•Three proposed solar powered humidification–dehumidification (HDH) water desalination systems are investigated and presented.•Parametric study of the hourly and daily systems performance is ...presented at various operating parameters.•Mathematical model of the HDH water desalination system is developed and solved numerically using EES software.•The performance of the proposed systems is maximized by changing between the open/closed mode of operations.
Transient performance analysis and investigation of three proposed solar powered humidification–dehumidification (HDH) water desalination systems: single stage (SS), double stages (DS) and modified double stages (MDS) systems are presented for hot and humid cities. Open/closed modes of operations of the systems are also investigated. A parametric study of the hourly and daily systems performance is presented at various operating parameters. The results reveal that: (i) inserting solar collectors in HDH systems for air and water heating substantially enhances the system’s performance; (ii) the fresh water productivity of all systems at open mode of operation increases with increasing the outdoor air temperature and humidity; (iv) at low outdoor humidity, the system’s performance at closed mode of operation is higher than those at open mode of operation and the opposite is true at high outdoor humidity; (v) comparing between the three systems, MDS system (open mode) can produce fresh water of 350 kg/day with GORday of 1.63 and its fresh water productivity enhances with 86.7% and 34% than SS and DS systems, respectively; and (vi) using control system, the performance of the proposed systems can be maximized by changing between the open/closed mode of operations according to the outdoor conditions.
•A quasi-2D transient model of PEMFC with anode recirculation is developed.•Anode recirculation leads to certain level of self-humidification.•Nitrogen accumulation causes performance decrease ...especially at anode downstream.•Current distribution non-uniformity of co-flow is improved by anode recirculation.•Anode recirculation avoids the severe fuel starvation compared to dead-ended anode.
Anode recirculation of proton exchange membrane fuel cell (PEMFC) is considered as an effective way for self-humidification and efficient hydrogen utilization, but nitrogen crossover presents a problem. To investigate the transient characteristics of PEMFC with anode recirculation, a comprehensive quasi-2D non-isothermal transient model is developed. The simulation results show that for a PEMFC initially operated with dry hydrogen, anode recirculation leads to certain level of performance increase in the beginning due to self-humidification effect, and then the performance decreases caused by nitrogen crossover, hydrogen dilution and the increase of anode activation loss. The nitrogen accumulation in anode downstream is severer than upstream, and the performance decline rate decreases with increasing cathode inlet velocity under the simulated operating conditions. The performance and current density distribution uniformity of counter-flow configuration is better than co-flow under low inlet humidity, but the difference becomes smaller with anode recirculation. Anode recirculation and dead-ended anode (DEA) operation both have self-humidification effect, and compared to DEA, the advantages of anode recirculation can be summarized as lower performance decline rate, less chance of local hydrogen starvation and better current density distribution uniformity.
In this paper, the performance of a novel HDH solar desalination system equipped with a combination of heat pipe (HP), evacuated tube collector (ETC) and air bubble column humidifier is ...experimentally investigated. This novel HDH system uses advantages of ETC-HP as a highly efficient thermal absorption and conductor device, and at the same time employs the advantages of an air bubble column humidifier, i.e. high interface area and effective mixing in order to heat the water and humidify the air, respectively. The effects of various parameters including incoming air flow rate into the humidifier, initial depth of water in the humidifier, and adding fluids such as oil and water in the space between the ETCs and heat pipes on the system performance were investigated. Results indicate that by adding oil in the space between the ETCs and heat pipes, daily fresh water productivity and daily efficiency increase significantly and reached 6.275kg/day·m2 and 65%, respectively. In addition, the optimum initial water depth in the humidifier is the same as the length of the heat pipe's condenser section in the humidifier. Moreover, daily production slightly increases as the air flow rate increases. The estimated cost of fresh water produced through the HDH system designed is 0.028$/L.
•The performance of a novel HDH system is investigated experimentally.•Effect of initial water depth in the humidifier and air flow rate is studied.•Adding oil in the space between ETCs and HPs increases productivity significantly.•The maximum daily productivity and daily efficiency are 6.275kg/day∙m2 and 65%.•The estimated cost of the fresh water produced is 0.028$/L.
•A novel heat pump coupled humidification-dehumidification system was proposed.•Heat pump evaporator was used as the air dehumidifier.•Sizes of the heat exchangers were obtained for fixed operational ...conditions.•System performance was balanced for fixed-size components at off-design conditions.•Characteristics method was utilized to study the system at off-design conditions.
In this study, the theoretical investigation of a novel heat pump assisted humidification-dehumidification desalination system has been carried out. The conventional dehumidifier of the humidification-dehumidification system was removed and the evaporator of the heat pump was utilized for direct dehumidification of air. Besides, the heat pump condenser was used as the saline water heater of the humidification-dehumidification cycle. First, the sizes of the components were calculated at design conditions for fixed input operational conditions namely the ambient air temperature, relative humidity, and inlet saline water temperature. However, the input operating conditions vary at off-design conditions resulting in changes in heat pump performance. Since the performance of a heat pump is a function of its refrigerant saturation temperatures, to examine the performance of the designed system at off-design conditions, the system performance balancing was carried out through the method of characteristics for a given set of fixed-size components. It was observed that the gained output ratio and hourly yield of the proposed system reach 2.476 and 0.91 kg/h, respectively. The economic analysis indicated that the cost per liter of the proposed system is 0.014 $/L.
•The designed evaporative cooler can cool the air to meet thermal comfort.•Desalination performance of integrated system is better than stand-alone systems.•Overall system efficiency is dominated by ...the humidification-dehumidification cycle.
Cooling and freshwater represent two fundamental demands in hot and arid regions. This paper reports the integration of an indirect evaporative cooler (IEC) and a humidification-dehumidification desalination cycle (HDH) for the simultaneous production of cooling and freshwater. To take full advantage of system integration, the purge air from IEC is supplied to HDH to promote water productivity. A pilot IEC unit is firstly designed and tested to achieve the temperatures and humidity of the outlet air steams. Results reveal that the IEC unit is able to cool down the supply air to below 25 °C under different outdoor conditions, and the purge air temperature is also 5–10 ℃ lower than the intake air temperature. Employing the IEC purge air as the working air, the HDH cycle is then investigated analytically. Under the operation ranges considered, the freshwater productivity and gain-output ratio (GOR) are 25–125 L/hr and 1.6–2.5, respectively, which are higher than other HDH configurations operating under the same conditions. Finally, the performance of the combined IEC-HDH system is evaluated. The overall coefficient of performance (COP) and Second-law efficiency are found to be 2.1–2.5 and 3–26%, respectively. Further improvement of efficiency can be achieved by integrating with adsorption or vapor compression refrigeration cycles.
•A solar desalination system heated directly by curved Fresnel lens concentrator.•Desalination system is based on the humidification-dehumidification process.•Four-stage multi-effect desalination ...system is proposed.•Condensation latent heat and residual heat in the brine are recycled and reutilized.•The maximum yield and GOR of the unit can reach 3.4kg/h and 2.1, respectively.
This study demonstrates a multi-stage humidification-dehumidification (HDH) solar desalination system heated directly by a cylindrical Fresnel lens concentrator. In this novel system, the solar radiation is sent directly into desalination unit. That is to say, the solar receiver and the evaporator of the system are a whole in which the black fillers in seawater directly absorb the concentrated solar lights to heat the seawater film to produce the evaporation. The configuration and working processes of the proposed design are described in detail. In order to analyze its performance, a small solar desalination prototype unit incorporated with a cylindrical Fresnel lens concentrator was designed and built in our laboratory. Using three-stage isothermal tandem heating mode, the variation of the fresh water yield rate and the absorber temperature with time were measured experimentally and were compared with theoretical calculations. The experimental results show that the maximum yield of the unit is about 3.4kg/h, the maximum gained output ratio (GOR) is about 2.1, when the average intensity of solar radiation is about 867W/m2. This study indicates that the proposed system has the characteristics of compact structure and GOR high. It still can be improved when the design and operation are optimized further.
•The different water mass flow rate on HDH is investigated.•Enhancement of the gain output ratio of the desalination unit.•Enhancing the thermal performance of the desalination unit with reusing ...rejected water from HDH.
A continuous solar still (SS) integrated with a solar humidification–dehumidification (HDH) is investigated. An evacuated solar water collector is utilized in the HDH. The idea of closed-air and open-water cycles is considered as the operating principle of HDH process. The different water mass flow rates (1.5, 2, 2.5 and 3L/min) of HDH is investigated. To avoid a large water loss during HDH desalination, a SS is fed with the exit warm water from HDH during day and night time. Therefore, the productivity of the SS with exit warm water from HDH is greater than that of the conventional solar still (CSS) by approximately 242% and the gain output ratio (GOR) is increased by about 39%. The daily water productions of the CSS, SS with exit warm water from HDH, HDH unit and continuous solar desalination unit are 3.9, 13, 24 and 37L/day, respectively.
•Thermoeconomic performance of various HDH configurations is compared.•Single, double, and triple humidifiers are examined and contrasted.•The effectiveness of humidifiers and dehumidifiers ...influences freshwater production.•The water-heated multi-humidifier HDH cycle generates more freshwater.•A double humidifier air-heated HDH cycle generates fresh water at 2.06 $/m3.
Drought has hit several water-stressed regions, with serious consequences for water and food security. In this regard, the low-cost and low-grade thermally driven humidification-dehumidification desalination cycle has good potential to provide freshwater in arid regions. From energy and economic point of view, different configurations of the humidification-dehumidification cycle have been proposed in the literature without a comprehensive comparison to identify the most suitable one. Thus, this paper investigates the thermal and economic performance of several multi-humidifier water- and air-heated humidification-dehumidification cycles. Thermodynamic models are structured to study the behavior of each cycle. The models rely on heat and mass transfer balances for each component. The effectiveness method is implemented for humidifiers and dehumidifiers to evaluate heat and mass exchangers. The models are validated using previously published data. The series seawater flow, triple humidifier, water-heated humidification-dehumidification cycle could deliver 151.2 kg/h of fresh water at a maximum temperature of 70 °C, with a gain output ratio of 1.995 and a specific cost of 2.73 $/m3. This cost rises by around 16% when a maximum temperature of 60 °C is employed. Results reveal that using a double-humidifier in an air-heated humidification-dehumidification cycle produces freshwater of 102.2 kg/h at the lowest price of $2.06/m3. It is concluded that the multi-humidifier water-heated humidification-dehumidification cycle produces more freshwater and less gain output ratio than the multi-humidifier air-heated humidification-dehumidification cycle.
Studies conducted at the beginning of this century revealed a warming and humidification trend in northwest China, which lasted for a short period, with some minor variations. The future trend ...remains uncertain, with uncertainty over the future duration of warming and humidification trends in the region. In this study, a comprehensive index was constructed to quantitatively characterize the degree of humidity. Using observation data, National Center for Atmospheric Research/National Centers for Environmental Prediction (NCAR/NCEP) reanalysis data, and future scenario prediction data we systematically analyzed the temporal and spatial evolution characteristics of the warming and humidification trend in northwest China and its possible causes. The results showed that the temperature in northwest China has been continuously increasing since 1961, and although there was no obvious precipitation trend from 1961 to 1999, it has been increasing nonlinearly since 2000. The wetness index considers the synergy of evaporation and precipitation, and had the same pattern of variation as precipitation. From a spatial perspective, the temperature has consistently increased throughout the whole northwest region, and the degree of warming has gradually increased. Precipitation displayed a trend of increasing in the west and decreasing in the east, but the area with increased rainfall expanded eastward over almost three climatic periods, and the area experiencing humidification extended across the whole of the northwest. There has been an obvious eastward expansion of climate humidification in northwest China since the start of this century. However, the current extent of climate humidification has not changed the basic climate pattern in northwest China. Over the next 30 years, the humidification trend will likely slow down significantly compared with the last 20 years. The eastward expansion of climate humidification since the start of this century may have resulted from the multi-decadal synergistic influence of westerly wind circulation and monsoon circulation.
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