RENEWABLE-POWERED REVERSE OSMOSIS DESALINATION WITH ACTIVE FEEDWATER SALINITY CONTROL FOR MAXIMUM WATER PRODUCTION EFFICIENCY WITH VARIABLE ENERGY INPUT

Abstract

Methods and systems for desalinating feedwater are disclosed. The system includes at least one feedwater source, a reverse osmosis module, an input feedwater stream fed to the reverse osmosis module, and a control module. The feedwater stream comprises water from at least one feedwater source, e.g., from two or more feedwater sources of different salinities. The control module analyzes the level of energy available to the system, and increases the salinity of the input feedwater stream proportional to an increase in available energy. Feedwater stream salinity can be adjusted to reach water demand targets and fully utilize variable power inputs from renewable sources.

Claims

1. A system for desalinating feedwater, including: at least one feedwater source; a reverse osmosis module; an input feedwater stream fed to the reverse osmosis module, wherein the feedwater stream comprises water from at least one feedwater source; and a control module analyzing the level of energy available to the system, wherein the control module increases the salinity of the input feedwater stream proportional to an increase in available energy.

2. The system according to claim 1, further including at least one power source selected from the group consisting of: a renewable energy source and the energy grid.

3. The system according to claim 2, wherein the renewable energy source is at least one of a photovoltaic energy source and a hydroelectric energy source.

4. The system according to claim 1, wherein the control module decreases the salinity of the input feedwater stream proportional to a decrease in available energy.

5. The system according to claim 1, further including an energy storage.

6. The system according to claim 1, wherein said at least one feedwater stream comprises a first feedwater stream of wastewater effluent and a second feedwater stream of seawater.

7. The system according to claim 1, wherein said at least one feedwater stream comprises a first feedwater stream and a second feedwater stream, wherein the second feedwater stream has a salinity higher than a salinity of the first feedwater stream.

Description

DESCRIPTION

[0014] In some embodiments, the present disclosure is directed to a RO desalination system for maximizing potable water production at minimal levelized water cost by actively controlling feedwater salinity and adapting to variable renewable power inputs. In some embodiments, the system includes at least one variable-speed pump. In some embodiments, each feed water supply is in fluid communication with at least one variable-speed pump. In some embodiments, a plurality of feedwater sources are in fluid communication with the system of the present disclosure, as will be discussed in greater detail below. In some embodiments, a RO module is in fluid communication with a plurality of feedwater streams. In some embodiments, each feedwater stream in fluid communication with the RO module represents a separate feedwater source. In some embodiments, at least two of the feedwater sources have different salinity. In some embodiments, the RO module is powered by a renewable energy source. In some embodiments, the system of the present disclosure includes a salinity adjustment module for identifying an optimal salinity for a feedstream to be sent to the RO module for desalination based on the available energy level of the system. In some embodiments, the salinity adjustment module combines feedwater streams from a variety of sources to create the feedstream for desalination at the RO module. In some embodiments, the system comprises a controller for controlling the various modules and flow streams, including operation parameters such conductivity, pressure, temperature, pH, backwashing frequency, chemical dosing rates, and the like. In some embodiments, the system includes an effluent stream of potable water.

[0015] In some embodiments, the RO system has adjustable flow configurations, allowing the system to switch between (by way of example) closed-circuit flow and 2-pass flow.

[0016] In some embodiments, the renewable power is any suitable renewable energy source. In some embodiments, the renewable energy source is photovoltaic. In some embodiments, the renewable energy source is hydroelectric. In some embodiments, the system includes at least one energy storage system.

[0017] In some embodiments, the present disclosure is directed to a method of adjusting feedwater salinity by utilizing two or more feedwater sources. In some embodiments, the adjustment is performed as needed and in real-time. In some embodiments, at least two of the feedwater sources have different salinity concentrations.

[0018] By adjusting the feedwater salinity level to match energy supply at that given time, the system achieves optimal power consumption. For example, when excess power is produced and must be curtailed (this incurs a cost), feedwater salinity would be intentionally raised, thereby utilizing the excess energy while producing water without adversely affecting reverse osmosis membranes (membranes have operational limitations relevant for avoiding significant membrane damage and/or excessive scaling/fouling); meanwhile, when available renewable power is very low, the feedwater salinity would be intentionally reduced to minimal levels to reduce the required energy for water production.

[0019] In some embodiments, the feedwater source is at least one of seawater, brackish water, or treated wastewater effluent. In some embodiments, a first feedstream is selected from seawater and brackish water, and a second feedstream is treated wastewater. Thus, in some embodiments, the present disclosure combines desalination and water reuse into one system. Such a system results in a higher utilization rate and is more reliable during and after extreme climatic periods, such as droughts. Seawater desalination plants alone can sometimes be unnecessary and less cost-effective once a drought passes. One example of this is in Australia, where seawater desalination capacity was rapidly increased because of a severe drought period; during heavy-rain periods, the desalination capacity was unnecessary and could not cost-effectively continue operation as intended. Further, having more than one feedwater source at different salinity concentrations (such as seawater and brackish water) can further enable brine dilution for release back to the sea. Multiple feedwater sources also ensure higher reliability and system utilization. Switching between different feedwater sources also helps reduce membrane fouling, which is known to inhibit overall system efficiency and increase cost.

[0020] Referring to FIG. 1, in some embodiments, a feedwater stream from one or more feedwater sources flow passes through a pretreatment stage to remove potential membrane foulants (see W2). In some embodiments, pretreated water is then stored in feedwater tanks (see W3) for subsequent supply to a pumpset (see W4). In this figure, energy flow is shown with solid lines and liquid flow shown with dashed lines.

[0021] In some embodiments, during periods when it is operating in low-energy mode, feedwater streams having relatively low salinity is fed across the RO membranes. In some embodiments, the low-salinity feedwater stream is treated effluent. However, with the use of renewable energy both in the system and on the larger grid, excess electricity is generated. During periods when an increasing ramp rate in power occurs, the pumpset increases speed to match until reaching an upper limit of power consumption for desalinating low-salinity water.

[0022] In some embodiments, if excess electricity is still available after reaching this upper limit, feedwater streams from higher-salinity feedwater sources are blended into the low-salinity water to increase feedwater salinity. In some embodiments, the higher-salinity feedwater source is seawater. In some embodiments, flow rates and operation pressures are also increased to take advantage of the available excess power. In some embodiments, as long as there is an excess in energy, feedwater salinity would be increased in accordance with increased pump flow and pressure until reaching maximum pump power and feed salinity limits.

[0023] In some embodiments, once a decreasing ramp rate in power occurs, feedwater stream flow rates are adjusted accordingly to decrease salinity (higher-salinity feedwater streams are slowed or stopped) and reduce power consumption.

[0024] Referring again to FIG. 1, in some embodiments, product water from the RO modules (see W5) is stored for distribution (see W6). In some embodiments, product water flows through a post-treatment stage. In some embodiments, brine flow passes through energy recovery devices to recover pressure and transfer it back to the feedstream (see W7). In some embodiments, brine is subsequently retained (W8) and diluted (W9) before disposal. In some embodiments, a reservoir providing hydroelectric power is used as a feedwater source (see W10).

[0025] Referring to FIG. 2, in some embodiments, a renewable energy source (such as photovoltaic plant E1) is used to generate power for the RO system (E2). In some embodiments, the energy source also provides power to at least one pump (see E3) and surplus energy for the grid (see E4). In some embodiments, pumps E3 are pumped-hydro reversible pumps. During the day, the reversible pumps lift water to an high-elevation reservoir, and function as turbines at night or when solar irradiance is insufficient (see E5). When operating as turbines, they generate hydroelectric power for the RO system (see E6) and surplus energy for the grid (see E7). In some embodiments, grid power is stored by pumped-hydro energy storage. In some embodiments, grid power is used to supplement RO operations as needed depending on the water salinity level (see E8). Steps E1-E4 are in order of priority for daytime operations consistent with some embodiments of the present disclosure. On a typical day, PV powers the RO plant, followed by the pumped-hydro pumps, and any excess energy goes to the grid. During late afternoon, pumped-hydro pumps shift to turbine mode, continuing to power the RO plant and/or selling any excess energy to the grid. In parallel, the water flows to the RO plant night and day, but the volume of the wastewater treatment plants relative to seawater would increase or decrease according to the grid's needs and energy prices.

[0026] In some embodiments, the RO system is in a location having at least one of the following attributes: high renewable energy potential, favorable market conditions (policy, regulation, prices), proximity to a coast for seawater access, proximity to thermal power plants and wastewater treatment plants, proximity to brackish water sources, proximity to high-elevation terrain (approximately 200 m or greater) with natural depressions or existing reservoirs for pumped-hydro, away from restricted areas (such as protected areas, private ownership, and the like), and near electrical lines or substations.

[0027] Overall, the proposed innovation is a flexible, renewable-powered, variable-salinity RO plant that provides potable water and options of selling excess energy to the grid, providing grid energy storage, storing excess power generated on site, and enhancing energy consumption controllability through variable power and variable-salinity response. Utilizing a system that can tolerate two salinity-distinct feedwater sources achieves a wider electric load profile for operation. Furthermore, the concept offers the strong potential of retrofitting existing desalination plants and utilizing other existing energy or water infrastructure to reduce energy consumption, decrease capital and operating costs, and invoke flexibility to help dampen current and future stresses on the grid. Lastly, using treated wastewater effluent as a feedwater source provides an additional, consistent low-salinity input and promotes water sustainability through direct potable reuse.

[0028] By way of example, California is an attractive location because of the high solar radiation (average annual global-horizontal-irradiation >5 kWh/m.sup.2/day), proximity to the sea, and abundant source of treated wastewater. Further, California has high-elevation coastal terrain for use with pumped-hydro energy storage. As shown in FIG. 3, California's load profile receives significant solar power penetration during the day, and storage can soften the grid's peak demand after sunset. Proximal thermal power plants with seawater intakes/outfalls can be used to reduce or eliminate RO seawater intake construction costs, and preheated water from once-through cooling systems can be exploited to increase membrane water permeability (i.e., produce more water). Nearby wastewater treatment plants can provide the minimum-salinity feedwater for the system; only a fraction of California's treated effluent is reused during the spring and summer mainly for irrigational purposes while the remaining flow is usually discharged to the ocean.

[0029] A techno-economic, hourly RO model developed by Columbia University, in combination with the HOMER energy model, was used to make initial estimates of the efficacy of the systems and methods of the present disclosure using California as the target. The systems and methods were found to produce 16,000 m.sup.3/day at 350 ppm TDS, with a 95% productivity factor. The system desalinated 5,000 ppm treated effluent at an 80% recovery rate for 75% of the time and 37,000 ppm seawater at a 50% rate for 25% of the time. The electric load varied between 0.4-1.2 MW. The on-site power system comprised 5 MW, one-axis tracking PV and 1.8 MW pumped-hydro energy storage connected to the grid. The estimated levelized cost of water was 37 cents/m.sup.3 (see FIG. 4). Energy accounted for approximately 35% of the overall water cost, but this is offset by services to the grid which reduce energy costs from about 15 cents/m.sup.3 to about 2 cents/m.sup.3.

[0030] The capital cost of 6 cents/m.sup.3 is based on an RO system base cost of $3,000/(m.sup.3/day) and additional financial factors. Total electricity sold to the grid was 8.8 GWh/year, yielding a 13 cent/m.sup.3 reduction allocated to grid services. Labor costs account for 11 cents/m.sup.3. The membrane replacement cost of 7 cents/m.sup.3 is based on an annual membrane replacement rate of 12.5%. Maintenance costs for spare parts are assumed to be 2 cents/m.sup.3. Chemical costs account for pretreatment and post-treatment. Monthly San Diego feedwater temperatures were assumed to undergo a 10 C. increase to simulate feedwater preheated by a thermal power plant. PV and pump-hydro system capital costs were set to $1,600/kW and $1,000/kW, respectively; annual operational and maintenance costs were assumed as 2% and 9% of capital costs, respectively. Industrial, scheduled tariff rates were used for the grid model, and sellback price was set equal to purchase price.

[0031] Non-limiting exemplary applications of some embodiments of the present disclosure include reverse osmosis desalination and water reuse systems for freshwater production; energy and water production systems, such as deployable systems for emergency and disaster responses that impact an area's drinkable water; and retrofitting conventional desalination plants to enable operational flexibility and reduced energy consumption.

[0032] Although the invention has been described and illustrated with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.