Energy consumption in the production process is deemed the largest ob­stacle to algal biofuel production and a positive energy balance is a neces­sity but difficult to achieve. There are now several LCA studies which have investigated the amount of energy consumed in each of the neces­sary processes and comparing this to the energy recovery potential. Some studies have suggested that a positive energy balance is possible others suggest the contrary. Lardon et al. [77] found that if algae was cultivated purely for biodiesel, a positive balance would be unattainable. However if the residual biomass were to be anaerobically digested, a positive balance could be achieved in the scenario of growing algae in low nitrogen media and processing wet biomass [86].

The majority of other life-cycle analyses conducted recently suggest that algal biofuel can be produced with a positive energy balance though possibly not as positive as some alternative biofuels. Clarens et al. [87] modelled the growth of algae in raceway ponds and compared the energy consumption and environmental impacts of the fuel produced to fuel from corn, canola and switchgrass. In terms of energy consumption it was found that algal biodiesel required a far higher input, at least four times as much) as the next highest, and a sensitivity analysis revealed that the energy con­sumption was mainly a result of fertiliser use and carbon dioxide produc­tion [88]. Sander and Murthy [95] conducted a LCA comparing the differ­ence between harvesting methods of filter pressing and centrifugation, and a positive energy balance for both methods was reported, with a higher net energy yield for the filter press (almost double that of the centrifuge). The mentioned study did not provide details of the modelled strain nor likely productivity rates; it assumed that the algae contained 30% lipids, which would be difficult to achieve for an outdoor cultured strain; and did con­sider year round production, which would also be a challenge. In a study by Stephenson et al. [96] air-lift tubular photobioreactors and raceway ponds were compared in terms of energy consumption and yield, and their results suggested that the majority of energy was consumed in the cultivation stage (i. e., the cultivation stage in bio-reactors required approximately 10 times more energy than raceway ponds), which is in contrast to previous studies [77][87]. This study has similarities with that of Jorquera et al. [55] who showed raceway ponds provided a far greater energy balance than bioreactors. A high energy consumption in the bio-reactors was attributed to the manufacture of the PVC material and circulation of the culture [55]. The majority of energy consumed from cultivation in raceway ponds was due to circulation using a paddlewheel. It was suggested that for raceway cultivation the anaerobic digestion of the residual biomass could offset the energy required from cultivation, however this was not the case for the tubular photo-bioreactors [55].

In a further study conducted by Clarens et al. [97], different process chains and how these affect the energy balance or Energy Return on In­vestment (EROI) were compared, particularly the study looked at the vari­ous end-products from the algae (i. e., anaerobic digestion (AD) to electric­ity, biodiesel and AD to electricity, biodiesel and combustion to electricity and direct combustion) as well as source options for CO2 (i. e., virgin CO2, carbon capture, flue gas) and nutrients (wastewater supplementation). In each case direct combustion of the biomass to electricity produced the highest EROI and the best option was direct combustion of the biomass with direct compression of flue gas providing a source of CO2 [97]. The EROI for this scenario was 4.10, a similar scenario using flue gas and wastewater supplementation provided an EROI of 4.09. According to Cla­rens et al. [97] a value greater than 3 is considered sustainable, comparing well to canola (2.73), but not to switchgrass (15.90). Table 8 provides a summary of the best energy balances produced through the main studies conducted investigating energy recovery from algae.

Current production of biodiesel from algae without some other form of energy recovery will usually give a negative energy balance. To overcome this, it is necessary to include another form of energy recovery such as anaerobic digestion or combustion. It may even be far more beneficial to ignore biodiesel and to recover energy directly from anaerobic digestion or combustion as their input requirements are significantly lower. Energy reduction measures in each process will further improve the viability of biofuel from algae whatever the process stream used. Further work is re­quired to find the optimal recovery method that can compete with the en­ergy balance of conventional biofuels.