Accumulator tank stratification - stratified heat store UK

Accumulator tank stratification

Stratification is another word for layering, and this is exactly what you want the water in your tank to do. You would like the hottest water to be at the top of your accumulator tank (also known as a heat store), the cooler water at the bottom of the tank, and as small as possible a mixing layer between the two.

The reason for this is that the accumulator is to act as a heat store - which means storing up heat (in the form of hot water) from, say, your boiler stove and solar panels when they are hot, and then letting you use this heat later on. The hot water at the top of the accumulator tank will be used for your hot water and heating. If the water at the top starts out at, say, 75 ºC, then it will easily provide you with hot water for a long time. As you use the hot water the idea is that the layer of hot water gets smaller rather than the whole layer cooling down. This means that the tank can provide usable hot water for a long time.

As you heat the water in the tank up the idea is that the layer of hot water at the top gets bigger (and the cold water layer at the bottom gets smaller).
If the temperature of the water in the tank was totally uniform then the tank would stop providing usable hot water long before the tank with good stratification.

As an example lets imagine we have a 1000lt accumulator tank which has a 250lt layer of hot water at the top which is at 80 ºC. The rest of the water in the tank is at 20 ºC. This tank will continue to provide hot water for some time, we are talking quite a lot of showers. Now lets mix the water in the tank up. By mixing the 250lts at 80 ºC and the 750lt at 20 ºC the temperature of the whole accumulator tank is now 35 ºC, and will not provide us with hot water - well not hot enough for me anyway!

Heat transfer coils for Domestic Hot Water (DHW) in the tank are run so that the water enters at the bottom of the coil. This means that when the water gets near the top of the coil that it has already picked up a fair bit of heat from lower down in the tank, which means that it cools the water at the top of the tank less.

Similarly in solar accumulator tanks with a solar coil at the bottom of the tank the hot water from the solar panels enters at the top of the coil and runs downwards. This means that most of the heat is transferred through the upper part of the coil into water higher up in the tank.

Because the solar coil is at the bottom of accumulator tanks it can lead to mixing because it heats the cooler water at the bottom of the tank, which then rises and creates a current. These currents can get pretty fast and so some solar accumulator tanks have a baffle inside them to help reduce them. The baffle separates the tank into a top and bottom half with the solar coil in the bottom half where the current is allowed. In the middle of the baffle is a hole which allows the hottest water from the lower half of the tank up into the top, but minimises the currents in the top half to reduce mixing.