Let the area of the flat plate solar collector be A, average insolation per unit area be S, the length of time that the flat plate solar collector is exposed to sunlight be T, the fraction of solar energy absorbed by the phase change material be F, and the latent heat of the phase change per unit mass of the phase change material be L, and the required mass of phase-change material be M. Then once the flat plate solar collector reaches the phase change temperature, LM = ASTF or M = ASTF/L.

Could you send me a paper to more explanation??? 

I haven't written a paper on this subject. But I corrected an oversight in my answer, accounting for the length of time T that the flat plate solar collector is exposed to sunlight.

Ok, how to calculate this factor "the fraction of solar energy absorbed by the phase change material be F"??

The reflectivity or albedo, or equivalently F = 1 - albedo, of many materials is probably available in handbooks on the subject and online. To measure F, rearrange M = ASTF/L to F = ML/AST = (L/AS) X (M/T). L, A, ans S are known, so multiply by L/AS by the time rate M/T at which the phase change material undergoes phase change.

This paper would help you I think.

DSC ANALYSIS OF COMMERCIAL PARAFFIN WAXES FOR ITS USE AS SOLAR ENERGY STORAGE MATERIAL

https://www.researchgate.net/publication/316740689_DSC_ANALYSIS_OF_COMMERCIAL_PARAFFIN_WAXES_FOR_ITS_USE_AS_SOLAR_ENERGY_STORAGE_MATERIAL

Article DSC ANALYSIS OF COMMERCIAL PARAFFIN WAXES FOR ITS USE AS SOL...

Thank you very much