The use of water vapor selective membranes can reduce the energy requirement for extracting water out of humid air by more than 50%. We performed a system analysis of a proposed unit, that uses membranes to separate water vapor from other atmospheric gases. This concentrated vapor can then be condensed specifically, rather than cooling the whole body of air. The driving force for the membrane permeation is maintained with a condenser and a vacuum pump. The pump regulates the total permeate side pressure by removing non-condensable gases that leak into the system. We show that by introducing a low-pressure, recirculated, sweep stream, the total permeate side pressure can be increased without impairing the water vapor permeation. This measure allows energy efficiency even in the presence of leakages, as it significantly lowers the power requirements of the vacuum pump.
Such a constructed atmospheric water generator with a power of 62 kW could produce 9.19 m3/day of water (583 MJ/m3) as compared to 4.45 m3/day (1202 MJ/m3) that can be condensed without membranes. Due to the physical barrier the membrane imposes, fresh water generated in this manner is also cleaner and of higher quality than water condensed directly out of the air.
The increasing water scarcity, due to ongoing desertification, salinization of fresh water sources and a still increasing global population poses a major challenge for society. Access to safe drinking water is so substantial that it was made one of the United Nations “Millennium Development Goals”. Most approaches for generating new sources of fresh water focus on desalination techniques to make use of the seemingly unlimited water body of the oceans [1] and [2].
Another possibility, which has found less application up to now, is the extraction of water from the air humidity. The amount of water that can be present as vapor strongly depends on the temperature. Therefore, when a body of air is cooled down far enough, it will result in the condensation of the excess vapor which can then be collected. This cooling can either occur naturally (dew collection [3] and [4]) or it can be achieved by investing energy [5].
Due to the relatively high latent heat of water, the energy requirements to condense water are usually orders of magnitude larger than the energy required for water purification methods. Thus, atmospheric water vapor processing can only be remunerative in the presence of natural or existing heat sinks (radiative cooling [6] and [7], deep sea water [8] and [9], otherwise unused heat-sinks [10]) or in remote areas where the energy balance changes significantly when transportation is taken into account. In such locations a humidity harvesting unit may be driven by renewable energy, like solar [11], [12] and [13] or wind [14] energy, so it could be a stand-alone application, independent of existing infrastructure.
Besides the energy required for the condensation, a significant part of the energy demand in humidity harvesting is needed for cooling the body of air in which the water vapor is embedded at atmospheric conditions. If a cubic meter of air of 30 °C with a relative humidity of 50% is cooled down to 2 °C, only 43.6% of the cooling power is used for condensing water (9.66 g), while the remaining 56.4% is almost entirely spent on cooling air. A way to circumvent this sensible heat requirement is to concentrate the water vapor by the use of desiccants [15], [16] and [17]. However, even with the recent discovery of advanced desiccant materials [18] the main disadvantage of this process is that a desiccant system works in cycles, reducing the maximum water output as a continuous process is not possible. Another method that brings about the same advantages, but allows for a continuous process is the use of water vapor selective membranes to separate the water vapor from the other gases prior to the cooling process [19]. The driving force for the permeation is the partial pressure difference across the membrane. This force is maintained with a condenser and a vacuum pump that displaces the inflow of non-condensable gases. Due to the use of a dense polymer membrane that is highly selective for water vapor, no pollutants or pathogens can pass the membrane, making the condensed water very pure. Also the membrane maintenance should not pose a major challenge as only air and vapor (therefore no scaling) are used as a feed and the absence of sunlight and aqueous environment does not favor bacterial or algae growth, which poses the greatest challenges to other common membrane technologies.
In this paper we analyze the effects of operational and meteorological conditions (like temperature, humidity, permeate side pressure or water vapor pressure) on the water production and the energy efficiency of a system working with membrane separation. According to these and the membrane characteristics, such as permeability and selectivity, we suggested a system design and provide operational parameters, that can be used to significantly reduce the energy demand for the production of potable water.
In this paper we have shown that the energy gain when using membrane technology in humidity harvesting is significant. The effect of membrane characteristics, temperature, humidity and leakage flows resembling realistic conditions has been evaluated. We have shown that when there is an inflow of non-condensable gases into the system, the energy balance can be improved significantly by non-trivial combinations of system pressure and a low-pressure recirculated sweep stream.
It has been demonstrated that for a given input power twice the amount of freshwater can be produced with membranes, which is of superior quality.
We expect that this enhancement can have a large impact on technologies using atmospheric water vapor as a source of drinking water and can be a large benefit, especially for remote areas. Future research is focused on experimental evaluation of the model results.
