During the project, Skytree transitioned from a cyclic fixed-bed Temperature Vacuum Swing Adsorption (TVSA) process (“Cumulus”) to a continuous moving packed-bed TVSA process (“Stratus”). This pivot significantly improved scalability, energy efficiency, sorbent lifetime and CO2 capture capacity.
In 2024, innovative reactor concepts were developed and tested. Early results showed that the fixed-bed approach would not meet market-relevant energy targets. The project therefore adopted a gravity-fed adsorption reactor combined with a vacuum desorption reactor featuring advanced heat exchange. A proof-of-concept built in Q1 2024 identified key challenges such as airflow channeling and heat transfer inefficiencies, which informed the design of the Stratus Alpha system.
By Q3 2024, the 400 kgCO2/day capacity Stratus Alpha system was constructed, integrating adsorption, desorption and skid based modules. Initial validation confirmed that operational CO2 capture capacity and energy efficiency met foundational scaling targets.
A major redesign was required when resistive electrical heating proved unreliable. The desorption system was re-engineered using pillow-plate heat exchangers powered by a closed-loop pressurised hot water circuit. This solution improved reliability, safety, scalability and suitability for industrial production.
Throughout 2025, extensive system testing was performed, including multi-day continuous runs simulating field conditions. Key optimisation insights included:
Sorbent handling: Venturi vacuum transport was too slow and high-capacity blowers caused excessive attrition. Bucket conveyor systems provided stable transport with minimal sorbent degradation and are being further integrated.
Sorbent durability: Continuous cycling demonstrated minimal attrition, validating the combination of polystyrene-based solid sorbents with a moving-bed TVSA process.
Vacuum performance: To increase process speed and depth of vacuum, the system is being upgraded with a Central Vacuum System.
CO2 purity: >95% purity targets were challenged by sorbent interference with valve seals. Dedicated valve testing led to the transition to ball/gate valves or inflatable seals in next-generation systems.
Energy efficiency: Stratus Alpha is transitioning from separate heaters and chillers to an integrated heat pump system with dual-buffer heat recovery. This design simultaneously cools and reheats sorbent using recovered internal waste heat, significantly reducing specific energy consumption. The architecture is further optimised in Stratus Beta and future cascaded heat pump configurations.
Parallel to hardware development, an AI-driven control system was implemented. Using IoT sensors, machine learning, internal sorbent data and climate modelling, the system dynamically adjusts adsorption/desorption cycles and sorbent flow based on ambient conditions, user requirements and energy price forecasts. This improves capture efficiency, reduces energy consumption and extends sorbent lifetime. The modular framework enables deployment from greenhouse-scale to industrial multi-unit installations.
Due to increased system scale and complexity, Stratus Alpha was retained as a permanent “living laboratory” at Skytree’s Dutch test facility, allowing cost-efficient optimisation under semi-outdoor conditions. Customer-facing validation is shifting to Stratus Beta, a 750 kg/day transportable, serial-manufacturable platform engineered for field deployment in Canada from Q2 2026.
