GALLY, OPERATED BY FERMES DE GALLY (SAINT-DENIS, FRANCE)
Nicolas Brulard (Gally), Pierre Raulier (ULG)
One of the four GROOF pilot RTGs is located on the site of the “ferme urbaine de Saint-Denis”, near Paris (FRANCE). The farm is dedicated to production and education; it covers 2.3 hectares. It currently produces vegetables on a 1-ha area and in a 250-m² greenhouse. It also receives citizens, families, and companies eager to discover the basics of farming, healthy food, and nature, and how the farm works.
The farm is managed by Les Fermes de Gally, a French group of family companies specialised in landscaping and devoted to bringing nature and farming in and around the cities.
The 360-m² RTG is dedicated to vegetable production to increase the tomato and herb production of the farm. The goal is to promote the ambition of the GROOF project with a greenhouse solely dedicated to production. Visitors will be able to walk on a platform located on the east and south sides of the RTG. Inside the greenhouse, vegetables will be produced on hydroponic systems. This will create a flexible production tool, to change the crop if necessary, in order to follow the market. The stakeholders also want an easily manageable greenhouse, sustainable over time and in the way of producing vegetables.
Business – value creation
There is a small shop on the farm that sells fresh produce to local people, so successful that it quickly runs out of stock; so, the ambition is to increase production to meet the demand. Unfortunately, the property cannot be expanded because of urban constrains, so an RTG is a logical solution. Producing on roofs is a wonderful opportunity to increase production. The RTG will be installed on the only flat roof available for the farmers to work under controlled conditions, and then to get better yields than in the field without polluting the soil.
The soils of the farm are polluted with heavy metals, so that leafy greens cannot be produced. Leafy greens are produced in soilless systems in the greenhouses present on the farm. Due to the success of this production, the farm is in need of more greenhouse surface, but building a new greenhouse is not possible on the ground: the whole surface is currently used for mechanised production and teaching activities, and local rules forbid an extension of covered surfaces. An RTG would solve both space and urbanism issues.
The main strength of the project is that the greenhouse is not the only economic resource. The RTG belongs to a worldwide project that will soon be economically viable. The future building will support the project development. The philosophy of GROOF is already fully represented on the site of the “ferme urbaine de Saint-Denis”, i.e. farming, awareness, proximity, and sustainability on many aspects. This RTG will be fully appreciated by real customers.
The urban greenhouse on the farm is part of a corporate ecosystem. The objectives, the organisation of production and the economic model are linked to the fact that the greenhouse is included in the operation of a 2-hectare farm, itself included in the operation of all the farming and landscaping businesses of the Fermes de Gally.
Installing an RTG on a building to be refurbished gives great freedom to define a fully functional production by adapting the building to farming constraints. This will have a big impact on yields inside the greenhouse, as everything will have been designed for production.
However, the project also has some weaknesses. As yet it is hard to justify such an investment for tomato production. We chose to design a productive greenhouse with limited public access to lower investments. Combining several activities on the farm and developing both affordable and high-value products is a key project. Once again, we bet on the flexibility of the infrastructure and on our capacity to react to market evolutions.
The host building (an old steel-structure shed) turned out to be too weak to bear the greenhouse and farming activity. The building will therefore be partially destroyed to be rebuilt with a more resistant structure (a 500-kg resistant hollow core slab) and suitable for rooftop cultivation or a light greenhouse. The preserved part of the building will be insulated to improve its energy efficiency. Heat storage and exchanges are the core of the project, and the building structure will be made of concrete with large glass facades to collect heat.
Concerning the covering material, we chose an inflatable double layer greenhouse, with a multispan polycarbonate greenhouse. Polycarbonates have the advantage to provide a good balance between cost, energy efficiency and weight. The U value is one of the main advantages of glass covering for RTGs and makes it possible to achieve safety standards close to those of the glass roofs of public buildings.
By choosing a greenhouse dedicated to production and not intended for the public, we will be able to set up a greenhouse close to the greenhouses of peri-urban professional market gardeners. Through this project, we want to demonstrate that we can build efficient and economically viable RTGs. Another important advantage of polycarbonate is its low weight; this facilitates the load management of the greenhouse. Nevertheless, such greenhouses require good anchoring because their low weight does not compensate for the wind forces.
Gally was willing to work with a lightweight production system. In this context, the raft system was rejected. Thin hydroponic gutters will be used to grow tomatoes and leafy greens, as a lightweight and productive technique. We are studying the irrigation system to make closed-loop cultivation possible.
The concrete slab will be thickened to increase the inertia of the whole building. When the outside temperature is lower than the temperature in the greenhouse, the slab will start giving back the calories it has stored. Thicker slabs have increased heat storage capacities. Thus, the greenhouse will stay warm overnight.
The polycarbonate of the northern facade will be replaced by a concrete wall to improve the greenhouse insulation and decrease the loss of calories from the colder side of the building. This wall will save 20 % of energy. Moreover, it will store heat by catching the calories from the sun and the greenhouse, and therefore mitigate the temperature drop at night.
One major limitation of the approach was to find a greenhouse builder eager to build the greenhouse we designed. Even if the shape is quite traditional, our first choices of materials and specifications did not match with builders’ priorities, and we had to lower our ambitions to get the greenhouse built (for instance, no builder wanted to install ETFE over less than 2 hectares).
The goals of this greenhouse are to increase the production of the whole farm, and to make the farm more self-sufficient by producing all the plants it needs on site.
The building is a new one, so the greenhouse was designed as part of the overall project and was made to be a high-performance greenhouse.
No light or heating systems will be installed to grow vegetables in a sustainable way.
Vegetables will be grown on hydroponic gutters and on ebb-and-flow systems. That is why the trail of the hydroponic gutter is only mentioned for the moment. This system allows producing vegetables in NFT, in aquaponics, in aeroponics, and on a growing media, so it is suited for a lot of species. Then, to increase again the production range, it seems that we will install ebb-and-flow systems especially dedicated to herb and leafy green production. Finally, the greenhouse will be the place where all the young plants of the whole farm will be produced. This way, the farm will increase its self-sufficiency and economic viability.
Tomatoes and herbs will be produced in pots and sold locally.
We are currently training our farming team to be efficient in hydroponic systems.