Hello. I am making a gas generation plant on my farm and I would be very pleased if you can help me with making a heat pump the correct size.

I have made a round concrete tank in the ground that is diameter 4m and height 3m that will be full of liquid food waste. I need to heat the contents to 40 degrees celsius by use of a heat pump with a heat exchanger in the tank. The decomposing of the waste is exothermic and will require very little heat input once the process is started but I require the heat pump to be capable of making the initial heating during start up.

The tank is constructed of 250mm thickness 25MPa concrete and the surrounding ground is sandy and quite waterlogged in winter especially. The ground temperature I measured today (after a cold weekend) at 1 meter deepth is 13 degrees C, last week the same measurement was 17 degrees C.

I need to be able to raise the contents of the tank (approx 30 cubic meters) to 40 C in around 24 hours. I can calculate how much heat would be required to do this but I do not have any reference material to give me figures on expected heat losses through the tank itself. Are there standard figures available to work with or accepted ways of assessing these losses?