My green project for 2010 is to install a large thermal store in my home,
along with a solar array for hot water. This kind of panel:
http://www.navitron.org.uk/product.php?proID=115

One of the big issues with solar collectors (especially larger ones) is the
dumping of surplus energy once the store is "full". An issue for the
summer.

To manage the solar array I will be getting one of these:
http://www.resol.de/index/produktdet...d/3/sprache/en which
has a bunch of nifty features, amongst which is the ability to generate a
relay output on "over-temperature".

My thinking is that in an overtemperature situation a 3 port valve could
then divert the solar fluid (water/glycol) to some kind of heatsink. But
what kind of heatsink? (remember this will be running at 90-95C).

The obvious solution is to install some kind of radiator outdoors [caged
because of the potential temperatures!]. But that seems boring.

Other suggestions I have had:

* Heat a hot tub (don't have one. don't want one)
* Heat a swimming pool (great idea but somewhat over budget)
* Heat a greenhouse
* Lose the heat through some kind of buried pipe (Hassle and MDPE certainly
won't like the temperatures)

Any other suggestions?...bearing in mind this situation will likely only
occur on hot summer afternoons.


D

"Vortex4" wrote in message
...


Any other suggestions?...bearing in mind this situation will likely only
occur on hot summer afternoons.

Sell it to next door.
Install a blind on the panel.

"dennis@home" wrote in message
...


"Vortex4" wrote in message
...


Any other suggestions?...bearing in mind this situation will likely only
occur on hot summer afternoons.

Sell it to next door.
Install a blind on the panel.
Thanks,

Actually the blind idea has been suggested.

It's also been suggested I heat the pond and farm alligators.

One of the big issues with solar collectors (especially larger ones) is the
dumping of surplus energy once the store is "full". *An issue for the
summer.

It depends upon whether you want to use the spare heat, or simply
avoid overheating the tank. I've got a solar system (bought in kit
form from the extremely helpful http://www.barillasolar.co.uk/ )
coupled to a HeatWeb thermal store. When the heat store has reached
its maximum temperature, the solar controller (a Resol BS4) simply
stops pumping any more fluid around the solar circuit. The fluid that
is in the panel boils off, and the gasified solar fluid is absorbed
into an expansion vessel connected in the circuit. When the heat store
loses temperature again, or when the solar panel stops collecting
heat, the expansion vessel lets the gas condense back into the panel,
and things start off again. I think this is called `stagnation'. I've
not had any problems with this working over the last few years.

It would be great to use the spare heat in some way though, and any of
your other suggestions sound good (except heating the greenhouse -
that will be quite hot on a sunny day without extra help!). In my case
though I decided that it wasn't practical or worthwhile to do.

dan.

Vortex4
wibbled on Wednesday 25 November 2009 11:20

My green project for 2010 is to install a large thermal store in my home,
along with a solar array for hot water. This kind of panel:
http://www.navitron.org.uk/product.php?proID=115

One of the big issues with solar collectors (especially larger ones) is
the
dumping of surplus energy once the store is "full". An issue for the
summer.

To manage the solar array I will be getting one of these:
http://www.resol.de/index/produktdet...d/3/sprache/en which
has a bunch of nifty features, amongst which is the ability to generate a
relay output on "over-temperature".

My thinking is that in an overtemperature situation a 3 port valve could
then divert the solar fluid (water/glycol) to some kind of heatsink. But
what kind of heatsink? (remember this will be running at 90-95C).

The obvious solution is to install some kind of radiator outdoors [caged
because of the potential temperatures!]. But that seems boring.

Other suggestions I have had:

* Heat a hot tub (don't have one. don't want one)
* Heat a swimming pool (great idea but somewhat over budget)
* Heat a greenhouse
* Lose the heat through some kind of buried pipe (Hassle and MDPE
certainly won't like the temperatures)

Any other suggestions?...bearing in mind this situation will likely only
occur on hot summer afternoons.


D

One of the stock solutions is to have a 2 port valve between the DHW
(assuming youy heat the hot water from the store) and dump some down the
drain. Not optimal on wasting water mind, but it is a very simple solution
that can be implemented mechanically with a Danfoss RAVK remoted sensor
valve on a 2 port body. Sensor is strapped on the return from the solar
panel and the 2 port valve is between DHW and drain. It's probably the
method I would use.

The other standard method is to run the heating circuit up, ensuring there
is at least one radiator or UFH circuit that is always on and dump the heat
there.

I think dumping the HW is the simplest - the amount of water wasted is
dependent on getting enough sun to actually overheat the store - any idea
how likely that is in your case?

If you have a bypass radiator (no TRV or user valves) then the even more
simplest arrangment is use your relay output to switch on the rad pump.

If you wanted a custom heatsink - then how about a car radiator from a
scrappy mounted on a north wall? Again, how much power do you need to
lose?


On an aside - all of these solutions depend on the electricity not failing.

What happens if it does and your panels cook? I've always wondered about
that.

--
Tim Watts

This space intentionally left blank...

coupled to a HeatWeb thermal store.

+++++

Snap. Will be going for the new Xcel 2009 from www.heatweb.com The biggest
one is 475 litres.

Amazing piece of kit. Not cheap but I'm comfortable with that taking a 10
year view.

My summer gas usage (no cooking) is about 30kWh/day. That's the input
energy

The boiler is a 15 year old Potterton non condensing and non modulating so I
guess anly about 66% efficient...so I guess about 20kWh/day gets into the
current tank.

Will be deliberately oversizing the solar to maximise the length of the
season using a 30kWh/day array like:
http://www.navitron.org.uk/product_d...=126&catID=115

By my calculation from a room temperature start the store will be up at 90C
in a couple of days if no energy is drawn off.

Q. I assume your system is pressurised. Does your system not have a
safety over pressure valve? In which case do you not lose fluid every time
there is an overheat?

One of the stock solutions is to have a 2 port valve between the DHW
(assuming youy heat the hot water from the store) and dump some down the
drain. Not optimal on wasting water mind, but it is a very simple solution
that can be implemented mechanically with a Danfoss RAVK remoted sensor
valve on a 2 port body. Sensor is strapped on the return from the solar
panel and the 2 port valve is between DHW and drain. It's probably the
method I would use.

Thought about this. Got a water meter and quickly discounted it!

Still open to reconsidering.


The other standard method is to run the heating circuit up, ensuring there
is at least one radiator or UFH circuit that is always on and dump the
heat
there.

I thought about the bathroom towel rail, but I guess there could be 3kW to
lose!!


I think dumping the HW is the simplest - the amount of water wasted is
dependent on getting enough sun to actually overheat the store - any idea
how likely that is in your case?

If you have a bypass radiator (no TRV or user valves) then the even more
simplest arrangment is use your relay output to switch on the rad pump.

If you wanted a custom heatsink - then how about a car radiator from a
scrappy mounted on a north wall? Again, how much power do you need to
lose?


On an aside - all of these solutions depend on the electricity not
failing.

Agreed. I think I may need to get some kind of roller blind (you can get
them for greenhouses)
to cover the tubes when away for any period of time.

What happens if it does and your panels cook? I've always wondered about
that.

I would imagine the safety pressure valve would open and you lose expensive
antifreeze!

--
Tim Watts

This space intentionally left blank...

Snap. *Will be going for the new Xcel 2009 fromwww.heatweb.com*The biggest
one is 475 litres.

Amazing piece of kit. *Not cheap but I'm comfortable with that taking a 10
year view.

They are very good, and very well made. I've only got a 210l version,
as that was all I could fit in the space available, but on a sunny day
it will heat up enough to supply two day's worth of hot water.

The HeatWeb stores have overheat protection built-in as well, so they
dump to the C/H if it gets too hot. The Resol controller is set to a
lower temperature than the overheat thermostat so it should stop
heating the store before it reaches this point.

By my calculation from a room temperature start the store will be up at 90C
in a couple of days if no energy is drawn off.

Mine easily reaches 80c on good days. At 90c the store's own overheat
would kick in, but I've only ever seen that happen when my woodburner
with back-boiler is going full bore.

Q. * *I assume your system is pressurised. *Does your system not have a
safety over pressure valve? *In which case do you not lose fluid every time
there is an overheat?

It is pressurised, and there is an overpressure valve, but the
expansion vessel seems to cope with the overheat mostly. Over the
course of a year there is a bit of a pressure drop, which is probably
through the over-pressure value, but I only need to top the system up
about once a year.

The other advantage of this system is that it will fail-safe if there
is a power failure because the panel will simply stagnate.

dan.

On 25 Nov, 13:04, "Vortex4" wrote:

I would imagine the safety pressure valve would open and you lose expensive
antifreeze!

Catch tank for that. Saves contamination, saves antifreeze, notifies
you that you've over temperatured.

SMS or email messages from the controller are another option. Easy to
do these days.

The _big_ problem is in overheating the tubes themselves. Thermomax
tubes (at Thermomax prices!) have an internal widget that shuts down
the heatpipe when overheating, Navitron's don't. So you _must_ either
put a blind over the tubes (ideally weighted to fail safe) or you must
keep coolish water circulating through the tube header tank. Blinds
are good, because otherwise a pump / power fail in the Summer could be
expensively damaging.

As a heat dump, it seems popular to either overheat the heat store
(safely vented etc., and maybe with cold mains makeup water auto-
available) or else to divert to a coldwater heat dump, such as a
crude coil in a water butt or pond. This should ideally be a separate
circuit, in case of leakage.

crude coil in a water butt or pond. This should ideally be a separate


Water butt: Excellent idea.

Actually the location concerned would be ideal for a water butt....or better
still the (huge) galvanized loft tank which will be redundant after this
engineering is complete.

[That's assuming I can get it out of the loft]

The other advantage of this system is that it will fail-safe if there
is a power failure because the panel will simply stagnate.

dan.


Out of interest, the Resol solar controllers seem to have some quite
sophisticated anergy logging/monitoring capability.

I'm curious to know what is the difference in solar "yield" between a
midwinter sunny day and midsummer. Is this something you've ever looked at?

david

The _big_ problem is in overheating the tubes themselves. Thermomax
tubes (at Thermomax prices!) have an internal widget that shuts down
the heatpipe when overheating, Navitron's don't.

I installed my Barilla panel mid-summer, and didn't plumb it in until
the winter. The tubes lasted several months in direct south-facing
sunlight, with no circulating water and no blinds, and suffered no ill
effects whatsoever.

dan.

Vortex4
wibbled on Wednesday 25 November 2009 14:14


crude coil in a water butt or pond. This should ideally be a separate


Water butt: Excellent idea.

Actually the location concerned would be ideal for a water butt....or
better still the (huge) galvanized loft tank which will be redundant after
this engineering is complete.

[That's assuming I can get it out of the loft]

Angle grinder...


--
Tim Watts

This space intentionally left blank...

dent
wibbled on Wednesday 25 November 2009 14:26


The _big_ problem is in overheating the tubes themselves. Thermomax
tubes (at Thermomax prices!) have an internal widget that shuts down
the heatpipe when overheating, Navitron's don't.

I installed my Barilla panel mid-summer, and didn't plumb it in until
the winter. The tubes lasted several months in direct south-facing
sunlight, with no circulating water and no blinds, and suffered no ill
effects whatsoever.

dan.

I presume that is the principle behind gravity drain down systems? Pump
stops and water drains out of the panel into a tank... If I understand it,
that's a pretty failsafe system.

--
Tim Watts

This space intentionally left blank...

Out of interest, the Resol solar controllers seem to have some quite
sophisticated anergy logging/monitoring capability.

They do. I've got the extra sensor pack which measures additional
temperatures (e.g., flow and return temp) to figure out energy gains
more accurately, but after the initial burst of enthusiasm for
checking it daily, I've forgotten about it now. I think you can get
networked versions now which tie in to your computer to make it easier
to log, but mine doesn't do that.

I'm curious to know what is the difference in solar "yield" between a
midwinter sunny day and midsummer. *Is this something you've ever looked at?

'fraid not. The panel still makes a bit of a contribution to the
thermal store even in the middle of winter, but I've not measured how
much.

dan.

On 25 Nov, 14:14, "Vortex4" wrote:

better
still the (huge) galvanized loft tank which will be redundant after this
engineering is complete.

My first solar system was an old heating oil tank in a greenhouse
(antifreeze and a heat exchange coil inside), used as a pre-heater.

On 25 Nov, 14:31, Tim W wrote:

I presume that is the principle behind gravity drain down systems? Pump
stops and water drains out of the panel into a tank...

Yes, but it only works for flat panel systems, not heat pipe tubes.
Simplest of the lot are the solar-powered controllerless pump systems,
like Mary's.

Andy Dingley
wibbled on Wednesday 25 November 2009 15:00

On 25 Nov, 14:31, Tim W wrote:

I presume that is the principle behind gravity drain down systems? Pump
stops and water drains out of the panel into a tank...

Yes, but it only works for flat panel systems, not heat pipe tubes.
Simplest of the lot are the solar-powered controllerless pump systems,
like Mary's.

Ah - thank you. Always wondered about that...

--
Tim Watts

This space intentionally left blank...

On 25 Nov, 15:08, Tim W wrote:

Evaporative cooler? Power stations mostly still use cooling towers to
dump heat because they're most efficient. If there's any sort of
recirculating pond involved, there's legionella concerns. If the water
evaporated is hard, it will leave limescale deposits.

Or a dry cooler (finned coil, like a car radiator with a fan) which is
what most AC systems acquired when the owners got scared of
legionella. You could shift some heat by natural convection if you
installed a coil in a stack with cool air entering at low level. Any
redundant chimneys from ground floor fires?

In article ,
Vortex4 wrote:
My green project for 2010 is to install a large thermal store in my home,
along with a solar array for hot water. This kind of panel:
http://www.navitron.org.uk/product.php?proID=115

I have one of those...

One of the big issues with solar collectors (especially larger ones) is the
dumping of surplus energy once the store is "full". An issue for the
summer.

It will (probably) only be a problem if you don't have a few showers/bath
each day in the summer, or use any hot water - so basically when you're
on holiday.

My solution is to dump the store into the central heating system -
It's only going to happen when I'm away, so ...

However I'm in the process of building my own controller to let me do that
via the rather creative process of running the CH pump with the boiler off
and alternating the three way valve between CH and tank every 5 miuntes...

(Even just running it through the boiler dumps heat initially without
then shunting it through the radiators)

To manage the solar array I will be getting one of these:
http://www.resol.de/index/produktdet...d/3/sprache/en which
has a bunch of nifty features, amongst which is the ability to generate a
relay output on "over-temperature".

I have the TDC3 controller right now and it has a mode and a 2nd relay
output that can do that (I think)

My thinking is that in an overtemperature situation a 3 port valve could
then divert the solar fluid (water/glycol) to some kind of heatsink. But
what kind of heatsink? (remember this will be running at 90-95C).

Big radiator on the north side of the house.

The power issue that someone else mentioned is a real concern, but I
have a big UPS for that, and I have already run the CH off it during
short power cuts. (fortunately uncommon here)

Gordon

The power issue that someone else mentioned is a real concern, but I
have a big UPS for that, and I have already run the CH off it during
short power cuts. (fortunately uncommon here)

What size UPS do you use, and how long does it run for? I've thought
of doing that, but never sorted out which UPS to get. I've heard that
some have problems driving pumps - something to do with the sine-wave
modulation?

dan.

Gordon Henderson wrote:
My thinking is that in an overtemperature situation a 3 port valve could
then divert the solar fluid (water/glycol) to some kind of heatsink. But
what kind of heatsink? (remember this will be running at 90-95C).

Big radiator on the north side of the house.

Build a solar chimney and stick the radiator at the bottom of it?

On Wed, 25 Nov 2009 06:57:24 -0800, Andy Dingley wrote:

On 25 Nov, 14:14, "Vortex4" wrote:

better
still the (huge) galvanized loft tank which will be redundant after this
engineering is complete.

My first solar system was an old heating oil tank in a greenhouse
(antifreeze and a heat exchange coil inside), used as a pre-heater.

Was there any Science (tm) involved? i.e. calculations for size of tank,
coil, size of surrounding greenhouse etc. or was it a bit hit-and-hope?
(no disrespect intended if it was the latter)

I'm always intrigued as to how much planning goes into many of these
projects - and how much is either guesswork or simply using whatever
materials happens to be lying around :-) I'm sure a physicist would
produce pages of calculations first based on all sorts of variables - but
does the typical builder of projects like this bother?

On 25 Nov, 17:03, Jules
wrote:

My first solar system was an old heating oil tank in a greenhouse
(antifreeze and a heat exchange coil inside), used as a pre-heater.

Was there any Science (tm) involved?

Yes and no. I modelled the hell out of all of it, but then the design
was fundamentally limited by "found materials" anyway. Modelling did
suggest it was worth bothering though. Basically solar gain of the
greenhouse, minimal contribution from sun on the tank itself. The only
real cost or work involved was trenching and piping from the
greenhouse across the garden and into the house. The pump was an old
beer pump, the heat exchange coil was pulled from a scrap HWC and much
of the old oil boiler found itself being pressed into service.

In article ,
dent wrote:

The power issue that someone else mentioned is a real concern, but I
have a big UPS for that, and I have already run the CH off it during
short power cuts. (fortunately uncommon here)

What size UPS do you use, and how long does it run for? I've thought
of doing that, but never sorted out which UPS to get. I've heard that
some have problems driving pumps - something to do with the sine-wave
modulation?

I have a 1500 VA UPS. (It's an MGE model - double conversion true sine
wave which is nice, but a bit innefficient) More by accident than design
as it was about to be thrown out - a new set of batterys later and ...

In my experience over many years of using USPs, the typical run time is
15 miuntes on half load and 5 miunte on full load, and they're more or
less linear at half load and less, so 30 miuntes on quarter load.

The CH pump is 40 watts, so lasts for long enough. (The max. it's normally
supplying is 70 wats for servers, switches, adsl modem/router, phone)

Gordon

On Wed, 25 Nov 2009 09:21:15 -0800, Andy Dingley wrote:

On 25 Nov, 17:03, Jules
wrote:

My first solar system was an old heating oil tank in a greenhouse
(antifreeze and a heat exchange coil inside), used as a pre-heater.

Was there any Science (tm) involved?

Yes and no. I modelled the hell out of all of it, but then the design
was fundamentally limited by "found materials" anyway. Modelling did
suggest it was worth bothering though. Basically solar gain of the
greenhouse, minimal contribution from sun on the tank itself. The only
real cost or work involved was trenching and piping from the
greenhouse across the garden and into the house. The pump was an old
beer pump, the heat exchange coil was pulled from a scrap HWC and much
of the old oil boiler found itself being pressed into service.

Good stuff!

I'm happy to build stuff, but I went the eletronics & computing
route rather than physics, so the thermodynamics and whatnot is something
of an unknown to me (and lots of sites describing projects like this just
explain what the builder did, rather than what led them to their
final design; I can follow exactly *why* it works, just not why it works
as well - or not - as it does)

Maybe I need to do some more studying :-)

cheers

Jules

On 25 Nov, 19:35, Jules
wrote:
the thermodynamics and whatnot is something
of an unknown to me

* Assume that for low heat draw in a large greenhouse (40'!) you'll
have an adequate supply of water at greenhouse ambient temp.

* Use the 1983 EU conference rules for passive solar design (a fairly
common book in the '80s) to predict greenhouse ambient temps across
the year.

* Fudge the heat loss calcs for the pipe to the house.

* Use early generation PC & spreadsheet tech to do the rest of the
sums.

On 25/11/09 12:47, Tim W wrote:

On an aside - all of these solutions depend on the electricity not failing.

What happens if it does

The xcel seems to offer a power free method, a thermostatic valve that
at 95C allows cold mains water to run through the coil and dumps it via
a tundish to a drain.

On 25/11/09 12:58, Vortex4 wrote:

Will be going for the new Xcel 2009 from www.heatweb.com The
biggest one is 475 litres.

Amazing piece of kit. Not cheap but I'm comfortable with that taking a
10 year view.

Out of interest, how not-cheap?

On 25 Nov, 12:58, "Vortex4" wrote:

[clip...]
The boiler is a 15 year old Potterton non condensing and non modulating so I
guess anly about 66% efficient...so I guess about 20kWh/day gets into the
current tank.

Will be deliberately oversizing the solar to maximise the length of the
season using a 30kWh/day array like:http://www.navitron.org.uk/product_d...=126&catID=115
[clip...]

I'm honestly puzzled where the makers numbers come from. My (poor)
logic runs as ...
That panel has a 3.7mtr^2 'aperture' and at 100% conversion would give
3.7kW in midday sunshine.
But efficiency is more like 30%, so that's actually a 1kW panel?.

Average UK summer sunshine 5.3Hours per day. That implies 5.3kW hours
of panel power per day, which apparently is only 1/6th the makers
rating.
Taken over a UK summer/winter year that averages out at 3.5kW hours
per day and at say 3p per unit for gas, implies a 25 year payback just
for buying the panel, disregarding all the ancilliary kit and
installation costs.
I've obviously missed some technical aspect.

On Wed, 25 Nov 2009 15:22:27 -0800, john wrote:

I've obviously missed some technical aspect.

Yebbut it's Green, innit?

And it goes up to 11.

--
John Stumbles

87.5% of statistics are made up

On Wed, 25 Nov 2009 14:27:04 +0000, Tim W wrote:

Vortex4
wibbled on Wednesday 25 November 2009 14:14


crude coil in a water butt or pond. This should ideally be a separate


Water butt: Excellent idea.

Actually the location concerned would be ideal for a water butt....or
better still the (huge) galvanized loft tank which will be redundant after
this engineering is complete.

[That's assuming I can get it out of the loft]

Angle grinder...

And WD-40, which it would seem is the solution to all problems,

Perhaps it should have been called WD-42.


--
Frank Erskine

Frank Erskine wrote:
On Wed, 25 Nov 2009 14:27:04 +0000, Tim W wrote:

Vortex4
wibbled on Wednesday 25 November 2009 14:14


crude coil in a water butt or pond. This should ideally be a
separate


Water butt: Excellent idea.

Actually the location concerned would be ideal for a water
butt....or better still the (huge) galvanized loft tank which will
be redundant after this engineering is complete.

[That's assuming I can get it out of the loft]

Angle grinder...

And WD-40, which it would seem is the solution to all problems,

Perhaps it should have been called WD-42.

Groan, but I smiled.

On 25 Nov, 23:55, John Stumbles wrote:
On Wed, 25 Nov 2009 15:22:27 -0800, john wrote:
I've obviously missed some technical aspect.

Yebbut it's Green, innit?

And it goes up to 11.

--
John Stumbles

87.5% of statistics are made up



(I'm getting to think that Green stats are about 99.9% made up.)

"Andy Burns" wrote in message
o.uk...
On 25/11/09 12:58, Vortex4 wrote:

Will be going for the new Xcel 2009 from www.heatweb.com The
biggest one is 475 litres.

Amazing piece of kit. Not cheap but I'm comfortable with that taking a
10 year view.

Out of interest, how not-cheap?


£2-3k depending on size and options.

wrote:

I'm honestly puzzled where the makers numbers come from. My (poor)
logic runs as ...
That panel has a 3.7mtr^2 'aperture' and at 100% conversion would give
3.7kW in midday sunshine.
But efficiency is more like 30%, so that's actually a 1kW panel?.

Average UK summer sunshine 5.3Hours per day. That implies 5.3kW hours
of panel power per day, which apparently is only 1/6th the makers
rating.
Taken over a UK summer/winter year that averages out at 3.5kW hours
per day and at say 3p per unit for gas, implies a 25 year payback just
for buying the panel, disregarding all the ancilliary kit and
installation costs.

and maintenance - 25 years is a long time for everything to keep
working. Will the technology have moved on by then anyway?

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.

On Wed, 25 Nov 2009 23:15:20 +0000, Andy Burns
wrote:
On 25/11/09 12:47, Tim W wrote:

On an aside - all of these solutions depend on the electricity not failing.

What happens if it does

The xcel seems to offer a power free method, a thermostatic valve that
at 95C allows cold mains water to run through the coil and dumps it via
a tundish to a drain.


Wasting mains water, eh? Not very green, is it?

Mind you, I think anyone who thinks there is going to be such a vast
amount of waste heat from this system is suffering from delusions.

wrote in message
...
On 25 Nov, 12:58, "Vortex4" wrote:

[clip...]
The boiler is a 15 year old Potterton non condensing and non modulating
so I
guess anly about 66% efficient...so I guess about 20kWh/day gets into the
current tank.

Will be deliberately oversizing the solar to maximise the length of the
season using a 30kWh/day array
like:http://www.navitron.org.uk/product_d...=126&catID=115
[clip...]

I'm honestly puzzled where the makers numbers come from. My (poor)
logic runs as ...
That panel has a 3.7mtr^2 'aperture' and at 100% conversion would give
3.7kW in midday sunshine.
But efficiency is more like 30%, so that's actually a 1kW panel?.

Average UK summer sunshine 5.3Hours per day. That implies 5.3kW hours
of panel power per day, which apparently is only 1/6th the makers
rating.
Taken over a UK summer/winter year that averages out at 3.5kW hours
per day and at say 3p per unit for gas, implies a 25 year payback just
for buying the panel, disregarding all the ancilliary kit and
installation costs.
I've obviously missed some technical aspect.



I think your assumptions are a little pessimistic. Let's do a plausibility
check:

Solar radiation at the earths surface is circa 1.3kW/sq metre "normal" to
angle of incidence

Claim of 30kWh/day for (say) 6 sq. m implies 5kWh/sq. metre/day (round
numbers). Remember that is PEAK.

As an example this manufacturer claims 17MJ/sq metre/day:
http://www.solartubecompany.co.uk/solar-water-heating/

1kWh = 3.6 MJ

17/3.6 = 4.7kWh/day is the manufacturer claim for 1M^2

+++plausibility check passes.

++++++++++

Least year I "bought" ~25000 kWh of gas which cost about 800 quid (all
heating no cooking). Remember that's the input energy. Probably an
absolute maximum of 2/3 ended up in my radiators or water tank. Say 17000
kWh

It's reckoned that well implemented solar can deliver 1.3kWh/sq metre/day
year round average. Something like 2750 kWh/year with a my proposed setup.

I'm under no illusions. the gas usage might only go down 15% but I believe
it will be at least 25% for a variety of reasons. It's going to be
interesting.

David Mackays book is a good read on this subject. Look at figures 6.3 and
6.4 he http://www.inference.phy.cam.ac.uk/w.../page_39.shtml

I'm honestly puzzled where the makers numbers come from.

Lots of makers get their numbers from SPF tests (http://
www.solarenergy.ch/spf.php?lang=en&fam=41&tab=1). SPF are well-
respected independent testers of solar systems.

But efficiency is more like 30%, so that's actually a 1kW panel?.

30% is very low. 75 is more typical for a double-walled evacuated tube
and single-walled tubes can reach 90 or more. Thermal solar is much
more efficient than PV cells (IIRC PV efficiency is ~15%). Flat
thermal panels have similar (and some makers claim higher!)
efficiencies, but don't work well in overcast conditions, so you will
get better average performance in UK conditions from evacuated tube
collectors.

dan.

On Wed, 25 Nov 2009 15:22:27 -0800 (PST) someone who may be
wrote this:-

Average UK summer sunshine 5.3Hours per day.

Source?

Remember also that direct sunlight is not needed for the panel to
work.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000...#pt3-pb3-l1g54