Introduction and Potential of Exergy Analysis in Primary Energy Savings Research for Greenhouses

Until now, research on primary energy savings in greenhouse systems is primarily based on the analysis of energy and enthalpy state variables. It has been proven that these state variables are very useful for greenhouse climate modelling. However, the theory of thermodynamics indicates that these state variables are rather inaccurate for assessing primary energy saving options. In this respect, an analysis in combination with the second law of thermodynamics, which uses the concept of exergy (free energy), is a better approach. Therefore, the different main processes and in/outputs of the greenhouse system are presented to conduct an exergy analysis. Subsequently, exergy analysis is performed for both heating and active dehumidification processes. The exergy efficiency for dehumidification by ventilation is only 0.38 but still 4% for heating. This efficiency difference could not be detected by calculating traditional energy balances. As a consequence of this low dehumidification exergy efficiency, it is arguable that an exergy efficient dehumidification technology (like a vapour heat pump) is effective to obtain significant primary energy savings in greenhouses in the future. The Institute for Agricultural and Fisheries Research (ILVO) and the Department of Flow, Heat and Combustion Mechanics of UGent will use exergy analysis as a basis for the development of a vapour heat pump and an exergy efficient greenhouse prototype.