A simple, low-cost solar desalination still
by the CADDET Japanese National Team

A novel solar desalination system, capable of producing 10 litres of fresh water per m2 of heat-collecting area per day, has been developed in Japan. The simple, easy-to-maintain system, called AQUA KIDS, is low in both initial and operating costs, and will be the first of its kind to reach the marketplace.

An AQUA KIDS unit.

Introduction

In the 21st century, solar desalination technology for producing fresh water from seawater is likely to be one of the most important means of meeting global requirements. In particular, solar desalination stills, which can produce fresh water using heat from the sun, can provide water supplies for remote villages, on islands and in the coastal regions of developing countries where infrastructure is inadequate and economically difficult to build.

Any desalination system intended for use in such areas must be simple in structure, maintenance-free, easy to transport and install and, above all, low in both initial and operating costs, as well as high in efficiency. A solar desalination still that satisfies all these requirements has been developed by the Top Ecology Company Limited in co-operation with the Thermal Engineering Laboratory of Ryukyu University in Okinawa; the final work towards commercialisation is now proceeding. Known as AQUA KIDS, the system combines a basin still and a multiple effect still in a single unit.

Figure 1: Cross section of AQUA KIDS

 

Solar stills

There are three types of solar desalination still: basin stills, tilted wick stills and multiple effect stills. Until now, only small basin stills have been used on a commercial scale, but they are inefficient, producing only about 3 or 4 litres/m2/day.

Multiple effect stills offer better efficiency, but other problems are associated with them, such as contamination of the fresh water by seawater and a technically difficult manufacturing process.

AQUA KIDS

AQUA KIDS has a simple structure combining a basin still (heat collecting area) and a multiple effect still into a single unit that has the advantages of both types. As shown in Figure 1, the cross-section of the new still is shaped like a right-angled triangle, with a double-glazed aperture on the inclined plane facing the sun, a basin filled with seawater on the bottom, and a multiple effect still section arranged on the vertical side.

Sunlight entering the still through the double-glazed aperture heats and evaporates seawater in the basin, and also heats the first plate of the multiple effect section. Some of the water vapour rising from the basin condenses on the double-glazed aperture and its latent heat is discharged to the air; however, some vapour condenses and discharges its heat onto the first plate of the multiple effect section, adding to the solar heat directly absorbed (see Figure 1). A wick behind the first condensation plate carries seawater, which is in turn evaporated, to condense on the second plate. This process of evaporation and condensation is repeated at each of up to eight condensation plates in the multiple effect section (see Figure 2 an example showing just five plates). At each stage, the condensate (distilled water) is collected in a tank.

Performance

It is known that the heat transfer coefficients between two parallel plates are almost constant, whether they are set horizontally and heated from below (as in a conventional still) or vertically. Minimal heat loss, therefore, occurs due to the vertical configuration. In fact, this configuration has an advantage the upright position of the plates prevents them from sagging due to gravity, so that the plates can be set closer together than those in conventional multiple effect stills. The heat transfer coefficient improves as the space between the plates narrows. However, to avoid natural convection, the space should be no less than 6 mm.

In the basin section, the double-glazing in the aperture does not allow the transmission of heat. The basin is painted black, and the side walls are made of mirror-finished stainless-steel sheets to exploit fully the solar radiation coming into the still.

In tests conducted at Ryukyu University in November 1998, an AQUA KIDS still with a basin area of 4.5 m2 produced 48 litres/day (about 10 litres/m2/day) of distilled water. This distillation rate is about three times higher than a conventional basin still and 1.5 times higher than a bottom-heated multiple effect one. Even in winter, the performance of the still is expected to decrease only a little.

Figure 2: Evaporation mechanism in the multiple effect section

Commercialisation

A standard commercial AQUA KIDS system comprises three of the new, combination stills connected in series. With a total heat collection area of 20 m2 this system can produce 200 litres/day of fresh water with a distillation rate of 10 litres/m2/day in summer. The production capacity can be expanded easily by adding on more stills.

The capital cost is likely to be JPY 3 million (where JPY is the Japanese yen) for a system in which seawater is drawn up into a tank manually; and JPY 5 million for a system which includes a PV-powered seawater pump. Other costs will include transportation, installation and piping of a seawater tank, and the installation, piping and cleaning of a distilled water tank.

Top Ecology commissioned Nippon Kentetsu Company Limited to begin manufacturing AQUA KIDS in March 1999. Export of the product is scheduled to start from the end of this year, making AQUA KIDS the first fully-fledged, multiple effect solar desalination still to reach the world marketplace.

In parallel with their commercialisation activities, Top Ecology and Ryukyu University are continuing to develop the technology. Some of the areas for future improvement include better rust prevention, more durable components, and simpler control systems.

For more information contact the CADDET Japanese National Team in Tokyo.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.