(359b) Generic Modeling, Design and Operating Optimization of Hybrid Renewable Power Generation Flowsheets Including Hydrogen Storage

To address the intermittent nature of largely unpredictable environmental phenomena, renewable energy sources (RES) are often transformed into dependable power flows by simultaneous utilization of different types of conversion equipment and storage media (e.g. PV panels, wind generators, chemical accumulators etc.). Hydrogen-based technologies are also receiving considerable attention as they enable flexible and long-term power storage, increasing the capacity provided by conventional accumulators. Such features result in hybrid infrastructures combining multiple sub-systems of heterogeneous characteristics that need to operate smoothly and efficiently within an integrated flowsheet. Their optimum design and operation involves significant complexities due to the existence of a very large number of interactions affecting the overall system performance. In addition to numerous structural options, the intense variability of renewable energy sources inflicts a varying temporal behaviour resulting in significant operating transitions among temporally different flowsheet topologies. Optimization can be used to systematically address such complexities by treating system features and interactions as decision options. However, the efficient consideration of all the available alternatives requires systematic models for the exhaustive representation of structural and temporal flowsheet options.

Recently reported works address the structural design of power and/or heating/cooling co-generation systems through generic modeling concepts developed around flow characteristics which are independent of specific equipment functions (e.g. energy hub and power node concepts [1], matrix-based models [2]). In a similar context, superstructure-based approaches have also been considered in the design of extensive polygeneration flowsheets, focusing mostly on chemical conversion processes for the co-generation of chemicals, fuels and power [3, 4]. Clearly, there is a lot of scope in exploiting the merits of both approaches for the efficient modeling and design of flowsheets integrating renewable power generation systems including hydrogen/power storage. On the other hand, models addressing the varying temporal behavior of such systems are often case-specific. They mostly utilize conditional rules in the form of empirical, tree-like representations to determine the appropriate time–instance for activation/deactivation of the desired equipment, ultimately aiming to maintain feasible, stable and prolonged system operation. Although useful, this practise requires the manual addition of terms and conditions in the event that new options (e.g. streams, equipment, rules etc.) need to be considered in the performed calculations. Whilst marginally feasible for reasonably small systems, the exhaustive consideration of all possible temporal combinations becomes impractical as the number of incorporated structural options increases.

This work addresses the systemic modeling, design and operating optimization of renewable power generation and hydrogen/power storage flowsheets based on generic representations integrating structural, temporal and logical features. Building on existing methods, an inclusive synthesis model is developed in the form of a superstructure to capture structural and operating characteristics of RES-based flowsheets, regardless of processing tasks or utilized equipment. Generic concepts are used to describe flows and tasks of conversion and accumulation, integrating streams utilized in multi-component material/energy or purely energy conversions and facilitating the representation of operations at different resolution within the same flowsheet.

The above are coupled with the use of logical propositions assembled around generic temporal features of power generation and storage systems to support the flexible development of simple and complex conditional statements representing operating requirements as constraints. Such features are based on the hysteresis phenomena dominating the operation of systems that switch between activation and de-activation, within or between subsequent time instances. They are linked with the proposed structural concepts by defining hysteresis as a zone developed around the capacity of the accumulators where the converters are allowed to operate. In this respect, operating policies are easily represented as logical schemes that maintain the generic features of the superstructure. The resulting schemes are subsequently incorporated in the synthesis models, supporting the temporal evolution of flowsheet topologies based on the conditions associated with the considered constraints. This is approached by appropriate transformations inflicted upon the superstructure model.

The proposed developments are illustrated through case studies on the optimum design of an extended power generation and hydrogen storage flowsheet, considering equipment such as PV panels, wind generators, batteries, fuel cells, electrolyzers etc. Important structural and operating characteristics such as the state of the accumulators at different time instances (e.g. capacity of batteries and hydrogen storage tanks at different pressures), the hysteresis zones and the types of equipment are considered as design variables, hence giving rise to an extensive number of feasible alternatives of utilizing and storing power at different forms.

[1] Heussen, K., Koch, S., Ulbig, A., Andersson, G., Unified system-level modeling of intermittent renewable energy sources and energy storage for power system operation, IEEE Systems Journal (2011), 10.1109/JSYST.2011.2163020

[2] Chicco, G., Mancarella P., Matrix modeling of small-scale tri-generation systems and application to operational optimization, Energy 2009, 34 (3), 261-273.

[3] Liu, P., Pistikopoulos, E.N. (2010), A multi-objective optimization approach to polygeneration energy systems design, AICHE Journal, 56(5), 1218-1234.

[4] Baliban, R.C., Elia, J.A., Floudas, C.A. Simultaneous process synthesis, heat, power, and water integration of thermochemical hybrid biomass, coal, and natural gas facilities, Computers and Chemical Engineering (2012), DOI: 10.1016/j.compchemeng.2011.10.002