Optimized PCM/polymer alloys based on paraffin wax and polyolefin viscosity modifiers with regard to thermal efficiency,melt viscosity and cost in order to produce melt spun bi-component fibers with a latent heat greater than 60 J/g in the temperature range 27-36 and with sufficient strength for use in garments and other products (tenacity greater than about 20 cN/tex) has been demonstrated in industrial scale production.