From 7dbc3cf90b629f622c63c3b1e8bb7df0bea29a91 Mon Sep 17 00:00:00 2001
From: Alinson S Xavier <axavier@anl.gov>
Date: Fri, 14 Nov 2025 10:23:26 -0600
Subject: [PATCH] model: Capacity cannot decrease over time

---
 docs/src/problem.md          | 9 +++++++++
 src/model/build.jl           | 7 +++++++
 test/src/model/build_test.jl | 6 ++++++
 3 files changed, 22 insertions(+)

diff --git a/docs/src/problem.md b/docs/src/problem.md
index 7521cd8..933a4cd 100644
--- a/docs/src/problem.md
+++ b/docs/src/problem.md
@@ -249,6 +249,15 @@ The goal is to minimize a linear objective function with the following terms:
 \end{align*}
 ```
 
+- Plant capacity cannot decrease over time (`eq_capacity_nondecreasing[p.name, t]`):
+
+```math
+\begin{align*}
+& z^\text{exp}_{pt} \geq z^\text{exp}_{p,t-1}
+& \forall p \in P, t \in T
+\end{align*}
+```
+
 - Plant is initially open if initial capacity is positive:
 
 ```math
diff --git a/src/model/build.jl b/src/model/build.jl
index a8e1717..04a4e0a 100644
--- a/src/model/build.jl
+++ b/src/model/build.jl
@@ -370,6 +370,13 @@ function build_model(instance::Instance; optimizer, variable_names::Bool = false
         eq_keep_open[p.name, t] = @constraint(model, x[p.name, t] >= x[p.name, t-1])
     end
 
+    # Plants: Capacity cannot decrease over time
+    eq_capacity_nondecreasing = _init(model, :eq_capacity_nondecreasing)
+    for p in plants, t in T
+        eq_capacity_nondecreasing[p.name, t] =
+            @constraint(model, z_exp[p.name, t] >= z_exp[p.name, t-1])
+    end
+
     # Plants: Building period
     eq_building_period = _init(model, :eq_building_period)
     for p in plants, t in T
diff --git a/test/src/model/build_test.jl b/test/src/model/build_test.jl
index 2dda67a..3639082 100644
--- a/test/src/model/build_test.jl
+++ b/test/src/model/build_test.jl
@@ -128,6 +128,12 @@ function model_build_test()
           "eq_keep_open[L1,4] : -x[L1,3] + x[L1,4] ≥ 0"
     @test repr(model[:eq_keep_open]["L1", 1]) == "eq_keep_open[L1,1] : x[L1,1] ≥ 1"
 
+    # Plants: Capacity cannot decrease over time
+    @test repr(model[:eq_capacity_nondecreasing]["L1", 4]) ==
+          "eq_capacity_nondecreasing[L1,4] : -z_exp[L1,3] + z_exp[L1,4] ≥ 0"
+    @test repr(model[:eq_capacity_nondecreasing]["L1", 1]) ==
+          "eq_capacity_nondecreasing[L1,1] : z_exp[L1,1] ≥ 150"
+
     # Plants: Building period
     @test ("L1", 1) ∉ keys(model[:eq_building_period])
     @test repr(model[:eq_building_period]["L1", 2]) ==