Doctor Drivel wrote:
wrote in message
oups.com...

Preheat cylinders are used because
they are the one way to get
good efficiency from unconcentrated panels.

The pre-heat cylinder can be the bottom half of a thermal store, not heated
by anything but solar panels. The DHW drawoff can take off water from the
solar section and when too cool start to use the cooler solar heated water
for mixing, instead of much colder mains water. An depletion of hot water
in the upper section heated by a boiler, will be replaced by warmer water
from the lower section, not very cold mains water, hence keeping the burner
off for longer. Not difficult or expensive to do, using only a 3-way
blending valve with a remote temperature.

In principle its a good idea. To work the panels efficiently though you
need a lot of stratification, as panel efficiency drops rapidly as the
water they heat gets hotter. I'm not sure how much temp diff in the
tank one would get in practice.


Also any solar generated heat can be used for CH too.

not really hot enough. CH runs at close to flat panel stagnation temp,
making efficiency close to zero. Evacuated tubes would contribute, but
are high cost and lower output.

I suppose in principle one might try circulating the CH system via the
solar panel, with boiler off, when the room stat is already satisfied.
The lower heat output could then extend the time it takes before the
boiler refires due to the stat calling for more heat.

On 2nd thoughts a comfort zone would be the thing, let the solar heat
run when the stat is satisfied, but only as far as 1-2C above stat
setting.

This is only going to fly when the boiler off periods are long enough
for rad water to cool to below solar panel output temp, at peak heat
times the panels would contribute nothing.


Really quite a lot could be done with solar HW once you have a flexible
and capable multivalve controller panel. These are unheard of today,
but if they become low cost they could control the flow from multiple
inputs to multiple outputs, working out how to maximise return at all
times. For example the inputs might be:
- minimum cost solar collector, eg black radiator or hose panel
- medium cost flat panels
- high cost evacuated tube collectors
- boiler

and the outputs would be several layers in the HW heat store, each at
different temperature. The controller would monitor all the temps and
work out what to route where when to maximise output.

The controller would also automatically drain down collectors when
frost damage risk arose. This means an all direct system, with its
lower install cost, higher efficiency, and more routing flexibility.


Preheat cylinders are notorious for being inefficient to all one cylinder
thermals storage.

They extract heat from the panels efficiently, which is why they get
used, then they use that heated water inefficiently.


NT