On Wednesday, December 17, 2014 8:43:06 AM UTC, Dave Liquorice wrote:
On Tue, 16 Dec 2014 19:41:05 -0800 (PST), wrote:

With solar thermal you get a lower grade heat from flat panels
than
from evacuated tubes. The same amount of energy if the store is
cold
enough to make use of the lower grade.

Flat panel solarthermal yield much more than tubes per area and per
pound when heating cold water (and only when heating cold water).

Isn't the whole idea that the water doesn't stay cold?

I believe so

The bottom of a thermal store ought to be at 40 to 50 C, coolest.
Once the solar has been running for a while it'll be pushing 60 C or
more. This'll be the minimum temperature of the circulating water, it
needs to be highre to actually transfer any heat... Ours (tubes)
circulates at about 80 C.

Min temp a panel heats is mains cold water temp, maybe 5C in winter. In an ideal world one would optimise a system by using flat panel to raise cool water temp, then vac tubes to get it hot. In a large system this maximises return. In a small system the cost of a 2nd pump, piping, control etc isnt necessarily justified.


NT