fifth hit
"CO² enrichment has become common practice for many growers. Some use bulk CO² (carbon dioxide) from bottles or tank, but most of the larger operations use CO² from natural gas combustion. A heat storage tank can be employed to enable CO² enrichment when no heat is required. The a considerable amount of energy is used over the summer. This article aims to give some ball-park figures on CO² enrichment. A later article will give some further information about energy consumption for CO² and heat storage."
http://www.redpathaghort.com/bulletins/co2.html