Conditioning in an organic greenhouse

Organic growers in the Netherlands have to cope with ambitious goals for the emission of CO2, which in practice requires that less fossil fuel is used for heating. With conditioning, the greenhouse air can be cooled and/or dehumidificated mechanically, so that excess summer heat is collected and can be stored in aquifers for reuse with the help of a heat pump in winter. The required energy can be provided through green electricity instead of fossil fuel for heating. Experiences with closed greenhouses have shown that mechanical cooling lowers the need for ventilation, thereby decreasing the CO2 emission and pest pressure. In addition, conditioning can give a more balanced indoor climate and allows for a better management of humidity, which greatly reduces the chance of outbreak of Botrytis and similar diseases. Wageningen UR Greenhouse Horticulture is carrying out a dedicated research with an organic grower who operates a conditioned greenhouse. In this project, the main objective is to find out in what way the advantages of cooling are helpful, or can be improved for organic production, in view of the very strict requirements about application of plant protection chemicals. Conditioning an organic greenhouse may give higher yield and a better control of diseases and insects, but not enough references are available to give a significant comparison. INTRODUCTION Organic greenhouses in the Netherlands emit more CO2 per unit of produce than conventional greenhouses (Kramer et al., 2000; Spruijt-Verkerke et al., 2002). Although the energy use per unit of organic produce has decreased in the last decade (Raaphorst, 2009), organic growers still need to make more of an effort to show that their produce can be more environmentally friendly than conventionally-produced flowers and vegetables. That is why BiJo, a greenhouse company growing organic vegetables in the Netherlands, started a conditioned greenhouse. With conditioning, the greenhouse air can be cooled and/or dehumidified mechanically, so that excess summer heat is collected and stored in aquifers for reuse with the help of a heat pump in winter. In this way, the required energy can be produced using green electricity instead of fossil fuel for heating. Mechanical cooling lowers the need for ventilation, thereby decreasing pest pressure. In addition, conditioning can give a more balanced indoor climate and allows for a better management of humidity, which greatly reduces the chance of outbreak of Botrytis and similar diseases. Both these factors can be particularly helpful in organic production, in view of the very strict requirements for application of plant protection chemicals. In a greenhouse where CO2 fertilization is applied, this would have the additional advantage of decreasing CO2 emissions. Wageningen UR Greenhouse Horticulture, in cooperation with GreenQ, carried out a research project with BiJo. The use of energy, the effects on the indoor climate and the impact on the produce were the main topics. a marcel.raaphorst@wur.nl Proc. First IC on Organic Greenhouse Hort. Eds.: M. Dorais and S.D. Bishop Acta Hort. 915, ISHS 2011 186 MATERIALS AND METHODS The greenhouse of BiJo consists of several sections. The smallest and newest section is a 2.5 ha venlo-type greenhouse where fruit vegetables (tomato, sweet pepper and cucumber) are grown. These crops need to be heated, and the average air temperature is maintained at 18 to 20°C. If the greenhouse air temperature increases to a level that is higher than necessary for the growth (22–27°C, depending on the global radiation rate), or if the relative humidity of the greenhouse air exceeds 85%, the greenhouse is cooled or dehumidified with an air conditioning device. This air conditioning device can also support the low temperature (40°C) heating pipes with the distribution of heat in the greenhouse. The hose pipes can be moved vertically up and down, depending on whether heating or cooling is required. The largest and oldest section is a 7 ha wide-span greenhouse, where green vegetables that do not need much heat are grown. This section can be heated with a relatively low water temperature (±20°C), using heat exchangers with fans and large hose pipes. This section cannot be cooled or dehumidified mechanically, but only uses the low temperature heat that is harvested in the conditioned section with fruit vegetables (Fig. 1). Heat and cold are provided by three heat pumps which use green electricity. A boiler that can be heated with bio-oil is installed as a backup and is used for heat supply during peak demand. CO2 for the enrichment of the greenhouse air, to improve the photosynthesis rate, is bought from a supplier because an electric heat pump does not produce any CO2. The bio-oil burner can produce CO2, but CO2 from bio-oil is more expensive and gives a higher risk of polluting the air in a semi-closed greenhouse. All the climate and energy data were collected from the climate computer (Priva Integro) and analyzed using pivot tables (Microsoft Excel). RESULTS AND DISCUSSION Conditioning had a direct impact on the CO2-concentration, the humidity and the energy use of the greenhouse. Indirectly it affected yield, quality and pest control. Temperature A 24-h average temperature that exceeds 25°C can negatively affect the anthesis of fruit vegetables like tomato (Sato and Peet, 2005). Cooling the greenhouse with an air conditioning device can avoid problems with the anthesis. However, with a moderate outdoor climate near the coast of the Netherlands and a standard unconditioned greenhouse, the 24-h average temperature rarely exceeded 25°C. In fact, the temperature of conditioned greenhouses hardly differed from unconditioned greenhouses in the Netherlands. CO2-Concentration If the greenhouse was kept closed during the night, the CO2-concentration exceeded the range of the CO2-sensor (3000 ppm). This was partly caused by the CO2 emission of the crop and mainly caused by the emission of decomposition gases from the soil and the organic fertilizers. Because very high levels of CO2 and other decomposition gases have been shown to damage the crop (de Visser et al., 2004), the vents were opened to remove these gases if the CO2-concentration was higher than 2000 ppm. In greenhouses without active cooling possibilities, the vents had to be opened during periods of high global radiation and a high outside temperature. The CO2concentration in the greenhouse could not be kept much higher than the outside concentration. Five hundred ppm is a common CO2-concentration in a greenhouse with CO2-enrichment and wide open ventilators. In a conditioned greenhouse, the ventilators rarely have to be wide open, so that the CO2-concentration in the conditioned greenhouse can increase to a level of 800–1000 ppm. This high CO2-concentration during high global radiation is supposed to give a 12% higher photosynthesis level than at a level of 500 ppm (Nederhoff, 1994).