Renewable Energy: Help with 3 phase inverter choice

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- CommentAuthorjonc_uk
- CommentTimeAug 20th 2016 edited
Hi All,

I am new here, so please go easy on me!

I am looking for any advice on an inverter for my specific application.

At my holiday home in the Algarve, I have a Lorentz PS4000 solar borehole pump system. This is used to pump water for irrigation of my garden which is needed typically from May to the end of October in varying degrees. The system is powered by fourteen Solarworld Sunmodule Plus SW 285 Mono connected as seven in series, in parallel to another seven of the same (so 2x7 series-parallel if that makes sense).

I want to use the spare power from the winter/spring seasons to power my air source heat pump to keep the house warm. The ASHP is 3 phase and I can't alter the configuration of the panels which will provide a relatively low voltage, but high current:

Voc = 7x 39.7 = 277.9V
Vmp = 7x 31.3 = 219.1V
Isc = 2x 9.84 = 19.68A
Impp = 2x 9.20 = 18.4A

Over this period I expect most days will be full cloud with the system operating at below maximum power, but still brighter than the UK!

I know I have limited options being 3-phase - generally they are higher power units which means long strings and high voltages etc. which is not ideal. I intend to use a relay to switch from the pump controller to the inverter but I don't want to mess with the wiring at the panels which are ~40m away at the end of some very hefty cable.

Thanks in advance for any advice or suggestions,

Jonathan.
- CommentAuthorSteamyTea
- CommentTimeAug 20th 2016 edited
Slightly off the wall solution, could you change your HP to single phases ones?
- CommentAuthorEd Davies
- CommentTimeAug 20th 2016
Sorry, no idea about three-phase inverters for such relatively low power. I think you're right that it might be difficult, particularly with 200 V or so being towards the bottom end of the ranges at which such things tend to work.

But, why do you need a relay to switch between the inverter and the pump? Why not just switch on the one you want to use at any given time? Would something terrible happen (other than them not work properly) if you accidentally switched them both on together? I don't think so.

Alternatively, is there any chance of using each string into a separate single-phase inverter on two of your three phases to at least save 2/3 of the cost of running the ASHP?

I take it you're up-to-date on the legal situation with feeding power into the grid in Spain which is a bit tricky I understand. Or are you trying to keep this completely disconnected from the grid?
- CommentAuthorSteamyTea
- CommentTimeAug 20th 2016
So this is a 4 kWp system on two strings.
Each string at maximum power has a voltage or around 219V, which as Ed says is low.
To get 3 strings would mean adding one extra module, but the voltage would be lower at 156V. And that is in full light.
The only inverter that I know that can reliably use those sort of voltages is the Steca, not sure if they make a 3 Phase one though.

So I think that Ed is on the right track, feed in what you can, where you can.
- CommentAuthorjonc_uk
- CommentTimeAug 20th 2016 edited
Thanks for all your suggestions.

Posted By: SteamyTeaSlightly off the wall solution, could you change your HP to single phases ones?

I am happy to accept all off the wall solutions. In this case, the heat pump is a Panasonic 16kW model. The cost of replacing it would make this solution unviable I think. I also like the idea of keeping the phases balanced as much as possible (although obviously just turning on the kettle messes this up).

Posted By: Ed DaviesBut, why do you need a relay to switch between the inverter and the pump? Why not just switch on the one you want to use at any given time? Would something terrible happen (other than them not work properly) if you accidentally switched them both on together? I don't think so.

Actually, I am sure you are right.

Posted By: Ed DaviesAlternatively, is there any chance of using each string into a separate single-phase inverter on two of your three phases to at least save 2/3 of the cost of running the ASHP?

Well, if I am into changing the wiring config at the panels, I might as well have them as a single string of 14 panels instead. I was trying to avoid this. At the moment, the 2x7 panels have only 2 wires from the array to the controller. I suppose I could put the inverter with the panels in this case but I would then have to run a 3PN cable to the supply.

Posted By: Ed DaviesI take it you're up-to-date on the legal situation with feeding power into the grid in Spain which is a bit tricky I understand. Or are you trying to keep this completely disconnected from the grid?

This is Portugal - the rules are up to 1.5kWp with no payments (UPAC self-consumption) but mandatory registration. I have 1kWp under this scheme plus the 4kW borehole pump system plus 1kW pool pump system. The last two are not grid-tied.

The house has a large (for domestic) supply of 3 phase 40A per phase and my intention is purely self-consumption so I consider it within the spirit of the rules
- CommentAuthorjonc_uk
- CommentTimeAug 21st 2016
Ok, I think switching the array to a single string of 14 panels when the pump is not in use will probably be the best solution. This will give:

Voc = 14x 39.7 = 555.8V
Vmp = 14x 31.3 = 438.2V
Isc = 9.84A
Impp = 9.20A

And so on to choose an inverter.

SMA / Power One / Frontius / SolarX..

Any suggestions? I was thinking of over-speccing and putting in 5kW or thereabouts, depending on manufacturer. I plan on installing/configuring this myself next time I visit so this is a consideration. It will need to be configured for Portugal.

Thanks again, Jonathan.
- CommentAuthorbxman
- CommentTimeAug 21st 2016
Do the panels feed a grid tied inverter? or drive the pump directly somehow ?

In winter you are going to need a load of panels to feed a 18Kw heat pump.

why not get a diverter and feed surplus PV into a regular heater and hope that cuts down on the time the heat pump needs to run .

you are less likely to mess things up IMO.
- CommentAuthorjonc_uk
- CommentTimeAug 21st 2016
The pump system is not grid-tie; it is an independent system, but it is not really used in the winter.

I want to make use of the 14 panels when they sit idle, and use this to provide some heating to the house, which otherwise would be unheated and return to ambient temperature - typically around 10 degrees by mid February.

The Panasonic heat pump has an output of 16kW, but does not consume that much electricity.

I think a switched reconfig of the panels to a 14 panel string is the answer, but I need some advice on which inverter to buy.
- CommentAuthorEd Davies
- CommentTimeAug 21st 2016
Posted By: bxman: â€œwhy not get a diverter and feed surplus PV into a regular heater and hope that cuts down on the time the heat pump needs to run.â€

Hmmmâ€¦ if going that way then why bother with an inverter and diverter - why not go DC to heat more directly? Save any controversy about 1.5 kW feed in limits.

Posted By: jonc_uk: â€œVoc = 7x 39.7 = 277.9V; Vmp = 7x 31.3 = 219.1Vâ€

With appropriate DC switching you might get away with putting that into heaters designed for mains voltage.

https://edavies.me.uk/2016/07/mp-pv-heater/
- CommentAuthorbxman
- CommentTimeAug 22nd 2016
Do you have an more details on the Lorentz controller .
When it is not pumping water does it any electrical output ?
looks like the majority of their systems use the DC.

Do you propose to couple the panels to an inverter for winter use and then connect to the Lorentz controller when you need them for water pumping duties ?
- CommentAuthorjonc_uk
- CommentTimeAug 22nd 2016
Posted By: Ed Davies
Hmmmâ€¦ if going that way then why bother with an inverter and diverter - why not go DC to heat more directly? Save any controversy about 1.5 kW feed in limits.

Posted By: jonc_uk: â€œVoc = 7x 39.7 = 277.9V; Vmp = 7x 31.3 = 219.1Vâ€

With appropriate DC switching you might get away with putting that into heaters designed for mains voltage.

That's an interesting option, given that it's an inverter heat pump - however, it takes a 3 phase supply. Also, from a cabling point of view, it's the least convenient.

Posted By: bxmanDo you have an more details on the Lorentz controller .
When it is not pumping water does it any electrical output ?
looks like the majority of their systems use the DC.

The output from the controller is a sort of 3-phase DC to drive their own high-efficiency pumps. There is also level control so I would not be able to use their output.

Do you propose to couple the panels to an inverter for winter use and then connect to the Lorentz controller when you need them for water pumping duties ?

Yes, that's basically it.
- CommentAuthorEd Davies
- CommentTimeAug 22nd 2016 edited
Posted By: jonc_ukThat's an interesting option, given that it's an inverter heat pump - however, it takes a 3 phase supply. Also, from a cabling point of view, it's the least convenient.
That's not what bxman or I had in mind: rather than using the heat pump at all why not just use the output of the PV for heating directly? OK, you don't get the COP benefit but it could be a lot simpler and more legally sound. If it's just for background heating when the place is not occupied (much) then the COP loss might not matter much and saves the wear and tear on the heat pump.

My extension of bxman's suggestion was to skip converting to AC at all but to heat directly with the DC output of the panels. Thinking about it today, perhaps sending up to 280 or more volts (have you taken the higher Voc in cold conditions into account?) into an unattended heater built for 240 volts might be pushing your luck. Better maybe to have two 240â€¯V heaters in series running at derated power.
- CommentAuthorWillInAberdeen
- CommentTimeAug 23rd 2016 edited
(Reply deleted as I don't really know enough about this subject)
- CommentAuthorbxman
- CommentTimeAug 23rd 2016
Can you explain the system in Portugal in slightly greater detail I note you say you have 1Kw grid tied presumably one a single phase of your 3 phase supply is 1.5 the maximum you are allowed in total or on any single phase

It would appear the minimum size for quality 3 phase inverters is 5-6 Kw they would probably work satisfactorily with 4 kw of panels , however most diverters would work better if you could feed a single phase .

can you fit second hand inverters ?
if so I would look for a Power-One Aurora PVI-3.6-TL-OUTD

sub Â£200 now on the bay if money is a consideration .
- CommentAuthorWillInAberdeen
- CommentTimeAug 24th 2016 edited
(Reply deleted as I don't really know enough about this subject)
- CommentAuthorjonc_uk
- CommentTimeAug 25th 2016
Posted By: Ed DaviesThat's not what bxman or I had in mind: rather than using the heat pump at all why not just use the output of the PV for heating directly? OK, you don't get the COP benefit but it could be a lot simpler and more legally sound. If it's just for background heating when the place is not occupied (much) then the COP loss might not matter much and saves the wear and tear on the heat pump.

Ah - I see now. Maybe an in-line pool heater would do the job here.

My extension of bxman's suggestion was to skip converting to AC at all but to heat directly with the DC output of the panels. Thinking about it today, perhaps sending up to 280 or more volts (have you taken the higher Voc in cold conditions into account?) into an unattended heater built for 240 volts might be pushing your luck. Better maybe to have two 240â€¯V heaters in series running at derated power.

I think I will talk to a manufacturer and see what they say. There is a 4.5kW unit which might do the job.

Posted By: bxmanCan you explain the system in Portugal in slightly greater detail I note you say you have 1Kw grid tied presumably one a single phase of your 3 phase supply is 1.5 the maximum you are allowed in total or on any single phase

I have 4x 250w panels for this; a pair on a 500W inverter on one phase, and 2x 250W on the others. Each inverter sees only a single phase.
- CommentAuthorEd Davies
- CommentTimeAug 25th 2016
Posted By: jonc_ukI think I will talk to a manufacturer and see what they say. There is a 4.5kW unit which might do the job.
Apart from the somewhat higher voltages in your system than normal mains RMS voltages the big issue with DC heating is switching. Normal AC switches (such as in thermostats) don't like DC at all. They rely on the AC reversing direction 100 (or 120 for some places outside Europe) times a second to cut off any arc drawn. Staying within the rated current but with DC rather than AC they're likely to weld shut much too soon for comfort.

You either need a very beefy switch (contactor = big relay, usually) or some sort of semiconductor switch. Simple power FETs (field-effect transistors) work well up to 100 or 150 volts (which is why off grid MPPT charge controllers tend to work to such voltages [Â¹]) but are not so good for voltages like yours. There are switches for DC at higher voltages and currents but they're rather specialized so don't tend to be cheap.

[Â¹] Midnite Solar make a thing of working to higher voltages than most but still not to 300 V or so for routine operation.
- CommentAuthorbxman
- CommentTimeAug 26th 2016
Sorry I should have said what is permitted on the portuguese grid.

Heat pumps do not take to being switched on and off they are best run for long periods .

As you have acknowledged the sun is fickle or it's PV output is if you want to benefit from it you need a diversion device that feeds a resistive load that does not mind if it is 5 watts 50 watts or 4,500 watts because it will seldom be the same for more than 30 seconds .

If you switch on heater or heat pump without a diversion device because the sun is out it is going to cost you money because clouds come and go . Most days it is up and down like a yo-yo seldom meeting the current demand then feeding the grid when ever it exceeds the load .

I know I have watched my surplus PV production feeding into night storage heaters with an analogue clamp meter.

As Ed says Dc switching would not be wise especially with an unattended property.
- CommentAuthorjonc_uk
- CommentTimeOct 11th 2016
I appreciate the comments about handling high-current DC - I remember standing next to the operator of a DC-powered fairground ride 20 years ago and watching the arcing of the hand-operated speed control switch.

It occurred to me yesterday that the solution to this is probably better handled as 3x single phase systems. With galvanic isolation, I don't think I need to worry too much about the string connections. If I connect a micro inverter to each panel in addition to what is already there, I think I might get away with it. Possibly the MPPT would not be perfect with the 2 parallel connected strings but I suspect this would just reduce gains slightly.

All I need to do then is control when the inverters are powered up - a contactor powered by a float in my water tank should handle that.

bxman - the maximum allowed feed in to the grid in Portugal is technically 100% of your contracted power so in my case this would be 40A per phase (which is big for a domestic supply).

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