TP# 37: 9/85
UNDERSTANDING SOLAR STILLS
by Horace McCracken, Joel Gordes
- Technical Reviewers
- Daniel Dunham
- Jacques Le Nonmand
- Darrell G. Phippen
Published by: VITA 1600 Wilson Boulevard, Suite 500 Arlington, Virginia 22209 USA Tel: 703/276-1800 * Fax: 703/243-1865 Internet: firstname.lastname@example.org
Preface[edit | edit source]
This paper is one of a series published by Volunteers in Technical Assistance to provide an introduction to specific state-of-the-art technologies of interest to people in developing countries. The papers are intended to be used as guidelines to help people choose technologies that are suitable to their situations. They are not intended to provide construction or implementation details. People are urged to contact VITA or a similar organization for further information and technical assistance if they find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated almost entirely by VITA Volunteer technical experts on a purely voluntary basis. Some 500 volunteers were involved in the production of the first 100 titles issued, contributing approximately 5,000 hours of their time. VITA staff included Maria Giannuzzi as editor, Suzanne Brooks handling typesetting and layout, and Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Horace McCracken, is the president of the McCracken Solar Company in Alturas, California. The co-author, VITA Volunteer Joel Gordes, is currently the solar design analyst for the State of Connecticut's Solar Mortgage Subsidy Program. The reviewers are also VITA volunteers. Daniel Dunham has done consulting in solar and alternative sources of energy for VITA and AID. He has lived and worked in India, Pakistan, and Morocco. Mr. Dunham has also prepared a state-of-the-art survey on solar stills for AID. Jacques Le Normand is Assistant Director at the Brace Research Institute, Quebec, Canada, which does research in renewable energy. He has supervised work with solar collectors and has written several publications on solar and wind energy, and conservation. Darrell G. Phippen is a mechanical engineer and development specialist who works with Food for the Hungry in Scottsdale, Arizona.
VITA is a private, nonprofit organization that supports people working on technical problems in developing countries. VITA offers information and assistance aimed at helping individuals and groups to select and implement technologies appropriate to their situations. VITA maintains an international Inquiry Service, a specialized documentation center, and a computerized roster of volunteer technical consultants; manages long-term field projects; and publishes a variety of technical manuals and papers. For more information about VITA services in general, or the technology presented in this paper, contact VITA at 1815 North Lynn Street, Suite 200, Arlington, Virginia 22209 USA.
INTRODUCTION[edit | edit source]
Ninety-seven percent of the earth's water mass lies in its oceans. Of the remaining 3 percent, 5/6 is brackish, leaving a mere.5 percent as fresh water. As a result, many people do not have access to adequate and inexpensive supplies of potable water. This leads to population concentration around existing water supplies, marginal health conditions, and a generally low standard of living.
Solar distillation uses the heat of the sun directly in a simple piece of equipment to purify water. The equipment, commonly called a solar still, consists primarily of a shallow basin with a transparent glass cover. The sun heats the water in the basin, causing evaporation. Moisture rises, condenses on the cover and runs down into a collection trough, leaving behind the salts, minerals, and most other impurities, including germs.
Although it can be rather expensive to build a solar still that is both effective and long-lasting, it can produce purified water at a reasonable cost if it is built, operated, and maintained properly.
This paper focuses mainly on small-scale basin-type solar stills as suppliers of potable water for families and other small users. Of all the solar still designs developed thus far, the basin-type continues to be the most economical.
HISTORY OF SOLAR DISTILLATION[edit | edit source]
Distillation has long been considered a way of making salt water drinkable and purifying water in remote locations. As early as the fourth century B.C., Aristotle described a method to evaporate impure water and then condense it for potable use.
P.I. Cooper, in his efforts to document the development and use of solar stills, reports that Arabian alchemists were the earliest known people to use solar distillation to produce potable water in the sixteenth century. But the first documented reference for a device was made in 1742 by Nicolo Ghezzi of Italy, although it is not known whether he went beyond the conceptual stage and actually built it.
The first modern solar still was built in Las Salinas, Chile, in 1872, by Charles Wilson. It consisted of 64 water basins (a total of 4,459 square meters) made of blackened wood with sloping glass covers. This installation was used to supply water (20,000 liters per day) to animals working mining operations. After this area was opened to the outside by railroad, the installation was allowed to deteriorate but was still in operation as late as 1912--40 years after its initial construction. This design has formed the basis for the majority of stills built since that time.
During the 1950s, interest in solar distillation was revived, and in virtually all cases, the objective was to develop large centralized distillation plants. In California, the goal was to develop plants capable of producing 1 million gallons, or 3,775 cubic meters of water per day. However, after about 10 years, researchers around the world concluded that large solar distillation plants were much too expensive to compete with fuel-fired ones. So research shifted to smaller solar distillation plants.
In the 1960s and 1970s, 38 plants were built in 14 countries, with capacities ranging from a few hundred to around 30,000 liters of water per day. Of these, about one third have since been dismantled or abandoned due to materials failures. None in this size range are reported to have been built in the last 7 years.
Despite the growing discouragement over community-size plants, McCracken Solar Company in California continued its efforts to market solar stills for residential use. Worldwide interest in small residential-units is growing, and now that the price of oil is ten times what it was in the 1960s, interest in the larger units may be revived.
Although solar distillation at present cannot compete with oil-fired desalination in large central plants, it will surely become a viable technology within the next 100 years, when oil supplies will have approached exhaustion. When that day arrives, the primary question will be, "Which method of solar distillation is best?" Meanwhile, almost anyone hauling drinking water any distance would be economically better off using a solar still.