Efficiency of carbon dioxide enrichment in an unventilated greenhouse

In recent years, efforts have been made to utilise carbon dioxide (CO₂) enrichment and reduce the emissions of CO₂ from greenhouses. In this study, efficiency of CO₂ enrichment in an unventilated greenhouse was investigated based on balance measurements of CO₂ in the short-term and an estimate of CO₂ leakage in the medium-term. A greenhouse covered with plastic film (floor area 178m²) was used. Pure CO₂ was supplied to a tomato crop trained to a high-wire system, maintaining a CO₂ concentration nearly 1000μmol[CO₂]mol^-1[air] during daylight. For CO₂ balance measurements, the leakage rates, the amount of CO₂ leakage and crop uptake were derived hourly from a CO₂ balance equation of greenhouse air and leakage measurements using a tracer gas technique. The leakage rate of the unventilated greenhouse was within similar ranges found in literatures investigating other types of greenhouses. The amount of CO₂ leakage was comparable to crop uptake in windy or overcast conditions. The efficiency of CO₂ enrichment in the medium-term was estimated using the regression equation of the leakage rate, measurements of outside wind velocity, amount of CO₂ supply, and CO₂ concentrations inside and outside. The estimated results in the medium-term showed an average efficiency of 45.5%; the highest efficiency was around 50%. Higher efficiencies were achieved when it was less windy and there was higher solar radiation.