Technical Engineering Specifications¶

Ocean-Based sCO2 Cooling System for 100 MW Data Center¶

Document ID: CCNL-TES-2024-01 | Version: 1.1 | Date: December 2025

1. Executive Summary¶

This document provides detailed engineering specifications for a closed-loop supercritical CO2 (sCO2) ocean cooling system designed to reject 100 MW of thermal load from a hyperscale data center. The system leverages the cold waters of the Labrador Current (2-4°C year-round) to achieve passive heat rejection with minimal pumping energy.

Key Design Parameters: - Thermal capacity: 100 MW (341 MMBTU/hr) - Working fluid: Supercritical CO2 - Operating pressure: 80-100 bar - Ocean loop length: 8-10 km - Target PUE contribution: <0.05 (cooling only)

2. Thermodynamic Fundamentals¶

2.1 Supercritical CO2 Properties¶

Critical Point: - Critical Temperature (Tc): 31.1°C (304.13 K) - Critical Pressure (Pc): 73.8 bar (7.38 MPa) - Critical Density: 467.6 kg/m³

Operating Conditions (Design Point: 80 bar, 35°C):

2.2 Heat Transfer Advantage¶

The heat transfer coefficient for sCO2 near the critical point can be 2-3x higher than water due to:

1. Low viscosity - Enables higher Reynolds numbers at same flow rate
2. Property variation - Cp spikes near pseudo-critical temperature
3. Density changes - Drives natural convection enhancement

Nusselt Correlation for sCO2:

Nu = 0.023 × Re^0.8 × Pr^0.4 × (ρw/ρb)^0.3 × (Cp_avg/Cp_b)^n

Where: - Re = Reynolds number (typically 10^5 - 10^6) - Pr = Prandtl number - ρw/ρb = wall-to-bulk density ratio - n = 0.4 for heating, 0.3 for cooling

Calculated Heat Transfer Coefficients:

3. System Architecture¶

3.1 Overall System Schematic¶

┌─────────────────────────────────────────────────────────────────┐
│                     DATA CENTER (100 MW IT)                      │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐  ┌──────────┐        │
│  │ Server   │  │ Server   │  │ Server   │  │ Server   │  ...   │
│  │ Hall A   │  │ Hall B   │  │ Hall C   │  │ Hall D   │        │
│  │ (25 MW)  │  │ (25 MW)  │  │ (25 MW)  │  │ (25 MW)  │        │
│  └────┬─────┘  └────┬─────┘  └────┬─────┘  └────┬─────┘        │
│       │             │             │             │               │
│       └─────────────┴──────┬──────┴─────────────┘               │
│                            │                                    │
│                    ┌───────┴───────┐                            │
│                    │  PRIMARY HX   │ ← Liquid-to-sCO2           │
│                    │  (Titanium)   │                            │
│                    └───────┬───────┘                            │
└────────────────────────────┼────────────────────────────────────┘
                             │
              ┌──────────────┼──────────────┐
              │              │              │
        ┌─────┴─────┐  ┌─────┴─────┐  ┌─────┴─────┐
        │  PUMP 1   │  │  PUMP 2   │  │  PUMP 3   │  (N+1 Redundancy)
        │  (Duty)   │  │  (Duty)   │  │ (Standby) │
        └─────┬─────┘  └─────┬─────┘  └─────┬─────┘
              │              │              │
              └──────────────┼──────────────┘
                             │
                    ┌────────┴────────┐
                    │  MANIFOLD &     │
                    │  CONTROL VALVE  │
                    └────────┬────────┘
                             │
         ════════════════════╪════════════════════  SHORE LINE
                             │
                    ┌────────┴────────┐
                    │  SHORE CROSSING │
                    │  (HDD Conduit)  │
                    └────────┬────────┘
                             │
    ─────────────────────────┼─────────────────────  SEABED
                             │
              ┌──────────────┴──────────────┐
              │                             │
        ┌─────┴─────┐                 ┌─────┴─────┐
        │  SUPPLY   │                 │  RETURN   │
        │  (Hot)    │                 │  (Cold)   │
        │  35-40°C  │                 │  8-12°C   │
        └─────┬─────┘                 └─────┬─────┘
              │                             │
              │    ┌─────────────────┐      │
              │    │                 │      │
              └────┤  OCEAN LOOP     ├──────┘
                   │  (8-10 km)      │
                   │  2-4°C seawater │
                   └─────────────────┘

3.2 System Components¶

4. Thermal Design Calculations¶

4.1 Heat Load Analysis¶

Data Center Thermal Load:

Heat Balance:

Q_total = Q_servers + Q_storage + Q_network + Q_lighting + Q_UPS
Q_total = 85 MW + 8 MW + 5 MW + 1 MW + 1 MW = 100 MW

Q_cooling = 100 MW = 100,000 kW = 341.2 MMBTU/hr

4.2 sCO2 Flow Rate Calculation¶

Design Conditions: - Inlet temperature (from servers): 40°C - Outlet temperature (from ocean): 10°C - Temperature differential: ΔT = 30°C - Average specific heat: Cp = 3.0 kJ/kg·K (at 85 bar)

Mass Flow Rate:

Q = ṁ × Cp × ΔT

ṁ = Q / (Cp × ΔT)
ṁ = 100,000 kW / (3.0 kJ/kg·K × 30 K)
ṁ = 1,111 kg/s

Volume Flow Rate:

At 85 bar, 25°C: ρ = 700 kg/m³

V̇ = ṁ / ρ = 1,111 / 700 = 1.59 m³/s = 5,724 m³/hr

4.3 Ocean Heat Exchanger Sizing¶

Heat Transfer to Seawater:

Design basis: - Seawater temperature: 3°C (annual average) - sCO2 inlet temperature: 40°C - sCO2 outlet temperature: 10°C - Log Mean Temperature Difference (LMTD):

LMTD = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2)
     = ((40-3) - (10-3)) / ln(37/7)
     = (37 - 7) / ln(5.29)
     = 30 / 1.67
     = 18.0°C

Overall Heat Transfer Coefficient (U):

For pipe-in-seawater configuration with steel-core finned pipe (see Section 5.3): - sCO2 side: hi = 12,000 W/m²·K - Pipe wall (carbon steel): k = 50 W/m·K, t = 25 mm - External fins: 3x surface area enhancement - Seawater side (natural convection): ho = 500 W/m²·K

Baseline calculation (bare steel pipe):

1/U = 1/hi + t/k + 1/ho
1/U = 1/12,000 + 0.025/50 + 1/500
1/U = 0.000083 + 0.0005 + 0.002
1/U = 0.00258
U_bare = 387 W/m²·K

However, the seawater-side convection (ho = 500 W/m²·K) dominates the thermal resistance. With external longitudinal fins providing 3x effective surface area:

U_effective = 387 × 0.85 (fin efficiency) × 3 (area ratio) ≈ 24 W/m²·K (conservative)

Note: Previous versions calculated U = 7.9 W/m²·K assuming 50mm HDPE walls (k=0.4 W/m·K). The final design uses steel-core pipe (Section 5.3), which has 125x better thermal conductivity. The limiting factor is seawater-side natural convection, addressed through external fins.

Required Heat Transfer Area:

Q = U × A × LMTD

A = Q / (U × LMTD)
A = 100,000,000 W / (24 W/m²·K × 18.0 K)
A = 231,500 m²

Pipe Length Required:

For 16 parallel pipes at 500 mm OD:

Circumference per pipe = π × 0.5 = 1.57 m
Total circumference = 16 × 1.57 = 25.1 m
Length = 231,500 / 25.1 = 9,225 m ≈ 9.2 km per direction

Final Design Selection: - 16 parallel pipes, 500 mm diameter (steel-core with HDPE liner/jacket) - Total loop length: 10 km (5 km out, 5 km return) - Heat transfer area: 251,000 m² (provides 8% margin) - External longitudinal fins on seawater-exposed sections

5. Pipe Specifications¶

5.1 Material Selection¶

5.2 HDPE Pipe Specifications (Ocean Loop)¶

Selected: PE100 SDR 11

Issue: HDPE pressure rating insufficient for sCO2 at 80 bar

5.3 Revised Design: Steel Core with HDPE Liner and External Fins¶

For 80+ bar operation and enhanced heat transfer, use composite finned design:

External Fin Configuration: - Type: Longitudinal fins (continuous along pipe length) - Quantity: 8 fins per pipe - Height: 50 mm (extends from pipe surface) - Thickness: 3 mm - Material: Carbon steel, welded to pipe - Surface area enhancement: 3x bare pipe area - Fin efficiency: ~85% (calculated for natural convection)

Pressure Rating:

Hoop stress: σ = P × D / (2 × t)
For P = 100 bar, D = 500 mm, t = 25 mm:
σ = 10 MPa × 500 / (2 × 25) = 100 MPa

Steel yield strength (X52): 360 MPa minimum
Safety factor: 3.6 ✓

Design rationale: Insulation layer removed from original concept. For a cooling system, thermal resistance is counterproductive. External fins address the limiting factor (seawater-side natural convection) while the steel core provides pressure containment.

5.4 Pipe Flow Analysis¶

Velocity Check:

Flow area = π × (0.818)² / 4 = 0.526 m²
Per pipe (8 parallel): V̇ = 1.59 / 8 = 0.199 m³/s
Velocity = 0.199 / 0.526 = 0.38 m/s

This velocity is low. Optimize with smaller pipes:

Revised: 16 pipes of 500 mm diameter

Pressure Drop Calculation:

Darcy-Weisbach: ΔP = f × (L/D) × (ρ × v²/2)

For smooth pipe, Re = 2.5×10⁶: f = 0.01
L = 10,000 m, D = 0.4 m, ρ = 600 kg/m³, v = 0.79 m/s

ΔP = 0.01 × (10,000/0.4) × (600 × 0.79²/2)
ΔP = 0.01 × 25,000 × 187
ΔP = 46,750 Pa = 0.47 bar

Acceptable pressure drop.

6. Primary Heat Exchanger Design¶

6.1 Server-Side Cooling Architecture¶

Cooling Distribution:

6.2 Primary Heat Exchanger Specifications¶

Type: Shell and Tube (TEMA BEM)

6.3 Heat Exchanger Arrangement¶

    WATER/GLYCOL FROM DATA CENTER
              │ 50°C
              ▼
    ┌─────────────────────┐
    │   ┌─────────────┐   │
    │   │  TUBES      │   │
    │   │  (Ti Gr 2)  │   │
    │   │             │   │◄── sCO2 IN (10°C)
    │   │    ~~~~     │   │
    │   │    ~~~~     │   │
    │   │    ~~~~     │   │
    │   │             │   │──► sCO2 OUT (40°C)
    │   └─────────────┘   │
    │     SHELL (CS)      │
    └─────────────────────┘
              │ 35°C
              ▼
    WATER/GLYCOL TO DATA CENTER

7. Pumping System¶

7.1 Pump Requirements¶

Hydraulic Calculations:

Pump Power:

P_hydraulic = ρ × g × H × Q
P_hydraulic = 600 × 9.81 × 13 × 1.59
P_hydraulic = 122 kW

At 75% efficiency:
P_shaft = 122 / 0.75 = 163 kW

With motor efficiency 95%:
P_electrical = 163 / 0.95 = 172 kW

Total pumping power: ~200 kW (0.2% of cooling load)

7.2 Pump Specifications¶

Type: Multistage centrifugal, hermetically sealed

7.3 Pump Arrangement¶

                    ┌──────────┐
    FROM HX ───────►│  PUMP 1  │───┐
                    │  (Duty)  │   │
                    └──────────┘   │
                                   │
                    ┌──────────┐   │
    FROM HX ───────►│  PUMP 2  │───┼───► TO OCEAN
                    │  (Duty)  │   │
                    └──────────┘   │
                                   │
                    ┌──────────┐   │
    FROM HX ───────►│  PUMP 3  │───┘
                    │ (Standby)│
                    └──────────┘

8. Ocean Environmental Conditions¶

8.1 Labrador Current Characteristics¶

8.2 Site Selection Criteria¶

8.3 Seasonal Temperature Profile¶

The deep water temperature is remarkably stable year-round, providing consistent cooling capacity.

9. Installation Methodology¶

9.1 Shore Crossing¶

Method: Horizontal Directional Drilling (HDD)

9.2 Offshore Pipe Installation¶

Method: Towed or floated installation

Sequence: 1. Fabricate pipe strings onshore (500 m sections) 2. Weld/fuse sections on beach or floating barge 3. Attach buoyancy modules 4. Tow pipe string to installation corridor 5. Controlled flood and sink to seabed 6. Position with ROV assistance 7. Connect sections with mechanical connectors 8. Bury in trench (1-2 m depth) or protect with rock dump

Installation Vessels: - Pipe-lay barge or DP vessel - Support tugs (2-3) - ROV support vessel - Survey vessel

9.3 Installation Schedule¶

10. Control System¶

10.1 Control Philosophy¶

Primary Control Loop: Temperature-based flow control - Measure: Server return water temperature - Setpoint: 50°C maximum - Output: sCO2 pump speed (VFD)

Secondary Controls: - Pressure maintenance (expansion tank) - Flow balancing (control valves) - Emergency shutdown (ESD)

10.2 Instrumentation¶

10.3 Control System Architecture¶

┌─────────────────────────────────────────────────────────┐
│                    SCADA / HMI                          │
│              (Control Room Display)                     │
└─────────────────────────┬───────────────────────────────┘
                          │ Ethernet
          ┌───────────────┼───────────────┐
          │               │               │
    ┌─────┴─────┐   ┌─────┴─────┐   ┌─────┴─────┐
    │  PLC 1    │   │  PLC 2    │   │  PLC 3    │
    │ (Cooling) │   │ (Pumps)   │   │ (Safety)  │
    └─────┬─────┘   └─────┬─────┘   └─────┬─────┘
          │               │               │
    Field │         Field │         Field │
    I/O   │         I/O   │         I/O   │

11. Safety Systems¶

11.1 Hazard Identification¶

11.2 Safety Instrumented Functions¶

11.3 Emergency Scenarios¶

Scenario 1: Ocean Pipe Leak 1. Fiber optic DTS detects temperature anomaly 2. System isolates affected pipe section 3. Remaining pipes handle reduced load 4. Alert maintenance team 5. ROV inspection within 24 hours

Scenario 2: Total Cooling Loss 1. Server inlet temperature rises 2. At 35°C: Reduce IT load 25% 3. At 40°C: Reduce IT load 50% 4. At 45°C: Graceful server shutdown 5. Backup air cooling maintains UPS/network

Scenario 3: CO2 Release Onshore 1. CO2 detectors trigger at 5,000 ppm 2. Ventilation fans activate 3. Non-essential personnel evacuate 4. System depressurizes to safe level 5. Investigate and repair

12. Reliability and Redundancy¶

12.1 System Availability Target¶

Target: 99.999% (Five 9s)

12.2 Failure Modes and Effects Analysis (FMEA)¶

Rating Scale: - Severity (S): 1=Negligible, 3=Minor, 5=Moderate, 7=Major, 10=Catastrophic - Occurrence (O): 1=Rare (<1/10yr), 3=Low (⅕yr), 5=Moderate (1/yr), 7=High (1/mo), 10=Frequent - Detection (D): 1=Certain, 3=High, 5=Moderate, 7=Low, 10=None - RPN = S × O × D (Action required if RPN > 100)

12.2.1 Ocean Piping System¶

12.2.2 Pumping System¶

12.2.3 Heat Exchangers¶

12.2.4 Control and Safety Systems¶

12.2.5 sCO2 Inventory¶

12.2.6 FMEA Summary¶

No failure modes exceed RPN 100 with proposed mitigations. Highest risks are fin integrity and fouling - address in detailed design phase.

12.3 Maintenance Strategy¶

13. Performance Summary¶

13.1 Design Point Performance¶

13.2 Seasonal Performance¶

13.3 Comparison to Alternatives¶

14. Cost Estimate¶

14.1 Capital Cost Breakdown¶

Cost context: At $127M for 100 MW cooling capacity, this represents ~12% of a $1.05B data center build. Conventional chiller plants cost \(50-80M but consume 15-25 MW of electricity annually (~\)9-16M/year at $0.07/kWh). The sCO2 system pays back the premium in 5-8 years through energy savings.

14.2 Operating Cost (Annual)¶

Comparison: Conventional chiller OPEX is dominated by electricity ($9-16M/year). The sCO2 system's $4.4M/year OPEX saves $5-12M annually vs chillers.

15. References¶

Technical Standards¶

• TEMA (Tubular Exchanger Manufacturers Association) Standards
• ASME B31.3 Process Piping
• API 610 Centrifugal Pumps
• DNV-OS-F101 Submarine Pipeline Systems
• ISO 13628 Petroleum and Natural Gas Industries - Subsea Production Systems

Research Papers¶

• NIST IR 6496: Heat Transfer of Supercritical CO2
• DOE/NETL: Supercritical CO2 Technology Program
• PPI Handbook: Marine Installation of PE Pipe

Data Sources¶

• NL Heritage: Cold Ocean Environment
• NOAA: Labrador Current Characteristics
• Wikipedia: Labrador Sea, Supercritical CO2

CCNL-TES-2024-01 v1.0