Water as thermal mass
Thermal mass is used in and around buildings to absorb or emit heat. Internal thermal mass, such as provided by water, in a well-insulated building reduces the temperature swings in the interior. Water has a high specific heat that makes it an ideal and readily available, cost-effective provider of thermal mass.
Thermal mass is often used in solar heating systems to absorb heat during the day and release it into the living areas during the night. The thermal mass can also help during longer periods such as overcast days. In simple solar heating systems, the thermal mass may directly absorb sunlight; in more complex systems, moving liquid or air is used to transfer heat between the collectors, the thermal mass and the living areas.