- Hot water storage tank
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. . 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.
Solar hot water storage tank
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.
Hot water storage tank with closed water circuit
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.
Stratified hot water storage tank with closed water circuit
- For an illustration - see reference
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:
- The different heating and cooling loops must send the heated or cooled water in with as low velocity as possible.
- This means that the heating and cooling loops each must have a velocity controlled pump - and the tube ports must have as big inner diameter as is feasible.
- The heat sources hot return water are send into the top - and the heat sources gets cooler water from the bottom.
- Heat usage gets hot water out at the top and cool return water is send in at the bottom.
- There can be "stratification-enhancing"-devices within the hot water storage tank, but if it the water inlet velocity is as low as possible this might not be needed.
- There must be used a more advanced heat control system
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. 
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. 
- ^ a b invest-tools.com: Graphs, Main page and description: Overview of the DHW system, The water tank (heat reservoir) Quote: "...With the tank there came a 10 cm "soft foam" isolation that fulfills the minimum insulation requirements...So after several months, I added another round of 16 cm aluminum coated glass wool, and now the tank stores enough heat for about 7 or 8 days when it is full loaded (temperature range is 95°C down to 40°C)..."
- ^ a b Solar heating system with stratified hot water storage tank with closed water circuit and the possibility of reading many current parameters: solar.webseiten.cc: Live-Daten Holzhausen
- ^ Technische Alternative: Freely programmable universal controller UVR1611, Manuals and firmware in english and german: downloads, Programing examples: UVR-Beispielsammlung, Practical examples on solar heating systems with hot water storage tanks with current measuring date: Livedaten
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