On Using Sunlight to Distill Sea Water

On Using Sunlight to Distill Sea Water

By Basil Gala, Ph.D.

From space the Earth is mostly blue water, veiled with white vapor; a few brownish yellow patches are the deserts: Sahara, Arabia, Gobi, Australia's northwest. Seventy percent of Earth's surface is water, oceans deep enough to drown the Himalayas. Yet, fresh water is a precious resource, getting scarcer with growing human populations. The sun boils up much water into clouds, which drift over land, raining down, sometimes in great torrents; but in many land areas, it does not rain enough to sustain life well. For those areas human devices can use sunlight to distill sea water at a minimal cost after installation.

Sunlight is free and and plentiful where we need fresh water most, in deserts and arid areas: Southern California, Baha California, the Sahara, Arabia, and Australia. Seawater is not far from the same areas, except the Gobi. To concentrate sunlight on water for rapid boiling, we need mirrors, lenses, or a combination of both. Semiconductors to produce electricity for our machinery are also needed. Mirrors, lenses, and semiconductors are all made from silica sands, also plentiful in desert areas.

We also need capital and labor to fashion our materials into efficient distillation equipment. Poor, work-hungry local people can supply the labor; the industrialized nations have plenty of excess capital they are eager to invest for profit.

That's reasonable, as my dad would have said, so let's do it. If it's reasonable, my mom, would have countered, it's not practical. Why aren't people doing it?

So what's keeping us from launching enterprises for solar water distillation? It may be that we cannot earn enough profit, or any at all, with the use of available technology. I'd like to generate new ideas for both the technology and the economics; good ideas will make this industry boom eventually, since the fundamentals are right for its development. Again, that's reasonable, if people want to be that way, instead of starting new genocides or wars.

Make ideas, not wars. What's given? The sun rises somewhere in the east and sets somewhere in the west, the exact locations depending on the seasons. Mirrors or lenses have two motions, vertical and horizontal, to follow the sun as it travels in the sky. Our installation is close to the coast with canals or pipes bringing seawater to it. Pipes, rail, or trucks carry the distilled water where it's needed; salty minerals are taken away to plants for further processing. Electric turbines produce power from the steam generated as it is cooled, another valuable byproduct.

Mirror systems focusing sunlight on a boiler already exist in Arabia for the production of electricity with steam turbines. I visualize such a system of mirrors in a semicircle, directing sunlight to three or more lenses, which heat rotating boilers to increasingly higher temperatures. When the last boiler achieves superheated steam, the steam feeds a turbine and the residual very salty water drains to outside drying tanks. The empty boiler fills with sea water. Stopping briefly behind each lens, the boiler gets heated again for steam production. In this system, production of electricity and fresh water is continuous.

With the system installed, energy, fresh water, and salts are produced without polluting the environment; moreover, since sunlight is free, there are no costs other than maintenance. With plenty of fresh water and energy, crops thrive in the sunlit desert. Baha California can become a verdant paradise, and so can many coastal areas in North Africa, the Middle East and Australia. Trees can also be planted which will absorb some of the sun's energy impacting the Earth causing planetary warming.

Forests cool the environment and increase rainfall, pushing back desertification.

We have jumbed ahead in our story; let's look at some of the details of our design. Three or more mirrors opposite each lens reflect sunlight to it right to the edges. The mirrors are parabolic, expressed as y=ax². The parameter “a” is fixed based on the distance from the mirror to the lens, deciding how “open” the parabola is. Any reflected sunlight falling outside the lens is absorbed by semiconductors, which provide negative feedback to correct the position of the mirrors.

Although the three mirrors are oriented somewhat differently, because they have different positions, their deviations are fixed, so the servo mechanism on the lens can control them together. The mirrors do follow the sun's movement in the sky, but the servo controls are still needed for perfect focusing of the reflected sunlight. The lens focuses sunlight on the surface of the water in the boiler. This is accomplished because the boiler is tilted upwards towards the lens.

The mirrors of our system are not the kind you have in your bathroom; they are crafted from glass and highly polished stainless steel backing which can stand up to the weather for many years. The big lenses are fashioned from thick glass.The tops of the boilers are from specially tempered glass, the bottoms from stainless steel. The boiler tops are also lenses to further concentrate sunlight on the water. The shape of the boilers is roughly conical, with the point of the cone next to the axis of rotation of the boiler assembly. The exit point for the steam is higher up on the cone on top, while the water exit and entry is lower, below the cone. Sea water comes in from the lower opening to the boiler after excess water has been discarded with the salts.

Granted that neither energy nor salt will be produced in large amounts by our system, as useful byproducts energy and salt will make the installation more cost effective, perhaps making it marginally profitable. A small profit margin is all that is required. The solar distiller is a major capital expenditure, which if properly constructed, can last many decades.

Thus, the solar distiller will qualify for a bond issue, and after the bonds have been paid off, like a bridge or toll road, it will be an asset belonging to the community, to be used at the cost of maintenance only. If proven profitable, development may be financed by private companies or investors. Local people will not object as much out of nationalism to foreign investments, as they often do with oil or gas reserves, because sunlight is a renewable resource which will eventually revert to the people. When the project has shown its financial feasibility, the World Bank and the International Monetary Fund will be confident in lending to credit-worthy poor countries which are interested in solar seawater distillation.

Very poor countries can distill seawater using low-tech equipment and manual labor. Imagine a metal bowl holding about 10 liters of seawater, covered tightly by a glass dome in the shape of a lens. A hose leaves the north side of the dome, spiraling down to an opaque bottle also of 10 liters, placed in the water of a trench, dug by the sea. The metal bowl is anchored on the ground next to the trench. When the bottle is full of water that has condensed from vapor, the villagers replace it with an empty one and refill the metal bowl with water. When the metal bowl has accumulated enough salt, the villagers scrape the salt off and package it for use or sale.

The water produced from evaporation is clean enough for drinking, washing, and bathing by people and animals; it may also be used to irrigate vegetable gardens.

Other low-tech ways exist for producing fresh water from seawater evaporation. On the west coast desert of Africa, a beetle spreads its gossamer wings in the morning fog by the sea, tilting its head down to the ground. Droplets from the fog condense and flow down to its head and mouth. Another instance of such distillation we find in Frank Herbert's fine science-fiction novel “Dune.” The native Fremen of the desert planet catch the evening fog, at the mouths of the caves where they live, with flimsy material like the beetle's wings, condensing it into bowls. The fog catching material can be weaved from inexpensive plastic fibers, framed like a fan to be opened and folded easily, and placed by the sea where early morning breezes pass, laden with moisture.

We have examined high-tech seawater distillation methods and low-tech ones. Now let's look at projections for super-tech approaches. Such methods might be weather control from space. Giant mirrors, lenses, and lasers in earth orbit could focus coherent light and energy to specific spots on the ocean to boil up water to steam. When clouds reach a desert area, immense shades in orbit could be opened to darken and cool the atmosphere so as to cause rainfall.

With weather control from satellites, artificial rainfall with cloud seeding, low tech and high tech distillation of seawater, all these technologies will alleviate fresh water shortages. Yet, all these methods will not suffice, nothing will suffice, as long as humans and their animals keep reproducing at current rates, as long as forests are burning in the tropics to uncover fragile soils for cultivation, as long as more trees are cut down for lumber than are planted, and as long as planetary warming goes on with the burning of fossil fuels.

We need to remember, taking to heart, the ecological disasters of Iceland, Easter Island, Taos, and the Aegean Isles, once thickly forested until people cut down too many trees, and then let loose grazing animals on the land. I remember as a boy in the Greek Youth Corps, planting baby pines and firs in a government program of reforestation. It was a happy experience for me. Today I plant trees on land I own whenever I get the chance. All of humanity should follow the example of the German and the Japanese people, who for centuries have looked after their forests with love and care.

Trees and other plants are natural humidifiers for the environment, holding rainwater in their roots, branches, and leaves, and releasing it to the
atmosphere slowly; they restrain heavy rains from washing down precious soil to the sea, permitting water to seep into the soil and appear further down as lovely
springs and rivulets. Whatever fresh water we have we can conserve better and use more wisely than we are doing now. In suburban neighborhoods I often see broken or defective sprinklers gushing the source of life, with the water police nowhere in sight. We could learn from the olive and the fig trees, also from the desert rat and beetle, how to handle water.

As much as I admire technology and such devices as I have described, I feel Nature on our blue-white planet is enough for us, if we respect and preserve it, and will give us enough fresh water for our needs—provided we keep our numbers in check.