A hot water storage tank (also hot water tank, thermal storage tank, hot water thermal storage unit, heat storage tank, hot water cylinder) is a water tank, that is used for storing hot water for space heating or domestic use. A heavily insulated tank can retain heat for days. [1]. Hot water tanks may have a built-in gas or oil burner system, or electric immersion heaters, or may use an external heat exchanger to heat water from another energy source such as district heating, Wood-burning stove, or a district heating system.
Water is a convenient heat storage medium, because it has a high specific heat capacity. Compared with other substances it can store more heat per unit of weight (and volume). Water is non-toxic and low in cost.
Water heaters for washing, bathing, or laundry have thermostat controls to regulate the temperature, in the range of 40 to 60°C, and are connected to the domestic cold water supply. Where the water supply has a high content of dissolved minerals such as limestone, heating the water causes the minerals to precipitate in the tank; a water tank may develop leaks due to corrosion after only a few years. Dissolved oxygen in the water can also accelerate corrosion of the tank and its fittings. A hot water storage tank is wrapped in heat insulation to reduce heat loss and energy consumption.
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In a solar heating system, a solar hot water storage tanks stores heat from solar thermal collectors. The tank has a built-in heat-exchanger to heat domestic cold water. These tanks have the same problems with limestone deposit and corrosion, which reduce the efficiency of the heat exchanger.
Another method to store heat in a hot water storage tank is to heat the water directly in one (or more) heat sources via external heat-exchangers. The heat can later be tapped again to hot water or heating via external heat-exchangers.
The good thing about avoiding sending domestic cold water directly into the hot water storage tank is that only a little limestone is deposited when water is replenished in the closed water circuit.
Another good thing is that the hot water storage tank water only seldom is added domestic cold water with oxygen, which relaxes the materials choices for the hot water storage tank and closed water circuits.
The external heat-exchanger used for domestic hot water will have limestone deposits and for that reason need to be exchanged or use some benign descaling agent (temporary pressurized CO2?) from time to time, but this is cheaper than exchanging the hot water storage tank.
Another method to store heat in a hot water storage tank is as above, but with some important changes. It have many names: Stratified hot water storage tank with closed water circuit, stratified thermal storage, thermocline tank and water stratified tank storage.
The important changes are as follow. Water mixing within the hot water storage tanks vertical water column must be minimized. These water layers typically has different temperatures. The purpose is to maintain the vertical stratification of the water column. This is achieved by:
When a stratified hot water storage tank has closed water circuits, the water temperatures can be up to 90...95°C at the top - and e.g. 20...40°C at the bottom.
Calm and undisturbed water is a relatively bad heat conductor comparable with glass, bricks, soil. This is illustrated by a lake, where the surface water is warm but deeper layers are cooler. An arbitrary volume of hot water can be stored, as long as the stratification is kept intact. No vertical metal plates or tubes may be used inside the hot water storage tank, because this would conduct heat between the stratified layers. The hot water storage tank is insulated.
The advantage is that water can be stored at 90...95°C in this hot water storage tank type, giving a higher energy density, and this energy can be stored a long time if the hot water is not diluted.
Optimizing water exchanges can allow different levels to be tapped within the tank, matching temperature to the required use. [2]
A practical solar heating system with many parameters can be read as a function of time - incl. the hot water storage tank temperature near the top. [1]