Encapsulation of high temperature molten salts
The present disclosure relates to a method of encapsulating microcapsules containing relatively high temperature phase change materials and the microcapsules so produced. The microcapsules are coated with an inorganic binder, film former and an inorganic filler. The microcapsules may include a sacrificial layer that is disposed between the particle and the coating. The microcapsules may also include an inner coating layer, sacrificial layer and outer coating layer. The microcapsules are particularly useful for thermal energy storage in connection with, e.g., heat collected from concentrating solar collectors.
1. A method of encapsulating microcapsules containing phase change material, comprising:
supplying a phase change material comprising an inorganic salt selected from nitrates, nitrites, and hydroxides of potassium, lithium and combinations thereof, having a melting point of 200° C. to 600° C., and a largest cross-sectional dimension of 0.5 mm to 100 mm;
applying a coating to said phase change material wherein said coating includes an inorganic binder, a film former comprising an organic polymer at a MW of greater than or equal to 5000, and an inorganic filler at a particle size of less than or equal to 50 μm and wherein said coating has a thickness of 50 μm to 1000 μm; and
heating said coating at a temperature in the range 200° C. to 400° C. wherein at least a portion of said organic polymer of said film former is removed to form said microcapsules, wherein from 0% to 5% of said microcapsules rupture at temperatures of 200° C. to 600° C.
2. The method of claim 1 wherein said binder is present in the range of 5% to 30% by wt. relative to the combination of binder, film former and inorganic filler, said film former is present in the range of 1% to 10% by wt. relative to the combination of binder, film former and inorganic filler, and said inorganic filler is present in the range of 50% to 90% by wt. relative to the combination of binder, film former and inorganic filler.
3. The method of claim 1 , wherein said binder comprises an inorganic aluminum colloid and is present in the range of 15% to 25% by wt. relative to the combination of binder, film former and inorganic filler, said film former comprises ethyl cellulose and is present in the range of 1% to 10% by wt. relative to the combination of binder, film former and inorganic filler, and said inorganic filler comprises mineral silicate and is present in the range of 70% to 85% by wt. relative to the combination of binder, film former and inorganic filler.
4. The method of claim 1 wherein 80% or more of the organic polymer of the film former is removed with heating.
5. The method of claim 1 wherein said inorganic binder comprises an inorganic solvent based binder applied to said phase change material comprising dispersed phase inorganic particles wherein said particles are present at a size of 1 nm to 1000 nm.
6. The method of claim 1 wherein said inorganic binder comprises dispersed aluminum particles.
7. The method of claim 1 wherein said inorganic filler comprises mineral silicates, clay, silica, and/or micronized stainless steel.
8. The method of claim 1 , wherein said microcapsule shell further includes a sacrificial layer, wherein said sacrificial layer is disposed between said phase change material and said coating layer and wherein said heating of said coating removes said sacrificial layer and provides a space as between said coating layer and said phase change material.
9. The method of claim 8 wherein said sacrificial layer comprises an organic polymeric resin that thermally degrades upon exposure to heat.
10. The method of claim 8 wherein said sacrificial layer comprises hydroxyl-propyl methyl cellulose, plant protein or ethyl cellulose.
11. The method of claim 8 wherein said sacrificial layer has a thickness of 1.0 μm to 500 μm.
12. The method of claim 1 wherein said microcapsules are packed in a storage tank containing a heat transfer fluid.