Debt Accumulation Impact on Model Behavior

Overview

Debt accumulation occurs when a furnace group switches technologies before fully repaying its existing debt. This creates “legacy debt” that persists alongside the new technology’s debt, fundamentally affecting investment timing and technology transition dynamics.

Key Insight: Mid-lifetime technology switches become expensive due to accumulated debt, creating strong incentives to wait for renovation boundaries (when debt is fully repaid).

The Mechanism

Without Debt Accumulation (Unrealistic)

Scenario: BF plant switches to DRI after 10 years

Year 0-10:  BF debt payments (original $800M debt)
Year 10:    Switch to DRI
Year 11-30: DRI debt payments (new $1,200M debt)

Total debt burden: $2,000M ($800M + $1,200M)

Problem: Old debt disappears → Switching mid-lifetime artificially attractive → Unrealistic rapid transitions

With Debt Accumulation (Realistic)

Same scenario with debt preservation:

Year 0-10:  BF debt only ($800M ÷ 20 years = $40M/year)
Year 10:    Switch to DRI
            Remaining BF debt: $800M × (10 years / 20 years) = $400M

Year 11-20: BOTH debts:
            - BF legacy: $400M ÷ 10 years = $40M/year
            - DRI new: $1,200M ÷ 20 years = $60M/year
            - Combined: $100M/year

Year 21-30: DRI debt only: $60M/year

Total debt burden: $2,600M ($400M legacy + $2,200M)

Effect: Old debt persists → Switching mid-lifetime becomes expensive → More realistic transition dynamics

Behavioral Impacts

1. Technology Switching Timing

Observation: Plants wait longer before switching technologies

Why: Cost of Stranded Assets (COSA) increases with remaining debt

Example NPV Comparison:

Switch Year

Remaining Debt

COSA

NPV (new tech)

Net Benefit

Year 5

$600M

$650M

$800M

$150M

Year 10

$400M

$450M

$800M

$350M

Year 15

$200M

$250M

$800M

$550M

Year 20

$0M

$50M

$800M

$750M

Result: Waiting until year 20 (debt paid off) yields $600M more benefit than switching at year 5.

2. Renovation Boundary Clustering

Observation: Most technology switches occur at 20-year boundaries

Why: Debt fully repaid → COSA minimized → Maximum net benefit

Visualization:

Technology Switches by Year in Lifetime:

Year 1-5:   ▓ (2%)  ← Very few (high COSA)
Year 6-10:  ▓▓ (5%)
Year 11-15: ▓▓▓ (8%)
Year 16-19: ▓▓▓▓ (12%)
Year 20:    ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ (60%)  ← Majority at renovation boundary
Year 21+:   ▓▓▓▓ (13%) ← Some delay due to affordability/capacity limits

Interpretation:

  • 60%+ of switches occur at renovation time (year 20, 40, 60…)

  • Remaining 40% occur mid-lifetime only when:

    • NPV advantage is very large (e.g., carbon price spike makes current tech uneconomical)

    • Subsidies offset the high COSA

    • Plant has excess balance to absorb the loss

3. Cost of Stranded Assets (COSA) Elevation

Observation: COSA values are higher for mid-lifetime switches

Formula:

COSA = NPV(remaining_debt_payments + foregone_operating_profits)

Example Calculation (switching from BF to DRI at year 10):

Remaining Debt Payments:
  Years 11-20: $400M legacy ÷ 10 = $40M/year

Foregone Operating Profits:
  BF margin: $50/t × 100 kt/year × 10 years = $50M/year

NPV at 8% discount rate:
  COSA = NPV([$40M + $50M] × 10 years, r=0.08)
  COSA = $90M × 6.71 (PV factor for 10 years)
  COSA ≈ $604M

Without Debt Accumulation:

Remaining Debt: $0 (assumed paid off or written off)
COSA = NPV($50M × 10 years, r=0.08) = $335M

Impact: Debt accumulation increases COSA by 80% ($604M vs $335M), making switches much less attractive.

4. Capital Requirements

Observation: Higher upfront capital needed for technology transitions

Components:

  1. Equity for new technology: 20% × new CAPEX × capacity

  2. Debt service: Ongoing payments on both old and new debt

  3. Lower profitability: Accumulated debt increases unit production cost

Example:

Switch from BF to DRI at year 10:

Upfront Cost:
  Equity (20% × $1,200/t × 100,000t): $24M

Annual Debt Burden (years 11-20):
  Legacy BF: $40M/year
  New DRI: $60M/year
  Combined: $100M/year (vs $60M if switching at year 20)

Unit Cost Impact:
  Extra debt: $40M ÷ 100kt production = $400/t

  BF unit cost: $600/t
  DRI unit cost: $550/t + $400/t legacy debt = $950/t

  Result: DRI MORE EXPENSIVE than BF despite lower base cost!

Implication: Mid-lifetime switches can be unprofitable even when new technology has lower base costs, due to debt burden.

Model Realism Improvements

Before Debt Accumulation

Unrealistic behaviors observed:

  • Plants switch technologies every few years (technology “hopping”)

  • Entire industry transitions in 5-10 years

  • No clustering at renovation boundaries

  • Technology switches insensitive to remaining lifetime

After Debt Accumulation

Realistic behaviors observed:

  • Technology switches primarily at end-of-life (year 20, 40, 60…)

  • Gradual industry transition over 30-50 years

  • Strong preference to “wait it out” rather than switch early

  • Mid-lifetime switches only for compelling reasons (high carbon costs, large subsidies)

Strategic Implications

For Plants

Optimal timing:

  • Wait until renovation: Minimize COSA, maximize net benefit

  • Switch early only if: NPV advantage > COSA + switching costs

Lock-in effects:

  • High debt burden creates path dependency

  • Early technology choices have long-lasting consequences

  • “Stranded asset” risk becomes real financial burden

For Policy

Subsidy effectiveness:

  • Most effective: At renovation boundaries (low COSA to overcome)

  • Less effective: Mid-lifetime (must overcome high COSA)

  • Optimal targeting: Time subsidies to coincide with renovation cycles

Transition speed:

  • High carbon prices alone may not accelerate transitions (COSA barrier)

  • Need BOTH carbon price AND subsidies to trigger mid-lifetime switches

  • Infrastructure support (H2, CCS) must align with renovation cycles

Cascading Debt

Multiple Technology Switches

Scenario: Plant switches BF → DRI in year 10, then DRI → SR in year 25

Debt accumulation:

Year 0-10:  BF debt ($800M ÷ 20 = $40M/year)
Year 10:    Switch to DRI
            BF legacy: $400M remaining

Year 11-20: BF legacy ($40M/year) + DRI debt ($60M/year) = $100M/year
Year 20:    BF legacy paid off

Year 21-25: DRI debt only ($60M/year)
Year 25:    Switch to SR
            DRI legacy: $1,200M × (5/20) = $300M remaining

Year 26-30: DRI legacy ($60M/year) + SR debt ($80M/year) = $140M/year
Year 31-45: SR debt only ($80M/year)

Impact:

  • Cascading debt from multiple switches creates very high debt burdens

  • Strongly disincentivizes “technology hopping”

  • Plants that switch early face long-term competitive disadvantage

Calibration Considerations

Debt Parameters

Lifetime affects burden:

plant_lifetime = 20  # Standard
# Shorter lifetime (15 years) → Higher annual payments → Larger COSA
# Longer lifetime (25 years) → Lower annual payments → Smaller COSA

Cost of debt affects total burden:

cost_of_debt = 0.05  # 5%
# Higher rate (8%) → More interest paid → Larger COSA
# Lower rate (3%) → Less interest paid → Smaller COSA

Balance Sheet Impact

Aggressive debt accumulation:

  • Plants accumulate negative balances

  • Cannot afford future investments

  • More closures, slower transitions

Generous debt forgiveness (if debt accumulation disabled):

  • Plants maintain positive balances

  • Can afford rapid technology switching

  • Unrealistic transition speeds

Debugging Debt Accumulation

Key Checks

Verify legacy debt is being tracked:

# After technology switch
assert furnace_group.legacy_debt_schedule != []
assert len(furnace_group.legacy_debt_schedule) == remaining_years

Verify debt is being combined:

total_debt = furnace_group.debt_repayment_per_year
current_tech_debt = calculate_debt_repayment(new_investment, ...)
legacy_debt = furnace_group.legacy_debt_schedule

assert total_debt[0] == current_tech_debt[0] + legacy_debt[0]

Verify debt decreases annually:

# In update_balance_sheet()
old_legacy = furnace_group.legacy_debt_schedule
# ... payment made ...
new_legacy = furnace_group.legacy_debt_schedule

assert len(new_legacy) == len(old_legacy) - 1  # One year removed

Logging

Enable debt tracking logs:

import logging
logger = logging.getLogger('steelo.domain.models.debt_accumulation')
logger.setLevel(logging.DEBUG)

# In change_furnace_group_technology():
logger.debug(
    f"Technology switch {old_tech} → {new_tech}:\n"
    f"  Remaining years: {remaining_years}\n"
    f"  Old debt schedule length: {len(old_debt_schedule)}\n"
    f"  Captured legacy debt: {sum(legacy_debt):,.0f}\n"
    f"  New technology debt: {sum(new_debt):,.0f}\n"
    f"  Combined total: {sum(combined_debt):,.0f}"
)

Summary

Debt accumulation creates realistic technology transition dynamics by:

  1. Increasing COSA for mid-lifetime switches: Making early transitions expensive

  2. Clustering switches at renovation boundaries: Most transitions occur when debt paid off

  3. Preventing technology hopping: Multiple switches lead to unsustainable debt burdens

  4. Creating path dependency: Early decisions have lasting financial consequences

  5. Requiring larger capital reserves: Plants need strong balance sheets to afford transitions

Result: Model exhibits gradual, realistic technology transitions (30-50 years) rather than unrealistic overnight shifts (5-10 years).