The form-stable phase change materials (PCMs) with fatty acid eutectics absorbed in and/or supported by the overlaid mats of electrospun polyacrylonitrile (PAN) nanoflbers or the derived carbon nanoflbers were explored for storage and retrieval of thermal energy. The carbon mats were made from the PAN mats through stabilization in air followed by carbonization in nitrogen. Three fatty acid eutectics of capric acid-lauric acid, capric acid-palmitic acid, and capric acid-stearic acid were studied as model PCMs; for comparison, the individual fatty acid of capric acid was also studied. The results indicated that electrospun nanofibrous mats were highly porous and capable of absorbing a large amount of PCMs, and the maximum absorption capacities of PAN and carbon mats were similar to 99 wt% and similar to 81 wt%, respectively. The composite materials (i.e., the form-stable PCMs) could well-retain their overall shapes when the PCMs were in the molten stage; i.e., they would not flow/leak from the mats. The conversion of PAN mats into carbon mats increased the thermal energy storage/retrieval rates of the resulting composite PCMs; nonetheless, the thermal treatments of stabilization and carbonization also made the mats to be less fluffy, leading to the reduction of absorption amount. Morphological structures, as well as the properties of thermal energy storage, absorption capacity, and thermal energy storage/retrieval rates, of electrospun nanofibrous mats and the resulting composite PCMs were studied by scanning electron microscopy, differential scanning calorimetry, and measurement of melting/freezing times. The results indicated that the melting temperatures and enthalpies of the prepared composite PCMs were in the range of 19.8-31.4 degrees C and 120.1-165.4 kJ/kg for PAN mats, and 18.8-30.7 degrees C and 92.1-128.0 kJ/kg for carbon mats, respectively. In comparison with melting times of CA-PA/PAN and freezing times of CA-SA/PAN, the melting times of CA-PA/carbon and freezing times of CA-SA/carbon were shortened by about 49.5% and 41.0%, respectively. It is envisioned that this innovative type of form-stable PCMs could be utilized for potential applications in building energy conservation. (C) 2012 Elsevier B.V. All rights reserved.