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
Partners: IBPSA-World and 42 organizations participating in IEA EBC Annex 60 and IBPSA Project 1
DOE Funding: $1,800,000 in FY20; $10,010,000 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, BOpTest, URBANopt

Project Objective

Control of HVAC, lighting, and other building systems plays a significant role in building energy performance. Advanced control algorithms have been shown to reduce annual energy use by over 30%.

Many modern BEM engines including EnergyPlus are not well-suited to evaluating advanced control sequences and are not well integrated with control applications and workflows in general. The split between control and BEM workflows creates significant friction and dampens the adoption of advanced control as design engineers struggle to find traditional algorithms that approximate advanced control or use workarounds, while control engineers must interpret and re-implement control sequences that are implicit in energy models.

Spawn-of-EnergyPlus (Spawn) is a next-generation engine intended to bridges the worlds of BEM and control workflows. The intent is for Spawn and EnergyPlus to live side-by-side, with the OpenStudio software development kit providing access to both and, to the extent possible, 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. Whereas EnergyPlus uses quasi-static load-based simulation of thermal loads and HVAC response, Spawn uses fully dynamic state-based simulation. Whereas EnergyPlus retroactively interprets and applies the actions of control sequences within the timestep, Spawn interprets and executes control sequences as they are implemented in physical building controllers.

The ability to directly simulate physical control sequences creates a bridge that allows simulated sequences to be directly ported to implementation. A companion project, Open Building Control, is fleshing out this connection on the control workflow side.

The use of the Modelica language separates domain-specific model development from domain-agnostic model compilation and numerical solution, allowing the project to leverage these overlapping but largely separate areas of expertise. Since 2012, BTO has supported the development of the Modelica Buildings Library, an international effort aimed at creating modular models of building components and physical phenomena. 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.

Spawn gains additional leverage by heavily leveraging the Functional Mockup Interface (FMI) standard for co-simulation, i.e., for simulation component and engine coordination. The use of FMI will allow Spawn to to integrate with other simulation engines that support the FMI standard, a number that currently stands at 89 and is growing. It will also allow Spawn 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.

The use of the Modelica and FMI standards provides a stable platform for buildings industry investment, and allows it to leverage technologies, tools, and investments from other related domains.


DOE Technology Manager: Amir Roth
Principal Investigator: Michael Wetter, LBNL


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