Patent Application
20190241444
This invention relates to methods and systems for solar humidification/dehumidification desalination of saline water based on solar thermal energy integrated with evacuated tube collectors as well as flat plate, evacuated flat plate collector, concentrated solar collectors and alike, wherein operating parameters such as the maximum temperature of the water heater and the mass flowrate of the humidifier dehumidifier components are optimized.
Consumable freshwater is progressively becoming a scarce resource, mainly affecting dry regions and rural areas with deserts. However, a common advantage in such areas is the availability of abundant solar radiation. The use of solar energy for producing freshwater in such regions is an available solution to address the scarcity of freshwater.
Humidifier dehumidifier desalination methods and systems have a relatively high gained output ratio (GOR) and require relatively low capital investment and involve relatively simple mechanisms. Such systems generally include components such as heat supply systems, condensers, and evaporators along with an option for thermal storage. The process involves direct contact between warm saturated air and warm raw water allowing the air to reach a preferred humidity level which is followed by extraction of water vapor from humid air using a condenser. It should be recognized that while a single Humidifier dehumidifier is mentioned, multiple such subsystems may be incorporated.
The solar-powered humidification-dehumidification desalination system includes a supply of saline/brackish water passing through a dehumidifier/condenser. The saline/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 respective humidifiers to evaporate the preheated saline/brackish water, and separating pure water vapor from the brine. The humid air is reheated and recirculated through the humidifying stages and solar heaters, and the desalinated water from the dehumidifier via condensation is collected and processed. The system recirculates the brine successively from each humidifier to the next for more efficient evaporation and less energy consumption.
The solar powered humidification-dehumidification desalination system comprises or consists of a supply of seawater or brackish water referred to hereinafter as saline water passing through a dehumidifier condenser. The saline water passes a dehumidifier/condenser and is preheated therein. A plurality of humidifying stages includes humidifiers and respective solar collectors. The solar collectors heat air and the heated air passes through the respective humidifiers to evaporate the preheated saline water and separating pure water vapor from the brine. The heated air is reheated and recirculated through the humidifying stages and the dehumidifier, and desalinated water from condensation in the dehumidifier is collected and processed. The system recirculates the brine from one or more humidifiers utilizing the heat content 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 one or more humidifiers in series after preheating by use of 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 from review of the following specification and drawings.
The invention will now be described in connection with the accompanying drawings wherein like reference numbers are used to identify like parts.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
A prior art solar-powered humidifier-dehumidifier desalination system, hereinafter referred to as a multistage air-heated humidifier-dehumidifier MSAH-HDH 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 conventional HDH systems, the MSAH-HDH desalination system 1000 uses the residual or latent heat in the saline/brackish water 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. 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 the heat loss. In contrast, the MSAH-HDH 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 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.
The two main categories of thermal desalination use either heating or freezing for the purpose. The first involves evaporation and condensation, while the latter involves freezing and melting. The most commonly used technique of the two is the evaporation and condensation method. Solar stills and humidification-dehumidification (HDH) processes are among the widely used thermal desalination processes. These are capable of operating at low temperatures as a result of the differences in the quantity of water vapor in the air stream [1].
As desalination and cooling systems in general require a huge amount of energy for operation, the majority of these systems are in operation at locations where solar energy is abundant (deserts or high temperature zones). The integration of solar energy with these systems is a major advantage to provide the supply to meet the increasing demand for such systems [2].
About 60% of the entire world's desalination market employs the reverse osmosis technique, while the other half uses thermal desalination processes [3]. Solar stills are a commonly used thermal desalination process that requires a larger solar collector area due to its significantly lower gained output ratio (GOR). This is mainly due to the fact that this technique combines all processes into a single unit operating as a complete system. The processes are namely evaporation, condensation, and water heating through solar energy collection [4]. The performance of solar stills relies on solar radiation, cloud cover, ambient temperature, and wind velocity. The output of these systems may also be affected by factors such as brine depth, insulation, and vapor leakage [5].
HDH desalination systems when compared to solar stills have a significantly higher GOR, thereby requiring a comparatively smaller area for solar energy collection. The required technical support and capital investments associated with HDH desalination systems are minimal, as it involves simple mechanisms and can operate with raw water of wide-ranging quality. The maintenance procedures involved are also far less complicated [6].
HDH systems in general include components such as, heat supply systems, condensers, and evaporators along with an option for thermal storage. The process involves direct contact between warm saturated air and warm raw water allowing the air to reach a preferred humidity level, which is followed by extraction of water vapor from humid air using a condenser [7].
The main factors affecting evaporation and condensation of an HDH system are the rate of evaporation of water and condensation, and the temperature of the cooling water. The rate of evaporation of water and condensation increases with increasing amount of evaporative raw water. The condensation rate is also higher at lower cooling water temperature [8].
The extent of availability of solar radiation, expected loads, type of auxiliary energy, economic feasibility, the rate of solar energy required to substitute conventional energy used, and the required reliability are some of the main factors that affect the optimum capacity of thermal storage [9].
Latent heat storage depends on the phase changes of materials from solid to liquid, liquid to gas and vice-versa. Phase changing process is isothermal, indicating that the temperature of the storage material does not change. The phase changes are supposed to take place with controlled super heating and super cooling. This method of storage may operate within small temperature ranges and have high storage capacities with relatively low mass and volume [9].
Summers et al. [10] pointed out that a constant heating temperature and constant heat output are important for HDH cycle performance. The use of phase change materials (PCM) was shown to provide consistent air outlet temperatures through day and night. In the proposed design the PCM was placed just below the absorber plate.
The technology of thermal storage using phase change materials is considered as one of the most useful thermal storage options, due to the constant temperature in storing and releasing heat, high density of heat storage, and ease of control. Heat storage and release in this type of systems is affected by the flowrate at the inlet and outlet of the storage component. During the day time if the temperature of the collector outlet exceeds the maximum allowed temperature, additional heat is directed to the thermal storage unit [11].
Most heat storage systems use oil, water, or air as the heat transfer fluid, whereas iron, ceramic bricks, earth, water, or stones are used as the storage medium. The high heat capacity of water makes it logical to be used as a storage medium for applications that require heating and cooling, although large quantities of water are required due to its lower density. Water storage tanks are highly recommended for 24-hour operation of humidification-dehumidification desalination plants. Storage materials such as rocks or ceramics have the capability of maintaining large temperature differences; however they have a relatively low heat capacity [12].
Phase change material (PCM) may be used to store thermal energy in the form of latent heat. The energy transfer occurs when a material changes from solid to liquid and vice versa. This is called a change in state or “Phase.” The heat energy is stored during transformation of the material from solid to liquid and is discharged when the material undergoes solidification. PCM is classified as organic (e.g. paraffin, formic acid), inorganic (e.g. salt hydrates) and eutectic (e.g. triethylolethane+water+urea).
The importance of thermal storage systems combined with solar thermal desalination systems is mainly due to the variation in heat input through time dependent solar radiation. This patent introduces a unique energy storage system that uses hot and cold storage tanks as two separate storage entities to provide constant heat input for the HDH system. Such an approach is expected to smoothen the fluctuations of energy input through renewable sources, where the diurnal variations of the heat gained through collectors can introduce thermal stresses and irregular water production rates. The design proposed in this study introduces a water-heated HDH system that uses evacuated tube collectors for thermal energy collection, along with thermal storage. The humidifier and dehumidifier units used within the system use packed beds that provide a highest efficiency of about 85% for the evaporation and condensation components. Detailed thermodynamic analyses were conducted to evaluate the performance of the proposed system, with the detailed performance evaluation conducted for Dhahran, Saudi Arabia. Furthermore, the analysis was extended and the performance of the proposed system were evaluated at six different locations in Saudi Arabia.
The proposed design for a closed-air/open-water (CAOW) HDH system integrated with an evacuated tube water heater and a unique thermal storage system is shown in
As illustrated in
The humidifier 214 directs heated seawater from a hot storage tank 213 and humidifier 214 to the dehumidifier/condenser 218. The proposed design for a closed-air/open water (C-A/OW CAOWHDH) system integrated with an evacuated tube water heater and a unique thermal storage system shown in
Thermal storage consists of a hot storage tank and a cold storage tank, where the hot storage tank is considered as an un-stratified water storage unit. When heat is demanded by the HDH, the control system measures the hot storage tank temperature and water from the cold storage is mixed with water from the hot storage at the heat exchanger in order to provide the required temperature to the water line between the humidifier and the dehumidifier.
While the invention has been defined in accordance with its preferred embodiments, it should be recognized that changes and modifications may be made therein without departing from the scope of the appended claims.
