TECHNICAL, ENVIRONMENTAL AND ECONOMIC ASSESSMENT OF ROOFTOP GREENHOUSES

Karsten Wilhelm (IfaS), Ulrike Kirschnick (IfaS), Zaira Ambu (HS Trier/IfaS), Nicolas Ancion (ULg), Nicoleta Schiopu (CSTB)

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INTRODUCTION

Future cities need solutions to improve quality of life, reduce greenhouse gas (GHG) emissions, and adapt to climate change. A symbiosis between urban environment and agriculture production can help fight climate change and improve overall living conditions in cities. The GROOF project aims to reduce GHG emissions by creating a synergy between rooftop greenhouses (RTGs) and buildings. It reduces transportation emissions by creating local food production. This assessment of RTGs provides information on a symbiosis between greenhouses and buildings and the economic aspects of urban food production.

TECHNICAL ASSESSMENT OF RTGS IN THE GROOF PROJECT

The synergies between RTGs and buildings offer a wide range of potentials corresponding to environmental benefits quantifiable in terms of reduced gas emissions. These synergies and their relative flows are illustrated in the figure below to provide a legible view of interrelationships among systems diagrams.
See figure below : Symbiotic relationship between an RTG and a building in which waste streams are interrelated, and reciprocal exchanges of flows are optimised.

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The GROOF project offers insights into the following CO²eq-mitigating synergies:

  • Energy (using waste heat from the building or greenhouse)

  • Using a PV system integrated in a greenhouse

  • Water (using rainwater)

  • CO² recovery from the building (derived from human activities)

This will be analysed and monitored in four pilot RTGs established in France, Belgium, Germany, and Luxemburg.

ENVIRONMENTAL ASSESSMENT

Life cycle assessment (LCA) was chosen as the suitable methodology to assess and demonstrate the potential mitigation of CO² emissions by RTGs in the different scenarios of the GROOF project in NWE. LCA is a useful tool to quantify environmental impacts of different kinds of systems (products, buildings, etc.) according to international standards (ISO 14 040 and 14 044). Methodological specifications of the LCA for the construction sector are given in European standards EN 15 804 and 15 978. The goal and scope of the LCA are in accordance with the objective given by the Interreg NWE funding program, i.e. to facilitate the implementation of low-carbon, energy, and climate protection strategies to reduce GHG emissions. The GROOF scenarios are compared with business-as-usual scenarios, considering the main contributors:

  1. Products needed to build/renovate the building and the RTG

  2. Energy consumption of the building and the RTG

  3. Fertilisers for crop production in the RTG

  4. Water consumptions of the building and the RTG

  5. Land use change as an indicator of the space efficiency benefits of RTGs

  6. Crop transportation from the RTG to the building (business-as-usual scenario: 150 km).

ECONOMIC ASSESSMENT OF RTGS

An assessment based on the life cycle costing (LCC) method is the following documents: 
Ref. Hunkeler et al. (2008), Swarr et al. (2011) and ISO (2008) Ref. Hunkeler et al. (2008), Swarr et al. (2011) and ISO (2008)
Peña, A. and Rovira-Val, M. R. (2020) ‘A longitudinal literature review of life cycle costing applied to urban agriculture’, International Journal of Life Cycle Assessment.
doi: 10.1007/s11367-020-01768-y.

CONCLUSIONS 

Greenhouse production can be energy-intensive: this depends on the greenhouse design and crop species. We compared synergetic rooftop production with conventional production systems to calculate GHG emission savings. For this reason, we analysed the energy and material flows of different commercial greenhouse designs (from highly efficient to inefficient) and crop production systems (lettuce to tomato).
The analysis showed that the highest CO² emission saving can be achieved by combining energy efficiency measures and the use of renewable energies. The following figure shows the CO² emissions of a conventional tomato production system.

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GHG emissions of the energy and material flows of conventional tomato production

Based on these results, the reduction of GHG emissions is strictly related to the greenhouse system type, the symbiosis between the building and the RTG and the use of renewable materials and energies. Possible components of such a system are:

  • recovering waste heat from the building and the greenhouse or using renewable thermal energy sources,

  • sustainable construction materials: recycled or renewable raw materials,

  • integrating a PV (photovoltaïc) system to use renewable energies.

Additional potentials include producing local fertiliser because efficient nutrient recovery is possible in urban buildings that host many users. For example, separate collection of urine can recover nutrients for fertilisation and reduce the fertiliser demand for the production system.

Overall, depending on each pilot scenario, it will be possible to save 10 to 20 t CO²eq per m² * year within the GROOF project framework as compared to conventional production systems.

More detailed results will be available at the end of the project, based on fine-tuned modelling and monitoring over an 18-month period.