Forum user Acuario has built himself a greenhouse that is usable for the entire year. It is roughly 140 square feet (13 square meter), and is built attached to a shed. In winter, the greenhouse is kept warm via its own solar gain, plus some solar hot water panels that circulate water through pipes buried in the dirt. The whole thing is controlled by a DIY electronic controller.
Construction isn’t incredibly complex. It has a brick base. The greenhouse itself was a kit that Acuario bought. It is mostly aluminum and polycarbonate. As you can see, the hydronic heating coils are just laid below everything.
With the greenhouse built, Acuario went about designing a control system to automate many of the greenhouse tasks. The controller is designed around a PIC16F877A development board. This takes care of nearly all the routine things from the solar heating, to the grow lights, and even watering the plants.
The one thing Acuario is still working on is a cooling setup for those hot summer days. The plan is to use some temperature sensors and fans to also automate the cooling system.
To top it all off, the greenhouse also requires no energy to maintain itself. In addition to the solar hot water heating setup, it also has solar PV (electric) panels to take care of powering the lights, pumps, and controller.
For more info on Acuario’s greenhouse, see his forum thread.