the air in a greenhouse is dehumidified using a direct - contact heat exchanger utilizing concentrated brine . the diluted brine produced when water vapor in the greenhouse condenses on the concentrated brine is regenerated in a fuel - fired boiler whose products of combustion , carbon dioxide and water vapor , are passed into the greenhouse . the products of combustion provide the desired level of carbon dioxide in the greenhouse ; and the excess water vapor in the products of combustion as well as a significant portion of the water vapor produced by plants , growing in the greenhouse are removed by the hygroscopic concentrated brine .