I have no experience but spent some time researching and designing ideas.

I think it would be pretty easy to heat in the winter using solar hot water. Maybe $200-400 in materials and no running cost. A tank or pond inside holds the water mass which heats up in the day and releases heat at night. Plus you can grow fish in the water. I've seen these used in the far north. We only have to get the temp up a little in winter so I think these are great. You can have plant tables and walkways over the top of the pond so there's no loss of floor space.

City water goes into the pond and you water the plants from the pond. This helps keep the pond clean, fish healthy and the water is better for the plants.

Some of the nurseries here use swamp coolers. They're they're pretty simple. One end wall is the media the water flows over and air is pulled thru with a big fan. I think they build it themselves. Work good. I think maybe more for raising humidity than lowering temp although the temp was lower when I've been in them.

Fun thread, DO keep us posted! :) I'm always envious of greenhouse growers wherever they are. We'd love updates and pics! :) Happy gardening!

What direction will it face? If south, you may want a solid roof rather than glass, to protect from high summer sun, while admitting low winter sun. I built a kind of half octagon made of french doors on each side, rolling screens so in winter it was a conservatory and in summer was a screen porch. I was very happy with it. Good luck with your project.

Thermal water mass: excellent idea! Tilapia too...