Realistic claims for solar pv

[billyorange007]

How realistic are the manufacturers claims for energy produced by their solar pv panels. If they claim 2kw howlikel is this to be true/realistic ?

[The Natural Philosopher]

realistic at midday in June.

Average about 20W

[John Rumm]

Its likely to be a true reflection of the maximum power delivery of the panel when in full midday sun in the summer.

In peak summer you might get a load factor as hight as 20% in the uk. In the winter it will be down in low single digits. Averaged across the year its usually estimated to around the range of 10% - 12%. Also peak delivery never aligns with peak demand.

[Martin Brown]

It is better than you might think in winter direct sunlight since to some extend the cold makes the panel a bit more efficient.

You can also push solar panels a bit harder at the risk of cooking them by adding a couple of mirrors either side. Some fold out portable battery chargers work that way.

Pessimist! The usual figure for the UK is about 900kWhr per 1kW peak of PV panel which in ballpark terms is about 100W average (taking a year as

9000hr). The further north you go the worse it gets.

Even so I am inclined to think that solar is a bit of a waste of time in the UK since it isn't really sunny enough to be worthwhile. At lower much sunnier latitudes where peak solar power generation coincides with peak aircon demand then having an extra layer on the roof providing shade and solar PV power serves a useful purpose.

A lot of UK solar generated electricity is immediately dumped into the homes immersion heater because of FIT market distortions. You get paid for half of what you generate irrespective of what you do with it!

[Tim Lamb]

Hmm.. My grazing tenant is considering solar PV panels for an unattended cattle watering set up.

12V bilge pumps seem readily available and adequate for the head and flow required. Lead acid battery not a problem but... how much panel do you need for Hertfordshire sunshine May to October? Hot weather conveniently requires more water which, last summer, was around 600litres/day at a head of around 5m.

My mathmatical capabilities do not stretch to this sort of problem:-)

[tony sayer]

Well right now. 30/4 at 12:14 not that sunny today Gridwatch is reporting 6.31 GW 16% which is a fair chunk of the UK demand..

[Andy Burns]

From 13GW of installed PV capacity.

[Pancho]

At 12:14...

The problem with solar in the UK is that it doesn't generate power when we need it most.

I'm still having mega problems understanding why we can't deliver nuclear more cost effectively. It seem we should be able to do that and keep both TNP and the global warming crowd happy. A reliable base load give us much more scope for load balancing via smart devices.

[Clive Page]

Well it's only worth-while because of the very generous subsidy for those of us who got in early, though the price of solar panels has of course dropped a lot since so I don't know whether it's still worth-while.

In our case the predicted outputs were pretty accurate. We've had a 4kW set of panels on our roof since 2011 and the output each year has been at least slightly above the predictions. No real sign of ageing so far - last year was our best year overall. I just noticed the other day that near mid-day we were getting the full 4kW while the sun was out, which doesn't happen much in winter for obvious reasons. So I'm a satisfied owner so far.

[Clive Page]

There are quite a few programs around that predict average results, but you need to give them lots of information about your location, the azimuth and elevation of the panels, etc.

[Martin Brown]

It might be OK over that period with something like a 300W PV panel to guard against several cloudy days in succession. The main worry will be people nicking it - even the batteries grow legs sometimes :(

mgh = 600kg x 10ms^-2 x 5m = 3000kJ needed each day

Roughly 8h daylight and 3600s in an hour = 29000s so in theory a 100W panel would just be able to keep up if it was sunny every day but allow at least twice that and you will need a charge controller too.

Every time I have tried to do solar powered irrigation (but on a slightly smaller scale) lugging a heavy SLA battery out to the field and taking the old one away to recharge has won hands down. Solar panels are a bit cheaper now but I doubt if it is truly worthwhile. OTOH if he has a solar panel just lying around it would extend the period between having to manhandle to battery and recharge it at home.

I only needed a 3m head of water and 100L delivered daily.

[F]

Full midday sun in the summer will reduce the maximum output as the panels are warmed. Maximum output is more likely in full sun on a cool/cold Spring day.

[Martin Brown]

It might or might not be worthwhile. You need to get some early adopters on board if the mass production cycle is ever to get off the ground.

I don't think the economics for solar PV stack up particularly well in the UK - we don't get enough sunshine and when we do in summer there is less not more demand for electricity. Solar thermal is better but also a bit borderline and isn't subsided in the UK so hardly anyone does it.

OTOH when thin film perovskite PV comes of age it might even be viable here if it drops the price per 1kW peak by an order of magnitude.

They have reached about 20% efficiency and are thermally stable now (although it is still a bit borderline).

[newshound]

Some years ago I tried to use solar lighting in winter for a small stables. Admittedly not with a very big panel. Not very much light is needed in the winter, but it still was a complete failure. Now, like Martin I manage fine with a SLA swapped out every week or so.

[John Rumm]

OK this is entirely guesswork, so make what you want of it...

If we work it out from the work required for that lift of water... So Force x Distance = 600 x 9.8 x 5 = ~ 30kJ of energy.

Now that ignores the losses associated with the motor, pump, and battery cycling etc. So let's make a few wild stabs... say 60% for the motor,

80% for the battery charge cycle. Pump efficiency is harder to call - could be 50 to 70% for a smaller one. Lets go with 60%.

so we could do 30,000 / ( 0.6 x 0.8 x 0.6 ) = ~105kJ Energy input required from the panels.

Let's say a typical roof top sized panel can give approx 250W peak. That's perhaps 1.2 kWh per day in summer (20% load factor). 1.2kWh in J is 4.3 MJ. In winter is could be 0.18 kWh (3% load factor) or ~650kJ.

That sounds like it ought to be plenty, although the variation from cloud cover can be significant - so some days you may only get a tenth of that - so you would still be relying on the battery to provide potentially several days worth of support worst case in the winter. (and if you have snow cover, then all bets are off!)

[Dave Plowman (News)]

But the nuclear lot want a power station running flat out all day every day.

[Dave Plowman (News)]

Then you adjust the output from the generators that don't come from solar?

After all, no generator has a constant load 24/7.

[Dave Plowman (News)]

Thus spake a true Tory. Except where that subsidy applies to him, of course.

[Martin Brown]

At a stables if you wanted then wind power isn't too bad and you can get

300-500W units intended for topping off batteries in sailing boats.

Solar in winter is pretty hopeless. There are bunch of solar powered active radar "please go round the dangerous bend" signs round here which kill their batteries stone dead every winter. In summer they are in fine fettle all day but on a frosty winters morning they are useless and they typically fail about two hours after sunset in mid-winter.

[Tim Streater]

Meaning that it was never really worthwhile. Subsidies are always a bad idea as they hide the true cost of something.