Alternative Power sources - Phase change material as a heat sink

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Phase change material as a heat sink

Pig Sick

2005-12-12, 8:21 pm

I'm a newbie here so please excuse if this topic is done to death
already.

I'm renovating a house here in Scotland and aim to heat it with
renewables. Underfloor heating (water pipes in concrete) supplied with
hot water from a large (ca 400gall) tank of water in an out-house which
is heated by a 2.5kW wind turbine and a 20 m2 solar collector (plus
other sources if needed). A direct system - same body of water
underfloor, in tank, in collector. Doing it all myself: laid the
pipework; made the solar collector; buying the turbine; searching for
the right tank (tall for a good temp gradient). All systems go until at
the weekend someone said 'Why not use phase change material?' Smaller
tank, less need to insulate. Sounds good, but I've no practical
experience of pcm. What to use? Organic or non-organic? Is supercooling
a practical problem? Is heat conductivity an issue in what will now be
an indirect system? What about materials compatability? Cost of pcm?
Stability? Eco-profile? Tank specifications?

If the main advantage is just that I could get off with using a smaller
tank to store the same amount of heat then it's probably not worth it -
I've plenty space in the out-house.

Anyone have any views - or even better, practical experience?

Thanks.

Derek Broughton

2005-12-13, 10:21 am

Pig Sick wrote:

> All systems go until at
> the weekend someone said 'Why not use phase change material?' Smaller
> tank, less need to insulate. Sounds good, but I've no practical
> experience of pcm. What to use? Organic or non-organic? Is supercooling
> a practical problem? Is heat conductivity an issue in what will now be
> an indirect system? What about materials compatability? Cost of pcm?
> Stability? Eco-profile? Tank specifications?

I have no personal experience - and the experiments I know of are now quite
old - but it just seems that the complexity makes it not worthwhile. How
much efficiency could you reasonably expect to gain? Calculate how much
efficiency you would _need_ to gain to make it break-even on cost.
--
derek

Loren Amelang

2005-12-13, 5:21 pm

On 12 Dec 2005 15:44:02 -0800, "Pig Sick"
<pigsick@squidink.demon.co.uk> wrote:

>I'm a newbie here so please excuse if this topic is done to death
>already.
>
>I'm renovating a house here in Scotland and aim to heat it with
>renewables. Underfloor heating (water pipes in concrete) supplied with
>hot water from a large (ca 400gall) tank of water in an out-house which
>is heated by a 2.5kW wind turbine and a 20 m2 solar collector (plus
>other sources if needed). A direct system - same body of water
>underfloor, in tank, in collector. Doing it all myself: laid the
>pipework; made the solar collector; buying the turbine; searching for
>the right tank (tall for a good temp gradient). All systems go until at
>the weekend someone said 'Why not use phase change material?' Smaller
>tank, less need to insulate. Sounds good, but I've no practical
>experience of pcm.
....
>Anyone have any views - or even better, practical experience?

I spent a day researching that question before staying with water. The
problems I found with phase change were cost and the fixed change
point. You pick a material based on its freezing/melting point, where
the phase change action takes place. That needs to be higher than the
working temperature of your output radiation, or you don't have useful
stored heat. But you can't store any heat until your collectors or
other heat sources are above the phase transition temperature.

I guess if you have surplus input heat, the built-in temperature
regulation of phase change storage is a feature. In my case, where I
have several output devices that need different working temperatures
(hot tub, DHW, hydronic under 1/2" of masonry, and hydronic under 1.5"
of wood), and variable input sources (flat panel solar, wood boiler,
gas boiler), it seemed to be a big limitation. With water for storage,
I can run the lower temperature outputs from solar even when it isn't
able to provide the working temp to deal with the thick wood floor.

You say you are building a "direct" system. Doesn't it freeze in
Scotland? And do you have perfectly soft mineral-free water to fill
your system with? I ran a direct system for about 15 years, and the
mineral build-up inside the collectors and pipes gradually reduced the
efficiency over that time. Finally the small passages in the solar
collectors were so clogged they began to freeze and burst. After
fixing several I gave up and started over with an indirect system
using glycol.

When I cut apart the old collectors, it was obvious that at every
pressure change and turbulence point (particularly where a smaller
tube emptied into a larger tube) there was a "delta" of built-up
minerals. Many of the smallest tubes were filled with a honeycomb of
mineral growth. These deposits gradually increased from the input
point of the array toward the hotter output.

Loren

Pig Sick

2005-12-13, 7:21 pm

Thanks, both. Loren, your day's research work looks like it's helped 2
people now: you and me. I guess the point about no heat storage until
the collector is above phase transition temperature is a critical one.
Sounds unnecessarily complicated too - my WT would have to heat water
in a separate system which I'd have to run through the pcm via an
indirect coil.

Does it freeze in Scotland? Well, yes, at times but on the west coast
where I am, warmed by the North Atlantic Drift (what's left of the Gulf
Stream) we don't get much frost at all. The solar collector is freeze
tolerant - EPDM pipework. And the water is really soft, low pH -
nothing furs up. So, fingers crossed, my big direct system should be
OK.

There's still a few things that I'm guessing on, and any comments would
be welcome: I'm guessing 400 gallons of water should be about right for
the accumulator. I'm guessing it probably doesn't matter too much where
(i.e. height) I stick the indirect coil from my woodstove boiler into
the accumulator.
Another thing I'm not clear on: I've got a temp diff switch to turn on
the pump for the solar collector (SC) - it will detect the temperature
in the SC and also the accumulator (Acc) and when the SC is hotter than
the Acc, on comes the pump. Thing is, where's the best point to measure
temp of the Acc? Top? three quarters the way up? half way up? And what
level is it best to introduce the SC-heated water in relation to the
Acc top and to the switch thermostat?

Thanks for any thoughts you may have.

Iain

Loren Amelang

2005-12-14, 3:21 pm

On 13 Dec 2005 15:02:57 -0800, "Pig Sick"
<pigsick@squidink.demon.co.uk> wrote:

>There's still a few things that I'm guessing on, and any comments would
>be welcome: I'm guessing 400 gallons of water should be about right for
>the accumulator. I'm guessing it probably doesn't matter too much where
>(i.e. height) I stick the indirect coil from my woodstove boiler into
>the accumulator.

Sounds a bit like my 600 gallon hot tub with 50' of 3/4 copper tube
inside as an exchanger. The size you need depends on how much heat you
need to soak up in the surplus season - or can you easily drain or
cover your solar collection if your storage is overheating? And of
course how much heat you want to store in scarcity season.

For exchanger location, I'd say definitely bottom of the storage. My
tub coil is about 2" off the bottom, and it is amazing how those
bottom 2" of water stay cold and undisturbed while the rest of the tub
circulates to an even temperature.

>Another thing I'm not clear on: I've got a temp diff switch to turn on
>the pump for the solar collector (SC) - it will detect the temperature
>in the SC and also the accumulator (Acc) and when the SC is hotter than
>the Acc, on comes the pump. Thing is, where's the best point to measure
>temp of the Acc? Top? three quarters the way up? half way up?

For solar, where the only cost is running the pump, I'd say you want
to take water out of your accumulator at the very bottom, and measure
temperature at that same level. If there is any water the solar could
be heating, it might as well be running.

When it involves burning fuel, I measure about halfway up the
accumulator, so I'm only committing to the expense if there isn't
enough heat in the accumulator for a comfortable shower. With your
larger accumulator you might measure even higher.

> And what
>level is it best to introduce the SC-heated water in relation to the
>Acc top and to the switch thermostat?

I put the hot in at the very top, for maximum layering. My indirect
output coil exits at the top, so the output can be higher than the
average accumulator temperature.

Loren

Pig Sick

2005-12-15, 6:21 pm

Thanks, Loren, that all makes sense and is basically what I was
planning. Right now, on a cold and wet night it's difficult to conceive
of having too much heat and perhaps that's what's driving my thinking
to go for a system that stores as much as I can - and if she boils in
the summer I'll just run some off and cook lobsters (and then drain
back the SC).

I'm interested in your 50' long heat exchanger though. I'm going to
have to make one for the wood stove I'm planning to connect up and am
trying to find out what sort of length of pipe to bend. Of course it'll
depend on all sorts of things like temp gradient across the pipe and
speed of flow, but I reckon to connect a bog-standard stove back-boiler
to a bog standard tank (albeit 400 gallons) of water and transfer as
much heat as possible by gravity circulation and 28mm (ca. 1") pipe.
Did you calculate it out at 50 ft or just say, 'What the Hell, I'll
make it 50 ft' ?

Iain

Loren Amelang

2005-12-16, 6:21 pm

On 15 Dec 2005 14:00:53 -0800, "Pig Sick"
<pigsick@squidink.demon.co.uk> wrote:

>Thanks, Loren, that all makes sense and is basically what I was
>planning. Right now, on a cold and wet night it's difficult to conceive
>of having too much heat and perhaps that's what's driving my thinking
>to go for a system that stores as much as I can - and if she boils in
>the summer I'll just run some off and cook lobsters (and then drain
>back the SC).

Please believe me that a solar collector boiling is a serious event.
I've now tilted all my collectors up to face the winter sun, and thus
be less likely to overheat in summer. Before, when they faced the
summer sun, if my heat usage dropped too low in summer the panels
would begin getting hot spots and boiling internally. The steam
pressure would back up to the T&P valve on the tank and max it out,
and then continue backing up through the plumbing to the open
(gravity) pressure tank 150' away. Typically the only way to stop this
event would be to run to the roof and open a direct vent valve at the
top of the collector array - shooting a 10' jet of live steam into the
air for several minutes.

One solution to this would be a much larger circulation pump, to
prevent the hot spots, but being off-grid I don't want to run that
full time to prevent a very rare occurrence. I have added a completely
redundant circulator and control system that will take over if the
normal pump or control fails, but even that is not powerful enough to
stop a boil event once it is happening.

Most solar water heat systems seem to be "assists" rather than primary
heat sources. I don't find much ready-made technology to deal with
overheating. My current fantasy is a rolled-up opaque cover along the
top of the collector array, restrained by an electrical latch that
could drop it if the system was overheating for any reason. Or if the
power failed...

>I'm interested in your 50' long heat exchanger though. I'm going to
>have to make one for the wood stove I'm planning to connect up and am
>trying to find out what sort of length of pipe to bend. Of course it'll
>depend on all sorts of things like temp gradient across the pipe and
>speed of flow, but I reckon to connect a bog-standard stove back-boiler
>to a bog standard tank (albeit 400 gallons) of water and transfer as
>much heat as possible by gravity circulation and 28mm (ca. 1") pipe.
>Did you calculate it out at 50 ft or just say, 'What the Hell, I'll
>make it 50 ft' ?

The 50' tub coil was a guess. Under the conditions it normally sees
now, fed water about 30F above the tub temperature by a 24 V DC
"El-SID" circulator, the coil has dropped to ambient about 2/3 of the
way through. It could handle a larger pump.

For the input side of my storage tank, which must accept the output of
my gas boiler (Taco 008 circulator), I needed more flow capacity and
less back pressure. Rather than go to 1.5" pipe, I built a pair of
manifolds and ran five 50' lengths of 1/2" copper between them.
Probably overkill - 50KBTU input causes only a 10F differential across
the exchanger.

My very first experiments with woodstove water heating used the "Holly
Hydro" internal boiler module in my cookstove. With a direct 10' rise
of 3/4" pipe to the storage tank, and no exchanger to fight, there
still wasn't enough thermosyphon to handle the heat of a really hot
wood fire. I think you'll find you want a pump!

Loren

Robert Morein

2005-12-16, 7:21 pm

In article bk96q1tkjg3bj0181k8brjd4mbjddvfga9@4ax.com, "Loren Amelang"
<loren@pacific.net> wrote...

> Most solar water heat systems seem to be "assists" rather than primary
> heat sources. I don't find much ready-made technology to deal with
> overheating.

That's because you're STUPIDLY looking for a technology solution, rather
than something smart.

A layer of shade cloth (purchased from your local Lowes) over the panels in
the summer is the most intelligent solution.

Pig Sick

2005-12-16, 9:21 pm

The lobster boiling was slightly flip, you're right. In fact I'm
thinking about a secondary heat sink for those few times when I reckon
I'll have too much heat in the accumulator. I've got plenty underfloor
heating pipe left over and may just dig a hole in the garden and feed
it in and circulate the water through that if and when I need to. Could
maybe put it into an insulated mass of concrete underground and keep
hold of the heat to re-use when the sun goes. My wife is keen on a hot
tub in the garden but the midges (no-see-ums) would make it hell.
Shading isn't really an option - we get to much wind here to have
something in situ ready to deploy - and I'd want to automate the
overheat failsafe and not depend on there being someone available to
cover up the SP with shading.
Good stuff on the heat exchanger. Sounds like about 35 feet needed in
your case and mine probably won't be that much different - certainly
much longer than I thought, and it will create a fair bit of back
pressure.. You're probably right about the desirability of a pump in
the system. Over here the regs stipulate that back boilers must work by
gravity, not pump, to avoid problems if the pump fails. A bit of
lateral thinking required... Thanks Loren, you've been very helpful.

Iain

Loren Amelang

2005-12-17, 5:21 pm

On Sat, 17 Dec 2005 08:43:42 +1000, Robert Morein
<nowhere@nowhere.com> wrote:

>In article bk96q1tkjg3bj0181k8brjd4mbjddvfga9@4ax.com, "Loren Amelang"
><loren@pacific.net> wrote...
>
>
>That's because you're STUPIDLY looking for a technology solution, rather
>than something smart.
>
>A layer of shade cloth (purchased from your local Lowes) over the panels in
>the summer is the most intelligent solution.

That presumes your summer usage is consistently low enough to be met
with the shadecloth in place, or that you are willing to climb up and
manually adjust the shadecloth as your usage varies.

At that time, we typically needed the full heat output, even in summer
- unless both teen girls were away from home for a couple of days...

Loren

2005-12-17, 5:21 pm

"Loren Amelang" <loren@pacific.net> wrote in message
news:bk96q1tkjg3bj0181k8brjd4mbjddvfga9@4ax.com...
> On 15 Dec 2005 14:00:53 -0800, "Pig Sick"
> <pigsick@squidink.demon.co.uk> wrote:
>
>
> Please believe me that a solar collector boiling is a serious event.
> I've now tilted all my collectors up to face the winter sun, and thus
> be less likely to overheat in summer. Before, when they faced the
> summer sun, if my heat usage dropped too low in summer the panels
> would begin getting hot spots and boiling internally. The steam
> pressure would back up to the T&P valve on the tank and max it out,
> and then continue backing up through the plumbing to the open
> (gravity) pressure tank 150' away. Typically the only way to stop this
> event would be to run to the roof and open a direct vent valve at the
> top of the collector array - shooting a 10' jet of live steam into the
> air for several minutes.
>
> One solution to this would be a much larger circulation pump, to
> prevent the hot spots, but being off-grid I don't want to run that
> full time to prevent a very rare occurrence. I have added a completely
> redundant circulator and control system that will take over if the
> normal pump or control fails, but even that is not powerful enough to
> stop a boil event once it is happening.
>
> Most solar water heat systems seem to be "assists" rather than primary
> heat sources. I don't find much ready-made technology to deal with
> overheating. My current fantasy is a rolled-up opaque cover along the
> top of the collector array, restrained by an electrical latch that
> could drop it if the system was overheating for any reason. Or if the
> power failed...
>
>
> The 50' tub coil was a guess. Under the conditions it normally sees
> now, fed water about 30F above the tub temperature by a 24 V DC
> "El-SID" circulator, the coil has dropped to ambient about 2/3 of the
> way through. It could handle a larger pump.
>
> For the input side of my storage tank, which must accept the output of
> my gas boiler (Taco 008 circulator), I needed more flow capacity and
> less back pressure. Rather than go to 1.5" pipe, I built a pair of
> manifolds and ran five 50' lengths of 1/2" copper between them.
> Probably overkill - 50KBTU input causes only a 10F differential across
> the exchanger.
>
> My very first experiments with woodstove water heating used the "Holly
> Hydro" internal boiler module in my cookstove. With a direct 10' rise
> of 3/4" pipe to the storage tank, and no exchanger to fight, there
> still wasn't enough thermosyphon to handle the heat of a really hot
> wood fire. I think you'll find you want a pump!

==================================

On the length of pipe for a heat X-changer, I had a gas boiler feeding a 90
imperial gallon tank for home heating, inside of this tank I had 150 feet of
3/4 copper tube coiled up which was fed with mains water at the bottom, and
out of the top of this coil came my domestic hot water at mains pressure.

So the 150 feet of 3/4 copper tube was immersed in the hot water inside of
the copper tank of hot water, IE it was a heat exchanger. The tank was
pressurised by a make up tank, so there was little pressure in it, but it
did need a seperate one inch expansion pipe rising from the top of the hot
water tank to above the make up tank in case it boiled.

You have to also consider how hard the water is that you are using for
domestic hot water, as it does tend to coat the inside of the pipe and
reduce the heat transfer over time, so if you are using very hard water, put
extra copper tube inside the tank to compensate, or otherwise you will need
to do an 'acid clean' of the pipe when it becomes less effective.

I ran the boiler pretty hot, up near boiling at times, and the domestic hot
water temperature was quite hot. Previously I had used around 100 feet of
3/4 tube, and found it was not enough to maintain a good domestic hot water
temperature.

I would not go below 3/4 pipe in this system, as the is a pressure drop in
the pipe and it would be noticeable when showering, as it was I could notice
a slight pressure drop in my system. That is if someone turned on a hot
water tap somewhere when one was showering, although both the hot water and
coldwater were mains pressure, you could notice a change in the shower
temperature. Of course this could have been just a percentage thing of hot
water to cold water mix with a drop in pressure in the system.

The domestic hot water was not pumped, and although the water through the
tank was pumped for heating the house purpose, I would think that there was
not a great deal of pumped flow over the 150 feet of copper in the tank. In
the summer there was no pumping.

The boiler to the hot water tank was thermo syphon using one and a half
inch pipes. With thermo syphon the larger the pipes the better. I would not
go below one and a half inch pipes unless the system is continuously pumped,
shutting down the pump could cause steam to form and the pipes or boiler to
rupture with disastrous results.

Previously the lesser amount of 3/4 tube did fracture inside the tank as it
was not fixed and tended to move about inside the tank when taps were turned
on and off. It caused the make up tank to overflow, hence the new copper
tank and different copper coil set up.

When I put the new tube in (into a new tank) I braised 3 copper channel
struts to the length of the coil of tube 120 degrees apart to hold it into
position so it could not move around and braised the struts to the inside
the tank holding the coil of 3/4 tube solid and not able to move around.

I had this system running for around 20 years with no problems.

Loren Amelang

2005-12-17, 5:21 pm

On 16 Dec 2005 17:04:40 -0800, "Pig Sick"
<pigsick@squidink.demon.co.uk> wrote:

> Over here the regs stipulate that back boilers must work by
>gravity, not pump, to avoid problems if the pump fails. A bit of
>lateral thinking required...

Hmmm... I guess that means they want your storage tank above your
stove. That was my original plan, after having seen a neighbors small
system seem to work well. Somehow when I scaled it up to an 80 gallon
tank on the second floor and a much bigger "back boiler", mine proved
less nice. The weight of the tank and water was too much for the wood
construction, even though the builder knew it was going to be there.
There was a constant series of small leaks, which typically found
their way outside the drip pan. The long, snaking drain for the drip
pan was too frequently clogged by insects and miscellaneous crud. When
the tank finally failed, the resulting mess spread over half the
house. (Granted, that installation was poorly done. So much for my
flirtation with trusting "professionals".)

But mainly, hot water really doesn't like having to flow downward. We
ended up with a circulation pump to get the hot water from the top of
the house down to the usage points in a reasonable amount of time.
Even then there was a serious pressure differential between hot and
cold supplies.

My new exchanger is below the usage points, and works ever so much
better. And the wood boiler is a dedicated outdoor unit, which solves
the problem of the gravity-run back boiler stealing all the heat from
the indoor cookstove until the water was hot, and not being able to be
easily turned off. If I ever used one of those again it would have
valves to quickly isolate and drain it when I wanted the heat for
myself.

Loren

Pig Sick

2005-12-19, 6:21 pm

Thanks, both. Looks like I may have to pump the water through the wood
stove boiler. It's going to be in an outbuilding and will hopefully
only have to be used infrequently - and I'll give it plenty of
opportunty to vent should it want to. So minimal risk of it causing
serious problems. I reckon too that I'll go for a multi-pipe heat
exchanger: I like the idea of the 4 x half inch pipes off a manifold -
easier to coil for a start.

Iain

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2005-12-20, 4:21 pm

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