# QUESTION

Discussion in 'Photovoltaics' started by [email protected], Oct 30, 2003.

1. ### Guest

Q1 : why does the peak daily insolation on a south facing vertical
surface, in the northern hemisphere, drop below it's peak during the
summer months.

2. ### George GhioGuest

Angle.

Have you ever seen a sun chart. Do you know how the sun moves across the
sky. i.e. Azimuth and Altitude.

If you know your Latitude and Longitude I can send you a sun chart.

George

3. ### Miguel GimenezGuest

The orientation (as explained in previous answers) is a factor, but the
module relaxation due to heat is another. If you have a sun tracker you'll
get less peak power in summer anyway. Each cell lowers its voltage 2 mV per
ºC, so a 36 cell module losses 72 mV per ºC.

Best regards
Miguel Gimenez

4. ### Guest

And in the northern hemisphere, the sun is 1.7% farther away from
the earth in summertime, so it's 3.4% less intense.

Nick

5. ### George GhioGuest

Can you explain this statement Nick?

6. ### daestromGuest

The earth's orbit is not circular. It is an ellipse with the sun at one of
the foci. It just happens that the earth is closest to the sun in dec-jan
and farthest from the sun in jun-jul. So in the northern hemisphere's
'summer', the earth is farther away from the sun than during the northern
hemisphere's 'winter'.

The difference is only about 1.7% (the ellipse is very nearly circular).
Since the radiation from the sun can be approximated as a point-source, it
follows the inverse of the distance squared law. When 1.017 times further
away, there is (1/1.017)^2 (~ 0.966) insolance.

Ain't physics fun ;-)

daestrom

7. ### Beneixida SolarGuest

Ante todo disculpas por el OFF TOPIC.

He creado un espacio para un foro en castellano. Hay uno genérico sobre
energías renovables y otro más centrado en la energía fotovoltaica. De
momento no hay mensajes, pero espero que dentro de poco no sea así.

La dirección es:

http://boards3.melodysoft.com/app?ID=RENOVABLES

o bien a través de

http://www.terra.es/personal5/xtz6gfdj/home.htm

y pinchando en el link 'Foro Energías Renovables'

Un saludo sostenible.
JESUS ZAPATERO
BENEIXIDA SOLAR

8. ### Guest

George can do a lot better than that

Nick

9. ### Guest

that always used to puzzle me. how come the sun is closer in the
winter than in the summer ?
i know it is to do with the tilt of the earth on its axis.
can you explain it more clearly ?

10. ### Guest

problem not solved merely reflected through the aixs of the equator.

11. ### Guest

frosty

im in london and the same thing happened in space of 10 days.
it was mid 20s and then it went to 10 or less in no time at all.
i remember being in prag in 1992 and the same thing happened.
late september it was scorching by mid october it was below freezing.
i think its been that way there for a long time but this is new in
london.

12. ### George GhioGuest

Thank you for helping Nick out. He seems to think that these things are
a big secret not to be shared with others.

George

13. ### daestromGuest

There are two different peculiarities you are mixing up. One is that the
orbit is an ellipse. As I mentioned, the sun is at one of the foci of an
ellipse, and the ellipse is *almost* a perfect circle.

The other issue is the earth's axis of rotation is tilted with respect to
the orbit. As you may recall from your school days, this tilt is
responsible for the seasons. The tilt is something like 23.5 degrees. The
axis stays pointing in the same direction in space as the earth orbits
around the sun (oh, allright, there is a ~26 000 year 'nutation', but lets
ignore that).

Seasons are caused by the tilted axis remaining oriented the same way in
space (relative to distant stars) as we orbit the sun. When the earth is on
one side of the sun, the north end of the axis is 'leaning' towards the sun.
Summertime in the northern hemisphere, later the orbit carries earth to
opposite side of orbit and the north pole is further away from sun than the
south pole (winter in northern hemisphere).

Now, the orientation of the axis in space, and the orientation of the
ellipse traced out by the earth's orbit, *ARE UNRELATED*. It is purely a
coincidence that in these modern times, the earth's axis is oriented for
summer in the northern hemisphere when at the closest point of approach in
orbit.

Indeed, as I mentioned earlier, the axis does 'wobble' (called 'nutation')
with a period of ~26 000 years. So in about 13 000 years (1/2 cycle from
now), the earth will be closest to the sun in winter (for the northern
hemisphere). The axis will have shifted to point to another distant star.
The 'north star' (Polaris) won't be very close to the north pole anymore.
But I won't have to learn any new celestial navigation because I won't be
here in 13 000 years ;-)

IIRC, the ellipse that is the earth's orbit also moves. But I believe that
movement is slower by several orders of magnitude from the planet's axis
nutation. But now we've reached my limit on celestial mechanics, I'd have
to go look it up to go further ;-)

There, does that help??

daestrom

14. ### Guest

didnt you just say summer in the northern hemisphere is the furthest
point of approach in orbit?

15. ### daestromGuest

OOPS!!!

Yes, the earth is furthest away from the sun in jun-jul. How incredibly
stoopid of me. Can only blame it on the trick-or-treaters interrupting me
;-)

daestrom

18. ### Nick PineGuest

The next perihelion happens on January 4, 2004. It gets later by 25 minutes
per year, or 1 day every 57 years, in a 21,000 year cycle. Steve Baer says
"Perihelion coinciding with winter in the Northern hemisphere is a big favor
we enjoy that will pass."
I think the difference is +/- 1.7%. If the average sun-earth distance is 1,
it's 0.983 in July and 1.017 in January, so the intensity difference is 7%.

Nick