https://www.academia.edu/23908215/Carbon_dioxide_enrichment_by_composting_in_greenhouses_and_its_effect_on_vegetable_production

Here is another trial on some green leafy veg by the same researchers with some very dramatic results, there seems to be a little more detail on the composting units too.

I have heard it mentioned that if a grower is supplimenting light then using co2 enrichment can reduced his need for light and achieve the same yield? I am in ireland where we have cool but not cold winters with fairly low light levels and I am thinking that this system might let me keep my tunnels closed and increase yields on my greens and early spring crops.