The main problem in the solar energy field is the storage of thermal energy. To divert this problem, it was suggested to use a flat-plat solar collector which also serves as a storage system; this solution will reduce the size of a refrigerating machine that we are studying. A high stored energy density is only possible if we through use latent heat of phase change. Thermal analysis has been developed for this type of storage collector for near-steady state conditions using a nanofluid heat storage substance depended on KNO3–NaNO3 binary salt mixture as PCM and a mix of Al2O3–SiO2 as nanoparticle, from which the new Hottel-Whillier-Bliss equations have been used for efficient flat plate collector. Computations were achieved for a large variety of parameters to verify the significance of the created model.