It is well recognized that the production phase is most significantly affected by its energy requirements. This is most marked for photobioreactors and contributes significantly to costs. Norsker et al. (2011) explored these interactions. In the hori­zontal tubular reactor, liquid circulation contributes to both capital costs through the pump required and to energy costs. By reducing the linear velocity from 0.5 m s-1 to the minimum value predicted of 0.3 m s-1, a reduction in cost per kilogram of 25% was achieved. Similarly, the aeration of the flat-plate reactor (or vertical tubular reac­tor) contributes significantly to capital and operating costs. In the former, a reduction in the aeration rate from 1 vvm (volume per volume per minute) to 0.3 vvm resulted in a 48% reduction in cost. The contributions of mixing and mass transfer in the flat — plate and tubular reactors were on the order of 52% and 30%, respectively.

Productivity significantly influences the cost of production. This is well demon­strated by the increased productivity with increasing light intensity, as correlated by Williams and Laurens (2010). Areal productivity increased from some 10 g m-2d-1 at an irradiance of 15 MJ m-2d-1 to 30 g m-2d-1 on doubling the irradiance to 30 MJ m-2 d-1. The impact of improved productivity under improved irradiance is illustrated by comparing the cost for production in Eindhoven (the Netherlands) and Bonaire (Dutch Antilles). In the three cases analyzed (Norsker et al., 2011), the cost decreased by 40% to 45% under conditions of increased illumination (sum­mer high of 7,000 and 5,000 W-h m-2d-1, respectively; winter low of 4,500 and <1,000 W-h m-2d-1, respectively, in Bonaire and Eindhoven).

The provision of CO2 can significantly impact the costing, based on whether the CO2 is provided “free” as a by-product of an adjacent process or requiring its purchase as a compressed gas (Williams and Laurens, 2010). Stephenson et al. (2010) further considered the compression energy, with associated costs, based on the CO2 concentration in the gas stream.

Williams and Laurens (2010) noted that their energy costs (typical of the US envi­ronment) were some sixfold higher than those estimated by an analysis conducted in British Columbia, Canada, where hydroelectric power was used. This highlights the potential for the use of renewable energy resources in conjunction with algal production.