Solar paired with phase change material

wklopf

I have been toying with an idea that I'd like to get some opinions on. I have a house which is heated and cooled with a ground-source (or geothermal) heat pump system. I have been thinking of adding some flat plate solar collector panels and use this heat to melt a phase change material which has the phase change at about 95 degrees. What I'm thinking of is hydrogenated coconut oil.This material has a heat of fusion of a bit over 100 BTUs per pound. I have seen that used 275 gallon totes are available for cheap prices. Since the oil should be quite non-reactive and 95 is not very hot, I was thinking of using one or two of these totes as my storage vessel. I would construct copper heat exchanger systems for these totes. I'm thinking of using an air handler-like system with a liquid-air heat exchanger and tap that into the existing duct system.

The water from the ground loop entering my heat pump is 54 degrees, while the return water is 49 degrees. The air from the heating vents is 85 degrees. The COP of this unit operating like this is about 4. With a phase transition at about 95 degrees, no heat pump would be required. Just circulate the warm liquid through the liquid-air heat eschanger. I'm not expecting to save bundles of money with a system like this. I'm a retired scientist. I taught chemistry att he university level. But, I grew on a farm. My father was a carpenter and since retiring, I have redone five houses, so I know which end of the nail goes into the wood, so to speak. Any thoughts would be appreciated.

Jamie Hall

Hadn't thought of coconut oil. But why not? 450 gallons would be a bit over a ton? Which could store -- if you could get it all active -- around 200,000 BTU. I think your biggest problem will be arranging your piping -- essentially a heat exchanger -- to get it all active, and then also the rate at which you could store, or release, the energy.

retiredguy

Many, many, years ago around 1982, I decided to construct a heat pump system using solar collectors and an ice bank storage. I got it to work on "paper only" so I do not really know if it would be practical. My idea was to use an antifreeze solution in the collectors to heat the water and design a heat pump that would switch from "air to air" to "water to air" when the outside ambient outside air temperature fell below about 30 degrees F. I used a 1000 gallon storage tank for my calculations and assumed that I could heat the stored water to 95 F. The water side of the heat pump would use the sensible heat of the stored water and the phase change "latent heat" part. I calculated that the sensible heat storage was 500,000BTU's and the latent storage was 1,200,000 BTU's. I designed the whole system and even the control system, everything, and then reality and life took over. My family grew and I lost direction. I still have all the paper work.

For your project you may want to consider a phase change solution or "eutectic solutions". I looked at these in addition to water. These may be an alternative to the coconut oil. You can search for "eutectic solutions" and a company I looked up and have information is, (www.pcmproducts.net) that is located in the UK. They have a lot of phase change stuff. Just curious, where are you located?

Hope this helps.

MaxMercy

A long time ago I also toyed with the idea of storing heat, and the phase change material I selected back then was paraffin. I think the best price I could find for a huge quantity came from India of all places.

hot_rod

95F would be a great operating temperature for solar in winter harvest. This formula shows how to calculate efficiency 
I would next define the energy required for changing the storage temperature, that would help determine how large of an array to consider 

Sol_Brother

I have no actual experience doing this but have been intrigued with the possibilities. I suspect one hard part would be preventing the phase change liquid from "icing up" the heat exchanger. That is, you don't want it to solidify and block circulation in or around the pipes. That implies you might want to move the heat from the tote to the air handler with water in a copper tubing loop (one end being a heat exchanger coil in the tote, the other a duct coil in the air handler) instead of trying to pump coconut oil that is going to want to solidify in the duct coil. And the same applies to the HX coil in the tote. You could eliminate both issues by encapsulating the coconut oil in smaller containers that are then packed (not too tightly) into a big drainback tank (like the nicely insulated ones made by American Solartechnics) filled with water. That way you get the benefit of the phase change without having to move the material.

But why not apply the heat to the water entering the heat pump, since it will be much cooler and able to absorb it easily? Of course, that means you still have to run the compressor to move the heat to the air, but the transfer should be much more efficient. In fact, if doing that you might want a material that changes phase much lower, like say 60 -70°.

Derheatmeister

How about a thermal battery bank using Sodium acetate trihydrate ?
Each Battery can be utilized/Triggered when needed.
I think Sometimes potassium chloride and/or sodium chloride was also utilized but i think the Heat exchanger design may be challenging.

Jamie Hall

"but i think the Heat exchanger design may be challenging." Got it in one, @Derheatmeister . The problem is that if the storage material is not in some interesting way crystals in suspension when it is in the solid phase, you become dependent on the heat transfer capabilities of the solid phase to get heat in -- or out -- of the liquid phase, which will be at some distance from your circulating fluid.

Given enough time, you could change the phase of any amount of material with even pretty crude heat exchanger designs -- but to get any reasonably rapid storage or release of heat, seems to me (just paper napkin thinking here) you are going to need a pretty fancy design with many very small diameter clustered tube bundles or interleaved flat plates.

Something to think about while I go plow some snow...

Derheatmeister

Jamie..Exactly as you said.
Many HX designs have failed in heat exchange efficiency and have also leaked due to the inherent phase change from a liquid to a solid.
A company i know from Germany is at this point using Sodium acetate trihydrate in their thermal Battery which i am guessing is more forgiving on the HX.
But this may be something Wklopf may be able to answer.
Is the BTU storage on Sodium acetate trihydrate better than potassium chloride and does it go from a liquid to a heavy viscos stage during the Phase change ?
Blue bird skies in CO so no snow plowing here on the Divide today.

ratio

Liquid/vapor instead of solid/liquid should simplify the HX. Google 'vapor phase soldering' for some interesting phase-change materials.

Derheatmeister

This is a US based company that uses a low environmental impact.Non flamable,non toxic,Non corrosive approach..https://phasechange.com/biopcm/

jumper

At today's cost of photovoltaic solar thermal doesn't compute. You can store energy in 250° water so that you have more useful energy with smaller storage. What to do with that energy is another issue. All depends on how much energy (electricity in OP's case) costs. With his geothermal that is probably a little more than $1/therm? Unless he has a big leaky house in cold climate.....

MaxMercy

Derheatmeister said:

This is a US based company that uses a low environmental impact.Non flamable,non toxic,Non corrosive approach..https://phasechange.com/biopcm/

Interesting claims, but there's no specifics at all on their website.

solsean

The Latento tanks use a parafin wax layer to increase the thermal storage. No reason it could not be done in an atmospheric tank. https://www.ivt-group.com/en/latento-solar-system/all-year-solar-system/

eclecticmn

Whatever happened to Glaubers Salts? I recall there were nucleation or settling issues that were solved and any patents have long since expired.

eclecticmn

I found this
https://www.youtube.com/watch?v=sLPrfG9btH0
https://solarthermalworld.org/news/denmarkbelgium-winner-video-research-competition-2014
Winner of Video Competition 2014: Storing solar energy using sodium acetate trihydrate and graphite

eclecticmn

I did a little digging about phase change materials. Back when I thought they showed great promise.
The best link is the last one. I posted over at r/solar but they keep getting taken down for some reason.
At the last link below they use passive solar for the salt tubes, no heat exchange. I wonder what metal could be used in contact with the salts.

I recall that Glaubers' Salts were considered for storing solar thermal heat in home heating applications. There were nucleation and settling problems that were fixed as I understood things. Nucleation meant that melting and re-crystalizing could not be repeated forever. Additives were used? Any such patents have long expired.

What happened?

https://www.extension.purdue.edu/extmedia/ae/ae-89.html

https://www.sciencedirect.com/science/article/abs/pii/0038092X80903321

https://www.osti.gov/biblio/6622821-thermal-energy-storage-encapsulated-glauber-salt-liquid-fluidized-bed

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1565&context=water_rep


Hmm more recent stuff

https://www.ingentaconnect.com/content/tcsae/tcsae/2016/00000032/00000020/art00027

also

http://www.allanstime.com/SolarHome/Eutectic_Salt/

As described on our web site, I can heat the north side of our home (passively) with these salts. Because of Dr. Telka's patented techniques, the salts cycle indefinitely. I have had mine for about 20 years.

My salt tubes are about 2 1/2 inches in diameter and 36 inches long and are black-heavy PVC -- sealed with caps on the ends. They work well. As you can see on our web site, I have them on the South side so that the winter sun gives them good exposure -- melting them during the day time, so that they will give off 90 degree F heat at night. If you had them in a circulating water bath, that would probably work as well.

A source
http://www.pcmenergy.com/products.htm?gclid=CJy7nI_YgpECFSUTIgodTXU0_Q

Jamie Hall

What happened? Unnecessarily complex -- like so many of the bright eyed concepts for solar heat.

jumper

Story from long ago. Guy actually built a geothermal heat pump for heating only running refrigerant pipes horizontally in his yard. After a few years soil froze up. So could he have used solar thermal in summer to store enough warmth in soil?

Jamie Hall

jumper said:

Story from long ago. Guy actually built a geothermal heat pump for heating only running refrigerant pipes horizontally in his yard. After a few years soil froze up. So could he have used solar thermal in summer to store enough warmth in soil?

Probably not. Keep in mind that there is a good deal of "solar thermal" falling on that yard all summer... for free, no moving parts. Now if the yard was heavily shaded, problem.

jumper

Jamie Hall said:

jumper said:

Story from long ago. Guy actually built a geothermal heat pump for heating only running refrigerant pipes horizontally in his yard. After a few years soil froze up. So could he have used solar thermal in summer to store enough warmth in soil?

Probably not. Keep in mind that there is a good deal of "solar thermal" falling on that yard all summer... for free, no moving parts. Now if the yard was heavily shaded, problem.

How deep does solar and warm air heat the ground? Enough to melt naturally frozen ground. Maybe just a bit more from energy from solar collector can sustain less expensive geothermal.

Jamie Hall

How deep? Depends on your climate and latitude, of course. In the northern states, by the time you get about five feet down, you are at a pretty constant temperature -- in the high forties. Deeper still constant. Freezing the ground is definitely a consideration with shallow geothermal -- and often not properly considered.

hot_rod

A school project I visited near Fairbanks had a dozen thermal panels tied to loops of Pex in the ground with a geo loop field. The just drove all the thermal into the earth in the summer, when the sunshine’s a lot😎 to boost the Hp performance,

jumper

hot_rod said:

A school project I visited near Fairbanks had a dozen thermal panels tied to loops of Pex in the ground with a geo loop field. The just drove all the thermal into the earth in the summer, when the sunshine’s a lot😎 to boost the Hp performance,

Tried using ground underneath parking lot for A/C condenser. Hopefully warmer ground melts snow. Some years needed auxiliary condenser and some years needed to hire plow. In years when things worked out I looked smart.

Mark Eatherton

THE best material for doing phase change energy storage is (drum roll please....) Di Hydrogen Monoxide. Yeah, water.

It requires the use of a water to water heat pump for heat extraction. It (H20) essentially goes from 1 btu/pound/degree F (roughly) between 211 and 33 degrees F), then when going from 33 degree water to 32 degree ice, it jumps to 144 Btu's per pound.

So, what's so exiting about having a frozen block of ice in a 1000 gallons polypropylene buried tank? ANYTHING with a thermal temperature potential greater than 32 degrees F becomes a "free" heat source. This includes, but is not limited to drain waste heat recovery, refrigerant waste heat recovery, ambient air, GSHP, well water, reverse driveway SIM, and solar thermal.

What would the thermal efficiency of a FP solar collector be with sun shining on it and a storage tank at 32 degrees F? Off current charts because they never considered this as a possibility. The WSHP can maintain efficiencies well above break even for a lot lower than 32 degrees F. On a seasonal basis, this same heat pump extraction system will probably need overheat protection at the end of the summer. Don't want to supply it with too high a heat or you will blow the compressor. There was a group in North Dakota a few years back that actually put numerous of these units in, and they worked beyond expectations. The guys name escapes me, might come back to me later, BIG cowboy with solar thermal and RFH background. Concept was called Thermal Storage Battery. Used poly prop tank, polyethylene tube heat exchangers in tank for input and extraction, off shelf controls/pumps etc. There are poly prop tanks available for FREE (totes). The tank can flex to accommodate expansion due to ice formation (20% increase in volume when frozen). We have the technology. Use it.

hot_rod

High Plains Drifter, Colin Wunder?
Energy Dynamics Inc.

Jamie Hall

Water is also an excellent refrigerant, with a number of very useful properties. The only disadvantage is its relatively high freezing point.

jumper

Jamie Hall said:

Water is also an excellent refrigerant, with a number of very useful properties. The only disadvantage is its relatively high freezing point.

I have somewhere proceedings from a '70's conference about solar powered A/C. Yes the refrigerant was water. Carrier and Sunstrand fabricated steam turbine powered turbocompressors. The solar collectors were not concentrated type. With concentrated collectors performance would be better.

The relatively high freezing point is actually an advantage for solar powered A/C. A PV powered ice maker enables off grid cooling.