Use of excess PV generation

[brackwell]

On the sma 4000TL the newer software includes settings for "Self Consumption" which allows power to be switched, via a built in Relay, to an Immersion heater or some other device under certain generation criteria. 

Me not being an electrician how do use this fact?   The AC goes from the inverter to the generation meter and after that its too late? or can you have more than one channel in a generation meter.  Do you need two generation meters or just forget the FITS on one of the outputs.

Assuming you have the bit of kit that is able to use just the excess (over and above house consumption) where in the system do you wire it?

As a example -you are generating 1000W and the house is consuming 500W and you wish to use the excess 500W for another purpose.  All AC goes to the consumer unit and after that one bit of electricity is much the same as another ie lost in the ether ? Puzzled

Ken 

[EccentricAnomaly]

Remember that this design might be appropriate to countries other than Britain. E.g., France where, as I understand it, you only get FiTs on actual export, not just generation, so it wouldn't matter how internally used power is routed.

Still, couldn't you just use the output from that relay to switch another relay which routes power on the other side of the generation meter?

[Fintray]

Assuming you have the bit of kit that is able to use just the excess (over and above house consumption) where in the system do you wire it?

Ken

If you have the said bit of kit that is able to use just the excess power it will be wired wherever you want to use the excess power as it should be measuring the total load along with the amount supplied from the grid and that produced by your PV to calculate if there is any excess.

[AlanIOW]

Hi Ken

The "Self Consumption" settings are not clever enough to know what the house is consuming, it just opens/closes a low voltage relay when the generation kW's is above a certain figure for a certain time. So something like, if the system is generating over 2kW for more than 20Mins the relay opens for 60Mins. Also as EA says the internal relay is low voltage which controls another external relay which can switch items that are after the FIT generation meter.

[Mike McMillan]

Is there anyway of using excess PV power instead of feeding back into the grid? Someone asked me this the other evening.

[brackwell]

EA and Alan,
Thanks for that it certainly answers the first part of the question but the 2nd part is a bit more difficult i feel.

If one minute you are producing surplus and the next deficit (over and above house consumption) then something needs to measure which one it is and if it is surplus to direct JUST the surplus to the device. This as been discussed here before and i am sure the bit of kit is now available to do this but where does it go? Maybe on the mains side of the consumer unit where it could detect import/export and if export throw a relay to switch something on eg kettle.  When the kettle switches off you need something else to switch on - and so on.  Dont know if the last bit is doable but it needs to be.   Thats well and good but if the kettle needs more power than is surplus the bit of kit will be constantly switching on and off.

Not easy
Ken

[Wickham]

With the sort of sky we usually have, which is part blue sky in open sun and scattered white clouds, the generation is 1kW one second and 2kW the next, continually fluctuating over short time periods, so this kit would be continually turning on and off a relay.

I can't see how it would work. If it switches the relay on for a set period as AlanIOW said, and the generation changes, what happens?

[RobertReadman]

Ok lets set a few things, if you have the settings so its on and off all the time you will reduce the life span of the Multifunction Relay in the SMA SB4000TL which is a minimum of 100,000 switching cycles, when adhering to the maximum switching voltage and maximum switching current.
At 24 switches a day that's only 10 years worth of use, if you have a 3kW immersion heater, and a 3.2kW system switching on and off as the sun makes the system peak, its easily going to be over 24 switches a day.

So that set's the life span, this means that you need a big system, or to reduce the load, putting a 2kW immersion coil in your heater is easy, it will just take longer to heat!

Lets say we want to switch our 2kW immersion heater on and off, when the system outputs 2.2kW or 2.4kW.

The built in multifunction relay can only switch 1A, which at 240V is only 0.24kW (or 240Watts) so what the MFR is designed to do is power an electrical contactor, which will then in turn switch the immersion.

First thing is to set the immersion heater up, you may have an analogue or digital thermostat for your immersion. So from your manual switch on the wall, a wire will go to the themostat, and from there to your immersion coil.
I have mine set to 55C, you can set yours to whatever temp you want on your thermostat range, usually 45C-75C. Once set your immersion should only switch on, when necessary to keep the water temperature at the set level (55C).

Next is to actually aquire your contactor, many are available and some come in the form of a standard module. Now is the important bit, you have the cable coming from your consumer unit, this should go to your contactor first, then to your switch, then from there the wire goes to the thermostat/immersion-coil.
The reason I think this is important, is that you can still switch your immersion heater on and off to safely set the thermostat in the future and have no risk of the contactor switch it on or off.

To sum up so far, Consumer Unit > Wire > Contactor(if module, then inside another small consumer unit) > Wire > Switch > Wire > Thermostat/Immersion.

The next is to connect the multifunction relay to the contactor, this depends on which contactor you have, for our 2kW immersion in this example it's a 240V 20A contactor, with a 12V coil (DC).
This means the contactor can switch on a load requiring up to 20A (4.8kW), and that power will come from the consumer unit, (i.e. AFTER your solar energy has gone through its generation meter).
The only consumption by the contactor will come directly from the inverter to open/close the switch at 12V.
This will look as followers, Inverter > Wire > Line Circuit Breaker > Wire > Contactor.

Once installed it's time to set the inverter.

Set the inverter multifunction relay to self consumption.

The MinimumOnPower by default is set to 1.5kW, we will change this to 2.4kW, so we know 400Watts over what the immersion would consume are spare for other items in the house.

The MinimumOnPowerTime by default is set to 30minutes, this means that the system should be doing over 2.4kW for 30minutes before our contactor will switch on the immersion heater. We can change this down 1 minute, but for my example I am setting it to 5 minutes, so it must be producing over 2.4kW for over 5 minutes before the immersion switches on.

The MinimumOnTime by default is 2 hours, so even if the sun is out for 5minutes, and it switches on, and then the sun goes in, the immersion will stay switched on, even if its pulling from the grid.
For this setting I have reduced to 15minutes, so that if the sun goes in and out, it will just stay on, just as long as the inverter does report a maximum power output of under 2.4Kw over the 15minute period  

From here on you now have your inverter, when it is producing over 2.4kW for 5minutes, switching on your 2kW immersion heater.
Now you can of course set the inverter to switch a 3kW immersion heater on when it gets over 1.5Kw, this would reduce the power required from the Grid. The inverter lets you set whatever settings you want.

Thanks for reading,

Rob.

[Laurence]

Robert's quite right, of course, but I have a simpler solution (having been tinkering with the darker corners of the 4000TL parameters only this evening).

Use the low power relay just to control a small (Low Energy, of course) light, behind an opaque screen with a picture of a smug git on it. Whenever the smug git's face can be seen, you can tell other electricity bill payers that they are subsidising your pocket.

An EVEN SIMPLER low-tech approach is to follow my gran's advice. When it's sunny, do your washing. She was right.

[M]

I'm probably going to start a fight and get shouted at by everyone, but I'll take a punt anyway.

Maybe I'm being a bit socialist here, but what's wrong with letting the excess go to the grid, isn't that a very 'we're all in it together' approach?

If the excess can be 'bottled' efficiently then fine, but if system losses are high isn't it more socially acceptable to send it round to your neighbours?

Please, please, please don't take this as an insult, I just wonder what is more efficient. If export was accurately measured can you get 3.1p worth of returns, or is it just the perception that all unmeasured export is technically free at the moment.

I have no agenda here, I'm genuinely interested.

Cheers

Martyn

[RobertReadman]

The way I see it, is that the FIT pays for generated electricity, grid connected or not, so even if your not grid connected, you get paid for what you generate.
You get paid 3.1p to export, and buying electricity back at 18.491p and that figure is going up all the time!
I paid for solar panels, I get the FIT, if I export I get an extra 3.1p, but at 18.5p to heat my water at night, letting all my electricity go to grid is a waste of money.

If you read back on my guide, I set the thermostat on the immersion at 55C. This means that even if the sun is out, and the contactor switches the immersion heater on, it WON'T consume because the water is hot enough.

On a fully sunny day in August I think I generated 22kWh. I am talking using 3-4kWh of that to the immersion, rather than exporting all day, then making the coal fired powerstation work harder at night to give me 4kWh to heat the hot water.

In countries like Germany where they have the FIT, they also reward for self-consumption, if setup correctly it fully reduces grid demand, and that is the main thing.

[billi]

In countries like Germany where they have the FIT, they also reward for self-consumption, if setup correctly it fully reduces grid demand, and that is the main thing.

they get 12-16 cent  for the self consumed unit  and 28,74  cent for  the unit feed back to the grid  at the moment

So the discussion over there is not as hot as here where  people  get the same (- 3 pence) if they self-use  the units  or feed the grid

I  suggest to get a Solar thermal install   , or a small DHW heatpump ( with  300 litre store)   for about 1200 pounds   on a timer during the day   and export the excess

And am with Martyn   that the FIT is something social as well

But i should shut up cause off grid

Billi

[M]

This is an interesting situation. I noticed that I wasn't be as well behaved when the sun was shining and PV was more than I needed. I'd started to take the attitude that it's mine and I'll use more even if I don't need to. I quickly made a mental note to stop being a prat. So I'm not judging anyone.

If you can use the power more efficiently than exporting it, then sounds great. Don't get me wrong, I try to switch on the washing machine, mow the lawns etc when the suns out. Just need to be careful that I'm not using leccy for the sake of it, just because it's there.

I guess the issue of using spare leccy for the sake of it will naturally be addressed in time with smart meters.

Just to take this discussion a bit further, does anyone know why the export price is so low? I appreciate that there are a lot of other cost factors resulting in a billed price of 12p or more, but 3.1p seems a tad low.

Martyn.

[RobertReadman]

I see what you mean, but I think the immersion thermostat is what stops energy being used for the sake of it, and in my method, you can still switch the whole thing off at your immersion switch even if the contactor switches, it won't go anywhere.
I think it's only 3.1p because the company already pays the FIT to you, so take advantage of that, I have read that some people negotiate the 3.1p higher, but usually involves getting your FIT from one supplier, and selling the electricity to another, but without an export meter, you really can't do it that well.

[EccentricAnomaly]

Just to take this discussion a bit further, does anyone know why the export price is so low? I appreciate that there are a lot of other cost factors resulting in a billed price of 12p or more, but 3.1p seems a tad low.

That seems to be around the wholesale price of electricity. E.g., this year-old article http://www.prnewswire.co.uk/cgi/news/release?id=291989 says that a peak price had been reached:

Power delivered between 07.00h and 19.00h on Monday was last traded on Friday at GBP56.25/megawatt hour (MWh), its highest closing price since 12 February 2009, according to ICIS Heren historical data.

AIUI, that's the price DNOs would be paying generators for electricity, give or take national grid transit charges.

Obviously, £56.25/MWh would be 5.6p/kWh so 3.1p/kWh for full time (not just peak) doesn't seem too bad.

It's a big pity that standing charges were dropped (well, wrapped into the usage) as it's harder to separate out the markup from the DNOs and suppliers.