Thermal Stores and Heat Banks

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In these systems a container of water is heated by the boiler or immersion heater etc. Mains pressure cold water passes through a heat exchanger where it is heated by the stored hot water and supplies the taps etc. The thermal storage container itself is under low pressure, sometimes only the equivalent of a few centimetres of water. With very low pressures the container does not have to be cylindrical but can easily be made in other shapes such as rectangular section, which can make better use of the space they are installed into. (However for the sake of clarity they wil be referred to here as cylinders.)

In a classic thermal store the heat exchanger is a coil of pipe with a large surface area within the cylinder itself. As cold water flows through the coil it is heated by the hot water surrounding it and as the surrounding hot water cools it sinks by convection bringing hot water from elsewhere in the cylinder into contact with the coil.

In another type of system, sometimes referred to as a heat bank, the heat exchanger is external to the hot water cylinder. When DHW is required hot water is pumped through the heat exchanger and back into the cylinder. The pump is usually controlled by a switch in the DHW pipework which senses when water is flowing to the hot taps etc. In this type of design the heat exchanger is usually a type comprising several plates of copper joined in a sort of multi-layer sandwich, known as a Plate Heat Exchanger (PHE).

Thermal Store Heat Bank
http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/thermalstore.jpg http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/heatbank.jpg

In this document when comparing the two types of system we refer to them as thermal stores and heat banks, but elsewhere (e.g. when comparing these kinds of system with unvented and conventional) we refer to them generally as thermal stores.


Thermal Stores and Heat Banks: Pros and Cons

Clearly a Thermal Store, with its internal heat exchanger and no moving parts, is simpler than a heat bank, with its pump, heat exchanger and flow switch. On the other hand thermal stores are supposed to be more prone to scaling up in hard water areas, so it may be advised to use heat banks in these locations. [Need to expand on this]

Since the heat exchanger and pump are outside the heat bank they can be sited remotely from the bank, potentially closer to the point of use, reducing wastage of hot water in dead legs of DHW pipework. One can also have multiple heat exchangers and pumps drawing from the same heat bank, e.g. supplying different areas of a building. Also a heat bank can be DIY-built out of fairly standard components rather than as a custom assembly.


Another difference between thermal stores and heat banks is in how efficiently they use the amount of hot water they contain to provide DHW. This concerns stratification and mixing of water in the store, a discussion of which follows (skip discussion).

Stratification

Assume (for the sake of illustration) that

  • the water in the store reaches 70C when fully heated
  • the incoming cold water is at 10C
  • the hot water must be at least 40C to be acceptable
  • the heat exchanger is perfect, i.e. there is no temperature drop across it.

As DHW is heated it will rise to 70C (when the store is fully heated) and will cool the water in the store to 10C. If the cooled water is thoroughly mixed with the hot water the whole store will progressively cool until, when it reaches 40C, the temperature of the DHW starts to suffer. However if the 10C cooled water is kept separate and only the water at 70C is fed to the heat exchanger then the DHW will continue to produce acceptable water until the whole store contains water at 10C. In the first scenario the amount of heat available by cooling the store from 70C to 40C is available for heating DHW, while in the second the amount of heat available is that of cooling the store from 70C to 10C. This gives much more DHW for a given volume of store at a given temperature. Stratification, where the cool water at the bottom of the store remains relatively separate from the hotter water further up, provides a means of approaching this goal.

Circulation of water in a a thermal store is driven by convection currents caused by the cooling effect on the stored water of the cold water flowing through the heat exchanger. These convection currents are relatively gentle, and proportional to the amount of heat being drawn from the store. Thus the cooled water tends to fall gently to the bottom of the store leaving hotter water higher up, nearer the heat exchanger. As the store continues to supply heat to DHW the cooler layers of water extend further up the cylinder until at some point, when much of the heat exchanger is surrounded by cooler water, the temperature of DHW drops below 40C. At this point there will still be some hot water near the top of the cylinder, so the thermal store is not using the stored hot water with perfect efficiency.

In a heat bank water can be drawn from the very top of the cylinder to pass through the heat exchanger, so all the stored hot water should be available for heating DHW. Unfortunately most DHW heat banks use a simple fixed-speed central heating circulator switched by a flow switch in the DHW pipework. This has to provide sufficient flow through the heat exchanger to provide satisfactory DWH temperature at maximum DHW flow rate, so even when the demand is less the pump is still circulating water at its maximum rate. This causes unnecessary mixing in the cylinder, destroying stratification. This disadvantage is not inherent: it would be possible to control flow rate through the heat exchanger to the rate necessary to give the required DHW temperature. This could be done either electronically, by controlling the pump speed, or mechanically by restricting the flow through the pump and heat exchanger (e.g. by a mechanism similar to that in thermostatic radiator valves).

Recovery

Conventional cylinders used with boilers and central heating systems are indirect types because the primary water must be kept free of oxygen and may be treated with corrosion inhibitor and other chemicals. However in thermal stores and heat banks the water in the cylinder heats DHW indirectly through a heat exchanger so the water in the store can be the primary water shared with the boiler etc. This allows the store to recover as fast as the boiler can generate heat. Also, by eliminating the temperature drop across the primary heat exchanger, the store can be kept at its desired temperature from a lower temperature of primary water. This results in better efficiency with high-efficiency condensing boilers. This arrangement does, however, require that the primary circuit be vented (since an unvented system would place the storage vessel under pressure, as in unvented DHW systems). In practice this means a small header tank is required above the highest point in the system i.e. above any radiators on the top floor. [picture]


Configurations

Four common configurations of thermal store are:

External header tank Internal header tank
Direct: electric only or sharing primary water with boiler http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/Thermal_Store_Direct_Cylinder_150.jpg http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/Thermal_Store_Direct_Combi_150.jpg
Indirect: thermal store separate from boiler's primary water http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/Thermal_Store_Indirect_Cylinder_150.jpg http://homepage.ntlworld.com/john.stumbles/DHW_FAQ/Thermal_Store_Indirect_Combi_150.jpg