About the mass fraction of Supercritical CO2 power cycle and the pressure drop of the connection

[Silencesy99]

Dear fwitte:
I have also established a supercritical carbon dioxide recompression cycle model following the example in your manual (https://github.com/fwitte/sCO2_exergy), but I want to set the split ratio of the cycle, but the operation result shows an error

unable to solve 1phase PY flash with Tmin=220.934, Tmax=3000 due to error: HSU_P_flash_singlephase_Brent could not find a solution because Hmolar [3844.71 J/mol] is below the minimum value of 4181.1870859 J/mol

The code is as follows.

nw = Network(fluids=['CO2']) 
nw.set_attr(
    T_unit='K', p_unit='MPa', h_unit='kJ / kg', m_unit='kg / s',
    s_unit="kJ / kgK") 

water_in = Source('Water source')  
water_out = Sink('Water sink')   

closer = CycleCloser('Cycle closer')  

cp1 = Compressor('Compressor 1', fkt_group='CMP')  
cp2 = Compressor('Compressor 2', fkt_group='CMP')  

rec1 = HeatExchanger('Recuperator 1', fkt_group='REC')
rec2 = HeatExchanger('Recuperator 2', fkt_group='REC')  
cooler = HeatExchangerSimple('Water cooler')
heater = HeatExchangerSimple('Heater')
turb = Turbine('Turbine')
sp1 = Splitter('Splitter 1', fkt_group='REC') 
m1 = Merge('Merge 1', fkt_group='REC') 

c1 = Connection(cooler, 'out1', cp1, 'in1', label='1')
c2 = Connection(cp1, 'out1', rec1, 'in2', label='2')
c3 = Connection(rec1, 'out2', m1, 'in1', label='3')
c4 = Connection(m1, 'out1', rec2, 'in2', label='4')
c5 = Connection(rec2, 'out2', heater, 'in1', label='5')
c0 = Connection(heater, 'out1', closer, 'in1', label='0')
c6 = Connection(closer, 'out1', turb, 'in1', label='6')
c7 = Connection(turb, 'out1', rec2, 'in1', label='7')
c8 = Connection(rec2, 'out1', rec1, 'in1', label='8')
c9 = Connection(rec1, 'out1', sp1, 'in1', label='9')
c10 = Connection(sp1, 'out1', cooler, 'in1', label='10')
c11 = Connection(sp1, 'out2', cp2, 'in1', label='11')
c12 = Connection(cp2, 'out1', m1, 'in2', label='12')

nw.add_conns(c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, c10, c11, c12)

power = Bus('total output power')
power.add_comps({'comp': turb, 'char': 1, 'base': 'component'},
                {'comp': cp1, 'char': 1, 'base': 'bus'},
                {'comp': cp2, 'char': 1, 'base': 'bus'})
heat_input_bus = Bus('heat input')
heat_input_bus.add_comps({'comp': heater, 'base': 'bus'})
nw.add_busses(power, heat_input_bus)

c1.set_attr(p=7.7, T=34+273.15, m=75, fluid={'CO2': 1})
cp1.set_attr(eta_s=0.86)
turb.set_attr(eta_s=0.89)
heater.set_attr(pr=0.97)
cooler.set_attr(pr=0.97)
cp2.set_attr(eta_s=0.86)
c6.set_attr(T=753.15)
c6.set_attr(p=20)
rec2.set_attr(ttd_l=10, pr1=0.97, pr2=0.97)
rec1.set_attr(ttd_l=10, pr1=0.97, pr2=0.97)
c11.set_attr(m=Ref(c9,0.34,0))
nw.solve(mode='design')

nw.print_results()

In addition, I saw your example the network is calculated twice under the condition of resetting some parameters ,

nw.solve(mode='design')
rec2.set_attr(ttd_l=5)
rec1.set_attr(ttd_l=5)
c11.set_attr(T=Ref(c12, 1, 0))
c3.set_attr(T=None)
c6.set_attr(T=None)
c14.set_attr(T=None)
c1.set_attr(m=None)
power.set_attr(P=-100e6)
nw.solve(mode='design')
# print results to prompt and generate model documentation
nw.print_results()

I'm curious about the function of it. I have also tried that some parameters can be calculated when given before the second calculation, and errors will be reported when given before the first calculation.
Moreover, I would like to set the pressure drop ratio between connections. My innital idea is to set pipes between components.Is there any better advance?

[fwitte]

Hi @Silencesy99,

I can look into your example tomorrow. There is also the online user meeting at 17:00 CET https://tespy.readthedocs.io/en/main/regular_meeting.html. You can join if you are interested and I can elaborate a bit, on the two steps of calculation. The basic ideas is, that the solver shows much more stability once good starting values are provided from a converged solution. In that sense you could make your modifications after the first calculation.

Also, I am not yet sure, if I understand your problem correctly. You want to set the mass flow with a reference? And pressure losses between what components?

Best

Francesco

[Silencesy99]

Thanks, Francesco,

I understand that the first calculation can create an environment that makes the solver more stable, right?
While, I want to set the ratio of branch mass flow to total trunk mass flow. While, I solve the network twice after I set the mass flow ratio at the second calculation. But when I want to change the ratio value to explore the relationship between cycle efficiency and mass flow split ratio, there is an error that says
"Invalid value for ttd_u: ttd_u = -3.7303532469156835 below minimum value (0) at component Recuperator 1."

And I want to set the pressure losses of connections between each components of the cycle. For example, my ideal situation is to set the pressure loss along the way when the fluid comes out of the Compressor and enters the Recuperator 1.