World-wide water scarcity, especially in the developing world, indicates a pressing need to develop inexpensive, decentralized small-scale desalination technologies which use renewable resources of energy. This paper provides a comprehensive review of the stateof-the-art in one of the most promising of these technologies, solar-driven humidification-dehumidification (HDH) desalination. Previous studies have investigated many different variations on the HDH cycle. In this paper, performance parameters which enable comparison of the various versions of the HDH cycle have been defined and evaluated. To better compare these cycles, each has been represented in psychometric coordinates. The principal components of the HDH system are also reviewed and compared, including the humidifier, solar heaters, and dehumidifiers. Particular attention is given to solar air heaters, for which design data is limited; and direct air heating is compared to direct water heating in the cycle assessments. Alternative processes based on the HDH concept are also reviewed and compared. Further, novel proposals for improvement of the HDH cycle are outlined. It is concluded that HDH technology has great promise for decentralized small-scale water production applications, although additional research and development is needed for improving system efficiency and reducing capital cost.

Humidification dehumidification desalination (HDH) is a promising technology for small-scale water production applications. There are several embodiments of this technology which have been investigated by researchers around the world. However, from a previous literature review [1], we have found no study which carried out a detailed thermodynamic analysis in order to improve and/or optimize the system performance. In this paper, we analyze the thermodynamic performance of various HDH cycles by way of a theoretical cycle analysis. In addition, we propose novel high-performance variations on those cycles. These high-performance cycles include multi-extraction, multi-pressure and thermal vapor compression cycles. It is predicted that the systems based on these novel cycles will have gained-output ratio in excess of 7 and will outperform existing HDH systems.

Carrier gas based thermodynamic cycles are common in water desalination applications.These cycles often require condensation of water vapor out of the carrier gas stream. Since the carrier gas is most likely a non-condensable gas present in very high concentrations (60-95%), a large additional resistance to heat transfer is present. We propose to reduce the aforementioned thermal resistance by condensing the vapor-gas mixture in a column of cold liquid rather than on a cold surface by using a bubble column heat exchanger. A theoretical predictive model for estimating the heat transfer rates and new experimental data to validate this model are described. The model is purely physics based without the need for any adjustable parameters, and it is shown to predict heat rates within 0% to -20% of the experimental values. The experiments demonstrate that heat transfer rates in the proposed device are up to an order magnitude higher than those achieved in existing state-of-the-art dehumidifiers.Keywords: condensation, bubble column, non-condensable gas, thermal resistance model, dehumidification, moist air, carrier gas * Corresponding author c p specific heat capacity at constant pressure (J/kg·K) c p,g average specific heat capacity at constant pressure of the vapor-air mixture (J/kg·K) T air local energy-averaged temperature of the air-vapor bubble (T coil local temperature of the coil surface (T column local energy-averaged temperature of the liquid in the column (T coolant local energy-averaged temperature of the coolant in the coil (

Boiling heat transfer using nanofluids has been a subject of a few investigations in the past few years and incongruous results have been reported in literature regarding the same. Conflicting explanations for deterioration of pool boiling heat transfer coefficient at higher concentrations (4–16wt%) have been presented by various researchers. Recently, a few works have reported a significant enhancement in pool boiling heat transfer coefficient at lower concentrations (0.32–1.25wt%) and the physical reasons for this have not been explained. The present work is aimed at removing these ambiguities. Experiments have been carried out by using stable water based nanofluids containing alumina nanoparticles (average sizes of 47 and 150nm) with vertical tubular heaters of various surface roughnesses (48, 98, and 524nm). It has been observed that with the rough heater (Ra=524nm), heat transfer is significantly enhanced and the enhancement reaches ∼70% at a particle loading of 0.5wt%. With the smooth heater (Ra=48nm), heat transfer is significantly deteriorated and the heat flux reduction reaches ∼45% at a particle loading of 2wt%. Further, it has been observed that a parameter which is the ratio of average size of the particle to the average roughness value of the heater can explain the reported controversy in the pool boiling behavior of these suspensions.

Given simultaneous heat and mass transfer and a multiplicity of possible temperature and flow configurations, the optimisation of humidificationdehumidification desalination systems is complex. In literature, this optimisation has been tackled by considering moist air to follow the saturation curve in the humidifier and dehumidifier of a closed air water heated cycle. Under similar conditions and the same pinch point temperature differences, energy recovery was shown to improve with an increasing number of stages. In the present work, the limits upon the energy recovery and the water recovery (product water per unit of feed) of closed air water heated cycles are investigated. This is done by considering heat and mass exchangers to be sufficiently large to provide zero pinch point temperature and concentration differences with the humidifier and dehumidifier. For cycles operating with a feed temperature of 25• C and a top air temperature of 70 • C, GOR is limited to approximately 3.5 without extractions (i.e. single stage system) and 14 with a single extraction (i.e. dual stage system) while RR is limited to approximately 7% without extractions and 11% with a single extraction . GOR increases and RR decreases as the temperature range of the cycle decreases, i.e. as the feed temperature increases or the top air temperature decreases. A single extraction is shown to be useful only when heat and mass exchangers are large in size. In addition, the effects of salinity and the validity of ideal gas assumptions upon the modelling of HDH systems are discussed. * lienhard@mit.edu Keywords: desalination, water treatment, humidification-dehumidification, saturation curve, multiple stage, extraction, injection 2 MotivationHumidification-dehumidification (HDH) systems are commonly viewed as robust liquid purification systems, driven by low-grade heat and capable of operating on a small scale decentralized basis, [1,2]. In desalination applications, HDH systems purify saline feed water. From another perspective, the role of HDH systems could be to concentrate industrial waste water streams.The premise underlying HDH is that a volatile solvent containing a nonvolatile solute, may be separated by humidifying a suitable carrier gas with that solvent and subsequently condensing it in pure form. achieve this by transferring heat from the dehumidifier to the humidifier via a common heat transfer wall. HDH systems, by contrast thermally separate the humidification and dehumidification processes, as shown in Fig. 1.[ Figure 1 about here.]The system described in Fig. 1 is a closed air water heated HDH process. Air enters the humidifier and is heated and humidified by a counterflow of feed. The moist air then flows to the dehumidifier, in which the air is cooled and water vapor is condensed due to heat transfer to a counterflow of liquid feed. This feed is heated in the dehumidifier and then in the heater, before being cooled and partially evaporated in the humidifier. The feed remaining at the bottom of the humidifier, known a...