On 18/12/2014 20:05, Dennis@home wrote:
On 18/12/2014 13:52, wrote:

When hot water is heated, it starts stone cold, and progresses up to
piping hot. Flat plate work better at the cold to warm region, tubes
work better in the warm to hot region. Hence the upside of using both
together with 2 water zones, one hot one warm.

I have tubes, with a compound reflector behind them, I can see no reason
why a flat plate of the same area would work any better.
I can see several reasons why flat panels wouldn't work as well.

But then you always were rather dense.

Assuming the same grade of absorber with efficiency 1-a
(a 1 probably around 0.04 in a typical matt black finish)
eg
http://www.aviantechnologies.com/pro...fuse_black.php

And a typical aluminised mirror with an efficiency 1-b
(b 1 but probably around 0.08 or worse)
eg
http://en.wikipedia.org/wiki/Reflect...eflectance.png

Flat plate efficiency = 1-a ~ 96%

Mirror tube efficiency = (1-a)(1-b) ~ 88%

Acting against that is that with evacuated tubes there is no convection
robbing heat off the tube based designs so long as their vacuum holds.

At higher temperatures the tubes win out with lower convection losses.

t makes me wonder what panels were being compared and what assumptions
were made.

That the mirrors are not perfect ones - which is *very* true.

A typical production mirror scatters or absorbs about 8% of the incident
light where as a decent matt black paint scatters less the 4%.

The concentrators allow you to trade some minor loss of efficiency for a
much higher output temperature which is usually worth it.

There are additional losses off the air-glass surfaces of the front
cover plate as well but they are common to both designs.

--
Regards,
Martin Brown