HCTGS v31 — The Desalination Tower That Pays for Itself

Abstract This concept reframes the Hydro-Core Tower Gravity Systems (HCTGS) series around a single question: what is the least a tower must do to pay for itself? The answer is a short, self-paying heat cascade built on the magnesium–chlorine (Mg–Cl) hybrid thermochemical water-splitting cycle, whose maximum process temperature near 550°C makes it, with the copper–chlorine cycle, the lowest-temperature route of its class. Magnesium chloride from the concentrated brine meets steam at 450–550°C to give magnesia and hydrogen chloride (MgCl₂ + H₂O → MgO + 2 HCl); electrolysis of the hydrogen chloride releases the hydrogen and frees the chlorine, which returns to reform the magnesium chloride. Hydrogen and cement therefore come from one cycle: the magnesia is calcined at 700–1,000°C into light-burned feedstock for Sorel-class green cement, designed to bind CO₂ as it cures by the reactive-magnesia carbonation route, while the chlorine circulates internally as the mineral cascade’s reagent in place of being sold. Sorel’s own limitation is stated with its strength: magnesium oxychloride softens under prolonged wetting, which sets its application to interior, precast, unreinforced and sealed-lining work. The heat is then spent down a temperature ladder of five to eight stages — boron/aluminium alloying at the top, the splitting cycle at 550°C, steam-raising for the vortex, desalination and cooling at the base — one joule working many times. That the ladder pays five to eight times is an architectural property, not a thermodynamic one. The magnesium supplies only the high-temperature exergy at a small hot core (~5% of the flow); the bulk evaporation heat is the sun-warmed surface layer of the sea, so the sun pays for the heat and the cascade pays for the plant. Water is a near-zero-marginal-cost byproduct, clean by non-volatility (micro/nanoplastic has no vapour pressure) while the 1,500°C core mineralises PFAS and dioxins on the concentrate side. The bulk power circuit runs 60°C to 4°C (Carnot ceiling ~17%, ~6–8% net); the ~1,700:1 condensation collapse (far larger at low pressure) drives the flow through a closed, degassed, pure-vapour circuit, the swirl set by a tangentially mounted jet. The 600-metre head is spent as electricity or, through a closed U-pipeline to an elevated lake or cistern, as pump-free gravity distribution over 200–300 km — the mechanism on which an additive land-value transformation (dryland to farmland to city) depends. Two magnesium populations are kept separate (cement stream and alloy-metal stream, split explicitly); minerals are worked for maximum profit and intermediates handed to existing industry below primary-source cost, turning competitors into customers. At scale, on the order of eleven thousand towers would supply the current world freshwater withdrawal without rivers or rain — holding in the climate worst case — with three to four times current world electricity at the conservative per-tower figure. A dedicated section sets the concept against the regulatory calendar: the EU Packaging and Packaging Waste Regulation, whose core obligations apply from 12 August 2026 and which bans PFAS in food-contact packaging from that date, sets binding beverage reuse targets of 10% by 2030 and 40% by 2040 and lifts recycled content in single-use plastic bottles from 30% to 65%. Because a prohibition relocates a substance and almost all installed PFAS treatment merely concentrates it, the mineralisation band above 1,100°C — set by the carbon–fluorine bond at ~485 kJ/mol — becomes the market position: the tower holds that temperature for alloying and calcination, and evaporation delivers the concentrate to it, so separation and destruction share one machine and one already-paid heat. The released hydrogen fluoride is captured by the plant's own magnesia (MgO + 2 HF → MgF₂ + H₂O), returning magnesium fluoride as optical, flux and corrosion-conversion material as a saleable solid; seawater's own 1.3 mg/L fluoride adds 390–700 t MgF₂ per tower-year. This article introduces no new novel contributions: it is a synthesis drawing the self-paying economic logic out of concepts already on the public record across the HCTGS series — the heat cascade and magnesium populations (v22, v8.0), element-by-element brine crystallisation (v18), the reverse-osmosis brine symbiont and Janus interface (NC-UHR-13, v30; v26), and the gravity distribution and land-value transformation (Rivers Above v2, NC-ATI-9). A dedicated section sets out the working figures — the flash balance, the GOR regeneration that cuts the heat input from ~26 GW single-pass to ~3–5 GW, the low-temperature power bound, the vapour volume-flow, and the material and fluorine mass balances — for independent checking. All parameters are design estimates requiring independent validation; the decisive open figure is the electrolysis step, since the cycle undercuts direct water electrolysis only where the hydrogen chloride is captured dry (≈1.2–1.4 V, ~32–37 kWh/kg H₂) in place of aqueously (≈1.8 V, ~48 kWh/kg). Published as open prior art under CC BY-NC-ND 4.0; commercial use by separate agreement.