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Doctor Drivel wrote:
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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.

You are only guessing here.

CH runs at close to flat panel
stagnation temp, making efficiency
close to zero.

The solar section is at the bottom with the DHW top, CH middle and solar
bottom. No CH return to the solar section, only the middl;e CH section.
Using a plate heat exchanger the return from the DHW can be directly into
the bottom of the cylidner in the solar section. That is because the
efficiencies of the plate is phenominal. There can be 80C into the plate
and the temp of the pipe coming out is so low you can grab the pipe
infefinately. So, cold/very cool water enters the solar section. A tall
thin cylinder helps in stratification.

Evacuated tubes would contribute, but
are high cost and lower output.

http://www.navitron.org.uk

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.

No. Just heat the solar section. Any heat extracted from the centre CH
section (cool return water) will be replaced by hotter solar water in the
lower section (hot water rises).

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

May as well fit the optimum cost/perfoanmnce panel rather than inefficient
black rads.

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.

Mixing and prioritising water use is the problem. A preheat may have very
useful hot water at the top, yet this only enters the run cylinder when it
draws-off hot water. Then this hot ware mixes with the run cylinders cooler
water cooling the water overall. Not what you want. An all one thermal
store can prioritise the use of solar water very easily with one blending
valve and mix the solar water with hotter DHW boiler heated water.