Re: [TRNSYS-users] Water-Water heat pump (Type 668)

["Jeff Thornton" <thornton@tess-inc.com>]

<I am using a water-water heat pump (Type 668) to simulate a domestic
application. I have various space heating demand profiles for the
dwelling, therefore I use an equation with the formula Q=m*Cp*ΔT to
calculate the returning temperature of the fluid in the load loop for each
time step. When I connect this returning temperature to the input load
temperature of the heat pump then both inlet and outlet load temperatures
don't make any sense and they vary from -10000 to 10000 degrees C !>

It doesn't surprise me at all that this system does not converge.  Your
simple model of the load (your equation) has no real physics behind it. 
Your equation assumes that the system can meet the load no matter the
temperature of the fluid entering the load....  There's also no
capacitance at all so the system will oscillate wildly.  Also I'm pretty
sure that your heat pump data file has no point that shows the capacity of
the equipment going to zero at extreme entering fluid conditions.  So no
matter the conditions, the heat pump will continue to work and your
building will continue to demand the smae amount of energy.  Imagine this
simple scenario:

Heating load = 1000 kJ/h
Heat pump capacity = 800 kJ/h
Fluid capacitance rate = 50 kg/h
Fluid specific heat = 4 kJ/kg.K
Initial temperature to load = 20 C

Iteration 1: Tout_load = 20-1000/4/50 = 15
Iteration 1: Tout_hp = 15 + 800/4/50 = 19

Iteration 2: Tout_load = 19-1000/4/50 = 14
Iteration 2: Tout_hp = 14 + 800/4/50 = 18

Iteration 3: Tout_load = 18-1000/4/50 = 13
Iteration 3: Tout_hp = 13 + 800/4/50 = 17

Iteration 4: Tout_load = 17-1000/4/50 = 12
Iteration 4: Tout_hp = 12 + 800/4/50 = 16

and the iterative process continues until the convergence limit is
reached....  See the problem?

So how do you work around the problem?  It starts with the fact that the
loads you are providing aren't based on what's happening in the system. 
"Load" is a rather artificial and outdated way of simulating systems but
luckily there are ways in TRNSYS of imposing loads from another source
(program) onto the system.

1) Impose the heating load on a "lump" and control the heat pump operation
based on the lump temperature.
2) Make sure your heat pump data file is realistic and has data points
that have little or no capacity at extreme temperatures
3) Include capacitance in your system; pipes etc.
4) Carefully consider your controls, timestep and tolerances

Hope this helps.

Jeff










Jeff Thornton
President - TESS, LLC

22 North Carroll Street - Suite 370
Madison WI 53703 USA

Phone: 608-274-2577
Fax: 608-278-1475
E-mail: thornton@tess-inc.com
Web: www.tess-inc.com