project - Research and innovation
Thermochemical fluids in greenhouse farming. TheGreefa
Thermochemical fluids in greenhouse farming
Objectives
Greenhouse farming and energy intensive water recovery systems need new innovative applications to reduce their energy consumption. TheGreefa has the potential to significantly reduce the energy consumption in greenhouse climate control as well as in crop drying and will provide an alternative to energy intensive water desalination in arid regions. The uptake, conversion and storage of solar heat from greenhouses even provides the perspective to turn protected intensive horticulture from an energy/water consuming to an energy/water producing method, allowing to secure the important market of food production and food processing and to extend it to new regions.
Objectives
See objectives in English
Activities
The project demonstrates the technology in two demonstrators (Switzerland and Tunisia) and prepare a path to the market. Lab tests explore the processes and materials involved. Development of improved knowledge on modelling of the involved processes, the simulation and control of specific applications and the development of control strategies are further activities to provide a bright insight into the novel approach. Economic and environmental assessments improve the economic and environmental potential of the technology by reducing costs and increasing energy efficiency and allow to develop strategies to bring the technology to market as well as appropriate policies.
Project details
- Main funding source
- Horizon 2020 (EU Research and Innovation Programme)
- Horizon Project Type
- Multi-actor project
€ 4651865
Total budget
Total contributions including EU funding.
10 Practice Abstracts
The overall Greenhouse system model of TheGreefa is based on an innovative use of absorption processes in the greenhouse air-conditioning (also referred as sorptive air conditioning). The energy carrier is an aqueous magnesium chloride solution (MgCl2). To predict and check the behaviour, energy efficiency and further properties of systems using the technology, the use of simulation is inevitable. Therefore, methods for transient simulation of a node-based model are being developed. This system simulation is sufficient for most of practical problems in this project, such as the development and testing of smart control strategies. The aim is to maximize energy efficiency, crop production and water production, and take full advantage of fluctuating renewable energy, by reasonable and intelligent control of variables, such as indoor/outdoor temperature and humidity, etc. For this purpose, the simulation of temperatures and humidity in the greenhouse and its control by the new technology is crucial. The Modelica library of developed component models has already been developed and validated, which includes the absorber model, desorber model and thermal chemical fluid network model. Besides, the CFD simulation is considered, if air conditions in the green house are not constant and there are differences of conditions in greenhouse and at absorber inlet. If necessary, in order to integrate the CFD model and system model, the reduced order model of CFD model will be developed to increase the computational efficiency.
Das Gesamtsystemmodell Gewächshaus von TheGreefa basiert auf einer innovativen Nutzung von Absorptionsprozessen in der Gewächshausklimatisierung. Die Energie wird durch eine wässrige Magnesiumchloridlösung (MgCl2) transportiert. Um das Verhalten, die Energieeffizienz und weitere Eigenschaften von Systemen mit Hilfe der Technologie vorherzusagen und zu überprüfen, ist der Einsatz von Simulation unumgänglich. Daher werden Methoden zur transienten Simulation eines knotenbasierten Modells entwickelt. Diese Systemsimulation reicht für die meisten praktischen Probleme in diesem Projekt aus, wie z. B. die Entwicklung und Erprobung intelligenter Steuerungsstrategien. Ziel ist es, die Energieeffizienz, die Pflanzenproduktion und die Wasserproduktion zu maximieren und die schwankenden erneuerbaren Energien voll auszunutzen, indem Variablen wie Innen-/Außentemperatur und Luftfeuchtigkeit usw. sinnvoll und intelligent gesteuert werden. Zu diesem Zweck die Simulation von Temperaturen und Luftfeuchtigkeit im Gewächshaus und deren Kontrolle durch die neue Technologie entscheidend. Die Modelica-Bibliothek entwickelter Komponentenmodelle wurde bereits entwickelt und validiert, darunter das Absorbermodell, das Desorbermodell und das Modell des thermisch-chemischen Flüssigkeitsnetzwerks. Außerdem wird die CFD-Simulation berücksichtigt, wenn die Luftbedingungen im Gewächshaus nicht konstant sind und es unterschiedliche Bedingungen im Gewächshaus und am Absorbereintritt gibt. Um das CFD-Modell und das Systemmodell zu integrieren, wird bei Bedarf das Modell reduzierter Ordnung des CFD-Modells entwickelt, um die Berechnungseffizienz zu erhöhen.
The stakes of agriculture (and horticulture) are well-known: produce more on less surface, with a smaller environmental and energetic footprint, a better quality and a broader social involvement, considering sustainability in all its aspects. To achieve those challenges the symbiosis between nature and technology must be total to get the best from both worlds.
TheGreefa H2020 project embodies this union with an all-in-one technological package through a single process powered by low heat renewable energies. Moreover, the project deals with pre (heating, cooling and humidity control) but also post harvesting technologies, such as water recovery or food drying process.
The three innovative integrated solutions developed in the project to address temperature control, humidity management and water conservation in enclosed greenhouses are not limited to indoor agriculture. In fact, these technologies could be applied to any enclosed environment requiring de-/humidification and temperature control. For this reason, other sectors that make extensive use of HVAC systems have been targeted such as building construction with air control.
Air conditioning is energy intensive and polluting, it contributes to global warming because of leaks or poor recycling of refrigerants, which are powerful greenhouse gases.
All sectors combined, TheGreefa could allow a significant reduction of the carbon footprint and the struggle against global warming. Its energy savings (gas, oil), its circularity (water), its use of solar heat (green energy) and ambient heat, translate into a low environmental impact.
Les enjeux de l'agriculture sont connus: produire plus sur moins de surface, avec une empreinte environnementale et énergétique moindre, une meilleure qualité et une implication sociale plus large, en considérant la durabilité sous tous ses aspects. Pour relever ces défis, la symbiose entre la nature et la technologie doit être totale pour tirer profit du meilleur des deux mondes.TheGreefa incarne cette union avec un ensemble technologique tout-en-un à travers un processus unique alimenté par des énergies renouvelables à faible chaleur. De plus, le projet traite des technologies de pré(chauffage, refroidissement, contrôle de l' humidité) mais aussi post-récolte, comme la récupération d'eau ou le processus de séchage des aliments.Les 3 solutions innovantes développées dans le cadre du projet pour contrôler la température, la gestion de l'humidité et l'économie d'eau dans les serres ne sont pas limitées à l'agriculture d'intérieur. En fait, ces technologies pourraient s'appliquer à tout environnement fermé nécessitant une déshumidification et un contrôle de la température. D'autres secteurs utilisant les systèmes CVC ont donc été ciblés, tels que la les bâtiments avec contrôle de l'air. La climatisation est énergivore et polluante, elle contribue au réchauffement climatique à cause des fuites ou du mauvais recyclage des fluides frigorigènes,puissants gaz à effet de serre.Tous secteurs confondus,TheGreefa pourrait permettre une diminution significative de l'empreinte carbone et la lutte contre le réchauffement climatique. Ses économies d'énergie (gaz, pétrole), sa circularité (eau), son utilisation de la chaleur solaire (énergies vertes) et de la chaleur ambiante, se traduisent par son faible impact environnemental.
High product quality is one of the key issues in food drying industries. During the process, food structure, appearance and aroma can change through the modification of important bioactive constituents and the nutrients can deteriorate.
The selection of parameters, as temperature and duration, influences the quality of dried food. Some agricultural goods like herbs and fruits must be dried immediately after harvesting, in order to avoid their deterioration and rotting. To guarantee the quality, many agricultural drying processes are preferably operated at low temperatures (e.g. max 35°C). The used of hygroscopic fluid salt solutions (called thermo-chemical fluid, TCF) allows to reduce the absolute air humidity of the drying air, without direct use of thermal energy, but by adsorption of water vapour of the air. The temperature of the drying air is not so increased and the drying process can take places at low temperature. During this process the TCF becomes diluted and when its water content is too high, it loses its hygroscopic properties. To be reused, the TCF shall be regenerated evaporating part of its water content. This regeneration process needs heat, temperatures around 40°C-70°C are enough: solar heat or waste heat, otherwise not used, fulfil this purpose. The regenerated TCF can be then store without any energy losses, and without time limitation. In this way, the drying process can be driven by renewable energy stored in form of regenerated TCF, independently from fluctuation or periodically availability of the renewable energy source. The quote of usage of renewable energy can be increased in an economically way. The absorption-based drying systems are investigated in TheGreefa and in SONITRO (Swiss Federal Office for Energy).
La qualità del prodotto è una fondamentale nelle industrie di essiccazione degli alimenti. Durante l'essicatura, la struttura, l'aspetto e l'aroma degli alimenti possono cambiare per la modifica di costituenti bioattivi e le sostanze nutritive deteriorarsi.
La selezione dei parametri, quali la temperatura e la durata, influenza la qualità degli alimenti essiccati. Alcuni prodotti come erbe e frutta devono essere essiccati subito dopo la raccolta, per evitare che si deteriorino. Per garantire la qualità, molti processi devono avvenire a bassa temperatura (max 35°C). L'uso di soluzioni saline igroscopiche (dette fluido termochimico, TCF) consente di ridurre l'umidità assoluta dell'aria di essicatura, senza impiego diretto di energia termica, ma mediante assorbimento del vapore acqua. La temperatura dell'aria di essicuratura non è così aumentata e il processo di essicatura può avvenire a temperatura neutra. Durante questo processo il TCF si diluisce e quando il suo contenuto di acqua è troppo alto, perde le sue proprietà igroscopiche. Per essere riutilizzato, il TCF deve essere rigenerato evaporando parte del suo contenuto d'acqua. Questo processo di rigenerazione necessita calore, temperature intorno ai 40-70°C sono sufficienti: il solare termico o il calore di scarto, altrimenti non utilizzati, soddisfano questo scopo. Il TCF rigenerato può essere stoccato senza perdite energetiche e senza limiti di tempo: l'energia usata nell'essicazione è energia rinnovabile che è stata stoccata in forma di TCF rigenerato, indipendentemente dalla fluttuazione o dalla periodicità della fonte di energia stessa. I sistemi di essiccazione ad assorbimento sono studiati in TheGreefa e in SONITRO (Ufficio Federale Svizzero dell'Energia).
2022 has come loaded with new realities.
We already had great challenges to face in Europe: Reduction of around 55% in greenhouse gas emissions by 2030, Climate-neutral by 2050 ; Paris Agreement objective to keep the global temperature increase to well below 2°C ; European Green Deal and inside Farm to fork Strategy.
In addition, the instability in the supply of gas due to the war in Ukraine, the dependence on countries outside the EU and the increase in the frequency and intensity of extreme weather events.In a shifting geopolitical environment, the EU needs to continue strengthening its resilience and open strategic autonomy in critical sectors linked to the transitions. In the energy sector, intensified efforts are needed on green energy sources, replacing our reliance on fossil fuels.
In this context, TheGreefa is a technology that uses an inexpensive material and contributes both the reduction of emissions gases and the use of renewable energy. TheGreefa takes advantage of solar energy and waste heat to achieve a level of cooling, air humidity control and water recovery using salt water.
Essentially it is a process that takes advantage of the source of solar thermal energy in greenhouses, and the water condensation that occurs inside it to recreate an ideal atmosphere for crops at the temperature and humidity level, through a technological innovation BAT-NEC, for greenhouse farming.
In this way, TheGreefa contributes against climate change, in an elementary model of circular economy and renewable energy sources.
Solar thermal technologies and other renewable energies benefit from several laws and decrees which facilitate their installation or financially support investors.
El 2022 ha llegado cargado de nuevas realidades.
Ya teníamos grandes retos que afrontar en Europa: Reducción de alrededor del 55% en las emisiones de gases de efecto invernadero para 2030, Neutralidad climática para 2050, El objetivo del Acuerdo de París de mantener el aumento de la temperatura global muy por debajo de los 2°C, Pacto Verde Europeo y estrategia interna de "Farm to Fork".
A ello se suma la inestabilidad en el suministro de gas debido a la guerra de Ucrania, la dependencia de países extracomunitarios y el aumento de los fenómenos meteorológicos extremos.
En un entorno geopolítico cambiante, la UE necesita seguir fortaleciendo su resiliencia y abrir su autonomía estratégica en sectores críticos vinculados a las transiciones. En el sector energético, se necesitan esfuerzos intensificados en fuentes de energía verde, reemplazando nuestra dependencia de los combustibles fósiles.
TheGreefa es una tecnología que utiliza un material económico (agua salada) y contribuye tanto a la reducción de emisiones de gases como al uso de energías renovables. TheGreefa aprovecha la energía solar y el calor residual para mantener la temperatura del invernadero y también logra un nivel de refrigeración, control de la humedad del aire y recuperación de agua mediante conversión termoquímica.
Esencialmente es un proceso que aprovecha la fuente de energía solar térmica en los invernaderos, y la condensación del agua que se produce en su interior para recrear un ambiente ideal para los cultivos a nivel de temperatura y humedad, a través de una innovación tecnológica BAT-NEC, para invernadero agricultura.
De esta manera, TheGreefa contribuye contra el cambio climático, en un modelo elemental de economía circular y fuentes de energía renovables.
Tunnel greenhouses are simple constructions for protected crop production used all over the world. They can be covered by just one sheet of plastic foil, allowing to create a sealed surface without complicated means of construction. A modified version of a tunnel greenhouse is developed in The Greefa, allowing to be used to effectively create a closed atmosphere, by only using one piece of foil from one side to the other at a length of an entire roll of foil. By the reduction of foil connectors, losses of (elevated) CO2 and water vapor from the closed atmosphere are effectively reduced. For climate control in a closed greenhouse, a huge roof surface area is required, as all heat entering as solar radiation needs to be withdrawn by heat conduction within a 24 hours period. An increased surface is realized by a zig-zag structure between high points along construction bows and low points along tension belts. By using this structure, the surface area can be increased by a factor of 2-3, also allowing to withdraw 200-300% of heat, compared to a standard design. A secondary advantage relates to the strong slope between the high points and the low points, allowing to collect condensation droplets from the roof area during nighttime. This hinders a fallback of the droplets onto the vegetation, which would cause hygiene problems, mainly related to fungal growth on the leaves. The construction of a closed greenhouse can be provided by this design at low cost. The cost can even be lower than a standard tunnel greenhouse, as no ventilation flaps and openings have to be included.
Tunnel Gewächshäuser sind einfache Konstruktionen. Sie können mit nur einem Stück Plastikfolie abgedeckt werden, so dass eine geschlossene Oberfläche geschaffen wird. The Greefa entwickelte eine modifizierte Version eines Tunnelgewächshauses und schafft eine geschlossene Atmosphäre, indem nur ein Stück Folie von einer Seite zur anderen verwendet wird, und zwar bis zu der Länge einer ganzen Folienrolle. Durch die Reduzierung der Folienanschlüsse werden die Verluste von CO2 und Wasserdampf aus der geschlossenen Atmosphäre wirksam reduziert. Für die Klimatisierung eines geschlossenen Gewächshauses ist eine große Dachfläche erforderlich, da die gesamte, durch Sonneneinstrahlung eintretende Wärme innerhalb eines Zeitraums von 24 Stunden durch Wärmeleitung abgeführt werden muss. Eine vergrößerte Oberfläche wird hierbei durch eine Zick-Zack-Struktur zwischen Hochpunkten entlang von Konstruktionsbögen und Tiefpunkten entlang von Spanngurten realisiert. Durch diese Struktur kann die Oberfläche um den Faktor 2-3 vergrößert werden, so dass im Vergleich zu einer Standardkonstruktion 200-300% der Wärme abgeführt werden kann. Ein weiterer Vorteil ist das starke Gefälle zwischen den Hoch- und Tiefpunkten, das es ermöglicht, nachts Kondensationstropfen aus dem Dachbereich aufzufangen. Dies verhindert ein Zurückfallen der Tropfen auf die Vegetation, was zu hygienischen Problemen führt, vor allem im Zusammenhang mit dem Pilzwachstum auf den Blättern. Der Bau eines geschlossenen Gewächshauses ist mit dieser Konstruktion zu geringen Kosten möglich. Sie können sogar niedriger sein als bei einem Standard-Tunnelgewächshaus, da keine Lüftungsklappen und -öffnungen eingebaut werden müssen.
A closed greenhouse qualifies for plant production at elevated CO2 levels with the advantage of enhanced photosynthesis and increased production rates. In a normal greenhouse, cooling is mainly provided by the evaporative cooling of the plants and withdrawal of humid/hot air combined with supply of dryer and colder ambient air. In a closed greenhouse, cooling works totally different. An increased greenhouse surface allows to withdraw heat by conduction from hot greenhouse air through the surface to the colder ambient air without air exchange between inside and outside. A large portion of heat needs to be stored from daytime to nighttime, to compensate the lower cooling capacity of the heat conduction process and to use the heat conduction from inside to outside within all 24 hours of a day. Thermochemical solutions allow to uptake a huge amount of heat during the hot period of the day by using the phase change between water vapor and water. Evaporative cooling of the plants is combined with the absorption process, which allows air de-humidification in the greenhouse atmosphere and heat transport from the air to a solution storage. Heat and water from the air is captured and is released back to the greenhouse volume during nighttime. In this second period, greenhouse air is heated. The hot air remains at moderate relative humidity and is distributed between the vegetation, hindering condensation in this area, while the cold greenhouse surface forces condensation and allows water recovery. Condensation droplets can be captured by a specific design of the roof area. Up to 85% of the irrigation water can be recycled. The desiccant is re-generated and cooled in this process which qualifies for the next daytime period.
Ein geschlossenes Gewächshaus hat einen erhöhten CO2-Gehalt, d.h eine verbesserte Photosynthese und Produktionsraten. In einem normalen Gewächshaus erfolgt die Kühlung hauptsächlich durch die Verdunstungskühlung der Pflanzen und Luftaustausch. In einem geschlossenen Gewächshaus funktioniert die Kühlung anders. Eine vergrößerte Gewächshausoberfläche ermöglicht eine Kühlung durch Wärmeleitung von der heißen Gewächshausluft durch die Oberfläche an die kältere Umgebungsluft ohne Luftaustausch. Ein Teil der Wärme muss zwischen Tag- und Nachtbetrieb gespeichert werden, um die geringere Kühlleistung des Wärmeleitungsprozesses zu kompensieren und die Wärmeleitung von innen nach außen über die vollen 24 Stunden zu nutzen: durch den Phasenwechsel zwischen Wasserdampf und Wasser absorbieren thermochemische Lösungen Wärmemenge während der heißen Tageszeit. Die Verdunstungskühlung der Pflanzen wird mit dem Absorptionsprozess kombiniert, der die Entfeuchtung der Luft und den Wärmetransport aus der Luft in einen Lösungsspeicher ermöglicht. Wärme und Wasser aus der Luft werden aufgefangen und während der Nacht wieder an das Gewächshaus abgegeben. In der Nacht wird die Gewächshausluft erwärmt und bei mäßiger relativer Luftfeuchtigkeit zwischen der Vegetation verteilt, wodurch die Kondensation in diesem Bereich verhindert wird. Auf der kalten Gewächshausoberfläche wird Kondensation erzwungen und so eine Wasserrückgewinnung ermöglicht. Durch eine gezielte Gestaltung der Dachfläche kann Kondenswasser aufgefangen werden, bis zu 85% des Bewässerungswassers können wiederverwendet werden. Das Trockenmittel wird in diesem Prozess neu regeneriert und gekühlt, was Voraussetzung für die nächste Tagesperiode ist.
Thermochemical solutions allow to uptake water from the air without additional mechanical cooling. Depending on the specific liquid desiccant (usually solutions of salt and water), air can be dehumidified to a level between 35% (MgCl2) to 10% (LiCl2) relative humidity. This means that air with humidity beyond these values can be captured for the purpose of water production.
If ambient air relative humidity is above these values, humidity can be captured within the solution, while also the released heat from the phase change energy can be stored. Water and heat can be captured e.g., from daytime to nighttime. During night, the heat can be used to evaporate the water again back to the air (desiccant de-sorption process) and within a second process, the water can be captured within a condensation process.
As nighttime temperatures are usually lower, the process can potentially be driven without mechanical cooling, but passively by the cool of ambient air. This process only works at specific climatic conditions of daytime and nighttime temperature and air humidity. To extend toward a universal solution, the desiccant can be heated during daytime by a solar thermal collector and can be further cooled (after the de-sorption process) during nighttime within the same collector, working as a sky radiator then.
A further variation relates to a process, in which the desiccant absorbs water during nighttime, using the higher relative humidity during this period. Directly after the absorption, the solution is heated well above the ambient temperature by solar heat stored from daytime to nighttime, allowing to evaporate the water in a parallel process, again using the condensation driven by low nighttime temperatures.
Thermochemische Lösungen ermöglichen die Aufnahme von Wasser aus der Luft ohne zusätzliche mechanische Kühlung. Flüssigen Trockenmittel (Salz und Wasser) kann die relative Luftfeuchtigkeit auf einen Wert zwischen 35% (MgCl2) und 10% (LiCl2) reduzieren. Das bedeutet, dass Luft mit einr Luftfeuchtigkeit, die über diesen Werten liegt, für die Wasserproduktion in Frage kommt: die Feuchtigkeit in der Lösung kann aufgefangen werden, während gleichzeitig die freigesetzte Wärme aus der Phasenwechselenergie gespeichert werden kann. Wasser und Wärme können z. B. zwischen Tag und Nacht angereichert werden. Während der Nacht kann die Wärme genutzt werden, um das Wasser wieder zurück an die Luft zu verdampfen (Desorptionsprozess). In einem zweiten Ablauf kann das Wasser durch Kondensation aufgefangen werden.
Da die Temperaturen in der Nacht in der Regel niedriger sind, kann der Prozess unter bestimmten Bedingungen passiv durch die kühle Umgebungsluft betrieben werden. Im Sinne einer universellen Lösung kann das Trockenmittel tagsüber durch einen solarthermischen Kollektor erwärmt und nachts (nach dem De-Sorptionsprozess) im selben Kollektor weiter abgekühlt werden, wobei der Kollektor dann als Himmelsstrahler fungiert.
Eine weitere Variante ist ein Verfahren, bei dem das Trockenmittel während der Nacht Wasser absorbiert und dabei die meist wesentlich höhere relative Luftfeuchtigkeit während dieser Zeit nutzt. Unmittelbar nach der Absorption wird die Lösung durch vom Tag bis zur Nacht gespeicherter Sonnenwärme weit über die Umgebungstemperatur hinaus erwärmt, so dass das Wasser in einem parallelen Prozess verdampft werden kann, wobei wiederum die durch die niedrigen Nachttemperaturen bedingte Kondensation genutzt wird.
Crops transpiration produces water vapour inside greenhouses which needs to be removed to maintain suitable humidity. In continental climate, the excess humidity is removed venting the indoor air opening windows and simultaneously is heated to compensate the heat losses due to the venting and to reduce relative humidity increasing air temperature, but without modify absolute humidity. This system is very energy intensive.
The use of hygroscopic salt solutions (called thermo-chemical fluid, TCF) allows to reduce the energy consumption:
1) the TCF reduces the absolute air humidity absorbing the water vapour of the air. There will be a zeroing of the heat loss for ventilation because the air is recirculated, and the humidity removed by the TCF;
2) furthermore, during this absorption process the water vapour is converted in water liquid releasing heat used for heating the greenhouse.
A further advantage of the TCF’s use is the control of the air temperature independently from the control of the air humidity: the temperature of the TCF determinates the air temperature, while its content of salt (concentration of TCF) determinates the air humidity
Different TCFs are available, the main aspects to be considered are their hygroscopic properties, cost, availability, crystallisation point and toxicity. The main candidate for greenhouse air conditioning is the aqueous magnesium chloride solution (MgCl2); its hygroscopic potential allows to dehumidify the air down to 30% relative humidity at 20°C. The calcium chloride solution (CaCl2) has similar properties, but a more complicated production process. The magnesium chloride solution has been used in TheGreefa project (better performance/cost ratio) for the air control in the greenhouses.
La traspirazione delle piante produce vapore acqueo all'interno delle serre che deve essere rimosso per mantenere un'umidità adeguata. In clima continentale, l'umidità in eccesso viene rimossa ventilando l'aria aprendo le finestre e contemporaneamente viene riscaldata per compensare le dispersioni di calore e per ridurre l'umidità relativa aumentando la temperatura dell'aria.. Questo sistema è molto energivoro.
L'utilizzo di soluzioni saline igroscopiche (dette fluido termochimico, TCF) permette di ridurre i consumi energetici:
1) il TCF riduce l'umidità assoluta dell'aria assorbendone il vapore acqueo.La dispersione termica per la ventilazione è azzerata perché l'aria viene ricircolata,e l'umidità rimossa dal TCF;
2) durante questo processo di assorbimento il vapore acqueo viene convertito in acqua cedendo calore utilizzato per il riscaldamento della serra.
Un ulteriore vantaggio è il controllo della temperatura dell'aria indipendentemente dal controllo dell'umidità: la temperatura del TCF determina la temperatura dell'aria, mentre il suo contenuto di sale determina l'umidità.
Diversi TCF possono essere selezionati in base alle loro proprietà igroscopiche, il costo, la disponibilità, il punto di cristallizzazione e la tossicità. Il principale candidato per il condizionamento dell'aria in serra è la soluzione di cloruro di magnesio (MgCl2); il suo potenziale igroscopico consente di deumidificare l'aria fino al 30% di umidità relativa a 20°C. La soluzione di cloruro di calcio (CaCl2) ha proprietà simili, ma un processo di produzione più complicato. La soluzione di cloruro di magnesio è stata utilizzata nel progetto TheGreefa (miglior rapporto prestazioni/costo) per il controllo dell'aria nelle serre.
Crops transpiration produce water vapour inside greenhouses which need to be dehumidified to maintain suitable humidity. The main humidity control methods used in greenhouses are ventilation, heating, condensation on cold surfaces and adsorption by hygroscopic materials. In greenhouses of warm regions, as Mediterranean countries, ventilation is the most common method used due to its low cost. In colder climates, as Continental Europe, heating systems are used to reduce relative humidity increasing air temperature, but without modify absolute humidity.
The use of heat exchanger with cold surfaces for dehumidification allow to capture and re-use the latent heat released in condensation. A heat pump can be used as a dehumidifier to prevent condensation on the crop. In cold areas, the use of a heat pump can reduce 3-8 times the energy consumed by venting-heating dehumidification and to increase 2–3 times the coefficient of performance of conventional air-conditioning systems.
The use of hygroscopic fluid salt solutions (called thermo-chemical fluid, TCF) also allows to reduce absolute air humidity by adsorption of vapour and convert the latent heat of condensation to sensible heat used for heating the greenhouse. The difference of absolute humidity between air inside the greenhouse and outside is the main factor affecting the desiccant dehumidification. Water vapour adsorption onto silica-gel, activated carbon powder and activated carbon fibre can be used for climate control with solar operated air-conditioning system 5. An aqueous magnesium chloride solution (MgCl2) has been used in TheGreefa project (better performance/cost ratio) for the air control in the greenhouses.
La transpiración produce vapor de agua dentro de los invernaderos que se tienen que deshumidificar para mantener una humedad adecuada. Los principales métodos de control de humedad en invernaderos son la ventilación, la calefacción, la condensación en superficies frías y la adsorción mediante materiales higroscópicos. En invernaderos de regiones cálidas la ventilación es el método más usado por su bajo coste. En climas fríos la calefacción se utilizan para reducir la humedad relativa aumentando la temperatura del aire, pero sin modificar la humedad absoluta.
El uso de intercambiadores de calor con superficies frías para la deshumidificación permite capturar y reutilizar el calor latente liberado en la condensación. Se puede utilizar una bomba de calor como deshumidificador para evitar la condensación sobre el cultivo. En zonas frías, el uso de una bomba de calor puede reducir 3-8 veces la energía consumida por la deshumidificación con ventilación-calefacción y aumentar 2-3 veces el rendimiento de los sistemas de climatización convencionales.
El uso de soluciones salinas higroscópicas (llamadas fluidos termoquímicos) también permite reducir la humedad absoluta del aire por adsorción de vapor y convertir el calor latente de condensación en sensible y utilizarlo para calentar el invernadero. La diferencia de humedad entre el interior y el exterior es el principal factor que afecta a la deshumidificación con desecantes. La adsorción de vapor de agua sobre gel de sílice, polvo de carbón activado y fibra de carbón activado se puede utilizar para climatización basada en energía solar. En el proyecto TheGreefa se ha utilizado una solución acuosa de cloruro de magnesio (MgCl2) (mejor relación rendimiento/coste) para el control climático.
Air humidity is an important factor in the greenhouse climate as it affects the processes of transpiration and photosynthesis, and can cause the development of fungal diseases. In crops with commercial leaf value, such as lettuce and ornamental plants, an increase in humidity can contribute to the loss of production, quality and commercial value.
One of the main reasons for controlling humidity in greenhouses is to avoid the incidence of fungal diseases.
Under unsuitable humidity conditions the growth of some crops can decrease, anatomical changes and alterations or delays in the development of plants can occur.
A moderate humidity (55-75%) allows to increase the rate of net assimilation of plants due to the rise in stomatal conductance that facilitates the processes of exchange of water vapour (transpiration) and CO2 (photosynthesis) between plants and air.
A high humidity (75-95%) can produce beneficial effects, such as an increase in the individual surface of the leaves, although it can also cause adverse effects on flowering, setting and fruit growth of crops such as pepper. Relative humidity between 50-70% is considered optimal for tomato pollination, since high values close to 90% can decrease the viability of pollen due to thermal stress.
La humedad del aire es un factor importante en el clima del invernadero ya que afecta a los procesos de transpiración y fotosíntesis, y puede favorecer el desarrollo de enfermedades fúngicas. En cultivos con valor comercial de las hojas, como lechuga y ornamentales, un aumento de la humedad puede contribuir a la pérdida de producción, calidad y valor comercial.
Una de las principales razones para el control de la humedad en los invernaderos es evitar la incidencia de enfermedades fúngicas.
Bajo condiciones no adecuadas de humedad el crecimiento de algunos cultivos puede disminuir, se pueden producir cambios anatómicos y alteraciones o retrasos en el desarrollo de las plantas. Una humedad relativamente baja (55-75%) permite aumentar la tasa de asimilación neta de las plantas debido al incremento de la conductancia estomática que facilita los procesos de intercambio de vapor de agua (transpiración) y CO2 (fotosíntesis) entre las plantas y el aire.
Una humedad alta (75-95%) puede producir efectos beneficiosos, como un incremento de la superficie individual de las hojas, aunque también puede originar efectos adversos sobre la floración, el cuajado y el crecimiento de frutos de cultivos como el pimiento. Una humedad relativa del 50-70% se considera óptima para la polinización del tomate, ya que altos valores cercanos al 90% pueden disminuir la viabilidad del polen por estrés térmico.
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