PCM can be used in many ways for the cold storage. You can use PCM as the side wall of the clod storage chamber or you can use with condenser. Depending of the temperature that you would like to maintain in your chamber, you have to select the PCM phase change temperature. Also, take consideration of highest latent heat of vaporization of your PCM. PCM container is another factor. For more information please see the article: Article Conventional Refrigeration Systems Using Phase Change Materi...
when we know water is an excellent PCM which stores and releases heat by changing its phases like solid to liquid, then why we try for other different other PCM's?
We try other PCM to serve our purpose. The melting point of water is 0 degree C at NTP, where as if we need higher melting point PCM then we have to search other PCM.