Solar power during a power cut - could this work?

[winston1]

I think the biggest flaw in this is that power cuts rarely happen in periods of good weather with bright sunshine.
They invariably happen at times like the present weather and often at night.

 

[bernardgreen]

The turbine then sees no load and shuts down after a time. The site therefore still has to maintain a backup generator. Trying to explain this to the MD was quite a task. When power is restored, the 11kV RMU is electrically switched, the turbine sees a supply/load and begins to generate again.

If the MD was prepared to give up the income from the Feed In Tariff and install a fairly simple load allocation system then his turbine would be able to provide power during a power cut during windy weather.

When power is available power from the turbine is allocated to as much of the site as the available power can feed and the rest of the site is fed from the incoming mains supply.

 

[westie101]

The other issue with "islanding" generation from the grid is getting it back in syncronisation following restoration of normal supplies.

It can be done where there is good control of speed on say diesel generators but on other types it is difficult or impossible to re-synce to the grid. This would mean a short shut down to change over

 

[bernardgreen]

If the local source and local load are an "island" then there is no real need to run in synch with the supply network. If there has to be supply transferred to or from the "island" then using DC for the transfer would remove the need to run in synch. ( as in fact happens between France and England power transfers and many other places )

 

[AdrianUK]

You'll probably find that the 'cheap' grid tie inverters that are used on 'domestic' solar installations are not capable of 'free running'.

A simple technique which can be used to place power from a DC source back onto the mains uses forced communtation of thyristors. This method doesn't require a clock - the stiff source provides that. Take away the AC source and the thyristors simply can't fire - the design is incapable of 'free running'.

 

[JohnD]

but what sort of AC source could you use to make it work?

 

[AdrianUK]

but what sort of AC source could you use to make it work?

A stiff source that acts as an infinite bus - ie. the mains.

Lets not forget the subtle difference between exporting power back into the grid & running a local load....

When sync'd onto the grid... the grid is 'large' compared to your inverter.... it will happily cobble up every single Watt that you can throw at it. The inverter 'decides' how much power it can export - it always runs 'full throttle' as it were.

That's not the same when supporting a local load - the load decides how much power it needs, not the inverter. The inverter needs to adjust its power output as the load demands. Inverters designed to be grid tied cant normally do this, why should they, they don't need to, they always run at 'full power'.

 

[ban-all-sheds]

You'll probably find that the 'cheap' grid tie inverters that are used on 'domestic' solar installations are not capable of 'free running'.

But you wouldn't want them to.

FIT contract Ts'n'Cs aside, technically there is no reason why you could not have a changeover switch between the solar array and two inverters. One a grid-tie, the other a normal one which supplies your house. Would need to be a multi-pole one which also disconnected your house from the grid.

But for dealing with occasional outages, rather than going completely off-grid, a small genny has got to be more cost effective than a solar array and a bank of batteries.

Anyone heard how they are getting on with alternatives to batteries for storing power from solar/wind/etc sources? ISTR they were trialling something in Scotland using thermal storage, or hydrogen fuel cells... I know that using phase-change materials to capture and then release thermal energy looks to have a lot of potential.

 

[JohnW2]

That's not the same when supporting a local load - the load decides how much power it needs, not the inverter. The inverter needs to adjust its power output as the load demands. Inverters designed to be grid tied cant normally do this, why should they, they don't need to, they always run at 'full power'.

You say that - but, provided there is voltage regulation, isn't it inherent in all inverters (indeed all 'power supplies') that the 'power output' will necessarily be determined by the size of the load (from zero up to the maximum it can service)? If, as with the grid, the load in 'near infinite', the inverter (or PSU) will, indeed, work 'at full power' (in relation to the source power available to it) but if there is a modest finite load, then voltage regulation will dictate that it runs at lower output (and input) power, won't it?

Kind Regards, John

 

[ericmark]

We also wondered if a grid tie inverter could be used to boost the output of a stand alone inverter.

But the problem is the grid tie want to deliver all the power it can. So if the load was 1kW and the grid tie can produce 500W then it could work. But if the grid tie can produce 501W then the voltage would rise and it would auto switch off.

So with say a 3kW base inverter you may be able to us a grid tie to boost that to 4kW as long as base load never drops below 1kW.

There are it seems some special grid tie inverters for when the supply can't cope with the load and they will use a battery to supplement the mains supply so you can use 4A from the mains and 10A off the inverter when required and when the load drops it then recharges the battery.

I see on holidays to Turkey many generators and leads with two plugs but clearly this is dangerous both forgetting to switch off the main isolator first and picking up a plug which has power on it. Also very easy to overload the generator.

Using a single plug and two sockets works far better. All stuff required in an emergency comes from a consumer unit it's self powered from a socket on the main consumer unit. In the event of a power cut you can unplug from main consumer unit and plug into generator instead.

 

[ban-all-sheds]

 

[JohnW2]

Using a single plug and two sockets works far better. All stuff required in an emergency comes from a consumer unit it's self powered from a socket on the main consumer unit. In the event of a power cut you can unplug from main consumer unit and plug into generator instead.

Indeed, that's a variant of what I have here. The generator supply feed dedicated rudimentary lighting throughout the house, plus a few scattered sockets (neither ever used with grid power), hence avoiding any change-over switching. The CH system (boiler, controls etc.) is normally plugged into a socket from the normal CU. Next to that socket is a generator-supplied one - so, if it's ever necessary to fire up the genny, one only has to move that plug from one socket to the adjacent one to get the heating going!

Kind Regards, John

 

[westie101]

I'm glad talk has turned to sensible change-over concepts.

I can still remember after "Hurricane Fish" in 1987, when working near Petersfield near the south coast, the shock that went through the Electricity Board staff when we were told of the linesman that had dies as a result of a lash up of a generator connection. There was even talk of all the imported staff just walking off the job and leaving the folk in the area to their electricity less fate!

 

[Lectrician]

The turbine then sees no load and shuts down after a time. The site therefore still has to maintain a backup generator. Trying to explain this to the MD was quite a task. When power is restored, the 11kV RMU is electrically switched, the turbine sees a supply/load and begins to generate again.

If the MD was prepared to give up the income from the Feed In Tariff and install a fairly simple load allocation system then his turbine would be able to provide power during a power cut during windy weather.

When power is available power from the turbine is allocated to as much of the site as the available power can feed and the rest of the site is fed from the incoming mains supply.

No. Not in the configuration requested by the DNO. Wouldn't be possible. It's also not a good idea trying to use a supply which is variable to feed portions of a site depending on the varying output.

A generator can give a fixed and predictable output.

The generator was there prior to the turbine. The cost implications of trying to use the turbine during a power cut would not be viable.

 

[EightyTwo]

Anyone heard how they are getting on with alternatives to batteries for storing power from solar/wind/etc sources? ISTR they were trialling something in Scotland using thermal storage, or hydrogen fuel cells... I know that using phase-change materials to capture and then release thermal energy looks to have a lot of potential.

I heard that someone was using kinetic energy. They had built flywheels in evacuated cylinders about the size of an oil drum. They spun them up to something like 10,000rpm, if I remember correctly, and then stored them. Because the flywheel is in a vacuum, they stayed spinning at really useful speeds for a long time. I wish I could remember more of the details... I heard it on R4 within the last year.