Dave Liquorice wrote:
On Thu, 28 Feb 2008 04:57:02 -0800 (PST), wrote:
To get any useful output you need to get water hot. Tepid isnt too
useful.
Surely the important measurement is the delta T across the panel not the
absolute T of the outflow?
It is that delta T which measures the amount of energy transfered to the
circulating fluid and thus into the storage (less losses).
It is, but the delta T varies hugely according to stagnation temp,
and its stagnation temp thats the Big Issue with flat panels. With
a flat panel, you can get 80% or more efficiency with a cold
cylinder, but as the cylinder warms up the efficiency always falls
gradually... all the way to zero. When cylinder temp = stagnation
temp, efficiency is 0%, and for flat plate panels:
* in bright summer sun stagnation temp roughly = target hw temp
* in winter, stagnation temp is far below target hw temp
This is why absolute temp delivering ability, or stagnation temp, is
so important.
If stagnation temp were a non-issue, no-one would use vac tubes,
since they perform worse than flat plates in every other respect.
NT