MANAGEMENT AND FARMING SYSTEMS OF ROOFTOP GREENHOUSES (RTG)
Guillaume Morel-Chevillet (ASTREDHOR), Veronica Arcas (UAB)
Growing plants on a rooftop implies in most cases using soilless growing techniques, from substrate-based solutions up to hydroponics, aeroponics and aquaponics systems.
As explained in the diagram on the left, choices are mainly guided by project objectives, yield expectations, the roof bearing capacity, and the technical skills of the team.
SUBSTRATE-BASED PRODUCTION
Using a substrate-based production is the easiest way to grow plants. Depending on the substrate used this system might differ more or less from a hydroponic system, needing additional fertilization through irrigation or, on the other hand using soil and natural occurring structure and nutrients. With the use of soil, a larger reservoir of organic nutrients can be obtained effortlessly, while most substrates also will serve as good water storages needing less stable irrigation conditions. Nevertheless, it requires a specific quality of material in order to get a good agronomic growing medium (nutrient, structure, water retention, etc.), a mixture of raw materials, like the ones utilized as media in green roofs. Drip irrigation systems and a supply of slowly released fertilization is necessary if good yield is expected. As well a good water flow to reduce possible nutrient accumulation and greater salinity if nutrients are given through irrigation. The total substrate weight, when saturated with water, is a critical data point and needs to be known prior starting the project because generating loads between 200 to 400 kg/m². Moreover, after one or two growing seasons, a part of the substrate needs to be renewed (settling, loss of nutrient, etc.). Expected yield is less than 5 kg/m²/y for common vegetables. Nevertheless, using a substrate-based production would require less monitoring so less working force. It can be a good solution to create its own mixture of material from urban wastes for e.g. Prices are relatively low, between 10 € to 20 € per m², depending of the equipment (irrigation and fertilization). Possible substrates can be divided into two categories: organic and inorganic. The first category entails all substrates generated form organic materials like compost, coco fiber, pine bark, sawdust and peat moss. The second category can be again divided into inorganic media from natural sources, like sand, gravel, volcanic tuff, rock wool fiber, expanded clay and perlite. Or can be generated in a synthetic manner like foam or plastic.
Substrate based solutions on container can provide secure and sucessfull yields on lettuces, e.g., of the JB Hydroponics systems tested in ASTREDHOR Grand Est Station. Credits: Solène Batard.
HYDROPONIC/ AEROPONIC PRODUCTION
Hydroponics soilless growing systems recirculate nutrient solutions based on synthetic and mineral inputs (organic fertilizer remain a rare exception). It is important to have a great knowledge on the nutritional requirements of the crops as well as the capacity to routinely be able to check for the incoming and outgoing nutrients. This is even more relevant in case of water and nutrient recirculation. There are different forms of hydroponic systems:
The Flood-and-Drain system where plants are grown in aggregate-filled containers placed in watering beds.
The Drip-irrigation system commonly used to grow vine crops such as tomatoes, cucumbers or peppers. Plants sit in growth-medium-filled containers and nutrient solution drips onto the plant’s root ball.
The raft culture or deep-water culture which involves the submersion of plant roots in a nutrient bed solution.
The Nutrient Film Technique (NFT), one of the most productive and frequently employed technics for leafy greens. Plants roots are suspended in a long narrow trough through which the nutrient solution trickles.
Hydroponic techniques provide good yield and are lightweight. They are also adapted to many plant species, e.g. GHE systems on lettuce, chard and peppers. Photo credits: Solène Batard.
While in hydroponics systems the plant root supply is based on fertilized water, in aeroponic they get water and nutrient from a fog. Both systems are lightweight (less than 50 kg/m²) and provide high yields, between 30 to 70 kg/m²/y depending of the plants species. Water consumption is also very low in comparison to substrate-based solutions. Investment cost is about 50 € up to 200 € (depending of the equipment quality and the installation procedure). Maintenance requires a good agronomical skill and in case of technical problem (such as power cut) the production can be quickly damage because of the lower resilience of this system. This lower resilience is due to the lack of any substrate or matter capable of holding water and nutrients in case of a lack of irrigation.
AQUAPONIC PRODUCTION
Aquaponics systems combine the benefits of highly controlled hydroponic systems with the qualities that natural, organic growing systems offer. A closed-loop nutrient system replaces the mineral and synthetic fertilizer use, with the self-fertilization based on fish wastes decomposed by bacteria. This system needs specific skills balanced between plants and fish production due to the complex systemic interactions. The fish storage requires at least 800 kg/m², whereas plant production bearing and the yields (30 to 70 kg/m²/y) are like the hydroponics system. Added up to the yield of crops, fish production can be implemented into the business plan as well. Cost for plants production are like hydroponic ones, but fish production required a specific investment (between 800 to 1 200 € / m²) and a good mastery about fish production.
Aquaponic production includes fish production and plant cultivation. It can provide multiple incomes for urban farmers with high technical skills. Here: APIVA (Aquaponie, Innovation Végétale et Aquaculture) pilot system in Astredhor - Aura station. Photo credit: Guillaume Morel-Chevillet.
CROP SELECTION
Plant choices must be done carefully in respect to the local market, consumer expectations, agronomic conditions, growing systems and technical skills. Plants cultivated for urban agriculture on RTG purpose can be divided like this:
Microgreens, leafy greens and aromatics
This range of plants is focused on leaves production (not flowers or fruits).
The first leaves harvesting, called “Microgreens production”, is a niche market. A large spectrum of species can be grown such as sunflower, beans, wasabi or mustard and the production circle is short (from 10 to 20 days).
Leafy greens are also often grown by urban farmers because of the short expiry date and the freshness needs by the market. Lettuce, Kale cabbage, spinach or arugula are the most famous ones. The production schedule is likewise short (less than one month) and the same plants can be harvested a few times.
Aromatic plants cover a large selection of the harvest. From the most common ones like basil, mint or Persil to the more original ones such as the Ice plant, Artemisia or Agastache. They can be sold fresh (cuttings leaves) or in small pots.
These products are grown is many urban farms because of their ultra-fresh quality and their high high-value prices (between 10 € up to 70 € / kg for micro-green for example). Under certain circumstances these aromatic plants may receive a BIO label which can lead to a higher economic value on the market.
Fruit vegetables (tomatoes, cucumbers, peppers, vine crops...)
Fruit vegetables are more complex to grow. They need more time and more maintenance, especially as regards fertilisation, but the final product (their fruit) is easily saleable and expected by customers. One of the main current challenges is finding species combining good yield with original colours and flavour. Old varieties or new hybrids are grown for this purpose, but most of them have not been tested yet in soilless systems.
Other plants: plants for pharmaceutical, nutraceutical, landscape uses
Medicinal plants (calendula, comfrey, etc.) can also be an interesting niche urban market, and so can plants used for industrial purposes such as tinctorial plants (chamomile, fenugreek, etc.). Rare plants on the market (vanilla, saffron, etc.) are grown by a few urban farmers. Finally, ornamental and landscape plants (on balconies, terraces or in small gardens) can be a good solution to target the urban gardener market. Edible flowers are also original products that should be taken into consideration.
DEFINITION OF THE ENVIRONMENT CONTROL SYSTEM
The reason why we use a greenhouse is to generate optimal growing conditions for the plants. Inside the greenhouse they are more protected from negative natural influences. To get to even more stable climate conditions in the greenhouse several technical possibilities have been introduced into the greenhouse business, which help to mitigate existing problems or finding solutions to grow plants in otherwise too harsh environments (early spring to grow early vegetable or fruits, etc. ).
The difficult task designing a greenhouse is to find the optimal equipment and settings while staying within the boundaries of the project budget. In general, it can be advised to always focus on techniques which prevent a problem instead of getting rid of it. Additionally, passive systems, which use already existing energy flows are preferable.
It has to be mentioned that in a lot of cases of greenhouse construction projects the aspect of energy is neglected. But operating a greenhouse might become really energy intensive and therefore expensive. Reduced cost of the equipment can then lead to an expensive operation as well as technology failure.
INSULATION
Plants grow in limited temperature ranges and it is important for the RTG to offer controlled conditions
One important aspect is the installation of permanent or removable insulation. Insulation consists of materials, which have a low heat conductivity. Therefore, heat loss is progressing slower than without the insulation. In general, it can be stated that the thicker the material the better the insulating properties.
Using insulation is an often-overlooked possibility to reduce heat loss and therefore mitigate the heating demand in a greenhouse since insulation materials are usually opaque and reduce the total light transmissions of the outer shell. But insulation is important to keep the heat inside of the greenhouse. It is necessary to find a good balance between light and insulated walls. Sidewalls can usually be equipped partially with insulation, walls facing north can be closed completely without losing any light.
There are several possibilities for insulating materials ranging from artificial (sandwich panels with PU (Polyurethane) , EPS (Expanded Polystyrene), mineral wool) or natural (wood wool, hemp, straw or cellulose). Using natural insulating materials leads to a reduced CO2 impact, but comes along with a more complicated installation and often with higher cost. There are some transparent insulating materials available but usually either the insulating properties are not good enough to serve as a replacement, the transparency is quite low, or the price is too high to make the project economically feasible.
For the transparent openings double layered films or double skin sheets are available using traditional greenhouse materials or double-glazed coatings if glass is used.
Additionally, there heating screens are a possible solution. Here, an internally mounted screen is drawn out reducing the transmissivity heat losses. Such systems can also be installed for the side walls.
VENTILATION
One of the largest obstacles with operating a greenhouse is the overheating in summer. The easiest technically solution is to open up the greenhouse with vents. These are normally mounted at the highest point in the greenhouse. Heated air will rise and can escape through the ventilation openings. To make use of natural air movements it can also help to create flaps at the bottom side of the greenhouse to generate continuous flow of hot air escaping at the top while colder air gets in at the bottom. Depending on the rooftop situation and the style of the greenhouse, this very simple technique alone can be enough to achieve fitting growing conditions.
Next to natural or free ventilation it is possible to equip the greenhouse with forced ventilation systems, for example using fans to push in or suck out air. While it is a lot easier to control the forced ventilation also more energy is used, and the equipment is expensive.
It is important to mention that using ventilation also reduces the humidity in the air which gets transpired from the plants. This will lead to a loss of irrigation water to the environment.
SHADING SYSTEMS
Heat curtains and screens are a proven method of reducing heat gain in an RTG.
Furthermore, plants need warmth to grow on the one hand, on the other hand, too strong radiation leads to plant stress and reduced growing capabilities.
A simple solution is the installation of a shading screen which gets installed the same way as a heating screen. If a critical radiation threshold is overstepped the greenhouse control system automatically closes the screen. Thus, direct sunlight decreases, increasing the growing conditions for the plants and decreasing the necessity to cool the greenhouse.
Shading screens are normally made of a partially reflective and transmissivity material since it is not wanted to shut out sunlight completely. Some cultivation methods of certain plants need more controlled light. light deprivation system can be installed which offers complete darkness and should not be confused with a shading system. But in climates with intense sunlight over noon it is possible to use a light deprivation system to help the plants prevent overheating damage.
COOLING SYSTEMS
If either ventilation or shading was not enough to reduce the heat in the greenhouse, it is possible to use cooling methods.
First, the easiest and not very energy intensive method is to use adiabatic cooling. This technique uses the stored energy in the changes in the physical state of water to vapor, the same effect which cools the skin after leaving a swimming pool. Adiabatic cooling can be done with evaporation pads or misting devices. Evaporation pads are made from cardboard and are installed in the side or back walls of a greenhouse. The cardboard gets soaked with water and air is sucked through the fabric. The water evaporates and cooler but more humid air enters the greenhouse. Misting devices scatter water into very small droplets inside the greenhouse which evaporate quickly achieving the same effect. Adiabatic cooling devices have the disadvantage that they use a lot of water which gets lost with ventilation, also they only work in dry climate conditions. In comparison to chillers or cooling units adiabatic cooling has a lower energy demand.
Chillers use electrical energy to transfer heat to a thermal reservoir. In the case of a greenhouse this means that the heat from inside the greenhouse gets transferred to the outside air via heat exchangers. This process is energy intensive and the cooling units themselves are expensive which is why usually they are not used in horticulture.
Chillers can also be used for dehumidification. On the cold surfaces of the heat exchanger inside the greenhouse water condensates and can be collected. The water condensing heats up the heat exchanger with the reverse effect described before, leading to more energy demand for the cooling unit. In hot climates this can be used in combination with photovoltaics to prevent water scarcity while maintaining an energetically optimized operation.
SUPPLEMENTARY LIGHTING
Besides warmth, plants need enough light to grow. In winter or the transition periods in early mornings or late evenings there is not enough late to trigger plant growth.
Achieving better growing conditions can be done by using assimilation lights. Here, possibilities like modern LED techniques or plasma lamps are possible. In the greenhouse business sometimes high-pressure sodium lamps are used.
Generally, the energy demand is high using assimilation lights, which renders their usage uneconomic if the crop yield or the revenue is not high enough.
In addition, it has to be mentioned that rooftop greenhouses, especially in residential areas are bound to maintain a low light profile in the evening or at night. Without shading, using assimilation lights might be too intrusive for the surrounding areas.
ENERGY CONNECTION TO THE BUILDING
One of the key aspects of rooftop greenhouses is the connection between the greenhouse and the rooftop. In the support building, heating, ventilation and cooling are most probably already available and it can be possible to use already existing HVAC systems (Heating Ventilation Air Conditioning).
Implementing the greenhouse into already existing flows might help to increase the energy efficiency of the already installed systems:
Reducing the return flow temperature of the heating system increases the efficiency of the boiler
The greenhouse can preheat the inlet air for the building ventilation (via heat exchanger)
Waste heat from the building (chiller, bakery, restaurants etc.) can be used year-round for the heating of the greenhouse
Using the heat generated inside of the greenhouse to heat the support building can help both the greenhouse and the support building (greenhouse as solar collector)
etc…
To reduce CO2 emissions through rooftop greenhouses it is crucial that we find possible synergies. With that the combination of support building and greenhouse can achieve a better efficiency and increase the usage of available forces then each building separately.
CO2 CAPTURE FROM THE BUILDING
Furthermore, especially in office buildings the air is enriched with CO2 over the day. Connecting the exhaust air to the greenhouse creates the opportunity to fertilize the greenhouse with CO2 enriched air which can be beneficial for plant growth.
PEST AND DISEASE MANAGEMENT (INTEGRATED BIOLOGICAL PROTECTION)
Before the cultivation process starts, pest and disease attacks need to be evaluated and anticipated by an agronomist. They will depend of crops selection, climate conditions, prophylaxis and other factors. Many techniques already exist to fight against pest and disease, like pests’ capture (pheromones, light, etc.), host plants for auxiliary insects (attraction, feeding, ...), association of plants, etc. At this stage, a good agronomic expertise is necessary.
illustration of the integrated biological crop protection by using glue trap. Credits: ASTREDHOR
HORTICULTURAL PROCESSES
The team skill will depend of the technical intensity of the project as well as the social and pedagogical objectives. Horticultural activities are time consuming, especially during spring and summer seasons and it must be anticipated for the teamwork planning (work during the weekend, etc.). After all, harvesting and packaging process need to be perfectly foreseen. A specific area for storage, sorting, packaging activities must be design.
Horticultural practices that might be planned in this agricultural area are:
preparation and sowing of seeds
preparation of growing media
preparation of the nutrient solution
Installation of crops and growth tracking
pruning and extraction of excess biomass
harvesting, sorting and packaging
etc...