K. Wilhelm; M.Sc. Dipl. Ing. Gartenbau (FH)
R. Semenova; B.A.
The pilot sites within the framework of the Interreg NWE project GROOF have different targets and opportunities to develop synergies of energy and material flows between the buildings and the rooftop greenhouses. This article provides an overview on such potentials for the pilot site of Les Fermes de Gally in France. The key research objective was to identify the CO2 reduction potentials through the energy flows: for example, constructing a greenhouse with high energy efficiency, using waste heat from the building, mounting a PV-plant to generate additional energy for the greenhouse etc. The reason for this is that the energy consumption has a high potential to reduce the GHG emissions in the protected vegetable production. This result is the outcome of “The Deliverable 2.1: Reference framework & baseline analysis (U. Kirschnick and K. Wilhelm)”
Les Fermes de Gally will create pioneer technologies of vegetable production in urban environment. In this context, urban roofs remain a poorly exploited opportunity for agriculture development, however, they provide sustainable solutions to urban food production as well as contain a potential for green cities. Considering these facts, the company will develop a building-integrated rooftop greenhouse. An investigation was carried out with the following rooftop requirements:
- waste heat from the building can be used;
- CO2 from the ventilation system can be used for the plant production;
- implementation of renewable energy is possible;
- access for the daily work or connection to the farm St. Denis is possible.
Farm Gally pilot recommendation
In the first phase, the project evaluated different rooftop sites to demonstrate challenges, advantages, and disadvantages of implementing an RTG. In this project period a qualitative assessment of rooftop sites was done, which includes design drawings and discussions for the implementation of a GROOF pilot. The locations in question were Paris city center, new building in Stains, grocery store in Montevrain and a redesigned building at the St. Denis farm. The target was to select the best pilot site to achieve the project goals which, in addition, should follow the concept of the Fermes de Gally.
The evaluation of these four building types resulted in the exclusion of 3 options: Paris city centre, new building in Stains and grocery store in Montevrain. The reason behind this decision is that these projects’ development goes beyond the GROOF timeline.
For this reason, it was decided to focus on the redesigned building at the Gally’s farm in St. Denis. In general, when it comes to newly designed buildings, the advantage is the integration of the rooftop greenhouse into the building concept, therefore, the synergies between the greenhouse production and the building can be exploited better. In this specific case, the given structure and cubature of the building is beneficial for the greenhouse, because it will result in an optimum ratio between the envelop and the surface area of the greenhouse. This leads to lower energy consumption by the integrated rooftop greenhouse (iRTG). In addition, the rear wall of the building (on the northern side) is made of concrete. This construction material has positive impact on the energy consumption because it can function as a solar collector. This means that it can store the heat of the sun radiation during the day and transmit the heat back to the greenhouse during the night. Furthermore, the concrete slab (building rooftop) can be used as heat storage, because the sun radiation heats up the greenhouse environment. This energy will be stored in the concrete slab and will be used in the building below and the greenhouse itself. This helps to heat up the building complex during autumn, winter and spring.
However, the described above benefits turn into disadvantages during the summertime. The effects of heat transfer are working during the summer just as much as the rest of the year so the overheating of the building below is possible. For this reason, a perfect natural ventilation system is needed. The utilization of an air-conditioning system should be avoided. To tackle this problem, a building-integrated PV system could be installed on the southern side of the building for both shadowing the office and electricity production.
Furthermore, different versions of the greenhouse design were considered to identify the most energy-efficient option. As a reference, the baseline scenario was created with fixed size of the greenhouse and defined envelop materials. The size of this RTG is around 380 m² (19,2 x 19,8 m). It will be built with galvanized steel frame and transparent walls (3 m high). This double chapleted greenhouse has a ridge height of around 4,5 m and the roof will be covered with a double PE film. In addition, a shadow / thermal screen system will be installed to improve the efficiency further. The energy demand of the greenhouse and the building was simulated with the software EnergyPlusTM 9.1.0 (Vassal, 2019). Production period from March to December with a temperature level of 14°C was assumed. During January and February (inoperative period), a temperature level of 5°C would be kept inside the greenhouse (Lacaton & Vassal, 2019). For the baseline scenario (polycarbonate [U = 5,7 W/m²*K] on walls and double film layer [U = 2,65 W/m²*a] on the rooftop) the heat energy requirement of 45 - 50 kWh/m2 was estimated. In order to increase the efficiency of the greenhouse and reduce the energy consumption, a simulation with the following modifications was performed:
- Scenario 1: Polycarbonate walls replaced by ETFE film
- Scenario 2: Concrete wall in the greenhouse on the northern side
In addition, a scenario with water / heat storage tank inside the greenhouse was investigated too. However, the analysis pointed out that it has no significant influence on energy savings and additional positive effects are not evident in this planning stage. Hence, this option will be not discussed further.
Scenario 1: Polycarbonate greenhouse walls replaced by ETFE film
In this scenario, the heat exchange of the envelop material was analysed. Comparing the performance of polycarbonate with the ETFE film (U= 2,65 W/m2*K) resulted in no significant effect in terms of energy saving. To be precise, the replacement leads to the energy consumption of around 45 kWh/m2. However, the ETFE film has the highest UV transmission of all the greenhouse cover materials and this has a positive impact on the plant production and product quality.
Scenario 2: Concrete wall in the greenhouse on the northern side
For higher energy efficiency, the northern wall (in front of the building) can be constructed with concrete. This material has a U-value of 3,3 W/m²*K, which is significantly better than, for example, polycarbonate plates (5,7 W/m²*K) (Lacaton & Vassal, 2019). Furthermore, the concrete wall can be used as a heat storage, because the material can absorb radiation heat from the sun during the day and transmit it to the greenhouse at night. These effects lead to a higher efficiency. In this case, the energy consumption of the greenhouse could be reduced by up to 20% (35 - 40 kWh/m2) in comparison with the baseline scenario. Furthermore, the concrete wall has no significant influence on the greenhouse temperature during the summer but has a benefit in the colder periods of the year when the temperature level increases by up to 2°C compared to the baseline scenario (Lacaton & Vassal, 2019).
The scenario 2 demonstrated that the energy demand could be reduced to 35 - 40 kWh/m2 by installing a concrete wall on the northern side of the RTG. (Lacaton & Vassal, 2019). Furthermore, it is recommended that sandwich panels with a thickness between 40 - 60 mm are installed up to a height of 0,6 - 0,9 m on the western, southern and eastern sides of the greenhouse. The sandwich panels have a lower U-value (0,35 to 0,55 W/m²*K) than other envelop materials (e.g. polycarbonate, foil, etc.) and it brings the energy efficiency of the greenhouse to higher level.
Four sites were considered for the iRTG of Gally. Three options were excluded due to the significant implementation timeline differences with the GROOF project. As a result, the redesigned building at St. Denis farm was chosen as the pilot site.
The redesigned building has a beneficial design and materials such as concrete slab, which leads to lower heat energy consumption in the greenhouse during the heating period.
The size of the RTG will be 380 m² (19,2 x 19,8 m). It will be built with galvanized steel frame and transparent walls (3 m high) made of polycarbonate sheets (U = 5,7 W/m2*K). This double chapleted greenhouse has a ridge height of around 4,5 m and the roof will be covered with a double PE film (U = 2,65 W/m2*K, thickness of 2 x 200 micrometres).
The analysis demonstrates the heat energy consumption of 45 - 50 kWh/m2 for the design specified above. Based on this scenario, different energy efficency measures were simulated and it turned out that the energy demand could be reduced to 35 - 40 kWh/m2 by installing a concrete wall on the northern side of the RTG. This result is based on the fact that the concrete wall has a lower U-value than the polycarbonate layer and this massive wall is used as a solar collector. This leads to a 2°C temperature increase inside the greenhouse during the wintertime.
There are additional possibilities to increase the energy efficiency: installing a shadow / thermal screen system and using sandwich panels on the western, southern and eastern greenhouse walls up to a height of 0,6 - 0,9 m. This material has a lower U-value (0,35 to 0,55 W/m²*K) than others (e.g. polycarbonate, foil, etc.).
In order to achieve even greater synergy between the building and the greenhouse, it is advised to install the building-integrated photovoltaic system on site. Based on the climate data from St. Denis, the system of 7 kWp capacity would generate about 5.000 - 5.500 kWh/a of electricity per year.