Three years of research on in the greenhouses of the Innovation and Demonstration Centre (IDC) on Energy in Bleiswijk have provided a large amount of experience and performance data on low energy greenhouse cultivation. On November 7th, 2013, around 40 people, ranging from advisers, horticultural suppliers, and growers, attended the meeting where the results were presented.
Three researchers from Wageningen UR Greenhouse horticulture , Jan Janse, Frank Kempkes and Feije de Zwart, presented their findings with highly productive, but yet low energy consuming cultivations of tomatoes in the Venlow Energy Greenhouse and the Next Generation Semiclosed Greenhouse. The commercial tomato grower Ted Duijvestijn, who has been involved in the research for all these years, presented how he has used this experience in the development of his new 1 hectare greenhouse which is currently built next to his 13.5 hectares of existing greenhouses.
The Venlow Energy Kas achieves its low energy consumption by using a high insulating double glass covering and a balance ventilation system for dehumidification with a regain of ventilation energy. The Next Generation Semiclosed Greenhouse is cladded with single glass, but uses multiple screening systems to reduce heat loss. What’s more, this greenhouse is equipped with an internal dehumidification system that enables to regain most of the energy used for evaporation and dehumidification. Both greenhouses applied low temperature heating systems and diffused the solar radiation by a hazed light transmission.
Very good growing conditions
Jan Janse concluded that both greenhouses showed to be very productive. The Venlow Energy Greenhouse produced 7 to 10% more than relevant references and the Next Generation Semiclosed Greenhouse added another 8% of extra production on top of that, due to the very high light transmission of the glass applied. In all three years there were only short periods with small problems with botrytis and other diseases. At the start is was expected that tomato growing would be hampered by high insulation levels, but the experiments showed the contrary instead. Also the diffused light showed to increase production.
Frank Kempkes presented the figures on energy of both the greenhouses. In the past three years, the Venlow Energy Greenhouse showed an every year decreasing heating demand, achieved by improving the growing strategy. This year the energy consumption was only 16 m3 natural gas equivalents per m², including the energy consumption for electricity to run the fans of the balance ventilation system. This is around half the amount used by practical growers, who put a reference energy consumption of 33 m³/m² year. In winter, it was the high insulating glass of the Venlow Energy greenhouse that contributed to the savings and in summer it was the low energy growing strategy lowering the demand. The Next Generation Semiclosed Greenhouse can be operated with an energy consumption of 19 m³ of natural gas per m² per year. In this greenhouse it is the complete closing during winter and the use of summertime heat surpluses, after seasonal storage, by a heat pump in winter that realizes the 33% decrement of energy consumption. The low temperature heating systems did their job and with a 10 to 20 °C lowering of the heating water temperature needed without any adversive effects in crop response or ripening speed. With such lowering of temperature, the efficiencies of boilerhouse equipment like a boiler or a heat pump increase substantially.
Feije de Zwart listed some economic qualifications of both the greenhouses. Based on assumptions about energy prices for the near future and expected manufacture costs of the equipment applied, the Venlow Energy Greenhouse is expected to have a payback time of some 8,8 years. The Next Generation Semiclosed Greenhouse without seasonal storage has lower yearly revenues, but also lower investments, resulting in a payback time around 7,6 years. In case of the application of the full concept with seasonal heat storage, the payback time increases slightly to 8,2 years. The economic figures presented have to be treated as indications because subsidies and changing energy markets have large effects on revenues, investment costs and payback times.
Ted Duijvestijn elaborated on his learning during the frequent visits to the IDC in the past three years. This has resulted in the design of the completely new greenhouse building approach which is currently getting shape on his and his brothers premises. This new design combines the technologies tested on the IDC in the past years. The new greenhouse will be glazed by huge 2x6 meters double, tempered, diffused glass. Along the gables air treatment units will be placed for an equally distributed dehumidifation and the heating system will be comparable to the low temperature system applied in the Venlow Energy Greenhouse. Next year, Duijvesteijn and researchers will monitor the performance of this new greenhouse design and publish the results.