Arcasi, Alice (2023) Thermo-economical characterization and simulation of innovative systems for the agrifood sector. [Tesi di dottorato]

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Abstract

Nowadays, in the agricultural sector the reduction of the water, energy and chemical substances use is a crucial and fundamental aspect. Moreover, the arable land crisis, as well as the ongoing climate change and the constant global population growth aspects are all strictly related among them and the efforts of the next few years will be aimed to correct manage the link between food, energy and water in the agricultural sector. In this context, one of the primary goal of the Global Agenda of the United Nations is to promote, implement and develop sustainable and healthy food production systems able to increase the productivity, safeguard the ecosystems and able to adapt themselves to the critical conditions caused by the ongoing climate change such as drought, floods and other natural disasters. With the aim of energetic and environmental sustainability, the vertical farm systems represent an innovative, interesting and alternative solution to the traditional crop production systems due to their lower water consumption and pesticides use. On the other hand, due to the higher energy consumption of such systems, the primary challenge in the research lies in evaluating their viability and techno-economic feasibility. The present Thesis is focused on the thermo-economic characterization and simulation of such systems: in order to quantify their energy consumption and distribution among the system’s components – HVAC, lighting system and fans – a thorough model of a VF system is presented by considering its growth chamber with the main goal to establish a relationship between its energy consumption and produced yield. It is worth noting that all the influencing factors which affects the growth phase of the crops and consequently the energy consumption of the whole systems are taken into account, i.e. investigated climate conditions (Naples, Riyadh and Stockholm) and operating parameters – leaf temperature, lighting system efficiency, lighting system intensity and economic scenarios. The main aim of the research activity carried out during the Ph.D. program and consequently the principal objective of the present PhD Thesis is to provide general guidelines regarding the energy consumption and the energy cost related to the growth chamber for each product harvested in a VF system. In detail, a multi-level model of the whole food production in a vertical farm is discussed which allows to allocate the primary energy demand as well as the energy costs to the produced unit of product harvested. The purpose of this approach is to establish a general method for assessing the specific energy expenditure associated with any type of product unit. It aims to emphasize the various factors contributing to the overall energy cost of a product, thereby pinpointing the most energy-intensive elements that require optimization. In detail, once the environmental and energetic context of the agricultural sector is presented and depicted in the CHAPTER 1 in terms of energy and water consumption, arable crisis and global population growth, along with the ground, soil and water contamination due to the significant use of pesticides and chemical substances of the sector. The traditional farming methods – open-fields and greenhouses – are presented and discussed in the CHAPTER 2 as well as the vertical farm systems. A comparison between them is carried out by considering all the aspects of interest, such as energy and water use, arable land and pesticides use as well as their environmental impact in terms of greenhouse gas emissions (GHG) and finally economic aspects are discussed. The limits and potentialities of vertical farms are presented and the need of a model which is able to evaluate their energy consumption including all the influencing factors (the crop type, the internal environmental conditions, the lighting system efficiency and the lighting system intensity) is discussed. With this aim, in the CHAPTER 3 the state of the art of the VFs’ thermal loads modelling is depicted, by including the ones used for the evaluation of the thermal load through the building envelope, the ones regarding HVAC system, the crop energy balance models and the interaction between the lighting system and the crops during their growth phase. Once all the thermal loads models are illustrated, the CHAPTER 4 focuses on the developed thermo-economic model for the chosen vertical farm, by including different climate regions, different lighting system efficiencies and intensity levels. All the developed models for the sub-systems which constitute the cultivation chamber of the plant are presented, including the one for the crops transpiration and energy balance, the one for the heat transfer with the external environment, the air conditioning system model and the economic one through which is possible to attribute the specific cost for energy (namely SCE) to the kilogram of each harvested product. Finally, in the CHAPTER 5 both the energy and economical analysis are carried out, by considering different climate zones – Naples, Stockholm and Riyadh – different values of lighting system efficiency, intensity and leaf temperature. The effect of the abovementioned operating parameters on the primary energy consumption, on the produced yield and the specific energy consumption is investigated along with the effect of the operating parameters on the specific cost for energy. The results unveil a strong dependence of the total energy consumption and the specific cost of the product on external conditions, such as the climate, the efficiency and the energy cost. Notably, in Riyadh the energy consumption (11.4 GWh/year) is up to 100% higher than in Stockholm and 45% higher than in Naples. By means the optimization of the internal conditions, such as the lighting system efficiency (LED efficiency) and intensity (PPFD), and the target leaf surface temperature during the illumination period, the rate of cost for energy for the lettuce produced in the VF, attributable solely to energy expenditures in the growth chamber, is 1.09 €/kg in Stockholm, 1.45 €/kg in Naples and 1.98 €/kg in Riyadh.

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