HCTGS v15.0 Atlantis — Building a New River at the Coast

ABSTRACT HCTGS v15.0 "Atlantis" — A Land-Based Coastal Infrastructure for Flat-Coast Desalination, Mineral Extraction, Urban Planning, and Optional Atmospheric Coupling HCTGS v15.0 "Atlantis" extends the HCTGS series into flat coastal regions where classical mountain-anchored architectures reach their structural limits. It introduces a land-based gravity and thermal infrastructure that occupies the first 5–10 km behind a flat coastline and converts warm seawater into freshwater, mineral products, and electricity without dependence on rivers, groundwater, or external power grids — and without requiring any natural topographic feature as a prerequisite. The reference architecture consists of twelve Mega shafts — each 50 m in diameter and up to 600 m tall — arranged in a linear two-channel layout that allows progressive coastal extension without redesigning the base platform. Each shaft operates on buoyancy-assisted natural draft, producing approximately one million cubic metres of freshwater per day at reference throughput. The 12-tower cluster delivers 12 million m³/day, equivalent to a medium to large natural river. Seawater feed is supplied not by pressure pipes but by managed coastal channels 50–150 m wide — a sustained flow of 800–1,200 m³/s at river scale, handled by standard coastal civil engineering. The water multiplication effect significantly amplifies effective system yield: advanced-treated municipal wastewater is routed back into the channel system as warm secondary feed, increasing total water available for agriculture by 80–90% beyond primary output, while intercepting nutrients for agricultural mineralisation rather than discharging them to rivers or the sea. Because wastewater recycling partially satisfies agricultural demand, a larger fraction of Atlantis primary output can be allocated at drinking-water price rather than irrigation price — increasing revenue per tower without increasing CAPEX. The mineral backbone (Module C) receives concentrated brine from twelve shared gravity funnels and processes it through parallel extraction trains for NaCl battery-salt, magnesium, MgO-based products, and potash. Process heat is supplied by the four-stage Mg thermal cascade established in HCTGS v14 "Phoenix" (DOI: 10.5281/zenodo.19773264): Mg powder burns at 2,500–3,100 °C (Stage 1), with cascading heat recovery through two HRSGs (Stage 2: 800–1,500 °C; Stage 3: 150–400 °C) and seawater preheating (Stage 4). Where on-site electrolysis draws on system surplus electricity at 0.5–1 ct/kWh marginal cost, effective Mg production from seawater brine drops to $50–100/t versus $800/t market price — a 20–40× reduction. Each tonne of Mg yields 497 kg MgO, 2,300 kg distilled water, 7,000 MJ surplus heat, and optional green hydrogen. Total value: approximately $2,200 from $50–100 internal production cost. After 2–4 weeks startup, no external fuel is required. A tower cascade extension (Module F) allows a second shaft to receive residual steam from the first, reheated by Mg combustion waste heat or concentrated solar, reaching 1,200–1,500 m total condensation altitude where mountain topography permits. At two-stage cascade, delivery pressure reaches 110–120 bar and horizontal distribution extends to 400–450 km, irrigating agricultural plateaus at 800–900 m elevation without pumping. Combined with the wastewater loop, national irrigation coverage of 60–80% becomes achievable for water-stressed coastal nations with 2–5 installations. A structurally significant zero-cost side effect: when seawater flash-evaporates, steam carries no particles. Nanoplastics, microplastics, and polymer fragments concentrate in the brine and are actively collected as solid residue in Module C — not by dedicated filtration but as a physical consequence of the process. A 12-tower complex processing seawater at reference throughput removes an estimated several hundred tonnes of plastic particle mass annually from the coastal water column. Operational continuity is achieved through segmented underground pressure reservoirs connected by passive U-pipe loops — 24–48 hours of city-scale buffer without idle reserve towers. Where adjacent elevated terrain exists, surface reservoirs at 200–400 m double as gravity batteries for peak-power dispatch. An optional atmospheric interface (Module E) allows selected shafts to vent steam columns freely, creating inland moisture anchors. Waterspout Mode is fully reversible; in standard Condensation Mode the system operates as a conservative desalination and mineral plant. Surplus production beyond city and agricultural demand feeds Waterspout Mode or artificial reservoirs — towers always run at full capacity, as CAPEX efficiency requires continuous operation. For a 12-tower block, indicative CAPEX lies in the 5–14 billion USD range, with net annual revenues across water, minerals, and electricity sufficient for payback under 2 years at design-capacity operation. All architectures and specific combinations disclosed herein — including the linear two-channel geometry, the sea-to-ring managed waterway at river scale, the segmented underground U-pipe pressure storage system, the active nanoplastic extraction mechanism, the wastewater closed-loop secondary feed, the water multiplication economic model, the Module F tower cascade to 1,200–1,500 m, and the dual Condensation/Waterspout Mode — are published as prior art under the HCTGS/Seraphim ecosystem (OSIL v1.2). This document is a concept of proof at TRL 2–3. It is not an engineering specification or investment advice. Novel contributions of this work include: · Flat-coast gravity desalination without mountains — first in published literature · Sea-to-ring managed waterway at river scale (800–1,200 m³/s) as primary intake — replacing pressure pipes entirely · Linear two-channel tower geometry for indefinite modular coastal extension · Segmented underground U-pipe pressure reservoirs as operational baseline — replacing idle reserve towers · Active nanoplastic extraction from ocean water as zero-cost structural consequence of flash evaporation — not a filter, not an add-on · Tower cascade extension to 1,200–1,500 m via thermal booster stages — serving agricultural plateaus at 800–900 m without pumping · Wastewater closed-loop secondary feed increasing agricultural water availability 80–90% beyond primary output · Water multiplication economic model — wastewater recycling shifts more primary output to drinking-water price, increasing revenue without additional CAPEX · Desert oasis network as hard-wired infrastructure output — 500 oases, gravity-fed, climate-independent · Planned city template explicitly decoupled from water geography — structural argument against megacity formation · Marine surface cooling as zero-cost co-benefit of hot-water intake. The intake of overheated near-shore surface water (28–30°C) for flash evaporation simultaneously reduces coastal sea surface temperature by 4–6°C, increasing dissolved oxygen, reducing coral bleaching risk, and generating a cooler sea breeze — without dedicated cooling infrastructure. · On-site programmable premium mineral water production as a zero-raw-material-cost revenue stream. The combination of Module C mineral fractions, precision dosing, and Mg-alloy bottle production from the same brine stream creates a vertically integrated premium water brand with $0 feedstock cost and margins of 40–160× over production cost — a revenue architecture not described in desalination or mineral water literature. · Worst-case coastal hazard planning as explicit design principle. Atlantis is the first published large-scale desalination concept to explicitly specify platform elevation against four simultaneous hazard categories — sea level rise (high-emission tail risk + engineering margin), storm surge (1-in-500-year return period), tsunami run-up (maximum credible event from seismic modelling), and ground subsidence (relative sea level at end of design life) — rather than against current conditions or median projections. This planning discipline adds a fraction of a percent to CAPEX and eliminates the relocation risk that makes conventional coastal desalination a stranded-asset liability under accelerating climate scenarios. Where HCTGS v15 builds on prior work without claiming novelty: flash evaporation physics, steam chimney buoyancy, Mg cascade thermodynamics (v14, DOI: 10.5281/zenodo.19773264), Sorel cement chemistry, SOM electrolysis, U-pipe hydraulics (v2, v4), spine-and-rib distribution (v3), ring and funnel geometry (v4–v7), Waterspout atmospheric coupling (v6, v9), and NaCl battery feedstock (v8–v9). Ilir Mehmetaj