PowerSwitch

We've had our off-grid PV two years now (details here). In the first year I was quite hesitant about how much to run off it, not wanting to damage the batteries by discharging them too deeply. In the second year I found I could easily run our fridge off-grid from Spring to Autumn, running overnight off the batteries without draining them very much, and the difference showed up in our mains electricity usage. We also occasionally took the freezer off grid too, though usually only in daylight hours. I'm considering my strategy for this year, and would appreciate some opinions...

Here's what we have:
- solar thermal and internet router run off a 180W pure sine inverter all year. They are mostly draw power in daylight. This inverter is switched off when we go to sleep, and back on in the morning. It is also used to power a laptop sometimes.
- 350W pure sine inverter wired to run the fridge, but fridge plug has to be switched from one socket to another to do this.
- 700W modified sine inverter wired to run the freezer, again needing to switch the plug over manually.

The upside of what we did last year with the fridge was that it was minimal hassle, no need to move the plug from one socket to another. The downside is that power drawn from the battery overnight is less efficient than power drawn in the day. I was considering taking both fridge and freezer off-grid, but only for about 12 hours including all daylight. This would be more efficient, and save money too as we have Economy 7, but is a bit of hassle with all the manual plug switching. (We're careful not to switch them while the appliance compressor is running, to avoid damage)

There IS another option though. I've seen relatively cheap (£70-100) grid-tie inverters that can simply plug into a mains socket: http://www.ebay.co.uk/itm/160784470002. So, I was wondering about getting one of these, so all the PV output feeds into the mains, and using my mains battery charger to keep the batteries on float in case of a power cut - at which point I would throw a couple of switches and move some plugs to take the whole thing off-grid again.

What would you do? It'd probably enable us to knock £10-20 a year off the electricity bill, so would take a while to pay back.

Is it possible to put the fridge on a timer so that it runs more during the day (without compromising the temperature)? You might have to experiment a bit to get the best timings.

Also, are you thinking about how much (if anything) you get paid for feeding your PV o/p into the mains?

RenewableCandy wrote:Is it possible to put the fridge on a timer so that it runs more during the day (without compromising the temperature)? You might have to experiment a bit to get the best timings.

Also, are you thinking about how much (if anything) you get paid for feeding your PV o/p into the mains?

We'd get no FIT, as it is a DIY system - it would just be avoided consumption - effectively what we get now with it off-grid, just done through the house wiring instead.

If we tried the timer thing, we'd probably do it with the freezer rather than the fridge, though the inverter would still be drawing about 6W when the freezer was off.

If energy is put into a battery and then used later a proportion of this energy is lost as the battery has losses.
Also repeated cycling of the battery may* reduce its life.

Losses and battery cycling may be reduced whilst times are normal by use of PV in the day and grid power at night.

Put the fridge and/or the freezer on PV during the day and on grid power at night. I would automate this by means of a timeswitch.
Keep an eye on the state of charge of the battery and if this be lower than desired then simply use grid power all day until it has recovered.

There is little point in displacing cheap economy 7 electricity with PV, better by far to use the PV when grid power is expensive, and to use grid power when it is cheap.

Thanks Adam - I usually reckon you can lose up to 1/3 of the power by going in and out of the battery, which is why I'm keen to avoid it.

I'm interested in your mention of using a time switch to move the fridge/freezer between mains and off-grid - is there anything off the shelf that can do this?

A switch setup would have to compare well price-wise with that £75 plug-in inverter to make it worth it. Unless such inverters are unreliable? I realise they are made in China and not branded...

I would be very doubtful about £75 chinese inverters, also your electricity meter may measure power flow REGARDLESS OF DIRECTION that is if you had no load but were generating and backfeeding 200 watts, the meter would record this as 200 watts consumed !

Do you have any circuits that are energised ONLY during economy 7 hours, being controlled by the electricity companies timeswitch ? If so a changeover relay could be used energised by this supply.

Alternatively wire a dedicated circuit for the freezer and fridge.
Power this circuit via a heavy duty changeover relay such that it may be powered by inverter or grid.

Wire as if for mains failure protection
Grid present=load supplied by grid.
Grid failed=load supplied by inverter.
Now connect the relay coil not direct to the mains, but via a timeswitch.
When the timeswitch is "off" then the relay "thinks" that there is a power cut and transfers the load to to the inverter.

Adjust the setting of the timeswitch as needed so as to fully utilise the available PV.
Take care that the time switch is allways on during economy 7 hours.

The relay coil consumes energy, but not much, and such as it does consume is largely during economy 7 hours.
The timeswitch contacts only handle the small current used by the relay coil and should therefore have a long and reliable life.
Any failure of the timeswitch or of the relay wont leave the fridge or freezer without power, though the power might be from the wrong source until the fault is rectified.
If a power cut occurs then the load is automaticly transfered to the inverter regardless of the time of day.

This arrangement does however subject the appliances to a brief interuption when changing over, this is liable to kill compressors. It would therefore be advisable to fit each appliance with a fridgegaurd or similar.

Thanks Adam - that's all beginning to sound like a lot of work/expense...

If I knew someone with one of these plug-in inverters I could borrow it and see what happens! Maybe I'll just keep an eye out for a second-hand one on ebay, or I guess I could buy a new one and return it if it doesn't actually work...

In the meantime I think I'll try switching both fridge and freezer off-grid manually during peak rate hours, and see how my meter readings compare to last year.

I'm about to order the components for our DIY grid-tie system (600Wp). I was also looking at the cheap ebay-sourced inverters, but have decided to go for a Mastervolt Soladin 600. These are also "plug and play", have the full certification (including "islanding"), but seem to be a more established, known quantity. Navitron are selling them for £250 plus VAT.

We'll be using 8 x 80W 12v panels, configured in 2 x 48v strings (The Soladin 600 needs 48v input to run), but this would allow us to readily reconfigure the panels into a 12v off-grid system in the event of long-term grid-loss. I anticipate we'll use much of the output to cover our base-load from fridge, freezer, laptop, etc. However, our meter is one of the old types without the little "ratchet" symbol, so I guess it will run backwards if we generate more than we use. The panels will be on a movable, free-standing frame in the garden to catch the best of the sun and avoid listed-building planning hassles.

Separately, we'll have a 12v deep-cycle battery bank, trickle charged from the mains, with an inverter for temporary power-outages. This equipment can be re-purposed and connected to the re-configured panels through a charge controller if we decide we need to go off-grid.

We're going down this route because I don't really want the complexity of dual off-grid and on-grid circuits in the house during BAU, want to get a financial return on the installation, but also have a long-term off grid system if SHTF. (In which case having temporary off-grid circuits strung up around the house would be better than no electricity at all!).

I'm estimating the grid-tie system will cost about £1,000 to put together. If we use all we produce, we should achieve a return in excess of 10%, plus pay back the capital cost over the life of the panels. If it works, I'll build an additional one next year.

Ah, I'd forgotten that Mastervolt one... It's plus VAT though, so £300 from Navitron. A bit cheaper here: http://www.solar-pv-systems.com/soladin ... rrency=GBP

It does look like a better product, especially the 100ms cutoff, which is fast enough for someone pulling the plug out. It'd be over-specced for our current 240W of PV, but I've been wondering about upgrading this to 480W, so that would be a better fit. Interestingly, it would also be cheaper than the Morningstar MPPT charge controller I would have got to handle the upgraded panels...

Though I suppose for power cuts I'd still want this, as it can handle the same voltage input range as the Mastervolt: http://www.morningstarcorp.com/en/tristar%20mppt

And cheaper still here, though out of stock at present: http://www.cromptoncontrols.co.uk/produ ... oladin-600