Jouleflow... it sounds like your 'energy storage tank' is a thermal store type solution, which is fantastic ! The idea with these is that they hold a large amount of water (water has a high 'specific heat capacity' so is ideal for this) that stores heat energy from a number of sources (log burners, solar pv and tubes, off peak grid etc etc.) that is then used for whatever it needs to be used for. The tank is designed to 'stratify', i.e. very hot at the top and cooler as you go down. Therefore, heat is taken out at the appropriate height depending for the temperature required. E.g. underfloor heating uses low temperatures, so it takes heat low down, but traditional wall hung radiators need high temps, so they should take heat higher up.
A coil of pipe inside is used to input or extract heat.
Therefore... what you want is 'technically' achievable as you just want to run your shower supply from a very low level in the thermal store, with some temperature control attached to it, so as not to have water >28°C
This is really a plumbing question rather than an electrical...