Issues with offdesign calculation

[ottosandrini]

Hi,

i'm trying to build a simulation of sorts of a real heat pump that's part of a lab at my uni. I built a small website as a GUI for the simulation and connected it to this tespy model running in the background.

In the GUI Users have the ability to change;

• compressor power and efficiency
• the superheat achieved (though that could be fixed)
• the temperatures of the heat-source and -sink connected to the heat exchangers

The tespy model should then calculate the system parameters with the specified parameters.
I have spent quite a bit of time trying to understand tespy and got the design calculation to produce good results, but i cannot get the offdesign simulation to run without error.
I would be grateful for any input on my approach.

# PARAMETER LIST
working_fluid = "R410A"  # Kaeltemittel
high_pressure = 2480000  # pa
low_pressure = 1040000   # pa
high_temp = 273.15 + 56.3
t_cond = PropsSI("T", "Q", 1, "P", high_pressure, working_fluid)    # verdampfungstemperatur Hochdurck
t_evap = PropsSI("T", "Q", 1, "P", low_pressure, working_fluid)    # verdampfungstemperatur Niederdurck
print("t_cond: " + str(t_cond))
print("t_evap: " + str(t_evap))
compressor_power = 880          # Leistungsaufnahme Verdichter [W]
isentropen_koeffizient = 0.7    # isentropische effizienz Verdichter                                
design_heat_output = 5070       # design Leistung der Wärmequelle
design_heat_input = 4158        # Leistung des Verdampfers (inkl superheater)
condenser_pressure_ratio = 0.98
evaporator_pressure_ratio = 0.98
superheater_pressure_ratio = 0.99
superheat = 4.8                                         # K
compressor_entry_enthalpy = PropsSI("H", "P", low_pressure, "T", t_evap + superheat, working_fluid)
compressor_output_enthalpy = PropsSI("H", "P", high_pressure, "T", high_temp, working_fluid)
print("compressor_entry_enthalpy: " + str(compressor_entry_enthalpy))
print("compressor_output_enthalpy: " + str(compressor_output_enthalpy))
# falls notwendig:
superheater_entry_enthalpy = PropsSI("H", "P", low_pressure / superheater_pressure_ratio, "T", t_evap, working_fluid)


# # --- HEATING SYSTEM PARAMETERS ---
wq_massflow_data = 23                                   # kg/min
ww_massflow_data = 16.1                                 # kg/min        
wq_massflow = wq_massflow_data / 60                     # kg/s
ww_massflow = ww_massflow_data / 60                     # kg/s
ww_inflow_temp = 273.15 + 35.5    
water_pressure = 200000 # pa
condenser_wq_pressure_ratio = 0.99
superheater_wq_pressure_ratio = 0.99
# Heatpipe compensation
t_eingang = 273.15 + 14.2                # K
t_ausgang = 273.15 + 11.7                # K
h1 = PropsSI("H", "P", water_pressure, "T", t_eingang, "water")
h2 = PropsSI("H", "P", water_pressure * superheater_wq_pressure_ratio, "T", t_ausgang, "water")
dh = h2 - h1            # Enthalpieverlust der Wärmequelle 
Q = wq_massflow * dh    # von der Wärmequelle übertragene Wärme

Q_ges = -design_heat_input    # gesamte Leistung der Wärmeaufnahme
dh_ges = Q_ges / wq_massflow    # Enthalpieverlust der Wärmequelle wenn gesamte Leistung von Wärmequelle kommen würde
h1_virt = h2 - dh_ges
t_compensated = PropsSI("T", "P", water_pressure, "H", h1_virt, "water" ) # theoretische Einganstemperatur der Wärmequelle
wq_inflow_temp = t_compensated

class Heatpump():
   def __init__(self):
       # --- HEATPUMP PARAMETERS ---
       self.working_fluid = working_fluid
       self.compressor_power = compressor_power
       self.isentropen_koeffizient = isentropen_koeffizient
       self.design_heat_output = design_heat_output
       self.condensation_temp = t_cond
       self.evaporation_temp = t_evap
       self.low_pressure = low_pressure / 100000
       self.condenser_pressure_ratio = condenser_pressure_ratio
       self.superheat = superheat
       self.compressor_out_enth = compressor_output_enthalpy / 1000
       self.high_pressure = high_pressure / 100000 
       # --- HEATING SYSTEM PARAMETERS ---
       self.wq_massflow = wq_massflow                  # Massenstrom Wärmequelle        (kg/s)
       self.wq_inflow_temp = t_compensated             # Wärmequelle Rücklauftemperatur (K)
       self.ww_massflow = ww_massflow                  # Massenstrom Warmwasser         (kg/s)
       self.ww_inflow_temp = ww_inflow_temp            # Warmwasser Rücklauftemperatur  (K)

       self.condenser_w_pressure_ratio = condenser_wq_pressure_ratio
       self.superheater_w_pressure_ratio = superheater_wq_pressure_ratio
       self.superheater_pressure_ratio = superheater_pressure_ratio
       self.evaporator_pressure_ratio = evaporator_pressure_ratio
       # --- Network ---
       self.heatpump = Network()
       self.heatpump.set_attr(T_unit='K', p_unit='bar', h_unit='kJ / kg', iterinfo=True)
       # Components
       # --- Heatpump ---
       self.cc = com.CycleCloser('cycle_closer')
       self.evaporator = com.HeatExchanger('evaporator')
       self.condenser = com.HeatExchanger('condenser')
       self.expansionvalve = com.Valve('expansionValve')
       self.compressor = com.Compressor('compressor')
       self.superheater = com.HeatExchanger('superheater')
       # --- Sources ---
       self.tank_in_ww = com.Sink('tank_in_ww')
       self.tank_out_ww = com.Source('tank_out_ww')
       self.tank_in_wq = com.Sink('tank_in_wq')
       self.tank_out_wq = com.Source('tank_out_wq')
       # Connections
       # --- Heatpump ---
       self.conn0 = Connection(self.cc, 'out1', self.compressor, 'in1')
       self.conn1 = Connection(self.compressor, 'out1', self.condenser, 'in1')
       self.conn2 = Connection(self.condenser, 'out1', self.expansionvalve, 'in1')
       self.conn3 = Connection(self.expansionvalve, 'out1', self.evaporator, 'in2')
       self.conn4 = Connection(self.evaporator, 'out2', self.superheater, 'in2')
       self.conn5 = Connection(self.superheater, 'out2', self.cc, 'in1')
       # --- Water system ---
       self.ww1 = Connection(self.tank_out_ww, 'out1', self.condenser, 'in2')
       self.ww2 = Connection(self.condenser, 'out2', self.tank_in_ww, 'in1')

       self.wq1 = Connection(self.tank_out_wq, 'out1', self.superheater, 'in1')
       self.wq2 = Connection(self.superheater, 'out1', self.evaporator, 'in1')
       self.wq3 = Connection(self.evaporator, 'out1', self.tank_in_wq, 'in1')
       # --- Add Connections and parametise ---
       self.conn_list = [self.conn0, self.conn1, self.conn2, self.conn3, self.conn4, self.conn5]
       self.parametise()
       self.heatpump.add_conns(self.conn0, self.conn1, self.conn2, self.conn3, self.conn4, self.conn5, self.ww1, self.ww2, self.wq1, self.wq2, self.wq3)


   def parametise(self):
       # --- PARAMETRISATION ---
       # COMPONENTS
       self.compressor.set_attr(eta_s=self.isentropen_koeffizient, P=self.compressor_power)
       self.compressor.set_attr(design=['P', 'eta_s'], offdesign=['P', 'eta_s'])

       self.condenser.set_attr(Q=-self.design_heat_output, pr1=self.condenser_pressure_ratio, pr2=self.condenser_w_pressure_ratio)
       self.condenser.set_attr(design=['Q', 'pr1', 'pr2'], offdesign=['kA_char', 'zeta1', 'zeta2'])
       
       self.superheater.set_attr(pr1=self.superheater_w_pressure_ratio, pr2=self.superheater_pressure_ratio)
       self.superheater.set_attr(design=['pr1', 'pr2'], offdesign=['kA_char', 'zeta1', 'zeta2'])
       self.evaporator.set_attr(pr1=self.superheater_w_pressure_ratio, pr2=self.evaporator_pressure_ratio)
       self.evaporator.set_attr(design=['pr1', 'pr2'], offdesign=['kA_char', 'zeta1', 'zeta2'])
       
       # CONNECTIONS
       self.conn1.set_attr(p=self.high_pressure)

       self.conn3.set_attr(p=self.low_pressure)
       self.conn3.set_attr(design=['p'])

       self.conn4.set_attr(x=1, fluid={working_fluid: 1})

       self.conn5.set_attr(T=self.evaporation_temp + self.superheat)
       self.conn5.set_attr(design=['T'])

       self.ww1.set_attr(T=self.ww_inflow_temp, p=2, m=self.ww_massflow, fluid={'ethanol': 1})
       self.wq1.set_attr(T=self.wq_inflow_temp, p=2, m=self.wq_massflow, fluid={'ethanol': 1})

   def parametise_offdesign(self):
       compressor_power = 600
       compressor_efficiency = 0.77
       p_conn1 = 0.02756776456653455*compressor_power + 1.2160357159127209 # lineare Regression von Betriesdaten
       h_comp_out = 0.05909297981868707*compressor_power + 6.368557241127974
       #print(f"p_conn1: {p_conn1}")
       self.conn1.set_attr(p=p_conn1, m0=0.2, h0=h_comp_out)
       #self.conn3.set_attr(p0=self.low_pressure)
       self.compressor.set_attr(P=compressor_power, eta_s=compressor_efficiency)


   def solve(self, design):
       if design:
           self.reset()
           self.heatpump.solve(mode='design', init_path="design_state.json")  
           self.heatpump.save('design_state.json')
           self.heatpump.print_results()
       else:
           print("solving in offdesign mode!")
           self.parametise_offdesign()
           self.heatpump.solve(mode="offdesign", design_path='design_state.json')
           self.heatpump.print_results()

Currently i cannot get rid of this error;

2025-07-03 15:48:54,838 - INFO - network - Network initialised.
2025-07-03 15:48:54,838 - INFO - network - Starting solver.
2025-07-03 15:48:54,838 - DEBUG - network - Number of connection equations: 1.
2025-07-03 15:48:54,838 - DEBUG - network - Number of bus equations: 0.
2025-07-03 15:48:54,838 - DEBUG - network - Number of component equations: 14.
2025-07-03 15:48:54,838 - DEBUG - network - Number of user defined equations: 0.
2025-07-03 15:48:54,838 - DEBUG - network - Total number of variables: 15.
2025-07-03 15:48:54,838 - PROGRESS - network -  iter  | residual   | progress   | massflow   | pressure   | enthalpy   | fluid      | component  
2025-07-03 15:48:54,838 - PROGRESS - network - -------+------------+------------+------------+------------+------------+------------+------------
2025-07-03 15:48:54,862 - DEBUG - connection - Enthalpy out of fluid property range at connection condenser:out2_tank_in_ww:in1, adjusting value to -428612.9244121148 J / kg.
2025-07-03 15:48:54,862 - DEBUG - connection - Enthalpy out of fluid property range at connection cycle_closer:out1_compressor:in1, adjusting value to 98453.49662698088 J / kg.
2025-07-03 15:48:54,862 - DEBUG - connection - Enthalpy out of fluid property range at connection evaporator:out1_tank_in_wq:in1, adjusting value to 2277662.21673056 J / kg.
2025-07-03 15:48:54,862 - DEBUG - connection - Enthalpy out of fluid property range at connection evaporator:out2_superheater:in2, adjusting value to 106336.91227700001 J / kg.
2025-07-03 15:48:54,862 - DEBUG - connection - Pressure out of fluid property range at connection expansionValve:out1_evaporator:in2, adjusting value to 50000000.0 Pa.
2025-07-03 15:48:54,863 - DEBUG - connection - Enthalpy out of fluid property range at connection superheater:out1_evaporator:in1, adjusting value to -428612.9244121148 J / kg.
2025-07-03 15:48:54,863 - PROGRESS - network -  1     | 2.14e+10   | 0 %        | 1.05e+00   | 9.97e+08   | 6.25e+06   | 0.00e+00   | 0.00e+00   
2025-07-03 15:48:54,863 - ERROR - network - Simulation crashed due to an unexpected error:
unable to solve 1phase PY flash with Tmin=199.999, Tmax=300.775 due to error: HSU_P_flash_singlephase_Brent could not find a solution because Smolar [40.7017 J/mol/K] is below the minimum value of 40.8066662174 J/mol/K
Traceback (most recent call last):

[fwitte]

Hi @ottosandrini

thank you for reaching out! I will have a look on this once I find the time for it and then come back to you. Very cool project by the way, I would like to learn more about it :). Let me know if you are interested in an exchange on the topic!

Best

Francesco

[ottosandrini]

Hi Francesco,

thank you for taking the time to look at my project. I reached out to you via e-mail if you want to know more about what i'm doing with my project. The repo is public on my github, though not in a very complete state at the moment

[fwitte]

Thanks for the info!

I have looked into it, there is one main point: You change the compressor outlet/condenser inlet pressure for offdesign but you do not change the temperature level of the heat sink. With that the kA_char cannot produce reasonable results, because the temperature level on the condensation side is fully below the warm water temperature level. I would expect, that the compressor adjusts operation based on the desired temperature level (through lower or higher pressure at its exit governed by the expansion valve) and desired heat production (e.g. by controlling rotational speed). Let's talk about the setup and measurement data you have, then I can understand better.

Best

Francesco

[fwitte]

Quick update here:

Since there are measurement values, which can be utilized to create regressions/relationships between the different model parameters, it is not necessary to use offdesign mode for this specific case. The compressor power can be calculated using a UserDefinedEquation as function of the condensation pressure (and potentially pressure ratio of condensation to evaporation pressure if there are measurement values at variable evaporation temeprature levels). The condensation and evaporation pressure values can be controlled through assumed (and later also through regression functions) temperature differences to the heat source and heat sink fluid respectively. For this, it is advisable to use the td_pinch function of the MovingBoundaryHeatExchanger class. Compressor efficiency and heat exchanger pressure drops can be specified with fixed value assumptions.