Seawater Desalination Using Thermal, Solar, and Hybrid Systems, Including Humidification Desalination

More than 1.1 billion people do not have reliable access to clean drinking water and more than 2.4 billion lack improved sanitation systems. In 2000, unsafe water mortality amounted to 80 million years of lost life. Water is a particular problem for the developing world, but water scarcity impacts industrial societies as well—this, of course, is well known in the Kingdom of Saudi Arabia and in the southwestern United States. Desalination and water purification technologies are already widely used to convert seawater or brackish ground water into drinking water, but the challenge is to do so sustainably, with minimum cost and energy consumption, and to develop technologies that are accessible and economical for all who need them.

This project will focus on thermal and solar driven desalination of seawater and brackish water. Our initial focus will be solar-driven humidification-dehumifidication (HDH) desalination. There are various realizations of this technology. In a typical case, solar energy is used to heat air to a temperature of about 80°C. This air is humidified by contact with seawater that is injected into a packed bed in countercurrent flow, possibly followed by several stages of reheating and humidification (multistaging). The water is condensed from the humidified air stream using incoming seawater as a coolant. Various arrangements for regeneration and energy recovery have been considered in HDH systems.

Multistaging in particular allows much higher humidification than can be practically realized in a single stage system. It is theoretically possible to load air with a substantial amount of vapor (e.g., to 15 wt%) by intensive heating to very high temperatures (about 500°C) followed by adiabatic injection of seawater into the hot air; that process cannot be economically realized in a single stage by means of solar energy and because of the potential for hard scale at high temperatures. Multistaging allows high humidity to be achieved while limiting the top temperature to manageable levels. In addition, the lowered top temperature enables the use of less expensive materials, such as polymers, in both the solar collectors and the humidifier.

At present, systematic comparisons of competing HDH system designs are few, and much of the work reported in the literature has been incomplete. This project will develop comparisons of the performance of various configurations using thermodynamic analyses of the cycle (considering gained output ratio and exergy), and it will examine the thermoeconomic performance of different HDH cycles.

Major components of HDH systems include a packed bed humidifier and solar heaters. For the packed bed, we will develop analytical models that enable assessment of both structured packings (low pressure drop packing) and assessment of optimal inlet conditions for the air and water streams. The latter question is critical to understanding the choice between solar heating of the incoming air and direct heating of the seawater. At present, air heating appears to be the best choice owing to the potential for low cost plastic based solar collectors and the elimination of all scaling problems from the solar collectors. However, the full cycle analysis described above is necessary to fully assess these choices. We will develop optimized designs for solar air heaters as a part of this task; these designs will consider the use of plastics and traditional materials, as well as convective enhancement of the absorber plate.

Once the design trade-offs cycle and scientific uncertainties of the components are understood, laboratory experiments and a small-scale outdoor facility at KFUPM beach will be developed. The purpose of the experiments will be to test the proposed sea water desalination system and components under controlled laboratory conditions as well as under the actual operating conditions of Dhahran. It is expected that the experimental work will be conducted in the 3rd, 4th, and 5th years of this project.

This project will also examine the application of HDH and other desalination technology in the developing world, using an ongoing project in Petit Anse, Haiti as a test bed. In order to determine if solar humidification desalination is appropriate for similar communities in Saudi Arabia, a cost analysis will be performed in conjunction with the theoretical analysis of the system. This will also determine what size plant is appropriate in these environments. Another aspect of the project that will be investigated is the use of locally manufacturable components for use in the packed-bed humidifier. Of particular interest will be packing materials made from recycled plastic, especially the plastic from disposable water sachets that are common in many parts of the world.