I've been doing lots of work lately on passive solar homes and sizing of HVAC systems and trying to answer questions like-- how much air can you reasonably pass through an 8 feet deep pile of loose rock? All this is in the interest of passive solar thermal design.
There's all sorts of funky designs, such as Trombe Wall (an eponym), Water Walls, rock bin storage, direct gain, sun space, and the ever mysterious exotic phase change materials, like Crisco or Paraffin.
For the lay reader all these are means of storing solar heat during the evening hours when you most need the heat.
I've perused through all sorts of energy modeling engines, and user groups, and listservs, and any other forum I could find, to try to find the most easy to use thermal modeling engine for Passive Solar Design.
Among those that I've reviewed--
E-quest/DOE 2.1.-- No dice, as it can't really handle the second law of thermodynamics. Energy in=energy out when it comes to thermal storage.
Energy Plus-- it looks like it can, but the challenge actually setting the down model up. It can actually handle thermal storage, but as with anything, one would have to fuss indefinitely.
SunREL: Now free from the NREL website. It can supposedly do simple passive solar thermal systems, and after about 16 hours of fussing, I have yet to get a model properly working in it.
Micropas/EnergyPro: Mostly useless in terms of passive solar and thermal storage. Good for lots of other basic modeling tasks, especially if you're going to try to prove code compliance in California.
Awesome. Take a long time to set up, but they do the trick for understanding other modeling output. Some would say that these are essential in addition to all others.
So there's-- that's a blog post! It took 10 minutes and shows it, but still has some relevance in fact and opinion.