Grid Tie / Battery Question

[DominicJ]

I was just curious, is it possible (well, I know its possible, is it cost/legaly feasible) to have a grid tied solar/wind/whatever system that, under normal circumstances sells excess and buys shortages of power from the grid and in the event of grid failure, reverts to a stand alone system.

[Andy Hunt]

I think it can be done but is VERY expensive, you need 2 inverters and some fancy switching equipment. Plus the fact that if your batteries are not in regular use, when they come to be needed they might not work.

You might be interested in my setup, it is one step away from what you describe, can be switched to take its power from the mains in the winter and acts as a UPS in a power cut, and makes the most of the sun in the summer.

http://www.thebiggreenidea.org/news/new ... kup-system

It's not the most efficient use of the solar PV or the batteries but it's a damn sight cheaper than the system you are describing, which is also inefficient in fact.

[adam2]

A system such as the one linked to above has much to commend it since it can use PV, and supplement this with grid power.

Apart from the cost, the main drawback of such a system is that it is not fully automatic. The user must monitor the state of charge and select grid power when the battery is low.
If the battery is allowed to get too low before charging then its life will be reduced, and little stored energy will be available if the grid fails just when mains charging was being started.
If the grid input is left on all the time then the battery be allways be full, but no use will made of the PV input.
Still well woth considering though.

Some years ago I installed a somwhat similar, but not identical system.
This consisted of a large 24 volt battery, charged by a PV array.
The battery supplied a 3KW inverter.
All esiential loads were connected to a changeover unit that could be set for
1) grid power 24 hours a day unless it fails in which case inverter is selected automaticly.
2) grid power during "off peak tarrif hours" and inverter power the rest of the time.
3)inverter power continually.

In normal summer weather, position 3 was used all summer thus powering all small loads entirely off grid.

In dull weather, if the battery dropped below about 80% charge, then position 2 was selected, thereby transfering the load to the grid when grid power is very cheap.

Following a power cut, or in prolonged dark weather, position 1 would be used, load entirely grid powered, with the entire PV output slowly recharging the unloaded battery bank.

This worked well but was rather complicated and expensive.

[Andy Hunt]

A good summary of the drawbacks of my system adam2, all valid points, but I'm still quite happy with it to be honest.

little stored energy will be available if the grid fails just when mains charging was being started.

As we tend to only use the system in 'off grid' mode in the summer, the hope is that the PV will make the batteries run down relatively slowly. We currently have 330Wp, soon to be 495Wp which should make a difference.

The other thing is that power cuts would be more likely in the winter (although not exclusively so of course).

The inverter/charger I use is programmable for low voltage, so I have set it so that the batteries are not fully discharged when the warning 'beep' starts to sound. When I hear it, I switch over manually to mains charging using the appropriate switch on the consumer unit.

In the event of a power cut, the inverter/charger automatically switches over to batteries. On one occasion, we hadn't even noticed there had been a power cut until I went to the corner shop, where everything was in darkness. They told me there had been no power for several hours!

[contadino]

The inverter/charger I use is programmable for low voltage, so I have set it so that the batteries are not fully discharged when the warning 'beep' starts to sound. When I hear it, I switch over manually to mains charging using the appropriate switch on the consumer unit.

Could I ask which one your have Andy? I'm looking for something smaller than 1.5kw but with all the gubbins of the PowerMaster unit...

Ta.

[Andy Hunt]

The inverter/charger I use is programmable for low voltage, so I have set it so that the batteries are not fully discharged when the warning 'beep' starts to sound. When I hear it, I switch over manually to mains charging using the appropriate switch on the consumer unit.

Could I ask which one your have Andy? I'm looking for something smaller than 1.5kw but with all the gubbins of the PowerMaster unit...

Ta.

I have the 1.5kW Powermaster one, I don't think they do anything smaller with all the features.

Wasn't cheap at £900 either.

[contadino]

OK thanks. And if I attached a charge controller direct to the battery bank from a wind turbine in the future, it wouldn't be a problem..? Looks like I'm gonna have to break open the piggy bank.

[Andy Hunt]

OK thanks. And if I attached a charge controller direct to the battery bank from a wind turbine in the future, it wouldn't be a problem..? Looks like I'm gonna have to break open the piggy bank.

I doubt it would be a problem.

To be honest contadino, in retrospect I sometimes think I would have been better off putting in a low-voltage setup instead of running one of my ring mains off it.

The inverter itself uses a good chunk of the power supplied by the PV panels, and of course there was the cost of the inverter too.

I'm glad in a way we have the system we have, because it runs things like the central heating pump, solar hot water system etc etc in a power cut. But for someone with a room heater wood stove only, and a SHW system which has its own PV panel to run the pump, I think a 12V or 24V system running LED lights around the house, and mobile 'phone and laptop for communications, TV and internet, would probably be a cheaper and more efficient way of doing it.