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

celerity

EricMears wrote: »

No point wetting panels other than at periods of high generation.

Actually this may not be the case.
I'd encourage anyone interested in this to read the (long) links I posted as there are a lot of real world accounts of the effect of water cooling.

/\dam

larkim

grahamc2003 wrote: »

Is your cold water tank is situated higher than your panels?

What cold water tank? Don't have one of those, and have two separate rising mains in the house (for no good reason, it seems) both fitted with pressure reducing valves; we've got more pressure than we know what to do with!

Matt

EricMears

celerity wrote: »

Originally Posted by EricMears
No point wetting panels other than at periods of high generation.


Reply by Celerity
Actually this may not be the case.
I'd encourage anyone interested in this to read the (long) links I posted as there are a lot of real world accounts of the effect of water cooling.

/\dam

I don't think spraying panels with water would ever raise output from a point where there wasn't enough power to run the pump to a point where there was.

zeupater

grahamc2003 wrote: »

Is your cold water tank is situated higher than your panels?

As to cooling the panels efficiently, I expect in 10 years hybrid PV/Thermal panels will be the norm - killing two birds with one stone, cooling the panels while scavenging the heat.

Hi

The problem with the theory of hybrid pv/thermal panels is that for the thermal element to work efficiently the panels need to be encouraged to operate at higher temperatures or the system simply 'warms' the bottom of the tank and then stops ....

To provide hot water heating the panels would normally need to operate within a range between 4C and 10C higher than the water in your cylinder, which you would normally expect to reach 60C+, so the panels would be running at up to 70C+, which means that you would need to insulate the panels to retain the heat (not good for pv generation efficiency) or operate the panels without insulation, in which case there would be little/no benefit once the cylinder temperature reaches somewhere around 35C .....

In summary, if you want thermal and pv, buy separate thermal and pv systems to maintain efficiency from both instead of a compromise which will likely never satisfy and would probably cost considerably more ....

HTH
Z

grahamc2003

zeupater wrote: »

Hi

The problem with the theory of hybrid pv/thermal panels is that for the thermal element to work efficiently the panels need to be encouraged to operate at higher temperatures or the system simply 'warms' the bottom of the tank and then stops ....

To provide hot water heating the panels would normally need to operate within a range between 4C and 10C higher than the water in your cylinder, which you would normally expect to reach 60C+, so the panels would be running at up to 70C+, which means that you would need to insulate the panels to retain the heat (not good for pv generation efficiency) or operate the panels without insulation, in which case there would be little/no benefit once the cylinder temperature reaches somewhere around 35C .....

In summary, if you want thermal and pv, buy separate thermal and pv systems to maintain efficiency from both instead of a compromise which will likely never satisfy and would probably cost considerably more ....

HTH
Z

Well, as is the vogue, I've thunk more about it, and the more I think the more it makes more sense (at least in theory - there may be practical constraints, but no show stoppers as far as I can see).

The point I think you are missing is that pv is quite inefficient, <20% iirc (I'm sure someone can google to get the latest efficiencies). So that leaves >80% of the energy available to extract as heat. So worst case is the solar thermal part of a hybrid will be 80% of the efficiency of plain old thermal panel - all other things being equal. So yes, the tank water may not get as hot as a dedicated thermal panel of the same dimensions, but not, I suggest, as much as you suggest. And even if the tank doesn't get as hot, there is a valuable contribution to the heating, leaving only a top up from the boiler or electricity (which of course may be done by the solar pv part). There may be design constraints which lower the thermal part below 80% of course, but atm I'm just looking at the theoretical position.

Add in the gain in pv output due to cooler cells, and I can't really see how the idea isn't a goer, and a very sensible goer at that (all other things being equal). If the cells are kept to 25C instead of 50C due to thermal extraction, then theres a typical 12% gain in pv output, so a significant increase there.

Of course, imv, it still makes no sense ex-subsidies, but it seems i theory (with no obvious practical show stoppers afaics) if the same level of grants were available for hybirds as they are for pv and to a lesser extent thermal, then it appears to be quite a decent improvement on both dedicated technologies.

A hybrid would simply extract say 80% (imv) of the energy available rather than the <20% of pv and the <70% thermal.

zeupater

grahamc2003 wrote: »

Well, as is the vogue, I've thunk more about it, and the more I think the more it makes more sense (at least in theory - there may be practical constraints, but no show stoppers as far as I can see).

The point I think you are missing is that pv is quite inefficient, <20% iirc (I'm sure someone can google to get the latest efficiencies). So that leaves >80% of the energy available to extract as heat. So worst case is the solar thermal part of a hybrid will be 80% of the efficiency of plain old thermal panel - all other things being equal. So yes, the tank water may not get as hot as a dedicated thermal panel of the same dimensions, but not, I suggest, as much as you suggest. And even if the tank doesn't get as hot, there is a valuable contribution to the heating, leaving only a top up from the boiler or electricity (which of course may be done by the solar pv part). There may be design constraints which lower the thermal part below 80% of course, but atm I'm just looking at the theoretical position.

Add in the gain in pv output due to cooler cells, and I can't really see how the idea isn't a goer, and a very sensible goer at that (all other things being equal). If the cells are kept to 25C instead of 50C due to thermal extraction, then theres a typical 12% gain in pv output, so a significant increase there.

Of course, imv, it still makes no sense ex-subsidies, but it seems i theory (with no obvious practical show stoppers afaics) if the same level of grants were available for hybirds as they are for pv and to a lesser extent thermal, then it appears to be quite a decent improvement on both dedicated technologies.

A hybrid would simply extract say 80% (imv) of the energy available rather than the <20% of pv and the <70% thermal.

Hi

So, how do you keep the panels at 25C with water from a hot water cylinder when the water at the bottom of the cylinder soon reaches a temperature of 25C+ ? .....

Solar thermal, even in a hybrid system, will need a thermostat which controls a pump with a start/stop differential temperature range between the panels and the bottom of the cylinder (coldest part) usually of around +10CStart/+4CStop ... this would mean that the temperature of the panels would be above 25C when the water at the bottom of the tank is ~15C .... that's only a few degrees warmer than the cold water supply in mid/late summer ..... for information, the whole of our HW cylinder was 68C earlier this evening and would therefore be incapable of providing any form of cooling to hybrid panels.

I would strongly suggest that, unless there is a technical breakthrough which allows pv cells to operate at significantly higher temperatures without performance degradation, using hybrid panels to provide domestic hot water and improve the efficiency of the pv generation would simply be a waste of money ... however, transferring the heat to a 'guaranteed' low temperature heat dump such as an outdoor pool could be a possibility, but that means that you'd need to add the cost of a pool to the price of the hybrid system ....

HTH
Z

zeupater

zeupater wrote: »

So, how do you keep the panels at 25C with water from a hot water cylinder when the water at the bottom of the cylinder soon reaches a temperature of 25C+ ? .....

..... for information, the whole of our HW cylinder was 68C earlier this evening and would therefore be incapable of providing any form of cooling to hybrid panels.

Hi

Update for information ...

It's 10:30 in the morning and after a couple of showers earlier, with around 1kWh of heat input since, the temperature at the bottom of our cylinder is 41C. The air temperature outside is 6.7C and the sky is probably around 80% cloud so it's unlikely that the pv panels would currently be anywhere near 25C and would therefore be operating above nominal performance .... a hybrid panel without insulation would not be warm enough to provide heat and one with insulation would need to be around 50C to provide any heat benefit to the HW cylinder with a nominal pv performance 'improvement' of around -10%, so a loss of efficiency, not a gain .... this would be pretty typical for the UK

HTH
Z

celerity

An interesting debate, I'm siding with Z at the moment .

Martyn1981

Can I chip in my two pence worth. And I'll admit to knowing very little about solar thermal.

Zeup, I understand what you're saying, and that the water would probably heat, not cool the panels. I also assume that as the panels hit max (normally around noon on average, though every day, and every system will be different), the water is probably also starting to get hot(ter). But .... come on, you knew there was going to be a but .... what about available roof space? Isn't that a factor important enough to influence the economics too?

I think you mentioned sometime back that you have a large roof, but most people probably won't have much or any spare from a 4kWp system. I'm also assuming that PV maximisation is important, given economies of scale, and its simplicity.

So if roof space is limited, would 'doubling up' some of the space, perhaps 2 to 4 panels worth (PV) be sensible?

I'm asking the question, as I'm sure this is going to get more complex. The affected panels could muck up the rest, so different strings, maybe separate inverters as matching voltages might also be a pain and so on. However, all that aside, might the performance loss from gradually warming some of the panels, be worth it in regard to doubling up the utility of the roof as Graham is suggesting?

I've got a gut feeling that this is getting a little complicated, but stranger things have happened.

Mart.

BobA

John_Pierpoint wrote: »

If I have a set of panels on my roof and so does my neighbour; how, at noon on a sunny day, does my inverter recognise a power cut and then turn itself off.
Would not it assume that my neighbours panels were creating the mains supply and likewise their inverter would recognise that power was being supplied from my inverter and assume that was the mains power supply?

Even the next morning could one in theory get the whole estate started up by plugging in a small "Honda" generator, to create a cascade of awakening PV installations.:huh:.

have you had your inveter cut-out. Mine started to cut out occasionally a few weeks ago - apparently it was due to high a/c grid voltages and my Inverter cuts out at 262v - the a/c grid should be less that 253v max - power company are investigating waht is causing the high grid voltages in my area - my inverter is working correctly so it is the power network that is causing the cut outs.