Hot water storage tank
Encyclopedia
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.
s. 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.
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
, brick
s, 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.
Water tank
A Water tank is a container for storing water. The need for a water tank is as old as civilized man, providing storage of water for drinking water, irrigation agriculture, fire suppression, agricultural farming, both for plants and livestock, chemical manufacturing, food preparation as well as many...
, 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
District heating
District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...
, Wood-burning stove
Wood-burning stove
For a list of stove types see Stove .A wood-burning stove is a heating appliance capable of burning wood fuel and wood-derived biomass fuel. Generally the appliance consists of a solid metal closed fire chamber, a grate and an adjustable air control...
, or a district heating
District heating
District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water 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
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
such as limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, 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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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 collectorSolar thermal collector
A solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air...
s. 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
Descaling agent
An anti-scaling agent, also known as descaling agent, anti-limestone, anti-limescale, anti-lime or anti-scale are preparations to prevent the buildup or to remove limescale and fouling...
(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
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
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
Stratification
Stratification is the building up of layers. Stratified is an adjective referring to the arranging of layers, and is also the past form of the verb stratify, to separate or become separated into layers...
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
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, brick
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...
s, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
. 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
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
, 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.