So now I have a solar PV system how do I make the most of it???

KevinG

EricMears wrote: »

It may be 'elegant', but relying upon light sensing can make it more difficult to use your 'free' electricity for something else. For instance, boil a kettle for a drink and you could be paying to do so unless you remember to go and turn off the automatic immersion heater connection.

The alternative being ... don't have a hot drink? I think this gets a bit silly, kettles are only on for a couple of minutes at a time and immersion heaters have thermostats anyway so are not on constantly.

EricMears

KevinG wrote: »

The alternative being ... don't have a hot drink? I think this gets a bit silly, kettles are only on for a couple of minutes at a time and immersion heaters have thermostats anyway so are not on constantly.

Kettle was just the first thing that came to mind. They may only be on for a few minutes but they could well be using 3kw whilst they're running. I don't think I suggested foregoing the hot drink; simply pointed out that you'd have to pay for it when it might have been free.

I might have used a vacuum cleaner as an example. Sometimes that's also 'just a few minutes' but could easily be much longer if you're doing a couple of rooms at the same time. Lots of modern vacs are well in excess of a KW.

What about popping the oven on for a Sunday lunch ? Or even a couple of hotplates to boil veg ?

The basic point is that there are a lot of appliances that can only run on electricity so if you're deliberately using the free supply for DHW you'll lose much of the benefit.

By all means use a light sensitive method for showing when the immersion could be used but it would be best to include an over-ride switch for the many occasions each day when you really need the power for something else.

Of course, a current sensitive switching method leaves you in control; switch on the oven and the immersion takes a rest, oven reaches working temp so immersion goes back on etc etc

Martyn1981

edwink wrote: »

Thank you so much for all of your help!!! I'll wait to hear from you!! I hope it is good news!!

Morning Ed.

Well, I've been having fun. Decided to download the SMA Sunny Design software and have a play. Been meaning to do this for some time - yes I am that sad.

Good news, to get a handle for how it works, I ran my two systems through it, and it gave both of them top marks for energy suitability. Phew, that means the nice guys I bought pasties and doughnuts for knew what they were doing.

So confident I knew what I was doing (well about 50% confident - one eyed man in the land of the blind stuff here) I tried your system.

Possibly bad news, didn't even enter different roof angles, simply tried to split the system 5 + 5. It didn't like that, and suggested losses "there will be a drop off as the PV voltage is lower than the minimum DC voltage of the inverter at the selected grid voltage".

Don't panic, at this stage, assume the problem is me (untalented amateur confusing things), not your system. I'm trying to refine my skills and get some more help. I'll be back!

For now keep monitoring generation to get a feel for performance, and comparisons to any similar installs nearby.

Anyone else out there know enough to help? From Ed's description of the wires, it certainly sounds like two strings.

Mart.

sly_dog_jonah

EricMears wrote: »

Of course, a current sensitive switching method leaves you in control; switch on the oven and the immersion takes a rest, oven reaches working temp so immersion goes back on etc etc

The current-sensing method for the device I linked to yesterday can only be used to measure generation, not consumption. Therefore it won't turn off the immersion when you turn the oven on. Current clamps can't detect direction of energy flow, even if you have a cable where you can detect consumption. On the PV cable though, reverse power is only ever very low current (when the Inverter is in standby) so this will not trigger the current-sensing switch.

You set the threshold for the switch to whatever level you choose. Set it at 1kW and anything else you use in the house will cause electricity to be imported while the immersion is on. Set it at 3kW and you'll always have 2kW 'spare' for other appliances when the immersion is on, but you'll limit the 'heating window' considerably as few systems will kick out >3kW for very long each day on average. Somewhere inbetween there will be a sweet spot where the risk of import is reduced, and excess export is also minimised.

In terms of manual overriding, my idea is for a 3-way rotary switch to allow a householder to control the immersion controller as follows:

• Position 1: 'Sensed On' - the Immersion will be powered whenever the PV generation exceeds a set value, eg 2.5kW (until the thermostat setting is reached, or generation drops again)
• Position 2: 'Always On' - the immersion is powered irrespective of PV output (bypassing the current sensing switch) and stays on until the thermostat temperature is reached. This covers the scenario where your boiler packs up and can't heat water.
• Position 3: 'Off' - the immersion never comes on - this can be used if you're cooking a meal during the daytime. If you're going on holiday this can reduce wasted heating.

According to my somewhat rusty electronics knowledge, this would be pretty easy to adapt into the circuit diagram I linked to previously.


Of course the immersion circuit can also be isolated at the fuse box (if it's on a dedicated MCB), and switched off manually in the airing cupboard (if there is a switch there). Our setup has both override options to turn it off. In fact this means that we wouldn't need the 3rd position for the switch above, assuming we are happy to walk to the airing cupboard to turn off the immersion supply.

sly_dog_jonah

rogerblack wrote: »

Certainly - it's possible to wire another switch across the relay.
It would be prudent to find an illuminated switch that could indicate the heater circuit was live, even when 'off' of course.
And you need to make sure that any electrician is informed of this, otherwise it may be dangerous.

Our immersion switch in the airing cupboard has an indicator light.

No self-respecting electrician would work on a circuit without isolating power at the fuse box.

sly_dog_jonah

pwllbwdr wrote: »

Of the two approaches (current sensing and light sensing), I think the latter is the more elegant as it doesn't involve fiddling around at the fuse box.

For me the situation is more straightforward as I don't have gas. The difference between oil heating and peak imported electricity is much less than with gas. So I think I'll just (!) get a 1kw immersion and run a time to have it come on during daylight hours in summer. That system will only be on when the CH boiler is off.

Wherever you connect the controller, you'll need to do 'fiddling' - or rather an electrician would. The fusebox is ideal for current sensing as the Solar PV cable and Immersion circuit (if on a dedicated circuit) are co-located.

Out of interest how much is oil per kWh compared to gas?

On a grey day in summer you'll be heating water with your immersion, which could cost around 7kWh depending on the size of the tank, insulation, start temperature and thermostat setting. That's about 70p/day by my reckoning. How would oil heating cost compare?

EricMears

sly_dog_jonah wrote: »

The current-sensing method for the device I linked to yesterday can only be used to measure generation, not consumption.

Afraid I didn't see that link from yesterday.

My Wattson meter is set up to measure (separately) generated power - using a clamp-on sensor inside the isolator box for the solar panel system - and used power - using similar sensors on (a) the supply from the non-RCD to the RCD side of the consumer unit and (b) from the non-RCD side of CU to a junction box feeding several circuits.

(it's a standard Wattson function to add the two consumption figures together and deduct generation current)

That gives a useful indication of 'nett power' by keeping the meter itself in the kitchen but afraid it's not automated. The standard Wattson software ("Holmes" ) can offer those figures to a computer but only historically rather than in real time so it's not really very useful. Apparently there are ways around this but they generally involve running Linux operating systems and a Wattson application in 'C'. If I ever get round to installing that sort of system, it would be a very simple next step to use a computer controlled relay to switch immersion heater on when there is surplus power available.

zeupater

Martyn1981 wrote: »

Morning Ed.

Well, I've been having fun. Decided to download the SMA Sunny Design software and have a play. Been meaning to do this for some time - yes I am that sad.

Good news, to get a handle for how it works, I ran my two systems through it, and it gave both of them top marks for energy suitability. Phew, that means the nice guys I bought pasties and doughnuts for knew what they were doing.

So confident I knew what I was doing (well about 50% confident - one eyed man in the land of the blind stuff here) I tried your system.

Possibly bad news, didn't even enter different roof angles, simply tried to split the system 5 + 5. It didn't like that, and suggested losses "there will be a drop off as the PV voltage is lower than the minimum DC voltage of the inverter at the selected grid voltage".

Don't panic, at this stage, assume the problem is me (untalented amateur confusing things), not your system. I'm trying to refine my skills and get some more help. I'll be back!

For now keep monitoring generation to get a feel for performance, and comparisons to any similar installs nearby.

Anyone else out there know enough to help? From Ed's description of the wires, it certainly sounds like two strings.

Mart.

Hi

The system was described as having 'two black cables' from the inverter to the DC isolation switch but 4 cables running to the panels which suggests that, unless the inverter to isolation switch cables are twin core, the two panel groups are wired as one string of 10 panels within the isolation switch ....

If the above is correct, depending on how the circuits have been wired, you will either have two (6+4) parallel DC circuits or a single (serial) 10 panel DC circuit. If the unbalanced circuits are in parallel I would have thought that there will be a voltage difference which would likely effect/negate the power contribution from the smaller circuit and reduce the overall output .... it would be advisable to check this out with a qualified and experienced installer, but this would seem to explain the system as described and, if parallel, the low performance.

HTH
Z

sly_dog_jonah

EricMears wrote: »

Afraid I didn't see that link from yesterday.

My Wattson meter is set up to measure (separately) generated power - using a clamp-on sensor inside the isolator box for the solar panel system - and used power - using similar sensors on (a) the supply from the non-RCD to the RCD side of the consumer unit and (b) from the non-RCD side of CU to a junction box feeding several circuits.

(it's a standard Wattson function to add the two consumption figures together and deduct generation current)

That gives a useful indication of 'nett power' by keeping the meter itself in the kitchen but afraid it's not automated. The standard Wattson software ("Holmes" ) can offer those figures to a computer but only historically rather than in real time so it's not really very useful. Apparently there are ways around this but they generally involve running Linux operating systems and a Wattson application in 'C'. If I ever get round to installing that sort of system, it would be a very simple next step to use a computer controlled relay to switch immersion heater on when there is surplus power available.

Ah yes the old chestnut of taking multiple current-sensed values, and doing some arithmetic in real-time to work out what your consumption and export is. I'm yet to see a solution for this that doesn't require a great deal of h/w and/or a PC to do the control (although it should be possible to program such a simple operation into a programmable IC). A difficulty is that many home's electrical layout don't allow consumption to be measured accurately - there simply isn't a single cable available to clamp around.

Martyn1981

zeupater wrote: »

Hi

The system was described as having 'two black cables' from the inverter to the DC isolation switch but 4 cables running to the panels which suggests that, unless the inverter to isolation switch cables are twin core, the two panel groups are wired as one string of 10 panels within the isolation switch ....

If the above is correct, depending on how the circuits have been wired, you will either have two (6+4) parallel DC circuits or a single (serial) 10 panel DC circuit. If the unbalanced circuits are in parallel I would have thought that there will be a voltage difference which would likely effect/negate the power contribution from the smaller circuit and reduce the overall output .... it would be advisable to check this out with a qualified and experienced installer, but this would seem to explain the system as described and, if parallel, the low performance.

HTH
Z

Thanks Z. I think it's fair to say that I've run out of at talent at this point.

The 4 cables into the isolation switch, then 2 into the inverter, seems a little odd. Still no better than an absolute beginner on Sunny Design, but it wasn't happy with 5+5 or 6+6. In fact 7+7 was still too low a voltage, thought this did at least get a suitability score sadly it was 45%. 8+8 got 93%.

Like you, I also suspect that the isolation switch is being used to connect the two sets of panels as one string of 10. In order to get a high enough voltage to operate the inverter. The less suitably aligned panels then acting as a brake on the system.

Ed, at this stage, depending on your relationship with the installer, it might be worth just waiting and getting some more time under your belt. Though not fun, the system is working, and it may take a month to get enough comparative data. Unless of course you can find a nearby system with a similar orientation. Size doesn't matter (stop sniggering!), as you can just proportionalise output.

Anyone with recommendations?
BDPV, PVOutput etc?

Hopefully, there is absolutely nothing wrong, but I'm struggling to understand how a single string can operate across two differing roof angles (let alone potentially differing shading). Fingers crossed the fault lies entirely with my limited knowledge, not with the system.

So give it a bit more time, I'll keep digging, as I'm enjoying learning more about strings, inverters etc. If things still look wrong then it's time to have a chat with the installer.

You could also in the meantime, contact SMA, here's the site, send them an e-mail and ask for their opinion on the suitability of a single inverter for 2 roof pitches.

http://www.sma.de/en

Mart.