Pre-print for the tenth Congress of International Maritime Association of the Mediterranean, Crete, May 2002. SPRAY TURBINES TO INCREASE RAIN BY ENHANCED EVAPORATION FROM THE SEA

It seems that one of the effects of global warming is to increase rainfall in some parts of the world while reducing it in others. With the growth in demand for water the effects of any reduction on political stability and human happiness are profound. Several regions in the Mediterranean are vulnerable. The amounts of energy involved in weather systems are so very large that it is difficult for engineers to influence them. However the evaporation of water from the sea surface is slow and inefficient because of the need for large amounts of latent heat and because the perpendicular component of turbulence in the air vanishes at the surface leaving a stagnant humid layer (Csanady 2001). The wind has to blow over thousands of kilometres of warm sea before it can bring rain. Saudi Arabia is dry because the Red Sea and the Persian Gulf are narrow. Chile is dry because the Humboldt current is cold. Calculations show that some remediation may be possible using a mechanism that can be controlled to suit local needs. This paper describes the design of a floating, vertical-axis wind turbine which pumps sea water through the humid stagnant layer and sprays it in fine droplets with a large increase in surface area. The spray release height is chosen to give time for a large fraction of the water to evaporate when mixed with air in the turbine wake. The distance from land is chosen so that residual very salty drops fall into the sea. The humidified air is likely to produce rain when it reaches rising ground. The technique allows evaporation from narrow stretches of sea with winds blowing over the fetches associated with daily sea breezes caused by rising air ashore. Narrow waters can be made to behave like much wider oceans with seasonal prevailing winds but control remains in the hands of the turbine owners. PREVIOUS RAIN-MAKING METHODS Surface tension forces on very small drops make them become unstable and collapse. Even when the air is supersaturated with water vapour it is still necessary for there to be nucleation particles of the right size and chemical nature before rainfall can be triggered. Most of the attempts at rain-making have employed aircraft to seed supersaturated clouds with substances such as silver iodide or solid carbon dioxide. Very small quantities of the seeding substance are needed so that the aircraft operation costs more than the silver. Seeding has a sound scientific basis but it is difficult to do repeatable demonstrations. It works only with supersaturated air. It certainly generates extreme hostility from people down wind who will believe, with good reason, that their rain has been stolen. Furthermore once an event with the energy of a heavily super-saturated cloud has been triggered, the rain may come in uncomfortably large amounts, leading to flash floods. In the desert more people die by drowning than from thirst. TURBINE DESIGN The horizontal axis wind turbine has decisively beaten the vertical axis configuration for electricity generation. The differences are interesting. In a constant wind speed, horizontal axis machines have moderately steady aerodynamic loading but reversing gravitational loads, while vertical axis ones have reversing aerodynamic loads and constant gravitational ones. Horizontal axis blades can have the more efficient asymmetrical foil sections while vertical axis ones have to be symmetric. The vertical axis blades go through an idle point twice each rotation and experience a wide variation in the angle of incidence of airflow, while horizontal axis blades can use chord variation and blade twist to stay closer to ideal values. This gives them an advantage in efficiency that outweighs the cost of a tower and the difficulty of having power conversion plant far above ground. It would be possible to use a horizontal axis machine for spray generation but a boost pump would be needed to lift water to hub height. If the amount of water in the blades were ever different, there would be serious balance problems so that there would have to have reliable flow control between blades. The central part of the proposed vertical-axis spray turbine rotor has an approximation to the shape of the troposkien (Reis 1975) as shown in figure 1. This is the shape adopted by a flexible spinning body like a skipping-rope. It allows all forces to be taken as pure tensions rather than bending moments. A spray turbine would have a larger ratio of diameter to height than a land-based electricity-generating one because it is not necessary to release water from a very great height and we are concerned about stability in heel. Faired cross arms join the blades to the central shaft. The blades are hollow and will be filled with water pipes. For spray generation the vertical axis configuration has the overwhelming advantage that it provides an extremely efficient pump with no moving parts, almost free. This is shown in figure 2. The lower part of the blade breaks through the water surface close to the axis at a shallow slope. The angle of this slope is critical to the operation of the pumping action. Consider the forces on an element of water in the pipe at the water surface. It will feel the force of gravity vertically downwards and also a centrifugal force horizontally outwards. If the resultant of these two forces makes an angle less than 90 degrees to the line of the blade in the direction away from the axis then water will tend to move outwards. As the radius increases so will centrifugal forces and so the slope can increase. It is desirable that the rapidly moving parts of the blades are above wave height. This mechanism can easily produce a pressure of 10 bar, far more than is needed to lift water to the release height. The pump has no accurate machined parts, pistons, valves or sliding seals. Figure 1. Plan and elevation views of the spray turbine.