Patent Application
20140197022
1. Field of the Invention
The present invention relates to water treatment systems, and particularly to a solar-powered humidification-dehumidification desalination system that provides environmentally friendly and energy-efficient desalination of seawater and brackish water and increased production thereof.
2. Description of the Related Art
Small- to moderate-scale water desalination systems are expected to be vital for hot and arid areas, where natural sources of water are absent and access to sweet water pipelines is considered challenging, either due to lack of energy sources to run a desalination system or to isolated geographical territories. These locations have an abundance of solar energy that provides a suitable environmentally friendly energy source.
One of the moderate-scale water production systems that utilizes solar energy is the humidification dehumidification (HDH) system. HDH systems have received significant attention from researchers within the last decade. These units have a significant benefit over solar stills, where solar collection, water heating, evaporation, and condensation are all integrated in a single “box”. The solar still configuration results in considerable thermal inefficiency and produces a limited amount of desalinated water in the range of 5-7 L/m2 per day.
Humidification-dehumidification (HDH) desalination uses separate components for each of the thermal processes, allowing each component to be independently designed and allowing much greater flexibility in the design of the thermodynamic cycle for vaporizing water into air and subsequently condensing the vapor. The advantage of HDH over a solar still is a significantly higher Gain Output Ratio (GOR), which is the amount of fresh water produced per thermal energy added per latent heat of vaporization. This results in a smaller total area of solar collectors for a given water demand. More broadly, HDH systems are regarded as having an advantage over some other technologies, such as reverse osmosis, since they involve relatively simple, inexpensive components and can operate over a wide range of raw water quality without the need for pretreatment or complex maintenance operations. This makes HDH more suitable for deployment in the developing world, where capital investment and technical support may be more limited.
One of the concerns of the HDH system is that the thermal energy requirements are still relatively high in comparison with other technologies, i.e., the GOR is less than other thermal desalination processes, such as Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED). HDH cycles may be classified according to whether air or water is heated and according to whether the air or water circuit is open or closed.
Examples of an air-heated, closed-air, closed-water cycle of the prior art are shown in
The layouts for the above systems are similar. They include solar collectors 12, 112, 212 for heating air. Heated air 13, 113, 213 from the solar collectors 12, 112, 212 passes through respective first-stage humidifiers 14, 114, 214, second-stage humidifiers 16, 116, 216, and in some systems, third-stage humidifiers 130, 230. Preheated brackish water or seawater 19, 119, 219 is sprayed inside the humidifiers 14, 16, 114, 116, 130, 214, 216, 230, allowing the brackish water 19, 119, 219 to evaporate. This evaporation separates the sweet water from the brine 17, 117, 217. The brine 17, 117, 217 from each humidifier is collected in a brine tank 20, 120, 220. In these closed loop systems, the brine 20, 120, 220 supplies the brackish water for treatment and the collected brine therein is cycled towards the dehumidifier/condenser 18, 118, 218 via the supply line 21, 121, 221.
When the heated air 13, 113, 213 passes through the humidifiers, the air becomes humid air 15, 115, 215 due to the moisture collected during the evaporation. This humid air 15, 115, 215 is reheated via the adjacent solar collector 12, 112, 212 to provide the necessary hot air for either the humidifying process or the condensation process in the dehumidifier.
In the dehumidifier 18, 118, 218, the incoming brackish water or seawater 21, 121, 221 is at a much lower temperature than the humid air 15, 115, 215. Thus, heat exchange between the seawater and the humid air produces condensation and the desalinated water therefrom is collected through the desalinated water line 23, 123, 223. The cooled air 11, 111, 211 from the dehumidifier is fed back to the solar collector 12, 112, 212.
In the alternative humidification dehumidification desalination system 200 shown in
It will be noted that in the three systems shown in
Several studies have been conducted relating to water-heated cycles and air-heated cycles that suggest the above. It has been shown by thermodynamic analysis that the addition of more stages may increase the desalinated water productivity slightly. However, it decreases the parameter used for cycle performance assessment, i.e. GOR. In other words, while the prior art of
In light of the above, it would be a benefit in the art of water treatment systems to provide a desalination system that maximizes GOR for a given energy input. Thus, a solar-powered humidification-dehumidification desalination system solving the aforementioned problems is desired.
The solar-powered humidification-dehumidification desalination system includes a supply of seawater or brackish water passing through a dehumidifier/condenser. The brackish water is preheated in the dehumidifier/condenser due to the condensation process. A plurality of humidifying stages includes respective humidifiers and respective solar collectors. The solar collectors heat air, and the heated air passes through the respective humidifiers to evaporate the preheated seawater or brackish water, separating pure water from the brine. The humid air is reheated and recirculated through the humidifying stages and the dehumidifier, and the desalinated water from condensation in the dehumidifier is collected and processed. The system recirculates the brine from each humidifier, utilizing the latent heat therein for more efficient evaporation and less energy consumption.
In the present system, seawater or brine released from the brine tank is circulated through the multiple humidifier stages in series (after preheating by use as a heat exchanger in the dehumidifier), from the last humidification stage in sequence to the first humidification stage before returning to the brine tank.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The solar-powered humidification-dehumidification desalination system, hereinafter referred to as the multi-stage air-heated humidification-dehumidification MSAHHDH desalination system, utilizes latent or residual heat energy in the brine to increase thermal efficiency and desalinated water production in the desalination process. As shown in
Unlike the prior art conventional HDH systems, the MSAHHDH desalination system 1000 uses the residual or latent heat in the saline/brackish water or seawater to conserve energy required for the desired vaporization. In the prior art systems, the preheated saline/brackish water is supplied in parallel to all the humidifiers from the same source, i.e. through the dehumidifier/condenser 18. For any given temperature of the saline/brackish water, there is some heat loss prior to reaching the humidifiers due to the common source of the preheated saline/brackish water and the length of travel thereof which plays a contributing factor to said heat loss. In contrast, the MSAHHDH desalination system 1000 minimizes any heat loss, since the preheated saline/brackish water is supplied from a closer source and maintained at relatively higher temperature than conventional systems. For example, the preheated saline/brackish water 1019 for the second-stage humidifier 1016 is supplied directly from the dehumidifier 1018, while the preheated saline/brackish water 1025 for the first-stage humidifier 1014 is supplied directly from the brine of the second-stage humidifier 1016, the brine being the remainder of the saline water that has not evaporated. In the latter case, the brine 1025 is already at an elevated temperature as a result of the humidifying process performed on the preheated seawater or brackish water 1019 from the dehumidifier/condenser 1018. Due to the above, the preheated saline water is at a higher temperature than in the conventional system. This translates to a smaller temperature difference to overcome in order to humidify the incoming air in the first-stage humidifier 1014, thereby making the process more energy efficient by reducing energy consumption required to reach the desired temperature for maximal evaporation in the humidifiers.
As the brine 1025 circulates from the second-stage humidifier 1016 to the first-stage humidifier 1014 for further humidification, the resultant brine is collected in one place, viz., the first-stage humidifier 1014. The collected brine 1017 flows in to a collection tank, such as the brine tank 1020, via gravity. In this closed-loop system, the brine tank 1020 holds the brine 1017 from the humidifiers 1014, 1016, as well as the main supply of saline water to be processed, such as seawater. Since the seawater will be at a much lower temperature than the brine, mixing of both will also significantly lower the temperature of the brine 1017. This forms the main saline water supply 1021 piped into the dehumidifier/condenser 1018.
In the dehumidifier/condenser 1018, pure water vapor is separated by condensation from the moist air 1015. The condensation occurs through thermodynamic heat exchange between the cold incoming saline water supply 1021 and the incoming hot, humid air 1015 from the second-stage humidifier 1016. In this embodiment, the saline water supply 1021 is admitted through tubes in the dehumidifier/condenser 1018, and the hot, humid air 1015 condenses on the outside surface of the tubes. The condensed, desalinated water 1023 is collected and pumped out of the dehumidifier/condenser 1018 to an exterior holding tank. The cooled air 1011 from the condensation process cycles back to the solar collector 1012 associated with the first-stage humidifier 1014, repeating the humidifying dehumidifying process.
As noted above, the process described above can be applied to Nth degree of stages.
As with the MSAHHDH desalination system 1000, the desalination process begins with saline water from the brine tank 1120. The saline water 1121 can be primarily seawater or a mixture of seawater and brine from the first-stage humidifier 1114. This saline water 1121 becomes the preheated saline/brackish water 1119 supplying the humidification process in the third-stage humidifier 1130. The brine from the third-stage humidifier 1130 becomes the preheated saline/brackish water 1125 for the second-stage humidifier 1116, and the brine from the second-stage humidifier 1116 cycles into the first-stage humidifier 1114, where the resulting brine 1117 recycles back to the brine tank 1120. Pure or desalinated water 1123 condenses within the dehumidifier 1118 and flows to a collection tank.
As with the MSAHHDH desalination system 1000, the MSAHHDH desalination system 1100 utilizes thermal energy more efficiently by maximizing the latent heat recovery in the saline water from the dehumidifier 1118 and the brine from the second and third humidifiers 1116, 1130 to the respective humidifiers. The energy required to heat the air for the evaporation process is much less than in conventional systems when assisted by this residual heat.
The MSAHHDH desalination system 1200 shown in
As mentioned above, in this embodiment, the brine and the seawater are held in separate tanks, e.g., the brine tank 1220 (preferably, the brine tank 1220 is insulated to maintain the brine at elevated temperature) and the seawater tank 1240. The brine tank 1220 facilitates collection of the brine 1217 from the first-stage humidifier 1214 and circulates the same through the second-stage humidifier 1216, and then in series to the first-stage humidifier 1214. The brine processed through this sub-system maintains elevated temperatures conducive for efficient humidification in the humidifiers 1214, 1216, since the main heat loss for the brine occurs within the humidifiers 1214, 1216 rather than through the dehumidifier/condenser 1218.
Subsequently, the seawater processing sub-system mainly recirculates the seawater through the dehumidifier/condenser 1218. The seawater tank 1240 provides the incoming seawater 1241 for the dehumidifier/condenser 1218 and circulates the same from the dehumidifier 1218 as outgoing seawater 1243 back to the seawater tank 1240. This permits a more efficient and productive condensation to occur within the dehumidifier 1218 due to the incoming seawater 1241 being maintained at a relatively constant colder temperature than the hot, humid air 1215 passing through the dehumidifier 1215, i.e., the temperature difference between the humid air 1215 and the seawater 1241 is high. In contrast to the other MSAHHDH desalination systems 1000, 1100, the seawater does not mix with the brine, which would cause the cooling medium, e.g., the seawater and brine mixture, to be at an equilibrium temperature, the equilibrium temperature effectively being lower than in the current MSAHHDH desalination system 1200. The condensation is collected in the dehumidifier 1218, and the desalinated water 1223 is piped for further processing.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
