Patent Application
20210384865
Embodiments of the subject matter disclosed herein generally relate to a method and system for generating electricity and clean water production using solar energy, and more particularly, to a system that uses a solar panel as a photothermal component for simultaneously generating clean water and electricity.
Water and energy are inextricably linked and the intimate water-energy nexus is being felt globally, as water security is becoming a threat to energy security and vice versa. In the United States and Western Europe, about 50% of the water withdrawals are for energy production. On the other hand, clean water production, especially seawater desalination, consumes a large amount of electricity. For example, in the Arab countries, more than 15% of the total national electricity is consumed by the fresh water production industry. It has been reported that 1-10% of the clean water produced in the electricity-driven seawater desalination process is fed back to the power plant to generate the electricity consumed during the desalination process. The negative ramifications of the water-energy nexus have been felt especially in the arid and semi-arid regions.
The current share of nonrenewable fossil fuels in the global energy mix is still larger than 82% and the burning of fossil fuels leads to massive CO2 emissions, which is regarded as a major threat to the global sustainability. Sustained efforts are being made to develop and implement renewable energy sources, among which the solar energy has shown its potential to meet the world's future energy demands given its abundance and free availability. Large amounts of photovoltaics (PV) panels (>400 GW) have been installed all over the world to generate electricity from solar energy with minimal CO2 emission and water consumption.
In this regard note that for each MWh of generated electricity, the PV technology consumes only 2 gallons of water while the conventional thermal power plants, which use coal or nuclear fuel as the main source of energy, consume 692 and 572 gallons of water, respectively. However, solar irradiation has a quite low energy intensity, generally in the range of 4-8 kW·m−2 per day for the most parts of the world. Moreover, only about 10-20% of the energy from sunlight can be converted to electricity by the state-of-the-art commercial PV panels. As a result of this low efficiency, for a medium-sized solar power plant of 400 MW, it would need to collect sunlight from at least 2 million m2 land area. Besides the cost of the solar panels and land procurement, the mounting system supporting the panels on such a large area adds further capital cost to the solar power plant. Thus, for these reasons, the solar energy generation is still facing cost barriers.
Solar distillation has recently attracted considerable attention and has demonstrated promising potential in various processes aimed at seawater desalination, potable water production from quality-impaired water sources, wastewater volume reduction, metal extraction and recycling, sterilization, etc. However, similar to the solar-to-electricity generation process discussed above, the inherent low-energy intensity of the solar irradiation leads to a small fresh water production rate in conventional solar distillation facilities, for example, 0.5-4.0 kg·m−2 for a whole day, which is equivalent to a water production rate of 0.3-0.7 kg·m−2·h−1 under the standard of one Sun illumination condition (1 kW·m−2).
This low productivity requires a large land area and the installation of a mounting system to support the distillation setup, which constrains its economic vitality and benefit, similar to the case of the PV-based power plants discussed above. Recently, solar-driven multi-stage membrane distillation (MSMD) devices have been reported with a much higher clean water productivity, for example, 3 kg·m−2·h−1 in a 10-stage device under one Sun illumination, by recycling the latent heat released during the vapor condensation in each stage as the heat source for the next stage.
The concept of simultaneous production of clean water and electricity from solar energy has been recently investigated by several groups [1-3]. However, in these attempts, the solar distillation was utilized for clean water production and some side effects of the solar distillation were utilized for electricity generation, which led to low solar-to-electricity energy efficiency (<1.3%). The low-electricity generation efficiency of these strategies makes it uneconomical to apply them in commercial power plants.
Thus, there is a need for a new system that simultaneously produces fresh water and generates electricity with a high-efficiency, based on solar power, so that large-scale applications are economically viable.
According to an embodiment, there is an integrated solar PV panel-membrane distillation system that includes a solar photovoltaic panel having a front face for receiving solar energy and a back face, opposite to the front face, and a membrane distillation device attached directly to the back face of the solar photovoltaic panel. The solar photovoltaic panel is configured to simultaneously generate electrical energy and transfer heat to the back membrane distillation device for generating fresh water from contaminated water.
According to another embodiment, there is a method for simultaneously generating electrical energy and clean water. The method includes generating electrical energy from solar energy with a solar photovoltaic panel having a front face and a back face, which is opposite to the front face, transferring heat from the solar photovoltaic panel to a multi-stage membrane distillation device, which is attached directly to the back face of the solar photovoltaic panel, and generating fresh water from contaminated water with the multi-stage membrane distillation device, based on the heat from the solar photovoltaic panel.
According to still another embodiment, there is an integrated solar PV panel-membrane distillation system that includes a solar photovoltaic panel having a front face for receiving solar energy and a back face, opposite to the front face, a membrane distillation device attached directly to the back face of the solar photovoltaic panel, and an evaporative crystallizer layer attached to the membrane distillation device, the evaporative crystallizer layer being configured to cool down a bottom layer of the membrane distillation device. The solar photovoltaic panel is configured to simultaneously generate electrical energy and transfer heat to the back membrane distillation device for generating fresh water from contaminated water.
For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a system that uses a PV panel and a membrane-based device to simultaneously generate electrical power and fresh water. However, the embodiments to be discussed next are not limited to such a system, but they may be applied to a system that uses another type of fresh water generation device.
Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
According to an embodiment, a system that simultaneously produces fresh water and electricity is an integrated solar PV panel-membrane distillation (PV-MD) system in which the PV panel is employed as both (1) the photovoltaic component for generating electricity and (2) as the photothermal component for clean water production. In a typical solar cell, 80-90% of the absorbed solar energy is undesirably converted to heat, and thereafter passively and wastefully dumped into the ambient air. In this embodiment, different from the existing devices, a MSMD device is directly integrated on the backside of the PV panel to directly utilize its waste heat as a heat source to drive the water distillation. Under one Sun illumination, the water production rate of the novel PV-MD system was measured to be 1.79 kg·m−2·h−1 for a 3-stage MSMD device, which is three times higher than that of the conventional solar stills. At the same time, the PV panel generates electricity with an energy efficiency higher than 11%, which is the same as that recorded for the same PV panel without the back MSMD device. The benefit of the integration of the PV panel and the water distillation device is the highly efficient co-generation of clean water and electricity in a single system at the same time, having the same land footprint, which directly reduces the land area requirement for running such systems and also reducing the cost associated with the mounting system, as compared to two physically separate systems (PV and solar distillation systems). In one application, by using commercial solar cells for such a system makes the PV-MD system more appropriate for practical applications. This integrated system provides a solution for transforming a solar-based electricity generation plant, from otherwise a water consumer, to a fresh water producer.
One or more of the benefits of this new integrated PV-MD system are: (1) the electricity generation efficiency is much higher than those reported clean water-electricity cogeneration devices in literature; (2) the clean water production rate is much higher than those systems reported in literature; (3) in some cases, because the heat generated from the solar panel is used for water distillation, the temperature of the solar panel is reduced and the energy efficiency of the solar panel is accordingly increased; (4) because both the electricity and clean water can be efficiently generated with the same device and with the same mounting system, this novel system reduces the cost of land as well as the cost of the mounting system for such a system; and (5) because the water distillation system is physically and thermally sealed, the system can operate even in constant windy conditions. This novel system is now discussed in more detail with regard to the figures.
According to an embodiment illustrated in
To increase the efficiency of the system 100, in one application, a transparent cover 112 may be attached to the top of the solar PV panel 110 so that a chamber 114 is formed between the solar PV panel 110 and the transparent cover 112. In one application, vacuum is generated in the chamber 114 to eliminate the conduction heat loss. The transparent cover 112 may be made of a material that possesses a high-transmittance and low-thermal conductivity, such as glass, polyethylene terephthalate, Polycarbonate, polymethyl methacrylate and/or polyvinyl chloride.
Another optional feature of the system 100, for reducing the heat loss of the system to the environment, is a thermal insulator 116 that can be placed around the whole or only a part of the system 100. The thermal insulator 116 may include a low-thermal conductivity material such as glass fibers, silica gel, asbestos, inorganic porous insulation materials, polyurethane foam and/or polystyrene foam.
The system further includes an input 132 and an output 134 for each single-stage membrane distillation device 130. The input 132 provides contaminated water (for example, salt water) to a corresponding single-stage membrane distillation device 130, while the output 134 supplies the clean water to the outside of the system. The contaminated water can include not only seawater, by also lake water, river water, groundwater, industrial wastewater, brine, brackish water, etc. These source waters can be of impaired quality and can be contaminated with heavy metals, organics, radioactive materials, pesticides, or any other chemicals with health and environmental concerns.
The input 132 is fluidly connected to a water evaporation layer 140 for supplying the contaminated water for evaporation. The water evaporation layer 140 is, in this embodiment, located in direct contact with a heat conduction layer 142. The heat conduction layer 142 is in direct contact with the solar PV panel 110 and is configured to thermally transfer the heat from the back of the solar PV panel 110 to the water evaporation layer 140.
The solar PV panel 110 can be any kind of commercial or laboratory solar cell (such as amorphous Silicon solar cell, polycrystalline silica solar cell, monocrystalline solar cell, cadmium telluride solar cell, copper indium gallium selenide solar cells, dye-sensitized solar cell, gallium arsenide germanium solar cell, thin film solar cell, etc.). When a transparent solar cell is used, a black material can be placed under the solar cell to enhance the absorption of the sunlight. The heat conduction layer 142 could be made of a material that possess good thermal conductivity, such as, for example, copper (401 W/mK), zinc (116 W/mK), aluminum (237 W/mK), brass (109 W/mK), bronze (110 W/mK), graphite (168 W/mK), Ag (429 W/mK), silicon carbide (360-490 W/mK), iron (73 W/mK), stainless Steel (12-45 W/mK), or tin (62-68 W/mK). Other materials may be used. The water evaporation layer 140 may include a hydrophilic material that should possess hydrophilicity and a porous structures. An example of such a material may be non-woven fabrics, silica fibers, glass fibers, etc.
A hydrophobic layer 144 is placed below the water evaporation layer 140, as shown in the figure. The hydrophobic layer 144 includes a material that is hydrophobic and porous so that liquid water 150 from the water evaporation layer 140 cannot pass it. However, the hydrophobic layer 144 is configured to allow water vapor 154 to pass through. In one application, to increase the temperature gradient, the hydrophobic layer should also possess a low-thermal conductivity, or it can be composed of two or more kinds of materials, some of which possess low-thermal conductivity, such as polystyrene membrane, polyvinylidene fluoride, poly tetra fluoroethylene, etc.
A condensation layer 146 is placed below the hydrophobic layer 144 and is configured to condensate the water vapor 154 that passes through the hydrophobic layer 144, to form the fresh water 156. The condensation layer 146 may include a material that possess hydrophilicity and a porous structures, such as non-woven fabrics, silica fiber, glass fiber, etc.
The next single-stage membrane distillation device 130 has a similar structure, and for simplicity, only the heat conduction layer 142 of this second device 130 is shown in
Note that the configuration shown in
The system 100 works as now discussed. The source or contaminated water 150 can be adsorbed into the water evaporation layer 140, by capillary and transpiration effect, from a water source 152, or driven by the gravity or a pump. The water source 152 can be a container or a natural water reservoir. The heat from the solar PV panel 110 is transferred through the heat conduction layer 142 to the water evaporation layer 140. The contaminated water 150 from the water evaporation layer 140 evaporates due to the heat, leaving behind any solid contaminant that is present in the contaminated water. The condensation layer 146, which is a hydrophilic porous membrane, is insulated from the water evaporation layer 140 by the hydrophobic layer 144, which ensures that the high-salt or contaminant water 150 does not enter the condensation layer 146.
The water vapor 154 formed in the water evaporation layer 140 is forced to flow downward and gets condensed as condensed clean water 156 in the condensation layer 146. The condensed water 156 is then transported through the output 134 to a collector 158, by the gravity. The latent heat of the vapor 154, which is released during the condensation process, is used by the next single-stage membrane distillation device 130 as the heat source. The entire process is then repeated for each single-stage membrane distillation device 130.
As discussed above, the entire system 100 may be sealed by the thermal insulation material 116 to prevent vapor and heat loss. A larger temperature gradient between the water evaporation layer and the corresponding condensation layer will lead to a higher clean water production rate.
The system 100 discussed with regard to
Another embodiment of an integrated solar PV panel-membrane distillation system 300 is shown in
Another embodiment of an integrated solar PV panel-membrane distillation system 400 is shown in
Another embodiment of an integrated solar PV panel-membrane distillation system 500 is shown in
To be able to remove the crystalized salt, in this embodiment, each water evaporation layer 540 is fluidly connected through a corresponding connecting pipe 560 to a next water evaporation layer (i.e., the water evaporation layers are connected in series) and the last water evaporation layer 540 has an output 134, as shown in
Further, the system 500 may have the condensation layer 546 formed with no hydrophilic membrane, but rather with an air gap, similar to the system 400. However, in one application, a hydrophilic membrane may be placed inside the condensation layer 546. In this specific embodiment, the condensation layers 546 are also fluidly connected to each other (in parallel) with corresponding pipes 562 so that the clean water 156 formed in each of them collectively arrives at the container 158.
While this embodiment shows the water source container 152 placed higher than the top most single-stage membrane distillation device 130, one skilled in the art would understand that instead of placing the water source container at that position, it can be placed lower than the most single-stage membrane distillation device 130, and another means for supplying the contaminated water 150 to the system may be used, for example, a pump.
An overall view of a system 600, which is similar to system 500, is shown in
A variation of the system 600 is shown in
The evaporative crystallizer layer 614 can be used as a cooler to lower the temperature of the bottom condensation layer 146. Water can be wicked by capillary effect or transported by pump to the evaporative crystallizer layer 614 and then evaporated to consume the heat recycled from the bottom condensation layer 146. The salt from the waste water 603 can be crystallized and then collected on the outside of the layer 614. For example, the crystalized salt 615, formed on the outside of the layer 614, can fall off the layer 614 driven by its own gravity, or the salt 615 can be taken together with layer 614 from system 600 and a new layer 614 is added. The water 603 used in this case can be the concentrated water produced from the system 600, as shown in
The evaporative crystallizer layer 614 may be made, in one embodiment, from a porous hydrophilic material, which may be nylon 6, nylon 66, cellulose products, Polyvinyl Alcohol, non-woven fabrics, silica fibers, glass fibers, polyvinyl acetate, etc.
In one embodiment, each stage of the membrane distillation device 120 includes four separate layers: a heat conduction layer 142, a hydrophilic porous layer as a water evaporation layer 140, a hydrophobic porous layer as a membrane distillation 144 for vapor permeation, and a water vapor condensation layer 146. In one implementation, an aluminum nitride (AlN) plate was used as the heat conduction layer 142 because of its extremely high thermal conductivity (>160 109 W·m−1·K−1) and its anti-corrosion property in salty water. The hydrophobic porous layer 144 was made in one embodiment of an electrospun porous polystyrene (PS) membrane. The water evaporation layer 140 and the water condensation layer 146 were made of the same material, a commercial hydrophilic quartz glass fibrous (QGF) membrane with non-woven fabric structure.
In each stage of the membrane distillation device 120, the heat is conducted through the thermal conduction layer 142 to the underlying hydrophilic porous layer 140. The source water 150 inside the hydrophilic porous layer 140 is then heated up to produce the water vapor 154. The water vapor 154 passes through the hydrophobic porous membrane layer 144 and ultimately condenses on the condensation layer 146 to produce the liquid clean water 156. The driving force for the water evaporation and vapor condensation is the vapor pressure difference caused by the temperature gradient between the evaporation and condensation layers.
In each stage 130, the latent heat of water vapor, which is released during the condensation process, is utilized as the heat source to drive water evaporation in the next stage 130. The multistage design 120 ensures the heat can be repeatedly re-used to drive multiple water evaporation-condensation cycles. In a traditional solar still, the heat generated from the sunlight via photothermal effect only drives one water evaporation-condensation cycle, which sets up an upper theoretical ceiling of the clean water production rate to about 1.60 kg·m2·h−1, under one-Sun condition. The multistage design 120 makes possible to break this theoretical limit.
In the embodiments discussed herein, two contaminated water flow modes were presented, namely, the dead-end mode (
In the cross-flow mode shown in
The water production performance of a 3-stage PV-MD system (i.e., a system that includes 3 single-stage membrane distillation devices 130) was investigated by connecting the solar PV panel to an external circuit with different resistances. When the system was working under one-Sun illumination with pure water as the source water, the temperature of the solar PV panel 110, which is slightly affected by the external resistance, was measured to be approximately 58° C. Because the performance of the solar PV panel is affected by its working state temperature, the J-V curve (which plots the generated current density versus voltage) of the panel 110 at the working state (58° C.) was measured under one-Sun illumination condition with simultaneous clean water and electricity production operation, as illustrated in
When the panel 110 was connected to a resistance equal to its optimal load of 1.3Ω, the same PV-MD system exhibited a water production rate of 1.79 kg·m−2·h−1 (see
The clean water production performance of the 3-stage dead-end PV-MD system under solar illumination with different light intensity was also investigated and the results are presented in
One targeted application of the PV-MD system is to generate electricity and at the same time produce clean water from various source waters with impaired quality, such as seawater, brackish water, contaminated surface water, and/or groundwater. When 3.5% NaCl aqueous solution was used as a seawater surrogate, the clean water production rate was 1.77 kg·m−2·h−1 for an open circuit state and 1.71 kg·m−2·h−1 for the optimal load state (1.3Ω). These two values are both lower than those recorded when pure water was used as the source water, which is attributed to the decrease of the saturation vapor pressure of the salt water. When the system 100 is operated in the dead-end mode, the salt concentration of the source water in the evaporation layer would gradually increase during operation, leading to a slight decrease in the clean water production rate. The concentrated source water inside the system 100 can be sucked out of by a dry paper via capillary effect. Although not all the NaCl salt was removed in this way, the performance of the system could be nearly fully recovered in the next operation cycle. In this regard,
In another experiment, the PV-MD system 100 in the dead-end mode was used to produce clean water from a heavy metal-contaminated seawater. The PV-MD system exhibited a clean water production rate of 1.69 kg·m−2·h−1 under one-Sun illumination. The concentrations of the ions in the contaminated water source and clean water product were measured and are shown in
In a PV-MD system operated in the dead-end mode, the salts from the source water will continuously accumulate inside the evaporation layer during operation, as mentioned above, which may cause failure and damage if salt crystals block the pores of the MD membrane. Although the salt can be cleaned out of the system by frequent regeneration operations, as previously discussed, it is not practical for long-term operation and large-scale application.
Therefore, the PV-MD system 500 or 600, which can be operated in a cross-flow mode, would solve the salt accumulation problem. For the system 600, a contaminated water flow layer 610 was added at the bottom of the membrane distillation device 120 to recycle the heat for the purpose of pre-heating the source water before entering into the first evaporation layer 140. When the water outlet 134 of the 3-stage cross-flow type PV-MD system 600 was blocked, i.e., it was operated in a dead-end mode with no water flowing out of the system, the clean water production rate was 2.09 kg·m−2·h−1 with pure water as the source water, which is 7% higher than that recorded pure water on the dead-end type device under the otherwise same conditions (1.96 kg·m−2·h−1). This result suggests that adding a contaminated water flow layer at the bottom of the system to recycle the heat can improve the clean water productivity.
When the water outlet 134 of the 3-stage cross-flow type PV-MD device was opened and the flow rate of the contaminated water was controlled to be 5 g·h−1, which is about two times the water production rate in the dead-end mode, the clean water production rate was slightly decreased to 1.93 kg·m−2·h−1. This observation can be explained by the fact that some sensible heat was carried away by the outgoing water flow at the outlet 134. When the flow rate of the contaminated water was increased to 6 and 7 g·h−1, the clean water production rates were further decreased to 1.83 and 1.76 kg·m−2·h−1, respectively. These results indicate that the clean water production rate was only slightly affected by the flow rate of the contaminated water because the outgoing water contains only a small amount of sensible heat.
The seawater desalination performance of the 3-stage PV-MD system 600 with the cross-flow mode was then evaluated. The flow rate of the contaminated water was controlled at 5 g·h−1 to avoid continuous salt accumulation inside the system and the system exhibited a very stable clean water production rate of 1.65 kg·m−2·h−1 under one-Sun illumination in a 3-day continuous test. In this case, a continuous concentrated contaminated water stream steadily flowed out of the system, keeping the salt concentration at a steady state inside the system. The salt concentration of the contaminated and concentrated seawater was 3.8 wt % and 8.7 wt %, respectively. Although the clean water production rate was slightly lower when the device was operated under these conditions, comparing to the dead-end mode, its long-term clean water production stability outweighs its slightly reduced rate.
A method for simultaneously generating electrical energy and clean water with an integrated solar PV panel and a membrane distillation device is now discussed with regard to
In one application, the multi-stage membrane distillation device includes plural single-stage membrane distillation devices connected to each other, and each single-stage membrane distillation device includes a heat conduction layer, a water evaporation layer, a hydrophobic layer, and a condensation layer. The method may further include a step of heating the contaminated water with heat from the heat conduction layer, a step of evaporating the contaminated water in the water evaporation layer with the heat from the heat conduction layer, a step of allowing water vapor to pass the hydrophobic layer to the condensation layer, but not the contaminated water, and a step of condensing the water vapor into the clean water in the condensation layer.
In one application, each single-stage membrane distillation device has an input and no output fluidly connected to the water evaporation layer so that the contaminated water cannot pass through the water evaporation layer. In another application, each single-stage membrane distillation device has an input and an output fluidly connected to the water evaporation layer so that the contaminated water passes through the water evaporation layer.
The disclosed embodiments provide an integrated solar PV panel and a membrane distillation device that simultaneously generate electrical energy and uses the generated heat to clean contaminated water for producing clean water. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.
This application claims priority to U.S. Provisional Patent Application No. 62/767,647, filed on Nov. 15, 2018, entitled “DEVICE FOR ELECTRICITY GENERATION AND WATER DESALINATION BY SOLAR LIGHT,” and U.S. Provisional Patent Application No. 62/883,878, filed on Aug. 7, 2019, entitled “METHOD AND SOLAR-BASED SYSTEM FOR SIMULTANEOUS ELECTRICITY AND FRESH WATER GENERATION,” the disclosures of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2019/058562 | 10/8/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62883878 | Aug 2019 | US | |
| 62767647 | Nov 2018 | US |
