Air Source Heat Pump

[AditiBIlanthodi]

Hello TesPy community. I am trying to model an air source heat pump water heater but I am having trouble making the right connections in the network.

from tespy.networks import Network
from tespy.connections import Connection
from tespy.components import (CycleCloser, Compressor, Valve, HeatExchangerSimple, Source, Sink, HeatExchanger)
import pandas as pd
import numpy as np

# create a network object with R134a as fluid
fluid_list = ['R134a', 'air']
my_plant = Network(fluids=fluid_list)

# set the unitsystem for temperatures to °C and for pressure to bar
my_plant.set_attr(T_unit='C', p_unit='bar', h_unit='kJ / kg')

# create components
cc1 = CycleCloser('cycle closer 1')
cc2 = CycleCloser('cycle closer 2')
co = HeatExchangerSimple('condenser')
ev = HeatExchanger('evaporator')
va = Valve('expansion valve')
cp = Compressor('compressor')
ahx = HeatExchanger('air source heat exchanger')

# create connections
c1 = Connection(cc1, 'out1', ev, 'in1', label='1')
c2 = Connection(ev, 'out1', cp, 'in1', label='2')
c3 = Connection(cp, 'out1', co, 'in1', label='3')
c4 = Connection(co, 'out1', va, 'in1', label='4')
c5 = Connection(va, 'out1', cc1, 'in1', label='5')
c6 = Connection(ahx, 'out1', ev, 'in2', label='6')
c7 = Connection(ev, 'out2', cc2, 'in1', label='7')
#c8 = Connection(cc2, 'out1', va, 'in2', label='7')

# add connections to the network
my_plant.add_conns(c1, c2, c3, c4, c5, c6, c7)

# specify fluid properties and initial state
c2.set_attr(fluid={'R134a': 1, 'air': 0}, T=20, p=5, m=0.5)
c4.set_attr(fluid={'R134a': 1, 'air': 0}, T=80)
c6.set_attr(fluid={'R134a': 0, 'air': 1}, T=11)

# specify air-side heat exchanger parameters
ahx.set_attr(pr1=1, pr2=1, kA_char=1000)

# specify component parameters
co.set_attr(pr=0.98, Q=-4e6)
ev.set_attr(pr1=0.98, pr2 = 0.98)
cp.set_attr(eta_s=0.85)

# solve the network
my_plant.solve(mode='design')
my_plant.print_results()

# plot results
import matplotlib.pyplot as plt

# make text reasonably sized
plt.rc('font', **{'size': 18})

# plot COP vs heat demand
Q_array = np.linspace(0, 5e6, 50)
COP = []
for Q in Q_array:
    co.set_attr(Q=-Q)
    my_plant.solve('design')
    COP.append(abs(co.Q.val) / cp.P.val)

I'm sure this has quite a few errors starting with a missing outgoing connection from the cycle closer 2. Any help associated with modeling an air source heat pump is helpful!

[fwitte]

Dear @AditiBIlanthodi,

can you give a little bit more context of what you are exactly trying to do, e.g. a process flowsheet and a description of what you want to investigate? It is easier to help with more context.

Thank you!

[AditiBIlanthodi]

Hello @fwitte ,
Thank you for the reply! I am trying to implement an air source heat pump water heater. I have attached an image to describe the process better. Correct me if I am wrong, but I believe the basic heat pump model example on the TesPy website is a water source heat pump? I am curious about how I can alter this to mimic an air source heat pump. I would like to simulate a process wherein heat is drawn from the air into the refrigerant in the evaporator which passes through the compressor and finally releases the heat to water via the condenser. The ultimate goal of my study is to analyze the COP values of an air source heat pump given a time series of ambient air temperature values as well as a time series of 'Q' (heat demand') values. Hope this helps. Happy to clarify further!

[fwitte]

@AditiBIlanthodi: You can just change the fluid in your model, i.e. "air" instead of "water".

Then, should the heat demand be provided by the heat pump in every timestep as is? Or should it be optimized in a sense, that a setup with storage, heat pump (maybe other heat sources) and the demand, the provision of the demand should be at minimal cost in over a specific period of time? Specifically this was done (will be actually) in this workshop: https://fwitte.github.io/oemof-workshop-osmses-2023/. We have written quite an extensive documentation to show how things work.

[AditiBIlanthodi]

Thank you for the reply! I have changed the fluid to "air". Regarding the heat demand - I am running off-design calculations for part load COP by using a time series of heat demand values. However, I was also wondering : How can I input air temperature values and observe the corresponding change in COP of the heat pump?

[fwitte]

Well for that you will have to build a loop, change the temperature value in the model, rerun it and calculate the COP. Basically, very similar as it is done in the link I posted earlier. It really depends on the overall system assumptions you want to make. For example, should the compressor's efficiency be the same at any point or should it change e.g. with mass flow, or should the air flow rate kept constant or the air temperature reduction by the evaporator. These are design decisions you have to make. I really recommend you have a look at the link, there a couple of these things are actually explained and implemented.

[AditiBIlanthodi]

Yes, I plan to build a loop and change the temperature value. However, I was finding it hard to understand which parameter value to change. The 'HeatExchanger' component that is used to model the evaporator and condenser does not seem to have a 'Tamb' parameter. I will take a look at the link too, thanks for sharing!