IfaS: K. Wilhelm; M.Sc. Dipl. Ing. Gartenbau (FH)
IfaS: R. Semenova; B.A.
ebf-GmbH: D. Volk; M.Sc. Physics (TU)
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 ebf GmbH in Germany. 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, installing a PV shadowing system to generate additional energy for the greenhouse etc. The reason for this is that reducing the energy consumption results in reducing the overall 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)”
Ebf GmbH will develop and construct a modern and futureproof greenhouse concept which will focus on the synergies between the support building and the growing systems.
This lays a basis for research on urban roofs as sustainable solutions to urban food production as well as their potential for green cities. For ebf, the following rooftop requirements were necessary:
- Floorplan of the roof is fitting the projected greenhouse shape;
- Waste heat sources from the building are available;
- CO2 from the support building can be used in plant production;
- The roof is easily accessible for ebf;
Ebf GmbH pilot recommendation
Ebf had made contact to a project in Darmstadt next to a waste-to-energy plant. It was located in a small building with previous industrial usage which was up for renovation and remodelling. Next to the proximity of readily available waste heat, the contacts surrounding this project seemed to be promising. The rooftop, on the other hand, was quite small and also partly covered by shadow from an adjoint building part of the year. Therefore, winter production would have been made quite complicated. Additionally, as the GROOF project moved on and first results of other projects were gathered, it became obvious that the unclear structure of the property situation was too unreliable to guarantee a successful rooftop greenhouse. Similar projects had failed in the past.
This led to an assessment phase in which several other potential rooftops were examined. One of the restricting problems was the fact that in some cities or municipalities in Germany no commercial horticulture business is allowed to be operated in a residential area. It is possible to build a rooftop greenhouse but the implementation of regular horticulture operations is then not allowed. A few projects, located for example on a school in Freiburg, could therefore not be taken into consideration for a possible continuation of the GROOF project.
The next step in the evolution of the project was the discovery of the rooftop in Mannheim. The building was from the 1970-s and contained a rooftop parking lot. Due to the nature of the roof, the theoretical load capacity was ideal for the installation of an RTG and the implementation of an urban farm with a social aspect.
This parking lot simplified a few additional aspects: just to name a few, access was already established and a plain area, which needed no further treatment, was available without interruption or clutter.
Building up on this already good baseline, the building harbours a lot of different sources with a waste heat potential all year round, easily accessible as the equipment’s are located on the roof. Additionally, part of the building area belongs to school with a centralized HVAC (Heating, Ventilation and Air Conditioning) system without heat recovery, also placed on the roof. This led to a large waste heat and CO2 potential, giving an ideal basis for the results of the GROOF project.
After having found this new potential project site, a lot of new stakeholders became involved which made a larger social connection possible. This provides a different food value chain for the RTG than previously estimated. Possible connections could have been made between the Asian restaurant and the RTG, moreover, the school could have included the greenhouse into their curriculum. Early talks about that were already set in place.
The challenge in this case was the owner of the building: despite being very supportive from the beginning, he suggested a very high renting fee. This made economical operation of such a pioneer project impossible. In addition, further solutions to reduce operation costs (e.g. voluntary worker for the greenhouse production) could not have been identified in this short timeframe. For this reason, in order to avoid insolvency of the pilot greenhouse, the search for the suitable site continued.
The ebf GmbH owns a nursery in Bürstadt and this site as well as its infrastructure area was rented to a farmer. Recent development of this horticulture farm resulted in reduction of the area occupied by the farmer. Hence, the former packaging hall became vacant. This led to the conclusion to build the rooftop greenhouse on top of this building. In the near future this building will contain social rooms for the farm workers, a workshop and a packaging area including a refrigeration cell for the new concept of the whole nursery.
Since the greenhouse is now built on the property of ebf, a more ambivalent approach to the planning, construction and operation can be utilized. This results in a constantly evolving project with changing parameters adapting to the needs of ebf while maintaining the main goal of the GROOF project. The location provides synergies to overcome the economic challenges in terms of operation costs, therefore, successful development is secured.
The greenhouse design is developed by ebf in the last few years which has been slightly adjusted to serve as a rooftop greenhouse. The main difference here is the reinforced structure to abide to the building code, which is usually a lot leaner for a greenhouse than for residential or industrial buildings. It is based on a lean-to style, which means the northern and the side walls are opaque as well as part of the roof structure. The south-oriented arch structure is transparent and made of double-layered ETFE F-Clean film. Despite being double layered, the transmission still reaches ~89% with a U value of 3,0 W/m²*K. The inner layer of the film is diffuse to generate equal light distribution inside the greenhouse. Due to the opaque roof, the light inside the greenhouse is reduced at the back side, which generates a gentler light profile for low intensive culture.
The greenhouse has a size of 155 m² and its supporting structure is made of galvanized steel. The walls are built with renewable materials such as wood board and hempcrete. The hempcrete is a mixture of limestone and hemp and which has a significantly lower U-value (0,33 W/m²*K) than other greenhouse envelop materials (e.g. polycarbonate: 5,7 W/m²*K). Part of the greenhouse rooftop is insulated with hemp straw, generating even better insulation properties (U = 0,2 W/m²*K) than other materials. The use of natural materials reduces the ecological impact of the greenhouse too.
The materials and the structure are optimized to guarantee a highly energy efficient greenhouse design. The greenhouse has an estimated energy demand of around 6.700 kWh/a (40 – 45 kWh/a*m²) based on the simulation done by ebf using DesingBuilder 6.1.8. Due to the nature of the greenhouse, it is estimated that there will be a year-round operation, maintaining an operational temperature of 20°C.
In order to decrease the energy consumption two main techniques are being used:
- Installing an outside thermal blanket
- Using the greenhouse as a solar collector.
The thermal blanket will be rolled out in colder nights. Due to the curvature of the large southern transparent surface, it is possible to cover the film with additional insulation. An estimation suggests that it is possible to decrease the U-value to 1,0 W/m²*K for this greenhouse part. Further research has to be conducted to validate these numbers. Furthermore, on winter days with no sunlight, the blanket will be used to reduce the heat losses during the day and an assimilation lighting system will help avoid negative impacts on the plant production.
One more way to reduce the energy demand is to use synergies between the greenhouse and the building. In this case, the greenhouse provides thermal energy to the building, and it will be used as a solar collector. Especially in the transition period, the greenhouse can warm up enough to overheat. Instead of using ventilation to cool the greenhouse, it is possible to use a heat pump to cool the greenhouse and heat up the support building. This reduces the energy demand of the support building and the greenhouse likewise. The same effect can be used in summer and winter, only reversed in the latter case. Here, the large thermal mass of the support building and its concrete ground floor are helpful to keep a balanced interior. It is estimated that approximately 5.800 kWh of solar energy per year can be used to heat up the support building.
During the summertime, additional cooling and shading of the greenhouse are required. The main system for keeping the greenhouse cool are the ventilation flaps, which open at the bottom of the southern wall and at the highest point of the roof. The natural convection creates a chimney effect which draws fresh and cool air in, which heats up, rises and exits at the roof. To support the natural ventilation, ventilators are installed at the upper sidewall. They will be mainly used with a smaller ventilation flap at the backwall and an adiabatic fogging system. Here, water is sprayed into the greenhouse through a nozzle where it evaporates and cools the sucked in air. The ventilators can also be used with the lower ventilation flaps open without the adiabatic cooling system. The gradual escalation of more intense cooling methods gives a smooth control over the greenhouse temperature.
As a shading solution, a PV system is developed. In this case, the shading system is a multifunctional tool, compared to the conventional shadow system. The transparency of film with PV cells is used to produce electrical power inside the greenhouse. This system is light weighted and the panels are installed directly on a rotation system which moves them towards or against the sunlight. Therefore, if there is not enough sun for both electricity production and growing plants inside the greenhouse, the control system will retract the panels for the sun radiation to reach the plants directly. In case of highly intensive sunlight, the PV panels can be used to reduce the radiation by 60% while generating electricity. In a first estimation the PV panels are projected to produce 4.400 kWh/year.
Further energy saving potentials could be realised on the location. For example, the heat pump could be used to heat up the greenhouse with waste heat from different sources. In this case, there are potentials from the residential building next to the greenhouse or the refrigerator cell in the packing hall. This could increase the energy optimization further.
Three sites were considered for the RTG of ebf GmbH. The two first options were excluded due to unreliable property structure and high rent. At those locations it could not be guaranteed that the GROOF goals are fulfilled. The solution was to place the RTG at ebf’s own farm.
The greenhouse design had already been developed by ebf and was slightly adjusted to serve as a rooftop greenhouse. The construction is a lean-to type which means the north and side walls are opaque, as well as part of the roof structure. The transparent south-oriented arch structure is made of double-layered ETFE F-Clean film. The transmission of this material reaches ~89% with a U value of 3,0 W/m²*K. The inner layer of the film is diffuse to generate equal light distribution inside the greenhouse.
The greenhouse size is 155 m² and its supporting structure is made of galvanized steel. The walls are built with renewable materials such as wood board and hempcrete. The hempcrete is a mixture of limestone and hemp and it generates a significantly lower U-value (0,33 W/m²*K) than other greenhouse envelop materials. Part of the greenhouse rooftop is insulated with hemp straw, generating even better insulation properties (U = 0,2 W/m²*K). The use of natural materials reduces the ecological impact of the greenhouse too. The materials and the structure of the greenhouse are optimized to guarantee a highly energy efficient greenhouse design. The estimated energy demand is around 6.700 kWh/year (40 – 45 kWh/a*m²). The energy efficiency will be improved by a thermal blanket, because it will be rolled out on the large transparent southern surface in colder nights. An estimation results in the decrease of the U-value to 1,0 W/m²*K for this greenhouse part.
A further opportunity to reduce the energy demand is to use synergies between the greenhouse and the building. In this case, the greenhouse provides thermal energy to the building and it will be used as a solar collector. It is estimated that approximately 5.800 kWh of solar energy can be used annually to heat up the support building.
As a shading solution, a PV system is developed. It will be executed as multifunctional tool, compared to the conventional shadow system. At times with highly intensive sunlight the PV panels can be used to reduce the radiation by 60% while generating electricity. In a first estimation the PV panels are projected to produce 4.400 kWh/year.
Further energy saving potentials could be realised on the location. Waste heat from the residential building next to the greenhouse or the refrigerator cell in the packing hall can be used to supply the greenhouse.