The society is eagerly looking for economically viable bioenergetic solutions to the near future limited supply of fossil fuel. Plant biomass derived fuel need to maximize production with minimal energy input to cultivation and harvesting operations. Microalgae present several key advantages over higher plants as renewable source of sustainable biofuel (Khan, Shin, & Kim, 2018). Nevertheless, inevitably there is a demand for successful advances in cultivation tank design, so it can also be scaled up with more reduced cost and that has attracted the interest of several groups (Posten, 2009; Salama et al., 2017; Kubelka et al 2017). Of the entire microalgae biomass production chain, about 27% of the total cost are due to the energy consumption by the culture tank mixing/circulation system (Pires, Alvim-Ferraz, & Martins, 2017). Despite the importance of other variables like culture media, light, temperature and pH, the adequate control of the airflow rate could enable a favorable balance between energy cost and microalgae biomass production (Arudchelvam & Nirmalakhandan, 2012).
Decreasing airflow rate in the cultivation tank results in a marked reduction in the surface velocity of air getting into the culture tank (Kulkarni & Joshi, 2005). This may result in higher settling of microalgae cells forming dead-zones (Rampure, Kulkarni, & Ranade, 2007), and leading to a decrease in biomass productivity (Kubelka, Pinto, & Abreu, 2017). On the other hand, low airflow rate leads to a reduction in the energy cost to produce microalgae biomass (Norsker, Barbosa, Vermuë, & Wijffels, 2012). The application of the Energy Return On Investment (EROI) analysis allows accurately establishing a proper equilibrium between productivity and energy-saving. Additionally, it is an opportunity to compare with other energy sources (Hall, Lambert, & Balogh, 2014). Values of EROI greater than 1.0 indicate the economic potential of an energy source as an adequate substitute for fossil fuel. For instance, gasoline presents an Energy Return On Investment between 5.0 and 10.0 (Cleveland, 2005; Hall et al., 2014). Bioenergy sources stand back with values of 0.42 in the EROI analysis performed in a microalgae cultivation raceway (Colin M Beal et al., 2012) and 0.8 from terrestrial plants (Hall et al., 2014).
Biofuel from microalgae biomass have low EROI values, compared to fossil fuel, mainly due to the lack of adequate energy-efficiency improvements in the cultivation systems. Most of the reported improvements addresses increasing productivity only (Ho, Ye, Hasunuma, Chang, & Kondo, 2014), but rarely consider the most adequate cost-benefits of the system. The economic viability of a microalgae biofuel plant stands on producing the most, consuming minimum energy and low-cost cultivation resources (Borowitzka & Moheimani, 2013; Chua & Schenk, 2017; Slade & Bauen, 2013). In this context, the Microalgae Production Laboratory (LPM) at the Federal University of Rio Grande, works in the development of a low-cost biomass production system. To achieve this, the laboratory began seeking methods to improve cultivation conditions without increasing energy requirements (Kubelka et al., 2017; Kubelka, Roselet, Pinto, & Abreu, 2018), or indeed reducing production costs (Faé Neto, Borges Mendes, & Abreu, 2018). Harvesting procedures also were developed by LPM so biomass production costs were also reduced (Roselet, Burkert, & Abreu, 2016).
Currently, LPM uses 330 L (aspect ratio = 4.54 – see Table 1) bubble column photobioreactors (PBR) to produce massive volumes of an oil rich marine microalga, Nannochloropsis oceanica. These vertical tubular PBR, used by LPM, after appropriate productivity and energy-efficiency improvements show high potential for commercial scale up for biofuel production. This is principally, because of the simplicity and the easiness of reproducibility of the tank and the air-injection system, which have been pointed out as the most critical barriers in the proper use of PBR to commercial microalgae biomass production (Seo et al., 2012). However, improvements can be pursued by employing Computational Fluid Dynamics (CFD) which saves valuable time and prolonged effort including cost in constructing several PBR air-injection systems and successive experimental procedures.
Table 1
Dimensions of the closed cylindrical bubble column photobioreactor (PBR) and the nozzle the installed in the PBR.
D (m) | h (m) | V (m− 3) | NInj | DInj (mm) | Dbm (mm) |
0.55 | 1.25 | 0.33 | 9 | 1 | 6.5 |
Previously, studies developed in our laboratory have already improved productivity without an increase in energy consumption in these PBR through the reduction of air nozzles sizes and, consequently improving mixing condition inside PBR (Kubelka et al., 2017; Neto, Zhu, M.ASCE, & Rajaratnam, 2008). Naturally, the next step consists of optimizing the volume of air supplied, so there is no energy waste by the air blower. Simultaneously to the use of CFD, the Energy Return On Investment (EROI) analysis was implemented to assess the influence of increased efficiency in microalgae biomass production and its energy-cost in the context of biofuel production.
In the present study, in silica analysis of the different airflow rates and its potential maximization in the PBR production of N. oceanica is presented. The validity of the CFD results was checked by cultivation experiments. Furthermore, the EROI analysis was carried out to estimate the energy-efficiency of three airflow rates chosen through the CFD tool as most effective air injection configuration.