[TRNSYS-users] Energy balance issue with Type534

[Marie-Claude Hamelin]

Hi all,
I am having an issue with a GSHP system: Type668 water-to-water HP 
connected to a vertical storage tank Type534 (and of course a borehole, 
house Type56 and other components connected to it). Type668 sends hot 
water to a coil heat exchanger in the storage tank, which is then used 
for hydronic heating.
I have integrated the heat transfer to load for Type668, and then the 
heat tranfer from coil heat exchanger to tank, using Type28. The issue 
is that HP heat transfer to load flow gives me 8575 kWh, and heat 
exchanger heat transfer is 6539 kWh over a full year. The input and 
outpout temperatures for both components are directly connected, so 
there is no loss to the environment through piping.

I have studied the temperature variations for all 4 nodes of my water 
tank, as well as inlet and outlet temperatures for the coil heat 
exchanger (which has 4 nodes located between node 3 and 4 of the tank). 
It seems that when the HP is not operating, inlet and outlet 
temperatures for HX are identical, and heat transfer occurs only 
between  the HX and node 4 of the tank (delta T of heat exchanger for 
each time step is directly proportional to delta T between HX and node 
4). This does not seem like an optimal model but I don't see any 
particular problem here.

The real problem seems to be that for periods when heat pump is off, the 
heat transfered from the HX to the tank does not equal  the heat gained 
by the tank plus losses to the environment. For a given period of 9 
hours when the HP was off (sampled from a full year simulation), the 
energy variation was calculated for the volume occupied by the HX as 
well as the rest of the tank, using m x Cp x deltaT for each of the 4 
nodes. While the HX lost 1299 kJ (to tank I assume, since it's its only 
point of contact), the water tank also lost a net 155 kJ. Since losses 
to the environment were only 612 kJ (obtained from Type534 output), this 
means 687 kJ were lost to something else than environment or coil. I do 
not have any miscellaneous heat sources in the tank.

I cannot think of any explanation for that and wonder if it could be a 
mistake in Type534.
Thank you for your help,

Marie-Claude Hamelin