Matt,
great link, thanks for sharing.
I wish more of us in the "northern climes" would be aware of/embrace this design, makes tons of sense to me: - especially with the optional addition of cold frames to the south as additional growing space/heat collectors.
Hoops may be relatively cheap, but...I like the above design because it can be well insulated and can easily incorporate recycled materials...
Here's another interesting integrated approach for a system by New Alchemy
http://builditsolar.com/Projects/Sunspace/NewAlchemycompost.pdf
again, it's all about the concept; understanding the fundamentals, and local conditions/climate/site/needs....
Andor aka mechartnik