Green Houses - Phase Change Materials « Alternative Technology Assn Forums

Alternative Technology Assn Forums » Energy

Green Houses - Phase Change Materials

(18 posts) (5 voices)

Started 6 years ago by cherax01
Latest reply from Eco

Tags:

PCMs

cherax01
Member

Greetings All
this is my first post. I am going to trial Phase Change Materials (PCMs) in order to punch out latent heat through the course of a nighttime (cold in Canberra)
The science and plan is really quite simple. Solids convert to liquids and then resolidify when it gets colder. In doing so generates heat) Around the internal periphery I place black 25L drums filled with either paraffin wax, or better still Calcium Chloride hexahydrate (a complex salt) which appears cheaper than beaded wax (ease of funnel pouring) Neither are noxious either.
I can appreciate that Im dealing with a lot of people that understand PCMs, so my apologies.

Basically I am interested in talking to any parties, getting ideas and pitfalls
many thanks in advance, andrew

Posted Monday 28 Mar 2016 @ 10:06:07 pm from IP #
Annette
Member

I have always thought of PCMs as more useful for dealing with hot weather rather than cold.

Will the drums be sitting in sunlight? Have you already improved the insulation in the house?

Posted Tuesday 29 Mar 2016 @ 12:35:51 am from IP #
cherax01
Member

Hi Annette
Thanks for your reply.
Yes Indeed. Its a twin walled poly carb and I am also looking at bubblewrapping (apparently it lifts ambient temp) and the (black) drums will be oriented to maximise solar exposure
andrew

Posted Tuesday 29 Mar 2016 @ 12:39:46 am from IP #
Annette
Member

What are you referring to that is twin walled polycarb? The drums?

I have used bubble wrapping and it did seem to slow down heat loss from the windows.

I also had an air gap between the windows and bubble wrap and it was well sealed to the architraves with cloth duct tape. (However it did not raise the room temp.)

Posted Tuesday 29 Mar 2016 @ 12:49:45 am from IP #
cherax01
Member

Hi Annette
again thank you for your thoughts!
Yes its a twin walled polycarb. Single wall black cubic 25L containers. Garden variety that you see at most chains.
Are you by chance a user/grower or engineer.?
I seem to have come unstuck chemistry wise in my nearest supplier is stating this:
"no calcium chloride in the hexahydrate form, only as dihydrate or anhydrous."
Does mean greek to me. Otherwise will road test paraffin
many thanks andrew

Posted Tuesday 29 Mar 2016 @ 12:57:17 am from IP #
cherax01
Member

Annette
sorry the whole GH structure
A

Posted Tuesday 29 Mar 2016 @ 1:01:41 am from IP #
cherax01
Member

My next question to chemical engineers is:
what structural difference is ther with Calcium Chloride hexahydrate as opposed to dihydrate?
Apols in not being and industrialist perienced with PCMs
andrew

Posted Tuesday 29 Mar 2016 @ 4:30:28 am from IP #
Eco
Member

cherax01 said:
what structural difference is ther with Calcium Chloride hexahydrate as opposed to dihydrate?

No idea. But I bet they have different melting points.

I doubt whether what you propose will work. You will have trouble getting sufficient heat into and out of the PCM material in a drum - even with direct sunlight. Would need much thinner containers. Have you considered larger straight water drums to even out the day/night temps.

Posted Tuesday 29 Mar 2016 @ 9:31:36 am from IP #
axess
Member

cherax01,

What about secondhand hydronic heating panel radiators as containers for your experimental fluids?

Posted Tuesday 29 Mar 2016 @ 9:30:42 pm from IP #
Johnnojack
Member

Eco said:
No idea. But I bet they have different melting points.

I doubt whether what you propose will work. You will have trouble getting sufficient heat into and out of the PCM material in a drum - even with direct sunlight. Would need much thinner containers. Have you considered larger straight water drums to even out the day/night temps.

I think the drums will work as I have tied it in the past, but I doubt using PCM will be worth the extra expense. You can always set it up as you propose, fill the drums with water and see how it performs.

Posted Wednesday 30 Mar 2016 @ 1:56:00 am from IP #
cherax01
Member

Thanks for your thoughts, ECO, Axess and Johnnojack
I understand your reservations though the science is certainly there.
Initially going full PCM I will road test both pure paraffin and Calcium chloride hexahydate.
(My doubt there is that a "nucleating agent in salt hydrate systems has proven v successful"
(read badly need a chem engineer. Where not mixing paint here)

For the record the chems here are way cheaper than wax that includes 300km freight
btw I am striving to maintain an GH ambient temp of 5C whilst outside temp can get as low as -5C
(Sorry weve been metric in Australia. Been so since I was a kid! and Im young.
many thanks andrew

Posted Wednesday 30 Mar 2016 @ 6:27:16 am from IP #
Eco
Member

PCMs are not magic. They don't generate any additional heat. All they do is store heat when it is available (lessening max temps) and release it when there is less heat around (increasing min temps) - provided there is sufficient correct material. However water also does this. As does any thermal mass. The feature that can be advantageous with PCM is that it stores a lot of heat at a particular temperature and it can store a lot of heat in a smaller volume over a smaller temp range (provided that temp range includes the melting point). So for certain functions it can be very useful.

By all means try it out if you want to. Place a 25L black container of the PCM in the sun in your green house in winter and see whether there is sufficient heat to melt all of the PCM inside. (Go for the Calcium chloride hexahydate - MP 29C - in preference to Paraffin Wax - MP 37C.) Just don't buy 2 tonne of the stuff straight away.

By the way - Calcium Chloride Dihydrate - MP 176C, calcium chloride anhydrous - MP 772C.

Posted Wednesday 30 Mar 2016 @ 10:41:15 am from IP #
cherax01
Member

Thanks ECO
on the money there.
You obviously have a chemistry background....
It has been written that "Nucleaing agents in salt-hydrate systems have proved very successful"
Any thoughts on the factor?
cheers andrew

Posted Wednesday 30 Mar 2016 @ 11:19:30 pm from IP #
Eco
Member

How's your maths Andrew? A few very rough calcs for you to consider. (I thought I would do a few calcs to see if the gut feel opinion from my first post was justified.)

Calcium Chloride Hexahydrate has a latent heat capacity of 187 kJ/kg and a density of 1.7 kg/litre - so 25 litres has a total latent heat capacity of 187*1.7*25 = 7947 kJ. This is the energy needed to achieve a total melt of the PCM.

Now some back of the envelope calcs. Say the 25 litre container has a surface area exposed to the sun of 0.2 sqm (2 sides of a 25L cube - 1 side would probably more accurate). Say there is 600 Watts of heat available from the sun for 6 hours of the day (unlikely for a winter's day through double walled polycarb). So energy hitting the container = 0.2sqm * 600W * 6h * 60mins * 60secs/1000 = 2592kJ. About a third of what is required for a complete PCM melt- if it all gets into the container, which it won't. And this calc is a significant overestimate.

Before the PCM melts it has to get up to 29 degrees so the amount of melt of the PCM you get may be quite small. And if the PCM is not melting you may as well use normal thermal mass.

My assessment - go for water. Will water achieve your stated aim of stopping the temp falling below 5C? I don't know. But water is cheap.

Posted Thursday 31 Mar 2016 @ 10:06:21 am from IP #
cherax01
Member

Hey thanks Eco for your analytical thoughts. Wish I was a thermal engineer.
I am going to road test a few options. (incl water)
What are your thoughts on adding "nucleating agents" to inorganic salts. I have read that they can be quite beneficial indeed.
What in the hell are they? Hate to demonstrate my ignorrance here
thanks andrew

Posted Thursday 31 Mar 2016 @ 8:49:50 pm from IP #
axess
Member

cherax01,

Is your greenhouse open to the rest of your house? If not, shouldn't there be a significant rise in ambient heat even on a cold but sunny winters day in the greenhouse?

The PCM activation temperature of 29 degrees might be reached with help from the surrounding air temperature as well as incoming solar radiation.

Posted Thursday 31 Mar 2016 @ 10:14:09 pm from IP #
cherax01
Member

Hey axess
thats the big plan for it.
Its currently in a rental in Caberra after moving from Sydney
But yes in a nutshell
andrew

Posted Thursday 31 Mar 2016 @ 11:50:10 pm from IP #
Eco
Member

axess said:
The PCM activation temperature of 29 degrees might be reached with help from the surrounding air temperature as well as incoming solar radiation.

Water/Ice is a PCM with a melt point of 0C. How long do you think it would take for a block of ice to change phase (that is melt) if it was put into a room that had a temperature of 1C? Quite a while I would guess. What if the room was 10C? It would be quicker for sure. So translate this to the 29C melting point PCM. If the green house was at 30 degrees it would take quite a while. If the green house was at 40 degrees it would be quicker - but in no way instantaneous. But a green house at 40 degrees for a significant time in winter!! Is that likely?

Posted Friday 1 Apr 2016 @ 10:49:12 pm from IP #

RSS feed for this topic