Ever since the positive prospects of cultivating algal cells for biofuel production began being extensively deliberated in the literature (Chisti, 2007; Singh et al., 2011; Singh and Gu, 2010; Wijffels and Barbosa, 2010), recent active research and development have further propelled this industry a step closer to scaling up and commercialization. However, the issue of energy balance in the entire system boundary of algal biofuels is not clearly addressed, mainly due to limited availability of commercial cultivation plants for technical assessment. Based on several life-cycle assessments (LCAs) of algal biofuel production, most of the studies unfortunately revealed a negative energy balance in their assessments, especially when algae were cultivated in closed photobioreactors (Jorquera et al., 2010; Razon and Tan, 2011; Stephenson et al., 2010). Although some important parameters (biomass yield, lipid pro­ductivity, specific growth rate) assumed in the LCA studies were predominantly based on findings from laboratory scale and might be irrelevant for large-scale production, it gives a baseline to visualize and to verify energy balance-related problems in the algal biofuel production system. As a result, several precautionary steps could be suggested to further improve the energy conversion efficiency of algal biofuel production before commencing the commercialization stage.

Energy-efficiency ratio (EER) is usually used as an indicator to address the energy con­version efficiency for the entire biofuel production process. The EER is defined as the ratio of total energy output to total energy input, where a ratio higher than 1 designates net positive energy generated, and vice versa (Lam and Lee, 2012; Lam et al., 2009). Table 12.1 shows a comparative study on EER for biodiesel derived from various energy feedstocks such as oil palm, jatropha, rapeseed, sunflower, and algae. The values presented in the table are a rough indicator because all the LCA studies were conducted based on different assumptions and system boundaries. From the information presented in the table, it can be observed that biodiesel derived from oil-bearing crops is much more energy efficient than biodiesel derived from algae. All the EER values for biodiesel derived from oil-bearing crops are more than 1, whereas algal-derived biodiesel has an EER value as low as 0.07. These quantitative results showed that the cultivation of algae for biodiesel production does not necessarily produce a positive energy output but, worse still, could pose a critical risk of unsustainable biodiesel production. In addition, several issues such as reusability of water to recultivate algae, the possibility of using contaminated wastewater as a nutrient source, and the extraction and transesterification conversion efficiency have not been clearly accounted for in those LCA studies. If these factors are taken into consideration, the EER value is expected to decrease significantly.

However, there are exceptional cases where the EER values are positive, such as those studies performed by Lardon et al. (2009), Batan et al. (2010), Jorquera et al. (2010), Sander and Murthy (2010), and Clarens et al. (2010). These studies highlighted the importance of choosing suitable cultivation methods (e. g., nutrient deficiency to increase lipid productivity), nutrient sources (e. g., wastewater), open pond/photobioreactor design, and downstream biomass

processing options that can enhance the EER value of algal biodiesel. In the following sections, several energy-related problems in producing algal biofuels are comprehensively elaborated to propose possible strategies to commercialize this renewable fuel.