readers to try this for themselves when
Sirius appears again this winter.
In hot water, again
Regarding Norman Willcox’s letter about
the problems of using solar panels for
domestic heating (August p21), I also have
thermal solar panels installed. However,
contrary to his disappointing experience,
I have found that they provide my family
with a useful amount of hot water. In our
system, the solar energy is used to heat a
store of water, which has no other source of
heat. Mains-pressure cold water passes
through this store via a heat exchanger,
removing heat from it and warming up.
If the water becomes warm enough, an
unpowered thermostatic valve allows it to
go straight to the hot taps (mixing it with
cold if it is too hot). However, if it is not hot
enough, then the water is directed first
through our previously installed gas-powered combination boiler and then to
the taps.
This year, the first day we did not require
the combination boiler for water heating
was in March. Between May and August,
we used the boiler only about 20% of the
time, despite a wet July. As a result, our
combined gas and electricity bill (for a
family of four) is now £ 28 per month.
Had I gone with the standard set-up that
our installer was offering, I am sure that
the savings would be nothing like as good:
it pays to do your homework before
specifying a system. Electric immersion
heaters should be avoided, since,
environmentally, it is poor form to use a
high-grade energy source such as electricity
for heating. Also, given that a kilowatt-hour of electricity costs about four times as
much as a kilowatt-hour of gas, it does not
make economic sense. The solar industry
(at the installation level) could do with a
good physics lesson.
Alastair Basden
Durham, UK
a.g.basden@durham.ac.uk
I have been using solar water heating for
20 years, so although I am not a physicist,
I feel able to comment from years of
experience. My system works in
combination with gas central heating in
winter and an electric immersion heater in
the summer. The pump supplying the hot-water storage tank kicks in when there is a
6 °C difference between the temperature
of the panels and the bottom of the tank,
and it cuts out when the difference falls to
2 °C. By the time the central heating is
switched off sometime in April, the solar
panels are providing the domestic hot
water except on very wet days, when I
manually switch on the immersion heater
for a short time.
It is difficult to know precisely how
much money the system saves. However,
before I installed it, I kept the gas boiler
running for most of the year just for hot
water. Now it is turned off for at least five
months, which has presumably extended
its useful life. Moreover, after I installed it,
the gas company queried the very low
gas consumption in summer and
continued for three or four years to send
estimated bills for far more than the
system had consumed.
If you insist on having a full tank of
piping hot water at all times, then solar
panels are not for you. Whatever your
system, the output will vary from day to day
and season to season, and you must
monitor it to get the best out of it. But I
would be sorry to be without either the
solar water system or the photovoltaic
system I have been using to feed energy
back to the grid for the past two years.
Sheila Watkins
Wincanton, Somerset, UK
sheila.watkins@homecall.co.uk
Next month
in Physics World
Recipes for planets
With new observations of exoplanets pouring in,
astronomers are obtaining valuable information about the
temperature and composition of these new worlds that
may help us understand how planets form
Unravelling the threads
Mystery still surrounds a work of art created in 1913 by the
artist Marcel Duchamp, who dropped three threads exactly
a metre long from a height of a metre on to a piece of canvas
Beyond the quantum
Quantum mechanics is a hugely successful theory, but it is
also strange and incomprehensible. Now, though, some
physicists think there may be another level of reality
beyond the quantum world
Plus News & Analysis, Forum, Critical Point, Feedback,
Reviews, Careers and much more
physicsworld.com
C Carreau, ESA
