Mitigating the environmental impacts of urban agriculture: innovative materials, GHG emissions analysis and new by-products /
- Autores: Pere Llorach Massana
- Directores de la Tesis: Juan Ignacio Montero Camacho (dir. tes.), Francisco Javier Peña Andrés (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Jordi Bartrolí i Molins (presid.), Francesc Castells Piqué (secret.), Rosario Vidal Nadal (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Ambientales por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Resumen
Rooftop greenhouses (RTGs) are an urban agriculture (UA) modality which allows intensive food production on the roof of cities. RTGs can be connected with the building they are placed on to exchange water, heat or CO2 flows. These types of RTGs are named integrated RTGs (i-RTGs). i-RTGs can use the rainwater harvested by the building to irrigate crops, take advantage of the thermal inertia of the building to warm crops without using heating systems or use the residual air of the building, with high CO2 concentration due to human respiration or other processes, to increase the CO2 concentration of crops. These strategies are of great interest to mitigate the environmental burdens of i-RTGs.
In comparison with conventional decentralized agriculture, previous studies show that i-RTGs could reduce the environmental implications of feeding cities. Benefits are mainly obtained due to reduced food transportation distances, minimized food losses during transportation and improved packaging logistics which allows its reutilization. However, no advantages were detected for the other life cycle stages; therefore, further research is still lacking in this area. This doctoral thesis aims to fill this gap by addressing the following research questions:
1. Can passive systems made with phase change materials (PCMs) replace conventional heating in greenhouses and reduce the carbon footprint of i-RTGs? 2. Can the residual air of a building be used for CO2 enrichment in i-RTGs? 3. Could the GHG emissions of i-RTGs be calculated with more accuracy? 4. Is the creation of new by-products, with UA wastes, a strategy to sink the CO2 emissions captured by crops grown in i-RTGs? Life cycle assessment (LCA) was the main method used to answer these questions. In addition, other specific methods and materials (i.e.; open chamber system, pyranometers; anemometers; temperature sensors; CO2 sensors and gas or liquids chromatography) were used according to the requirements of each specific research line.
With the aim to answer the first research question, a theoretical study was carried out to determine the technical and environmental feasibility of using phase PCMs, as passive system, to replace conventional heating systems for greenhouses. Later, this study was expanded with a practical study to verify first data obtained and increase its precision. Main results demonstrate that PCMs will not be of interest to reduce the energy consumption of conventional heating systems until its prices decrease and the efficiency of its production increases, as more than the 90% of the environmental impacts and costs are generated during the production stage of PCMs. However, its technical feasibility is high, since the application of PCMs in the root zone of soil-less crops does not hinder agricultural maintenance tasks. Nonetheless, in cloudy days with low solar radiation, PCM may not accumulate enough thermal energy to heat crops during night. Therefore, it can be necessary to use complementary systems to heat crops in a timely manner.
The second research question was studied by collecting data from the ICTA-ICP building and its i-RTG. The design of the building allows to inject residual air from the laboratories into the i-RTG. CO2 concentrations of residual air and the i-RTG were measured to assess if residual air can be used for the carbon enrichment of crops grown in the i-RTG. The CO2 concentration measured in the residual air was equal or lower than 500 ppm. So, the CO2 concentration of the residual air is not high enough to allow the carbon enrichment of the i-RTG from the ICTA-ICP building. Nevertheless, due to the high CO2 concentration in household and office buildings, which is between 350 and 2.500 ppm, the residual air from their ventilation systems could be used for the carbon enrichment of i-RTGs.
Nowadays, N2O direct emissions from conventional crops and for i-RTGs are being quantified by using generic emission factors, like that of IPCC (0.0125 kg N2O-1 per kg N). As previous research demonstrates, emission factors vary according to the type of soil used, irrigation frequency or daily solar radiation, among others. So, the application of generic emission factors to quantify the direct N2O emissions from crops, which have a climate change potential 298 times higher than CO2, can cause the over o underestimation of crop’s carbon footprint. With the purpose of determining the error that the use of generic emission factors can generate on the carbon footprint calculation of i-RTGs, it was measured the emission factor of a soil-less crop grown in the i-RTG of the ICTA-ICP building through a nitrogen balance. The emission factor measured was 0.0079 kg N2O-1 per kg N, that is significantly lower than that of the IPCC. This result responds to the third research question and demonstrates that the carbon footprint of i-RTGs, which have been calculated until today with generic emission factors, has not been estimated with accuracy. The application of non-specific nitrogen emission factors can have caused the overestimation of i-RTGs carbon footprint by 7.5%.
Finally, the fourth research question have been analyzed through the study of the technical and environmental feasibility of producing biochar and insulation materials with tomato plant residues. When waste biomass generated in i-RTGs is used to produce new by-products, biomass waste management is avoided. Moreover, if efficient production systems are used and transportation distances are minimized, the emissions fixed within agricultural wastes could be higher than the emissions released during the obtaining process of the by-product. Producing by-products with tomato plants residues, such as biochar or insulation materials, has the potential to fix between 450 y 550 kg de CO2 per ton of dry waste reused. The results evidence that the creation of by-products with UA wastes can be the strategy studied with the higher feasibility to reduce the environmental implications of i-RTGs. This strategy is also of great interest to reduce the environmental impact of conventional agriculture.
In the near future, further research would be of interest to address in more detail the new topics studied during this dissertation and develop research on other methodological aspects detected.
