Doctor Drivel wrote:
wrote in message
oups.com...
Also any solar generated heat can be used for CH too.
not really hot enough.
You are only guessing here.
no, as explained he
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.
DHW is usually not as hot as CH, 65 versus anything upto 82, so an
unusual choice. Why choose to heat dhw to above its use temp, to extend
heat capacity?
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.
return from DHW? whats that?
where does the plate exchanger connect in your scheme (it has 4
connections)
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.
hard to know what to think until you explain your setup clearly.
A tall
thin cylinder helps in stratification.
yes
Evacuated tubes would contribute, but
are high cost and lower output.
http://www.navitron.org.uk
20" of panel for £220 is steep (10x 2" tubes). Evacuated tubes also
dont perform well in winter in Britain, flat plate works better. Tubes
are good with direct sun only, plates work ok on diffuse IR too.
BTW when are people going to start cutting fl tubes and putting copper
or plastic pipe thru them? 10 tubes for a tenner that way.
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).
But its not cool return water, CH return is still not that far from
panel stagnation temps. Except of course with vac tubes, but those are
just too expensive to pay their way.
You still need to change the thermostat setup to make use of the solar
heat, otherwise the pump only comes on when heat is needed, and the
solar part wont supply high enough water temp to avoid the boiler
firing. IOW the system would not heat the rads between boiler firings.
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.
The point is that each type of panel has its own advantages, and is the
best choice for some and only some of the work.
Tubes are best for the highest temp water input, theyre the only ones
that will heat water thats nearly hot enough with any efficiency. But
their cost is excessive for any other task, and they dont collect well
in winter.
Flat plates are the middle ground, mid price, mid efficiency, no use at
higher temps, and unnecessarily costly for heating cold water. But the
best option for a fair range of water heating temps.
Black rads will preheat cold incoming water effectively, and do so at
less cost than the others, but are not for medium or high temps. Their
inbuilt water storage means no need to allocate more tank capacity for
the lowest temp water layer, that temperature layer is effectively done
in the rads. Adding 15-20C to the tank's incoming water temp means more
piping hot water out the other end, and at minimal cost. That means
savings can be made elsewhere in the system for the same overall system
performance, giving better energy/£ ratio.
Most simply, rads can be connected between header tank and water tank,
though thats not what I'm suggesting doing here.
The whole point of a mixed panel system is to get the highest output
with the least cost - which is the only way solar dhw will ever pay.
Using vac tubes to heat cold incoming water is sheer folly - yet its
often done.
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.
Yes... if you can maintain enough stratification.
Also a separate preheat cylinder means more surface area per volume
stored, meaning more heat loss or more insulation expense, or both.
NT