Thermodynamics of greenhouse systems : a new approach leading to new proposals for sustainable production

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

Greenhouse production systems produce in the Netherlands and Flanders economical important quantities of vegetables, fruit and ornamentals. Control of the crop environment has led to a high primary energy use (1500MJ/m²/year). This high primary energy use affects the economical and environmental sustainability. Research projects in the Netherlands and Flanders are launched to achieve sustainable greenhouse systems with high crop yields and low primary energy use. Until now, these projects didn’t result in the desired primary energy savings. On the contrary, the civil building industry succeeds by the passive house technology in a magnitude’s reduction for primary energy use. The primary energy saving research in greenhouse systems was mainly based on analysis of energy balances. However, the thermodynamic theory indicates that an analysis based on the concept of exergy (free energy) and energy is preferred. Such analysis could reveal possibilities for primary energy savings. The different processes and in/outputs of the greenhouse system are outlined. The appropriate equations for exergy calculations are developed. First, analyses are performed on the processes of transpiration and ventilation. Further, a total system assessment is performed. The exergy analysis indicates that primary energy saving in greenhouse systems could be the same as in the building industry, but adapted technologies need to be developed. The Institute for Agricultural and Fisheries Research (ILVO) will use this exergy analysis as a basis for the development of an exergy efficient greenhouse prototype (EXEkas).