(209c) Flue Gas to Food Security: Radical Reductions in Greenhouse Water Usage through Direct Utilization of CO2

Agriculture is mankind’s single largest usage of water, comprising 70% of all water usage. Optimizing water usage in agriculture is therefore crucial to ensuring global water security. Only a tiny fraction of water taken up by a plant’s roots is ever incorporated into plant matter; the vast majority is lost through transpiration. There are essentially two types of transpiration; useful transpiration, a necessary mechanism for the transport of nutrient transport. The other type, useless transpiration, involves unnecessary evaporation from leaf and other surfaces. Useless transpiration is estimated to exceed useful transpiration in most agricultural conditions and can therefore be considered mankind’s largest avenue of water wastage.

In this research, a greenhouse is quantitatively modelled as a bio-reactor and it is shown that the high air-flow rates required to supply adequate CO₂ necessarily impose a high rate of evaporation, as air leaves the greenhouse in the same warm and humid conditions as the air inside the greenhouse. These evaporative losses make up a clear majority of mankind’s total water usage and can be mitigated by enrichment of CO₂ fed to the greenhouse, which reduces the airflow required to supply sufficient CO₂, reducing evaporative losses commensurately.

It is shown that using a typical gas-fired power station’s flue gas composition significantly reduces the required airflow and therefore evaporative losses by as much as 95% in many cases. Savings of this magnitude will not be entirely achievable because some amount of transpiration is necessary but these results nevertheless indicate that useless transpiration can be largely eliminated, drastically reducing water requirements for agriculture. Energy balance analysis also reveals that large amounts of energy are consumed by the heating and humidification of the air streams and that CO₂ enrichment therefore also promises to reduce heating requirements in cold-climate greenhouses.