Working fluids, such as drilling fluids, may remove heat from other fluids, tools, equipments and environments and transfer it to other locations by using reversible phase change elements. The heat removal occurs through the absorption of heat by one or more phase transitions or a sequence of phase transitions in the elements of the working fluid. For instance, heat is absorbed when the phase change portions of the reversible phase change elements change phase including, but not necessarily limited to, a change from solid to smectic liquid crystal, from solid to nematic liquid crystal, from smectic liquid crystal to isotropic liquid, from nematic liquid crystal to isotropic liquid, from solid to isotropic liquid, and sequences and combinations thereof. Heat is released when the phase change reverses. These phase changes are first-order transitions and are associated with a latent heat or enthalpy.