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Author Topic: 'Surplus' PV panels - ideas?  (Read 1380 times)
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« on: March 06, 2016, 01:27:56 PM »

Hello everyone - first post here.  I've read a few similar topics here and on greenbuilding forum but confess they got too technical so posting my scenario for ideas:

We have surplus 4.2kw PV panels installed (25x170w thin film) -we couldnt get agreement to export limiter with our DNO in timescale to install before cut off/drop in tarrifs in January. We are all electric and use off peak for hw and part heating (mix of wet rads/amptec boiler and storage heaters). We intend to install a thermal store in the summer to connect up a boiler stove and have two immersions, probably 4-500l TS depending on what we can get upstairs. There is an existing solar cylinder (160l) with an immersion at top, boiler connection in middle (not used) and solar coil at bottom (not working at present and not inclined to expend money trying to get it to work for a number of reasons - not least have 4.2kw panels sitting on the roof twiddling their thumbs....).

The grid tied PV is 6.8kw on a 4.6kw (2x2.3kw) inverters (installer checked design parameters all ok).  We will at some time upgrade to 3 phase at which point DNO will accept full 11kw system grid tied but its a lot of money we dont have - could be years before we do this. As we need hw heating all year round it makes sense to use the excess panels 'off grid'. I was attracted to 'PV direct to dc immersion' option but either its seems potentially dangerous and definately difficult and / or inefficient.

So options are:
1. cheap off grid inverter - no batteries (prefered, swap out when 3 phase upgraded, but whats cheap and reliable?)
2. cheap off grid inverter - cheap batteries (forklifts? Not convinced batteries are cost effective/sustainable)
3. better quality dual inverter - on-grid / offgrid - if there is such a thing? Can retain when upgraded to 3 phase - or unlikely anything can be both single and 3 phase?

Back up for powercuts would be a 'nice to have' and may allow an 'enhancement' of budget but not sure a battery back up will do anything than a lighting circuit.  The other factor that may have relevance is the PV is on a workshop roof and approx 20-25m from inverters to consumer board, so there will be 'losses' across any cable direct to immerion. Also not sure if we will keep existing cylinder or use it as a preheat for the thermal store.  No heat dump rads at moment but plan to have them when we connect up the boiler stove (Brosley Evolution 26). We have a Elios 4 you set up (max 3kw) for grid tied - should this go on  the 'top' immersion and the 'direct/ offgrid' immersion to bottom when we install the new thermal store?

(Sorry very long post - just remembered we may opt for 2 no seperate cylinders rather than a single thermal store- incase it makes a difference to any of the replies.  Partly logistics of installation but also possibly seperate hw and heating systems.).

Suggestions welcome...
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« Reply #1 on: March 19, 2016, 10:54:34 AM »

Hi Smudger,

Please excuse the ramble.

All my experience is offgrid so my answers will be weighted in that direction, and I'm thinking of trying roughly the same thing soon.

Hugh Piggott has a design using relays and capacitors for 'direct' connect option.
(If you disappear down the rabbit hole that is his site for many hours don't blame me!) You could investigate a similar solution with a Tristar?

On/offgrid inverters are available. The most commonly available would be a UPS system. But the Sunny Island is a more renewables orientated option and I believe a couple of the other high end inverters like Xantrex have this capacity too. You can also bodge a cheap powerjack LF inverter into doing it.

Batteries. Nice to have if you get powercuts. Would live a long and happy life if kept at float the whole time and only used for lighting/phone charging in a power cut and you could buy a cheap <500w PSW inverter to cover all your lighting needs if you have CFLs or LEDs.

The only other way I can think of is a grid tied solution using a solarmiser/PVrouter?

The guy who came up with the PVrouter this is a very clever chap by the name of Robin who I believe frequents here occasionally.

My solution to the same problem (opportunity) is.

Trick a GTI inverter into syncing with battery inverter. Use a voltage sensing relay (and maybe an AC current sensor) to switch an appropriately sized immersion heater on and off when surplus power is available. Also required in my case would be additional overcharge protection in the form of an additional dumpload/turning off the PV input so I don't boil the batteries when they are fully charged and the thermostat clicks off.

Sorry if this is zero help. But its something to chew over at least.


Outback FM60. EPsolar 30a MPPT and a bunch of Tristar 45's. Hodge Podge of solar ~ 4500w. Various generators and 1000ah 24v forklift battery.

Turkish Turnip
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« Reply #2 on: March 19, 2016, 02:30:14 PM »

 You could use a temporary copper cylinder for dhw.  It could be plumbed into the feed to the present cylinder.   Cylinders with no coils, just immersion,  are often removed  when  systems are updated.  Pre  cylinders have gone out of fashion  because they are a bit inefficient but  simple and inexpensive to install.

DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
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« Reply #3 on: March 19, 2016, 08:29:44 PM »

If i understand correctly at the moment you have 6.8kwp connected to 4.6 kw inverters and this 4.6kw is the max the DNO will presently allow.  If so, you do appreciate that all the DNO is interested in is the 4.6kw max going down the wires and has no interest in how much kwp of panels you have. Therefore you can connect all your panels,11kwp, to the inverters as long as they are restricted to the 4.6kw max output.  At first glance this sounds silly but remember that the panels never produce the rated kwp and during the winter months when you need the most leccy anyway the 11 kwp panels would rarely reach 4.6 kw. This could be further improved by mounting the panels on a vertical surface ie increasing winter output and decreasing summer output.You can see the effect of this by using

The problem will arise with how you approach the FITS as one is now unable to claim FITS for an extension i believe.  However i think you can prorata your production reading and still claim for the original FITS but i would like someone like TED and others to confirm this.

I would be wary of using a preheater tank because when i did this with solar thermal i was disappointed with the result which i put down to heat losses.

You do not need to worry about small wire losses do you?

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« Reply #4 on: March 19, 2016, 09:39:58 PM »

So options are:

a heatpump and a battery


1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
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