A MULTIFUNCTION DEHUMIDIFYING HEAT PUMP FOR GREENHOUSES

Abstract

mprovements in greenhouse structure and equipment with the aim of saving energy (double walls, thermal screens) have led to better insulation and airtightness but have also induced microclimatic changes, especially on nighttime humidity levels. During the last decade, dynamic climate procedures such as temperature integration have been frequently implemented to save energy in greenhouses. Temperatures may reach low levels and the greenhouse ventilation may remain particularly weak when using these types of systems, but the humidity level is always high. Consequently, the occurrence of fungal diseases and physiological disorders increases and greenhouse air dehumidification is required. In order to cope with this problem, a multifunction compression heat was developed, first, for dehumidifying the greenhouse during the night and, second, for heating the greenhouse in the same way as that of a conventional air-source heat pump, in addition to the boiler. To reach these objectives, a second evaporator is added to draw the heat from the outside to the inside (air-air). A water condenser (air-water) may also be added to provide a low-temperature heating system. The activation of the heating mode depends on the coefficient of performance (COP) and on the economic viability (bi-energy system: depending on the main energy source used). The dehumidification heat pump is designed for use at night as a function of the vapour pressure deficit (VPD) set point. A dynamic model of water vapour exchanges was developed to evaluate during the night the water vapour transfer between the different sub-systems (heat pump evaporator, crop, inside air, roof, outside air) considered as sources and sinks of water vapour. This model makes it possible to determine the power of the dehumidification systems according to the characteristic parameters of the greenhouse, the crop and the outside climate. This device is being implemented in a glasshouse in the west of France. Adjustments are, however, still necessary to validate the model and to optimise energy efficiency.