Melt spun bi-component fibers with PCM to optimized alloys based on and polyolefin viscosity modifiers with regar thermal efficiency, melt viscosity and cost in order to produce melt spun bi-component with a latent heat greater than 60 J/g in the temperature range 27-36 oC and with sufficient strength for use in garments and other products (tenacity greater than about 20 cN/tex) has been demonstrated in industri scale production. An example of PA6 fibers with PCM in core is shown in Figure 2.