HCTGS v31 — The Desalination Tower That Pays for Itself

Abstract This concept reframes the Hydro-Cascade Turbine 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. As an architectural rather than a thermodynamic property, the heat is 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. 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. The magnesia that the splitting cycle produces is not confined to green cement: it feeds a ladder of five established markets — refractory linings for heavy industry, alkaline treatment in environmental engineering, high-value electrical insulation, agricultural magnesium, and additive-grade oxide for advanced polymers — which together give the byproduct a market deep enough to absorb the output of eleven thousand towers rather than one cement stream alone. 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). Two ladders carry the value beyond the core. The magnesia ladder splits one byproduct stream by temperature and purity across five markets — refractories, environmental treatment, electrical insulation, agriculture and elastomers — where the light-burned grade leaves the cycle directly and the refractory grade needs a separate re-firing; magnesia from seawater is a built process, run for decades in Ireland, Japan and Israel, that lost to mined magnesite on the calcination energy alone. The cold ladder sells the same depth water three times before it is spent, through district cooling and computing-cluster cooling into the cascade feed, and against 45°C ambient the coefficient of performance rises from about 2.5 to 6, which for a hot-climate city of a million is some 37 per cent of its electricity. On the packaging side the same regulation converts a market measured in units: some 500 billion PET beverage bottles are made each year worldwide and about 120 billion inside the European Union, so the 2030 reuse requirement puts roughly 12 billion European containers into circulating systems and the 2040 goal about 48 billion — at forty per cent worldwide, six million tonnes of PET a year that is never made, against the eight to eleven million tonnes of plastic of every kind reaching the ocean annually. A reusable one-litre container formed at 0.8–1.2 mm from a 5083-class alloy at 4.5 % magnesium carries 6.8–10.2 g of it, which makes the European 2040 quota a single tower-year and a worldwide forty per cent about five; each tonne of that magnesium arrives alongside twenty-one tonnes of purchased aluminium. The carbon comparison is given with its threshold: reuse passes single-use PET above roughly 25 cycles against primary aluminium at the world average, nine against hydro-made primary and two against recycled metal. On the land side, one tower commands 487–1,329 km² of irrigated ground and a cluster of ten 4,870–13,290 km², bounded by topography and by drainage: the leaching fraction returns to the brine cascade as feedstock, so the salt leaves as product and the aquifer stays out of it. 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; CAPEX is stated as a band by variant — deep shaft, hybrid 300/300 or freestanding tower — with the long-distance gravity main treated as public water infrastructure and not as plant cost, and with towers two through ten in a cluster sharing intake, tunnel, cistern and chemical plant at roughly a third less each; 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.