1. Practical considerations for pumping sulphur dioxide or other materials to the stratosphere H.E.M. Hunt & K.A. Kuo University of Cambridge Engineering Department

2. Stratospheric Particle Injection for Climate Engineering (SPICE) WP1. Evaluating candidate particles WP2. Delivery systems WP3. Climate and environmental impacts

3. Proposed delivery system

4. Engineering challenges Aeroelasticity: large-scale deflections, small-scale vibrations Tether-balloon interactions Scale of manufacturing Wind profile, especially jet streams, gusts Launch & recovery Pumping pressure Temperature gradients Long-term deployment issues (UV degradation; gas losses; microdamage) Abrasion inside pipe Heat transfer Dispersion of particles Lightning Drag reduction, fixed balloon orientation Winch & pulley design Tether material & winding Terminations Safety & failure mechanisms Maintenance

5. What pumping pressure is required to deliver materials to 20km? Sulphur dioxide Solid particles Hydrogen

6. Key concepts High pressure Low tension 1. Pressure decreases with height due to: Friction Decreasing static head (weight of fluid) Change in fluid’s momentum 2. Fluid density changes with pressure & temperature 3. Tension increases with height Low pressure High tension Pump

7. Case 1: Sulphur dioxide At 250K, density is ~1530kg/m^3 Static head is 3100bar Total pressure required, say 4000 bar But: freezing occurs at 3000bar the pipe’s strength fibres only take 990bar (PBO) – 1390bar (K-49)

8. Case 1: Sulphur dioxide Increase temperature to 500K to lower the density (now a gas) Density varies along pipe with pressure Static head is 1.1bar Total pressure required: 2.1bar But: elevated temperatures cause creep in the strength fibres, leading to structural failure

9. Case 2: Nitrogen-based slurry TiO 2 proposed as alternative cooling agent Solid particles require carrier fluid: N 2 Can pump as gas at sub-zero temperature Expanding gas requires pipe of changing radius

10. Case 3: Hydrogen Use to replenish balloon & to power fuel cells Only a small amount required (70g/s) At 250K: 126 bar, 15mm pipe

11. What we don’t know What is the spatial and temporal variation of the ambient conditions (temperature, pressure, wind speed, wind direction)? How sensitive are these calculations to the assumed ambient conditions? What are the heat conduction properties of the pipe and the surrounding air? What are the desired conditions at 20(?)km (particle size, temperature, pressure)? Can we reliably produce the desired conditions at 20km by pumping only from the base? How do we monitor & control what is happening inside the pipe? Do our best models reflect what will actually happen if we put up a pipe?

12. Conclusions It would be extremely difficult to pump sulfur dioxide up a 20km pipe It may be easier to pump solid particles or gases There is still much work to be done before we can answer the question ‘is this possible?’