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 178 m2) was used. Pure CO2 was supplied to a tomato crop trained to a high-wire system, maintaining a CO2 concentration nearly 1000 μmol [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.