A couple of dumb questions...

[Alan Ambrose]

(1) PVGIS seems to give me 67% power output for North (vs. South) for Suffolk. Is that really right?

 

                   Azimuth           kWh p.a. / kWp      % vs. S

Westerly        125                 780                          74%

Easterly         -55                 986                          93%

Dead North                          703                          67%

Dead South                       1,057                        100%

 

This is for 20% roof slope.

 

 

(2) What happens if you don't draw the max available power from your panels at any time? And you don't actively divert it to heat hot water or send it back to the grid?

 

Does it end up as heat at the inverter or maybe in the panels themselves? Or something else?

 

 

 

[billt]

With very low roof angles that's probably correct.

 

As a general rule the inverter should reduce the current drawn from the panels to match the load. IOW energy is not collected if there is no load for it to go to.

[Drellingore]

9 minutes ago, Alan Ambrose said:

What happens if you don't draw the max available power from your panels at any time?

 

I'll be interested to read the answer to this from someone better informed than I!

[S2D2]

13 minutes ago, Alan Ambrose said:

(1) PVGIS seems to give me 67% power output for North (vs. South) for Suffolk. Is that really right?

Seems reasonable, 20° is a very low slope so will be in view of summer/midday sun. Indeed if you have a 0° slope you'll get 100% of South facing energy by facing them North 😉

[Radian]

2 minutes ago, billt said:

With very low roof angles that's probably correct.

 

Yes, 20^o from horizontal is flat enough to get some direct irradiation in the summer. Even in the winter there's a small amount of power from indirect illumination typically from cloud bounce.

 

17 minutes ago, Alan Ambrose said:

(2) What happens if you don't draw the max available power from your panels at any time? And you don't actively divert it to heat hot water or send it back to the grid?

 

Does it end up as heat at the inverter or maybe in the panels themselves? Or something else?

 

It's called potential energy for a reason, i.e. no power is transferred without a load. Just as you electricity meter doesn't 'spin' when nothing's plugged in to your sockets.

[SteamyTea]

5 minutes ago, Radian said:

Yes, 20^o from horizontal is flat enough to get some direct irradiation in the summer. Even in the winter there's a small amount of power from indirect illumination typically from cloud bounce.

 

It's called potential energy for a reason, i.e. no power is transferred without a load. Just as you electricity meter doesn't 'spin' when nothing's plugged in to your sockets.

Yes.

No load, no problem.

 

The only time the inverter gets really warm is at maximum power. So when pumping back into the grid in June.

Why you want the inverter in a cold place with plenty of airflow. Not stuck up in a loft.

[Drellingore]

I'd be interested if anyone knows what the physical answer to question two is. Electricity is push rather than pull, right? Appliances don't 'pull' power, you open a circuit and the charge difference on the other side of that circuit then makes electricity flow through the appliance. I probably incorrectly imagine it as the appliance is dammed off from a river pushing water against the dam.

 

In the case of solar PV, what happens when a photon hits an electron if that electron doesn't have anywhere to 'go'? Is there some sort of buffering done by the inverter, or does the electron not get 'knocked off' in first place?

[joth]

32 minutes ago, Alan Ambrose said:

 

 

(2) What happens if you don't draw the max available power from your panels at any time? And you don't actively divert it to heat hot water or send it back to the grid?

 

While the other replies are technically correct In most installs, there's no such thing as "actively" choosing to send it back to the grid. You connect the "grid tied" inverter to the consumer unit, and it generates as much AC as it can from the solar radiation available. The inverter doesn't care where that goes, whether it's either into your kettle or your neighbour's. At the end of the day all the sockets and appliances on your street are directly wired to one another via the mains supply down the street.

 

The only way to stop energy export is to have a non grid tied inverter (meaning your household electrics are not directly on the grid) or install active export limitation. Both of these require most equipment (so more installation cost) than just hooking up grid tie and letting excess go out to the grid. But yes, if you do this the unused solar power goes same place as it would have if you didn't have PV panels in the first place.

 

[billt]

An electrical circuit needs to be complete for current to flow. If your PV panel is not connected to anything the electron doesn't go anywhere as it has nowhere to go.

 

When the panel is connected to an inverter the inverter can, in effect, disconnect the panel from the load if there isn't enough load. It actually continuously varies the load presented to the panels.

[SteamyTea]

@Drellingore

 

It is all to do with exceeding electron shell energy levels and electron 'holes' in the atomic structure.

 

If you can imagine a book shelf and you are hurling books into it, some will hit the wall and bounce back (reflection), some will be too high and miss (jumped up 2 energy states/shells), and others will be too low and fall to the ground (below shell potential).

About, in perfect conditions, 40% will land nicely on the shelf. These are the photons that are in the Goldilocks zone.

Now the real fun starts.

As a photon is easiest to model as a mass less particle at the speed of light, and a massive particle at rest, two different physical principles have to be used.

When the photon hits the silicone it has to instantly change states and become a particle with mass. That mass then has to act like a snooker ball and knock an electron out of place. Sometimes this works and the now free electron has to find a hole to fill.

This happens about 50‰ of the time. That, and the 40% earlier, account for the efficiency of around 20‰ efficiency of PV.

When it does not happen, because the electrons have mass, and are moving, they have energy. There are two rules here, momentum and kinetic energy. Depending on the speed and angles involved, some will heat the module, reducing efficiency, but importantly, reradiating photons (why the module mass does not increase, apart from birdshit and wood smoke particles). 

Some of the particles will easily find a hole to make a temporary home in, others will have to travel further away.

Luckily the silicone is doped with other elements that means, in the lowest energy state, there is an excess of holes. On average, when the sun is shining, there is deficit of holes, so spare electrons find their way out via the wires and to any load in the circuit. If the module is unconnected, it acts like any mass with an energy input and rises in temperature. A similar cycle happens in the load, but in reverse. Why your light shines and your motor turns.

As for the 'direction of flow' this is where the very poor analogy with mechanical plumbing breaks down, it was never a good one anyway.

Electrons have a negative charge, empty atomic shells (the ones with holes in them) cause the atom to become positive.

This creates a 'field' that, like a magnetic field, can do work, but only where the field is disturbed I.e. the load.

 

So basically photons dislodge electrons, electrons then find a new hole, the ones that can't find one locally keep moving, and will eventually do some work, while re-emitting photons.

 

This is only a partial model of what happens. If it was easy to describe, it would not be worth a PhD.

Edited February 8, 2023 by SteamyTea

[ProDave]

1 hour ago, Alan Ambrose said:

(2) What happens if you don't draw the max available power from your panels at any time? And you don't actively divert it to heat hot water or send it back to the grid?

If it is a grid tied inverter, then the inverter WILL produce as much power as it can and any not used within the house WILL be exported to the grid.

 

Are you talking of a totally off grid system?  Probably with batteries?

[Drellingore]

Thanks for the high-effort response, @SteamyTea!

[SteamyTea]

6 minutes ago, Drellingore said:

Thanks for the high-effort response, @SteamyTea!

OK, I was sitting in the sun and getting warm, even though it is only 5.5°C.

Now time to load up with half a kilogram of carbohydrates and animal proteins.

[Alan Ambrose]

You guys are fantastic

 

>>> Are you talking of a totally off grid system?  Probably with batteries?

 

I was actually thinking of something like that i.e. without the 'luxury' of dumping excess power back to the grid. Seems then that the panels will just heat up more when the 'full load' is not extracted?

 

Thanks also for the answer to the PVGIS question - I was quite liking the 20 degrees roof slope anyway as it gives max internal space with a nominal 1.5 storey / 7m ridge height - so that adds another plus.

 

[SteamyTea]

4 minutes ago, Alan Ambrose said:

Thanks also for the answer to the PVGIS question - I was quite liking the 20 degrees roof slope anyway as it gives max internal space with a nominal 1.5 storey / 7m ridge height - so that adds another plus.

You get a secondary advantage when PV is on a roof.  As it takes energy way, that is energy that cannot heat the building below, as long as the energy has somewhere to go.

So with a bit of forward planning, if you heat water and charge batteries around the high production times, your house will be less hot.

[Alan Ambrose]

>>> So with a bit of forward planning, if you heat water and charge batteries around the high production times, your house will be less hot.

 

Whoo that's v. clever.

[joth]

24 minutes ago, Alan Ambrose said:

 

>>> Are you talking of a totally off grid system?  Probably with batteries?

 

I was actually thinking of something like that i.e. without the 'luxury' of dumping excess power back to the grid. Seems then that the panels will just heat up more when the 'full load' is not extracted?

I'm curious why you consider grid export a "luxury"?

To achieve the same level of utility, an off-grid inverter will cost more. 

[Alan Ambrose]

>>> I'm curious why you consider grid export a "luxury"?

 

Well tongue-in-cheek i.e. giving someone else power at a small fraction of the price they would sell me the same thing. I don't have much of an understanding of the electricity markets, but the general feeling here, I think, is that the DNOs are taking the ... mickey.

[JohnMo]

You are thinking too much.

 

The inverter produces a higher voltage and I think frequency than the grid coming in to the home, so PV generation is used before anything the grid is offering - basically the grid doesn't come in to the home, because its voltage is too low.  If you don't consume the PV generated electric, the electricity takes the next path of least resistance to the grid. You either get some money for the export or you don't.

 

Capping the electric export via software within the invertor, just plays with voltage so it can't escape to grid.

 

If you are using more than you generate, the voltage form PV falls and main electric comes in to your home.

[Radian]

23 hours ago, Alan Ambrose said:

Seems then that the panels will just heat up more when the 'full load' is not extracted?

 

No, I²R losses will make the opposite true. You still seem to think the power has to go 'somewhere'. It doesn't. If you take the DC leads from a PV module and short them together, no appreciable power will flow. Plenty of current, but no volts and no watts. 10A times 0V is 0W. Leave the panel unconnected and there's plenty of volts, but no amps and no watts. This is because power in watts (W) is the product of volts (V) times amps (A). This should help to get across the idea of potential energy.

[Dillsue]

51 minutes ago, Alan Ambrose said:

>>> I'm curious why you consider grid export a "luxury"?

 

Well tongue-in-cheek i.e. giving someone else power at a small fraction of the price they would sell me the same thing. I don't have much of an understanding of the electricity markets, but the general feeling here, I think, is that the DNOs are taking the ... mickey.

Maybe think along the lines of exporting surplus to the grid helps you local hospital or school run on greener electricity. Your DNO doesnt trade in electricity so they're not taking the mickey.

[SteamyTea]

Not exporting excess power to the grid could be considered the same as burning £20 notes in front of a homeless family.

[Nick Thomas]

4 hours ago, Alan Ambrose said:

giving someone else power at a small fraction of the price they would sell me the same thing.

 

The SEG rates are definitely taking the mickey, but 15p/kWh from octopus is ~50% of the retail price just now, which is in the general range of reasonable. I've started thinking of the grid as a low-efficiency battery as a result.

 

Might look a bit different from April, I guess.

[joth]

The energy market is a mess, but I'm not sure denying them a few "free" kWh per month is going to change anything. Paying more upfront to install a less efficient system in order to deny this export feels like sociopathy.

 

[PhilT]

5 hours ago, Alan Ambrose said:

>>> I'm curious why you consider grid export a "luxury"?

 

Well tongue-in-cheek i.e. giving someone else power at a small fraction of the price they would sell me the same thing. I don't have much of an understanding of the electricity markets, but the general feeling here, I think, is that the DNOs are taking the ... mickey.

you mean the power supply companies rather than the DNO. The latest average wholesale price they pay is around 5p/kWh so there is no incentive for them to pay us more than that. It's a bit like saying the supermarkets should sell us food at the same price they buy it

Edited February 9, 2023 by PhilT