The wiki article
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 it reaches 40C after which the temperature of the DHW becomes unacceptable. However if the water cooled to 10C 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 only 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.
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Comments here concern the thermal store.
I'd like to suggest that with stratified water, at end of useful hot water output one has a situation where the water at the top of the heat exchanger is 40C, and water below graduates down to 10C, rather than all water being at 10C. There isnt any way you could get hot water at
40C from a tank thats all at 10C, or 20 or 30. This still gives much more output than with a no-stratified tank.Second I have a question related to the thermal store diagram and how it might perhaps operate more effectively. I think we can accept that the water in teh cylinder will vary from bottom to top in a continous temperature progresion, ie 10,11,12,13,14 etc all the way to perhaps
65C or thereabouts. And we're assuming the water enters e heat exchanger at 10C for this example.The thermal store diagram is shown with the exchanger in the top half of the cylinder only. The water here would mostly be well above 10C.
Now, if the exchanger is in the top part of the cylinder, the mains pressure cold water comes into the bottom of the heat exchanger, which is halfway up the tank, and will meet tank water temp of somewhere between 10 and 65C, lets say maybe 35-40C. What this means is at end of useful hot output, the water below the heat exchnager will be about the same temp as it was before HW was drawn off, namely a gradient from 10C to 40C.
OTOH if one were to use a full height heat exchnager, with the cold fed to the heat exchange pipe at the bottom of the tank, not half way up, that bottom half of tank water prewarms the HW before it reaches the half way up the tank level in the heat exchange pipe. This means the top half of tank water has to do less work, and the heat, or just warmth, is being drawn out of the colder water in the bottom half. IOW you'll get more total volume of HW out, and the tank will end up with colder water in the bottom half than if it used a top section only heat exhanger.
I'm not sure if I've explained it too good!
NT