BATCH REVERSE OSMOSIS: EXPERIMENTAL RESULTS, MODEL VALIDATION, AND DESIGN IMPLICATIONS

In theory, batch reverse osmosis (RO) systems can achieve the lowest practical energy consumption by varying feed pressure over time. However, few batch RO syste ms have been built and operated. We have tested a bench-scale prototype of a true batch RO system using a bladder and a 2.5” (6.35 cm) spiral wound membrane element. Some practical issues in implementing batch RO include system start-up time, system depressurization, osmotic backwash during the reset phases, and lower permeate quality. This study is the first to validate batch models by measuring the hydraulic work of both the high pressure pump and the circulation pump. The experimental measurements agree well with the model (error ≤ 3 %) after accounting for concentration polarization. We used the validated model to calculate the energy savings of true batch systems at higher salinities and recovery ratios. We find that the energy savings achievable by true batch systems are less than previously thought, but still significant at relatively high recoveries. At 50% recovery of seawater feed, a batch RO plant could save 15% of the energy consumed by a continuous RO plant while still maintaining the same effective flux. Further studies should identify the additional costs associated with batch RO in order to identify the operating conditions where batch RO will be an economically favorable option compared to conventional continuous RO

Comparison of fouling propensity between reverse osmosis, forward osmosis, and membrane distillation

Resistance to fouling is often cited as an advantage of emerging desalination technologies such as forward osmosis and membrane distillation over the widely-used reverse osmosis process. However, the nature and magnitude of differences in fouling behavior between these three processes are not well characterized. This study directly compares the fouling and scaling behavior of reverse osmosis (RO), forward osmosis (FO), and direct contact membrane distillation (MD) in the same membrane module under identical hydrodynamic conditions (flux and cross-flow velocity). Fouling experiments were conducted using calcium sulfate as a model inorganic foulant and alginate as a model organic foulant. Although all three processes tolerated some degree of feed supersaturation for 36 h without inorganic fouling (scaling), FO exhibited the greatest scaling resistance, withstanding a feed of 33 ± 2 mM CaSO₄ (approximately twice saturation) without significant flux decline. Scaling occurred at similar concentrations at the membrane between MD and RO; however, while MD tolerated a more concentrated bulk feed due to reduced concentration polarization, flux decline after fouling was considerably more severe in MD. In contrast, MD tolerated organic fouling much better than FO or RO: despite accumulating a similar quantity of alginate gel over 18 h of operation, flux declined only 14% in MD versus 46–47% in RO and FO. These results are explained with respect to differences in temperature, membrane materials, and transport mechanisms between the three processes. Although FO and MD each exhibited superior resistance to one type of foulant, neither process outperformed RO in resistance to both organic and inorganic fouling. These findings inform a more nuanced approach to process selection for the treatment of complex water sources. Keywords: Desalination; Forward osmosis; Fouling resistance; Membrane distillation; Reverse osmosi

A Relational Approach to Shifting Gen Z and Millennial Environmental Beliefs

oai:poroi:id:31088While survey data identifies that most Gen Z and Millennials are anxious about climate change, are supportive of climate activists, and agree that climate change is anthropogenic, that same data fails to nuance these generations\u27 intersectional and relational environmental beliefs. The problem is both methodological and rhetorical, because assumptions built into closed-question public opinion surveys can fail to match younger generations\u27 perceptions on the environment. Additional research methods concerned with capturing these relations, including the cognitive interviews that survey designers already employ, could illuminate these environmental perspectives. We see models for this approach in the preliminary interviews used in large-scale surveys, in the field of climate psychology, and in arguments for ecological rhetoric in communication studies. Building from these fields, we provide example questions that are emblematic of these relational environmental and argue for increasing numbers of smaller, qualitative studies which investigate the many relations that younger generations already experience

Wave Energy From Idea, to Concept, to Converter: Structuring Methods for Design and Evaluation of Wave Energy Devices

The amount of energy we use and the ways that we get that energy sit on the edge of dramatic change as the carbon budget which can keep the planet under 1.5C of global average temperature increase gets smaller (Allen, 2018). In response, we continue to research and develop renewable energy technologies. Among these technologies are a diverse set of devices intended to convert the mechanical energy of wind-driven ocean waves to usable energy, typically in the form of electricity. Currently, researchers and developers work on wave energy devices for grid-scale energy applications as well as other emerging markets, such as ocean observation or desalination. Despite the large scope of potential uses for the technology, it is not currently being used as an energy source for any market. For many applications, the price remains too high and the technology too new. \par The unique challenges for wave energy converter design---integrating complex and uncertain technological, economic, and ecological systems, overcoming the structural challenges of ocean deployment, and dealing with complex system dynamics---have led to a disjointed progression of research and development. There is no common design practice across the wave energy industry and there is no published synthesis of the practices that are used by developers. This lack of established process likely contributes to the slow forward motion of the wave energy industry. \par In this body of work, I have integrated knowledge of engineering design processes with research in current wave energy converter (WEC) design challenges and pathways, in order to better understand and improve WEC design practice. The results from these studies reveal the dominance of point-based design approaches in the field of WEC design, the areas of WEC design in which methodological improvements are most necessary, and the need for significantly better ways of distinguishing between the performance potential of WEC concepts. Despite the significant attention given to late-stage design optimization by academic researchers, developers are in need of improved tools for earlier in the process. Point-based design, even with late-stage optimization is not sufficient or entirely appropriate for the field of wave energy. Set-Based Design, multi-attribute utility analysis, the improvement of holistic performance assessments, and the conversion of those assessments for use in the conceptual design stage are the four methods which I examine in this work to improve early-stage WEC design.Chapter of this thesis were peer reviewed and published in the Journal of Marine Science and Engineering and Chapter 3 peer reviewed and published at the European Wave and Tidal Energy Conference in 2019. Chapters 4 and 5 were submitted to the U.S. Department of Energy

The Wave Energy Converter Design Process: Methods Applied in Industry and Shortcomings of Current Practices

Wave energy is among the many renewable energy technologies being researched and developed to address the increasing demand for low-emissions energy. The unique design challenges for wave energy converter design—integrating complex and uncertain technological, economic, and ecological systems, overcoming the structural challenges of ocean deployment, and dealing with complex system dynamics—have lead to a disjointed progression of research and development. There is no common design practice across the wave energy industry and there is no published synthesis of the practices that are used by developers. In this paper, we summarize the methods being employed in WEC design as well as promising methods that have yet to be applied. We contextualize these methods within an overarching design process. We present results from a survey of WEC developers to identify methods that are common in industry. From the review and survey results, we conclude that the most common methods of WEC design are iterative methods in which design parameters are defined, evaluated, and then changed based on evaluation results. This leaves a significant space for improvement of methods that help designers make better-informed decisions prior to sophisticated evaluation, and methods of using the evaluation results to make better design decisions during iteration. Despite the popularity of optimization methods in academic research, they are less common in industry development. We end this paper with a summary of the areas of WEC design in which the testing and development of new methods is necessary, and where more research is required to fully understand the influence of design decisions on WEC performance

The Wave Energy Converter Design Process: Methods Applied in Industry and Shortcomings of Current Practices

Wave energy is among the many renewable energy technologies being researched and developed to address the increasing demand for low-emissions energy. The unique design challenges for wave energy converter design—integrating complex and uncertain technological, economic, and ecological systems, overcoming the structural challenges of ocean deployment, and dealing with complex system dynamics—have lead to a disjointed progression of research and development. There is no common design practice across the wave energy industry and there is no published synthesis of the practices that are used by developers. In this paper, we summarize the methods being employed in WEC design as well as promising methods that have yet to be applied. We contextualize these methods within an overarching design process. We present results from a survey of WEC developers to identify methods that are common in industry. From the review and survey results, we conclude that the most common methods of WEC design are iterative methods in which design parameters are defined, evaluated, and then changed based on evaluation results. This leaves a significant space for improvement of methods that help designers make better-informed decisions prior to sophisticated evaluation, and methods of using the evaluation results to make better design decisions during iteration. Despite the popularity of optimization methods in academic research, they are less common in industry development. We end this paper with a summary of the areas of WEC design in which the testing and development of new methods is necessary, and where more research is required to fully understand the influence of design decisions on WEC performance.</jats:p

Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation

In theory, the batch reverse osmosis (RO) process achieves the lowest practical energy consumption by varying pressure over time. However, few batch RO systems have been built and operated. We have designed, built, and operated the first “true” batch RO prototype using a flexible bladder. The flexible bladder serves as the high-pressure variable-volume tank that is inherent to true batch RO designs (as opposed to batch RO with energy recovery devices). We experimentally validated a model of batch RO energy consumption (≤2.7% difference) by measuring the hydraulic work of the high pressure and circulation pumps. We find that batch RO energy consumption will be greater than expected mostly due to salt retention, a problem neglected by most previous studies. However, despite operating at elevated salinity and flux conditions, batch RO can still save energy relative to single-stage and multi-stage continuous systems. For a seawater desalination plant (35 g/kg intake, 50% recovery, 15 L m [superscript−2]  h [superscript−1]), our newly-validated model predicts that batch RO would save 11% energy compared to a single-stage continuous RO plant. Our work demonstrates that batch RO is an energy-efficient process with the potential to reduce the cost of water desalination. Keywords: Desalination; Reverse osmosis; Batch reverse osmosis; Salt retention; Energy efficienc

True batch reverse osmosis prototype: model validation and energy savings

In this study, we tested a bench-scale prototype of a true batch reverse osmosis (RO) system using a flexible bladder and a 2.5 in. (6.4cm) spiral wound membrane element. In theory, batch RO systems can achieve the lowest practical energy consumption by varying feed pressure over time. However, this is the first study to validate batch models by measuring the hydraulic work of both the high pressure pump and the circulation pump. The experimental measurements agree well with the model (error < 3%) after accounting for concentration polarization. We used the validated model to calculate the energy savings of true batch systems at higher salinities and recovery ratios. Previous studies assumed that a batch RO plant would operate at the same flux and steady-state feed salinity as a comparable continuous RO plant. In order to match the permeate production of a continuous RO plant, a batch RO plant must operate at an elevated flux to offset its intermittent permeate production. A batch RO plant will operate at a steady-state feed salinity higher than the plant’s intake feed salinity due to salt retention between batch cycles. As a result of these practical inefficiencies, the energy savings achievable by true batch systems are less than previously thought, but still significant at relatively high recoveries. At 50% recovery of seawater feed, a batch system could save 11% of the energy consumed by a continuous RO system while still maintaining the same level of permeate production. We have demonstrated the successful operation of a true batch system and shown that it can indeed reduce energy consumption. Keywords: batch reserve osmosis; true batch; energy efficiency; energy savings; system desig