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 options 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 sites of IFSB (Institut de Formation Sectoriel du Bâtiment) in Luxembourg and ULg (Université de Liège) in Belgium. The analysis demonstrates synergies between the building and the greenhouse. The key research objective was to identify CO2 reduction potentials in terms of 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 outcome of “The Deliverable 2.1: Reference framework & baseline analysis (U. Kirschnick and K. Wilhelm)”
The IFSB pilot aims to create a rooftop greenhouse for the urban environment and use synergies between the building and the RTG:
- waste heat from the building to heat the greenhouse on the rooftop;
- use CO2 from office exhaust air (human respiration) for plant fertilization;
- achieve a better insulation of the rooftop through the greenhouse.
The ULg pilot greenhouse is designed to assist the university’s research activities and cooperation with other ongoing projects. By building an integrated rooftop greenhouse, the university aims to raise and answer the following questions:
- How you can install a greenhouse on the rooftop;
- How local food can be produced in a small-scale greenhouse;
- What kind of production system can be used on rooftops;
- What are the possibilities and resources for plant fertilisation;
- How GHG emissions can be reduced in rooftop greenhouses.
IFSB pilot recommendation
In the first phase, the project investigated the opportunities to utilise synergies between the greenhouse and different rooftops on site: steel structure warehouse, concrete office building (main building) and new steel structure canteen. The investigation of the options comprises the structure of the building and the static load capacity of the rooftop. Furthermore, an analysis of the synergies between the buildings and the greenhouses was conducted. Finally, an individual greenhouse was designed for each rooftop in order to discuss solutions for possible issues that may come up.
The evaluation of these three buildings resulted in the exclusion of the industrial warehouse: it has a low load capacity of the rooftop, which affected the cost effectiveness. The high investment is a result of the effort to connect the building to the greenhouse. Moreover, the location does not have access to waste heat sources from the building.
For the main building, an intensive analysis in terms of the energy demand and the usage of waste heat (canteen; offices) has been done. This first estimation demonstrated high energy demand (200 – 250 kW/m²). One reason is that the rooftop of the building has a non-beneficial dimension, which results in a poor ratio (2,4) of envelop to surface area of the greenhouse. In addition, the first estimation has demonstrated that the quantity of the waste heat cannot cover a large part of the energy consumption of the greenhouse. Furthermore, the analysis has shown an additional risk: the installed capacity of the current boiler system is not sufficient for all buildings.
The greenhouse above the restaurant has a beneficial cubature, a good orientation to the south as well as a concrete wall on the north side, which leads in general to lower energy consumption. Furthermore, the greenhouse will be developed in an energy-efficient way, which reduces the fuel demand. The waste heat in this greenhouse could be used in a more optimal way than at the main building. This rooftop greenhouse will be designed in accordance with the Eurocodes from the building sector and will have a dimension of 18 m width, 20 m length and 7,20 m height. The steel frame will have a grid dimension of 3,20 m to 6,88 m on the left side, 3,20 m to 6,87 m on the right side and 3,20 m to 5,00 m in the middle of the greenhouse (Arnaud De Meyer, 2019). The floor of the greenhouse (from greenhouse to the canteen) will be constructed with screed, concrete and mineral wool. The first estimation of this structure has shown a total U-Value of 0,19 W/m²*K (Benoit Martin; Construction Certification S.A (COCERT), 2020). It was concluded that the IFSB RTG construction can be done using double glazing and one thermal screen. In this case, the walls and the rooftop are constructed with aluminium profiles and a double glass layer. The glass layer consists of Planiclear 4 mm and Planitherm XN 4 mm glasses with an intermediate filling of argon. In this case a heat coefficient of 1,16 W/m²*K for the glazing material can be achieved. This glazing structure has a light transmission rate of 82% (Benoit Martin; Construction Certification S.A (COCERT), 2020). Based on these values, the RTG has an energy consumption of around 20.000 kWh/a. This result is based on the assumption that the RTG is unused from January until March and a heat supply is not necessary during this time. With regard to the RTG area of 380 m², a specific energy demand of 45-50 kWh/m² is analysed. The simulation shows a peak load capacity of around 45-60 kW for the RTG.
In addition, the analysis of energy and material flows has been done to improve the GHG emission savings of the IFSB pilot greenhouse. It demonstrated further measures to improve the symbiosis between the urban environment and the rooftop greenhouse. Hence, the rainwater collection and the potentials of fertilizer and renewable energy production around the IFSB area were analysed.
It is estimated that the current IFSB’s water consumption is approximately 1.220 m³ per year: it comprises fresh water (60%) and rain water (40%). The fresh water is mainly used for taps and showers as well as for the canteen. Currently, the rainwater is entirely used for toilet flushing and during the construction training courses, e.g. to prepare concrete. In the potential analysis the whole roof area of the building (approximately 2.765 m2) is used for rainwater collection. The calculation used an annual precipitation of 830 mm according to (Administration des Services Techniques de l'Agriculture, 2020) for the Luxemburg city. The exploitable potential is set to 90% in order to consider losses of rainwater e.g. in case of extreme rain. In the case of the IFSB, 700 to 900 m³/a could be collected and it covers the future rainwater demand (approx. 500 m³ for toilet flushing and during the construction training courses and approx. 350 m³ for the crop production).
Renewable energies are an additional potential of the building. The possible areas for a PV system are around the terrace and in front of the canteen. Hence, this PV system has a multi-usage because it would shadow the canteen, serve as balcony balustrade and produce electricity. A total capacity of around 4,4 kWp could be installed with an energy production of around 3.000 kWh/a. This PV system can cover the estimated energy consumption (2.500-3.000 kWh/a) of the greenhouse.
Urine is a valuable waste flow rich in key nutrients NPK for plant fertilization. Considering the occupational period and number of users, a theoretical amount of 24,5 m³/a can be collected at the IFSB building. This urine has a nutrient potential of around 43,91 kg N; 2,65 kg P and 22,00 kg K on annual basis. For the treatment of urine the VUNA technology developed by (Etter & Udert, 2016) is selected. Comparing the nutrient demand of tomato production in the IFSB RTG with the nutrient availability from urine, approximately 90% of the nitrogen demand could be covered. For P and K fertilizers this coverage is less than 30%. Additionally, 23,8 m³ of distilled water becomes available after the treatment. In terms of the GHG emission abatement through closing material loops, not only the substitution of mineral fertilizers and water recycling can help to reduce the global warming impact, but also emissions and costs from wastewater treatment can be avoided.
Further potential of the urban environment is the CO2 content of the exhaust air stream. Carbon dioxide can be used for plant fertilisation during the day. For this reason, IFSB has carried out a short analysis of the CO2 content in the airflows of the classrooms and offices. The analysis shows that the exhaust air has a CO2 content between 450 and 700 ppm during the day (Benoit Martin; Construction Certification S.A (COCERT), 2019). During the weekend, the carbon dioxide content is at the same level as the outside air. Depending on the crops, a CO2 level between 450 and 700 ppm in the greenhouse is desired for optimal content. In this case, a detailed investigation of the use of this option and integrating it in the rooftop greenhouse has yet to be carried out.
ULg pilot recommendation
The focus of the ULg rooftop greenhouse is creating a research tool for the urban agriculture sector. Unlike at IFSB, at ULg only one building was considered for the greenhouse. Hence, the analysis went through different planning stages for the technical building rooftop. The main aim of this investigation was to identify a greenhouse structure for different kinds of research purposes and crops.
The rooftop of the technical building offers a great location for an RTG. This place has a south-east orientation and allows enough light for the plant growth as well as the load capacity of the rooftop (400 kg/m²) sufficient to build a greenhouse on it. However, this building has disadvantages as well. In terms of the construction process, two challenges are present:
- The building wall (northern side of the greenhouse) is built as a ventilated facade.
- The RTG has to be built in a non-invasive procedure on the rooftop.
The solution to overcome these disadvantages is an installation of lean-to greenhouse. This greenhouse type has a light structure and it could be installed on a so called "floating foundation" on the insulation of the existing flat roof. Concerning this matter, the foundation has to be heavy enough to compensate the "pull up" forces coming from wind as well as it has to integrate an evacuation system for runoff water from the northern to the southern side of the building. Regarding the ventilated façade, a solution with an extra wall (made of sandwich panels) was identified. This RTG wall will be implemented at a distance of 0,1 m in front of the building. Further challenges can be avoided because such construction leads to air space between the sandwich panel and the ventilated façade and it allows the air circulation inside the cavity wall. At the end, the greenhouse is not physically connected with the building. This greenhouse type has additional advantages such as high light transmission into the greenhouse, easy ventilation process and low ratio between the envelope and the base area.
Another argument in favour of this building complex is the potential of recovering waste heat from the cooling system (chillers) to heat up the greenhouse. These chillers are the largest producers of waste heat in the building, but the heat reuse potential has a low temperature level (30-40°C). Comparison of the estimated heat demand from the greenhouse (14.000 – 18.000 kWh/a) and the assessment of heat recovery potential from chillers (120.000 – 600.000 kWh/a) shows that the waste heat potential could be sufficient to heat up the greenhouse. An energy recovery of 120.000 – 600.000 kWh/a is equivalent to 12.000 – 60.000 l heating oil. This zero emission energy source would reduce the GHG emissions of the greenhouse production system. However, the use of the cooling system, low temperature level of the heat source and the distance between the chiller and the greenhouse are the main challenges in this case. For this reason, several aspects have to be additionally investigated in detail:
- consumer behaviour,
- the possibility to install a pipe system from the chiller to the greenhouse,
- the heat distribution system with a special heater design (for low temperature levels).
Hence, a more thorough investigation of the heat recovery potential from the chiller is necessary.
In terms of the energy savings potentials, an investigation of envelop materials was made. The design of the walls was an additional issue. Hence, to increase the energy efficiency of the greenhouse, several options are identified:
- Using sandwich panels (thickness between 40 – 60 mm) up to a height of 0,9 m on the western, southern and eastern sides of the greenhouse. This material has a lower U-value (between 0,35 to 0,55 W/m²*K) than other envelop materials (e.g. glass, foil, etc). This solution is possible because the crops growing system is installed at a height of up to 0,6 – 0,8 m.
- Furthermore, heat protecting glasses (U-value 1,1 W/m²*K), 25 mm polycarbonate sheets (U-value 1,3 W/m²*K) or f-clean foil (U-Wert 3 W/m²*K) could be used as transparent material.
- In addition, thermal screens should be used.
Based on these options, target indicators for the greenhouse envelop material were identified:
- a U-value of 1,6 W/m²*K,
- a minimum light transmission of 70%.
The f-clean foil has an extremely high light transmission of 87%, which is positive for the crop production but the material has a higher U-value (3 W/m²*K) than the target value. For this reason, additional energy efficiency measures (e.g. thermal screens) could be used to avoid high energy demand of the greenhouse. This tool is used to limit energy loss during the night and it could further limit the sunlight during summer days. Furthermore, the target U-value can be achieved with heat protecting glass (U value of 1,1 w/m²*K) (DEFORCHE CONSTRUCT NV, 2019) or 25 mm polycarbonate sheets (U-value 1,3 w/m²*K) (EBF GmbH, 2018). Due to the light transmission rate of 70%, the heat protecting glass is a preferred envelop material. The 25 mm polycarbonate sheets have a light transmission of just 49% (EBF GmbH, 2018) which doesn’t fulfil the set requirements. However, every greenhouse design should be equipped with thermal screen.
There are further potentials such as photovoltaics around the RTG. The analysis with the software PV*Sol identified a potential installed capacity of around 50 kWp. The system would generate 49.000 kWh of electricity per year, which can be directly used on site. This energy source could also help reduce the GHG emissions in greenhouse production.
Moreover, the Ulg campus has potentials for urine collection and treatment. Hence, we analysed how the building infrastructure can change to implement a separation system for urine collection. The amount of available urine depends on the number of users and occupational period of the research centre.
The investigation shows that there are potentials for synergies between greenhouses and buildings. Both pilots demonstrate potentials to reduce the GHG emissions from the production processes. The investigation of the renewable energy sources such as waste heat usage or electricity production via PV-systems is a first step to a zero-emission rooftop greenhouse production system in the future.
The IFSB pilot will demonstrate the integration of a rooftop greenhouse into urban environment both from the engineering point of view and aesthetically. The rooftop of the canteen building was chosen for the RTG implementation due to its numerous advantages: high load capacity, energy efficiency, access to waste heat and access to CO2 from ventilation system for the fertilization of plants. Additionally, the potentials for a PV-system, urine and rainwater collection are available for this building.
The ULg pilot will serve the research activities at the university and showcase the possibilities of crop production in a small-scale greenhouse. It will be constructed on a rooftop of TERRA building. A lean-to structure was chosen as opposed to a gabled greenhouse. Lean-to type has advantages such as high light transmission into the greenhouse, easy ventilation process, and low ratio between the envelope and the base area. The project can utilize the available synergy potentials: waste heat supplied from the cooling system and electricity from the PV-system.
Furthermore, additional potentials could be developed from the combination of the greenhouse and the building environment. In the future, the rooftop greenhouses could use further potentials from buildings such as grey water, organic waste etc. The greenhouses themselves can also provide potentials to the urban environment, e.g. energy to the building. In this case, the RTG has a heat potential to heat up service water, because the challenge in the greenhouse production is the heat input into the greenhouse during the summer time. In the commercial production system, this solar energy potential remains unused. In addition, any concurrence of renewable solar energy usage doesn’t exist on the building. Furthermore, the climate conditions in the greenhouse will be improved. For the future iRTG system, the usage of all potentials surrounding both systems should be developed to create the symbiosis between greenhouse and building.