Sub-Surface Thermal Loop (SSTL) for Data Centre Cooling - Version 2.0

Executive Summary The Sub-Surface Thermal Loop (SSTL) is an open-hardware architecture designed to provide passive, zero-parasitic cooling for high-density AI data centres and quantum computing infrastructure. By leveraging a deep closed-loop geothermal thermosiphon, the system uses the natural thermal gradient of the earth as a cold sink to reject heat without active mechanical pumping. The design prioritises capital cost efficiency by retrofitting existing brownfield assets such as suspended oil and gas wells, potentially eliminating up to 90% of new drilling costs. This document provides the system architecture, detailed component specifications, engineering calculations, and phased implementation guidance. Addendum v2.0.1 — Engineering Refinements & Boundary Conditions Following initial feedback on Version 2.0, the following technical clarifications have been added to the system boundaries: Fluid Dynamics & Natural Circulation under Low Delta T Even with a modest operational temperature differential (Delta T approx 30-60°C) in industrial deployment, strong thermosiphon circulation is maintained through the 1,000 m vertical height of the U-borewell. The buoyancy driving pressure (Delta P = Delta rho * g * H) scales directly with height, providing sufficient head even with relatively small density changes. Auxiliary Power Integration The inclusion of a secondary co-generation feature is intended primarily to power local telemetry, safety actuators, and monitoring systems. Capital Cost Mitigation through Brownfield Repurposing To address drilling costs, the architecture strongly prioritises retrofitting existing non-producing oil and gas wells, suspended wells, or dry geothermal boreholes. This approach can eliminate up to 90% of new drilling expenditure, dramatically improving project economics and accelerating deployment for high-density computing operators. Technical Supplement: Thermodynamic & Fluid Dynamics Sizing Model To validate the physical mechanics of the Sub-Surface Thermal Loop (SSTL) architecture under Phase II field boundaries, the following steady-state thermofluid mass/energy balance establishes baseline performance profiles. System Boundary Assumptions Vertical Borewell Depth (H): 1,000 m (Total loop circuit length L = 2,000 m) Temperature Profile: Max reservoir production temperature (T_hot) = 45°C; Surface/TEG cold sink interface (T_cold) = 5°C (Delta T = 40 K) Pipe Roughness (epsilon): 1.5 x 10^-5 m (Commercial Duplex/316L Stainless Steel) Gravity (g): 9.81 m/s^2 TEG Efficiency: Baseline 5% conversion efficiency via commercial Bi2Te3 modules. Buoyancy Head & Driving Pressure Natural circulation is driven by the density differential (Delta rho) between the ascending hot leg and descending cold leg. Using a standard fluid baseline approximation (Delta rho approx 7.7 kg/m^3): Delta P_buoyancy = 7.7 kg/m^3 * 9.81 m/s^2 * 1,000 m approx 75,537 Pa (0.076 MPa / 0.75 bar). Frictional Losses & Mass Flow Rate Iteration Equilibrium flow velocity (v) occurs where driving buoyancy matches Darcy-Weisbach frictional loop losses. Iterating via the Blasius friction factor across varied nominal pipe diameters yields the following operational limits: Case A: 25 mm (1-inch ID) Riser Core Steady-State Velocity (v): approx 0.231 m/s Mass Flow Rate (m_dot): approx 0.113 kg/s (approx 406 kg/h) Thermal Power Transported: approx 18.9 kWth Net TEG Electrical Output: approx 944 W per bore Case B: 50 mm (2-inch ID) Riser Core (Optimized Scale) Steady-State Velocity (v): approx 0.435 m/s Mass Flow Rate (m_dot): approx 0.852 kg/s Thermal Power Transported: approx 142.5 kWth Net TEG Electrical Output: approx 7.1 kW per bore Surge Vessel / Accumulator Sizing To safely dampen thermal transients, manage fluid density swings, and prevent downhole vapor cavitation, a dedicated surface accumulator volume is sized against the total loop internal volume. For a 25 mm ID loop, the recommended accumulator capacity is 12 to 20 Liters, rated significantly above maximum system hydrostatic pressure. Limitations & Future Validation This design is an initial engineering proof-of-concept. The following areas require further validation before any physical deployment: Transient Fluid Modelling: Current calculations are steady-state. Future work should include dynamic simulation of startup, shutdown, and potential flow instabilities. Site-Specific Geotechnical Analysis: Performance is highly dependent on local ground temperatures, thermal conductivity, and geology. Working Fluid Properties: Detailed modelling with real fluid properties (supercritical CO2 or Propane using REFPROP/CoolProp) is needed to account for compressibility and phase behaviour. Safety & Regulatory Compliance: Full relief sizing, hazardous area classification, and regulatory approvals for deep-well retrofitting must be completed on a site-by-site basis.