Choosing Components¶

This guide helps you select the right flixOpt component for your modeling needs.

Decision Tree¶

graph TD
    A[What does this element do?] --> B{Brings energy INTO system?}
    B -->|Yes| C[Source]
    B -->|No| D{Takes energy OUT of system?}
    D -->|Yes| E[Sink]
    D -->|No| F{Converts energy type?}
    F -->|Yes| G[LinearConverter]
    F -->|No| H{Stores energy?}
    H -->|Yes| I[Storage]
    H -->|No| J{Transports between locations?}
    J -->|Yes| K[Transmission]
    J -->|No| L[Consider custom constraints]

Component Comparison¶

Component	Purpose	Inputs	Outputs	Key Parameters
Source	External supply	None	1+ flows	`effects_per_flow_hour`
Sink	Demand/export	1+ flows	None	`fixed_relative_profile`
SourceAndSink	Bidirectional exchange	1+ flows	1+ flows	Both input and output
LinearConverter	Transform energy	1+ flows	1+ flows	`conversion_factors`
Storage	Time-shift energy	charge flow	discharge flow	`capacity_in_flow_hours`
Transmission	Transport energy	in1, in2	out1, out2	`relative_losses`

Detailed Component Guide¶

Source¶

Use when: Purchasing or importing energy/material from outside your system boundary.

fx.Source(
    'GridElectricity',
    outputs=[fx.Flow('Elec', bus='Electricity', size=1000, effects_per_flow_hour=0.25)]
)

Typical applications: - Grid electricity connection - Natural gas supply - Raw material supply - Fuel delivery

Key parameters:

Parameter	Purpose
`outputs`	List of flows leaving this source
`effects_per_flow_hour`	Cost/emissions per unit
`invest_parameters`	For optimizing connection capacity

Sink¶

Use when: Energy/material leaves your system (demand, export, waste).

# Fixed demand (must be met)
fx.Sink(
    'Building',
    inputs=[fx.Flow('Heat', bus='Heat', size=1, fixed_relative_profile=demand)]
)

# Optional export (can sell if profitable)
fx.Sink(
    'Export',
    inputs=[fx.Flow('Elec', bus='Electricity', size=100, effects_per_flow_hour=-0.15)]
)

Typical applications: - Heat/electricity demand - Product output - Grid export - Waste disposal

Key parameters:

Parameter	Purpose
`inputs`	List of flows entering this sink
`fixed_relative_profile`	Demand profile (on flow)
`effects_per_flow_hour`	Negative = revenue

SourceAndSink¶

Use when: Bidirectional exchange at a single point (buy AND sell from same connection).

fx.SourceAndSink(
    'GridConnection',
    inputs=[fx.Flow('import', bus='Electricity', size=500, effects_per_flow_hour=0.25)],
    outputs=[fx.Flow('export', bus='Electricity', size=500, effects_per_flow_hour=-0.15)],
    prevent_simultaneous_flow_rates=True,  # Can't buy and sell at same time
)

Typical applications: - Electricity grid (buy/sell) - Gas grid with injection capability - Material exchange with warehouse

LinearConverter¶

Use when: Transforming one energy type to another with a linear relationship.

# Single input, single output
fx.LinearConverter(
    'Boiler',
    inputs=[fx.Flow('Gas', bus='Gas', size=500)],
    outputs=[fx.Flow('Heat', bus='Heat', size=450)],
    conversion_factors=[{'Gas': 1, 'Heat': 0.9}],
)

# Multiple outputs (CHP)
fx.LinearConverter(
    'CHP',
    inputs=[fx.Flow('Gas', bus='Gas', size=300)],
    outputs=[
        fx.Flow('Elec', bus='Electricity', size=100),
        fx.Flow('Heat', bus='Heat', size=150),
    ],
    conversion_factors=[{'Gas': 1, 'Elec': 0.35, 'Heat': 0.50}],
)

# Multiple inputs
fx.LinearConverter(
    'CoFiringBoiler',
    inputs=[
        fx.Flow('Gas', bus='Gas', size=200),
        fx.Flow('Biomass', bus='Biomass', size=100),
    ],
    outputs=[fx.Flow('Heat', bus='Heat', size=270)],
    conversion_factors=[{'Gas': 1, 'Biomass': 1, 'Heat': 0.9}],
)

Typical applications: - Boilers (fuel → heat) - Heat pumps (electricity → heat) - Chillers (electricity → cooling) - Turbines (fuel → electricity) - CHPs (fuel → electricity + heat) - Electrolyzers (electricity → hydrogen)

Key parameters:

Parameter	Purpose
`conversion_factors`	Efficiency relationship
`piecewise_conversion`	Non-linear efficiency curve
`status_parameters`	On/off behavior, startup costs

Pre-built Converters¶

flixOpt includes ready-to-use converters in flixopt.linear_converters:

Class	Description	Key Parameters
`Boiler`	Fuel → Heat	`thermal_efficiency`
`HeatPump`	Electricity → Heat	`cop`
`HeatPumpWithSource`	Elec + Ambient → Heat	`cop`, source flow
`CHP`	Fuel → Elec + Heat	`electrical_efficiency`, `thermal_efficiency`
`Chiller`	Electricity → Cooling	`cop`

from flixopt.linear_converters import Boiler, HeatPump

boiler = Boiler(
    'GasBoiler',
    thermal_efficiency=0.92,
    fuel_flow=fx.Flow('gas', bus='Gas', size=500, effects_per_flow_hour=0.05),
    thermal_flow=fx.Flow('heat', bus='Heat', size=460),
)

Storage¶

Use when: Storing energy for later use.

fx.Storage(
    'Battery',
    charging=fx.Flow('charge', bus='Electricity', size=100),
    discharging=fx.Flow('discharge', bus='Electricity', size=100),
    capacity_in_flow_hours=4,  # 4 hours at full rate = 400 kWh
    eta_charge=0.95,
    eta_discharge=0.95,
    relative_loss_per_hour=0.001,
    initial_charge_state=0.5,
)

Typical applications: - Batteries (electrical) - Thermal tanks (heat/cold) - Hydrogen storage - Material buffers

Key parameters:

Parameter	Purpose
`charging`, `discharging`	Flows for in/out
`capacity_in_flow_hours`	Size (or use `InvestParameters`)
`eta_charge`, `eta_discharge`	Round-trip efficiency
`relative_loss_per_hour`	Standing losses
`initial_charge_state`	Starting level (0-1 or `'equals_final'`)

Transmission¶

Use when: Transporting energy between different locations.

# Unidirectional
fx.Transmission(
    'HeatPipe',
    in1=fx.Flow('from_A', bus='Heat_A', size=200),
    out1=fx.Flow('to_B', bus='Heat_B', size=200),
    relative_losses=0.05,
)

# Bidirectional
fx.Transmission(
    'PowerLine',
    in1=fx.Flow('A_to_B', bus='Elec_A', size=100),
    out1=fx.Flow('at_B', bus='Elec_B', size=100),
    in2=fx.Flow('B_to_A', bus='Elec_B', size=100),
    out2=fx.Flow('at_A', bus='Elec_A', size=100),
    relative_losses=0.03,
    prevent_simultaneous_flows_in_both_directions=True,
)

Typical applications: - District heating pipes - Power transmission lines - Gas pipelines - Conveyor belts

Key parameters:

Parameter	Purpose
`in1`, `out1`	Primary direction flows
`in2`, `out2`	Reverse direction (optional)
`relative_losses`	Proportional losses
`absolute_losses`	Fixed losses when active
`balanced`	Same capacity both ways

Feature Combinations¶

Investment Optimization¶

Add InvestParameters to flows to let the optimizer choose sizes:

fx.Flow(
    'Heat',
    bus='Heat',
    invest_parameters=fx.InvestParameters(
        effects_of_investment_per_size={'costs': 100},  # €/kW
        minimum_size=0,
        maximum_size=1000,
    )
)

Works with: Source, Sink, LinearConverter, Storage, Transmission

Operational Constraints¶

Add StatusParameters to flows for on/off behavior:

fx.Flow(
    'Heat',
    bus='Heat',
    size=500,
    status_parameters=fx.StatusParameters(
        effects_per_switch_on={'costs': 50},  # Startup cost
        on_hours_min=2,  # Minimum runtime
        off_hours_min=1,  # Minimum downtime
    )
)

Works with: All components with flows

Non-Linear Efficiency¶

Use PiecewiseConversion for load-dependent efficiency:

fx.LinearConverter(
    'GasEngine',
    inputs=[fx.Flow('Fuel', bus='Gas')],
    outputs=[fx.Flow('Elec', bus='Electricity')],
    piecewise_conversion=fx.PiecewiseConversion({
        'Fuel': fx.Piecewise([fx.Piece(100, 200), fx.Piece(200, 300)]),
        'Elec': fx.Piecewise([fx.Piece(35, 80), fx.Piece(80, 110)]),
    }),
)

Works with: LinearConverter

Common Modeling Patterns¶

Pattern: Parallel Redundant Units¶

Model N identical units that can operate independently:

for i in range(3):
    flow_system.add_elements(
        fx.LinearConverter(
            f'Boiler_{i}',
            inputs=[fx.Flow('Gas', bus='Gas', size=100)],
            outputs=[fx.Flow('Heat', bus='Heat', size=90)],
            conversion_factors=[{'Gas': 1, 'Heat': 0.9}],
        )
    )

Pattern: Heat Recovery¶

Model waste heat recovery from one process to another:

# Process that generates waste heat
process = fx.LinearConverter(
    'Process',
    inputs=[fx.Flow('Elec', bus='Electricity', size=100)],
    outputs=[
        fx.Flow('Product', bus='Products', size=80),
        fx.Flow('WasteHeat', bus='Heat', size=20),  # Recovered heat
    ],
    conversion_factors=[{'Elec': 1, 'Product': 0.8, 'WasteHeat': 0.2}],
)

Pattern: Fuel Switching¶

Model a component that can use multiple fuels:

flex_boiler = fx.LinearConverter(
    'FlexBoiler',
    inputs=[
        fx.Flow('Gas', bus='Gas', size=200, effects_per_flow_hour=0.05),
        fx.Flow('Oil', bus='Oil', size=200, effects_per_flow_hour=0.08),
    ],
    outputs=[fx.Flow('Heat', bus='Heat', size=180)],
    conversion_factors=[{'Gas': 1, 'Oil': 1, 'Heat': 0.9}],
)

Next Steps¶

Building Models — Step-by-step modeling guide
Examples — Working code examples
Mathematical Notation — Constraint formulations