An analytical study is described of a freezing/melting phenomenon of a phase change material encapsulated in a pipe with the heat transfer fluid flowing in the annulus of a concentric pipe. The moving front problem is solved numerically using the boundary immobilization technique. Parameters such as the Stefan number, the Fourier number and the diameter ratio of the two concentric pipes to obtain sufficiently large heat transfer rates to make the system suitable for thermal energy storage are examined. © 1992.