Decentralized algal energy system design at various urban densities and scales

Abstract

Algal biotechnology seems promising to be implemented as a decentralized urban energy system due to its high biomass productivity and its utilization of urban waste streams. Current literature on algal energy systems mainly focuses on algal strain, reactor, and process improvements, neglecting the systems level consequences arising when algal biotechnology is integrated into the urban system as a part of renewable energy resources. The overall performance of the integrated system can vary due to different availability levels of urban nutrient, solar and CO2 resources, and the cost can vary due to different settings of material transports and processing. This study explores two critical questions: How does the urban density affect overall system performance? At what spatial scale could the optimal system performance be achieved? The study investigates these questions through test cases in real urban settings in Atlanta, Georgia, USA. Four neighborhoods are selected with different building densities representing the urban core, urban, suburban and rural areas, with scales of 0.5, 1, and 2 km radiuses as the system boundaries. Decentralized algae energy system design is proposed for each site, while available vacant lands are identified to place such energy systems. Based on relevant literature and validated seasonal cultivation data from the Algae Testbed Public-Private-Partnership (ATP3) project, as well as established data and methods for urban resource availability assessment and transport calculations, the overall energy performance of the decentralized system is estimated. The results show that higher density leads to higher energy performance while scale has no obvious effects on the energy performance. At the same time, the negative net energy production suggests that under current algal technologies, it is still far from feasible to implement the system in real urban settings.