For millennia, Nature has used solar energy to evaporate water from the ocean and let the wind transport the vapor over mountains where it (hopefully) precipitates. This process supplies needed water to mountainous regions, and the surplus flows downhill to other communities. Now this naturally occurring mountain precipitation is inadequate for our needs. The remedy proposed here is the large-scale employment of floating solar-heated evaporators to increase the production of water vapor and the careful placement of these evaporators to maximize the recovery of the vapor as useful precipitation.
Orographic precipitation is produced when moist air is lifted as it moves over a mountain. The rain and snow considered here is caused by water vapor from the ocean being carried by winds to mountains. In rising over the mountains, the air expands, cools, water vapor condenses, coalesces on airborne nucleation centers, and precipitation results. The following is a partial list of mountainous regions that have depended on orographic precipitation: The USA western states, Australia's New South Wales, China, North Africa's Atlas Mountains, the Ethiopian Highlands, Mt. Kilimanjaro and other mountains surrounding Lake Victoria, the Himalayas, Mt. Olympus on Cyprus, the European Alps, the Pyrenees, parts of the Andes, Israel's Golan Heights, Turkish mountains that feed the Tigris and Euphrates rivers that supply Iraq. Precipitation in these areas is now insufficient and we have worsening drought in all areas that depend on runoff from these mountains. Disease is spreading, people are starving, and more wars will result.
The decline in orographic precipitation is to some extent caused by global warming, but another cause may be a worldwide increase in aerosols that provide nuclei for the condensing water. More aerosol particles result in smaller water particles or ice crystals and this reduces the ability of these particles to coalesce into larger drops or ice crystals that are heavy enough to fall to earth. If the humidity of the rising air is increased, there will be more condensate, and hence larger drops for the same number of aerosol particles.
It may be possible to increase the humidity of the air by increasing the evaporation from the ocean. This can be done with dark-colored porous rafts whose top surface is covered with a thin layer of water that attains a higher temperature than water in the unmodified ocean. This is because the solar energy is totally absorbed by the thin layer in contrast to the unmodified ocean, where the sunlight must heat the entire depth that it illuminates. Also the raft body insulates the evaporation surface from the water under the raft to further increase the temperature and the resulting evaporation rate. Some preliminary calculations indicate that the domestic water requirements of the entire population of New South Wales might be supplied by an evaporator area 45 kilometers by 45 kilometers located off the coast of Sydney. This is an enormous area compared to a rooftop, but it is only a small dot on the map of Australia. The big question is how much of this fresh water distilled out of the sea will condense and precipitate, and will it precipitate where it can be collected and used?
There are many challenges. The rafts must survive storms, and must maintain a wet surface while being tossed about by waves. Locations must be chosen to maximize the precipitation on land compared to that which falls uselessly into the sea. The geographic areas listed previously have different wind regimes, such as easterly trade winds in the tropical belt, and prevailing westerlies in the temperate zones. The western US has westerlies at 40 degrees latitude and local monsoons further south. China has a monsoon which brings moisture in from the sea during the warm season. It has a long coastline and we will need to find locations for evaporator rafts that will maximize rainfall in the mountains while minimizing flooding in other areas. The microclimate in each location must be studied to optimize the evaporator raft placement.
There is a companion application for floating solar reflectors to minimize evaporation in certain locations and prevent solar heating of the ocean. The reflectors would not be located in the same place as the evaporators. The reflectors could also offset the global warming caused by wave-powered pumps bringing nutrients and CO2 up through the thermocline. The CO2 would outgas to the atmosphere but its greenhouse effect could be offset by the cooling effect of the reflectors. Reflectors and wave-powered pumps are another subject and must be considered elsewhere.
For drought remediation in mountainous areas, there may be no alternative to using the ocean as a source of fresh water, using solar energy to distill water vapor from the ocean, and using the wind to transport the vapor to the mountains where it is needed. This will help to lower sea level and recharge aquifers which have been pumped dry.
The raft is composed of modules factory-assembled into mats which are carried on a barge to the deployment site. At the site, the mats are connected end-to-end to form a strip, which is moored in place.
The present concept for the raft module is a somewhat resilient foam block a few inches in thickness with a wetable fabric or open-cell foam top surface supported by a closed-cell or syntactic foam bottom layer. The block floats at a level such that the top surface can be kept wet in the ocean. Too high a position will allow it to dry out, and too low will allow waves to wash over it and cool it. A small module size (1 m x 1 m) will allow adequate strength to be obtained with a thickness small enough to keep the top surface wet while the foam module floats on the water. The top surface must be a dark color to absorb sunlight, but must not be degraded by ultraviolet rays. The thickness of the porous top surface must be great enough to store water while the raft is tilted by wave motion, and the open cell pore size must retain the water in the tilted position. Somehow, we must make it sufficiently strong and durable to survive, and remain attached to all the other modules in the fabric of the raft. Individual modules would be 1m x 1m with a square wire frame around the periphery and with loops at the corners for attachment to corresponding loops on bordering modules. Wire diagonal braces welded to the corners and at the middle will allow the adjacent modules to exert tension, shear, and compression forces in any direction with no panel distortion. The wire cross section must be chosen to prevent buckling in compression. The wire frame would be molded into the foam block at its mid-plane to insure permanent attachment. Protection of the metal frame from corrosion is a problem that must be solved. Perhaps a frame made of plastic tubing would have adequate strength and solve the corrosion problem.
Links connect the modules together at their respective corners to form a mat 20 m wide by about 50 m long. The links should have enough play to allow the mats to conform to the ocean wave surface and also allow a mat to be wound around a drum 4 m in diameter for deployment off the stern of a barge. There would be about 10 cm open space between modules. A mat 18 modules wide will fit on a drum 20 m wide. The drum would have end walls to keep the mat from sliding sideways.
Mats would be stacked flat on a platform that could be lowered into the barge hold and raised up as the pile was deployed. I have not yet estimated the weight of each mat and compressive loading that the foam modules will withstand. The more we can stack, the faster we can build the raft. The deployment drum travels on an overhead trolley. Mats are stacked bottom-up. The top mat is wound loosely on the drum while the drum travels from stern to bow. The mat is fed over the top of the drum so that its top surface is on the outside of the roll. The drum with wound-up mat then moves to the stern and the mat is let out onto the water as the barge moves forward. The first mat end to go overboard is attached by horizontal ropes to a row of buoys anchored to the ocean bottom. The buoys have sufficient volume to prevent submergence by stresses on the anchor ropes due to wind, tides, and ocean currents. The horizontal separation between mooring buoys and mat will allow the panels to float on the ocean surface and work as evaporators.
Mat #1 must be joined to Mat #2 after #2 is wound on the drum, so we have to devise a method of connecting them together on site. Should they be joined together with short links, or do we want some connecting ropes to separate the panels? Also, we must determine how many mats can be connected together between the end anchor points. There will be anchors and buoys along both sides of the strip of mats in order to resist sidewise forces.
We will require open space between adjacent strips of mats to allow a small workboat to pass between them. This can be accomplished by joining the side buoys of adjacent strips together with ropes of sufficient length to allow their respective centers to be pulled down by anchor lines. The side anchors would be spaced at least 20 m apart along the strip. The workboat will have fore, aft, and side thrusters flush with the hull so that it can pass over or between the tie ropes without entanglement.
The moorings at the ends of the strip are designed to withstand the forces parallel to the length of the strip and prevent shearing the side moorings. The longitudinal forces are proportional to the number of mats in a strip. This sets a limit on the number of mats in a strip between end moorings and breaks the evaporator array into many separate patches. There will be open space between patches. In addition to the open spaces between patches, the rope ties will leave large open spaces between adjacent strips and possibly smaller spaces between the mat ends. There will be smaller open spaces between modules. The area and pattern of the various open spaces between modules is relevant to the ability of a tropical cyclone to gain energy from the warm water under the raft. This may improve the ability of the raft to survive such storms. The rafts will cover large areas of the ocean. It may be possible to incorporate damping mechanisms in the moorings and on the periphery to decrease the forces exerted by storm waves.
The work described above is preliminary and will remain incomplete until many other people examine it, decide that it may be valid, and lend their expertise to changing speculation into reality. In a world of competition, suspicion, hostility and outright warfare, the demands on wisdom, diplomacy, good will, and leadership will be difficult to satisfy, but it is hoped not impossible.
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