ok, then you are in the lucky/sunny parts of southern Europe.
This changes things to your advantage of course.
Well you need to get your definition of measurement units in order, I think.
A PV with 3kWp installed will produce an certain amount of “Energy”, like 7kWh per winter day (on average). So Power → kW and Energy → kWh are two things.
It is Power…look at the data sheet. A classic heatpump inverter will run at a constant power draw Some more modern inverters, are able to be operated with variable output (hence variable input). Again look ate the data sheet.
Energy consumed will be, of course, the power drawn during the time interval it is switched on.
A 2kW heatpump (non-variable inverter) will consume 2kWh per hour of operation - and produce 4-10kWh of heat during that time. hence what you need to know is the average heat consumption of your home (maybe your architect or vendor supplied that - this is required by law in the parts of Europe where I live). .But you cannot assume that the amount required per day is a constant…it will depend on your behaviour and weather, of course. hence your battery should be sized to at least cater for 3 days of average consumption/operation, when you cannot afford to run out of power at any time (without a generator as backup or “hybrid”/supplemental supply). If your wood stove will also heat the water in your tank, you have some kind of backup, which is good. Remember that you still need power to operate the controls, valves and pumps (besides lights, kitchen-stove, -fridge, Internet-router, TV , …)
You need to check the data sheet of the heat pump. In Island mode the MPII-5000 can supply up to 9kW peak for a very short time, according to its data sheet. Your heatpump should stay a good portion below that during start-up (the multi need to supply the base load of your home at this moment as well).
Well, yes…over supply during summer needs to be taken into account.
I do not have such problems as my PV array with its inverter is AC connected and I am running 100% on-grid with a good quality of supply. Everything (PV, Home, Heatpumps, Wallboxes) is connected to AC-In…I am running an ESS.
Why do you say “over supply during summer needs to be taken into account.”?
Why is PV over supply a problem?
As far as I know the mppt will cut the PV energy input when battery bank is full or there is not load demand.
Actually my 3kw PV grid is only working at full capacity during a couple of hours of the day to recover the battery capacity spent during the night. The rest of the hours the PV production only covers few hundred wats load. I over sized the PV roof grid to cover future needs and I have not had any problem after 2 years with the over sized PV… Now in summer could produce more than 15kwh per day and I am only using 2kwh
Yes, that is my assumption, too.
I was refering to a scenario, where you install an even larger PV array, where a DC-coupled MPPT is not availble or feasible…some installation seem to use an AC-coupled PV-inverter in that case, which can - to my knowledge - only be controlled by frequency shifting (which sometimes does not work as expected, when looking into some feedback from other users here).
However, I am not an expert with off-grid systems, nor am I a professional victron installer.
Over all your initial request was on how to integrate a heat pump