Water balance and energy partitioning in a semi-closed greenhouse

The concept of a closed (or semi-closed) greenhouse that is used to harvest solar energy can be attractive to reduce the fossil fuel input or for an increment of water use efficiency. To examine the concept, a 550 m2 greenhouse was built in The Netherlands and experiments were carried out with a pepper crop. This paper reports only on the water balance during the summer period of such a greenhouse and examines the energy partitioning of the solar radiation in the greenhouse. Results show that the percentage of drainage out of the irrigation changed significantly among the days but was on average 34%. The major water input to the greenhouse came from irrigation and only a small fraction was due to fogging in some of the days. The remainder of irrigation minus drainage was transpired by the plants. On average, 15% of the water was lost from the greenhouse by exchange of vapor with the outside air. This was partly by leakage, but also deliberately when the greenhouse was ventilated with outside air to remove a moisture excess during night. The rest condensed, mainly on the heat exchangers of the cooling units, but partly on the roof and side walls as well. Thus, roughly 85% of water used for irrigation was recaptured. The results further show that the global radiation inside the greenhouse was on average 0.57±0.06 of the outside radiation. The heat load on the greenhouse was mainly removed by the overhead cooling units who removed sensible and latent heat with an average sensible to latent ratio of 0.62. This ratio changed among the days and increased approximately linearly with solar radiation. The energy extracted by the coolers was about 0.73±0.18 of the solar radiation entering the greenhouse, which means that it was 42% on average of the outside solar radiation