Paraffins and salt hydrates store a large quantity of heat in a narrow temperature band, which is exactly what a thermal store should do. Their weakness is transport: thermal conductivities of a few tenths of a watt per metre-kelvin mean the charge and discharge rate, not the capacity, sets what the store can actually deliver. This open access review in Energies surveys graphene-based composite phase change materials, which are the most direct attempt to fix that.
The mechanism and its price
Graphene and graphene nanoplatelets carry heat along the basal plane extremely well. Dispersed or, better, assembled into a continuous porous scaffold, they create conduction paths through an otherwise insulating wax, raising effective conductivity by an order of magnitude at modest loadings. Foams and aerogels outperform loose dispersion because they build a percolating network rather than isolated islands, and they also suppress leakage of molten material — a persistent nuisance in encapsulated designs.
Every improvement is paid for in capacity. Graphene stores no latent heat, so each percentage point of filler displaces active material and lowers the enthalpy of fusion per kilogram of composite. Higher loadings also increase supercooling in some systems and can shift the transition temperature. The design problem is therefore an optimisation, not a maximisation: enough network to carry the heat in and out at the required rate, and no more.
Where it should be used
The composites make most sense where power density, not capacity, is binding — battery thermal management, electronics cooling, short-cycle building fabric storage. For seasonal or day-length stores, where charge and discharge take hours, the untreated material's conductivity is often adequate and the filler is wasted money.
The exergy view
Conductivity is an exergy variable disguised as a heat transfer variable. A poorly conducting store needs a large temperature difference to move heat at the required rate, and that difference is destroyed work at both ends of the cycle: the source must run hotter than necessary to charge it, and the load receives heat colder than the store nominally holds. Adding a graphene network narrows the difference and recovers that lost quality. The corollary is that the filler earns its capacity penalty only in duties where the rate is binding. Chosen on a first-law view of storage capacity, these composites will usually look like a downgrade; chosen on the temperature difference they eliminate, they often pay.
