Using a solid thermal mass is discussed on the linked wiki: https://fridge0.branchable.com/thermal_mass/ https://fridge0.branchable.com/thermal_mass/ compositi

Using a solid thermal mass is discussed on the linked wiki: https://fridge0.branchable.com/thermal_mass/ https://fridge0.branchable.com/thermal_mass/ composition Water is the easy choice. A material that can be frozen would be better, because it takes a lot of extra heat to melt a frozen material and so more cold could be stored. However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall. This is an open research area.