On Wed, 17 May 2006 13:04:51 +0100, David Hansen
wrote:
On Wed, 17 May 2006 10:01:07 +0100 someone who may be Peter Parry
wrote this:-
The "government report" also signally failed to mention that on a
cold sunny day with little solar gain the device also happily pumps
your gas heated hot water into a nice big radiator on the outside of
the roof to cool it down. What is the energy cost of this?
Already covered by http://www.solartwin.com/technical_faq.htm
However not covered,measured or even mentioned by the report you
quoted.
"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 households want hot water in the morning which means the tank
will start hot (unless someone has a remarkably effective water
consumption predictor or the last person likes a cold wash).
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.
What about the more realistic situation of less than 100% sun where
the pump is running slowly but the heat gain is minimal? Today with
an air temperature of about 15deg and solid overcast the solartwin
pump is managing to fitfully pump a trickle of water but the inlet
temperature and outlet temperature are the same (26 deg +- 0.1deg).
If it was cold that would mean hot water would be circulated through
the panel and heat loss (the "well insulated" front is simply
twinwall polycarbonate).
Whenever an advantage can be gained from a figure it appears -
whenever it's a disadvantage we get subjective descriptions.
Suppliers of gas boilers have to give comprehensive performance data
to buyers and their efficiency is independently checked. Why are
"alternative" suppliers not obliged to do the same and give
measurements of performance? More to the point why are they so
unwilling to supply this data if their products are as good as they
say?
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!
Which rather defeats the advertising about a "simple to install"
system.
BTW, unless variable speed pumping is in use it takes
more than a few watts to run the pump.
The tiny little Solartwin pump I have at the moment appears to be
about a 5W max unit,
The panel is limited to 4W output and the pump under-run. However,
the fact that this system uses low power pumping does not mean that
they all do.
If 4W is adequate to transmit all the power collected by the solar
panel on a bright day in midsummer why do others need (or chose) to
use much larger pumps?(My 30 year old system used a mains powered 15W
pump made specifically for solar hot water pumping).
Even with variable speed pumping I suspect that 50-100W
is a fairly typical electricity consumption for a mains powered
system.
That seems rather high - a controller should be no more than 3W and
the pumps I've seen recently are generally about 30W.
I have left in what you said, "little or no mention of the fact that
for most people living east of a line drawn roughly from Lincoln to
Bristol you also need to install an ion exchange water softener".
In response I pointed to the most well known supplier of solar
pumped panels, where they explicitly talk about water hardness.
Therefore your statement that they make, "little or no mention", is
incorrect.
Ah, I had misunderstoop your meaning, I had assumed you were talking
about the relative numbers who could use phosphate dosing rather than
an ion exchange softener.
--
Peter Parry.
http://www.wpp.ltd.uk/