On Sun, 09 Dec 2007 01:13:36 +0000 someone who may be Derek Geldard
wrote this:-
It was less than 3C (Got a constant frost warning in the car) and
gloomier than The Wreck of the Hesperus at 1-00 pm in Leeds, and dark
by soon after 3-00 pm.
What does the external air temperature have to do with a properly
designed solar heating system?
As it says at
http://www.navitron.org.uk/solar_collector_panel.htm
"The vacuum tube solar panel has been around for over 10years, and
has proved to be reliable and dependable. The vacuum tubes consist
of a double wall glass tube (made from strong borosilicate glass ie
pyrex) with a space in the centre which contains the heat pipe. The
sun's radiation is absorbed by the selective coating on the inner
glass surface, but prevented from re-radiating by the silvered
innermost lining. This is in effect like a one-way mirror which has
been optimised for infra-red radiation. In fact it is very
efficient, of the sun light's energy hitting the tube's surface, 93%
is absorbed, whereas only 7% is lost through reflection and
re-emission. The presence of the vacuum wall prevents any losses by
conduction or convection - just like a thermos flask. Because of
this, the system will work even in very low temperatures, unlike
traditional flat plate collectors. This is why our system can be
used to heat up water at the South Pole Antarctic Science Base -
where ambient air temperatures can drop below -40°C"
Or, as it says at
http://www.solartwin.com/technical_faq.php
"Q: During the winter months when most of the hot water will be
generated by the boiler, what happens if there is enough sun to set
the pump in action. At which time there is not enough thermal energy
to heat up the water sufficiently so that when it is returned to the
hot water tank it reduces the temperature there?
"A: The pump runs only on solar energy in the form of electricity.
It has no temperature sensor, only a high pressure bypass in case
the panel or its pipes are frozen. In response to your question:
First - best not to have the hot water system on all day since this
is wasteful anyway and does not allow for optimum solar performance.
Most boilers have separate timers for this, but not all. Ideally
time the boiler to add heat to the domestic hot water after 4pm.
Second, even in winter some hot water is made by Solartwin, not all
by the boiler as you say. Third - the panel is well insulated and so
will still raise the temperature of water going into it since it
collects heat from the sun and not the air. Fourth - at 100% sun and
a water input temperature of 50C and air temperature of freezing our
mathematical model (based on extensive tests at Napier University)
suggests that the water will still leave the panel at least 10C
hotter than when it went in. Fifth - if they really want to put cold
water in under these circumstances they can connect a second
cylinder behind the first and draw water off it! This will also
allow for more summer hot storage and is a neat solution for people
with AGAs and Rayburns."
--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54