Hi Asyraf,

There will be some details depend on your specific application, however, generally speaking if you are using the fins to enhance heat transfer (as commonly used) then you would look for materials with higher thermal conductivity (k) e.g. copper. However, if you want to enhance heat transfer and store heat then, you are in trade off between thermal conductivity (k) and specific heat (Cp).

I hope this helps  

Your question is incomplete in detail.

(1) Is it latent heat storage or sensible heat storage?

(2) You state "The fins is attached to tube along in the thermal storage." It is still not clear. If it means that fins are on the side of heat storage medium, then are you talking about latent heat storage? Fins are likely to be useful only if you are using latent heat storage. Fins will not be cost effective on waterside or the sensible heat storage side

(3) Is the heat stored in the fin itself important to you?

(4) Is the water passing through tubes inside the storage cylinder of 30 cm dia? How many such tubes are you going to have? Or is it a coil? If yes, put in multiple coils and put the fins on the outside of the water carrying tubes.

Though copper is the best, aluminium is more economical. Since you are presumably talking about temperatures < 100 C, aluminium is a good choice.

IMPORTANT: Do not use stainless steel. Make sure there is NO contact resistance between fin base and the tube surface.

Better to purchase tubes with readymade extruded fins. If you are doing in-house fabrication, theoretically speaking, you can TIGweld the fins but you need a lot of fins for effective heat storage and discharge, so self-manufacture may be inadvisable..