"Andy Champ" wrote in message
...
Mary Fisher wrote:
"Andy Champ" wrote in message
...

...
It occurs to me that using a solar system as a pre-heater, and fossil to
bring it up to useful heat, might be a good compromise. Except of
course you need two tanks...

Why?

Mary
Andy


Where do you put the pre-heated water without another tank?

I don't understand. We only have one tank. The water heated by the solar
panel is stored in the tank. It's not an indirect system.

Care of Mr Legionella, we're advised to keep the hot over 60.

That's wasteful.

I don't want to pump the 60 degree water onto my cold roof (OK warm, but
not *that* warm) so I use the roof panel to feed a preheating tank which
on a good day will be up to... ISTR you said 44 a few weeks ago? ...

It varies from day to day.

so the fossil only has to heat from 44 to 60, a *big* saving over the
normal 10-60 requirement.

Quite.

Mary

Andy

Dave Liquorice wrote:
On Fri, 29 Feb 2008 21:16:33 +0000, Andy Champ wrote:

Doesn't it get a bit less efficient (possibly 0%!) when you're pumping
water at 47.5 into a panel on the roof?

The losses are higher but you still get the same delta T (more or less).

Assuming the energy input to the panel and the flow through is constant
then it doesn't matter what the inlet temp of the flow is. The water still
collects the energy and thus the same delta T. True, the whole thing gets
hotter so losses rise which ultimately limit the maximum temp that a given
energy input can acheive.


On a day like today, which is probably a fairly average day - bright but
not much sun, temp. about 10 - what's the equilibrium temperature of the
panel?

Mary,

The PV pump will only mean I don't waste electricity running the pump.
It won't help if the panel is doing no heating.

Andy

David Hansen wrote:
On Fri, 29 Feb 2008 21:17:32 +0000 someone who may be Andy Champ
wrote this:-

Twin coil? So there are two indirect coils?

Two or three coils, depending on the number of heat sources. More
than three becomes interesting.

http://www.newarkcyl.freeserve.co.uk/solar%20page.html gives some
(external) photographs and a drawing.


Ah I see. But how long before the finned coils scale up?

Doesn't that mean there
isn't much room for hot water above the boiler fed (presumably hotter) coil?

If sized correctly there is enough room above the boiler coil.

This arrangement also has the advantage that solar can heat the
whole cylinder and thus store more heat.


Gotcha. It'd work for me, I have a full height airing cupboard, and
it's a better than 2 cylinders.

I'm still far from convinced it's cost effective at current prices.
(And the best thing I could do for the environment would be in
transport, not heating)

Andy

"Andy Champ" wrote in message
...


Mary,

The PV pump will only mean I don't waste electricity running the pump. It
won't help if the panel is doing no heating.

If the panel isn't heating the p[u,p won't be running, I don't understand
your point.

Mary

"Andy Champ" wrote in message
...
David Hansen wrote:
On Fri, 29 Feb 2008 21:17:32 +0000 someone who may be Andy Champ
wrote this:-

Twin coil? So there are two indirect coils?

Two or three coils, depending on the number of heat sources. More
than three becomes interesting.

http://www.newarkcyl.freeserve.co.uk/solar%20page.html gives some
(external) photographs and a drawing.


Ah I see. But how long before the finned coils scale up?

Doesn't that mean there isn't much room for hot water above the boiler
fed (presumably hotter) coil?

If sized correctly there is enough room above the boiler coil.

This arrangement also has the advantage that solar can heat the
whole cylinder and thus store more heat.


Gotcha. It'd work for me, I have a full height airing cupboard, and it's
a better than 2 cylinders.

I'm still far from convinced it's cost effective at current prices. (And
the best thing I could do for the environment would be in transport, not
heating)

Do both. And more.

Mary

Andy

On Thu, 28 Feb 2008 06:30:39 +0000, David Hansen
wrote:

On Wed, 27 Feb 2008 22:21:39 +0000 someone who may be Peter Parry
wrote this:-

My
home made one runs about 10% -20% more effectively than the one of the
same size from the same manufacturer your conjoined twin uses
(although it lacks the nuclear fusion device to generate heat out of
nothing hers seems to have).

No mystery there Peter, the Nuclear Fusion Device was our nearest star
and provided the energy to grow plants and forests 100 million years
ago. What's left of them now is a collection useful fuels, one of
which is eminently suitable to be burnt in a 30Kw boiler to do the
washing up for three people via a 120 litre hot water cylinder and
leave enough hot water for a "Deep bath" the following morning.


Excellent, abuse. Do keep it up.

Peter, those two "Don't do numbers".

On Sat, 01 Mar 2008 16:02:30 +0000, Andy Champ wrote:

On a day like today, which is probably a fairly average day - bright but
not much sun, temp. about 10 - what's the equilibrium temperature of the
panel?

Fecj knows I haven't got one and it would vary from system to system as
pipe lengths insulation levels etc would all be different.

BTW air temp is not particulary relevant apart for affecting the loss
rate. Solar panels can produce useful energy is sub zero temperatures,
they absorb solar radiation not heat from the air.

--
Cheers
Dave.

Mary Fisher wrote:
"Andy Champ" wrote in message
...

Mary,

The PV pump will only mean I don't waste electricity running the pump. It
won't help if the panel is doing no heating.

If the panel isn't heating the p[u,p won't be running, I don't understand
your point.

Mary


If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.

Andy

Dave Liquorice wrote:
On Thu, 28 Feb 2008 04:57:02 -0800 (PST), wrote:

To get any useful output you need to get water hot. Tepid isnt too
useful.

Surely the important measurement is the delta T across the panel not the
absolute T of the outflow?

It is that delta T which measures the amount of energy transfered to the
circulating fluid and thus into the storage (less losses).

It is, but the delta T varies hugely according to stagnation temp,
and its stagnation temp thats the Big Issue with flat panels. With
a flat panel, you can get 80% or more efficiency with a cold
cylinder, but as the cylinder warms up the efficiency always falls
gradually... all the way to zero. When cylinder temp = stagnation
temp, efficiency is 0%, and for flat plate panels:
* in bright summer sun stagnation temp roughly = target hw temp
* in winter, stagnation temp is far below target hw temp

This is why absolute temp delivering ability, or stagnation temp, is
so important.

If stagnation temp were a non-issue, no-one would use vac tubes,
since they perform worse than flat plates in every other respect.


NT

On Sat, 01 Mar 2008 23:57:50 +0000 someone who may be Andy Champ
wrote this:-

If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.


The answer to this question for the Solartwin has already been given
several times http://www.solartwin.com/technical_faq.php


"Q: During the winter months when most of the hot water will be
generated by the boiler, what happens if there is enough sun to set
the pump in action. At which time there is not enough thermal energy
to heat up the water sufficiently so that when it is returned to the
hot water tank it reduces the temperature there?

"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 boilers have separate timers for this, but not all. Ideally
time the boiler to add heat to the domestic hot water after 4pm.
Second, even in winter some hot water is made by Solartwin, not all
by the boiler as you say. 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. 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! This will also
allow for more summer hot storage and is a neat solution for people
with AGAs and Rayburns."


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Sat, 01 Mar 2008 23:40:38 +0000 (GMT) someone who may be "Dave
Liquorice" wrote this:-

On Sat, 01 Mar 2008 16:02:30 +0000, Andy Champ wrote:

On a day like today, which is probably a fairly average day - bright but
not much sun, temp. about 10 - what's the equilibrium temperature of the
panel?

Fecj knows I haven't got one and it would vary from system to system as
pipe lengths insulation levels etc would all be different.

It also depends on the control system. In systems with more
complicated controls than the Solartwin nothing would happen until
the panel has been warmed up to a suitable temperature above the
storage. Once the pump has been started, what happens next depends
on the sun and the control system.

BTW air temp is not particulary relevant apart for affecting the loss
rate. Solar panels can produce useful energy is sub zero temperatures,
they absorb solar radiation not heat from the air.

People in this newsgroup would argue against such a statement if a
few of us made it. Let's see what they do with you having made it.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Sat, 01 Mar 2008 16:05:39 +0000 someone who may be Andy Champ
wrote this:-

http://www.newarkcyl.freeserve.co.uk/solar%20page.html gives some
(external) photographs and a drawing.

Ah I see. But how long before the finned coils scale up?

That depends on your local water quality. If this is bad then there
is a lot to be said for a thermal store with plate heat exchangers
(AKA a heat bank). The latter are the things which will scale up and
they can be replaced without replacing the store.

I'm still far from convinced it's cost effective at current prices.

In terms of simple payback period at current fuel priced it is a
long term investment. However, compared to other things people do,
for example a new kitchen or new double glazing....

(And the best thing I could do for the environment would be in
transport, not heating)

As Mary has said, you could do things in both areas. You could do
things in other areas too.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

"Andy Champ" wrote in message
...
Mary Fisher wrote:
"Andy Champ" wrote in message
...

Mary,

The PV pump will only mean I don't waste electricity running the pump.
It won't help if the panel is doing no heating.

If the panel isn't heating the p[u,p won't be running, I don't understand
your point.

Mary


If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.

Why should you do that?

Mary

Mary Fisher wrote:
"Andy Champ" wrote in message
...
Mary Fisher wrote:
"Andy Champ" wrote in message
...

Mary,

The PV pump will only mean I don't waste electricity running the pump.
It won't help if the panel is doing no heating.
If the panel isn't heating the p[u,p won't be running, I don't understand
your point.

Mary


If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.

Why should you do that?

Mary


To stop it freezing?

On Sat, 1 Mar 2008 17:03:28 -0800 (PST), wrote:

It is that delta T which measures the amount of energy transfered to
the circulating fluid and thus into the storage (less losses).

It is, but the delta T varies hugely according to stagnation temp,
and its stagnation temp thats the Big Issue with flat panels.

Can you explain "stagnation temp", all I get in my mind is static pool of
black smelly water, which doesn't fit with a circulating system with an
energy input.

--
Cheers
Dave.

On Sun, 02 Mar 2008 11:16:46 +0000 someone who may be The Natural
Philosopher wrote this:-

If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.

Why should you do that?

To stop it freezing?

Why?

The Solartwin heats the domestic hot water directly. If it freezes
then it freezes, the panel tolerates this without problems. When the
sun comes out next it will soon unfreeze the panel.

With systems which heat the domestic hot water indirectly the solar
circuit can be provided with a suitable amount of anti-freeze. There
are also drain-down systems. Even if people haven't done either of
these, the pump will generally run for not many minutes even on the
coldest nights.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

"David Hansen" wrote in message
...
On Sun, 02 Mar 2008 11:16:46 +0000 someone who may be The Natural
Philosopher wrote this:-

If I have hot water in my cylinder, and there's not much sun, I may
lose
heat by pumping the hot water out onto the roof.

Why should you do that?

To stop it freezing?

Why?

The Solartwin heats the domestic hot water directly. If it freezes
then it freezes, the panel tolerates this without problems. When the
sun comes out next it will soon unfreeze the panel.

The piping isn't affected by freezing, it's a flexible silicone material,
perhaps the poster you're replying to didn't understand that.

But I still don't understand how hot water could be pumped onto the roof ...

Mary

David Hansen wrote:
On Sat, 01 Mar 2008 23:57:50 +0000 someone who may be Andy Champ
wrote this:-

If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.


The answer to this question for the Solartwin has already been given
several times http://www.solartwin.com/technical_faq.php


"Q: During the winter months when most of the hot water will be
generated by the boiler, what happens if there is enough sun to set
the pump in action. At which time there is not enough thermal energy
to heat up the water sufficiently so that when it is returned to the
hot water tank it reduces the temperature there?

"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 boilers have separate timers for this, but not all. Ideally
time the boiler to add heat to the domestic hot water after 4pm.
Second, even in winter some hot water is made by Solartwin, not all
by the boiler as you say. 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. 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! This will also
allow for more summer hot storage and is a neat solution for people
with AGAs and Rayburns."

IOW in some cases yes, you will lose heat. However what you
gain far outweighs what you lose annually


NT

Dave Liquorice wrote:
On Sat, 1 Mar 2008 17:03:28 -0800 (PST), wrote:

It is that delta T which measures the amount of energy transfered to
the circulating fluid and thus into the storage (less losses).

It is, but the delta T varies hugely according to stagnation temp,
and its stagnation temp thats the Big Issue with flat panels.

Can you explain "stagnation temp", all I get in my mind is static pool of
black smelly water, which doesn't fit with a circulating system with an
energy input.

Stagnation temp is the temp the panel will eventually reach with no
circulation. Flat panels stagnate at around 60C or a bit above, so
by the time your HW has reached 60 the panels are running at
almost zero efficiency. And in winter when stagnation temp might
only be 35 or 40C, youre getting little thats any use from them.
Stagnation temp is the big issue with flat panels.

This is why flat panel performance can be greatly improved if you
can add a little concentration, eg by reflectors. This raises
stagnation temp, and shifts panel operation into significantly
improved efficiency. Hence on top of extensions can be a good
place to site such panels, as one can then use wall above and
roof under to reflect a bit more sun onto the panel.


NT

David Hansen wrote:
On Thu, 28 Feb 2008 05:10:59 -0800 (PST) someone who may be
wrote this:-

Agreed. Vacuum tubes have the edge in cloudy conditions, but
commercially supplied flat panels will do almost as well in bright
sunshine.

I'm not sure where you get that from.

It's common knowledge,

Flat panels yield far more output per area and per cost in bright
sunshine, basic physics tells us that.

Common belief doesnt make something accurate. That belief comes
from comparing systems rather than comparing equal collector
cost or equal collector area. I'm sure you'll have noticed that tube
and flat panel systems tend not to be comparable cost-wise or
performance design-wise

FWIW old systems are always flat panel, and modern are mostly
tube, and much more expensive systems too. Its no surprise that
a higher cost system with better specs can produce more return, .
but it would be unsound to conclude vac tube gives greater ROI
than plate. It isnt that simple. We havent even started on the
significant variation in flat plate designs and performance.


NT

On Sun, 2 Mar 2008 17:04:03 -0800 (PST) someone who may be
wrote this:-

Common belief doesnt make something accurate.

Not a term I used.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54