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Avoid CHP Binaries When Not Needed #556

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a52f4f1
Avoid binCHPIsOnInTS binary dV array if not needed
Bill-Becker Dec 15, 2025
b86fcf9
Add temp dev test file for avoiding CHP binary
Bill-Becker Dec 15, 2025
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94 changes: 54 additions & 40 deletions src/constraints/chp_constraints.jl
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Original file line number Diff line number Diff line change
@@ -1,17 +1,11 @@
# REopt®, Copyright (c) Alliance for Sustainable Energy, LLC. See also https://github.com/NREL/REopt.jl/blob/master/LICENSE.
function add_chp_fuel_burn_constraints(m, p; _n="")
# Fuel burn slope and intercept
fuel_burn_slope, fuel_burn_intercept = fuel_slope_and_intercept(;
electric_efficiency_full_load = p.s.chp.electric_efficiency_full_load,
electric_efficiency_half_load = p.s.chp.electric_efficiency_half_load,
fuel_higher_heating_value_kwh_per_unit=1
)

function add_chp_fuel_burn_constraints(m, p; _n="", binary_created=true, fuel_burn_slope, fuel_burn_intercept)
# Fuel cost
m[:TotalCHPFuelCosts] = @expression(m,
sum(p.pwf_fuel[t] * m[:dvFuelUsage][t, ts] * p.fuel_cost_per_kwh[t][ts] for t in p.techs.chp, ts in p.time_steps)
)
# Conditionally add dvFuelBurnYIntercept if coefficient p.FuelBurnYIntRate is greater than ~zero
# Note: if intercept exists, binary_created will always be true (checked in add_chp_constraints)
if abs(fuel_burn_intercept) > 1.0E-7
dv = "dvFuelBurnYIntercept"*_n
m[Symbol(dv)] = @variable(m, [p.techs.chp, p.time_steps], base_name=dv)
Expand Down Expand Up @@ -40,15 +34,9 @@ function add_chp_fuel_burn_constraints(m, p; _n="")
end
end

function add_chp_thermal_production_constraints(m, p; _n="")
# Thermal production slope and intercept
thermal_prod_full_load = 1.0 / p.s.chp.electric_efficiency_full_load * p.s.chp.thermal_efficiency_full_load # [kWt/kWe]
thermal_prod_half_load = 0.5 / p.s.chp.electric_efficiency_half_load * p.s.chp.thermal_efficiency_half_load # [kWt/kWe]
thermal_prod_slope = (thermal_prod_full_load - thermal_prod_half_load) / (1.0 - 0.5) # [kWt/kWe]
thermal_prod_intercept = thermal_prod_full_load - thermal_prod_slope * 1.0 # [kWt/kWe_rated


function add_chp_thermal_production_constraints(m, p; _n="", binary_created=true, thermal_prod_slope, thermal_prod_intercept)
# Conditionally add dvHeatingProductionYIntercept if coefficient p.s.chpThermalProdIntercept is greater than ~zero
# Note: if intercept exists, binary_created will always be true (checked in add_chp_constraints)
if abs(thermal_prod_intercept) > 1.0E-7
dv = "dvHeatingProductionYIntercept"*_n
m[Symbol(dv)] = @variable(m, [p.techs.chp, p.time_steps], base_name=dv)
Expand Down Expand Up @@ -92,16 +80,20 @@ Used by add_chp_constraints to add supplementary firing constraints if
p.s.chp.supplementary_firing_max_steam_ratio > 1.0 to add CHP supplementary firing operating constraints.
Else, the supplementary firing dispatch and size decision variables are set to zero.
"""
function add_chp_supplementary_firing_constraints(m, p; _n="")
thermal_prod_full_load = 1.0 / p.s.chp.electric_efficiency_full_load * p.s.chp.thermal_efficiency_full_load # [kWt/kWe]
thermal_prod_half_load = 0.5 / p.s.chp.electric_efficiency_half_load * p.s.chp.thermal_efficiency_half_load # [kWt/kWe]
thermal_prod_slope = (thermal_prod_full_load - thermal_prod_half_load) / (1.0 - 0.5) # [kWt/kWe]

function add_chp_supplementary_firing_constraints(m, p; _n="", thermal_prod_slope, thermal_prod_intercept)
# Constrain upper limit of dvSupplementaryThermalProduction, using auxiliary variable for (size * useSupplementaryFiring)
@constraint(m, CHPSupplementaryFireCon[t in p.techs.chp, ts in p.time_steps],
m[Symbol("dvSupplementaryThermalProduction"*_n)][t,ts] <=
(p.s.chp.supplementary_firing_max_steam_ratio - 1.0) * p.production_factor[t,ts] * (thermal_prod_slope * m[Symbol("dvSupplementaryFiringSize"*_n)][t] + m[Symbol("dvHeatingProductionYIntercept"*_n)][t,ts])
)
# Note: dvHeatingProductionYIntercept only exists when thermal_prod_intercept > 0
if abs(thermal_prod_intercept) > 1.0E-7
@constraint(m, CHPSupplementaryFireCon[t in p.techs.chp, ts in p.time_steps],
m[Symbol("dvSupplementaryThermalProduction"*_n)][t,ts] <=
(p.s.chp.supplementary_firing_max_steam_ratio - 1.0) * p.production_factor[t,ts] * (thermal_prod_slope * m[Symbol("dvSupplementaryFiringSize"*_n)][t] + m[Symbol("dvHeatingProductionYIntercept"*_n)][t,ts])
)
else
@constraint(m, CHPSupplementaryFireCon[t in p.techs.chp, ts in p.time_steps],
m[Symbol("dvSupplementaryThermalProduction"*_n)][t,ts] <=
(p.s.chp.supplementary_firing_max_steam_ratio - 1.0) * p.production_factor[t,ts] * thermal_prod_slope * m[Symbol("dvSupplementaryFiringSize"*_n)][t]
)
end
if solver_is_compatible_with_indicator_constraints(p.s.settings.solver_name)
# Constrain lower limit of 0 if CHP tech is off
@constraint(m, NoCHPSupplementaryFireOffCon[t in p.techs.chp, ts in p.time_steps],
Expand Down Expand Up @@ -174,13 +166,32 @@ end
Used in src/reopt.jl to add_chp_constraints if !isempty(p.techs.chp) to add CHP operating constraints and
cost expressions.
"""
function add_chp_constraints(m, p; _n="")
# TODO if chp.min_turn_down_fraction is 0.0, and there is no fuel burn or thermal y-intercept, we don't need the binary below
@warn """Adding binary variable to model CHP.
Some solvers are very slow with integer variables"""
@variables m begin
binCHPIsOnInTS[p.techs.chp, p.time_steps], Bin # 1 If technology t is operating in time step; 0 otherwise
end
function add_chp_constraints(m, p; _n="")
# Calculate fuel burn and thermal production slopes and intercepts which are passed into some constraint functions
fuel_burn_slope, fuel_burn_intercept = fuel_slope_and_intercept(;
electric_efficiency_full_load = p.s.chp.electric_efficiency_full_load,
electric_efficiency_half_load = p.s.chp.electric_efficiency_half_load,
fuel_higher_heating_value_kwh_per_unit=1
)

thermal_prod_full_load = 1.0 / p.s.chp.electric_efficiency_full_load * p.s.chp.thermal_efficiency_full_load
thermal_prod_half_load = 0.5 / p.s.chp.electric_efficiency_half_load * p.s.chp.thermal_efficiency_half_load
thermal_prod_slope = (thermal_prod_full_load - thermal_prod_half_load) / (1.0 - 0.5)
thermal_prod_intercept = thermal_prod_full_load - thermal_prod_slope * 1.0

# Check if binary variable is needed which creates binCHPIsOnInTS binary decision variable and activates other constraints
binary_needed = (abs(fuel_burn_intercept) > 1.0E-7) ||
(abs(thermal_prod_intercept) > 1.0E-7) ||
(p.s.chp.min_turn_down_fraction > 1.0E-7) ||
(p.s.chp.om_cost_per_hr_per_kw_rated > 1.0E-7)

if binary_needed
@warn """Adding binary variable binCHPIsOnInTS to model CHP.
Some solvers are very slow with integer variables"""
@variables m begin
binCHPIsOnInTS[p.techs.chp, p.time_steps], Bin # 1 If technology t is operating in time step; 0 otherwise
end
end

m[:TotalHourlyCHPOMCosts] = 0
m[:TotalCHPFuelCosts] = 0
Expand All @@ -189,17 +200,20 @@ function add_chp_constraints(m, p; _n="")
m[:dvRatedProduction][t, ts] for t in p.techs.chp, ts in p.time_steps)
)

if p.s.chp.om_cost_per_hr_per_kw_rated > 1.0E-7
add_chp_hourly_om_charges(m, p)
end

add_chp_fuel_burn_constraints(m, p; _n=_n)
add_chp_thermal_production_constraints(m, p; _n=_n)
add_binCHPIsOnInTS_constraints(m, p; _n=_n)
# These constraints are always needed
add_chp_fuel_burn_constraints(m, p; _n=_n, binary_created=binary_needed, fuel_burn_slope=fuel_burn_slope, fuel_burn_intercept=fuel_burn_intercept)
add_chp_thermal_production_constraints(m, p; _n=_n, binary_created=binary_needed, thermal_prod_slope=thermal_prod_slope, thermal_prod_intercept=thermal_prod_intercept)
add_chp_rated_prod_constraint(m, p; _n=_n)

# These constraints and decision variables created within the functions are only needed if binary_needed or more specific parameters is non-zero
if binary_needed
add_binCHPIsOnInTS_constraints(m, p; _n=_n)
end
if p.s.chp.om_cost_per_hr_per_kw_rated > 1.0E-7
add_chp_hourly_om_charges(m, p; _n=_n)
end
if p.s.chp.supplementary_firing_max_steam_ratio > 1.0
add_chp_supplementary_firing_constraints(m,p; _n=_n)
add_chp_supplementary_firing_constraints(m, p; _n=_n, thermal_prod_slope=thermal_prod_slope, thermal_prod_intercept=thermal_prod_intercept)
else
for t in p.techs.chp
fix(m[Symbol("dvSupplementaryFiringSize"*_n)][t], 0.0, force=true)
Expand Down
205 changes: 205 additions & 0 deletions test/test_avoid_chp_binary.jl
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Original file line number Diff line number Diff line change
@@ -0,0 +1,205 @@

using Revise
using REopt
using JSON
using DelimitedFiles
using PlotlyJS
using Dates
using Test
using JuMP
using HiGHS
using DotEnv
DotEnv.load!()

############### CHP Binary Creation Tests ###################
# Tests to verify that binCHPIsOnInTS is only created when needed

@testset "CHP Binary Creation Tests" begin

@testset "CHP with min_turn_down_fraction > 0 (binary SHOULD be created)" begin
# Test that binCHPIsOnInTS gets created when min_turn_down_fraction > 0
data = JSON.parsefile("./scenarios/chp_unavailability_outage.json")

# Set min_turn_down_fraction to a non-zero value
data["CHP"]["min_turn_down_fraction"] = 0.5

# Remove outage to simplify the test
delete!(data, "ElectricUtility")
data["ElectricUtility"] = Dict("net_metering_limit_kw" => 0.0, "co2_from_avert" => true)

s = Scenario(data)
inputs = REoptInputs(s)
m = Model(optimizer_with_attributes(HiGHS.Optimizer,
"output_flag" => false,
"log_to_console" => false,
"mip_rel_gap" => 0.01))
results = run_reopt(m, inputs)

# Verify the binary variable was created
@test haskey(m.obj_dict, :binCHPIsOnInTS)
binary_var = m[:binCHPIsOnInTS]

# Verify model solved successfully
@test results["CHP"]["size_kw"] > 0.0

println("✓ Test 1 passed: Binary created with min_turn_down_fraction = 0.5")
finalize(backend(m))
empty!(m)
end

@testset "CHP with min_turn_down_fraction = 0 (binary should NOT be created)" begin
# Test that binCHPIsOnInTS is NOT created when all conditions are zero/equal
data = JSON.parsefile("./scenarios/chp_unavailability_outage.json")

# Set min_turn_down_fraction to zero
data["CHP"]["min_turn_down_fraction"] = 0.0

# Ensure no intercepts by making efficiencies equal (no part-load curve)
# When electric_efficiency_full_load == electric_efficiency_half_load, intercept = 0
data["CHP"]["electric_efficiency_full_load"] = 0.35
data["CHP"]["electric_efficiency_half_load"] = 0.35
data["CHP"]["thermal_efficiency_full_load"] = 0.45
data["CHP"]["thermal_efficiency_half_load"] = 0.45

# Ensure no hourly O&M cost
data["CHP"]["om_cost_per_hr_per_kw_rated"] = 0.0

# Remove outage to simplify
delete!(data, "ElectricUtility")
data["ElectricUtility"] = Dict("net_metering_limit_kw" => 0.0, "co2_from_avert" => true)

s = Scenario(data)
inputs = REoptInputs(s)
m = Model(optimizer_with_attributes(HiGHS.Optimizer,
"output_flag" => false,
"log_to_console" => false,
"mip_rel_gap" => 0.01))
results = run_reopt(m, inputs)

# Verify the binary variable was NOT created
@test !haskey(m.obj_dict, :binCHPIsOnInTS)

# Verify model still solved successfully
@test results["CHP"]["size_kw"] > 0.0

println("✓ Test 2 passed: Binary NOT created with min_turn_down_fraction = 0 and no intercepts")
finalize(backend(m))
empty!(m)
end

@testset "CHP with fuel_burn_intercept > 0 (binary SHOULD be created)" begin
# Test that binary is created when fuel burn has non-zero intercept
data = JSON.parsefile("./scenarios/chp_unavailability_outage.json")

# Set min_turn_down_fraction to zero
data["CHP"]["min_turn_down_fraction"] = 0.0

# Create non-zero fuel burn intercept by having different efficiencies
data["CHP"]["electric_efficiency_full_load"] = 0.40
data["CHP"]["electric_efficiency_half_load"] = 0.30 # Different from full load
data["CHP"]["thermal_efficiency_full_load"] = 0.45
data["CHP"]["thermal_efficiency_half_load"] = 0.45 # Keep thermal same to isolate fuel burn

# Ensure no hourly O&M cost
data["CHP"]["om_cost_per_hr_per_kw_rated"] = 0.0

delete!(data, "ElectricUtility")
data["ElectricUtility"] = Dict("net_metering_limit_kw" => 0.0, "co2_from_avert" => true)

s = Scenario(data)
inputs = REoptInputs(s)
m = Model(optimizer_with_attributes(HiGHS.Optimizer,
"output_flag" => false,
"log_to_console" => false,
"mip_rel_gap" => 0.01))
results = run_reopt(m, inputs)

# Verify the binary variable was created
@test haskey(m.obj_dict, :binCHPIsOnInTS)

# verify model solved successfully
@test results["Financial"]["lcc"] != 0.0

println("✓ Test 3 passed: Binary created with non-zero fuel_burn_intercept")
finalize(backend(m))
empty!(m)
end

@testset "CHP with thermal_prod_intercept > 0 (binary SHOULD be created)" begin
# Test that binary is created when thermal production has non-zero intercept
data = JSON.parsefile("./scenarios/chp_unavailability_outage.json")

# Set min_turn_down_fraction to zero
data["CHP"]["min_turn_down_fraction"] = 0.0

# Create non-zero thermal prod intercept by having different thermal efficiencies
data["CHP"]["electric_efficiency_full_load"] = 0.35
data["CHP"]["electric_efficiency_half_load"] = 0.35 # Keep electric same
data["CHP"]["thermal_efficiency_full_load"] = 0.40
data["CHP"]["thermal_efficiency_half_load"] = 0.50 # Different from full load

# Ensure no hourly O&M cost
data["CHP"]["om_cost_per_hr_per_kw_rated"] = 0.0

delete!(data, "ElectricUtility")
data["ElectricUtility"] = Dict("net_metering_limit_kw" => 0.0, "co2_from_avert" => true)

s = Scenario(data)
inputs = REoptInputs(s)
m = Model(optimizer_with_attributes(HiGHS.Optimizer,
"output_flag" => false,
"log_to_console" => false,
"mip_rel_gap" => 0.01))
results = run_reopt(m, inputs)

# Verify the binary variable was created
@test haskey(m.obj_dict, :binCHPIsOnInTS)

# Verify model solved successfully
@test results["Financial"]["lcc"] != 0.0

println("✓ Test 4 passed: Binary created with non-zero thermal_prod_intercept")
finalize(backend(m))
empty!(m)
end

@testset "CHP with om_cost_per_hr_per_kw_rated > 0 (binary SHOULD be created)" begin
# Test that binary is created when hourly O&M cost is non-zero
data = JSON.parsefile("./scenarios/chp_unavailability_outage.json")

# Set min_turn_down_fraction to zero
data["CHP"]["min_turn_down_fraction"] = 0.0

# Make efficiencies equal (no intercepts)
data["CHP"]["electric_efficiency_full_load"] = 0.35
data["CHP"]["electric_efficiency_half_load"] = 0.35
data["CHP"]["thermal_efficiency_full_load"] = 0.45
data["CHP"]["thermal_efficiency_half_load"] = 0.45

# Set non-zero hourly O&M cost
data["CHP"]["om_cost_per_hr_per_kw_rated"] = 0.01

delete!(data, "ElectricUtility")
data["ElectricUtility"] = Dict("net_metering_limit_kw" => 0.0, "co2_from_avert" => true)

s = Scenario(data)
inputs = REoptInputs(s)
m = Model(optimizer_with_attributes(HiGHS.Optimizer,
"output_flag" => false,
"log_to_console" => false,
"mip_rel_gap" => 0.01))
results = run_reopt(m, inputs)

# Verify the binary variable was created
@test haskey(m.obj_dict, :binCHPIsOnInTS)

# Verify model solved successfully
@test results["Financial"]["lcc"] != 0.0

println("✓ Test 5 passed: Binary created with om_cost_per_hr_per_kw_rated = 0.01")
finalize(backend(m))
empty!(m)
end

println("\n===== All CHP Binary Creation Tests Passed! =====\n")
end
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