By integrating multiple energy systems at a community level, it is possible to use local renewable energy supplies and share energy resources between various producers, consumers, and storages. For heating and cooling needs, this integration occurs through district energy systems (DES), which can capture waste heat, allow multiple buildings to share thermal energy resources, and provide renewable energy conversion and resource sharing opportunities. However, before industry practitioners can leverage DES benefits for communities and cities, we need to address critical gaps in both the foundational energy modeling tools and the whole-system design approach. As such, the objective of this work is to enable district heating and cooling as linkages with waste-heat recovery potential for integrated energy systems. To meet this objective, this dissertation presents several projects on the comprehensive design of DES and their controls, which are made possible through advancements in Modelica modeling and novel engineering applications of ecosystem biomimicry.

To address fundamental gaps in modeling software and methods for complete DES simulation, this dissertation first focused on existing district cooling (DC) and steam-based district heating (DH) systems. For DC systems, we developed a methodology for DC model-based retrofits, released new open-source DC models in the Modelica Buildings Library, and leveraged these models for a real-world case study at the University of Colorado Boulder. Multi-dimensional analysis of the validated model led to several energy saving strategies, including control setpoint optimization, equipment modification, and pump setpoint adjustments. Results indicated that a combination of the studied measures can save the campus annually 84.6 MWh of energy, 8.9% of electricity costs, 58.0 metric tons of carbon dioxide emissions, while the waterside economizer cuts down chillers’ run times by 201 days/year, reducing maintenance costs and extending chiller life.

For steam-based DH systems, this work improved the performance of water/steam thermodynamic calculations for heating applications, released numerous DH models in the Modelica Buildings Library, and evaluated the accuracy and numerical performance of our new modeling approach from fluid mediums to complete districts. We improved the numerical performance of water/steam thermodynamic calculations by coupling a numerically efficient liquid water model (that represents enthalpy as a function of temperature only) with an industrial water/steam model (i.e., IAPWS IF97). Through this “split-medium” approach, we broke costly algebraic loops in the problem formulation by decoupling mass and energy balance equations. Compared to district models with the commonly adopted IF97 water/steam model for all phase regions, the new implementation improves the scaling rate for large districts from cubic to quadratic with negligible compromise to accuracy. For an annual simulation with 180 buildings, this translates to a computing time reduction from 33 to 1-1.5 hours. These results are critically important for industry practitioners to simulate steam DH systems at large scales.

To address the need for whole-system design approaches for integrated energy systems, we proposed an innovative ecosystem biomimicry approach. This approach functionally analyzes biological systems, abstracts those functions into models, and translates the models to create new technologies. Because ecosystem biomimicry is an emerging yet promising field, this dissertation includes an interdisciplinary literature review to synthesize trends across case studies, evaluate design methodologies, and identify future opportunities when applying ecosystem biomimicry to engineering practices, including cyber, physical, and cyber-physical systems. Through bibliographic analysis, statistical analysis, and deep dives into engineering applications and design methodologies, we synthesized trends in ecosystem biomimicry and identified opportunity spaces. For future studies, this review provides a comprehensive reference for ecosystem biomimetic design practices and a pplication opportunities across multiple engineering domains.

Following the critical review on ecosystem biomimicry, we then pursued case studies for district-level integrated energy systems. Ecological network analysis (ENA) was identified from the review as a promising whole-system analysis methods for cyber-physical systems that had not yet been applied for complex dynamic engineering systems. To fill this gap, this work first translated ENA from biological sciences for engineering applications. Then, we demonstrated the novel ENA method to design the heating and cooling systems for an office building and data center, coupling the buildings together via ambient-loop district energy. For the models to suit integrated building energy systems, which have multiple energy types and non-negligible dynamics, we were the first to formulate ENA (1) on an exergy basis and (2) dynamically for non-biological applications. For a case study that iteratively redesigned building heating and cooling systems, the proposed approach reduced source energy by 15% and HVAC site energy by 84% with negligible sacrifice to thermal comfort. Counter-intuitively, the redesign also reduced the exergy efficiency of the total system from 60% to 34%. This indicates that ENA and other network metrics that classify system organization can outperform traditional efficiency-based metrics for integrated energy systems.

@phdthesis{hinkelman2023modelica,
  title={Modelica Modeling and Ecosystem Biomimicry of District Energy Systems},
  author={Hinkelman, Kathryn},
  year={2023},
  school={The Pennsylvania State University}
}