On 20/08/2019 07:27, Chris Hogg wrote:
On Mon, 19 Aug 2019 21:47:42 +0100, Chris Green wrote:

Chris Hogg wrote:
On Mon, 19 Aug 2019 19:59:04 +0100, Chris Green wrote:

I know only too well that a (for example) 250 watt solar panel doesn't
produce a continuous 250 watts for 24 hours a day every day of the
year. There's those things call nights, not to mention dull winter
days, and even dull summer days.

I've searched diligently but I can't find a site providing what seem
to be realistic 'ball park' figures for UK regions. I suspect there's
a special word for this percentage which I haven't found.

So, can someone either provide a figure for the average output of
(say) a 250 watt panel over the year in eastern England or point me at
a site that will tell me this. (I seem to remember the figure is
something like 20 to 25 percent, so I'll get an average of 60 watts or
so from a 250 watt panel).

I think Harry gets about 15%, and that's pretty good. He's about 10
miles SW of Worcester. The average for commercial solar farms in the
UK is about 9%, 20-25% in Summer, down to about 4% in Winter, IIRC.
South of England better than the north, obviously, but east versus
west probably makes little difference. Optimum orientation and lack of
shadowing more important. It's questionable whether solar panels in
Northern Europe actually produce more energy in their lifetimes as is
used in their manufacture. South of France, North Africa, California,
yes, fine there, but not in Northern Europe. Without subsidies, or
misleading claims by mfrs and installers, it's doubtful whether anyone
in the UK would install them.

Some links to individual records/comments/experience:
http://www.viridis.net/energy/solar-pv.html
http://tinyurl.com/q2993y9
http://www.jaharrison.me.uk/Misc/Solar/index.html

Thanks, *that's* the sort of stuff I wanted to know. So my 20-25%
figure is a summer figure. The winter figure means I can only run a
10 watt or so pump from a 260 watt panel and even that requires enough
battery storage to keep going for several days I would guess.

I think averages are a bit misleading in this context. On a sunny day
in mid-winter, the output might be quite high, but only for a few
hours, after which time you get next to nothing, and you get next to
nothing for the full 24 hours on dull cloudy winter days.

That is true - on a clear winters day the cold conditions mean that the
PV array runs at higher efficiency, but not by enough to make up for the
very low sun. To first order you can predict output in clear sun to vary
as cos of the angle between normal to the panel and the sun and derate
output by 0.3% for each degree C the unit is above 20C.

The other rough heuristic is that the flux arriving directly from the
sun and the diffuse scattered component averaged by month are roughly
equal provided you have the collector at a sensible azimuth and angle.

There is a slight advantage to pointing ESE or SE rather than due south
since fair weather clouds in summer afternoons can rob you of power.

That's the problem with any renewable power supply; they're
intermittent, and unless you have battery back-up of sufficient
capacity to tide you over the dull days and darkness, and sufficient
renewable power not only to supply what you're trying to run but also
to recharge those batteries, you're stuffed!

It is even more of a problem for such things like the radar activated
"please go round the dangerous bend" on remote hillsides. They work
brilliantly in midsummer but are stone dead two hours after sunset in
winter and *NEVER* working on those cold frosty mornings. The batteries
in them are ruined completely every winter without fail.

I note that most (all?) of those solar powered garden lights you see
in garden centres etc these days, are useless in winter. The
dullness/darkness is too long and the capacity of the photocell is
insufficient to recharge the battery in the few hours of sunshine you
might get once or twice per week, so the device never works.

Again you are better off switching them off and putting them in the
garage. The combination of hard frosts and the batteries being brutally
discharged for a long period does them no favours at all.

Incidentally most of them are not that hot at our high latitude either
since they come on too early in our long twilight. They work much better
at lower latitudes when sunset to darkness is much faster.

--
Regards,
Martin Brown