Composite phase change materials (CPCMs) improve the practicality of phase change materials by limiting leakage, providing structural support, and often increasing thermal conductivity. However, many CPCMs exhibit insufficient heat-transfer rates and limited long-term stability. Incorporating nanoparticles can further modify thermal transport, nucleation, interfacial behavior, and structural performance. Although CPCMs and nano-enhanced PCMs have been reviewed extensively, the ternary architecture comprising a PCM, supporting matrix, and nanoparticles has received little focused attention. This review critically analyses 51 studies of nano-enhanced CPCMs (NCPCMs), distinguishing the effects of matrix incorporation from the subsequent effects of nanoparticle addition. Nanoparticle incorporation increased thermal conductivity in 97.7% of the reported formulations, with a median improvement of 47%. Median changes in apparent latent heat and supercooling were −1.4% and −8.3%, respectively, indicating that phase-transition responses were less consistent than improvements in heat transport. Selected studies reported leakage reductions of up to 80% and melting- or freezing-time reductions of approximately 30–50%. Available cycling results showed a median latent-heat retention of 96.7%, although only nine studies tested at least 500 cycles. The evidence identifies carbon nanomaterials as effective conductivity enhancers at low loadings, oxides and ceramics as suitable for nucleation control and cycling stability, and metals as useful where electrical or other added functions are required. Typical starting ranges are 0.1–3 wt% for carbons, 1–5 wt% for oxides and ceramics, and 5–25 wt% for metals. Overall, NCPCM performance depends on nanoparticle characteristics, matrix architecture, and interfacial compatibility, highlighting the need for standardized testing, interfacial characterization, and long-term system-level validation.
