Enriching the air feed to greenhouses using inorganic silica membranes can help to increase yield and reduce water consumption, University of South Africa Materials and Process Synthesis Research Group researchers Dr Neil Stacey, James Fox and Diane Hildebrandt propose.

In their paper, titled ‘Reduction in Greenhouse Water Usage through Inlet CO₂ Enrichment’, published in February in the American Institute of Chemical Engineers journal, they model a typical greenhouse as a continuously stirred tank reactor and calculate its mass and energy balance.

The bulk of the water supplied to a conventionally aspirated greenhouse is lost in the form of humidity. For most plant species, warm and humid conditions are ideal and yields can drop significantly if parameters vary outside an optimal range.

The air must be warmed up and humidified to reach suitable growing conditions, which means that significant quantities of water evaporate to make up the difference in water content.

This evaporation can be reduced with carbon dioxide (CO₂) enrichment, which reduces the amount of air required accordingly. Elevated CO₂ also increases crop yield. Growing a particular mass of plant material requires a particular mass of CO₂ to be absorbed from the air. The CO₂ in air is highly diluted – 400 parts per million (ppm) – and, therefore, plants require large volumes of air.

Reducing the air flow will reduce the water and heat leaving the greenhouse in the exit stream. Even a low selectivity separation can be expected to improve performance – merely raising the CO₂ concentration from 400 ppm to 600 ppm would just about halve the airflow required, reducing heat and water loss commensurately.

“Inlet CO₂ enrichment using existing membrane materials can reduce the air feed rate required to supply adequate CO₂ for photosynthesis, thereby mitigating evaporative losses,” the researchers state.

The full extent to which evaporative losses can be reduced will ultimately depend on the plant species, because a minimum amount of evaporative loss is necessary for transpiration as the primary means of nutrient transport in plants.

The model indicates that evaporative losses can be reduced by as much as 95% in many scenarios, leaving the transpiration requirement as the principal constraint on water needs.

In cold weather, large quantities of inert air must also be heated to the greenhouse operating temperature without subsequently participating in the photosynthesis reaction.

In adequately warm weather, these energy inputs are supplied by solar energy. In these circumstances, the flow of inert air and the evaporation of water that is lost in the form of humidity have a necessary cooling effect on the greenhouse.

Operating with much-reduced air and water inputs, as a result of inlet enrichment, will necessitate a commensurately large cooling duty under many circumstances.

“In practice, this creates a process control challenge, where overall inlet feed will comprise [a] combination of enriched air and unenriched air, with water supply modulated accordingly. “The ratio of flow rates from the two feed sources would, therefore, be used as a process control variable for managing the greenhouse temperature,” the researchers highlight.

This also implies that inlet enrichment is best suited to high-intensity cultivation techniques that maximise plant growth relative to surface area and minimise solar heating relative to yield, they add.

While very low selectivity membranes can still produce significant water savings, they do so at the expense of substantial compressive work. However, membranes with very high selectivity for CO₂ are commercially available and can offer drastic reductions in water use with minimal compressive work required.