INNOVATIVE WATER/ENERGY NEXUS: OPTIMIZING RENEWABLES BY COMBINING SEAWATER PUMPED STORAGE, HYDROPOWER, AND DESALINATION

Transcription

1 INNOVATIVE WATER/ENERGY NEXUS: OPTIMIZING RENEWABLES BY COMBINING SEAWATER PUMPED STORAGE, HYDROPOWER, AND DESALINATION Mark Allen, Oceanus Power & Water, LLC, 900 High St, Palo Alto, CA Ph: Sandra Walker, Oceanus Power & Water, LLC, Palo Alto, CA Neal Aronson, Oceanus Power & Water, LLC, Palo Alto, CA YuJung Chang, AECOM, Inc., Seattle, WA David Prasifka, AECOM, Inc., Los Angeles, CA Abstract: As the global water community is experiencing an intensified deficit in fresh water resources, developing seawater desalination infrastructure has become a necessity for many regions. Over the decades, technology advancements have made seawater desalination a reality, however high energy demand remains an issue, both in terms of operating cost and significant CO2 emissions. Finding an effective way to integrate renewable energy into energy-intensive water treatment processes has become a critical challenge for the development of new water supplies. Oceanus Power & Water (OPW) has partnered with AECOM to develop an innovative concept integrating a seawater reverse osmosis (SWRO) desalination plant with a seawater pumped storage hydropower (SPSH) facility. The system operates by pumping seawater up to a high elevation storage reservoir during periods of low power demand or excess supply. This stored water is later released for hydroelectricity generation during peak hours. OPW s system uses the potential energy of the stored water to continuously produce freshwater from the SWRO system, without the need for additional high pressure booster pumps. The system may also provide low-cost power for other energy-intense water operations, such as finished water conveyance or wastewater reuse. OPW s innovative system reduces the energy cost for advanced treatment processes, while also storing renewable energy. Substantial savings in construction and O&M costs can be realized from the facility co-location approach. This paper will present results from the feasibility design of the seawater intake and outfall structure and the core desalination process, and describes an innovative No-Pump desalination approach

2 I. Introduction 1.1 Project Overview Commercial scale seawater desalination capacity has increased significantly over the past decade, catalyzed by advances in seawater reverse osmosis (SWRO) technology and steady reductions in equipment cost. Today seawater desalination is recognized as a core strategic solution for ensuring water security, particularly in regions where water supplies are vulnerable to urbanization, population growth, and climate-related changes in rainfall patterns. The economics of seawater desalination have also benefitted from extensive research and development into energy efficient processes, with energy intensities now typically falling in the KWhr/m 3 range. Meanwhile global energy prices have stabilized and in some cases dropped over the past decade, with the unsubsidized levelized cost of energy (LCOE) from wind and solar energy now undercutting traditional thermal sources in the U.S. A fundamental limitation of wind and solar energy, however, is that they are not fully dispatchable, making standalone renewable generation unsuitable for powering conventional desalination processes which require a stable, uninterrupted power supply. This implies that if renewable energy sources are to be used to power seawater desalination they would either have to be combined with an energy storage system, or integrated into an extensive electricity grid which meets regional energy demand while achieving aggregate renewable portfolio targets. Oceanus Power and Water, LLC (OPW) is a clean energy and water development business specializing in providing energy storage and resilient sources of fresh water to drought-prone regions. OPW s mission is to develop saltwater facilities for energy storage, while securing emissions-free freshwater supplies. The company s focus is on leveraging proven technology, while employing new concepts in the design and configuration of their facilities. OPW has worked with AECOM, a leading engineering firm, to develop an integrated seawater pumped storage hydro (SPSH) and SWRO process which offers significant economic and environmental advantages that cannot be achieved by standalone SPSH or SWRO facilities. 1.2 Desalination Energy Challenges A typical modern seawater desalination plant requires over 11 MW of energy supply per m 3 /sec of fresh water processing capacity, representing a significant base load on power generation infrastructure. Today s U.S. electricity generation portfolio comprises a combination of thermal energy sources such as natural gas and coal, and emissions-free sources such as nuclear, hydro, wind and solar. With aggregate emissions in the U.S. currently averaging 0.52 Kg of CO2 per KWhr, a 50 MGD (189,000 m 3 /day) desalination facility operating at an efficiency of 3.0 KWhr/m 3 would be responsible for 107,000 tonnes of annual CO2 emissions, equivalent to adding over 900,000 automobiles to U.S. roads. Emissions of this magnitude are typically cited by environmentalists as a major objection to seawater desalination projects, prompting some project developers to mitigate emissions by contracting clean energy from specialized power providers. A desalination solution entirely powered by renewable energy sources would therefore go a long way towards reducing environmental opposition to new projects. In some regions a further energy-related challenge is the capacity of existing transmission and distribution infrastructure to deliver the incremental power required for desalination. For example, in high density urban areas such as Los Angeles or San Diego, existing transmission - 2 -

3 and distribution networks currently approach their capacity limits during peak demand periods, typically in the early morning and evening. The construction of large new desalination facilities near such locations would increase the peak load threshold, potentially requiring high cost combustion turbines or utility scale battery installations to service the peak hours of energy demand if transmission and distribution assets cannot be expanded. By contrast, if a desalination facility had the capability to store energy during off-peak periods and time-shift the stored energy to drive peak-time operations it could potentially avoid costly grid infrastructure upgrades. A final concern is the cost of baseload energy supply to the desalination facility. Typical commercial electricity rates in California currently average $0.10 per KWhr. By contrast, daytime energy can be procured from the California Independent System Operator market for an average of $0.05, and at times significantly less. Established energy storage systems such as pumped hydroelectric facilities take advantage of within-day price swings to exploit short-term differentials between off-peak and peak energy prices by time-shifting energy. If a desalination process could be integrated with energy storage, then the energy costs should be significantly lower than if the energy were continuously purchased from the grid. 1.3 Desalination Brine Disposal Challenges Reverse Osmosis Seawater Desalination facilities must dispose of the high salinity waste brine, which is usually pumped back out to sea. Due to the brine s relatively high density, effluent streams form broad plumes along the sea floor, which can be harmful to marine species. In California, the State Water Resource Control Board (SWRCB) issued an updated Ocean Plan in 2015, stipulating tight restrictions on desalination effluent salinity, requiring that samples taken as close as 100m from the point of discharge must be within 6% of baseline open sea salinity. California developers preferred solution to ensure that effluent salinity complies with SWRCB s new regulations has been to locate desalination facilities alongside once through cooling (OTC) channels. These are high flow rate seawater conduits, constructed at many legacy coastal power stations for the purpose of conveying seawater through the plant for thermal heat dissipation. The 50 MGD Carlsbad desalination plant is one such example, achieving an aggregate discharge salinity within 10% of ambient salinity at the point of discharge by diluting waste brine with the OTC seawater flux averaging over 500 MGD. California s future desalination opportunities will be limited by the SWRCB s 2010 OTC policy however, which mandates a state-wide phase out of all OTC operations by Instead, SWRCB s guidance for desalination developers is to deploy subsurface seawater intake galleries to minimize entrainment and impingement risks, despite such designs incurring significantly higher construction and operating costs. Furthermore, brine discharge salinity limits will require proactive diffusion measures to achieve the 6% salinity threshold, adding further cost and energy penalty. In summary, the prospects for seawater desalination in California are challenged by both strong environmental opposition and stringent regulatory guidelines. OPW s integrated SPSH and desalination design attempts to tackle these barriers to entry by reducing CO2 emissions, achieving low salinity effluent discharge without reliance upon legacy OTC infrastructure, and lowering the unit cost of produced water

4 II. OPW Desalination Solution 2.1 Design Principle Oceanus Power & Water, LLC (OPW) and AECOM have collaborated on the development of an integrated seawater pumped hydro energy storage and desalination facility (Figure 1), which brings economic and environmental benefits through the integration of two similar but independent processes. Figure 1. OPW Integrated SPSH and SWRO system layout OPW s system is designed around a seawater-service reversible, variable speed hydroelectric pump turbine which operates in two duty cycles Pump and Generate. During the pump cycle the hydroelectric motor takes electrical power from the grid to pump seawater via a large diameter conveyance pipeline, or penstock, up to a high elevation reservoir where it is stored. This mode is selected during low energy price periods when grid demand is low, for example at night or during daytime when surplus renewable generation sources may have to be curtailed. The system generate cycle is selected during periods of mid-peak or peak energy demand. During the generate cycle seawater is drained from the storage reservoir via the turbinegenerator, from where it is discharged via the tailrace tunnel and intake/outflow structure back to the ocean. The high value energy generated during this process is returned to the grid. During both pump and generate cycles, the motor-generator provides the grid operator with high value ancillary services to assist with frequency and voltage stabilization. The pumped hydro system is also able to provide fast-ramping standby generation capacity, a critical service for grid operators in case of unanticipated generation interruptions. The system is designed to switch between Pump and Generate mode, and vice versa, within minutes, while offering a round trip energy storage efficiency of up to 80% (energy produced / energy invested). The embedded potential energy in the seawater stored at 300m above sea level is 0.93 KWhr/m 3, representing significant potential to supply an RO desalination process. Modeling studies performed by AECOM demonstrate that the hydraulic head from a 300m elevation reservoir is sufficient to achieve an RO fresh water yield in excess of 40%, assuming that an efficient energy recovery device is installed to recover energy from the high pressure SWRO concentrate discharge

5 The desalination facility is located at the base of the seawater penstock, immediately above the pump-turbine powerhouse. Assuming that the upper reservoir is always kept at least partially full of seawater, this reservoir will provide a reliable source of high pressure feed water. A seawater feed line is therefore flanged to the base of the pumped hydro penstock, from where it is routed to the desalination facility, supplying high pressure seawater at over 430 psi (30 Bar). The arriving seawater undergoes a high pressure filtration pre-treatment process, before being pressure boosted by an energy recovery device to approximately 800 psi (55 Bar). The filtered, high pressure sea water proceeds through a conventional two-stage SWRO process, yielding approximately 40% permeate by volume, with the waste brine simultaneously directed to power the energy recovery device. Once the waste brine exits the energy recovery device it is held in a brine storage tank located adjacent to the desalination facility. The contents of the brine storage tank are periodically transferred to the seawater pumped hydro tailrace during SPSH generation cycles, allowing the saline concentrate to blend with the seawater flow exiting the turbine-generator. Due to the significantly higher flow rate of seawater from the pumped hydro operation (up to 100 m 3 /second) versus the brine discharge rate (up to 10 m 3 /second), an aggregate effluent salinity within 10% of ambient ocean conditions can be achieved. 2.2 Integration Benefits The integration of seawater pumped hydro and desalination offers many significant benefits to the project, which are discussed in further detail below: Lower energy costs The seawater pumped hydro system is designed to acquire and store energy during the lowest price periods, translating this energy into hydraulic head. Some of this stored hydraulic energy is continuously transferred in the form of a high pressure water supply to the desalination process, at a significantly lower cost than electrical energy purchased from the grid. Since energy purchase costs typically represent up to 40% of desalination Opex, OPW s energy storage design provides the potential to materially reduce the desalination unit costs. Environmental Benefits OPW s system is able to deliver desalinated water with lower CO2 emissions than conventional RO processes which are dependent upon continuous grid generation. Due to the dynamically responsive nature of the hydroelectric pump-turbine, which can switch between Pump and Generate modes within minutes, and many times per hour, the system is ideally suited to operating with intermittent power from renewable generation sources, which may otherwise have to be curtailed or stored at comparatively higher cost. OPW s approach to brine effluent dilution, as outlined in Section 2.1, represents a significant system benefit over raw offshore disposal or reliance upon Once Through Cooling to achieve sufficient dilution. Lower Capital Costs The opportunity to shared infrastructure between SPSH and SWRO facilities results in significant capital cost reductions, including: - 5 -

6 Shared seawater intake & discharge structure Shared substation and grid connection Shared common utilities, access and security infrastructure Shared electrical, mechanical and control systems Shared engineering, procurement, construction, project management and permitting costs Reduced Operating Costs The opportunity to shared infrastructure between SPSH and SWRO facilities also results in significant operating cost reductions, including: Minimized energy costs for desalination operations Shared staffing and management resources Optimized maintenance planning III. Project Delivery Having completed a technology feasibility evaluation study, OPW and AECOM are now embarking on the detailed design of a commercial scale demonstration facility, for which a number of sites have been identified with the appropriate combination of market requirements and suitable topography for the world s first integrated SPSH and SWRO system. OPW and AECOM share a long-term vision of constructing multiple SPSH and SWRO systems on suitable sites requiring resilient water supplies, and where energy markets seek to expand their renewable generation portfolio. The objectives of the joint study were to determine whether the integration of SPSH and SWRO would: (1) demonstrate that a combined project could economically provide electricity arbitrage and ancillary services to satisfy a representative Power Purchase Agreement (PPA); and (2) to demonstrate that the project could provide desalinated water at an acceptable cost to fulfil a competitive Water Purchase Agreement (WPA). OPW s integration of energy and water systems offers the opportunity for low energy transfer costs, which may serve as a catalyst for extending the infrastructure footprint beyond seawater desalination. For example, wastewater treatment plants employing energy intensive treatments, such as reverse osmosis (RO) and advanced oxidation processes (AOP) could also benefit from access to cheap energy, particularly in regions developing water reuse programs. 3.1 Water Energy Nexus Climate change is driving the need to secure and deliver more emissions-free renewable energy and resilient water supplies. The integrated SPSH/DS approach is designed to accommodate variable generation resources, such as wind or solar, at the utility scale, providing a strategic level of high reliability dispatchable energy storage at the lowest unit cost and with the longest life. Concurrent with the resurgence of seawater desalination, OPW recognizes that SPSH offers the best economic and technical solution to integrate intermittent renewables. Pumped storage hydro (PSH) is a proven technology which currently provides 97% of the world s energy storage. The construction of significant additional energy storage capacity is vital to achieving the renewable - 6 -

7 portfolio standards (RPS) declared by many nations and states around the world under the United Nations Framework Convention on Climate Change. IV. General Assumptions and Basis of Design The working assumption for the conceptual study is that there is potential for significant savings in the design, construction, operation, and environmental benefits from the integration of SPSH and DS based on the major project elements listed below: 4.1 Seawater Pumped Storage Hydro: 300 MW Reservoir and dams Water Conveyance systems Power Station and Associated Equipment 4.2 Seawater RO Facility and Associated Equipment: 50 MGD Pre-treatment and filtration system RO treatment and energy recovery systems 4.3 Common to Both Plants Finished water conveyance system Intake/Outfall Common Structure Switchyard/Interconnection Access road, buildings, and utilities V. Seawater Intake/Outlet System 5.1 General Description The seawater intake/outlet system controls the seawater that enters and exits the SPSH system and subsequently the desalination system. Each of these systems has different demands for the intake and outlet of seawater. The SPSH system requires much higher flow rates (approximately two orders of magnitude) than the desalination system, and therefore sets the size of the intake/outlet components. The seawater intake/outlet system consists of three primary components: 1. Intake Screens To prevent sea life, trash and other debris from entering system. 2. Outlet Valves To discharge water from the SPSH, which may be at a higher rate than the intake, therefore supplemental exit points are required. 3. Connection Pipeline Connecting the intake screens and outlet valves to the tailrace of the SPSH pump turbine Various Intake/Outlet configurations were considered, including both shore-side and deep water locations. It is assumed the intake system could be built in two types of geological formations: rock or sandy sea floor. Special consideration was given to the seismic performance of various types of intake locations/configurations. Four different variants of offshore intake structures screens were also developed and evaluated

8 VI. Seawater Pumped Storage Hydro Plant A typical pumped storage hydro project consists of an upper and lower reservoir, connected by a water conduit. During off peak periods, water is pumped from the lower reservoir to the upper reservoir. During peak energy demand periods, water flows from the upper reservoir to a powerhouse to generate hydro-electric energy. In a seawater pumped storage hydro project, the ocean replaces the lower reservoir. The evaluation of the seawater pumped storage hydro plant was conducted based on generating capacities of > 100 MW. Each generation capacity was combined with both short tailrace and long tailrace options to reflect variations in coastal topography, resulting in four alternative design concepts. In addition, a design concept eliminating major excavation and locating the powerhouse and pipelines above ground was also evaluated. The seawater pumped storage alternatives are based on the desalination plant located in the vicinity of the upper reservoir. An evaluation of other locations for the desalination plant was included in the original conceptual study. VII. Seawater Desalination Facility Desalination technology has progressed significantly in the past two decades, catalyzed by the advent of affordable and efficient RO membranes. The viability of large-scale reverse osmosis deployment has been demonstrated in a number markets, particularly Israel and Australia which have invested heavily in the technology to mitigate risk of supply shortfall from conventional water sources. Reverse osmosis (RO) technology employs semi-permeable membranes to separate salts from water under high pressure conditions. RO remains energy intensive, however, which typically results in higher costs for desalination water than for conventional supplies. For this reason, desalination plants have predominantly been deployed to date in drought-prone regions where conventional water supplies are insufficient to satisfy demand from the local population. Modern RO plants consume 3-6 KWhr per m 3 of water produced, while yielding high salinity waste brine which must be disposed of. Challenges faced by today s desalination projects include minimizing energy consumption, for example by using renewable energy resources, and dealing with waste brine in an environmentally responsible manner. In this project an SWRO plant is proposed and designed as an integral component of the overall project s seawater hydraulic system, interfacing with the seawater pumped hydro storage architecture described in Section IV. The design basis assumes a 50 MGD desalination facility, which represents a small fraction of the seawater throughput of the 300 MW SPSH facilities. The desalination facility will be designed to operate continuously, unlike the SPSH plant which operates on a cyclical basis to address fluctuating power markets. For this reason, a significant amount of time was spent developing concepts to optimally integrate the desalination plant within the SPSH system, ensuring continuous provision of feed water and power to the facility. A further important design consideration was the method for disposing of the waste brine via the - 8 -

9 seawater outlet structure, achieved by mixing with the far larger seawater volumes from the SPSH system during periods of power generation. 7.1 SWRO Facility Location During the design of the integrated facility selecting suitable locations for the desalination process was one of the most critical tasks. Three different potential locations were considered, including near the seawater upper reservoir; close to the powerhouse above ground, and close to the powerhouse below ground. Factors considered for the SWRO location include the ease of pretreatment configurations; maximizing the embedded value of available hydrostatic head of the seawater, construction cost, ease of continuously disposing RO reject brine without interrupting the RO process, and the conveyance of the finished desalinated water. Each option presents its pro s and con s, which were methodically reviewed and analyzed. The ultimate selection of the SWRO site was to locate it adjacent to the power house above ground, due to the ease of access to raw seawater, ease of brine disposal, overall project cost and system reliability. 7.2 Major SWROF Components Major SWROF components include: Seawater feed transfer pipeline from the source (SPSH penstock) Pre-treatment system (including filtration, buildings and equipment) Seawater desalination system (with chemical system, cartridge filters, RO skids, energy recovery, and control systems) Post-treatment system (including disinfection and corrosion control) Clearwell (to provide adequate virus inactivation disinfection contact time (CT) and storage requirements for onsite operations) A dedicated brine disposal line, either returning to the upper reservoir or flanged onto the tailrace adjacent to the powerhouse Raw Seawater Transfer Line This line is capable of transferring up to 110 MGD of raw seawater from the reservoir or penstock to the SWROF assuming, 50 MGD finished potable water 45% RO recovery Pre-chlorination Chlorine is added at the SWROF intake line to prevent biological growth throughout the entire pretreatment system. This is a typical SWRO practice to avoid the growth of algae and attachment of hard-shelled marine animals (e.g., barnacles and mussels) on the intake, pipeline, pumps, and other equipment. The pre-chlorination system will comprise chlorine storage, feed pumps, and a dose control/monitoring system. A de-chlorination system with sodium bisulfite dosing system prior to RO will also be included. Pretreatment Pretreatment is required to remove particulates through either conventional multimedia filters or MF/UF membranes. Antiscalant and cartridge filters will also be required

10 SWRO System The SWRO skids will include the following components: RO high pressure feed pumps Regular 8 RO elements Energy Recovery Devices Chemical Clean in Place (CIP) o High ph cleaning o Low ph cleaning o Flushing o Chemical waste neutralization Post Treatment The following treatments are assumed for permeate disinfection: Sodium hypochlorite (chlorine gas or onsite hypochlorite generation) Potential need of UV for 0.5 log Giardia and Cryptosporidium disinfection The produced water will also be stabilized and controlled for corrosion prior to entering the existing distribution system Lime (slacked lime or limestone for ph and hardness adjustments) CO2 (for alkalinity adjustment) Sodium Hydroxide (NaOH) for ph adjustment Orthophosphate (for corrosion control) Clearwell The clearwell serves three purposes: Provides adequate disinfection contact time (CT) for virus and residual chlorine Provides potable water to the entire facility Provides adequate water supply for onsite operation and emergency use Waste Disposal (from both filtration and RO) Assume sewer discharge if finished water can be used for backwash, otherwise backwash wastewater might be further treated due to its high salinity. Connection to Existing Distribution System Finished water conveyance was excluded during this desktop study, but it is anticipated that major investment in this conveyance pipeline may be required depending on the location of the SWRO. Buildings for Pretreatment and SWROF The following equipment, buildings and services will be required: Power supply (substation, emergency generators, etc.) Architecture (No actual drawing Electrical system HVAC Plumbing

11 Laboratory Maintenance & Storage Offices; operation/control room, meeting rooms, parking, etc. VIII. Cost Analysis Detail cost analysis was conducted for each of the key components of this project, including intake/outfall, pump/storage/hydropower, seawater desalination, and integrated operations. Results suggest that the integrated SPSH/SWRO approach can achieve substantial overall project Capex and Opex savings compared to a scenario where both facilities are separately designed, constructed, and operated. Due to current project development status, sensitive cost information will be available upon request. IX. Conclusions As a thought leader in green energy and sustainable desalination, Oceanus has created a unique integrated system that combines an innovative concept for renewable energy storage with a novel design for sustainable desalination. This system combines a seawater pump storage hydro (SPSH) facility and a seawater reverse osmosis plant (SWRO) into a single, integrated facility. Results from this feasibility study include both technical and financial assessments which suggest that this synergetic design is technically feasible and commercially attractive. Significant savings can be realized through the integrated design, construction, and operation of both facilities. Subsequent studies and site investigations are currently underway. X. ACKNOWLEDGEMENTS The authors would like to acknowledge significant contributions from the entire project team, including Steve Johnson, Craig Smith, Joseph Ehasz, John Chamberlain, and Martin Hammer. XI. REFERENCES Kenny, J.F., et al Estimated Use of Water in the United States in Reston, VA: U.S. Geological Survey. Neubauer, J., et al Electrical Energy Storage Applications and Technologies (EESAT) Conference. San Diego, CA: National Renewable Energy Laboratory. United Nations Framework Convention on Climate Change, 2016, Lazard, 2015, Lazard s Levelized Cost of Energy Analysis Version 9.0,