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Solar paired with phase change material

wklopf Member Posts: 44
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
    Jamie Hall Member Posts: 15,350
    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.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • retiredguy
    retiredguy Member Posts: 350
    edited February 8
    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, ( that is located in the UK. They have a lot of phase change stuff. Just curious, where are you located?

    Hope this helps.
  • MaxMercy
    MaxMercy Member Posts: 151
    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
    hot_rod Member Posts: 14,552
    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 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Sol_Brother
    Sol_Brother Member Posts: 8
    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°.
    Sol Brother
  • Derheatmeister
    Derheatmeister Member Posts: 997
    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
    Jamie Hall Member Posts: 15,350
    "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...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Derheatmeister
    Derheatmeister Member Posts: 997
    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
    ratio Member Posts: 2,727
    Liquid/vapor instead of solid/liquid should simplify the HX. Google 'vapor phase soldering' for some interesting phase-change materials.
  • Derheatmeister
    Derheatmeister Member Posts: 997
    This is a US based company that uses a low environmental impact.Non flamable,non toxic,Non corrosive approach.. :)
  • jumper
    jumper Member Posts: 1,593
    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.....
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