Improved phase-change material formulations for heat-storage applications

Abstract

Energy storage is an important tool in combatting misalignment in energy supply and demand. Increased use of such storage can help to improve the flexibility of the energy grid, especially as variable-output renewables such as solar and wind energy become more common. Storage can range from short-term (e.g. balancing between night and day) to seasonal. Energy-storage systems take in energy, store it for a period of time, then release it in a charge-store-discharge cycle. Heat-storage technologies are designed to store excess thermal energy to be used at a point in the future. Depending on the technology used, the storage system could be used for a single process or building, or a site or region.

Salt hydrates are a class of material that has great potential for use as thermal-energy-storage materials. They have a large latent heat of storage, making them useful as phase-change materials (PCMs), where energy is stored over a first-order phase transition. They also have high energy densities and a range of melting points suitable for heat-storage applications. However, to be commercially suitable, they must be able to work effectively over thousands of melt-freeze cycles.

Sodium sulfate decahydrate (Na2SO4·10H2O) melts incongruently, decomposing to the anhydrous salt, which precipitates out and can take no further part in the storage. Building on previous work, poly(methacrylic acid) was identified as a promising additive candidate. Temperature cycling was performed, and showed improved performance, retarding the growth of large crystals of anhydrous salt. Physical agitation methods also led to improvements in cycle stability with over 200 cycles completed with no sign of segregation, due to suspension of and improved re-dissolution of the anhydrous salt. Xanthan gum was identified as a potential thickener for the system, and a very desirable heat profile and no sign of segregation were seen over 90 cycles in a stirred system. An unstirred system showed signs of segregation and increased sub-cooling after 15 cycles, but work is ongoing with a higher percentage of xanthan gum. Cell-scale cycles are expected to be undertaken in the near future.

Calcium nitrate tetrahydrate Ca(NO3)2·4H2O melts congruently, but fails to nucleate, instead sub-cooling up to 60 K below its freezing point. It also has a very slow rate of crystallisation. Both homogeneous and heterogeneous methods were trialled. Two homogeneous methods of crystallisation were developed; one where a crystallite is maintained under pressure in cracks in a metal disc, and the second where calcium nitrate tetrahydrate is formed in situ from anhydrous calcium nitrate added to a calcium nitrate solution to give a water-poor formulation. A novel “cascade nucleation” mechanism comprising anhydrous strontium nitrate as a nucleator for magnesium nitrate hexahydrate, and magnesium nitrate hexahydrate for calcium nitrate tetrahydrate was also identified. The preparation method, composition and tolerance to conditions were determined, and the initiation of crystallisation was tracked in real-time using hot-stage microscopy. Cell-scale cycles showed consistent nucleation over multiple cycles and the system is now commercially available.

A metal disc with machined grooves can hold seed crystals under pressure and prevent them from melting at high temperatures. These seed crystals can then be ejected into a sub-cooled bulk solution to initiate crystallisation. This nucleation method is commonly used for sodium acetate trihydrate, and this work demonstrated its successful use with calcium nitrate tetrahydrate and sodium thiosulfate pentahydrate. Pressure-temperature phase diagrams were constructed mathematically and experimentally, and although the experimental technique could be refined, were shown to be broadly in agreement.