Energy saving: from engineering to crop management

In greenhouse horticulture, energy costs form an increasingly larger part of the total production costs. To increase both the profitability and the sustainability of the sector, energy use per kg produce but also per ha has to be reduced. Energy in the greenhouse is primarily used for temperature control, reduction of air humidity, increase of light intensity and to a lesser extent for CO2 supply. The use of fossil energy can be reduced by limiting the energy demand of the system and decreasing energy losses, by intelligent control of (micro)climate, by increasing the energy efficiency of the crop and by replacing fossil energy sources by sustainable ones. In this paper, recent developments concerning reduction of energy consumption in greenhouse production systems in different climatic areas will be presented, as well as the consequences for crop management. Energy requirement of the greenhouse can be lowered by using greenhouse covers with higher insulating values and the use of energy screens. A prerequisite is that these materials should not involve considerable light loss, since this would result in a loss of production. Efficient screening strategies can save energy saving while maintaining crop production level. Recently, new covering materials have been developed that reduce energy loss in winter, heat load in summer and increase light penetration in the crop, thereby increasing crop production, while saving energy. In energy efficient greenhouse concepts, durable energy sources such as wind energy, solar energy or geothermal energy should be included. In (semi-)closed greenhouses, the excess of solar energy in summer is collected and stored in aquifers to be reused in winter to heat the greenhouse. In this concept, ventilation windows are closed, with specific benefits to the crop: high CO2 levels can be maintained, and temperature and humidity can be controlled to the needs of the crop. This poses new research questions, since the optimal climate for crops in various developmental stages is not known yet. Development of new greenhouse concepts is ongoing. Current examples are greenhouse systems which convert natural energy sources such as solar energy into high-value energy such as electricity. Given a certain technical infrastructure of the greenhouse, energy consumption can be further reduced by an energy efficient climate control. Essential elements are to allow fluctuating temperatures within certain bandwidths, allow higher humidities and create fluent transitions in set points. Consequences for plant growth are related to rate of development, photosynthesis, stomatal opening, assimilate distribution, transpiration and the occurrence of diseases or disorders. Next to physiological research, knowledge has to be gained on crop sensors, interpretation of sensor information and developing new microclimate control algorithms.