Search

Greenhouse production has increased over the last decades in the Mediterranean region. Greenhouses allow protecting crops from adverse climate conditions, creating microclimate conditions appropriate for obtaining high production with high quality all over the year. However, greenhouse production is generally associated with high environmental impacts due to the use of high amounts of resource inputs and the high quantity of waste generated. Sustainable greenhouse production is nowadays a goal and can be achieved by using appropriate technologies such as innovating crop practices and environmental control techniques allowing reducing agro-chemicals, water, and energy consumption. Organic farming is based on methods and practices that try to protect the environment (soil, water, air), which includes limitation on the use of synthetic pesticides and fertilizers and the use of on-site resources, such as livestock manure for fertilizer. Demand for food produced without using agro-chemicals has increased; usually these products are sold at high price and associated to a particular nutritional value, taste and health. It is generally associated with lower yield and better quality. Greenhouse organic farming is not widely applied as farmers believe that yields are strongly reduced. However, there is a market opportunity to organic farming as a result of higher product prices and consumers demand. In order to compare the overall efficiency of crop farming systems, it is vital to consider energy consumption and efficiency. In this work, a case study is analysed in terms of energy consumption and greenhouse gas emissions (GHG). The data were obtained directly from the grower and the main objective of this work was to characterize the organic greenhouse production system. The most important inputs were identified and compared with others obtained with conventional greenhouse production. Results showed that organic greenhouse uses less energy ha-1 and t-1 of produced tomatoes than the conventional greenhouses.

In heated greenhouses, large amounts of energy are used to optimize climate conditions (temperature, humidity). In conventional tomatoes production, the estimated annual energy consumption is 320 kWh m-2 in France, with large differences across regions, 400 kWh m-2 in Brittany and 240 kWh m-2 in the South (ADEME, 2007). In Switzerland, it varies between 245 and 500 kWh m-2 according to the regions. With increasing energy prices and environmental concerns, growers have to find solutions to reduce their energy use and to improve the energy efficiency. Several axes could be used to achieve this goals, one of them is climate management. Within the working group "Energy saving and neutral production" of the Cost Action FA 1105 "BioGreenhouse", a review on the potential for energy saving in heated greenhouse thanks to climate management was done. Basically, there are two ways to reduce energy consumption: related to temperature control and to humidity control. The energy saving potential of lowered day and night temperature set points, temperature integration (TI) and screen management will be presented in relation to the effects on production. In Switzerland, three trials from 2006 to 2008 in tomato crops showed that an energy saving potential of 15 to 30% could be achieved with TI compared to the standard temperature treatment. An energy saving between 23 and 30% with screens management based on external temperature and light intensity compared to a management according to the sunrise was obtained. To reduce energy consumption related to humidity control, dehumidification with heat recovery was studied. A traditional dehumidification (ventilation and heating) was compared with dehumidification with a heat pump in tomatoes crop. In 2013, 15% energy saving was achieved with the dehumidifier and in 2014 reached 25%. No difference in plant growth, yield and fruit quality was measured.

The majority of greenhouses in northern latitudes are heated, in the winter mainly for temperature control and year round to control humidity. Heating is accepted by most organic regulations in different countries; if heating efficiently and the energy source is predominantly renewable energy, heating fits well into the concept of organic production, since it is aligned with the idea of achieving maximum potential with available resources. It is a fact that energy use for humidity control is more important than for heating. Indeed, the improved thermal performance (insulation) of high-tech greenhouses has decreased heating requirements while decreasing the discharge pathways of vapour at the same time. The need to control humidity is especially important in organic greenhouses, given the limited options to fight fungal diseases once they develop. Excess vapour can be discharged in three ways: through exchange with dry outside air (ventilation), through condensation on a cold surface and through hygroscopic adsorption. Ventilation can be uncontrolled (natural) or controlled (forced), and in the latter case can be controlled by a heat exchanger, recovering sensible heat in the ventilated air. Even then, however, the latent heat contained in the vapour (the energy used for evaporation) will be lost. In those cases where the greenhouse is dehumidified by withdrawing internal moisture, the loss of latent heat via ventilation is prevented and condensation on an internal surface recovers the latent heat. Obviously, it costs energy to cool the condensation surface and/or regenerate the hygroscopic salt. Experiments with these systems have been performed during the last years. Some growers have installed these types of systems and they have been monitored for their effect on moisture control and energy saving. The results of these experiments and model calculations to compare them are presented. In case dehumidification systems are well controlled they can save significant amounts of the energy and with an increase of technology level the saving can be improved. There is no generally best possible solution for dehumidification. The optimum system and its operation is dependent on desired temperature and humidity level in the greenhouse.

Review of the major sources for energy consumption in organic greenhouse horticulture and analyse of the options available to reduce energy consumption or, at least, increase the energy use efficiency of organic production in greenhouses. At the moment, the best way to match demand and availability of electricity (solar and wind power) is through the grid, also in view of existing subsidies for electricity sale to the grid. There is some scope for local application of solar thermal energy for heating and/or humidity management, although this requires a large investment in thermal buffering. In combined organic farms there is scope for biomass burning and/or biogas production. Technology developments ensure that currently there are commercial greenhouse operations that benefit from the use of biomass and, in some cases, biogas. Such alternative energy sources are progressively replacing traditional (and more pollutant) fuels.