Water filled plastic milk containers as heat source for P/T

I visited an "very-green" couple some years back who were very proud of their P/T from recycled materials - I had to enquire why all the milk containers and was told they were filled with water and this was heat recovery for night-time.

Is this effective?   I'm in the process of building a 24ft long  tunnel and my maths is suggesting that if I have 48ft 's worth of bottles they would be storing 9 kWh of heat at 20 deg C.  As there is so many variables in the equations for this, what is the empirical experience of anyone who has tried this ?

Thanks
Rob

Have a look at (death of the rocket stove) youtube, it might help, cheers Dave