Greenhouse Humidity Research Articles

HORTITRANS, a Model for Predicting and Optimizing Humidity and Transpiration in Greenhouses

A new model is presented to predict humidity and transpiration directly as a function of the outside climate, with the particular objective of developing optimal control strategies for humidity in greenhouses. This model is of straightforward structure, while including the processes of transpiration, condensation, ventilation and humidification or dehumidification. The model allows the inside vapour pressure to be directly calculated as a function of the outside conditions and the greenhouse characteristics. It includes a linearized relationship for transpiration, which is a good approximation of a more detailed model. Condensation on the cladding is first calculated for the inside greenhouse air at saturation (100% relative humidity) and then corrected by a factor to account explicitly for the feedback effect of inside humidity on cladding temperature. Because of its simplicity, this model also explicitly determines the water and energy to be added to or extracted from the greenhouse air, in order to achieve given humidity or transpiration set-points. The HORTITRANS model predicted to within 8% vapour pressure, relative humidity, transpiration and condensation inside the greenhouse, when tested against measurements taken on a young and on a mature crop in a full-size greenhouse. The optimization of inside humidity shows that there is little need for dehumidification in a single glazed greenhouse. For double glazing, dehumidification could be cost effective at very low light levels, in particular just before sunset.

Performance of a greenhouse heating system with a phase change material

A greenhouse with a phase change material (PCM) heat storage system containing a quasi-eutectic mixture was tested with a classical lettuce-tomato rotation. Measurements on the greenhouse microclimate and assessment of the thermal storage performances were carried out during two heating seasons, with special attention paid to evapo-condensatioe processes. It is concluded that such a heat storage system, when used in the South of France, can keep a greenhouse roughly 10°C higher than outside during typical nights of March or April. It appears however that the present phase change storage medium, with a transition point as high as 22°C, is not adequate for periods with low solar gains (January or February). On the other hand, when the greenhouse temperature exceeds the melting point of the PCM (in late winter), the present (1.6 MJ m −2) storage capacity was found to be too low to cope with actual solar gains. Humidity measurements showed the positive effect of condensation on the storage. At night, as most of the condensates are drained away, the purely convective and weaker heat transfers from the storage contribute to keep the greenhouse humidity below saturation and preserve the crops from fungal diseases.

Night-time transpiration in greenhouses

High night-time humidity in greenhouses is characteristic of well insulated houses in mild weather. Using ventilation to remove excess vapour increases the energy requirement of a greenhouse in direct proportion to the amount of vapour to be removed which, in turn, increases linearly with canopy conductance. The night-time transpiration of several greenhouse crops was measured and their leaf conductance and resistance to vapour flux were determined by two methods: a weighing method was used for individual containers and a vapour balance method for a whole compartment. A few night-long comparisons between the two methods gave good agreement. A wide range of leaf resistances was obtained, from 500 to 2000 s m −1 for vegetables, from 2000 to 5000 s m −1 for most foliage and flower plants, with one crop reaching 20 000 s m −1.

Surface Heating Greenhouses with Power Plant Cooling Water

ABSTRACT A small greenhouse was heated with power plant cooling water during February and March, 1977. The heating system used nozzles to spray heated cooling water on the outside of the greenhouse. The ad-vantages of this system for utilizing heated cooling water are low capital costs for the system and no in-crease in greenhouse humidity because the green-house air does not contact the water. A flow rate of 0.094 L/s per m2 of greenhouse area is sufficient to heat the greenhouse to 15 °C using 30 °C water. With this efficiency the cooling water from an 1800 megawatt power plant could be used to heat ap-proximately 30 ha of greenhouses.

Condensation and Resultant Humidity in Greenhouses During Cold Weather

Condensation and Resultant Humidity in Greenhouses During Cold Weather

Condensation and Resultant Humidity in Greenhouses During Cold Weather

Condensation and Resultant Humidity in Greenhouses During Cold Weather