CHP model

[PabloMBarral]

Hi, everyone.
In first place, I'd like to express my gratitude for such an useful software.

Let me explain my need: I'm trying to implement Tespy in my thermal cycle course in the University of Buenos Aires. I'm starting this task by modelling a conventional CHP steam cycle (a steam turbine with a condenser and a controlled extraction). In order to do so, I'd like to know which component I should use to model a desuperheater.

Although I've noticed that there is a component with this name, I understand that the outlet is dry saturated steam. In my case, because it's technologically more convenient, the steam outlet should have a little superheating, let's say 10°C above saturation.

How could I model this desuperheater?

And one more question: I need to model a steam header, with more than two outlets. Is the splitter just fine for this application? I belive so, but I'd like to double-check it.

Thanks in advance for your guidance, perhaps I'd join to the Stammtisch (only as a listener, because I'm in my office time).
Regards,
Pablo

[PabloMBarral]

I realized I've posted an incomplete description. The desuperheater is an injection of water into the steam pipe. That is: it's an open heat exchanger, a mixing one.

Thanks!

[fwitte]

Dear @PabloMBarral,

thank you very much for reaching out. Could you post a flowsheet of what you want to model exactly? If you do want superheated steam at the outlet you could just use the normal HeatExchanger and set the temperature difference to boiling boint (Td_bp) at the respective outlet. With the injection I'd think I can give you a more precise answer, if you post the flowsheet.

Have a good Holiday Season and start for 2023!

Best

Francesco

[PabloMBarral]

I believe that I can model it with the 'merge' component. What I had in mind was:

I still have to watch the specifics of parameters' definitions (the outlet must not be saturated because it's not an easily controlled process).

Ultimately, I'd like to model this CHP PFD (I teach an undergraduate course, and I'd like to implement Tespy, because EES tends to be a little messy for students):

Personally, I really appreciate the work you and your colleagues are leading.

Have a good start for 2023 you too!
Regards

[fwitte]

Hi Pablo,

thank you for the update and your patience with mine.

I believe that I can model it with the 'merge' component.

Yes that is what you'd do in that case. I made a little script as an example:

from tespy.networks import Network
from tespy.components import Condenser, Source, Sink, Splitter, Valve, Merge
from tespy.connections import Connection


nw = Network(
    fluids=["water"],
    T_unit="C", p_unit="bar", h_unit="J / kg", m_unit="kg / s"
)

so1 = Source("live steam")
si1 = Sink("condensate")
pre = Condenser("preheater")

me = Merge("injection cooling")
sp = Splitter("feed water splitter")
va = Valve("control valve")

so2 = Source("feed water cold")
si2 = Sink("feed water warm")

c1 = Connection(so1, "out1", me, "in1", label="1")
c2 = Connection(me, "out1", pre, "in1", label="2")
c3 = Connection(pre, "out1", si1, "in1", label="3")
c11 = Connection(so2, "out1", pre, "in2", label="11")
c12 = Connection(pre, "out2", sp, "in1", label="12")
c13 = Connection(sp, "out1", si2, "in1", label="13")
c14 = Connection(sp, "out2", va, "in1", label="14")
c15 = Connection(va, "out1", me, "in2", label="15")

nw.add_conns(c1, c2, c3)
nw.add_conns(c11, c12, c13, c14, c15)

c1.set_attr(fluid={"water": 1}, p=15, T=300, m=1)
c11.set_attr(fluid={"water": 1}, p=50, T=30, m=5)

# ignore any pressure losses on the preheater
pre.set_attr(pr1=1, pr2=1)

# set superheating after injection cooling
c2.set_attr(Td_bp=10)

nw.solve("design")
nw.print_results()

I still have to watch the specifics of parameters' definitions (the outlet must not be saturated because it's not an easily controlled process).

Ultimately, I'd like to model this CHP PFD (I teach an undergraduate course, and I'd like to implement Tespy, because EES tends to be a little messy for students)

That sounds interesting. Building the model step by step is the easiest way in my experience. If you are encountering any issues, please do not hesitate to let me know :). I am happy to hear, you are planning to use TESPy in your university courses. Actually, there are currently ideas to create open-source educational resources for thermal engineering with TESPy. We are preparing a little "test-course" on exergy analysis but want to extend on that, if there is interest. Maybe you could also provide some valuable ideas or examples from your experience.

Personally, I really appreciate the work you and your colleagues are leading.

Thank you, it is even better, when I see people can make good use of that!

[fwitte]

@PabloMBarral, how are you doing? I just wanted to hint you to two new repositories/projects we started as these might be interesting for you:

• Dispatch optimization of an air source heat pump and using TESPy to generate necessary inputs based on (very limited) datasheet information: https://fwitte.github.io/oemof-workshop-osmses-2023/
• Exergy analysis introduction and examples https://fwitte.github.io/TESPy_teaching_exergy/ (still under construction)

[PabloMBarral]

Thanks for your reply!

It sounds interesting, I'll give it a detailed look.

About exergy analysis, my experience is that it's not so clear for students that combustion is a heavily exergy destructor, and there's nothing you can do there, obviously taking less excess into account. It's specially shocking in contexts where we teach about combustion, even H2.

Another aspect that I usually highlight is that, in CHPs, or industrial steam plants, first law efficiency is not quite useful to evaluate, and it tends to lead you to wrong conclusions (especially when selecting the gas turbine, and deciding how to split the steam production in the HRSG, by a poorer efficiency GT or by postcombustion). Obviously, when first law efficiency, every single industrial LP steam plant is marvellous. Only with 2nd law is when the difference comes to light.

https://drive.google.com/file/d/193mDhq6q6SMR7iGihkhqh1T7BaKcJatt/view?usp=share_link

That is my pdf (in spanish), the balance's been resolved with EES (I'll try tespy in near future, because it seems faster to design new configurations once you're used to the sintaxis).

I'll try to contribute somehow.

Stay well!

[PabloMBarral]

Hi.

I'm returning to this.

Although I've been practising, I still don't quite get the logic behind the cycle closer.

Currently, we (me and the student I'm tutoring in studying TESPy) are trying to implement something like this:

where 1 & 13 being a source and a sink, and 12 another sink.

The problem arises with the attemperation closed cycle.
We've tried using and not using the cycle closer, and it still doesn't work properly either way. Should we explore the possibility of improving the starting values to get convergence? O maybe something else is not functioning.

In any case, any hint will be indeed appreciated.

Regards!!

# Application notes:

from tespy.networks import Network

from tespy.components import (CycleCloser, Turbine, Source, Sink, Splitter, Valve, SimpleHeatExchanger, Pump,Merge)

my_plant = Network()
my_plant.set_attr(T_unit='C',p_unit='bar', h_unit='kJ / kg', m_unit='t / h', s_unit='kJ / kgK', iterinfo=False)

# Components

cc=CycleCloser('cycle closer')

steam_inlet = Source('hp steam')

st_ext = Turbine('extraction st',eta_s=0.9)
st_cond = Turbine('cond st', eta_s=0.9)
att_pump = Pump('att_pump', eta_s=0.7)
cond_pump = Pump('cond_pump', eta_s=0.7)

sp_1 = Splitter('st inner splitter')
sp_2 = Splitter('condensate header')
me_1=Merge('Attemper')
cond_st_cv = Valve('cond st cv')

process_steam = Sink('process IP steam')

condenser = SimpleHeatExchanger('condenser')

steam_outlet = Sink('Sumidero de prueba')
steam_outlet_2 = Sink('Sumidero de prueba 2')
steam_outlet_3 = Sink('Sumidero de prueba 3')

# Connections

from tespy.connections import (Connection, Ref)

N = 13   # number of states

c = [None] * N
names = [None] * N

c[0] = Connection(steam_inlet, 'out1', st_ext, 'in1', label='1') # hrsg outlet
c[0].set_attr(p=66, T=480, m=180, fluid={'water': 1})
names[0] = 'hp steam'

c[1] = Connection(st_ext, 'out1', sp_1, 'in1', label='2') # st inner splitter
c[1].set_attr(p=13)
names[1] = 'extraction st outlet'

c[2] = Connection(sp_1, 'out2', me_1, 'in2', label='3') # process steam
c[2].set_attr(m=130)
names[2] = 'Steam from TV to attemp'

c[3] = Connection(sp_1, 'out1', cond_st_cv, 'in1', label='4') # cond st valve inlet
names[3] = 'cond st cv inlet'

c[4] = Connection(cond_st_cv, 'out1', st_cond, 'in1', label='5') # cond st inlet
c[4].set_attr(p=11)
names[4] = 'cond st inlet'

c[5] = Connection(st_cond, 'out1', condenser, 'in1', label='6') # condenser inlet
c[5].set_attr(p=0.08)
names[5] = 'condenser inlet'

c[6] = Connection(condenser, 'out1', sp_2, 'in1', label='7') # condenser header
c[6].set_attr(x=0, p=0.08)
names[6] = 'condenser outlet'

c[7] = Connection(sp_2, 'out1', cond_pump, 'in1', label='8') 
names[7] = 'cond pump inlet'

c[8] = Connection(sp_2, 'out2', att_pump, 'in1', label='9')  #El caudal m=10 lo voy a sacar cuando defina x=1 para vapor a proceso
#c[8].set_attr(m=10)
names[8] = 'att pump inlet'

c[9]=Connection(att_pump,'out1',cc,'in1',label='10')
c[9].set_attr(p=13)
names[9]='att pump outlet'

c[10]=Connection(cc,'out1',me_1,'in1',label='11')
names[10]='cc outlet'
#c[10].set_attr(fluid={'water': 1})

c[11]=Connection(me_1, 'out1',process_steam,'in1',label='12')
c[11].set_attr(x=1)
names[11]='process steam'

c[12]=Connection(cond_pump,'out1', steam_outlet, 'in1', label='13')
c[12].set_attr(p=5)
names[12]='cond pump outlet'


for j in range(0,N):
    my_plant.add_conns(c[j])

my_plant.solve(mode='design')

df_results_for_conns = my_plant.results['Connection']
df_results_for_conns['denomination'] = names

results = df_results_for_conns[['denomination',
                                'p','p_unit',
                                'T','T_unit',
                                'h','h_unit',
                                's','s_unit',
                                'x',
                                'm','m_unit']]

#results = results.drop(results.index[0])

def impresion(__file__, results):
    results = results.round(3)

    import os

    path = os.path.dirname(os.path.abspath(__file__))

    text_file = os.path.join(path, 'results.txt')

    formatted_text = results.to_string(index=False, justify='left')

    with open(text_file, 'w') as file:
        file.write(formatted_text)

impresion(__file__, results)

[fwitte]

Hm... Maybe this topic should be better explained in the docs then. I redrew your cycle including all connections. The connection numbers are colored.

1. Every color indicates, that this is the same mass flow:

• If the structure leads to a recycle where at some point you end up where you started and the mass flow is still the same, you need something to cut that mass flow (i.e. cycle closer or a merge and a splitter somewhere which connect to a source and a sink respectively).
• That means in this specific setup: No cycle closer needed for mass flow

2. Every color inidcates, that this is the same fluid:

• Same here but for the fluid vector (even though we only have a single fluid here...). You can see, that the fluid is always the same fluid vector because splitters cannot change the fluid composition (as opposed to seperators).
• A merge can potentiall change the fluid composition, thus connection 12 represents a new fluid vector.
• Therefore: No cycle closer here as well.

It was a little bit different in the previous mark-up steam example from this comment: #444 (comment)

1. For mass flow objects:

• No recycle, with or without cycle closer

2. For fluid vectors:

• Without cycle closer: recycle at the merge (fluid vector at outlet of merge 5 propagates to the same component again via 6, 1, 3, 4)
• With cycle closer: no more recycle
• There might be an alternative, i.e. using a separator. I'll try to figure that out.

You might want to join one of the next user meetings, if you want to discuss these things directly :).

Best

[fwitte]

P.S.: code for the first example without the cycle closer is below. I believe, you had an overdetermination in pressure, so I had to remove one of the specified values.

from tespy.networks import Network

from tespy.components import (CycleCloser, Turbine, Source, Sink, Splitter, Valve, SimpleHeatExchanger, Pump,Merge)

my_plant = Network()
my_plant.set_attr(T_unit='C',p_unit='bar', h_unit='kJ / kg', m_unit='t / h', s_unit='kJ / kgK', iterinfo=False)

# Components

cc=CycleCloser('cycle closer')

steam_inlet = Source('hp steam')

st_ext = Turbine('extraction st',eta_s=0.9)
st_cond = Turbine('cond st', eta_s=0.9)
att_pump = Pump('att_pump', eta_s=0.7)
cond_pump = Pump('cond_pump', eta_s=0.7)

sp_1 = Splitter('st inner splitter')
sp_2 = Splitter('condensate header')
me_1=Merge('Attemper')
cond_st_cv = Valve('cond st cv')

process_steam = Sink('process IP steam')

condenser = SimpleHeatExchanger('condenser')

steam_outlet = Sink('Sumidero de prueba')
steam_outlet_2 = Sink('Sumidero de prueba 2')
steam_outlet_3 = Sink('Sumidero de prueba 3')

# Connections

from tespy.connections import (Connection, Ref)

N = 13   # number of states

c = [None] * N
names = [None] * N

c[0] = Connection(steam_inlet, 'out1', st_ext, 'in1', label='1') # hrsg outlet
c[0].set_attr(p=66, T=480, m=180, fluid={'water': 1})
names[0] = 'hp steam'

c[1] = Connection(st_ext, 'out1', sp_1, 'in1', label='2') # st inner splitter
c[1].set_attr(p=13)
names[1] = 'extraction st outlet'

c[2] = Connection(sp_1, 'out2', me_1, 'in2', label='3') # process steam
c[2].set_attr(m=130)
names[2] = 'Steam from TV to attemp'

c[3] = Connection(sp_1, 'out1', cond_st_cv, 'in1', label='4') # cond st valve inlet
names[3] = 'cond st cv inlet'

c[4] = Connection(cond_st_cv, 'out1', st_cond, 'in1', label='5') # cond st inlet
c[4].set_attr(p=11)
names[4] = 'cond st inlet'

c[5] = Connection(st_cond, 'out1', condenser, 'in1', label='6') # condenser inlet
c[5].set_attr(p=0.08)
names[5] = 'condenser inlet'

c[6] = Connection(condenser, 'out1', sp_2, 'in1', label='7') # condenser header
c[6].set_attr(x=0, p=0.08)
names[6] = 'condenser outlet'

c[7] = Connection(sp_2, 'out1', cond_pump, 'in1', label='8')
names[7] = 'cond pump inlet'

c[8] = Connection(sp_2, 'out2', att_pump, 'in1', label='9')  #El caudal m=10 lo voy a sacar cuando defina x=1 para vapor a proceso
#c[8].set_attr(m=10)
names[8] = 'att pump inlet'

c[9]=Connection(att_pump,'out1',me_1,'in1',label='10')
c[9].set_attr()
names[9]='att pump outlet'

# c[10]=Connection(cc,'out1',me_1,'in1',label='11')
# names[10]='cc outlet'
#c[10].set_attr(fluid={'water': 1})

c[11]=Connection(me_1, 'out1',process_steam,'in1',label='12')
c[11].set_attr(x=1)
names[11]='process steam'

c[12]=Connection(cond_pump,'out1', steam_outlet, 'in1', label='13')
c[12].set_attr(p=5)
names[12]='cond pump outlet'


for j in range(0,N):
    if c[j] is not None:
        my_plant.add_conns(c[j])

my_plant.solve(mode='design')
my_plant.print_results()

[PabloMBarral]

Wow, this is terribly excellent.

I realize that I misunderstood the merge. I mistakenly thought that I had the freedom to specify pressures for both inlets. Consequently, the outlet pressure would also need to be set.

I need some time to study the answer in great detail, but I do want to participate in the Stammtisch. Firstly, to fully understand the logic of coding, and offer my user experience. And perhaps later contribute to the documentation and this forum. Contributing to the code is, unfortunately, beyond my current programming capabilities, although I want to be able someday.

I have some questions and ideas. For example, when performing a thermal balance in an HRSG, there's a need to specify where the pinch occurs, whether it's in the evaporator or the economizer. Usually, I solve this by using a min() function like this:

pinch = 8°C
pinch = min(temp diff @ evaporator, temp diff @ economizer)

I'm not quite sure how to implement this in Tespy. I've thought about using an if or a flow control like that, but it's a structure that's not part of TESPy, and it involves solving two problems when just one could be sufficient. Perhaps we could discuss this further.

Thanks again for your time!!