Water desalination by humidification and dehumidification of air: state of the art

The humidification–dehumidification process (HD) is an interesting technique adapted for water desalination when the demand is decentralized. This technique presents several advantages such as flexibility in capacity, moderate installation and operating costs, simplicity, possibility of using low temperature energy (geothermal, solar, recovered energy or cogeneration), etc. Although the common methods of desalination such as distillation and reverse osmosis are the subject of many investigations (archived data and principle of function validated), studies of the HD process are limited at the laboratory step or patents. The aim of this paper is to present the principle of functioning and the characteristics of this technique. The state of the art concerning the HD technique is presented. Several installations functioning worldwide with the HD technique are presented, analyzed and evaluated: solar and geothermal desalination units, a plant functioning by mechanical compression of humid air and an installation functioning by absorbing water from the humid atmosphere. Finally the perspectives of the HD process are evaluated and discussed.

Desalination is characterized by rapidly evolving technologies. Many new desalination plants are tested and installed each year worldwide. The desalination processes have reached maturity (technical an economic), allowing them to have a more important role in the future in water supply. Such installations can be constructed with different technical techniques and with great flexibility of dimensions and energy consumption (a function of energy resources and the situation). Multistage flash distillation (MSF) is the most commonly used process. About 50% of the installations in the world use this technique. On the other hand, for less production, the efficiency of the classical distillers decreases because it is difficult to realize a certain number of effects in small installations. For these smaller installations (rural regions, for example), classical distillers are not appropriate: the cost of the installation, the energy consumption and hence the water cost are very high. The HD process presents a very interesting solution for small units (hotels, rural regions, light industry, etc.), especially when new materials are used. The process is very convenient in cases where heat is available at low temperature at an attractive cost (cogeneration, solar energy, geothermal energy, etc.). Bourouni et al. [3] showed that the cost of water can reach $1.2 in the case of coupling a HD unit with geothermal spring. HD installations can be used for the lowtemperature part of classical distillers to avoid effects in which distillers have to function (vacuum). Coupling HD units with desalination collectors presents a very interesting solution; however, the water cost is relatively high. One reason for this is that solar heating necessitates significant investment in solar collectors and land. Another is that very large amounts of air need to be recirculated because the quantity of water normally contained in saturated air is minimal. Thus, air pumping alone may represent a prohibitive energy cost [16].