Alexander W. Dowling

A framework for multi-stakeholder decision-making and conflict resolution

Abstract We propose a decision-making framework to compute compromise solutions that balance conflicting priorities of multiple stakeholders on multiple objectives. In our setting, we shape the stakeholder dissatisfaction distribution by solving a conditional-value-at-risk (CVaR) minimization problem. The CVaR problem is parameterized by a probability level that shapes the tail of the dissatisfaction distribution. The proposed approach allows us to compute a family of compromise solutions and generalizes multi-stakeholder settings previously proposed in the literature that minimize average and worst-case dissatisfactions. We use the concept of the CVaR norm to give a geometric interpretation to this problem and use the properties of this norm to prove that the CVaR minimization problem yields Pareto optimal solutions for any choice of the probability level. We discuss a broad range of potential applications of the framework that involve complex decision-making processes. We demonstrate the developments using a biowaste facility location case study in which we seek to balance stakeholder priorities on transportation, safety, water quality, and capital costs.

Optimization of sub-ambient separation systems with embedded cubic equation of state thermodynamic models and complementarity constraints

Abstract A previously developed equation-based flowsheet optimization framework is extended and applied to design sub-ambient separation systems for oxy-fired coal power systems with carbon capture. Unlike most commercial flowsheet design and optimization tools, the proposed methods use exact derivatives and large-scale nonlinear programming algorithms to solve large flowsheet design problems with many degrees of freedom, including the simultaneous design of air separation units (ASUs) and their accompanying multistream heat exchangers. Emphasis is placed on additional model improvements regarding thermodynamic calculations. In order to maintain differentiability, complementarity constraints are used to model switches, including vanishing and reappearing phases. Nevertheless, these complementarity constraints may construct trivial phase equilibrium solutions, and a procedure based on embedded bubble and dew points calculations is proposed to avoid them. Furthermore, additional complementarity constraints for the cubic equation of state model are proposed to ensure correct phase identification in the supercritical region. Finally, the efficacy of these new models are demonstrated by optimization of the CO2 processing unit and compression train for an oxy-fired power plant.

Equation-Oriented Optimization of Cryogenic Systems for Coal Oxycombustion Power Generation

Abstract Efficient separation systems are essential to the development of economical CO 2 capture system for fossil flue power plants. Air Separation Units (ASU) and CO 2 Processing Units (CPU) are considering the best commercially available technologies for the O 2 /N 2 and CO 2 /N 2 , O 2 , Ar separations in coal oxycombustion processes. Both of these systems operate at cryogenic temperatures and include self-integrated refrigeration cycles, making their design challenging. Several researchers have applied sensitivity tools available in the commercial flow sheet simulators to study and improve ASU and CPU systems for oxy-fired coal power plants. These studies are limited, however, as they neglect important interactions between design variables. In this paper, we apply an advanced equation-based flowsheet optimization framework to design these cryogenic separations systems. The key advantage of this approach is the ability to use state-of-the-art nonlinear optimization solvers that are capable of considering 100,000+ variables and constraints. This allows for multi-variable optimization of these cryogenic separations systems and their accompanying multi-stream heat exchangers. The effectiveness of this approach is demonstrated in two case studies. The optimized ASU designs requires 0.196 kWh/kg of O 2 , which are similar to a “low energy” design from American Air Liquide and outperforms other academic studies. Similarly, the optimized CPU requires 18% less specific separation energy than an academic reference case. Pareto (sensitivity) curves for the ASU and CPU systems are also presented. Finally, plans to apply the framework to simultaneously optimize an entire oxycombustion process are discussed.

Rigorous Optimization-based Synthesis of Distillation Cascades without Integer Variables

Abstract In this paper we present a novel alternate model for distillation cascades without integer variables. This allows for optimization of highly nonlinear process flowsheets with embedded distillation columns, modeled with rigorous mass, equilibrium, summation and heat (MESH) equations. Instead of reformulating the famous MINLP distillation column model as an MPCC (mathematical program with complementarity constraints), bypass streams are added to the distillation cascade superstructure. If a tray is fully active the bypass streams are inactive (zero flow) and the standard MESH results are obtained. If a tray is completely inactive all of the flow bypasses the tray, resulting in no separation. As a result the interior of the bypass fraction (analogous to integer variables in the MINLP model) is well defined. In fact a partially bypassed stream is physically realizable (although inefficient). For this reason we postulate that this alternate model with bypass leads to well-behaved solution strategies.

Economic opportunities for industrial systems from frequency regulation markets

Abstract We analyze economic opportunities for industrial facilities provided by frequency regulation (FR) markets. We use classical frequency domain analysis techniques to characterize the harmonic content of FR signals and to analyze the impact of such harmonics on the response of dynamical systems. The analysis reveals that systems with slow dynamics, as those found in large industrial facilities, are suitable to provide FR capacity because they can naturally damp the dominant high-frequency harmonic content of FR signals. We also propose optimization formulations to quantify the maximum amount of FR capacity that can be provided by a system given its dynamic characteristics, its control architecture. A distillation case study demonstrates that significant economic potential exists for large industrial facilities.

Equation-Based Design, Integration, and Optimization of Oxycombustion Power Systems

The application of “systems-based tools’’ including exergy/pinch analysis and process simulation has facilitated increases in the thermal efficiency of ambient pressure oxycombustion coal-fired power systems with carbon capture from 36 to 39–40 %LHV, while also considering capital costs. This corresponds to a decrease in the energy penalty 10 %-points to 6–7 %-points (absolute), relative to reference air-fired coal power plants without CO2 capture (46 %LHV). These efficiency improvements are primarily due to tailored next-generation air separation systems and plant-wide heat integration. Furthermore, oxycombustion power systems are an ideal candidate for numerical optimization, given the complex interactions between its five subsystems. This chapter extensively surveys the oxycombustion literature and summarizes four key design questions. A new, fully equation-based, flowsheet optimization framework is then introduced and applied to three oxycombustion-related case studies: design of a minimum energy air separation unit to produce an O2 enriched stream for the boiler, optimization of the CO2 polishing unit and compression train to minimize specific energy, and maximization of thermal efficiency in the oxy-fired steam cycle using a hybrid 1D/3D boiler model.

Equation-Oriented Optimization of Cryogenic Systems for Coal Oxycombustion Power Plants

Efficient separation systems are essential to the development of economical CO2 capture system for fossil flue power plants. Air Se paration Units (ASU) and CO2 Processing Units (CPU) are considering the best commercially available technologies for the O2/N2 and CO2/N2, O2, Ar separations in coal oxycombustion processes. Both of these systems operate at cryogenic temperatures an d include self-integrated refrigeration cycles, making their design challenging. Sever al researchers have applied sensitivity tools available in the commercial flow sheet simulators to study and improve ASU and CPU systems for oxy-fired coal power p lants. These studies are limited, however, as they neglect important interactions between design variables. In this paper, we apply an advanced equation-based flowsheet optimization framework to design these cryogenic separations systems. The key advantage of this approach is the ability to use state-of-the-art nonlinear optimization solvers that are capable of co nsidering 100,000+ variables and constraints. This allows for multi-variable optimization of these cryogenic separations systems and their accompanying multi-stream heat exchangers. The effectiveness of this approach is demonstrated in two case stud ies. The optimized ASU designs requires 0.196 kWh/kg of O2, which are similar to a “low energy” design from American Air Liquide and outperforms other academic studies. Similarly, the optimized CPU requires 18% less specific separation energy th an an academic reference case. Pareto (sensitivity) curves for the ASU and CPU systems are also presented. Finally, plans to apply the framework to simultaneously optimize an entire oxycombustion process are discussed.

Economic assessment and optimal operation of CSP systems with TES in California electricity markets

The economics and performance of concentrated power (CSP) systems with thermal energy storage (TES) inherently depend on operating policies and the surrounding weather conditions and electricity markets. We present an integrated economic assessment framework to quantify the maximum possible revenues from simultaneous energy and ancillary services sales by CSP systems. The framework includes both discrete start-up/shutdown restrictions and detailed physical models. Analysis of coinci-dental historical market and meteorological data reveals provision of ancillary services increases market revenue 18% to 37% relative to energy-only participation. Surprisingly, only 53% to 62% of these revenues are available through sole participation in the day-ahead market, indicating significant opportunities at faster timescales. Motivated by water-usage concerns and permitting requirements, we also describe a new nighttime radiative-enhanced dry-cooling system with cold-side storage that consumes no water and offers higher effciencies than traditional air-cooled designs. Operation of this new system is complicated by the cold-side storage and inherent coupling between the cooling system and power plant, further motivating integrated economic analysis.The economics and performance of concentrated power (CSP) systems with thermal energy storage (TES) inherently depend on operating policies and the surrounding weather conditions and electricity markets. We present an integrated economic assessment framework to quantify the maximum possible revenues from simultaneous energy and ancillary services sales by CSP systems. The framework includes both discrete start-up/shutdown restrictions and detailed physical models. Analysis of coinci-dental historical market and meteorological data reveals provision of ancillary services increases market revenue 18% to 37% relative to energy-only participation. Surprisingly, only 53% to 62% of these revenues are available through sole participation in the day-ahead market, indicating significant opportunities at faster timescales. Motivated by water-usage concerns and permitting requirements, we also describe a new nighttime radiative-enhanced dry-cooling system with cold-side storage that consumes no water and offers high...

Pressure Swing Adsorption Optimization Strategies for CO2 Capture

Abstract Advanced process optimization technologies play a critical role in improving economics of carbon capture technologies. Pressure swing adsorption (PSA) has received recent attention as a potential process for economically removing CO 2 from flue and/or shifted syngas for carbon capture and storage. In this work, we present state-of-the-art methods for PSA optimization using a superstructure-based approach; this allows simultaneous selection of PSA cycle steps and optimization of operating parameters (feed flow rate, recycle fractions, bed pressures, etc.). The partial differential equation bed models are discretized with finite volumes in space and two flux limiters are compared. Sparse linear solvers are implemented to accelerate the integration of bed models and direct sensitivity equations, which are interfaced to MATLAB. The PSA optimization approach is demonstrated on a CO 2 /H 2 separation case study for an integrated gasification combine cycle power plant, and solved by sequential quadratic programming solvers.