In solar hot water systems, a water storage tank is used to store the heat generated by solar collectors, sometimes also known as a hot water storage tank. The use of liquids (especially water) for heat storage is the most mature, widely promoted, and applied theory and technology among various heat storage methods. It is usually desirable for the liquid used to have not only a high specific heat capacity, but also a high boiling point and a low vapor pressure. The former is to avoid phase transition (becoming gaseous), while the latter is to reduce the pressure on the thermal storage container. Among low-temperature liquid thermal storage media, water has the best performance and is therefore the most commonly used one. 
Advantages and disadvantages of using water as a thermal storage medium 
advantage 
① The physical, chemical, and hydrothermal properties are very stable, and people have a very clear understanding of them, with the most mature technology used; 
② Can serve as both a thermal storage medium and a heat transfer medium, eliminating the need for a heat exchanger in the thermal storage system; 
③ The heat transfer and liquid properties are quite good. Among commonly used liquids, it has the highest specific heat capacity, a small coefficient of thermal expansion, and low viscosity, making it very suitable for natural circulation and forced circulation; 
④ The temperature pressure relationship at liquid gas equilibrium is highly applicable to flat solar collectors; 
⑤ Rich sources and low prices. 
disadvantage 
① As an electrolytic corrosive substance, the generated oxygen is prone to corrode metals and is a solvent for most gases (especially oxygen), making it easy to corrode containers and pipelines; 
② During solidification (freezing), the volume expands significantly (up to about 10%), which can easily cause damage to containers and pipelines; 
③ Above medium temperature (over 100 ℃), its vapor pressure increases exponentially with the increase of hot water temperature. When using water to store heat, both temperature and pressure cannot exceed their critical points (373.0 ℃, 2.2 × 10Pa). In terms of cost, the cost of storing heat at 300 ℃ is 2.75 times higher than that at 200 ℃. 
When using water as a thermal storage medium, various materials such as stainless steel, enamel, plastic, aluminum alloy, copper, iron, reinforced concrete, wood, etc. can be used to make thermal storage containers. The shape can be cylindrical, box shaped, or spherical, but attention should be paid to the anti-corrosion and durability of the materials used. For example, when selecting cement and wood as materials for thermal storage containers, their thermal expansion must be considered to prevent cracks and water leakage caused by long-term use. 
A hot water storage tank is a device that can store both heat and cold. It was developed as a component in the system of supplying hot water, heating, and air conditioning to buildings, mainly used to regulate the imbalance between energy and consumption, in order to improve the thermal utilization efficiency of the system and meet the needs of heat load. 
Hot water storage tanks can be classified into various types due to their heat release characteristics (fully extruded flow, fully mixed flow, and partially mixed flow), pressure state (open and closed), number of tanks (single or multiple), installation methods (vertical or vertical and horizontal or horizontal), structural materials, and usage. Below, only the first two will be highlighted.