ESP-r is a research-oriented open-source building performance simulation software. ESP-r can model heat flow in thermal zones, fluid flow using networks or CFD, electrical power flow, moisture flow, contaminant flow, hygrothermal and fluid flow in HVAC systems, as well as visual and acoustic performance aspects within a modeled energy system/building.
ESP-r has been under development for the last 50 years, with contributions by researchers from several countries. ESP-r source-code was published in 2002, subject to the GNU General Public License. The current version (V13.3.17) was released in March 2024. Professor Joseph Clarke, of the University of Strathclyde, is the project archivist.
ESP-r is designed to work on Linux, but it can run on Windows using Windows Subsystem for Linux (or in any other operational system using a virtual machine). 
ESP-r`s holistic nature, flexibility, and range of features enable a well-informed user to optimize the energy and environmental performance of a building and/or associated energy systems. The user experience provided by ESP-r, however, cannot be compared to the one provided by commercial software. ESP-r learning curve is steep, but there is a growing amount of training material available online. 
ESP-r has been extensively validated. Among other projects, ESP-r was part of BESTEST, an IEA initiative that created a benchmark for quality assessment of energy simulation software. This benchmark was later incorporated on ASHRAE Standard 140 - Method of Test for Evaluating Building Performance Simulation Software.
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