Spawn-of-EnergyPlus (Spawn)

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Spawn-of-EnergyPlus software architecture diagram.
Spawn-of-EnergyPlus (Spawn) is a next-generation BEM engine that leverages open standards for equation-based modeling (Modelica) and co-simulation (FMI). Equation-based models can repurposed allowing Spawn to unify BEM with control workflows.
Amir Roth, BTO

Project Website:,, and


  • Lawrence Berkeley National Lab – Berkeley, CA
  • National Renewable Energy Lab – Golden, CO
  • Modelon, Inc. – Glastonbury, CT
  • Objexx Engineering, Inc. – Needham, MA
  • Big Ladder Software, LLC – Denver, CO


  • IBPSA-World and 42 organizations participating in IEA EBC Annex 60 and IBPSA Project 1

DOE Funding: $1,800 in FY19; $8,210,000 overall
Cost Share: In kind contributions from IEA Annex 60 and IBPSA Project 1 participants
Project Term: 2012 –
Funding Opportunity: Emerging Technologies Core Funding
Related Projects: EnergyPlus, OpenStudio, Open Building Control, Data Center Toolkit

Project Objective

Traditional simulation engines are “solver-based”—the governing equations of the system under simulation is implicit whereas the simulation engine itself embodies the solvers that evolve the equations in time. EnergyPlus is a traditional solver-based building energy modeling (BEM) engine—it contains equations for heat and mass balance, tightly intertwined with numerical solvers and program flow logic. Like other solver-based engines, EnergyPlus is difficult to maintain, extend, and link to other engines because it weaves together multiple solvers, each with its own structure and source-code. The tight coupling between equations and solvers also makes the models—i.e., the equations—impossible to extract and repurpose for other applications like control design and implementation. 

To address these limitations while opening up BEM to new applications, BTO is building a next-generation engine with the internal name Spawn-of-EnergyPlus (Spawn). The intent is for Spawn and EnergyPlus to live side-by-side, with the OpenStudio software development kit providing access to both and insulating users and client vendors from implementation differences.

Spawn reuses the EnergyPlus modules for lighting, building envelope, and loads but re-implements the HVAC and controls modules in the equation-based modeling language Modelica. In equation-based modeling, domain (BEM) experts can focus on write the governing equations of the system to be simulated. These equations can then be simulated by domain-agnostic tools. Equations are more compact than solvers and easier to write and maintain. Equation-based component and system models can be easily composed by linking interface variables—like temperature and air flow rate—in one set of equations to interface variables in another. Equation-based models are easy to repurpose—equations implicit in a solver can only be simulated, explicit equations can be analyzed, optimized, and even automatically turned into running code for direct execution in a different environment such as a building controller.

Modelica was developed primarily by the automotive and aerospace industries, but it is making headway in the buildings space where it is especially suitable to modeling HVAC systems and building controls. Since 2012, BTO has supported the development of the Modelica Buildings Library. Library development significantly advanced through the International Energy Agency (IEA) Annex 60 “New generation computational tools for building and community energy systems based on the Modelica and Functional Mockup Interface standards”. Development coordination has now transitioned to the International Building Performance Simulation Association (IBPSA) where 20 organizations contribute to IBPSA Project 1 “BIM/GIS and Modelica Framework for building and community energy system design and operation”.

The Modelica Buildings Library provides the domain-specific content for Spawn. BTO is working with commercial partners to provide domain-agnostic pieces like a Modelica translator and a simulation manager. As of spring 2018, BTO has an "alpha" prototype of a Spawn executable which combines EnergyPlus, Modelica Buildings Library components, the JModelica compiler, the g++ C++ compiler and the pyFMI simulator and can simulate simple buildings. A beta system is expected sometime in 2020 or 2021.

Project Impact

The use of Modelica and equation-based modeling over imperative languages like C++ and solver-based simulators holds significant promise for the future of BEM. Anticipated advantages include:

  • Ability to simulate real-world control sequences and to reuse control models in control design, implementation and verification workflows creating a natural bridge between BEM and building control. The related Open Building Control project is developing tools and processes that will form this bridge.
  • Improved model scalability including the ability to simulate very large models and to integrate models with different levels of fidelity.
  • Ability to leverage domain-agnostic simulation enhancements for speed and parallelization.
  • Reduced development and maintenance effort for BEM-specific code and ability to leverage external efforts in BEM development such as those of IBPSA Project 1.

SOEP gains additional leverage by building on the Functional Mockup Interface (FMI) standard for co-simulation, i.e., for simulation component and engine coordination. The use of FMI is expected to yield advantages including:

  • Ability to integrate with other simulation engines that support the FMI standard, a number that currently stands at 89 and is growing.
  • Ability to incorporate proprietary equipment models from manufacturers. Current solver-based BEM tools have to reverse-engineer models for new equipment or fit specific equipment performance characteristics to generic models, both approaches are costly and time consuming. FMI allows manufacturers to plug in-house component models into larger simulation frameworks without compromising intellectual property or competitive advantage.
  • More broadly, the use of standards provides a stable platform for buildings industry investment, and allows it to leverage technologies, tools, and investments from other related domains.


Related Publications

Journal Papers
Book Chapter
Conference Papers

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