Microencapsulated phase change material (MPCM) slurries were field-tested in ground source heat pump (GSHP) systems in the Southern United States to validate their thermal performance and durability as heat transfer fluids (HTF). MPCM particles consisted of methyl stearate (melting temperature: 39.5 degrees C) as phase change material (PCM), microencapsulated with polyurea as shell material. Experimental results showed that MPCM slurries transport more thermal energy than water at a constant pumping power due to the higher heat capacity associated with the PCM. Demonstration experiments showed that using MPCM slurries improved the heat load-to-pumping power ratio by up to 34% when using a coaxial heat exchanger. The coefficient of performance of the GSHP system was enhanced by up to 4.9% when using MPCM slurries. It can be concluded that MPCM slurries are viable HTF because of their higher heat capacity. In terms of durability, no significant damage of MPCM was detected under continuous pumping conditions after 123,252 pump-cycles or an estimated life span of 10.5 years. Results also suggest that progressive cavity pumps are more suitable than centrifugal pumps from the durability point of view. Long-term durability studies should be considered for the eventual implementation of MPCM technology in heating and cooling applications. (C) 2016 Elsevier Ltd. All rights reserved.