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In order to reduce the dependency on fossil fuels, the Dutch horticultural sector puts a lot of effort in the reduction of energy demand. By using multiple thermal screens, a modest temperature regime and allowing high humidities, the energy consumption of a greenhouse can be reduced substantially, but for additional savings, a considerable increase of the insulation is needed.
In heated greenhouses, attempts have been made to produce potted plants while reducing the amount of energy used. In fact, improvements in greenhouse structure and equipment (double walls, etc.) have led to better insulation and tightness. However, this tightness induces microclimatic changes, especially on night time humidity levels. The increase of indoor humidity leads to the development of condensation and dripping and, consequently, to the increase in the occurrence of fungal diseases and physiological disorders. It is therefore necessary to decrease the humidity in the air. To do this, growers have to dehumidify the air by ventilation-heating, involving a major increase in energy consumption. Humidity control is therefore a limiting factor for saving energy, making it necessary to find alternatives to the ventilation-heating method used to reduce indoor humidity. We analyzed the effectiveness of a dehumidifying heat pump installed in a 2350-m2 double wall inflatable greenhouse located in northwestern France, during spring 2010. Hydrangeas in containers were placed on the ground (heated). The temporal evolution of climatic, thermodynamic and ecophysiological data is presented and analyzed. The thermodynamic efficiency of the machine is also taken into account. Depending on the weather outside, the energy consumption of the device (power: 4W m-2) used to dehumidify the air is three to four times lower than that which would have been used by the ventilation-heating method. By condensing approximately 30 L of water per hour during the operating phase of the heat pump, the machine balances crop evapotranspiration and avoids condensation and the occurrence of fungal diseases.
Efficient use of energy in greenhouses has been subject of research and development for decades. The final energy efficiency, e.g. the amount of energy used per unit of product, is determined by improvements in energy conversion, reductions in energy use for environmental control and the efficiency of crop production. The new European targets on reduction of CO2 emission have resulted in a renewed interest in innovative technologies to improve energy efficiency in greenhouses designed for North- as well as South European regions. In this paper an overview of the recent developments is presented from both the Northwest European as well as the Mediterranean perspective. The developments range from new modified covering materials, innovative and energy conservative climate control equipment and plant response based control systems, to integrated energy efficient greenhouse designs.
A novel model of Latent Heat Convertors (LHC) or Agam units capable to reduce the air relative humidity in the greenhouse as well as supply energy required for greenhouse heating has been developed. The structure of Agam units is based upon direct contact of air with flowing downward hygroscopic solution known as brine. Two Agam units were installed in 120m X 30m greenhouse with an additional convential house used as a control. Each of the units is composed from separate heat exchangers A and B. Following contact of the humid greenhouse air with the brine of exchanger A, the vapor is condensed on the brine. The sensible heat of the condensation heats the brine and the warm brine heats the air which is introduced back with a lower RH into the greenhouse. The warm brine accumulated in the reservuar of exchanger A, pumped on top of exchanger B is distributed downward on the exchanger. The warm brine of exchanger B exchanges heat with the cooler outdoor air, the temperature of the outdoor is elevated and the air is introduced also into the greenhouse. The brine with reduced temperature following the exchange with outdoor air is accumulated in reservuar of exchanger B and pumped on top of exchanger A. The deluted brine is concentrated in a brine evaporator heated with hot water from the greenhouse general boiler while the heat from the brine evaporator is also introduced into the greenhouse.Two Agam units were exami in spring 2001 in a 3600 m2 (120m X 30 m) greenhouse. At ambient temperature of 11°C and RH of 90%, a continuous maintanence of 18°C during 12h night the combined heat from two Agam heaters and greenhouse boiler was 963 kWh, RH of 87%-88% in comparison to 1545 kWh and RH of 90%-95% in the neighboring greenhouse with a conventional heating system.
Energy consumption for heating a separate greenhouse insulated with opaque insulating material with high reflection was reduced by 28% compared to an uninsulated single glass greenhouse. To improve light distribution across the insulated greenhouse and to obtain the same light sum in both greenhouses an algorithm for controlling the artificial light was used. The electricity consumption of the artificial light was increased by 38% in the insulated greenhouse leading to an overall reduction by 25% in energy consumption because production time was equal in both greenhouses. Dendranthema grandiflora was grown at two densities of 36 and 45 plants m-2 under short day condition. In the insulated greenhouse the plant growth was influenced by plant density and high density increase the plant height. In the uninsulated greenhouse there was a spatial variation in plant height depending of plant density and position in the greenhouse. The spatial variation in growth of Dendranthema grandiflora was lesser in the insulated greenhouse owing to the equalization of light conditions and higher canopy temperature in the insulated part of the greenhouse.
More than 90% of the Dutch greenhouse area is covered with single glass. Energy losses through the covering are high during the heating period (winter) but energy requirements are also high during the cooling period (summer) in the case of semi-closed greenhouses. Until now, light losses of insulating coverings prevented growers from using double glass or plastic film. However, increasing energy prices allow new developments. Wageningen UR Greenhouse Horticulture studied the possibilities to use modern glass coatings to increase light transmission and save energy. Several glass types (standard glass, 90+ glass, low-iron glass) were covered with different anti-reflection coatings from different producers. Double glasses were produced; their optical properties were determined. It was possible to produce double glasses with new coatings having a higher light transmission than traditional single greenhouse glass (83-85% for hemispherical (diffuse) light, compared to 82-83% for traditional single glass) and a k-value of 3.6 W m-2 K-1 (compared to7.6 W m-2 K-1 of a traditional single glass). Other double glasses were produced using a combination of anti-reflection and modern low-emission coatings, reaching an even lower k-value of ≈2.4 W m-2 K-1, however, showing a slight light loss (78.5% for hemispherical (diffuse) light). Calculations of greenhouse climate (temperature, humidity, CO2) and energy consumptions year-round were carried out with a validated dynamic climate model. Additionally the effects on tomato production (dry matter) were calculated for the different prototypes of coated and insulated glass. Double materials show the highest energy saving with 25-33%, depending on the composition but also low-emission coatings on single glass decrease the energy use with 15-20%. Economic calculations with current tomato and energy prices showed that single and double glasses with anti-reflection coating currently have the highest potential.
Energy savings have always been a major concern in the greenhouse industry, particularly for northern tier growers. Better greenhouse insulation delivers an efficient method for energy savings. The injection of dynamic liquid foam insulation between two greenhouse membranes is a technology leading to significant increase in greenhouse insulation. The objective of this paper is to demonstrate the feasibility of this technology in a pre-commercialisation environment and to present its performances in energy savings and plant growth. Laboratory testing has demonstrated that the foam produces an increasing insulation factor which is proportional to the thickness of the foam injected between the two membranes. Results recorded during the winter season include comparative studies of climate conditions and energy savings of the prototype and non-retrofitted greenhouse. Within a full season of production, it would be possible to confirm the technical feasibility of injecting and circulating liquid foam between two greenhouse film membranes. Energy savings results predicted in laboratory conditions and scientific models are confirmed by this experiment in pre-commercialisation settings. Current data, collected in winter season, suggest energy savings of over 50%. Also, the impact of the use of such technology on plant growth would be obtained with a plantation of Tomato ( Lycopersicum esculentum).
In high tech greenhouse cultivation like in The Netherlands, large amounts of fossil energy are used to optimize climate conditions like temperature and humidity. To achieve a more sustainable greenhouse horticulture, a considerable reduction in energy use is needed, but tradeoffs with production and quality are not acceptable. Therefore, we developed a novel cultivation system for tomato to meet the goal: reduction of the energy input by 40% from 1.3 to 0.75 GJ.m-2.y-1, maintaining a production level of 60 kg.m-2.y-1. This concept, the Next Generation Greenhouse Cultivation, is based on the intensified use of thermal screens combined with control of humidity, maximizing the use of the integration capacity of the crop, growing at high humidity, improved efficiency of CO2 supply by reduction of ventilation and the use of cooling combined with a heat pump and an aquifer. In this approach, the main element is the prolonged use of screens with a high insulating value (>70%) to reduce the energy demand, combined with a forced ventilation system that injects relatively dry outside air into the greenhouse. The maximal use of the integration capacity of the crop implies a strong relation between daily radiation sum and diurnal mean temperatures, and a large difference between night and day temperatures on sunny days. This reduces the energy input and ventilation rate. Growing at increased humidity, achieved by misting, reduces ventilation rates because of the increased enthalpy of the greenhouse air. When supplying CO2, reduced ventilation results in higher CO2 concentrations and hence a higher energy efficiency. Cooling can be used in combination with a heat pump and an aquifer to reduce the need for fossil fuel. This approach, in which proven technologies are combined, enables growers to implement the elements step by step, leading to an easy acceptance.
For a more sustainable greenhouse horticulture, a considerable reduction of the use of energy is needed, but trade-offs with production and quality are not acceptable. The Next Generation Greenhouse Cultivation is a concept for energy saving, consisting of modules that can be implemented step by step into practice. Use of highly insulating screens, forced ventilation and the integration capacity of a crop are the main components of the concept. To validate the results of a desk study an experiment in a greenhouse of 1000 m2 equipped according to the concept was performed. The experiment proved that it was possible to produce 69 kg.m-2 truss tomatoes with an energy demand for heating of 750 MJ.m-2. In the Netherlands, this concept is currently applied in practical tomato cultivation and in several other crops.
The goal of the Dutch horticultural sector is to build new greenhouses in 2020 without the use of fossil energy and reduce CO2-emissions with 45% compared to 1990. Until now, energy losses through the covering are reduced by closing energy screens, mainly during night time. In winter even during daytime transparent screens are closed resulting in a significant reduction of light in the greenhouse. However, even higher insulation values can be reached by insulating double covering materials. In an earlier study we showed that new developed coverings using anti-reflection coatings to increase light transmission and low-emission coatings to reduce energy losses only show minor light losses and are suitable as double glazing. One of the glasses developed in an earlier study has been integrated in a total greenhouse concept of 500 m2, realized in summer 2010 at the research station of Wageningen UR Greenhouse Horticulture in Bleiswijk. The glass used has a light transmission of 88% perpendicular (τp) and 79% hemispherical (τh) and an uvalue of 1.2 W m-2 K-1 (compared to τp=90%, τp=82%, u=6.7 W m-2 K -1 of single glass). In order to develop the optimum growing strategy scenario calculations of greenhouse climate (temperature, humidity, CO 2) and energy consumptions yearround were carried out with a validated dynamic climate model. Effects on cucumber production (dry matter) were calculated. That way the optimum growing strategy in terms of low energy consumption combined with a high cucumber yield was developed. Model calculations showed that a cucumber production of 75 kg m-2 a year should be possible with a gas consumption of 12 m3 m-2, an electricity consumption of 8 kWh m-2 and a CO2 consumption of 43 kg m-2. That way the energy consumption per kg cucumber would be reduced from 0.53 m3 gas equivalents to 0.19. A first experiment in the new built greenhouse in Bleiswijk with an autumn crop of cucumbers seems to confirm the theoretical results.
Transpiration and water uptake play an important role in the growth of horticultural crops such as tomatoes. However, the transpiration rate is affected by the humidity level in the greenhouse. High levels of humidity restrict transpiration and lead to fungal diseases resulting in yield losses. Under northern latitudes, the use of more airtight structures combined with high levels of supplementary lighting increase the humidity level inside the greenhouses. To decrease humidity, growers have to dehumidify by ventilation leading to an increase in energy consumption. However, the literature does not report on the energy consumption needed to dehumidify.To evaluate this energy consumption, we used the GX software system which simulates the heat and mass exchanges in a greenhouse as well as the behaviour of the climate-control strategy. Evapotranspiration, condensation on the cladding and infiltration and ventilation rates were taken into account for the water balance. Based on one year of climatic data, three sets of simulations were made: (1) no dehumidification; (2) dehumidification by on–off ventilation (one air change per hour); and (3) dehumidification by proportional ventilation.Simulation results indicate that for an acceptable level of humidity, within a greenhouse containing a tomato crop (corresponding to a vapour pressure deficit above 0·5 kPa), the energy consumptions with on–off ventilation and with proportional ventilation are, respectively, 12·6 and 18·4% higher than without dehumidification. Proportional ventilation was also more effective than on–off ventilation for humidity control.
In this study, an experimental dehumidifying system for greenhouses is tested. The system uses finned pipes fixed under the gutter of the greenhouse. The pipes are cooled below the dewpoint of the greenhouse air by cold water. The humid air passes the pipe and fins by natural convection and condensation occurs reducing the humidity in the greenhouse. The performance of the system in relation to its location and dimensions are studied by computational fluid dynamics calculations. The total heat transferred and condensate removed are monitored as a function of the greenhouse conditions. The system removes 40 g of condensate per hour per square metre of greenhouse floor from the humid air sufficient during periods when heating is applied and ventilation is minimized. The heat transferred at the cold surface by condensation is less than one-third of the total heat removed by the system at a relative humidity of 80%.