On Fri, Toby Kelsey wrote:

Steve Firth wrote:
On Fri, 17 Nov 2006 12:58:52 -0000, Mary Fisher wrote:
"Joe Fischer" wrote in message
...
The roof pitch should be ignored, and the
panels mounted so they get the most direct sun,
but sometimes that makes an unsightly installation.

I disagree that solar water heating panels are unsightly, they look like a
large Velux window, flat and dark.

You appear to have misunderstood the observation about ignoring the roof
pitch. For best efficiency the panels need to be set at an appropriate
angle which can be steeper than the roof pitch. This means that the panel
must stand proud of the roof. Which *is* unsightly but it is also more more
efficient.

Is that "much more" efficient? I wonder.

In winter, maybe, and that is for a south slope
roof, the original poster has an east-west sloped roof,
which is a problem.

For someone in England the altitude of the sun is about
90 - 52 +-23.5 = 14.5 to 61.5 degrees with the lowest value in midwinter.

So about 38-11.75 = 26.25 degrees from vertical is a good panel angle for winter
heating.

Actually, vertical or almost vertical is better in
winter for space heating, and does not overheat the
panels as much in summer.
Also, mounted on a south facing vertical wall,
snow cover can give 50 percent more thermal energy
in winter, or even polished aluminum laying flat
can almost double the thermal energy.
And in a region where hail is likely, the vertical
mount protects any plastic or glass cover.

The efficiency varies roughly as cos(angle from optimum). For a roof
making an angle of 50 degree with the vertical, the efficiency is

cos(50-26.25 deg) = cos(23.75 deg) = 91.5%
(a vertical mounting has cos(26.25) = 89.7% efficiency)

Is that year round, or only in winter?

so you only lose about 8.5% efficiency with a flush roof mounting. Not too bad.
Toby

On the west slope roof there would be zero efficiency
until nearly noon, even if the panels have the north edge
of each higher than the south edge with the panels spaced
wide apart so they don't shade each other.

The big advantage of the south facing wall mount
is the near optimum efficiency in winter, with the
possibility of extra reflections to exceed 100 percent
rating of the panels, and the reduction of thermal
energy in summer. (This was discussed at length
with graphs in a 1959 ASHVAC proceedings
publication).

But that is for space heating (and it has a short
pay back time for low cost flat panel collectors), and
for domestic hot water, it may also be suitable, with
maybe one or two panels mounted for summer sun.

Joe Fischer