Urban heat islands and data center cooling represent >5% of global electricity demand and drive peak grid emissions. We present a fully open hardware reference design, the Planetary Thermal Siphon (PTS), that converts low-grade waste heat into three products: 1) passive cooling, 2) 24/7 carbon-free electricity, and 3) atmospheric water. The system uses a hub-and-spoke model: dimpled carbon nanofibre (CNF) ground mats collect heat in cities and transfer it via insulated, subterranean CNF pipes to 300–350 m CNF towers located on low hills outside flight paths. The 2. 7–3. 8 °C natural lapse rate drives a liquid thermosiphon, rejecting heat to the upper atmosphere. Electricity is co-generated by CNF solar skins with photon up-conversion (~35% efficiency) and Seebeck modules along the thermal gradient. Condensed water is harvested at altitude for irrigation and fire breaks. Techno-economic optimization shows peak ROI at 300–350 m tower height when sited on natural low hills 20–50 km from load centers. At 300–350 m, each tower rejects 8–15 MW of heat and generates 0. 75–1. 05 MW average continuous power. CAPEX is ~10. 3M per tower with 10–12 yr payback, followed by 80+ years service life. Because CNF is synthesized from biomass, each tower sequesters 3–8 kt CO2 in long-lived structure, enabling gigatonne-scale negative emissions if deployed globally. All designs, calculations, and test data are released under CERN Open Hardware Licence v2 – Strongly Reciprocal (CERN-OHL-S v2). We solicit replication of the 3 m benchtop demonstrator and invite peer review of the pressure-break and safety analyses. Keywords: carbon nanofibre, thermosiphon, urban heat island, data center cooling, PUE, open hardware, negative emissions, passive cooling, atmospheric water harvesting
David Seagal (Thu,) studied this question.