I’ve heard repeatedly about Spawn of EnergyPlus and seen this project in the DOE roadmap. What is the future of energy modeling going to look like for the industry? Can you provide a “Co-simulation 101” please?
-Curious about Co-Simulation
The future of energy modeling will be similar to the present of energy modeling—EnergyPlus is not going away. What is hopefully going to change and what Spawn is about is the relationship of energy modeling to control design and implementation, and to model use during operation.
Controls are an increasingly important part of building energy efficiency. HVAC systems increasingly rely on advanced control strategies to achieve efficiency. Controls are key to leveraging “fifth-generation” district heating and cooling systems that distribute water at near ambient temperature to buildings, thereby increasing the efficiency of heat pumps, allowing buildings to share waste heat and cold, and facilitate the integration of renewables. And controls are obviously necessary to allow buildings to provide grid services and help integrate renewable resources.
Unfortunately, there is currently a disconnect between BEM and control workflows. Modern BEM engines like EnergyPlus use load calculations to deduce HVAC operation from high-level descriptions. In each time step, they first calculate the internal and weather-driven thermal loads on the zone, then they calculate the heating or cooling the HVAC system can provide, and finally use the difference between these to calculate updated zone conditions. Physical control sequences are not defined in terms of loads, they are defined in terms of temperature readings and valve and damper positions. The control strategies modeled by EnergyPlus and other BEM engines cannot be directly extracted and executed on control hardware. They must be manually interpreted and re-written. Often, rather than interpreting a modeled control strategy, a controls engineer may simply fall back to a known and trusted control sequence.
Spawn is designed to bridge this gap. Spawn couples EnergyPlus’ simulation of the building envelope to HVAC simulation that can directly interpret control sequences as they are implemented in the real world. This HVAC simulation uses models implemented in the open-standard modeling language Modelica. Traditional, imperative HVAC and control models can only be simulated. Modelica models are declarative. They can be used for simulation, but they can also be translated for execution on commercial control product lines. OpenBuildingControl is a companion project to Spawn that is developing libraries of high-performance control sequences and translators for implementation of these sequences on actual product lines.
Spawn also builds on the Functional Mockup Interface (FMI) co-simulation standard. EnergyPlus has supported co-simulation for some time via a feature called External Interface. More recently, it added support for co-simulation using the FMI standard. Spawn uses FMI internally to co-simulate between its own modules, e.g., between the EnergyPlus envelope module and the various Modelica HVAC and control modules. This internal use of standard co-simulation protocols creates a modular architecture that allows Spawn to integrate externally developed component and system models, including data-driven machine learning models, and to export models for integration with other simulation tools or for use-cases such as hardware in the loop testing. One example of Spawn co-simulation is URBANopt, which couples multiple Spawn models with Modelica models of advanced district heating and cooling systems.
Michael Wetter, PhD
Lawrence Berkeley National Laboratory
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