Calculation of the stored energy for a heat storage tank

It is difficult to calculate the heat capacity because we have two regimens contributing to the temperature gradient inside the tank.

Heat conductivity of the water establishes a temperature gradient descending from the core of the tank to the tank wall which would cause slow convection up, and advection by the agitation of the circulating pump which causes a fast and likely turbulent flow of hot water down and cold water up.

The sensors' readings are not very helpful because of the spiral flow mixing the hot and cold water and disturbing a uniform predictable pattern of water circulation in the tank one would never know if the reading is of a random plume of water or a steady read.

A very rough estimate would be modeling the core temperature an average of the difference between the heat generator's output and the three sensors'.

Depending on the power of the circulation pump things will vary by a large amount.

I would recommend using simulation software.

I feel like this may be an X-Y problem. You are asking for the energy stored in a vessel, but what will you do with the information once you know?

Energy in water

A glass of room temperature tap water has an energy content, but few would find this information useful. Engineers tend to look at the change in energy either put in to water or that can be taken from it. Like how much home heating we could produce from a quantity of hot water, or how many gallons of 65 degree water would it take to give a 45 degree C shower. From a pure physics standpoint, the total energy in liquid water at atmospheric pressure is the energy required to heat it from absolute zero to its melting point as ice, the energy to melt the ice, and the energy required to heat it to its current temperature. But this isn't very useful because we are unlikely to encounter extremely low temperatures and so it would be difficult to extract all that energy. Remember that heat always flows from hot to cold.

Useful energy in water

Your calculation shows that a temperature change of 49 degrees C will be an energy change of 57 KW-hr. But what is that exactly? You wont be able to run a 1000 watt blow drier for 57 hours using that energy. You could use it to heat up some other volume of water, or possibly to heat up a room using a heat exchanger, but it becomes hard to extract all of that energy when the temperatures converge. You use 20 degrees in the calculation, but don't describe what that represents.

What we need

So what we need is to know what you are trying to do with this heated water. Is the circle in the diagram taking out heat and then pumping cold water back into the system? Once we know what you want to do with all the hot water we can better answer how much capacity the water has to do it. Also, let us know how accurate you need the answers to be. Moon shots and swimming pool heaters have different accuracy requirements ;^)