Performance of a controlled-ventilation open-top chamber for climate change research

An advanced controlled-ventilation open-top chamber (CVOTC) has been developed which exhibits reduced carbon dioxide consumption in comparison to current continuously-ventilated open-top chambers (OTC), as well as providing realistic ambient air speeds within the chamber. The measured performance of the CVOTC during field trials is reported and shows that a 5-fold reduction in carbon dioxide consumption is possible in comparison to OTCs. ?This low carbon dioxide consumption has been achieved in the CVOTC mostly by controlling the ventilation air flow in relation to the cooling requirement and by using a frustum with a 0.5 m diameter opening over the 3 m diameter chamber to limit air incursion; this small frustum opening resulted in the CVOTC nearly resembling a closed chamber. The average temperature in the CVOTC was 1.6 degrees C above ambient which represents a slight increase above the average temperature excess in conventional OTCs. Air speeds over the plants within the CVOTC were approximately 1 ms(-1) with the flow uniformity much improved in comparison to current OTCs. The results for the CVOTC show that a mean standard deviation in carbon dioxide concentration of about 40 mu mol mol(-1) is achievable when four permitted combinations (states) of ventilation/recirculation are employed, and that at least six ventilation states, or preferably a continuously-variable ventilation/recirculation combination, are required in order to achieve a narrow spread. During the CVOTC trials, the deviation between the mean carbon dioxide concentration and the target value was less than 6%.