If you are interested in (fast) Thermodynamics and Utility System Modelling, use MATLAB and like free stuff, JSteam Toolbox could be for you. See the JSteam Wiki for details on thermodynamics and modelling heat and power systems using MATLAB!
The best way to get JSteam Toolbox is to clone the master branch from GitHub (this page!) using git. I like GitKraken which makes it easy to use git without the command line. Once you have git/GitKraken/some other git client, simply click Clone or download (above, green button) and copy the address of this repository, and clone it to your computer. This way each time I push updates to the master branch, you can simply pull them down without having to redownload the entire JSteam Toolbox again!
If you don't use git, and don't want to learn (remember its easy and you should use it!), simply click Clone or download and click Download ZIP and save JSteam Toolbox somewhere you can find it.
Once you have cloned or downloaded JSteam to your PC, simply open MATLAB and navigate in MATLAB to the directory where you have placed JSteam Toolbox (hopefully somewhere permanent). Right click on JSteam_Install.m and click Run. Follow the prompts (Yes 'Y' is the normal response to every question) to install JSteam Toolbox - easy!
The best way to start using JSteam Toolbox is to have a look at the Examples on the JSteam MATLAB Interface Wiki. There are lots of problem and code examples (which all work) as well as helpful definitions.
JSteam Toolbox is released under the 3-clause BSD license, as detailed here. It is free software, and is released as an open-source interface to the closed-source (but free) JSteam package.